A Survey In Biological Science

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The Human Organism

and

the World of Life

The

Human Organism

and the

World of Life

Harper & Brothers

New York

London

A Survey in
Biological Science

by
Clarenpe \\f- Young

Assistant Professor of Psychology,
Colgate University

G. Ledyard Stebbins

Junior Geneticist, Experiment Station,
University of California

and

Clarence John Hylander

Assistant Professor of Botany,
Colgate University

Illustrations by

Clarence John Hylander

The Human Organism and the World of Life
Copyright, 1938, by Harper & Brothers
Printed in the United States of America

All rights in this book are reserved.

No part of the book may be reproduced in any

manner whatsoever without written permission.

For information address

Harper & Brothers

A-W

Contents

PREFACE vii

INTRODUCTION i

PART I
MAINTENANCE AND SURVIVAL

L THE SUBSTANCE AND STRUCTURE OF THE

HUMAN BODY 5

II. METABOLISM 23

III. CIRCULATION AND RESPIRATION IN THE

HUMAN BODY 40

IV. DIGESTION, ASSIMILATION, AND EXCRE-

TION IN THE HUMAN BODY 66

V. MAINTENANCE SYSTEMS IN ANIMALS 92

VI. THE BODIES OF PLANTS in

VII. THE WEB OF LIFE 129

VIII. COMMUNICABLE DISEASES 147

IX. FUNCTIONAL DISEASES 171

PART II
REPRODUCTION, INHERITANCE AND DESCENT

X. HUMAN REPRODUCTION 189

v XL REPRODUCTION IN PLANTS AND ANIMALS 221

XII. THE REPRODUCTIVE CYCLE 250

XIII. THE PRINCIPLES OF HEREDITY 263

XIV. THE FACT OF EVOLUTION 292
XV. THE OUTCOME OF EVOLUTION 329

XVI. WHAT CAUSES EVOLUTION? 368

XVII. HUMAN EVOLUTION 392

vi Contents

PART III

BEHAVIOR AND MENTAL ACTIVITY

XVIII. THE RESPONSE SYSTEM: THE EFFECTORS 411

XIX. THE NERVOUS SYSTEM 423

XX. THE SENSE ORGANS 444

XXI. INTERNAL ADJUSTMENTS 463

XXII. BEHAVIOR AND MENTAL ACTIVITY 478

XXIII. GROWTH RESPONSES IN PLANTS AND

ANIMALS 506

XXIV. MOVEMENT RESPONSES IN PLANTS AND

ANIMALS 519

XXV. THE DEVELOPMENT OF HUMAN BEHAVIOR 544

XXVI. THE BEHAVIOR OF THE INDIVIDUAL 566

XXVII. MENTAL ILLNESS AND MENTAL HEALTH 598

CONCLUSION 620

APPENDIX I: THE CLASSIFICATION OF

ORGANISMS 625

APPENDIX II: THE BRANCHES OF BIO-
LOGICAL SCIENCE 629
SUGGESTED READING 634
INDEX 639

Preface

Almost a decade ago, as a part of a general plan of curricular
reorganization, there was instituted at Colgate University a sur-
vey course in biological sciences which was made a part of the
work of every Colgate freshman. Its aim was to give every stu-
dent a broad view of scientific knowledge concerning the processes
of life; and in furtherance of that aim it was planned that it
should include a survey of the mental activities occurring in or-
ganisms as well as the processes that have more traditionally
been included within the realm of biological science. This book
is the outcome of our experience in teaching this course. In its
present form it is the result of much experimentation, throughout
which one question has remained uppermost in our minds : How
can this course and this book be made to yield the utmost pos-
sible value to the freshman student?

Our most important discovery has been that the student is
primarily interested in the life process as it displays itself in his
own species, and we have come to the conclusion that this is not
only a natural but a thoroughly wholesome prejudice on his part.
For human life, one's own life and that of one's companions, is
surely the aspect of the biological process that is of most profound
importance to every man and woman; and if one is to spend only
a brief period in the study of biological science, one will certainly
put in his time to best advantage in securing as good an under-
standing as possible of the ways of life among his own kind.

This centering of attention upon the human organism does not
necessarily make for a narrowness of outlook. In the first place,
the picture of human life cannot be complete unless' it includes
within it a portrayal of the relationship between the species Homo
sapiens and the whole organic world. Secondly, the activity of
protoplasmic systems is much the same throughout the two bio-
logical kingdoms, and a comprehension of the workings of one
organism provides the key to the understanding of all. Hence,
our account shifts back and forth from detailed description of
human processes to less detailed comparison with the structure
and function of plants and animals. Our experience indicates that

viii Preface

this approach does make the study of biological science meaning-
ful and interesting to the freshman student and that he finds that
it satisfies his own felt needs and desires for knowledge in this
field.

Early in our work with the course, we discovered that one of
the major difficulties our students experienced was the mastery
of biological terminology. To prevent this purely mechanical dif-
ficulty from standing between the student and the acquisition of
an understanding of the facts and principles which we are pri-
marily interested in conveying to him, we have eliminated as
much strange vocabulary as possible and have introduced a rather
complete glossary of new terms at the end of each chapter. It is
not intended that these glossaries be used for reference purposes.
Rather, they are to be employed in the study of each chapter, so
that the student can make certain that he has mastered the new
terminology before he passes along to sections where the words
may be used without explanation of their meaning. In short, these
glossaries are intended to serve much the same function as the
vocabulary lists attached to each lesson in an elementary course
in foreign languages.

While the authors did not confine their work entirely to spe-
cific chapters, the chief responsibility for the preparation of Chap-
ters II, V, VI, VII, and XI was in the hands of Dr. Hylander,
and for Chapters XII, XIV, XV, XVI, and XVII in the hands
of Dr. Stebbins. Dr. Young is chiefly responsible for the prepara-
tion of all other chapters as well as for the general editing of
the entire book.

In closing, we wish to express our appreciation to all those
who have helped us in the preparation of this book with friendly
counsel and criticism. Especially, we wish to thank Dr. W. M.
Chester, Dr. F. S. Keller, Dr. James Stauffer, Dr. Raymond J.
Myers, Dr. Oran Stanley, and Dr. G. H. Estabrooks, our col-
leagues at Colgate ; Dr. Jackson W. Thro, of Hamilton, N. Y. ;
Dr. H. D. Stebbins, of Brookline, Mass.; Dr. Edgar Anderson,
Dr. R. H. Wetmore, Dr. J. H. Welsh, all of Harvard Univer-
sity; and Dr. Ernest B. Babcock, Dr. Richard Goldschmidt, and
Dr. Alden Muller, all of the University of California.

Colgate University CLARENCE W. YOUNG

April 1 8, 1938

The Human Organism

and

the World of Life

INTRODUCTION

All living things, whether plant or animal, whether large or
microscopically small, are known as organisms. A man is an
organism; a stalk of corn is an organism; the "germs" that cause
smallpox, which happen to be so small that they cannot be seen
with the most powerful microscopes, are organisms.

All organisms have certain traits in common which more or
less distinguish them from objects that are not alive. Organisms
expend energy and wear their bodies away in the course of their
activities, yet they replenish their stocks of energy and replace their
wasted structure, either by consuming or by manufacturing food.
In addition, organisms grow, they reproduce, and they respond to
changes in their environment. Self -maintenance, growth, repro-
duction, and response may be termed universal organismic activi-
ties, since they characterize all living things, while non-living
things display them to only a slight extent if at all.

The relationships between organisms are so close that it is pos-
sible to look upon the entire world of life as a single great system
of activity. Life is unified in two ways. In the first place, organ-
isms are dependent upon one another for the chemical substances
which are essential to their existence, and each type of organism
plays a part in maintaining the entire world of life as a going
concern. In the second place, all organisms are related to one an-
other through a long line of evolutionary descent. A billion years
ago or more, the first organisms appeared upon the earth. All the
evidence indicates that they were simpler than even the humblest
of the organisms with which we may become acquainted by view-
ing them through the most powerful microscopes. Through the
long aeons that have passed since that time, the descendants of
those organisms have developed through evolution to become the
myriad of living forms both great and small which populate the
earth today.

The theme of this book is the role of the human organism in
the world of life. We shall see how all the great life activities are

2 Introduction

carried on by the members of the human race and compare the
way in which man maintains himself, grows, reproduces, and re-
sponds to his environment with the manner in which these func-
tions are carried on in plants and animals. We shall trace the line
of descent from the first inconceivably primitive living things
down to the human species of today.

Finally, we shall consider how man's capacity for speech, which
enables him to build up a body of knowledge and aspiration that
can be handed down to his descendants from generation to gen-
eration, makes of him an entirely unique sort of organism, pos-
sessing capacities far above those of any other form of life, and
facing problems with which no other organisms can be even re-
motely concerned.

PART I
MAINTENANCE AND SURVIVAL

A streamlined tree. (See page 512.)

CHAPTER I

THE SUBSTANCE AND STRUCTURE OF THE
HUMAN BODY

The Living Substance. — There is nothing in the world more
wonderful than the body of a living organism. It is a structure
in which occur all the intricate and remarkable activities which
constitute the miracle of living. All the achievements of mankind
have depended upon the conformation and structure of the hu-
man body, and all the life processes of plants and animals — which
appear the more marvelous to us the better we become acquainted
with them — are made possible only by the manner in which their
bodies are constructed. In this chapter we shall deal with the fun-
damental principles of bodily organization .in the : human being.

In the first place, the body is composed in large part of a certain
unique substance which forms Jhe most essential part of all living
s* from the tiniest bacterium to the tallest tree, but which

is found nowhere else in nature. It is, like time in the old proverb,
the very "stuff that life is made of." It is called protoplasm.

Not all the body is composed of protoplasm. The blood, for ex-
ample, is not protoplasm; neither are those solider substances
which give support and protection, such as the mineral part of the
bones, the hair, and the outer layer of skin. In fact, protoplasm is
u§ua!hLSQ completely enmeshed Jn^aon-livmg supporting and pro-
tecting structures that it is very difficult to isolate it in such
fashion as to make the study of it possible. An extremely minute
bit, however — a single cell — can be viewed under the microscope;
and it is possible to pick and tear at it with a very fine glass needle,
known as a microdissection needle, and thus get a conception of
what this living substance is like.

Under high magnification, it is a rather transparent grayish
stuff, something like uncooked white of egg, except that it fre-
quently appears to be full of small granules or bubbles. Often it is

5

6 The Substance and Structure of the Human Body

seen to flow restlessly round and round upon itself. It resembles a
globule of oil in that it does not mix with the water that usually
surrounds it, but forms a sharp boundary line between itself and
its environment.

While watching the tiny mass of protoplasm under the micro-
scope, one can carefully push the microdissection needle a little
way into it and then pull it out. A small bit of the living substance
sticks to the needle, stretches out, and, when it finally breaks loose,
moves back into the cell. In this way one discovers that protoplasm
resembles egg white in still other respects; it is sticky and elastic.
The needle also reveals that most of it is quite liquid, about the
consistency of a light oil, while certain parts may be as solid as
a soft jelly. There is a tendency for protoplasm to fluctuate be-
tween the jelly-like and the oil-like state.

The activity of this slimy, transparent, viscid, elastic, restless
material underlies all the activities of life. The growth of a tree,
the flying of a bird, the thinking of man — protoplasmic activity
is fundamental to them all. To explain how protoplasm is capable
of carrying on these activities would be equivalent to explaining
life itself. No scientist has ever been able to do it. But a large num-
ber of its properties may be attributed to the fact that it is a highly
con$f>lex colloidal system.

A cottwdal system exists where extremely fine particles of one
substance are held in suspension in another substance. For ex-
ample, gold may be broken up into particles so small that they
will not settle to the bottom in a jar of water but will remain sus-
pended throughout the jar. The particles are so minute that they
cannot be seen even under the highest-powered microscope, yet they
are considerably larger than the particles of a substance that is in
true solution in water. Ordinary smoke is another example of mat-
ter in the colloidal state. In this case, tiny solid particles of ash are
held in suspension in the air. There are, in addition, many every-
day examples of colloidal substances which are derived from the
bodies of living organisms, such as milk, butter, gelatin, agar-agar,
various jellies and glues. Although it is not alive, the white of egg,
which resembles protoplasm so closely, is also a colloidal system,
made up of the same substances that compose protoplasm.

Protoplasm itself is simply a special form of colloid in which
small particles of certain substances, known as proteins, are sus-

The Substance and Structure of the Human Body 7

fended in water. These proteins are the most complex chemical
compounds known. By this we mean that their molecules contain a
larger number of atoms than any other molecules yet discovered.
As nearly everyone has learned, the infinitesimally small particles,
known as molecules, which are the unit particles of any substance,
are themselves composed .of still smaller particles, the atoms of the
chemical elements. For example, two atoms of the element hydro-
gen, combined with one atom of oxygen, constitute a molecule of
water. Now the protein molecules are made up of the atoms of
hydrogen, oxygen, carbon, nitrogen, and a few other elements.

A B

FIG. i. — Diagram of a colloidal system. A, continuous phase (white) a liquid
(sol). B, continuous phase (black) a solid (gel).

But, unlike water molecules, which are composed of only three
atoms, protein molecules may contain thousands of them. These
may be held together in an almost infinite number of arrange-
ments, so that there are millions of different kinds of proteins
which, in suspension in water, produce millions of different kinds
of protoplasm. Indeed, the great range of differences between or-
ganisms is based to a large extent on the differences in the proteins
they contain. We are human beings, instead of being plants or
animals of some other sort, partly at least because the protoplasm
of which we are composed contains the proteins which are typical
of human beings. And if we are afflicted with hay fever, it is be-
cause "foreign" proteins from the pollen of plants have entered
our systems and started warfare against our own "native" pro-
teins.

8 The Substance and Structure of the Human Body
Y^^l&sm^teiL-h a^j&ltoiM^ys^

water and^pf Jhe complex ^ In addi-

tion, there are small quantities of mineral salts and of certain fat-
like substances which are essential to the formation of true proto-
plasm.

But why is protoplasm alive ?

Without going into the details of colloidal mechanics, we may
say that malter m Jhe cqllp^ of carrying on activ-

iti.es Jhat are impossible in any other condition. Many CQllQids ...are
capable of changing from a liquid to a jelly-like condition and
back again. Among non-living colloids, gelatin and a mayonnaise
salad dressing may be taken as familiar examples. The former
quickly becomes liquid when warmed, and solidifies again when
cooled, while the latter changes easily from the solid to the liquid
state and back again upon addition and removal of a little water.
Many biologists believe that protoplasm can be compared directly
in its structure to a combination of these two colloids. There is no
doubt, moreover, that many of the activities which we consider
to be the very essence of life, and peculiar to living things — such
activities as the movement of microscopic animals as well as
muscular contraction and nerve action in our own bodies — are
brought about by means of these reversible changes from liquid to
jelly and back.

Furthermore, non-living as well as livin^^QJlQids^cari. on ac-
count of the great chemical activity which is possible when matter
is in this state, build themselves up. from .simpler substances, thus
adding to their bulk. In other words, they can grow. For instance,
if we put a drop of one chemical, a copper salt, into a solution of
another, potassium ferrocyanide, a thin membrane of a colloid is
formed between these two substances, and this membrane will
grow for a long time by building itself up from the chemicals on
either side of it. It grows much as does a living membrane, except
that the process is chemically much simpler and will not continue
indefinitely.

Yet it should be emphasized that non-living colloids display only
the sjrnplest beginnings of those activities which constitute the
fundamental features of life. Protoplasm in action is vastly dif-
ferent from non-living matter in action. Science is not yet capable
of pointing out completely the reasons for that difference, but two

The Substance and Structure of the Human Body 9

ways, in which ^rptppjasm is unlike other colloids raay here be
mentioned.

First, it is exceedingly,, complex. Protoplasm is not a simple
suspension of one substance within another. Under certain cir-
cumstances, for example, it is thought that globules of fatty sub-
stances may have droplets of water suspended within them, while
the suspended water droplets may hold protein particles in sus-
pension within themselves. In other regions of the cell, or under
other conditions, the situation may be almost reversed. For to
add to all its complexity, protoplasm is never the same thing one
instant that it was the instant before. It is hardly accurate to speak
of it as the living substance. It is rather a mixture of many sub-
stances each in a continual state of flux, each continually trans-
forming itself from one thing into another, breaking down,
building up, physically restless and chemically unstable.

Secondly, protoplasm is organized into small, individual, self-
perpetuating systems known as cells. This is probably its out-
standing characteristic, which sets it apart from ordinary,
non-living colloidal systems, since cell organization enables the ac-
tivities of protoplasm to go on in the orderly, controlled fashion
that is essential if living things are to accomplish the acts necessary
to keep them alive. The cell is the unit of life, and it is also the unit
of structure in the human body.

What a Cell Is Like. — The body is composed entirely of these
organized bits of protoplasm called cells, and of the non-living
substances that they have built around themselves. They are so
small that they cannot be seen by the naked eye, and high-powered
microscopes are required to study them adequately. There are
many kinds, of the most diverse shapes and sizes, but all have cer-
tain characteristics in common. Fig. 2 is a diagram of a very
simple cell. It is not intended to be a representation of any actual
structure, since no cell in the human body is as lacking in speciali-
zation as this one. The diagram merely serves to point out the
parts that are characteristic of cells in general.

The nucleus is a more or less spherical body located toward the
center of the cell. In living cells it is often difficult to make out,
but in sections that have been stained by treating them with cer-
tain dyes, the nuclei of the cells are readily seen because they ab-
sorb dyes different from those absorbed by the cytoplasm. This

io The Substance and Structure of the Human Body

nucleus is a sort of "central office" for the cell, since it exercises
a directing influence over the cell's most vital activities, especially
those which have to do with the building up of structure, arid if it
is removed, the cell wears itself away without being able to recon-
struct itself in the way that a normal cell continually does.

The cytoplasm, which is simply the protoplasm outside the
nucleus, is the region in which the everyday work of the cell is
carried on. It is in the cytoplasm that the special structures which
distinguish one cell from another and which determine the special
functions of any cell are to be found. These special structures

Non-living
inclusions

Cytoplaim

Living _
inclusions

Cell membrane

Nucleus

.Nuclear
membrane

FIG. 2. — Diagram of a cell.

are of two kinds : those which are living parts of the cytoplasm
and which perform some special vital function, and those which
are not actively engaged in carrying on life processes.

The most important of the latter structures are stored particles
of food material. Chemically speaking, there are two major groups
of substances which the cell stores, the carbohydrates and the fats.
The carbohydrates are composed of carbon atoms combined with
hydrogen and oxygen in the same ratio as that in which they
appear in water, namely, two atoms of hydrogen to one of oxy-
gen; hence the name "carbohydrate," which means "carbon with
water." The simplest carbohydrates are the single sugars, which
are responsible for the sweetness in honey and most fruits. Their
molecules usually are composed of six atoms of carbon, twelve of

The Substance and Structure of the Human Body n

hydrogen, and six of oxygen (chemical formula, C6H12O6). Some-
what more complex are the double sugars, of which ordinary
table sugar is an example. Their formula is QoIIooOn- Still
more complex are the starclws, whose molecules may contain hun-
dreds of atoms, but always in the ratio of six carbon to ten hydro-
gen to five oxygen. It is a relatively easy matter for one carbo-
hydrate to be changed into another. Sugar molecules combine
readily to form the larger starch molecules, and the latter can be
split up to form sugar molecules. Starch is the form in which car-
bohydrates are stored, since the small molecules of sugar pass out
of the cells too readily to be stored therein. Hence, when a cell
has more sugar in it than it can immediately use, the sugar is
transformed into minute bits of solid starch which remain in the
cytoplasm until they are needed. Cells may use carbohydrates in
two ways. First, they may undergo chemical changes, usually com-
bination with oxygen, to furnish energy for the cell's activities,
just as coal and gasoline combine with oxygen when they are
burned to furnish energy for the running of machines. Second,
at least in plants, they may be combined with nitrogen to build up
the proteins of the cell structure. Animal cells cannot perform the
synthesis of proteins from carbohydrates, and hence must obtain
their proteins by devouring the bodies of other organisms.

Fats, like carbohydrates, are composed chiefly of carbon, hy-
drogen, and oxygen ; but they have less oxygen than the latter in
proportion to their hydrogen and carbon. They may also be used
as fuels to be combined with oxygen for the release of energy,
or they may be chemically modified to furnish the fat-like ele-
ments of the protoplasmic structure.

It is impossible for a cell to remain alive and not be active ; or, to
put it another way, activity, as well as cell structure, is an essential
condition of life; and if that activity stops for even a brief period,
the cell dies and cannot be revived. But all activity requires energy,
and hence it is very important for a cell to have stored food sub-
stances to provide this energy. If these are lacking, however, the
cell can meet the emergency by oxidizing the materials of the
protoplasmic structure. Hence, proteins as well as carbohydrates
and fats can be utilized as fuel. These three energy-yielding sub-
stances are spoken of together as organic foods. Water and min-
eral salts are called inorganic foods because they are found in na-

12 The Substance and Structure of the Human Body

ture apart from life, whereas the organic foods are formed
naturally only in the bodies of organisms.1

To return to our discussion of the structures in the cytoplasm
of cells, another frequent type of non-living structure is the water
vacuole, a small droplet of water, usually surrounded entirely by
cytoplasm, and containing various substances — notably salts, or-
ganic foods, and waste products — in solution or suspension. While
they are found in the cells of animals, water vacuoles are especially

Cell membrane

Nucleus

-Vacuole

Centrosome

Cytoplasm

FIG. 3. — Typical animal cell.

characteristic of plant cells. Fig. 4 shows a plant cell with a large
water vacuole and certain living cytoplasmic structures, known as
chloroplasts, whose function will be described in detail in the next
chapter.

Another important group of cell structures are the boundary
membranes which are formed on the surfaces between two dif-
ferent kinds of protoplasm. A somewhat similar membrane is
built up between water and oil wherever they come together, and
such membranes act as barriers to mixture between two different
kinds of substances. Hence, these membranes serve the function
of setting apart the various protoplasmic and non-protoplasmic

1 Chemists classify any substance that has carbon in it as an organic substance,
since the carbon atom is so essential to life that all such substances are likely
to have been derived ultimately from the bodies of living organisms. Carbon
dioxide (COa), however, a gas which is present in small quantities in the air
and which is the source from which organisms get their carbon, is often classi-
fied as inorganic.

The Substance and Structure of the Human Body 13

structures one from another and keeping substances in the environ-
ment from entering the cell, unless they can be formed into a part
of the protoplasm. Important among them are the nuclear mem-
brane, which separates the nucleus from the cytoplasm, and the
vacuolar membranes, which act as boundaries between the water
vacuoles and the protoplasm which surrounds them. Most impor-
tant is the cell membrane, which covers the entire surface of the
protoplasm, being located just inside the cell wall. All these mem-

Vacuoles

Chloroplasts

Cytoplasm

•Plasma membrane
Nucleus

acuolar membrane
Cell wall

FIG. 4. — Typical plant cell.

branes are part of the protoplasm and are alive, and they are
capable of the continuous physical and chemical changes char-
acteristic of protoplasm. It is at the membranes that the fat-like
substances of the protoplasm are found in highest concentration.
Built around this cell is a thick wall of non-living substance.
Such cell walls, along with other non-living parts of the plant or
animal body, such as the mineral matter of the bones, are manu-
factured through the activity of cell protoplasm, and laid down
outside the cell. Walls are particularly characteristic of plant cells,
where they are composed chiefly of a highly complex carbohydrate,
called cellulose. In animal cells, walls are largely protein in com-
position.

14 The Substance and Structure of the Human Body

Tissues. — The number of cells in the body of an prganism
varies enormously. There are thousands of different kinds of
plants and animals whose bodies are composed of but a single
cell, while it is said that the human body contains something like
a million billion of them. In unicellular organisms — plants and
animals that are composed of a single cell — it is obvious that one
cell must perform every function that is necessary to the life of
an organism. But even in our own bodies, cells are, as certain
writers have put it, "lesser lives within our life." Each cell is, in
a sense, an independent unit which carries on in itself all the es-
sential activities of living. 'Each takes in food that has been
brought to it by the blood stream, uses part of that food to build
up or repair its own structure, and burns part of it to furnish en-
ergy for its activities. The waste products, or "ashes/' from the
burning are given off or excreted into the blood. Indeed, certain
cells can be completely removed from the body and, if they are
put in the proper sort of solution and kept at the right temperature,
go right on living; their protoplasm continues its restless move-
ments, and the cells themselves may wander about and even divide
to form new cells.

But although each cell leads a life of its own, yet each must play
a part in the life of the whole organism. Each has a special task to
perform. In this respect, cells have of ten, been compared to workers
in a factory, where one group of mefi perform one operation,
other groups other operations, and all of these operations are re-
quired to complete the product which is being manufactured. The
work of maintaining the organism is done in a similar manner.
Muscle cells specialize in moving the body about; bone and
cartilage cells build up supporting structures for it; skin cells
furnish a protective covering, while gland cells specialize in man-
ufacturing liquids and pouring them forth at appropriate moments.

There is one way, however, in which the specialization among
the cells in the body differs from that among the workers in a
factory. Cells differ not only in what they do, but also in, the way
in which they are constructed, in their shape, size and. texture. In
other words, structure is specialized as well as f unction. }ln a shoe
factory the workers who cut the leather do not differ greatly in
appearance from those who sew or nail it together. Tl&ey are all
human beings, with the characteristic bodily structure of human

The Substance and Structure of the Human Body 15

beings. But muscle cells, skin cells, and nerve cells, while they all
possess nuclei and cytoplasm, show great differences in structure.
On the other hand, cells that perform the same function closely
resemble one another in structure.

A group or mass of cells that are similar to one another in
structure and function is known as a tissue. Many tissues have a
considerable amount of non-living material between the cells which
is also a part of the tissue. A good example is bone tissue in which
the cells are scattered throughout the hard substance which forms
the greater part of the tissue. The liquid part of the blood is also
an intercellular substance. There are many kinds of tissues in the
human body, but for purposes of summary they may be grouped
into four classes :

1. Epithelial Tissues. — These form the linings or coverings of
the body. The skin, the hair, the fingernails, and the membranes
which line the mouth, stomach, intestines and other internal or-
gans belong to this group. Glandular tissue is a specialized form
of epithelial tissue.

2. Connective and Supporting Tissues. — The bones, cartilage,
and tendons belong to this group, and, in addition, there is a mesh-
work of connective tissue which extends practically throughout
the body and which serves to give firmness to the organs and to
hold them in position. The blood is also classified as a connective
tissue.

3. Muscular Tissues.

4. Nervous Tissues.

Of these four groups, the muscular and nervous tissues are the
more highly specialized. Their cells are very complex and only
faintly resemble the simple, generalized cell that has just been
described. On this account, the description of nerve and muscle
cells will be reserved for later chapters.

How Cells Are Studied. — Some kinds of cells can be rather
clearly made out under the microscope in their living state. One-
celled organisms can be found in almost any drop of water taken
from a marshy pool or other place where there is decaying vege-
table material. It is easy to see them through the microscope and
to watch their activities. But the cells of the human body are
packed together so closely that special methods must be used to
make them visible at all. Since it is practically impossible to study

16 The Substance and Structure of .the Human Body

them while they are alive, the histologist, that is, the specialist who
studies tissues, proceeds in the following manner :

First he cuts from the dead body a small piece of the tissue
which he plans to study. He treats it with a preservative which
hardens all the protoplasmic colloids which have not already been
hardened by the processes of death. He places the small bit of tis-
sue in a machine, known as a microtome, which cuts it into ex-
tremely thin slices in much the way that bacon is sliced by ma-
chinery in the butcher shop. The slices of tissue are then mounted
on glass slides and treated with dyes which stain the nuclei, cell
walls, and other special structures so that they stand out clearly
from the cytoplasmic groundwork. The thin sections of tissue thus
prepared may be slipped under the microscope ; and, with the light
shining up through them from beneath the platform on which the
slide is placed, the structure of the individual cells can readily be
made out.

To give the reader some notion of what the various kinds of
cells in the body look like, we present here a few drawings made
from tissues prepared in the above manner.

Epithelial Cells.— Fig. 5 A shows a group of cells from the
skin of a frog that appear hardly more complex than the simple
generalized cell previously described. They fit tightly together, and
a cross section of them would show that they are quite thin and
flat.

Fig. 5 B, C and D shows cross sections of certain more highly
specialized epithelial cells from the lining of the windpipe. The
"fringe" seen at the top in Fig. 5 B is made up of minute, hair-
like threads of protoplasm known as cilia. They extend from the
surface cells out into the windpipe, and they keep up a continual
waving motion which sweeps dust and germs up through the
windpipe and out of the lungs. Between the ciliated cells are the
grayish, sac-shaped goblet cells. They manufacture a mucous
liquid which, every now and then, they pour out into the wind-
pipe, thus preventing its drying out on account of the continual
passage of air through it. The goblet cells, therefore, constitute
a very simple form of glandular tissue.

Connective Tissue Cells. — Connective tissues are made up of
cells which form around themselves thick layers of tough non-
living substances which serve to give strength and firmness to the

The Substance and Structure of the Human Body 17

body. Fig. 6 A shows a section of cartilage, such as that found in
the end of the nose, in which the cells, usually placed in pairs op-
posite one another, are embedded in a fibrous substance which they
themselves have manufactured.

FIG. 5. — Epithelial tissues. A, frog epithelium ; B, lining of windpipe ; C, lining:
of stomach; D, glandular. (C and D redrawn from Guyer's Animal Biology.)

In Fig. 6 B, young connective tissue cells are shown. At the
time they were prepared for microscopic demonstration, they were
busy building up the structure of a bone.

Fig. 6 C shows some cells that perform the duty of depositing
the mineral matter in our bones. They are embedded in little open-
ings within the bone structure. Tiny canals run from these open-
ings to the blood vessels that make their way through the bone,
and food materials pass through these canals to the cells.

Organs and Systems. — Cells are the smallest living units of
structure and function. The various tissues combine, however, to
form much larger units, namely, organs and systems.

An organ is a part of the body, usually composed of several
tissues, which possesses a certain degree of structural independence

1 8 The Substance and Structure of the Human Body

and which carries on a specific function or group of functions.
The heart is the organ which has the function of pumping blood
through the blood vessels ; the kidneys perform the task of filter-
ing certain groups of impurities out of the blood; the stomach is
responsible for a part of the digestive process; the lungs bring
oxygen to the blood and take carbon dioxide away ; the hand is the
organ for grasping and manipulating ; the brain is the organ which

A B C

FIG. 6. — Connective tissues. A, cartilage ; B, connective ; C, bone.

governs or integrates the responses we make to our environment ;
while the liver carries on so many functions that it is, in effect,
four or five organs in one.

A system is a group of organs that are joined one to another
and act as a unit in performing some major bodily function. The
following are the more important systems found in the human
body:

i. The digestive system is composed of the mouth, the esopha-
gus (the tube leading from the mouth to the stomach), the stom-
ach, the intestines, and certain glands which empty digestive secre-
tions into those organs. Its function is to prepare the food we eat
so that it may enter the blood and be carried to the cells of the
body.

The Substance and Structure of the Human Body 19

2. The respiratory system includes the mouth and nose, wind-
pipe, bronchial tubes, lungs, and the muscles which expand and
contract the chest. It brings oxygen to the blood and takes carbon
dioxide away.

3. The circulatory system includes the heart, blood vessels, and
lymphatic vessels. Its function is to carry substances to and from
the tissues.

4. The reproductive system is composed of the various male or
female reproductive organs.

5. The urinary system includes the kidneys, the bladder, and the
tubes or ducts which carry the urine from the kidneys to the blad-
der and from the bladder to the exterior.

6. The nervous system is made up of the brain, the spinal cord,
and the nerve trunks.

Both the structure and the function of these systems will be
dealt with more fully in succeeding chapters.

CHAPTER SUMMARY

The bodies of human beings, and of plants and animals as well,
are composed of two kinds of material : the living substance, proto-
plasm, and the non-living substances which the protoplasm has
built up around itself.

Protoplasm is transparent, but usually contains structures ap-
pearing like small granules and bubbles. It is viscid, sticky, and
elastic, and it does not mix with water. It resembles the uncooked
white of egg. It is a colloidal system composed of a suspension of
proteins in water, together with small amounts of mineral salts
and fat-like substances.

The proteins are chemically the most complex substances known,
being made up of thousands of atoms of hydrogen, oxygen, car-
bon, nitrogen and a few other elements. There are millions of dif-
ferent proteins, producing millions of different kinds of proto-
plasm, and one of the basic causes for the wide range of difference
among organisms is the difference in the proteins of which they
are composed.

Non-living colloids are capable of carrying on in a primitive way
many of the activities characteristic of living things. Nevertheless,
the activities of an organism are so intricate and marvelous that
they greatly transcend the very crude lifelike activities of non-liv-

20 The Substance and Structure of the Human Body

ing colloidal systems. This tremendous difference between living
and non-living activity may be partially explained by the great
complexity of protoplasmic colloids and especially by the fact that
protoplasm is organised into structures known as cells.

Cells are microscopic in size. They are units of bodily structure.
Although they differ greatly among themselves, all cells possess a
nucleus, which is a small rounded body in the center of the proto-
plasm, and a cytoplasm, which is the protoplasm outside the
nucleus. The nucleus exercises a directing influence over the activi-
ties of the cell, and the cytoplasm is the region in which those activ-
ities are carried on. Within the cytoplasm are two kinds of special
structures, those which are living parts of the protoplasm and
which perform special vital functions, and certain non-living inclu-
sions. Among the latter may be particles of stored food belonging
to two chemical groups, the carbohydrates and fats. Both groups
are composed of carbon, hydrogen, and oxygen; and they are
capable of combining with more oxygen to yield energy for the
activities of life. The carbohydrates are of three types : single
sugars and double sugars, whose molecules are small and soluble
in water, and starches, whose molecules are large. It is the latter
which are stored in the cells in the form of solid granules. An-
other type of non-living structure is the water vacuole.

Living protoplasmic membranes bound the nucleus, the cyto-
plasmic inclusions, and the protoplasmic part of the cell. They
separate the various kinds of protoplasm within the cell from
one another and also set the protoplasm off from the non-living
substances, both those within the cell and those in the surrounding
environment. Most cells possess thick walls of non-living substance
surrounding the protoplasm. In plants, these walls are composed
of a complex carbohydrate known as cellulose. In animals they are
usually of a protein nature.

The cells of the body are specialized in both structure and func-
tion. A group of similar cells is known as a tissue. There are many
kinds of tissues in the body, but they can be grouped under four
main headings : (i) epithelial tissue, (2) connective and support-
ing tissue, (3) muscular tissue, (4) nervous tissue.

A part of the body that possesses a certain degree of structural
independence and that carries on a special function or group of

The Substance and Structure of the Human Body 21

functions is called an organ. Most organs are composed of several
kinds of tissue. Organs are frequently found in special combina-
tions called systems.

A system may be defined as a group of organs which are joined
to one another and which act as a unit in performing one of the
major bodily functions. The chief systems of the body are the
digestive, respiratory, circulatory, reproductive, urinary and nerv-
ous systems.

The entire plan of structure of the body may be briefly summed
up as follows : The living substance, protoplasm, organizes itself
into microscopic structures, known as cells. A group of cells of a
given kind constitutes a tissue. Tissues combine to form organs.
Organs combine to form systems. And the* tissues, organs, and
systems, combined as they are, constitute the total organism.

QUESTIONS

1. Describe protoplasm. What are some of the characteristics of
protoplasm which help to account for the fact that it is alive ?

2. What is a cell? What are the structures which characterize cells?
What non-living organic food substances are stored in the cells?

3. Discuss the specialization of structure among the cells of the body.

4. What is a tissue? What are the four general groups of bodily
tissues ?

5. What is an organ? Give examples.

6. What is a system ? Give examples.

GLOSSARY

carbohydrate (car-bo-hi'drat) Name given to a group of organic
food substances which combine with oxygen to yield energy. Car-
bohydrates are composed of carbon, hydrogen, and oxygen, always
having two molecules of hydrogen to one of oxygen. There are
three kinds of carbohydrates that ordinarily serve as foods: single
sugars and double sugars, which have small, soluble molecules, and
starches, which have large, insoluble molecules.

cell The unit of living structure and function. It is composed of a
highly organized system of protoplasmic colloids, plus the cell wall
of non-living material which the protoplast builds around itself
and various non-living inclusions.

cellulose (cel'u-los) A carbohydrate substance which forms the walls
of plant cells.

colloidal system (ko-loi'dal) A condition of matter in which minute

22 The Substance and Structure of the Human Body

particles of one substance are held in suspension in another sub-
stance.

cytoplasm (si'to-plaz'm) That portion of the cell protoplasm that is
outside the nucleus.

epithelial (ep-i-the'li-al) Pertaining to the covering or lining tissues
of the body.

fat Name given to a group of organic food substances which com-
bine with oxygen to yield energy. Fats are composed of carbon,
hydrogen, and oxygen, but there is a lower proportion of oxygen
than in carbohydrates.

nucleus (nu'kle-us) A rounded mass of protoplasm found usually
near the center of the cell. It governs the growth and repair of
the cell.

organ A part of the body that possesses a certain degree of structural
independence and that carries on a special function or group of
functions. (Be careful not to confuse the terms organ and organ-
ism.)

organic compounds. Chemical compounds containing carbon. They
are derived, directly or indirectly, from the tissues of organisms,
living or dead.

organism Any living individual, whether plant or animal.

protein (pro'te-in) Name given to a group of very complex chemical
compounds that, together with water, constitute the chief structural
elements of protoplasm.

protoplasm (pro'to-plaz'm) The living substance.

system A group of organs which are joined to one another and which
act as a unit in performing one of the major bodily functions.

tissue A group of cells that are alike in structure and function.

water vacuole (vak'u-61) A droplet of water contained within the
protoplasm of a cell.

CHAPTER II
METABOLISM

What Is Metabolism? — In the previous chapter we have seen
that all organisms — whether plant, animal, or human — have in
common a substance and a structural organization unknown else-
where in the physical world. Yet these are as characteristic of
dead organisms as of living ones. Life is dynamic, not static.
It is far easier to define it in comparison with non-living matter
in terms of what it does rather than what it is. Thus, to under-
stand the phenomenon of life, we must think in terms of energy
as well as matter.

Just as protoplasm is the unique type of matter characteristic
of living things, so is metabolism the unique system of energy
changes associated with life. Living organisms are the scene of an
unceasing series of chemical and physical changes which collec-
tively manifest themselves in all the varied activities which we
designate as life.

Protoplasm, when alive, is in a continual flux, forever taking
in materials, transforming them, giving off wastes. It is like a
whirlpool in a river which maintains its apparent identity even
though at any two successive moments the individual water mole-
cules which constitute it are different. There is a continual flow
of energy into and out of protoplasm — which is only a special
form of matter capable of capturing, transforming and utilizing
energy in certain ways peculiar to the living world. It is this cap-
ture and transformation of energy which is known as metabolism —
the sum total of all the chemical and physical changes whereby
protoplasm builds itself up, secures the potential energy to be
expended in the ceaseless activities of life, and eventually con-
sumes itself.

There are two aspects to the process of metabolism. The first,
which involves the securing of food and the building up of proto-

23

24 Metabolism

plasm, is the constructive aspect termed anabolism. The second is
the destructive, or katabolic, aspect of metabolism. It involves the
oxidation of stored foods, the wearing away and oxidation of the
protoplasm itself, and the excretion— that is, the giving off or
elimination — of waste products resulting from the other katabolic
activities. Katabolism, if unbalanced by anabolism, causes a cell
to waste away and die ; it is the preponderant process during the
old age of organisms. In youth anabolic activities predominate,
resulting in growth rather than wastage of protoplasmic struc-
tures. But katabolism is as essential as anabolism, for while the
latter results in the storage of energy in the cell in the form of
food, the katabolic activity of oxidation is essential for the release
of that energy for the work of the cell : its growth, its movement
(if it is capable of movement), and the maintenance of that mini-
mum of protoplasmic activity which is essential if life is to con-
tinue. The oxidation which takes place in the cell is directly com-
parable to the burning of wood, coal or gasoline. Both are the
combination of oxygen with a carbon-hydrogen compound accom-
panied by the release of stored energy. The rapid oxidation of
substances in furnaces and engines is called combustion ; the slower
"burning" that goes on in cells is termed respiration.

The primary differences between organisms lie in the method
by which they secure their food. In fact, this is the distinction
between the typical plant and the typical animal. The plant king-
dom (with some exceptions) is characterized by organisms with
cells capable of manufacturing their own organic food out of com-
mon inorganic substances in the environment, which are absorbed
into the cells. The animal kingdom, on the other hand, consists
of organisms lacking this ability; they must secure their food in
ready-made form, usually ingesting it — i.e., taking it in — through
a mouth or similar opening.

The animal kingdom is therefore dependent upon the plant
kingdom for food, while plants are quite independent of other
organisms in this respect : they do not have to eat. We indicate this
difference by saying that animals are characterized by hetero-
trophic metabolism, wherein organic foods are essential for anabo-
lism to take place; and plants carry on autotrophic metabolism,
being able to manufacture their own food for anabolism from

Metabolism 25

inorganic substances in the environment. This generalization
holds as long as we confine ourselves to the ordinary green plants
with which everyone is familiar. Later on in this chapter certain
exceptions will be described.

The living activities of an organism are the sum total of all the
activities of the individual cells which constitute that organism.
Thus, no matter how complex the organism or its activities, its
life processes can be studied in simplified form within the
boundaries of a single cell. We can reduce metabolic aspects of life
to their ultimate fundamentals by confining our attention — as we
shall do in this chapter — to the energy changes which characterize
autotrophic and heterotrophic metabolism when they take place
within the bodies of unicellular organisms.

Green Plant Metabolism. — The common green plants (shrubs,
flowers, trees, ferns, grasses, etc.) typify autotrophic metabolism

Chloroplast

Nucleus

Cell wall-

Cytoplasm

FIG. 7. — Protococcus, a single-celled green plant.

at its greatest efficiency. Such metabolism is reduced to its simplest
expression in the unicellular organism known as Protococcus,
which forms a delicate green layer over the shaded and protected
surfaces of stone walls and tree trunks. Under the microscope,
this powdery green material is seen to be made up of many minute
spherical or elliptical cells, sometimes flattened where several cells
are packed closely together. Each cell is a complete and independent
organism. If we examine the cell carefully we discover that the
uniformly green color of the plant is due to the presence of a
green body in the cytoplasm, which is known as a chloroplast. In
Protococcus, there is one large chloroplast in each cell. Each
chloroplast is really a specialized portion of the cytoplasm satu-

26 Metabolism

rated with a mixture of pigments, bright green in color and known
as chlorophyll.

Chlorophyll is one of the most important substances found in the
living world. It is a compound of carbon, oxygen, hydrogen,
nitrogen and magnesium which, when associated with proto-
plasm, makes possible the synthesis of food from carbon dioxide
and water. The chlorophyll in each chloroplast intercepts the light
rays (in nature coming from the sun) and utilizes the captured
energy to dissociate the atoms of the carbon dioxide and water,
reassembling them into carbohydrates with energy added during
the process. Organisms with chlorophyll in their protoplasm have
the tremendous advantage of being able to utilize the vast amounts
of solar radiation continually reaching the earth. When sunlight
becomes transformed into the potential energy of food, metabolism
upon high levels is possible.

To return to Protococcus. The cell remains perfectly still, bathed
by sunlight and surrounded by an atmosphere containing water
and carbon dioxide. The chloroplast absorbs energy from the sun-
light and with it makes its own carbohydrate food. Sugar is the
first food manufactured by this process. As the carbon dioxide and
water in the cell sap of Protococcus become used up, new supplies
diffuse in from the atmosphere. Once sugar has been synthesized,
the manufacture of other foods is not difficult. Sugar molecules
become transformed into starch, and, in a somewhat more com-
plex fashion, fats are synthesized from the sugars. As a by-product
of this activity, oxygen is released. The following formula, indi-
cating what happens in the synthesis of sugar, demonstrates this
release of oxygen :

Carbon dioxide + water + sunlight > sugar + oxygen

Or, to put it in terms of a chemical formula :

6 CO2 + 6 H2O + solar energy > CoH^Oe + 6 O2

This introductory phase of autotrophic metabolism, involving the
use of chlorophyll, is called photosynthesis. It can take place only
in light, and only in an environment containing the essential raw
materials, namely, carbon dioxide and water. The importance of
this process of green plant fixation of carbon into foods essential
for animal existence will be discussed later when we study the
interrelations among organisms.

Metabolism 27

In order to manufacture proteins essential for protoplasm-
building, Protococcus must secure from the environment the ele-
ments nitrogen and sulphur ; these are absorbed in the form of the
soluble salts, nitrates and sulphates. The nitrogen and the sulphur
unite with the sugar molecules to form amino acids and these in
turn combine to form proteins.

In addition to food manufacture and protoplasm-building,
Protococcus carries on the destructive activities characteristic of
katabolism. These involve absorbing oxygen from the air, com-
bining it with foods, and thus releasing energy through respiration.

Chemically, this process is the direct opposite of photosynthesis.
In its simplest form, the oxidation of a single sugar, it can be
represented as follows :

Sugar + oxygen > carbon dioxide + water + energy

CcH^Oo + O2 > CO2 + H2O + energy

Thus the energy the plant takes into itself through photosynthesis
is released for use through respiration. Energy is stored up in a
form that is more convenient for the plant to use than the energy
of sunlight and one that is available both day and night, so that
the all-important continuity of protoplasmic activity is never
broken. In other fields, we know that energy-in-action (kinetic en-
ergy) is often transformed into potential energy in this fashion, to
be expended at a later time in a convenient manner. When an
engine raises the hammer of a pile driver to the top of the shaft,
the energy expended by the engine is stored up in the hammer for
use when it falls, driving the pile into the ground. The photo-
synthetic reaction is similar to the raising of the hammer. It is a
change requiring energy for its accomplishment, at the same time
converting the energy into a form capable of being released later
for special' use. The respiratory reaction is comparable to the fall
of the hammer; it is the employment of the potential energy to
accomplish work. Thus the two opposite chemical reactions are
equivalent, as far as energy exchanges are concerned, to the two
opposite movements of the hammer of the pile driver.

Just as the raw materials for food manufacture must enter the
cell of Protococcus during photosynthesis, so the products of
respiration — carbon dioxide and water — must leave the cell when-
ever respiration is proceeding more rapidly than photosynthesis;

Metabolism

other waste substances, such as the products of protein break-
down, are excreted through the cell wall from time to time. The
katabolic activities of respiration and excretion do not proceed
nearly as rapidly in a green plant as in an animal, since the motion-
less life of the plant cell does not call for a great expenditure of
energy.

The life of Protococcus is the life of the typical green plant.
An unexciting existence, but one in which vastly important activity
is quietly, invisibly, continually in progress. Animals, because of

KATABOLIC
PHASE

ANABOLIC
PHASE

Oxygen

, Carbon dioxide

/Respiration
' Jkr

Waste products x Excretion

\

o

1

\ Carbon dioxide

Photo- «( ^ \

Water

Protein ^ ' mmm

Minerals

synthesis ^/

FIG. 8. — Diagram of metabolic processes in a plant cell.

their type of metabolism, are essentially parasites and highway
robbers, deriving their sustenance by snatching food supplies from
other plants and animals. The plant, by comparison, is an indus-
trious citizen, producing food substance for the entire world of
animal life.

Animal Metabolism. — Animal metabolism is reduced to its
simplest expression in the unicellular organisms known as Proto-
zoa. There are several thousand different kinds of protozoans of
diverse forms, living their lives unseen in the numerous ponds
and puddles of the roadsides. We shall use as an example the
common slipper-shaped Paramecium.

The whole organism is a single cell, but a complex and highly
organized bit of protoplasm it appears to be as we look at it
through the microscope and compare it with Protococois. The

Metabolism

29

outermost region of the cytoplasm and the cell wall are modified
to form a mechanism for locomotion — hundreds of tiny hair-like
cilia are capable of vibrating in unison to make movement of the
organism possible. Paramecium can swim about rapidly, seemingly
continually prying about the debris in the water in search of food.
There is a groove running halfway down one side and terminating
in a mouth and gullet; it is in this portion of the cell that food is
caught and enters into the cell. Once within the Paramecium, the
bacteria or other unicellular forms of life serving as food become
surrounded by a portion of the cytoplasm in the structure known
as a food vacuole.

Excretory
vacuole

Nucleus

Cytoplasm

Food VaCUole Gullet

FIG. 9. — Paramecium, a single-celled animal.

This food contains carbohydrates, fats and proteins like the
food of all animals ; but they are not ready to be used immediately
by the Paramecium. Their complex molecules must be broken
down into smaller molecules soluble in water. Only then can they
"pass from the water in the vacuole into the actual structure of the
protoplasm and be used for fuel or building materials. The chem-
ical reactions essential for this breakdown are brought about by
certain substances called enzymes which are secreted into the
vacuole. Enzymes are organic catalysts ; that is, they increase the
velocity of chemical reactions. It is believed that organisms use
enzymes to produce nearly all the chemical changes which consti-
tute metabolic activity, but the role of enzymes that carry on
digestion is best known. In Paramecium the digestive enzyme ac-
tivity goes on in the food vacuole, just as in human beings it is
carried on in the digestive tract. Then the food materials, reduced

30 Metabolism

to a soluble form, are absorbed into the protoplasm and used to
build up the protoplasmic structure, or oxidized to yield energy.
As the oxygen in the Paramecium is exhausted by the res-
piratory process, new oxygen is absorbed into the cell from the
environment, where it is found dissolved in the water, since all
water in contact with the atmosphere contains a certain amount
of the gases of the air in solution. At the same time, the carbon
dioxide which is formed in excess in the cell diffuses out into the
water. This intake of oxygen and outgo of carbon dioxide is
termed external respiration to distinguish it from the respiratory

KATABOLIC
PHASE

ANABOLIC
PHASE

Organic food

FIG. 10. — Diagram of metabolic processes in an animal cell.

chemical reaction that goes on in the cell. It has already been
shown that external respiration goes on in Protococcus whenever
the rate of internal respiration exceeds that of photosynthesis;
but it is much more rapid in Paramecium, since the latter organ-
ism must oxidize a great deal of food to provide the energy for
its constant moving about.

With respect to oxygen, external respiration is an assimilative
process on a par with digestion and the absorption of food. With
respect to carbon dioxide it is excretory. The other excretory proc-
esses in Paramecium are carried on in a more complex fashion than
in Protococcus. Water and nitrogenous compounds formed by
the breakdown of proteins collect in small contractile vacuoles
which gradually enlarge until one edge touches the cell wall, where-

fetabolism 31

pon the vacuole contracts rapidly, forcing its contents out into
le surrounding water. Similarly, the food vacuole, after all the
igestible portions of the food have been absorbed, makes its way

> the cell surface and discharges the undigested residue into the
rater. These two processes are similar in both form and function

> urination and defecation in human beings.

Within the limits of a single cell, Paramecium exhibits all the
laracteristics of animal metabolism. On the katabolic side, they
iffer only in degree and complexity from those of Protococcus
nd other plants ; but with respect to anabolism, they are very
ifferent.

Colorless Plant Metabolism. — Although most of the common
(ants are green, because of the presence of chlorophyll, there are
lany plants which have no chlorophyll and hence cannot carry
n photosynthesis. These colorless plants are known as fungi and
acteria. The former include the mushrooms and bracket fungi
f rather everyday occurrence and many small and inconspicuous
lants, such as the molds, mildews, wilts, blights and rusts that
row upon living plants and frequently cause much crop damage,
'hen there is the vast assemblage of microscopic one-celled color-
:ss plants — the yeasts, classified among the fungi, which are ac-
ve in fermentation and bread-raising, and the bacteria which
ause diseases and decay.

Most of the colorless plants live upon the bodies of other or-
anisms and hence are heterotrophic ; but a few of the bacteria
re able to carry on a primitive type of autotrophic metabolism,
"he latter organisms may represent the forms of metabolic activity
thereby the first protoplasmic colloids lifted themselves definitely
ut of the realm of non-living matter to become organisms. Such
re the sulphur and iron bacteria, and the bacteria subsisting upon
mmonia and its derived products.

Sulphur bacteria are found in stagnant pools, where they form

scum on the surface of the water. The simple cells, lacking the
hloroplasts found in Protococcus, are united end to end in slender
liread-like filaments. In the water surrounding these bacteria are
arious inorganic substances, including carbon dioxide and sul-
hur (or hydrogen sulphide). The sulphur is absorbed into the
acterial cell and there oxidized; as a result, some energy is re-
used and the protoplasm uses this energy to transform carbon

32 Metabolism

dioxide into food. Here in the sulphur bacteria we find a process
of carbon synthesis taking place similar to photosynthesis in Proto-
coccus, except for one detail. Instead of sunlight, the energy for
the process comes from the oxidation of the sulphur or its com-
pounds.

Other autotrophic bacteria, also independent of light, oxidize
iron salts ; there are even some forms which subsist upon selenium
and methane. Most important of all, from the human point of
view, are the autotrophic bacteria which oxidize ammonia — the
end product of animal decay. Since they change ammonia into
nitrites during the process, these are known as nitrite bacteria.
.Still other bacteria, the nitrate bacteria, secure the energy necessary
for their food manufacture by oxidizing the nitrites into nitrates.

The heterotrophic colorless plants all get their nourishment from
organic material in the bodies of other organisms, living or dead.
The bacteria and fungi which cause decay utilize as food the
organic substance associated with dead protoplasm. Such scaven-
gers in the biologic realm are known as saprophytes. The colorless
protoplasm of the bacterial cell is not able to take in solid food
particles and digest them within the cell ; if this could be done,
bacteria would be classified as animals. Instead, the protoplasm
produces enzymes similar in function to those found in Para-
mecium; these enzymes are excreted by the cell into its environ-
ment and act upon the complex organic compounds of the dead
protoplasm, producing simpler substances capable of being ab-
sorbed into the bacterial cell and there utilized in its metabolism.

As an example, when a tree dies there is left a residue of wood
which is composed largely of cellulose. Decay fungi capable of
"digesting" wood produce several enzymes which eventually
change the cellulose into sugar. During the process, these fungi
secure organic material for their metabolism, and release carbon
dioxide and water as waste products of their katabolic activi-
ties. When an animal dies and its body decays, bacteria act upon
the carbohydrates present in the same way. In addition, the pro-
teins are broken down into ammonia and various organic acids.

Disease bacteria are unicellular plants of microscopic dimen-
sions; they are the smallest cellular forms of life known to the
biologist today. They have the same protoplasmic make-up as the
decay bacteria, but unlike the latter they usually feed upon or-

Metabolism 33

ganic substances associated with living protoplasm. Such a habit
of living is known as parasitism, and the organism is a parasite.
The carbohydrates and proteins in the host cells (human blood
and various tissues in the case of bacteria causing human diseases)
are, after digestion, absorbed into the bacterial cell and there
converted into bacterial protoplasm or used as a source of energy.
Waste products known as toxins, substances poisonous to human
tissues, are frequently produced as a part of the katabolic process
of bacterial metabolism.

A B

FIG. ii. — Single-celled colorless plants. A, yeast; Bf bacteria.

The yeast plant is also microscopic; the invisible single-celled
organisms are continually floating about in the air. Its food is gen-
erally sugar, which is abundantly present in most fruit juices.
When a cluster of grapes is picked, there may be yeast cells on the
skin of every grape. As the juice is squeezed out in a wine press,
some of the yeast cells go with the sugary solution. The yeast
protoplasm produces enzymes which break down the sugar mole-
cules into alcohol and carbon dioxide as end products. This
katabolic activity on the part of the yeast plant, resulting in the
production of alcohol, is a respiratory process, bearing the special
name fermentation. The same yeast plants, mixed with bread
dough, feed upon the carbohydrates in the dough, release some
alcohol and large amounts of carbon dioxide. This gas forms
bubbles in the mixture and causes the bread to rise.

The Difference Between Animals and Plants. — The differ-
ence between the two great kingdoms of life is fundamentally a
difference in their metabolic activities. Ordinarily we think of
animals as organisms which move about, whereas plants are footed

34 Metabolism

to the ground and are incapable of movement. But the biologist
knows that this distinction does not hold throughout the entire
realm of life. There are many one-celled green plants, and bacteria
as well, which swim about almost as freely as Paramecium ; while
one frequently comes upon types of animals, such as corals and
sponges, which remain attached to a single spot and obtain food
by simply setting up currents of water which carry it to their
mouths.

If we had to consider green plants and animals alone, we could
say that plants are autotrophic, and animals, heterotrophic organ-
isms. But the large number of heterotrophic colorless plants de-
stroys this distinction. Careful examination of the metabolic activi-
ties in both green and colorless plants, however, reveals that,
whether they take in inorganic or organic materials, they always
absorb them from the surrounding medium. They do not, like
animals, ingest them first and then carry on the processes of diges-
tion and absorption within their bodies. This explains why most
plants lead sedentary lives, dwelling within or upon the source of
their food material, while animals wander about in search of
things to devour.

The difference between ingestion and absorption as the initial
stage in anabolism may not appear very great, yet it is basic to all
the other obvious differences between plants and animals. A plant
is an organism constructed for the efficient absorption of nutritive
materials and, in the case of green plants, the utilization of the
solar energy. An animal is an organism especially adapted for
securing and ingesting foods. Although the higher, more complex
plants and animals seem very unlike in form and structure, among
the simpler, primitive representatives of both kingdoms there is
often little difference between a plant and an animal. There are
even some organisms which are both plant and animal. These are
found mostly among a group of unicellular organisms called
flagellates because they move about in the water by means of a
single, long, whip-like thread of protoplasm, called a flagellum,
which they stretch out and snap toward them, thus pulling them-
selves forward. Some flagellates are green plants whose metabolic
activities are in every respect similar to those of Protococcus;
some are animals, lacking chlorophyll and securing all their food
by ingestion. Finally, there are some flagellates that not only con-

Metabolism 35

tain chlorophyll and carry on photosynthesis, but are capable of
ingesting organic foods as well. These are true plant-animals, be-
longing, by right of their metabolic activities, to both kingdoms,
displaying in themselves the true unity of life. For wide as the
gulf may seem to be between ourselves and the plants, we are
fundamentally of the same nature as they, and with them share
membership in a single great world of life.

CHAPTER SUMMARY

All organisms have in common a unique system of energy
changes known as metabolism, whereby protoplasm takes in, trans-
forms and gives off energy. There are two aspects of this
metabolic activity: a constructive aspect known as the anabolic
phase, in which protoplasm is built up and energy stored, and a
destructive aspect known as the katabolic phase, in which energy
is released and protoplasm consumed. Katabolism usually involves
oxidation of protoplasm or organic substances within the proto-
plasm ; such oxidation within the cell is known as respiration.

The chief differences between organisms are related to their
types of metabolism, to the method by which they secure their
food during anabolism. There are three types of metabolism,
which differentiate living things into three major groups of or-
ganisms. These are (i) green plant metabolism, in which the
organisms manufacture their own organic substances from the
inorganic materials of the environment; (2) animal metabolism,
in which the organism ingests previously synthesized organic
materials from the environment, usually taking them in through
a mouth; (3) colorless plant metabolism, in which the organism
absorbs organic materials from the environment, materials which
previously made up the bodies of other plants or animals or even
were a part of living bodies at the time of absorption.

Both colorless plant metabolism and animal metabolism can be
carried on only when there has been previously green plant
metabolism ; for it is only by the latter process that organic mate-
rials— the basis of protoplasm and life — are synthesized from the
common inorganic substances found in the physical environment.
Green plants are independent of other organisms, while colorless
plants and animals are dependent upon green plants for their food

36 Metabolism

For this reason, green plants are called autotrophic organisms;
colorless plants and animals are known as heterotrophic ones.

These three types of metabolism involve rather complicated sets
of tissues and organs in the higher plants and animals, where the
organism is made up of millions of cells. Therefore, each type
can be seen to best advantage where the organism is a single cell.

Green plant metabolism is reduced to its simplest terms in the
single-celled Protococcus. The cell-organism is characterized by
a green structure in the protoplasm, known as a chloroplast; it
is the presence of such chloroplasts that gives plants their green
color. The importance of the chloroplast lies in the fact that it
intercepts solar radiation and uses the captured energy to break
up the carbon dioxide and water molecules, preparatory to re-
organizing the atoms as sugar and starch molecules. Thus green
plant metabolism requires (i) a source of energy such as sun-
lightj (2) chloroplasts, (3) raw materials in the form of water
and carbon dioxide. This process of synthesizing carbohydrates
from inorganic materials is called photosynthesis, and is an im-
portant phase of green plant metabolism. After manufacturing
this carbohydrate food, Protococcus absorbs nitrates from the
environment and changes the carbohydrates to proteins. Thus
materials for respiration and cell growth are provided by the two
synthesizing processes. All of the foregoing has been a part of
the anabolic phases of metabolism in Protococcus. Katabolism in-
volves taking in oxygen, combining it with the foods the plant
has manufactured, and thus releasing energy by respiration.

Animal metabolism is reduced to its most elemental form in
one of the single-celled animals, Paramecium. The cell-organism
in this case lacks the green chloroplasts and therefore cannot
carry on photosynthesis. It is dependent upon an external source
of organic food; such food is taken in, or ingested, through a
special part of the cell where there is a gullet and mouth developed
for this purpose. To get to the food supplies, Paramecium swims
about as a result of the rapid vibration of many small cilia all
over its surface. ^The food particles (minute plants or animals)
enter Paramecium through the mouth and become part of a food
vacuole, where enzymes produced by the protoplasm break down
the complex organic compounds into simpler ones. This is a
primitive sort of digestive process. When the food has been re-

Metabolism 37

duced to soluble form, it is assimilated by the protoplasm where
it can be used as a source of energy or of new protoplasm. The
katabolic phases of metabolism involve, as in Protococcus, the
absorption of oxygen and the oxidation of organic compounds,
with the accompanying release of energy and carbon dioxide.
The intake of oxygen and outgo of carbon dioxide is often called
external respiration to distinguish it from cell respiration, which
is the actual oxidation of protoplasm or food. Waste products
which are not gaseous are eliminated through special excretory
vacuoles not found in Protococcus.

Colorless plant metabolism, characteristic of the whole group
of fungi, is reduced to its essential characteristics in the bacteria.
A few of these are autotrophic, such as the sulphur, nitrite and
nitrate bacteria. Like Paramecium, they lack chloroplasts ; un-
like Paramecium, they do not need to feed upon plant and animal
remains but are able to get their energy by the oxidation of sul-
phur, ammonia or nitrites. Those that feed upon ammonia and
nitrites are important in the conservation of nitrogen salts in the
ground, where they can be used by green plants in synthesizing
proteins. Most of the bacteria are heterotrophic, and live either
as saprophytes or as parasites. They differ from unicellular ani-
mals in that they do not ingest their organic food, but instead
e^r£te...en_zymes into the environment which act upon proto-
plasmic materials, rendering them soluble and otherwise suitable
for absorption within the bacterial cell. After absorption the
organic substances are used for cell growth and cell oxidation.
Yeast is another unicellular organism carrying on colorless plant
metabolism; its food is generally sugar, which is changed to
alcohol by the yeast enzymes during katabolism.

In considering these types of metabolism, as they are simpli-
fied in the single-celled organisms, we can see that the essential
difference between the plant and animal kingdoms is one asso-
ciated with the type of metabolism. A plant is an organism which
absorbs its food — whether this is inorganic or organic — from the
surrounding environment. If the food is inorganic, the plant can,
by the process of photosynthesis, utilize solar energy to convert
the carbon dioxide and water into carbohydrates; if it is organic,
it is usually acted upon by enzymes outside the cell and absorbed
when soluble and usable. An animal, on the other hand, ingests

38 Metabolism

organic food and carries on the process of digestion and rendering
the food available for the cell, after the food is taken in. Both
plants and animals carry on respiration as the most important
aspect of katabolism.

QUESTIONS

1. Compare katabolism in Protococcus and in Paramecium.

2. What is the chief difference in metabolism between Protococcus
and a yeast plant?

3. What is the chief difference between a plant and an animal?

4. Are all heterotrophic organisms animals? Give examples.

5. Are green plants the only autotrophic organisms ? Give examples.

6. Describe the process of photosynthesis as it takes place in Pro-
tococcus.

7. Why is chlorophyll considered such an important substance?

8. Why is oxygen given off from plants ?

9. Oxygen is also taken into plants. Why ?

10. Describe the anabolic activities of Paramecium.

11. Compare the activity of a food vacuole and that of a contractile
vacuole in Paramecium.

12. How does external respiration differ from cell respiration?

13. What, if any, is the significance of autotrophic colorless plant
metabolism? What organisms exhibit such metabolism?

14. Describe anabolism in the heterotrophic bacteria.

GLOSSARY

amino acids (am'i-no) Nitrogenous substances out of which proteins
are built up.

anabolism (an-ab'6-lis'm) The constructive phase of metabolism,
whereby energy is stored and food accumulated for use in proto-
plasm-building or katabolism.

autotrophic (6'to-tro'fic) That type of organism or metabolism which
is able to transform inorganic materials such as carbon dioxide and
water into organic ones such as carbohydrates. All green plants are
autotrophic organisms.

bacterium (bak-te'ri-um) pi. bacteria. A very simple colorless one-
celled plant.

catalyst (ka'ta-list) A substance which speeds up a chemical reaction
without taking part in it.

chlorophyll (klo'ro-fil) Green pigment, made up of carbon, hydrogen,
oxygen, nitrogen and magnesium.

chloroplast (klo'ro-plast") A protoplasmic structure containing chloro-

Metabolism 39

phyll, necessary for photosynthesis and common to all green plants,
giving them their characteristic color.

cilia (sil'i-a) Minute hair-like outgrowths covering the cell wall of
many Protozoa; used for locomotion.

contractile vacuole Vacuole found in Paramecium and other Protozoa,
which functions as an excretory structure.

enzyme (en'zim) An organic catalyst.

external respiration Exchange of gases between the cell and the en-
vironment.

food vacuole Vacuole found in Protozoa, containing particles of in-
gested food.

heterotrophic (het'er-6 tro'fic) That type of organism or metabolism
which is dependent upon organic food, which is either absorbed
(colorless plants) or ingested (animals).

ingestion (in-jes'chun) The taking in of solid organic food, usually
through a mouth.

inorganic (in'or-gan'ic) Not containing carbon; sometimes also ap-
plied to substances not elaborated by living things.

katabolism (ka-tab'6-liz'm) The destructive phase .of metabolism,
whereby food is consumed and energy released.

metabolism (me-tab'6-liz'm) The sum total of all the physical and
chemical changes taking place in protoplasm, in the course of which
energy is taken in, transformed and utilized in living activities.

organic Substance containing carbon ; or one produced by living
organisms.

parasite An organism feeding upon the organic materials associated
with the living tissues of another organism.

photosynthesis (fo-to-sin'the-sis) The chemical reaction performed
by green plants in the presence of sunlight and with the aid of
cholorophyll, in which carbon dioxide and water are united to
form sugar.

protozoa (pro-to-zo'a) The group of one-celled animals.

respiration The process of oxidation of organic materials taking
place during katabolism within the cell.

saprophyte (sap'ro-fit) An organism feeding upon the organic mate-
rials associated with dead plants or animals ; most of the mushrooms
are saprophytes.

CHAPTER III

CIRCULATION AND RESPIRATION IN THE
HUMAN BODY

CIRCULATION

Human Cells and Their Environment. — Each of the million
billion cells of a man's body is busily engaged in living its own
life and performing the tasks whereby it serves its community,
the body as a whole. Each one carries on its own series of
metabolic activities in a manner similar to that in Protococcus,
Paramecium, or any other one-celled organism. There is one
major difference : unicellular organisms must secure the materials
for metabolism from a natural environment which has not been
constructed for their special benefit, whereas the cells of the
human body have an environment especially prepared for them
which provides them with everything they need. This environ-
ment, like that of Paramecium, is a liquid one. Every cell of the
body is surrounded by a liquid, known as the tissue fluid, although
in places where the cells are packed tightly together, this fluid
may be present only as a thin film between them. Dissolved in
the tissue fluid are the digested food substances and the oxygen
which must make their way into the cells, and, likewise, the
carbon dioxide and wastes which are continually being produced
in the process of metabolism. But the million billion cells are so
tightly packed together that food and oxygen are absorbed from
and waste products given off into the watery surroundings at an
extremely rapid rate. The tissue fluid would be choked with
wastes and starved for lack of food and oxygen if it were not
for the blood stream, which courses through all parts of the body,
carrying substances to and from each part, and which freshens
the tissue fluid as a brook freshens a pond through which it runs.

Circulation. — The blood does not enter the tissues itself, but is
carried about within a system of blood vessels. The walls of the

40

Circulation and Respiration in the Human Body 41

smallest of these blood vessels, the capillaries, are so thin that the
oxygen and foods which the blood brings make their way easily
through them and into the tissues, and the carbon dioxide and
other wastes diffuse into the blood and are rapidly carried away.

The circulatory., system is the gr^at transporting jn^rym, p f
the body. The blood runs through the lungs, where it gets its
supply of oxygen from the air and unloads carbon dioxide into
the air. It picks up foodstuffs from the intestines and carries
wastes to the kidneys, sweat glands, and liver. As its system of
canals serves the Dutch nation, so the circulatory system serves

the body, tiiQyjng materials ilQHL. one .place L_tOLjaynoth^_asJthe^

are needed. The flow of the blood which makes this movement
possible is maintained by the energetic pumping of the heart.
The channels through which it flows are the blood vessels — the
capillaries, arteries, and veins.

The Blood Vessels. — The vessels through which the blood
flows away from the heart are called arteries. The largest of them,
the aorta, which carries the blood as it leaves the heart to go to
the body tissues, is about an inch in diameter, but this artery
almost immediately sends off branches and becomes smaller. Ar-
teries branching off from the aorta run to all parts of the body.
On the average, they are about a quarter of an inch in diameter.
Their walls are thick and elastic. The arteries divide and divide
again, until they form extremely minute vessels known as arteri-
oles. The arterioles again divide to form the capillaries. These
latter form a thick network through every tissue of the body.
The average capillary is about six ten-thousandths of an inch in
diameter. Its walls are composed of cells that are inconceivably
thin. Through these walls materials can easily make their way
back and forth between the blood and the tissues. For practically
every cell in the body, there is a capillary flowing close by. Al-
though a single capillary is likely to be scarcely a thousandth of
an inch in length, it has been claimed that all the capillaries of a
man's body put end to end would reach two and a half times
around the earth.

The capillaries join one another to form venules, which cor-
respond in size to the arterioles. These combine to form veins,
which come together to form the few large vessels that empty
into the heart. In general, the veins are slightly larger than the

42 Circulation and Respiration in the Human Body

arteries, and their walls are much thinner and less elastic. The
blood is forced through them partly by the pressure of blood
coming through the capillaries and partly by the squeezing on
them of the other tissues, because of movements of the muscles.
The veins possess pocket-shaped valves which keep the blood from
running backward. (See Fig. 12.)

There is only one exception to the rule that arteries empty into
capillaries and capillaries into veins. The capillaries that collect
food from the small intestines join to form the portal vein which
runs to the liver and branches out into another capillary network,

FIG. 12. — Valves of a vein in action.

just as an artery might. These capillaries open into minute, cell-
lined cavities, known as sinusoids. Another set of capillaries takes
the blood from the sinusoids and carries it to an ordinary vein
which transports it toward the heart.

The Blood. — The blood which flows through these vessels may

be described as a r&pidly,,jHQVUig tissue. It is made up of cells

like any other tissue, but these cells are floating in a liquid — called
the blood plasma — and are carried through the circulatory system
in the swift current of that liquid.

The plasma is composed of water containing proteins held in
suspension. It also holds in solution food and waste materials and
a variety of chemical substances which play a part in regulating
the activities of the body.

The cells of the blood are of two kinds, the red corpuscles and

Circulation and Respiration in the Human Body 43

the white corpuscles. The former are disk-shaped bodies, hol-
lowed in on both faces, so that they are thicker around the edges
than in the center. They are very small, about three ten-thou-
sandths of an inch in diameter and eight hundred-thousandths of
an inch thick. Strictly speaking, they are not true cells, since they
do not contain nuclei, but they are derived from true cells located
in the marrow of the bone. Before they leave the bone marrow,
they lose their nuclei and become the mere disks that we find in
the blood.

When they are looked at under the microscope, these corpuscles
appear yellow, but in the mass they give the blood its red appear-

A B

FIG. 13. — Blood corpuscles. A, red corpuscles ; B, white corpuscles.

ance. The redness is due to the presence of a substance known as
hemoglobin which forms a great part of their structure. Hemo-
globin is a protein which contains four atoms of iron in each
molecule. It is extremely important in the economy of the body,
since the iron atom is capable of forming a loose chemical at-
tachment to oxygen. The hemoglobin thus serves to transport
oxygen ...from _the .jung^tp^^_pai^^fjtl^tedy. When it is com-
bined with oxygen the hemoglobin is red ; when it loses its oxy-
gen, it is a bluish purple. For this reason the blood in our arteries,
which is carrying oxygen to the tissues, is bright red, while
that in the veins, which has given up its oxygen to the cells of
the body, is blue.

If one turns a microscope upon the web of a frog's foot, one
may see the red corpuscles being carried through the capillaries
from the arteries to the veins. They are whirled along by the

44 Circulation and Respiration in the Human Body

blood current and are sometimes so thickly clumped together
that they appear almost to block the narrow capillary channels in
which they flow. Indeed, there is an incredible number of them
in the blood of as large an animal as a man. Usually blood
counts indicate that there is something like 75 billion to a cubic
inch, which brings the total amount in the body of a man of
average size to about thirty trillion.

Every day about ten per cent of the red blood corpuscles in
the body are destroyed, which means that every day three tril-
lion new ones must be produced in the marrow of the bones,
where there are cells which multiply at a rapid rate and finally
undergo changes which transform them into corpuscles. Naturally,
these cells must be provided not only with much protein — which
is necessary for the growth of all cells in the body — but also
with a considerable quantity of the iron that forms so essential
a part of every molecule of hemoglobin. In the course of their
breakdown, red blood corpuscles are engulfed by cells in the
spleen and liver. Much of their protein and iron is removed by
these cells and thus conserved by the body for its use. Neverthe-
less, some iron is lost, and hence foods containing iron are in-
dispensable in our diet. Spinach, whole wheat foods, eggs, and
lean meat are familiar dietary items which are rich in iron. Egg
yolks and molasses hold it in especially high concentration.

The so-called white blood corpuscles are really colorless in
appearance. They are considerably larger and only one five-hun-
dredth as numerous as the red corpuscles. They are true living
cells with nuclei. Indeed, most of them can move about independ-
ently by ameboid movement, so called because it is the type of
movement displayed by a certain one-celled animal known as
Ameba. In Ameba, the cell continually changes shape as it moves.
Small protrusions of protoplasm called pseudopodia (false feet)
flow outward from the body of the cell; then the body is drawn
forward until the pseudopodia become again a part of the general
mass; whereupon, another set of pseudopodia are projected, and
the process is repeated. Instead of having a special gullet through
which food passes to enter the cell, Ameba can project a num-
ber of pseudopodia around a food particle until the protoplasm
completely surrounds it and it is held in a vacuole inside the cell.

White blood corpuscles look and act very much like the free-

Circulation and Respiration in the Human Body 45

living Ameba. Not only do they move independently about in
the blood stream itself, but they may squeeze through the capil-
lary walls and wmfar PUt frmpng ...fog _ tissues^ Like Ameba, they
are,capa.bk_ ol engulfing, solid particles, which, in the case of the
corpuscles, are the bodies of bacteria and the particles that are
produced when cells are destroyed. Thus the body is freed from
materials which might harm it.

The white corpuscles that engulf bacteria are formed, like the

red corpuscles, in the marrowy o£ tjie_ bones. Those that engulf

cellular materials are the same cells which, in the spleen and
liver, take care of the broken-down red corpuscles. They have
merely broken loose from their moorings to wander about in the
blood stream and tissues. Wherever cell destruction occurs in the
body — because of a bruise, burn or some other attack upon the
tissues — these cells make their way to the spot. The pus formed
in abscesses is made up chiefly of white corpuscles that have
crowded about a place where destruction of tissue has occurred.

There are also certain white corpuscles formed in theJymfch
nodes (see below). They are incapable of independent motion
and do not engulf particles, but perform the function^ of manu-
facturing certain substances which are essential for growth in
the cells of the body..

The Clotting of the Blood. — The blood is a liquid which flows,
usually under pressure, within a closed system of tubes and cham-
bers, the heart and blood vessels. The slightest break in this sys-
tem allows this liquid to escape, just as water escapes from a
broken pipe, If the escape is not soon prevented the body loses
a greater part of its blood, and death ensues immediately, since
the cells are deprived of essential materials, notably oxygen. Some
method of plugging all leaks in the circulatory system is therefore
indispensable. The coagulation or clotting of the blood performs
this service. Clotting is brought about by a series, of chemical jre-
agtions between substances in solution in the blood which result_
in the synthesis of a solid protein substance, fibrin. Fibrin is de-
posited in the form of a network of fine elastic fibers, which holds
the corpuscles within its meshes and thus provides an obstruction
through which the blood cannot pass. Clotting does not take place
inside the circulatory system because of the absence of a substance,
thrombokinase, which is needed to start the chemical reactions

46 Circulation and Respiration in the Human Body

that yield fibrin. Thrombokinase is found in the tissues out-
side of the blood vessels and is also released through the disinte-
gration of certain minute particles in the blood called platelets.
Whenever the blood escapes from the circulatory system and wets
some foreign surface, platelets break down, yielding thrombokin-
ase, which, together with that in the tissues, sets going the process
of fibrin formation.

It takes about five minutes for the blood to clot sufficiently
to stop bleeding in a small wound in which only the capillaries
are cut. The blood oozes out rather slowly, and not much is lost
during the period that clotting is taking place. When an artery
is cut, however, the blood spurts out rapidly with each beat of
the heart, so that a clot does not form, and death from bleeding
will occur unless the flow of blood through the artery is stopped.
This can be accomplished by tying off the artery — as is done in
surgical operations — or, as a first-aid measure, by tying a tour-
niquet around the limb at a point above the place where the artery
is cut and twisting it tightly so as to shut off the flow of blood
until the wound can be treated surgically. A tourniquet should
always be loosened every five or ten minutes to allow some blood
to the limb, since otherwise the cells will die for lack of oxygen.

In some diseased conditions, notably in the hereditary disease
hemophilia, clotting takes place very slowly. If it is extremely
slow, the slightest wound may cause death. Sometimes bleeding
through the capillary walls occurs without known damage to
the capillaries, and a "bleeder" may die in this way without ever
receiving a wound. This suggests that the clotting reaction is
not merely a device for meeting emergencies, but that it is con-
tinually needed to reinforce the very thin walls of the capillaries.
Occasionally a clot may form inside a vein or artery as a result
of some damage to the walls of the vessel. Then, especially if the
clot occurs in a vein, the individual must be kept very quiet, lest
pieces of the clot break away and be carried to the capillaries
where they may completely shut off the circulation in some im-
portant region of the body. A clot in the vein of the leg may
cause death when particles of it stop circulation through the cap-
illaries of the lungs.

The Heart. — As nearly everyone knows, our body cavity is
divided into two parts by a muscular membrane known as the

Circulation and Respiration in the Human Body 47

diaphragm. Below the diaphragm, in the abdominal cavity, the
stomach, intestines and liver are located. Above it, in the chest
cavity, are the lungs and heart. The heart is placed just under
the breastbone and just above the diaphragm. It lies about in the
mid-line of the body, although most people believe it is on the
left side because the lower tip of it, which is the part that beats
the hardest, is turned toward the left. (See Fig. 15 A.)

The heart is a muscular bag, divided into four chambers. The
upper left chamber is called the left auricle', the upper right, the
right auricle; the lower left, the left ventricle; and the lower right,
the right ventricle. There are openings between the auricles and
ventricles, but the right and left sides of the heart have no connec-
tion with each other. The blood from the veins enters the auricles
— that which has come from the lungs, the left auricle; that which
has come from the body, the right auricle. The two auricles con-
tract together, pushing the blood into the ventricles. A fraction
of a second later, the ventricles contract and push the blood out
into the arteries. The right ventricle pumps it into the arteries
running to the lungs, the left ventricle into those running to
the bodily tissues. There are valves between the auricles and the
ventricles which close when the ventricles are contracting so that
the blood will not run back into the auricles. There are also valves
in the arteries that close while the ventricles are being filled with
blood, so that blood will not run back from the arteries into the
ventricles. The ventricles have much thicker walls than the auricles,
since they have the job of forcing the blood through the arteries,
capillaries, and veins and back again to the heart. The walls of
the left ventricle are the strongest of all, since it must pump blood
throughout the entire body.

The Circuit of the Blood. — Perhaps the best way to under-
stand the action of the heart and its connection with the whole
circulatory system is to take an imaginary ride on a red cor-
puscle as it makes a complete round of the system. Let us climb
aboard our corpuscle just as it enters the right auricle. It is, of
course, not really red just now, since its hemoglobin has been
completely robbed of oxygen by the tissues of the body, and it has
taken on a purplish hue.

The auricle has just been contracting, and the blood that was
about to enter it has been held up for a moment, but now the

48 Circulation and Respiration in the Human Body

Pulmonary artery
Right auricle

Superior vena cava /M

INTESTINES J?1ti

KIDNEYS

^«

LEGS

Hepatic vein

FIG. 14.— Diagram of the circulatory system. Light dots, oxygenated blood; dark
dots, deoxygenated blood.

Circulation and Respiration in the Human Body 49

auricle relaxes; its muscular walls are no longer squeezing in
on the auricular chamber ; the chamber increases in size and allows
the blood containing our little corpuscle to enter. About a half a
second passes, and then, suddenly, the auricle contracts. The
blood is driven through the opening into the right ventricle.

It has hardly time to enter before the walls of the ventricle
give it a terrific squeeze. A set of three valve flaps located at
the opening between the ventricle and auricle snaps shut, thus
keeping the blood from going back into the auricle. Its only
course out of the ventricle is through the artery which leads to
the lungs. Our blue little red corpuscle rushes out with millions
of its fellows. It comes to a place where the artery branches and is
whirled along down one of the two possible paths. There is
another branch, another, and another. The pathway is becoming
somewhat narrow; the blood is moving more slowly. Presently
the corpuscle is crowding through a minute capillary, located close
to the inner surface of the lungs. So small is the passageway that
the corpuscles cannot pass two abreast. Now the oxygen of the
lungs is combining with the hemoglobin of the corpuscle, which
begins to lose its bluish tinge and take on the crimson hue from
which the corpuscle derives its name. The capillary through which
it is passing joins with another capillary. The corpuscle makes
its way into a tiny vein. The vein combines with another to form
a larger one. Combination after combination takes place until
our little red corpuscle comes to the place where one of the veins
from the lungs empties into the left auricle of the heart. From the
auricle, it is pumped into the ventricle. The muscular walls of the
ventricle close down with even greater vigor than did those of
the right lower chamber. The blood is driven into the largest artery
of the body, the great aorta. The aorta leaves the heart in an
upward direction, but it soon bends over and takes it course down-
ward through the body cavity. Our little corpuscle, swept along
by the rapid stream of the blood in the aorta, passes one artery
after another branching off toward the various parts of the body.
First there are the arteries which lead to the muscles of the heart
itself, then those branching to the head and arms. Lower down
there are branches which go to the digestive organs.

Let us suppose that our corpuscle continues down the aorta,
into a large artery that passes down the leg, and finally into a

50 Circulation and Respiration in the Human Body

capillary mesh work in one of the muscles of the foot. Here, as
it slowly struggles through the narro\v cnannels, its oxygen leaves
it and diffuses through the thin capillary walls into the muscle cells,
and, having again assumed a bluish purple tinge, it makes its way
into one of the veins of the leg. In the veins, its progress is con-
siderably slower than it was in the arteries, but the pocket valves
keep it from backing up whenever the pressure becomes too
slight to force it forward. So, having no chance to retreat, our
corpuscle pursues a steady course up the leg, into the body cavity,
and finally into the large vein known as the inferior vena cava,
which takes it back to our starting place, the right auricle of the
heart.

The flow of blood from the right ventricle through the lungs
and back to the nght auricle is called the pulmonary circulation,
and the flow from the left ventricle through the body generally
and back to the right auricle is the systemic circulation. Blood in
the pulmonary arteries is blue and in the pulmonary veins, red;
while in the systemic circulation it is red in the arteries and blue
in the veins.

Rate of Blood Flow. — The average length of time spent by a
corpuscle in making a complete trip around the circulatory system
is probably about fLf^_J2££QJld3. The journey which we have just
outlined would take a little longer, since our corpuscle went all
the way to the foot and back. At any rate, the blood is driven
through the body with a surprising rapidity, and it consequently
serves as an efficient transportation system for bringing things
to the cells of the body and taking other things away.

The total amount of blood that flows through each part of the
system must always be the_same. since otherwise the blood would
pile up in the regions of slowest total flow. Hence it must move
more ...rapidly .through the. arteries than through the veins because
the artery channels are narrower. It flows most slowly in the
capillaries, since, although a single capillary offers a very narrow
passageway, the total width of all the capillaries if they were laid
side by side is much greater than that of the arteries and veins
laid side by side.

Blood Pressure. — Wl^^_a^,Hgi^dj|aws through a pipe, the
highest pressure is at the point where it Starts, and the pressure.
!]^^ I*1 tiie circulatory system,

Circulation and Respiration in the Human Body 51

blood pressure is highest at the point where the blood leaves the
ventricles, and lowest at the point where it enters the auricles.
The force which decreases the pressure is the friction offered by
the walls of the vessels through which it passes. This friction is
greatest in the walls of the arterioles, capillaries, and venules be-
cause these passageways are so narrow, and hence the pressure
drops most rapidly between the small arteries and small veins.

Since blood is pumped intermittently from the heart into the
arteries, the pressure in the arteries rises and falls with each
beat of the heart; and because of their elasticity the arterial walls
give way to the waves of blood that are sent out with the closing
of the ventricles, so that wherever an artery comes close to the
surface of the body we can feel it pulsating with each heart
beat. However, the elasticity of the walls tends to "smooth out" the
pulse, since the walls behind each wave of the pulse continue to
squeeze upon the blood even after the wave has passed by, so
that by the time the blood reaches the capillaries, the pulse has
entirely disappeared and the blood flows at a slow, even pace which
is determined by the amount of pressure the elastic arterial walls
exert upon it.

When the heart beats str qngly and rapidly, it forces more blood
intQjhe .aperies and stretches the walls; farther, thus increasing
their elastic pressure, just as the pull of the rubber band becomes
greater, the more it is stretched. In this way, the pressure on the
blood going through the capillaries is increased, causing it to
flow more rapidly through them, and much more blood passes
through the body in a given time than when the pressure is lower.

Whenever our muscles are active, the rate of metabolism in
their cells increases enormously, and the blood must carry more
substances to and from them than when we are resting. The body
immediately reacts to the situation by an increase in blood pres-
sure, which speeds up the circuit of the blood. This increase is
brought about in two ways : The heart beats more rapidly and
forces more blood into the arteries with each beat, and at the same
time the arterioles in the inner parts of the body contract, so that
there is less room for the blood to flow through these parts;
hence a larger amount of blood is forced into the arteries running
to the muscles of the limbs and trunk, and their elastic walls
squeeze the blood through the muscle capillaries at a rapid pace

52 Circulation and Respiration in the Human Body

The Lymph System. — The pressure of the blood against the
walls of the capillaries causes a considerable amount of the water
which forms the base of the plasma to seep through these thin
walls and to become a part of the tissue fluid. This surplus liquid
in the tissue fluid must be drained away in some manner. It is, in
fact, collected into a system of minute vessels known as the lymph
capillaries which come together to form larger vessels, the lymph
ducts. The lymph ducts finally combine to form two large vessels,
one on the right side of the neck and the other on the left, which
empty into the veins not far from the place where they enter the
right auricle of the heart. The lymph ducts, like the veins, are
provided with valves that keep the liquid from backing up ; hence
it is slowly pushed through them by the movements of the body,
especially those of breathing. At the points where the vessels empty
into the veins, the blood pressure is so low that the lymph is easily
forced into the circulatory system.

The lymph system is an auxiliary to the circulatory system, but
not a real part of it. Rather^ it is a one^wayjdrama^^ystem. Its
capillaries are believed to open at their ends. At any rate, various
solid particles — important among them, the bacteria which have
been attacking the tissues — make their way into it. While these
solid particles are microscopic in size, they are still too large to
make their way through the walls of the blood capillaries, and
the body would rapidly become clogged with them if the lymph
system did not carry them away. Scattered along the courses of
the lymph vessels are bunchesjc)f Jtissue . called _ lymph _nades or,
sometimes, lymph glands. Here the lymph is forced through
layers of cells which take up and destroy solid substances, so that
these substances become filtered out of the lymph before it makes
its way into the blood. The lymph nodes do much to keep disease
from spreading from one part of the body to other parts. Oc-
casionally, when they are overworked, they swell and become
hard. Nearly everyone has experienced at one time or another
these little kernels or lumps appearing in the neck, armpit, or
groin — the places where lymph nodes are found in greatest abun-
dance. The tonsils and "adenoids," which surgeons have to re-
move so frequently from the throats of small children, are
composed of the same sort of tissue as the lymph nodes. They
are iisefi.il in filtering p^t bacteria and other substances_riiat

Human lung. Microphotograph showing alveoli (the open spaces), bron-
chiole (the roughly star-shaped object toward the bottom), and a blood vessel
(the large, round object in the upper center).

Circulation and Respiration in the Human Body 53

so easily get into the tissues of thJJL.tcgJQP ; but, once diseased
themselves, they act as a source of infection, rather than a pro-
tection against it. Occasionally lymph nodes in the neck or other
regions become diseased and badly swollen and have to be removed.

RESPIRATION

The Respiratory System. — One of the most important func-
tions of the circulatory system is tp hejp bring Qxyggn t,Q. th^
cells 1^ In the one^

celled Paramecium, the process of external respiration involves
merely the diffusion of oxygen into and of carbon dioxide out
of the cell, together with the oxidative chemical reactions. In the
human body, the same sort of respiratory process goes on for each
cell ; but, in addition, oxygen and carbon dioxide must be brought
to and from the cell from the air outside the body, and this must
be done at a fairly rapid rate, since the cells are packed so closely
together and their metabolism goes on more rapidly that it does in
the cells of the lower organisms. The respiratory system functions
to bring air containing much oxygen and a little carbon dioxide
into sufficiently close contact with the blood that the latter may
take up oxygen from the air and transport it to the tissues while
it gives off the carbon dioxide which it has brought from the tis-
sues. This system is composed of the Jungs, plus the passages
running fromjhem tQ the exterior. Its parts are shown in Fig. 15.

The air we breathe, after it is taken into the nose or mouth,
passes through the l&tyjw, or voice box, and down the J£odk0,
otherwise known as the windpipe. A little less than halfway down
the chest cavity, the trachea divides into two branches, called
bronchi^ one going to the left and the other to the right lung.
The bronchi proceed to divide again and again into smaller and
smaller bronchi, until the air is making its way through tubules
that are scarcely a hundredth of an inch in diameter. Each of these
tubules, known as bronchioles, widens at its end to form a group
of air sacs, and the outer edges of each air sac are folded into
minute cup-shaped cavities, known as Muzali. A few' alveoli are
also found in the sides of the bronchioles themselves. The ar-
rangement at the end of the bronchiole is shown in Fig. 156.

The alveoli are crowded closely together in the lungs, and just
outside each alveolus is a thick network of blood capillaries. The

54 Circulation and Respiration in the Human Body

walls of the alveoli are nearly as thin as the capillary walls ; and
consequently it is very easy for oxygen of the air to make its
way through such thin membranes into the blood, and also for
the molecules of carbon dioxide to leave the blood and get into
the air that fills each alveolus. The blood which comes into the
lungs from the heart has given up a great deal of its oxygen to

-Trachea

-Aorta

Pulmonary
artery

•Heart

A B

FIG. 15. — Diagram of respiratory system. A, heart and lungs ; B, front view of
lungs and air passages.

the body tissues. There is a sort of "oxygen vacuum* ' in this blood,
and the oxygen in the alveoli rushes in to fill this vacuum.

The fact that oxygen makes a loose chemical union with
hemoglobin enables the blood to carry much larger amounts of
it than would be possible if it simply dissolved in the blood plasma.
The body could not possibly maintain itself on the oxygen that
might be brought to the cells in the latter manner. Nearly every-
one knows of the danger involved in running a motor car in a
closed garage. The carbon monoxide gas which escapes from the
exhaust has the property of combining with the hemoglobin in
such a way as to take the place of oxygen. The oxygen cannot be
transported to the tissues, and the body is asphyxiated as surely

Circulation and Respiration in the Human Body 55

as it would be if the windpipe were stopped and breathing shut
off completely.

When the blood, rich in oxygen, leaves the lungs, it travels
back to the heart, where it is pumped out through the aorta and
is carried to every part of the vast network of capillaries which
branches throughout all the tissues of the body. Every cell in
these tissues is continually using up oxygen ; and to make up the
shortage thus produced, the oxygen in the blood leaves the hemo-
globin and diffuses through the capillary walls into the tissue fluid
and thence into the cells themselves.

The burning of food substances in the body cells not only con-
sumes oxygen but produces carbon dioxide, which, becoming
thus concentrated in the tissue fluid, diffuses rapidly into the
blood. Here a series of chemical reactions enables it to be carried
in high concentration in the blood just as oxygen is. Upon
reaching the lungs, the carbon dioxide diffuses into the air in
the alveoli, since its concentration is lower in the alveoli than it
is in the blood.

The exchange that takes place in the lungs is indicated by the
difference between the ratios of gases in the air which we inhale
and in that which we exhale. Water vapor and certain other gases
being disregarded, the air that we breathe in is about 79 per cent
nitrogen, 20.96 per cent oxygen, and .04 per cent carbon dioxide.
That which we breathe out is 79 per cent nitrogen, 16.6 per cent
oxygen, and 4.4 per cent carbon dioxide.

How We Breathe. — The lungs are located in two air-tight cav-
ities, separated from each other by a region in the middle of the
chest which contains the heart, certain large blood vessels, and
various other structures. Each of the lungs is encased in an elastic
membrane, the visceral pleura, which encloses it on all sides like
a bag, except at a point on the inner side where the bronchus, blood
vessels, and nerves enter. Since a lung itself is a closed system of
tubes and sacs, open only at the point where the bronchus enters
the trachea, the whole arrangement is like a very large, thin sack
(the lung) crumpled up and stuffed inside a smaller sack (the
visceral pleura).

At the point where the bronchus enters each lung, the pleura
folds backwards and continues around on the outside, lining the
chest wall, the diaphragm, and the region between the lungs.

56 Circulation and Respiration in the Human Body

This outer layer is called the parietal pleura. It lies immediately
over the visceral pleura, and their surfaces are in contact so that
they rub back and forth over each other with each breath that is
taken. Inflammation of these surfaces is the cause of that very
uncomfortable disease, pleurisy, in which pain is experienced with
each breath that is taken into the body.

Since there is no air in the space between the two pleura, the
elastic lungs are forced to expand by the pressure of the outside
air, and the two pleural membranes are pressed closely together.
Any air, getting inside either of the lung cavities would cause the
lungs to collapse. In cases of tuberculosis, the chest wall is some-
times punctured and one of the lung cavities filled with nitrogen
gas. The lung collapses and is put entirely out of commission ; thus
it receives a complete rest which often puts an end to the progress
of the disease. Then the gas is drawn out and the opening in the
chest wall closed, so that the lung is forced to expand and fill
the cavity, whereupon the other lung may be collapsed and al-
lowed to heal.

Since the vacuum between the lungs and the wall of the chest
cavity allows the air outside to force the lungs tight against the
wall, any expansion of the chest cavity will result in the entrance
of more air into the lungs. Inhalation and exhalation of air are
brought about by increasing or decreasing the size of the chest
cavity. When we inhale, our rib muscles contract, causing the
ribs to rise and spread outward, thus increasing the diameter of
the chest cavity from front to back and side to side. At the same
time the diaphragm, which arches up in a sort of dome over
the stomach, liver, and other abdominal organs, is contracted
and flattens out, increasing the depth of the chest cavity and
pressing on the organs beneath it, thus causing the abdominal
wall to bulge outward. In ordinary quiet breathing, we exhale
simply by a relaxation of the muscles of inspiration, allowing
the ribs and diaphragm to return to their normal position; but
when we breathe hard or blow, it is possible to force air out of
the lungs by contraction of the muscles of the abdomen. Singers
are taught to breathe by drawing down the diaphragm as far
as possible, since filling the lungs in this fashion allows the ab-
dominal muscles to control the rate at which air is expired. Athletes
also learn this trick of "belly breathing/' for it enables them to

Circulation and Respiration in the Human Body 57

exhale more rapidly and thus to move air in and out of their
lungs at a faster rate.

MOVEMENT OF SUBSTANCES

Filtration. — It should now be evident that the chief problem the
body encounters in its task of maintaining the life of its cells is
that of moving substances in and out and from one part of the
body to another. This problem faces all organisms, plant and
animal, small and large. In general, these movements are of two
sorts, movements through passageways and movements through
the tissues. The former are illustrated by the flow of the blood
and the substances which it carries through the circulatory system
and the flow of air through the respiratory passages. In animals,
these movements are usually brought about through muscular con-
tractions, as in breathing or the beating of the heart, although
other means may be employed. The movement of liquids and gases
through passageways is always from a region of high to one of
low pressure. Not only is this true of circulation, but also of
respiration, for it is the difference between the air pressure in-
side and outside of the lungs that produces movements of air in
both directions. If the pressure inside a passageway is higher than
that outside, and if its walls are thin, the materials being moved
may be squeezed through the walls. Such a movement is called
filtration. It occurs in the circulatory system, where the pressure
coming from heart and arteries forces part of the water in the
blood out through the walls of the capillaries.

Diffusion and Dialysis. — The passage of substances through
the tissues of the body is brought about chiefly by the general
phenomenon of diffusion, which involves simply the movement
of molecules in a gaseous mixture or liquid solution from regions
of high concentration to regions where the concentration of the
substance being moved is low. For instance, if an odorous gas of
some sort is released in one corner of a room, the odor will gradu-
ally permeate the entire room as the molecules of the gas move
from the corner where they are in high concentration to all other
parts. Eventually, provided the weight of the gas is about equal
to that of the atmospheric gases, the odor will be as strong in
one part of the room as another, since diffusion will continue until
the concentration is equalized in all parts of the space provided.

58 Circulation and Respiration in the Human Body

The reason is that the molecules of a gas move about freely and
very rapidly in all directions, striking against and bounding off
one another like a set of billiard balls on a table; thus they
gradually wander away from any region of concentration and
become evenly distributed. Molecules in water solution (as well
as the dissociated particles of molecules, called ions) also move
about in this free manner and hence tend to distribute themselves
evenly throughout the solution. For instance, if you drop a pinch
of salt into a cup of water, it will sink to the bottom, and the
water at the top will have no taste of salt about it. But if you
leave it in the water for some time, the salt will dissolve and its
ions will diffuse throughout the cup, so that the water at the top
will be as salty as that at the bottom.

The substances which the cells of the body consume and pro-
duce during the course of metabolism are usually moved to and
from the cells by being held in solution in water and by diffusing
toward the cells as these substances are metabolically consumed
or away from them as they are produced. Thus carbon dioxide is
<:ontinually diffusing into the blood in the capillaries of the sys-
temic circulation, since its concentration in the tissues is con-
tinually increased by metabolic activity, while oxygen diffuses
out of the blood, since its concentration in the tissues is con-
tinually being decreased.

In both these instances, the diffusing substances must cross
membranes, namely, the capillary walls. At the same time there
are substances which are in different concentration on either side
of the capillary walls and which do not diffuse through the walls
because they cannot pass through them. The walls are semi-
permeable, allowing certain substances to go through and holding
others back. The diffusion of substances through semipermeable
membranes is called dialysis. Usually substances in true solution
can pass through a dialyzing membrane, whereas substances in
colloidal suspension cannot, because the submicroscopic openings
in the membrane are not large enough to allow colloidal particles
to pass through them. Thus, the protein substances which are
held in colloidal suspension in the blood are kept from passing
intQ the tissues, while oxygen, food material, and various other
substances dissolved in the blood pass out freely. The fat-like
membranes which surround the individual cells of the body will

Circulation and Respiration in the Human Body 59

allow fat-soluble substances to pass through them much more
readily than substances which are not soluble in fats. By means
of this selective permeability of dialyzing membranes, only those
substances which the cells can use are allowed to reach them or
enter them, and useless or harmful substances are kept out.

How Movements Are Facilitated. — There are many in-
genious devices whereby the movement of substances through the
body is caused to go on more rapidly than if the simple process
of diffusion were allowed to operate alone. The transport of
oxygen by the hemoglobin of the blood is a case in point. Oxygen
first enters the body by becoming dissolved in the water of the
blood plasma. Oxygen dissolved in water is the sole source of sup-
ply for plants and animals that live under water ; indeed, oxygen
dissolved in the tissue fluid is the sole direct source of it for
the individual cells of the body. But the oxygen that could be
carried in solution in the plasma would not be nearly enough to
furnish the body with all that it needs. Blood leaving the lungs
carries with it the equivalent of about twenty parts by volume of
gaseous oxygen to every one hundred parts of blood, while the
amount carried in solution is only one part in two hundred. The
rest is carried in chemical combination with the hemoglobin.
The amount of hemoglobin that will combine with oxygen de-
pends upon the concentration of oxygen in solution in the blood.
In the lungs, so much oxygen passes into solution in the blood
that nearly all the hemoglobin combines with oxygen. In the
tissues, the oxygen in solution diffuses rapidly out of the blood,
so that only about half as much remains dissolved in the plasma.
A great deal of oxygen immediately leaves its chemical associa-
tion with the hemoglobin and is also available for diffusion into
the tissues, and the hemoglobin returning to the lungs contains
considerably less oxygen than that which leaves them. Thus, with
only a small variation in the total amount of oxygen carried in
solution, there is a considerable variation in the total amount
carried by the hemoglobin.

Obviously, the reaction between the hemoglobin and the oxygen
in solution must go on very rapidly because the blood remains
only a second or two in the capillaries where all the chemical action
must be accomplished. Naturally, it can take place most readily
where the hemoglobin and the plasma come into direct contact,

60 Circulation and Respiration in the Human Body

namely, at the surfaces of the red corpuscles. The amount of
surface available for the reaction would not be nearly as great
if it were not for the minute size of the corpuscles, since the
smaller any body is, the larger is its surface in proportion to its
volume. Thus, a cube with a volume of a thousand cubic inches
has a surface of six hundred square inches, whereas a cube one-
inch in volume has six square inches of surface. In the frog,
where metabolism is not nearly as rapid as in a warm-blooded
animal like man, the red corpuscles are considerably larger than
ours because the hemoglobin-oxygen reaction does not need to
go on as rapidly as it does in our blood stream.

Dialysis of substances through membranes is frequently speeded
up considerably by providing small structures with large mem-
brane surfaces. The small size of most living cells is probably
in part accounted for by the fact that if sufficient amounts of
material are to diffuse into and out of them, there must be a large
surface in proportion to their total volume. Hence, all large
organisms are multicellular. The air sacs of the lungs, with their
cup-like alveoli, provide a large surface for the movement of
oxygen and carbon dioxide relative to the total volume of air in
the lungs. When filled to capacity, the lungs hold about two cubic
feet of air; the total surface of the alveoli is about two thousand
square feet. The small size of the capillaries in both the lungs and
the tissues again offers a tremendously large surface for dialysis
in proportion to the volume of the blood.

Osmosis. — Since water is continually being filtered out of the
blood because of the blood pressure on the capillary walls, and
since there is no pressure in the tissues to force it back in, it
would seem that before long the blood would be so completely
robbed of its water that the pressure would fall to zero. Indeed,
that is what would happen if it were not for the fact that the
blood proteins do not pass out with the water. The result is that
in the blood where the capillaries join the venules, the proteins
held in suspension are in high concentration. Conversely, the
water in this blood is in relatively low concentration — lower than
its concentration in . the tissues outside. Consequently the water
diffuses back through the capillary walls wherever the blood
pressure is so low that filtration does not take place. This passage
of water from a solution of low concentration (with a conse-

Circulation and Respiration in the Human Body 61

quent high concentration of water) through a membrane to a so-
lution of high concentration (low concentration of water) is
called osmosis. Obviously, it is simply another case of diffusion,
with the exception that in this case it is the water itself and not the
substances in solution that diffuses. It takes place when on one
side of a membrane there is a concentrated solution or suspension
of a substance that will not pass through the membrane. It is
possible to demonstrate osmosis by half-filling a membranous bag
with water holding some substance, such as egg white, in solu-
tion and placing the bag in a pan of pure water. If the substance
in solution is unable to pass through the membrane, and the
water does pass through it, the bag will gradually fill with water
until it is tightly distended. Indeed, the pressure inside may be-
come so great that the bag breaks. The pressure exerted when
water passes through a membrane into a solution of a substance
which cannot pass in the opposite direction is called osmotic
pressure. It is the osmotic pressure of the water entering the
circulatory system from the tissues that counterbalances the pres-
sure which causes it to filter out and maintains blood pressure at
approximately the same level over long periods of time. Osmotic
pressures of this sort play a part in countless vital processes. By
osmosis water is supplied to all our cells and tissues; and by
the proper balancing of osmotic pressures, the proper amounts of
water are allocated to each cell and tissue.

These balances must be maintained at all times. For instance,
after severe bleeding and in some disease conditions, it is necessary
to supply water to the blood by injecting it into the veins. When
this is done, the water must contain salts in solution in concen-
tration equivalent to that in the blood plasma, for otherwise the
water rushes into the corpuscles and the osmotic pressure becomes
so high that they burst. On the other hand, cells placed in water
where the concentration of solutes is greater than that of the
water in the cells will lose their water and shrivel up. In short,
a living cell requires a water environment that is balanced with
the water in the cell.

CHAPTER SUMMARY

Each cell of the human body lives in an artificial liquid environ-
ment, called the tissue fluid, from which oxygen and foods make

62 Circulation and Respiration in the Human Body

their way into the cell and into which carbon dioxide and waste
materials make their way in their passage from the body. The
chief function of the circulatory system is to provide rapid trans-
port for these substances to and from the tissues.

Blood is pumped through the circulatory system by the heart,
a muscular bag divided into four chambers, the right and left
auricles and the right and left ventricles. The blood leaves the
heart through the thick-walled elastic arteries, which branch until
they form minute arterioles; these branch further to form the
microscopic capillaries that are thickly distributed through every
tissue of the body. The capillaries join to form venules, which
unite to form the thin-walled veins that carry the blood back to
the heart. The blood leaves the heart under high pressure, and
passes in slowly dying pulsations through the arteries to the capil-
laries, through which it flows smoothly without a pulse. The
pressure gradually decreases as the blood flows through the sys-
tem until it reaches its lowest point where the veins enter the
heart. When the muscles are active, blood pressure is increased so
that the blood can carry the oxygen more rapidly to the active
cells.

The circulatory system is composed of two blood vessel circuits.
The first runs from the left ventricle of the heart through the
arteries to the capillaries in all regions of the body, and back
along the veins to the right auricle. The second runs from the right
ventricle along the pulmonary arteries to the capillaries of the lungs
and through the pulmonary veins to the left auricle. Passing
through the first circuit, the blood gives up oxygen to the tissues
and receives carbon dioxide. In the second circuit, it gives up
carbon dioxide to the air in the lungs and receives oxygen.

The basis of the blood is a liquid, the plasma, in which float
the red blood corpuscles, which transport oxygen by means of its
chemical union with the hemoglobin which they contain, and
ameba-like white blood corpuscles, which engulf the bodies of
bacteria and other waste solids. Clotting of the blood, which is
essential to keep the organism from bleeding to death, is effected
by the formation of a network of fibrin in the plasma. Throm-
bokinase, which is present in the tissues and is also released by

Circulation and Respiration in the Human Body 63

ihe platelets in the blood whenever bleeding starts, initiates the
series of chemical reactions that results in the formation of fibrin.

The lymph system is a one-way drainage system of capillaries
and ducts which removes solid wastes and bacteria from the tissues,
passing them through lymph nodes, where they are absorbed and
rendered harmless, and finally emptying into the blood stream
in the region of the neck.

The plan of the respiratory system is as follows : The trachea,
or windpipe, runs from the throat to a point halfway down the
chest cavity, where it divides into two bronchi, one going to the
right lung and the other to the left lung. The bronchi branch pro-
fusely in the lungs until they form millions of tiny passageways,
called bronchioles. At the ends of the bronchioles are air cham-
bers, the walls of which are creased so as to form smaller chambers
called alveoli. Each alveolus is surrounded by a capillary network,
so that exchanges of carbon dioxide for oxygen can take place
over a large surface area. When the walls of the chest are raised
and the diaphragm is pulled down, air is sucked into the lungs.
A contrary set of movements pushes it out.

Substances are transported through the body by movements
through passageways or through tissues. Movement through the
latter is effected by the following processes :

Filtration, the forcing of liquids through membranes by me-
chanical pressure. Blood pressure, for instance, forces water
through the capillary walls and into the tissues.

Diffusion, the movement of molecules or ions in a gas or in
solution from a region of high concentration to one of low
concentration.

Dialysis, the diffusion of substances in solution through a
semipermeable membrane.

Osmosis, the diffusion of water through a semipermeable mem-
brane from a region where it holds substances in dilute solution
to one where it holds them in concentrated solution.

When substances are transported on surfaces or through mem-
branes, movement is facilitated if the structures involved are very
small, so that their surfaces are large in proportion to their
volumes.

64 Circulation and Respiration in the Human Body

QUESTIONS

1. In what sort of an environment do the cells of the body live?

2. Describe the circulatory system and tell how the blood flows
through it.

3. Describe the blood.

4. Where is thrombokinase found, and why is it so important to
human life?

5. Why is there no pulse in the capillaries?

6. Describe the lymph system, and discuss its importance.

7. Describe the structure and functioning of the respiratory sys-
tem.

8. What is the importance of the vacuum that exists between the
visceral and parietal pleurae ?

9. Define each of the following and give an instance of its occur-
rence in the body: filtration, diffusion, dialysis, osmosis.

IO. What is the significance of the small size of each of the following:
the red blood corpuscles, the capillaries, the alveoli? Explain.

GLOSSARY

alveolus (al-ve'o-lus) pi. alveoli (al-ve'o-ll) A very minute air cham-
ber in the lungs.

Ameba (a-me'ba) A protozoan which moves about by means of
pseudopodia.

ameboid movement Movement by means of pseudopodia.

aorta (a-or'ta) Large artery which carries blood from the left ven-
tricle on its way to all parts of the body except the lungs.

arteriole (ar-ter'i-6l) A very small artery.

auricle (6'ri-k'l) One of the upper chambers of the heart.

bronchus (bron'kus) pi. bronchi (bron'ki) One of the branching
passageways for air in the lungs.

bronchiole (bron'ki-61) One of the very small passageways for air
in the lungs.

dialysis (di-al'i-sis) Diffusion of a substance through a semipermeable
membrane, leaving behind substances that cannot diffuse through
the membrane.

diaphragm (di'a-fram) A muscular membrane which separates the
chest cavity from the abdominal cavity, the movements of which
help to force air in and out of the lungs.

diffusion Movement of gases or substances in solution from regions
of high concentration to regions of low concentration.

fibrin (fi'brin) A solid protein, the formation of which brings
about the clotting of the blood.

Circulation and Respiration in the Human Body 65

filtration Movement of a liquid through a membrane as a result of
mechanical pressure.

hemoglobin (he'mo-glo'bin) A protein pigment which is the chief con-
stituent 6f the red blood corpuscles and which serves to transport
oxygen.

hemophilia (he'mo-fiTi-a) An hereditary disease characterized by the
failure of the blood to clot.

larynx (lar'inks) The voice box, located at the point where the trachea
enters the throat.

osmosis (os-mo'sis) Passage of water or other solvent through a semi-
permeable membrane from a region of low concentration of sub-
stances in solution to a region of high concentration of solutes.

osmotic pressure (os-mot'ic) Pressure produced by osmosis.

parietal pleura (pa-ri'e-tal plu'ra) The outer of the two membranes
which line the lungs.

plasma (plaz'ma) The liquid portion of the blood.

platelets Small, solid structures in the blood which disintegrate and
release thrombokinase when the blood wets a surface outside the
blood vessels.

portal vein The vein passing from the intestines to the liver.

pseudopodia (su'do-po'di-a) Temporary protrusions of protoplasm
employed by Ameba in locomotion.

sinusoid (si'nus-oid) One of the minute cavities in the liver into
which blood from the portal vein flows.

thrombokinase (throm'bo-ki'nas) A substance which initiates a series
of chemical reactions resulting in. the formation of fibrin.

trachea (tra'ke-a) The windpipe.

ventricle (ven'tri-k'l) One of the lower chambers of the heart.

venule (ven'ul) A very small vein.

visceral pleura (vis'er-al plu'ra) The inner of the two membranes
which line the lungs.

CHAPTER IV

DIGESTION, ASSIMILATION AND EXCRETION
IN THE HUMAN BODY

DIGESTION

Outline of the Digestive System. — Digestion in man is es-
sentially the same process that it is in Paramecium, the breaking
down of the large molecules of food substances so that they be-
come small enough to pass through membranes and enter the
protoplasm. But while in Paramecium digested food enters di-
rectly into the protoplasm from the simple food vacuole, in man
a complex digestive system is provided, whence the food enters
the blood stream to be taken to various parts of the body, where
it may be used in the cells immediately or, quite as frequently,
stored for use at a later time.

The digestive system is composed of a twisting, irregular tube,
the alimentary canal, and a number of glands which pour diges-
tive secretions into it. (See Fig, 16.) The structures of the ali-
mentary canal, in the order in which food passes through them,
are as follows : the mouth ; the throat ; the esophagus, a thin tube
extending from the throat to the stomach ; the stomach, a pear-
shaped bag in which the food is held for some time after being
swallowed; the small intestine, a narrow, coiled tube, about an
inch or two in diameter and some twenty feet in length ; the large
intestine, a somewhat wider tube, about five feet long and shaped
like an inverted U; the rectum, in which the feces, or waste
materials left after the passage of food through the alimentary
canal, are held until they are expelled through the opening to
the exterior, known as the anus.

The walls of the esophagus, stomach, small intestine, large in-
testine and rectum are composed of sheaths of muscular and
connective tissues, with epithelial linings inside and out. The
contractions of the muscles serve to push the food through the

66

Digestion, Assimilation and Excretion

Nasal cavity

Mouth cavity

Salivary glands

Opening into
trachea

Salivary glands

Gall bladder

Liver-

Pylorus

Large

intestine

(colon)

Appendix

Anus
FIG. 16. — The human digestive tract.

Esophagus

Rectum

68 Digestion, Assimilation and Excretion

digestive tract and to churn it about, breaking it up and thor-
oughly mixing it with the digestive secretions.

The glands which produce the digestive juices are : the salivary
glands, which empty into the mouth; the gastric glands, located
in the walls of the stomach; the liver, located somewhat to the
right, just below the diaphragm and above the stomach, the
digestive function of which is to secrete bile into the small in-
testine; the pancreas, a long, thin gland lying just below the
stomach, also emptying into the small intestine; finally, small
glands which line the walls of the small intestine and secrete their
intestinal juice into that organ.

The juices secreted by these digestive glands are watery fluids
containing a variety of substances in solution or suspension. All
of them, except the bile of the liver, contain enzymes, which, as
we have already said, are substances that bring about the chemi-
cal reactions of digestion.

The Process of Digestion. — Upon entering the mouth, food
is usually broken up through mastication and more or less thor-
oughly mixed with the saliva. This juice is produced by three
pairs of glands: the parotid, located at the corners of the jaws
just under the lobe of each ear; the submaxillary, just under each
jaw bone; and the sublingual, on each side of the floor of the
mouth. They empty into the mouth through small tubes, or ducts,
coming from the cheek in the case of the parotids and from just
under the tongue in the case of the submaxillaries and sublinguals.
The parotid gland is the one that becomes swollen and sore when
we have mumps.

These glands secrete slowly most of the time, thus keeping the
mouth moist; but when food enters, their secretion becomes
more copious, and, mixing with the food, lubricates it so that it
is easily swallowed. The chief enzyme contained in saliva is
ptyalin, which acts to break starches down into a double sugar.
Bread turns sweet if held in the mouth for a short time, because
the starch which it contains is converted into sugar.

After mastication, the food is swallowed by an upward move-
ment of the tongue, which pushes it back into the throat. Immedi-
ately muscles in the throat contract in such a way as to shut off
the openings into the nose and trachea and at the same time
push the food down into the esophagus. The muscles of the walls of

Model of the luunan body cavity, mirror image.

Digestion, Assimilation and Excretion 69

the esophagus contract just behind the food, making a ring-
like constriction of the passageway. This ring of constriction
moves forward, forcing the food along the tube into the stomach.
Such forward-moving waves of constriction in the alimentary
canal are called peristaltic waves, and they occur in the stomach
and intestines as well as in the esophagus. At the point where the
food enters the stomach there is a thick ring of muscle, called the
cardiac sphincter. Normally it is contracted so tightly that nothing
can pass through it; but as food coming down the esophagus
makes contact with it, it relaxes, allowing the food to pass
through, whereupon it immediately contracts so that the food
cannot pass backward from the stomach into the esophagus.
Occasionally this sphincter does not contract as tightly as it

FIG. 17. — A peristaltic wave.

should, and then some of the acid contents of the stomach makt?
their way back into the esophagus, causing the unpleasant sensa-
tion known as "heartburn."

The stomach is composed of two parts : a wide, rounded por-
tion, called the fundus, on the left side of the body from the
cardiac sphincter ; and a narrower, tapering section to the right,
the pylorus, at the end of which is the pyloric sphincter. This
sphincter does not contract tightly enough to hold back the liquid
portions of a meal, but it does keep back the solids until they have
been mixed with the gastric juice. When they have been pretty
well liquefied, it allows them to pass into the stor^acfi^ little bit
at a time, so that a meal does not entirely leave the stomach until
two to six hours after being eaten.

The gastric juice contains three important substances : pepsin,
an enzyme which begins the digestion of proteins by breaking
them down into less complex substances called proteases and pep-
tones; rennin, another enzyme which curdles milk and thus gets

70 Digestion, Assimilation and Excretion

it ready for the pepsin to act upon it ; and hydrochloric acid. This
last element of the gastric juice is as important as the enzymes ;
for, although it has no action whatsoever on the foods, it makes
the mixture of food and juices acid, and pepsin can act upon
proteins only in an acid medium. The gastric juice that is secreted
into the fundus contains very little acid; consequently protein
digestion does not take place in the food that is held there, but
rather there is a continuation of the digestion of starch by the
saliva that has been swallowed. The walls of the fundus exert a
steady pressure on the food, forcing it into the pylorus as the
latter is slowly emptied through the occasional relaxation of the
pyloric sphincter. In the pylorus, however, there is a continual
succession of peristaltic waves moving from the region of the
cardiac sphincter to the pyloric sphincter about once every three
seconds. Since each wave takes approximately ten seconds to
move from one end of the pylorus to the other, there are usually
three or four waves following one another at any moment. By
putting some substance that is opaque to X-rays in the food,
it is possible to take moving pictures of these waves. When the
pyloric sphincter is closed, the peristaltic waves cannot force the
food out of the stomach. Rather, they churn it up, mixing it with
the gastric juice and producing a semi-liquid mass of food and
juice called chyme. From ten minutes to half an hour after the
food has been eaten, the sphincter relaxes for a moment, and
a small portion of the chyme is squirted into the small intestine.
It is immediately replaced in the pylorus by more solid food
forced in from the fundus ; and from then on the sphincter relaxes
at frequent intervals, allowing small amounts of food to enter the
intestine, until the stomach is empty.

Although we ordinarily associate the stomach more than any
other organ with the process of digestion, it really plays only a
minor role in the actual chemical breakdown of foods. To be sure,
the gas.tr ic juice mit jatesjh^^ just, as saliva
initiates the digestion of starches ; but both mouth and stomach
digestion mark only the very beginnings of the process and, in-
deed, may be dispensed with entirely. There are many people
whose gastric juice contains almost no hydrochloric acid, so that
the action of pepsin is practically eliminated, yet they seem to
digest their meals as readily as normal individuals. The real

Digestion, Assimilation and Excretion 71

function of the stomach is .tQ_bold the food in storage so that a
whole meal is not immediately forced into the intestine and at
the same time to reduce its solidity* converting it into a mass of
small solid particles held in a liquid and thus exposed to the action
of the enzymes in the small intestine.

The first juices with which the food comes into contact in the
small intestine are the pancreatic juice and the bile of the liver,
which enter through a common duct a short distance below the
pyloric sphincter. The bile is formed by the cells of the liver and
stored in a small sac, the gall bladder, which is located on the lower
surface of the liver. When chyme enters the intestine, the gall
bladder contracts and the cells of the liver begin to secrete more
rapidly. At the same time the pancreas pours its juice into the in-
testine. Both these juices are alkaline and neutralize the acid
from the stomach. The bile contains no enzymes, but its salts mix
with the fats in such a way as to make them much more sus-
ceptible to lipase, the pancreatic enzyme which splits the fats into
two absorbable types of compounds, fatty acids and glycerol. The
pancreas also provides one or more enzymes which carry on pro-
tein digestion where the pepsin of the stomach has left off.
These enzymes reduce some of the proteins to ami-no acids, the
ultimate protein constituents that are absorbed into the blood
stream. Finally, the pancreatic juice contains an enzyme known
as diastase, which continues the action of the ptyalin, breaking
starches down into double sugars.

There is still some digestive action to occur. Certain proteins are
not completely reduced to amino acids by the pancreatic enzymes,
and their digestion is left to the action of enzymes found in the
intestinal juice. Furthermore, the double sugars must become
single sugars before the carbohydrates can be absorbed into the
body, and this is accomplished by three different enzymes of the
intestinal juice which act upon three different kinds of double
sugars.

The movements of the small intestine are of two kinds : peri-
staltic waves, which occur from time to time and move for a
short distance along the tract, carrying the chyme with them;
and somewhat similar contractions which, however, do not move
along. These latter serve to mix the food with the digestive secre-
tions and also to bring all parts of it into contact with the walls

72 ** Digestion, Assimilation and Excretion

of the Intestine, through which the digested food is absorbed into
the blood.

Digestion and, absorption of_jbod. .are completed in the small
intestine, except for the residue which is still very watery as it
passes into the large intestine. The mass now passes slowly
through the large intestine, where most of the water from it is
absorbed ; and finally the waste materials, or f eces, are passed into
the rectum, where they are held until they are finally expelled
through the anus.

A brief outline of this rather complex process of digestion
will help the reader to picture it as a whole. The entire process
may be divided into two aspects : the chemical aspect, that is, the
actual breakdown of the food substances by the action of enzymes ;
and the mechanical aspect , the fragmentation and liquefaction of
the food to prepare it for efficient enzyme action, and the mixing
of the food with the enzymes. Digestion takes place in three parts
of the alimentary canal : the mouth, the stomach, and the small
intestine. We may therefore outline it as follows :

DIGESTION IN THE MOUTH :

Chemical: Beginning of carbohydrate digestion by the ptyklin

of the saliva.
Mechanical: Fragmentation of the food through mastication,

mixture with the liquid saliva.

DIGESTION IN THE STOMACH:

Chemical: Continuation of the action of ptyalin in the fundus.

Beginning of protein digestion in the pylorus by the pepsin

and rennin of the gastric juice.
Mechanical: Liquefaction of food and mixture with juices

through peristaltic movements of the pylorus.

DIGESTION IN THE SMALL INTESTINE :

Chemical: Continuation of carbohydrate digestion by action of
pancreatic diastase. Completion of carbohydrate digestion by
the sugar-reducing enzymes of the intestinal juice. Comple-
tion of protein digestion by the enzymes of the pancreatic
juice and another enzyme in the intestinal juice. Digestion of

Digestion, Assimilation and Excretion 73

fats by the lipase of the pancreatic juice with the helf^Qf the
bile salts.

Mechanical: Mixture of digestive juices with chyme by station-
ary ring-like contractions of the small intestine.

Many people are curious to know the length of time it takes
to digest food. This problem can be studied indirectly by putting
some substance in the food that is opaque to X-rays and then
taking X-ray pictures of the meal as it passes through the digestive
tract. Knowing that the process of digestion is completed before
the food gets into the large intestine, we can judge just about
how long it takes. The meal does not go through in a lump, but
parts of it make their way far in advance of others. Some portions
leave the stomach within a few minutes after they are swallowed,
and there is a continuous exodus of small bits of the meal out of
the stomach on their way through the small intestine. At the end of
six hours, most of the meal will have entered the large intestine,
yet small parts of it will be strung out through the small intestine,
and some may still remain in the stomach. At the end of twelve
hours, the entire meal is usually completely digested.

ASSIMILATION

The Absorption and Use of Foods. — The wall of the small
intestine is not smooth, but thrown into a multitude of folds or

FIG. 1 8. — Small intestine, showing the folding of the inner wall.

ridges which run around it transversely, as shown in Fig. 18.
More important still, the surface of the mucous membrane lining
the small intestine is completely covered by closely packed little
projections which stand up on it like the pile on velvet. They are
much smaller than this, however, being about a fortieth of an

74

Digestion, Assimilation and Excretion

length and quite slender in proportion. When they are
examined under the microscope, each one is found to contain a
network of blood capillaries and a second network of lymph
capillaries. (See Fig-. 19.) Digested food is absorbed into these
minute blood and lymph channels. Here we have another example
of small structures affording a large surface for the passage of
materials across a membrane. The total surface afforded for the
absorption of foods in the intestine is many times greater than
that which would be present if the lining was entirely smooth.

Artery Vein

\

Epithelium of Villus

1
1

1

Lymph vessel

Intestinal gland
FIG. 19. — Structure of a villus. (After Hardy.)

The single sugars in the intestines make their way through the
thin walls of the villi and into the capillaries. Hence they are
carried into the portal vein, and through it to the capillaries of
the liver. After a meal rich in carbohydrates, the concentration
of sugar in the portal vein is quite high; but the blood which
leaves the liver to go to the heart has only a trace of sugar in it,
since all the surplus has been removed and stored as glycogen in
certain special storage cells of the liver. Glycogen is a form of
starch that is peculiar to animal bodies. The liver regularly holds
& considerable amount of it as a reserve supply of carbohydrate

Digestion, Assimilation and Excretion 75

for the body. As the tissues use up sugar and its concentrati^fl in
the blood becomes lower, the glycogen in the liver is turned into
sugar and sent back into the blood. By this means the concentra-
tion of blood sugar is kept almost constant, no matter if several
hours have passed since the last meal was eaten. Glycogen is
stored in the muscle cells, ready to be used for fuel whenever our
muscles are active.

Sometimes carbohydrates are taken into the body in such
excess that they cannot all be burned or stored as glycogen. In
this case two things may happen. First some of the surplus sugar
may make its way out of the blood and into the kidneys to be
excreted in the urine. Second, some of the carbohydrates may
be converted by certain cells of the body into fat and stored in that
form. Well-fed people store a great deal of food in this way ; con-
sequently, individuals who are trying to reduce must avoid not
only fats, but starches and sweets as well.

The products of the digestion of fats, glycerol and fatty acids,
are first absorbed into the epithelial cells which form the outer
linings of the villi. Here, within these cells, they are immediately
recombined to form fats. But while the fats in our food are not
of the type found in the human body (almost every species has its
own particular kinds of fats), those now formed are of the human
type. These recombined fats make their way from the lining cells
of the villi into the lymph capillaries and are carried through
the lymph system to the veins in the neck and emptied into the
blood stream. They float about in the blood in the form of ex-
tremely small, insoluble globules until they pass through the liver,
where they are combined with phosphoric acid to form a com-
pound known as lecithin. Lecithin is soluble, and hence can make
its way out of the blood stream and into the cells of the body.
Very small quantities of it may be used in the construction of
the fat-like parts of protoplasm. Most of it is either oxidized to
yield energy or else stored as fat. Fat storage takes place in certain
specialized connective tissue cells located just below the skin or
around such internal organs as the intestines, kidneys, and heart.
When not enough fuels are eaten to serve the energy require-
ments of the body, these fats leave the storage tissues, are recon-
verted to lecithin, and carried through the blood to the active
cells.

76 Digestion, Assimilation and Excretion

Protein materials are absorbed into the blood capillaries of the
villi in the form of ammo acids. These substances are then car-
ried to the cells of the body, where they may be recombined into
proteins to become part of the protoplasmic cell structure in
growth or to replace structures that have been worn away. There
are about twenty kinds of amino acids, but they can be put to-
gether in different combinations to form millions of different
proteins. Each kind of cell manufactures its own special brands
of proteins. The proteins found in vegetable foods contain fewer
of the amino acids used to build up the structures of the human
body than those found in such animal foods as meat, eggs, milk,
butter, and cheese; hence, an adequate quantity of the latter types
of food is desirable in any diet.

The average American diet contains four or five times as much
protein as is necessary to replenish worn-out cell structure. The
surplus is used for fuel. As the amino acids pass through the
liver, a chemical reaction takes place which splits them into sugar-
like substances and ammonia. The ammonia is rapidly trans-
formed into urea, which is excreted, and the sugar-like substances
are oxidized in the cells. In some way, the consumption of excess
protein causes an increase in the rate of oxidative metabolism in
the body. This results in the formation of heat. It is a well-
known fact that more meat is eaten in winter than in summer, for
people unconsciously increase their protein consumption as a
protection against cold. Good reducing diets contain little fat
and carbohydrate in proportion to protein. The protein raises the
rate of metabolism and thus causes the individual to oxidize the
fat that has been stored in the connective tissues of his body.

Metabolic Rates. — Every living cell in the body must oxidize
food substances at a slow rate if it is to maintain life at all. In
addition, even if we rest completely, some energy must go into
breathing, digesting food, the pumping of the heart, and similar
activities. The total amount of energy that must be expended
merely to keep the body alive is approximately 1,900 Calories
per day.1 This is called the rate of basal metabolism. Whenever
we move about, however, considerable additional energy is con-

*A Calorie is a standard unit of energy, the amount necessary to raise one
kilogram (2.2 pounds) of water one degree Centigrade (1.8 degrees Fahren-
heit).

Digestion, Assimilation and Excretion 77

sumed by the muscles. In a rowing race of about six minutes'
duration, an oarsman will consume about thirty Calories per
minute, which is seventeen or eighteen times the rate of basal
metabolism. But this rate can be maintained only by a trained
athlete over a brief period of time. The average metabolic rate
for people in sedentary or semi-sedentary occupations who take
just enough exercise to maintain health is 2,500 Calories, only
600 Calories above the basal rate. For manual laborers, the rate
runs from 3,500 to 5,000 Calories per day.

The greater part of all this energy is wasted as far as doing
any work is concerned, because it is transformed into heat. As
anyone knows who has had the slightest acquaintance with en-
gines, every mechanism which transforms potential energy into
the energy of movement loses much of that energy in the form of
heat, and the human body is no exception. But the heat liberated
in our bodies is not entirely wasted. An automobile engine runs
most efficiently when some of the heat which it generates has
warmed it up. And, similarly, metabolism goes on best at a
certain temperature; in fact, at just about the temperature that
is constantly maintained in the human body by the heat produced
in metabolic activity, namely, 98.6 degrees Fahrenheit.

Other Food Substances. — The proteins, carbohydrates, and
fats are the "big three" of the food substances and are the only
sources of food energy. But many other substances appear in our
foods. First there are the condiments, such as the spices, which
have no food value at all and merely serve to improve the palata-
bility of foods. Then there are the drugs — caffeine, alcohol, and
the like — which are taken partly for their taste and partly for
the effect they have on the nervous system. A certain portion of
any alcohol that is imbibed, however, is transformed into sugar
and serves as a carbohydrate food. In America, without a doubt,
most alcohol is taken for narcotic purposes ; but in many European
countries it is essentially a staple food product, being taken
regularly at meals in rather small quantities. Another important
element in our food is roughage, the indigestible portion of the
material that gives it bulk, enabling it to pass through the in-
testines readily, thus preventing constipation. Most of this rough-
age is cellulose, the indigestible carbohydrate which forms the
cell walls of plants. Finally, there are three groups of substances

78 Digestion, Assimilation and Excretion

taken in with our food that are absolutely essential to the metabo-
lism of the body. They are water, salts, and vitamins.

The Role of Water in the Human Body. — Water may be
looked upon as the basis of life. It is the matrix in which the busi-
ness of living is carried on. Life might be defined as the activity of
proteins, fat-like substances, and mineral salts in colloidal solu-
tion in water. Water is not only the chief constituent of proto-
plasm, but it forms the major part of the blood and tissue fluids
which are so important in transporting substances to the cells.

The daily bodily intake of water is very large. In addition to
that which we drink, we secure a great deal from our food, the
major portion of which is water. Such vegetables as lettuce and
cabbage, for example, are approximately 90 per cent water.
Furthermore, the oxidation of sugar results in the production of
considerable water within the body.

This great intake is necessary to balance the loss of water
through the lungs, sweat glands and kidneys. And thus water
serves a further purpose, for the daily stream passing through
our bodies washes along with it a great variety of impurities and
useless substances.

Mineral Salts. — The mineral salts of the body include the
chlorides, sulphates, nitrates, carbonates and phosphates of po-
tassium, sodium, ammonium, calcium, magnesium and iron. Salts
cooperate with the proteins in carrying on their activities. They
may be looked upon as the regulators of the activity of protoplasm.
The rate and nature of this activity depend upon the balance of
concentration of the various salts. They are also important in
keeping a proper chemical balance in the blood stream and other
parts of the body, and they enter into many vitally important
chemical reactions.

Salts are found in small quantities in nearly all foods, but es-
pecially in meats, dairy products, and leafy vegetables. The only
salt that needs to be added to a normal diet is ordinary table
salt, sodium chloride. Most vegetarian animals require some spe-
cial source for this salt, since plants are deficient in it relative to
the amount needed by animals. Grazing animals will travel for
miles to get to a "salt lick" where they can secure some of this
necessary substance. Human beings, however, usually eat much
more than is needed for metabolic purposes, and most of it

Digestion, Assimilation and Excretion 79

passes out of the body through the kidneys. With us it is essentially
a condiment.

Vitamins. — Vitamins are substances that exist in small quanti-
ties in our food and that are essential for normal and healthy
metabolic reactions. When there is a marked lack of one or more
vitamins in the diet, certain deficiency diseases appear. Less marked
vitamin deficiencies retard growth and reduce health and vigor.

The idea of vitamins as essential elements in metabolism was
advanced in 1912. Since then, great progress has been made in
discovering various types of vitamins, determining their concen-
tration in foods, working out their chemical composition, and even
manufacturing them by methods of chemical synthesis. It is un-
certain how many vitamins there are, since new ones are still being
discovered. Furthermore, there are instances in which severai
chemically different vitamin substances have been found which
have the same general effects on metabolism, although each may
show slight differences in its effects. These are classed as different
vitamers of the same vitamin.

In short, vitamin study is becoming highly complicated. To
simplify it, only the vitamins whose known effects on human
beings are of considerable importance will be described here. These
can be divided into two groups : the fat-soluble vitamins, A, D,
and K, and the water-soluble vitamins, including certain B-com-
plex vitamins and vitamin C.

Vitamin A is found in all kinds of animal fats except lard, being
most concentrated in halibut liver oil. The best sources of it in
normal diets are liver, eggs, cream, butter, and fortified oleomarga-
rine. There are also substances in green and yellow vegetables
which are changed into vitamin A in the animal body. Hence, such
vegetables are good sources for the vitamin.

A disease called xer ophthalmia, which in its extreme form re-
sults in practically complete destruction of the eyes, appears when
vitamin A is almost totally lacking in the diet. Less extreme vita-
min A deficiencies cause slow growth in children and produce
"night blindness" in adults. The latter condition makes driving
at night difficult, since the eyes are readily blinded by the glare
of headlights on approaching cars.

In xerophthalmia, there is a very high susceptibility to contagious
diseases ; hence, vitamin A has been termed an anti-infective vita-

80 Digestion, Assimilation and Excretion

min. Many people take it in cod liver oil or halibut liver oil to
avoid colds. Actually there is little evidence that taking more
vitamin A than is provided in regular diets is at all effective in
preventing colds or other infections.

Vitamin D has been called the "sunshine vitamin" because it is
formed in fatty substances that are irradiated by certain ultra-
violet rays in the sunlight. It is not found in great quantity in any
of the ordinary foods, and unless intentional provision is made
for it in the diet, more vitamin D is secured by irradiation of the
fat beneath the skin than from food. But in cloudy northern cli-
mates the sun is an undependable source ; hence the best way of
securing a good supply of this vitamin is to take regular doses of
it in capsules or fish liver oils.

Vitamin D regulates the amount of calcium and phosphates in
the blood. In young children, lack of it results in the formation
of soft bones which are easily deformed. The disease so produced
is called rickets ; it is very prevalent in northerly, cloudy regions.
There is much evidence also that decay of the teeth results, at
least in part, from lack of vitamin D during the period of early
growth. It is important, therefore, that expectant and nursing
mothers and all infants and growing children should take vitamin
D in some of its concentrated forms.

Nothing is known about the requirements of vitamin D for
adults, but it is generally assumed that an adequate supply will
make the body function better because of its regulation of the
mineral balance in the blood.

Vitamin D is the only vitamin that is occasionally taken in such
great quantity that it causes illness. A very concentrated form,
known as ergosterol, is usually responsible for overdoses. To be
on the safe side, ergosterol should not be taken except under a
doctor's direction.

Vitamin K is interesting in that human beings do not need it
in their diet because it is synthesized by bacteria which inhabit the
human intestine. There it is dissolved in 'the fats of the food and
enters the body through the intestinal villi.

When this vitamin is absent, the blood will not coagulate. In
cases of obstructive jaundice, where stoppage of the bile duct pre-
vents bile from entering the intestine, fats are not absorbed and
the vitamiti K formed by the intestinal bacteria fails to enter the

Digestion, Assimilation and Excretion 81

blood. Before operation to remove the obstruction, the vitamin
must be administered to prevent death from bleeding.

Vitamin K is sometimes absent in newborn children and must
be provided to prevent fatal bleeding.

The vitamin B complex is a group of vitamins, of which about
a dozen are now known, which are usually found together, al-
though not always in the same proportions in each food. The best
known are thiamine (B^, riboflavin (B2), and niacin.

Good sources of the B complex are milk, fresh fruits and vege-
tables, and the parts of wheat and rice that are removed in milling.

Oriental populations in which the great dietary staple is polished
rice are especially prone to the disease called beriberi, which is
characterized by pain and general weakness. This disease can be
cured by feeding the husks of the rice that are removed in polish-
ing and also by administration of pure thiamine.

Beriberi is by no means unknown in this country; indeed, al-
though the American diet usually contains enough of this vitamin
to ward off this disease, it is believed that thiamine is more likely
to be lacking than any other vitamin. Increased thiamine feeding
has been found to stimulate growth in children, improve appetite
and relieve digestive disturbances, eliminate aches and pains,
reduce irritability, and increase energy. Irritable, "run-down"
people, however, should not be led to expect that thiamine is a
sure cure for their disabilities.

Marked deficiency in riboflavin results in a disease called cheilo-
sis, characterized by cracking of the skin at the angles of the
mouth and other abnormalities of the epithelial tissues. Riboflavin
is usually deficient also in pellagra, although the most outstanding
deficiency in that disease is niacin. Heavy feeding of niacin can
relieve pellagra, but for permanent cures the patient must have
an adequate amount of riboflavin and probably other B vitamins
in the diet.

Pellagra is a very severe disease that often results in insanity.
It occurs frequently in our southern states among the poor who
depend largely on corn for their nutrition. Corn seems to be lack-
ing in the B vitamins which prevent pellagra. Delirium tremens
is essentially a form of pellagrous insanity brought about by the
fact that chronic alcoholics eat so little that they fail to obtain a
sufficient supply of the B complex.

82 Digestion, Assimilation and Excretion

Vitamin C is found most abundantly in citrus fruits. Tomatoes,
potatoes, cabbage, and various other fruits and vegetables are
good sources. There is little vitamin C in cow's milk, and hence
bottle-fed babies must be given it in the form of orange or tomato
juice.

Lack of vitamin C produces scurvy, characterized by weakness,
pain, and a readiness to bleed, especially in the gums. This disease
was once very common in Europe but disappeared with the intro-
duction of the potato in the European diet. It continued to ravage
the crews of ships on long voyages until it was discovered that
lemon or other citrus juices would prevent it.

Some authorities believe that mild lack of vitamin C is fairly
widespread among people today and that it results in lowered re-
sistance to infection. As with vitamin A, however, taking large
doses of vitamin C will not raise resistance to infection above
normal.

There is still considerable controversy as to whether the normal
American diet does or does not contain sufficient vitamins. A few
years ago many authorities felt that there was no need for vita-
mins above the amount required to prevent beriberi, scurvy, rickets,
and other deficiency diseases, and some still hold to this point of
view. But so many sound scientists who have carried on vitamin
research are convinced that ordinary diets do not contain enough
vitamins to maintain full health and vigor that wisdom would
seem to lie in the direction of overdoing the matter of adding
vitamins to the diet, rather than underdoing it.

There are three ways of attempting to guarantee a sufficient
vitamin intake. The first is for the housewife to make a careful
study of the vitamin content of foods and of ways of preparing
them so as not to lose vitamins in cooking and storage, and then
for members of the family to consent to eat plenty of whole wheat
bread, milk, butter, eggs, leafy vegetables, and fruits. This is un-
doubtedly the ideal method, and individuals who are wise about
their diets will do their best to change their eating habits and
methods of food preparation so as to improve their vitamin intake.
But such a program is difficult enough among the well-to-do, con-
scientious, and educated members of the population. Most people
lack the money, time, and self-control.

The second method of supplying vitamins is \o "fortify" or

Digestion, Assimilation and Excretion 83

"enrich" the foods that nearly everyone eats with synthetic vita-
mins or vitamin concentrates. Much progress has already been
made in this direction. It is reported that, as a result of enriching
part of the bread sold in New York City with thiamine, riboflavin,
and niacin, the incidence of beriberi and pellagra has been reduced
to one- fourth and one-third of what it had been in New York
City hospitals. Such enriched bread does not contain the whole
vitamin B complex and is doubtless inferior to whole wheat bread,
but it is certainly better than ordinary white bread.

Programs of food enrichment and fortification constitute the
most practical method of getting vitamins into the diets of the
general public.

Recently methods have been developed of producing a yeast
that can be mixed with cereals and other types of food to give
them a very high vitamin B-complex content along with consider-
able valuable protein. Here is a promise, at least, of a fairly easy
and inexpensive way of making certain that the ordinary diet con-
tains the vitamin B complex.

A third means of obtaining vitamins are the pills and capsules
sold in drugstores. There is much controversy over the advisa-
bility of buying vitamins in this form. No one doubts the value
of vitamin D concentrates for children ; but other drugstore vita-
mins are objected to because they are over-advertised, because they
are too expensive, and because it is claimed that they cause people
to feel less responsible about securing a good natural vitamin diet.
Actually they are not too expensive for many people and they
probably include the vitamins that are most important to human
health. Furthermore, it has been reported that giving synthetic
vitamins to factory employees results in a greater interest in nat-
ural diets that are good from the standpoint of protein and salt
as well as vitamin content. Thus, all three methods of increasing
vitamin intake are of use, although the latter two should be viewed
as supplementary to, rather than a substitute for the first.

EXCRETION

The Excretory System. — There are four avenues whereby
substances leave the body : the lungs, the sweat glands, the rectum
and anus, and the urinary svstem. The substances which leav^ 3~e

84 Digestion, Assimilation and Excretion

chiefly of two kinds : first, the products of katabolism, the carbon
dioxide and water formed by oxidation, and the products formed
from the wearing away of the protein structure of the cell ; second,
the food remnants, substances which we take in with our food, but
which leave the body without ever entering into the metabolic
activities of the cells. Among the latter are the undigested materials
in the feces, which never even enter the blood stream ; the urea
that is formed in the liver when ammonia is removed from the
amino acids to prepare them for use as fuel; and, finally, the
greater part of the water which passes through the body.

The reader is already familiar with the activities of the lungs.
It need only be added that, besides carbon dioxide, a considerable
amount of water vapor diffuses out of the blood capillaries into
the alveoli and is carried out of the body with each exhalation.

The sweat glands, located in the skin, carry on an excretory
activity that is accessory to that of the kidneys. The substances
which they excrete are much the same as those which leave the
body in the urine, but they are in more dilute solution. The ex-
cretory function ol ihe^weat^laads is not very important. Their
real function is that of cooling the body ; when ^^ it becomes_over-
heated.

The J_eces, which leave the body by way of the rectum and anus,
are not composed merely of undigested food substances. They
also contain, in slight concentration, various products of cell
breakdown which diffuse from the blood into the intestine. An
astonishingly large part of the feces, almost a full third, are com-
posed of the dead bodies of bacteria which live in the intestinal
tract. They also contain materials from the digestive juices. Im-
portant among the latter are the bile pigments formed from the
breakdown of red corpuscles. It is these pigments which give the
feces their characteristic brownish color.

The most important organs of the urinary system are the kid-
neys, which are bean-shaped structures situated just back of the
abdominal cavity, one on either side of the backbone and slightly
below the region of the stomach. From the inner side of each
kidney, a tube, known as a 1^1*1. carries the urine formed in the
kidney to the bladder, a small muscular bag at the base of the
abdomen. This holds the urine until enough has collected to be

Digestion, Assimilation and Excretion 85

passed out through the urethra, the tube that carries the urine to
the exterior. (See Fig. 20.)

When cut in half, the kidneys are seen to be solid bodies with
a small hollow portion near the point where the ureter enters them.
The solid part is filled with microscopic tubules which open into the

Cortex

•Kidney

Bladder-

=Ureters

-Urethra
FIG. 20. — Diagram of urinary system, posterior view.

hollow portion and from these openings run up into the outer
cortex of the kidney, branching as they go. Each tubule ends in
the cortex in a little pouch or cup which holds inside it a tangled
ball of capillaries. The blood enters the kidneys under consider-
able pressure, and some of the water and other substances in it
filter through into the tubules at their cup-like ends. Then the
blood passes on to another network of capillaries that surrounds
the tubule. As the solution of substances from the blood trickles

86

Digestion, Assimilation and Excretion

down through each tubule, the cells in the walls of the tubules
remove much of the water and other substances which are not
excreted and return them to the blood. Thus, a concentrated solu-
tion of excretory products, known as urine, is produced in the
kidney. This solution passes through the ureters and into the
bladder. There is a sphincter muscle at the opening into the urethra
which holds the urine in the bladder until it becomes distended.
When urination takes place, this muscle relaxes and allows the
urine to pass through the urethra.

The substances other than water that are carried out in the urine
are as follows :

Vein

Artery

FIG. 21. — Diagram of the end of a kidney tubule.

1. Urea. This is chiefly derived from the breakdown of amino
acids for fuel purposes, but a small amount may also be produced
in cell metabolism.

2. Products of breakdown of proteins in cells.

3. Salts. When more of a certain salt is eaten than the body
can use, the salt is excreted in the urine. Ordinary table salt
(sodium chloride) is eliminated in this fashion.

4. Excess sugar. Sometimes there is so much sugar in the blood
that it cannot all be stored in the form of glycogen. All but a cer-
tain percentage of it is then eliminated through the kidneys.

5. Useless food components. Alcohol, caffeine, and the like are
absorbed into the blood from the intestine and, since they are
not used by the body, must be eliminated through the kidneys.

CHAPTER SUMMARY

Our food is digested and absorbed while passing through the
*timentary canal, which is composed of the following parts in the

Digestion, Assimilation and Excretion 87

order in which the food passes through them: mouth, throat,
stomach, small intestine, large intestine, rectum, and anus. In the
mouth, the food is broken into particles by chewing and is mixed
with saliva from the salivary glands. The saliva contains an
enzyme, ptyalin, which starts the digestion of carbohydrates by
breaking them down into st^e sugars. The food is swallowed
and carried down the esophagus to the stomach, where it enters
the rounded fundus at the left end of the stomach and is passed
on into the narrow, tapering pylorus at the right. It is kept from
entering the intestine by the contraction of the pyloric sphincter,
and is mixed with the gastric; juice and reduced to a semi-liquid
mass, known as chyme, by the churning of peristaltic movements
in the pylorus. Peristaltic movements are ring-like contractions
which move down the alimentary canal, pushing the food ahead of
them. They occur in the esophagus and the small and large intes-
tines as well as in the stomach. The gastric juice, secreted by small
glands located in the walls of the stomach, contains the enzyme
rennin, which curdles milk, and another enzyme, pepsin, which
reduces proteins to proteoses and peptones.

The chyme passes through the pyloric sphincter a little at a time
into the small intestine, where it comes in contact with the bile
from the liver, the pancreatic juice from the pancreas, which is
a large gland situated under the stomach, and the intestinal_iiiice
from small glands located in the wall of the small intestine. The
salts of the bile make the fats ready to be split into fatty acids and
glycerol by the action of the pancreatic enzyme, lipase. Certain
pancreatic enzymes reduce proteins, proteoses, and peptones to
amino acids, and an enzyme in the intestinal juice completes the di-
gestion of other proteins not acted upon by the trypsin. The pan-
creatic diastase completes the reduction of starches to double sugar,
and three enzymes of the intestinal juice reduce double sugars to
single sugars. Food is completely digested and absorbed in the
small intestine, while, as the residue passes through the large
intestine, its water is absorbed. The remainder, known as the
feces, passes into the rectum and is expelled through the anus.

The food is absorbed from the small intestine through extremely
minute, close-packed projections, known as villj, which extend out
from the intestinal wall. Each villus contains a network of blood
and also of lymph capillaries. Single sugars make their way into

88 Digestion, Assimilation and Excretion

the bl^d^capillaries of the villi, then through the portal vein into
the liver, where they are converted into glycogen which is stored
in the liver cells, to be gradually reconverted into blood sugar as
the concentration of sugar in the blood falls. Some is also stored
in the muscle cells, ready for oxidation when the muscle becomes
active. Excess carbohydrates may be converted into fats and
stored in the fatty tissues of the body.

Glycerol and fatty acids are absorbed into the epithelial cells of
the jillu within which they are reconverted into fats characteristic
of the human body and passed on to the lymph capillaries of the
villi. They are carried through the lymph system to the blood
stream, through which they are carried to the liver, where they
are converted into a soluble compound, lecithin, in which form
they may pass into the body cells. Here they may be used for fuel
or to form the fat-like parts of the protoplasm, or they may be
stored in the cells of certain connective tissues until the body has
need of extra fuel.

Amino acids are absorbed into the blood capillaries of the liver
and carried to the cells, where they may "be recombined into pro-
teins for growth or to replace outworn structures. Or they may
be split into sugar-like substances and ammonia in the liver, the
sugars being used for fuel and the ammonia being converted into
urea. About four-fifths of the protein in our diet is used for fuel.
A high percentage of protein in the diet results in an increased
rate of metabolism.

In order to maintain the body at rest, a basal metabolic rate of
L9-QP Calories per day must be maintained. On the average, seden-
tary workers require 2,500 Calories of energy per day in their
food, and manual laborers from 3,500 to 5,000. Most of this
energy is expended for heat ; but, while this heat is wasted energy
as far as the accomplishment of work is concerned, it is useful
for maintaining the body at a temperature at which its activities
are carried on most efficiently.

In addition to proteins, carbohydrates, and fats, the following
substances are found in our food : ( i ) Condiments, which merely
improve flavor; (2) drugs, which improve flavor and have pleas-
urable effects on the nervous system; (3) roughage, the indigesti-
ble portion, which aids in causing the chyme to pass through the
intestines; (4) water, which forms an essential part of the proto-

Digestion, Assimilation and Excretion 89

plasmic structure and is also a medium for the movement of other
substances through the body; (5) salts, which are important in
maintaining a proper chemical balance in the body; and (6) vita-
mins, which are essential for the maintenance of health. Of the
many vitamins now known, the following have the greatest prac-
tical importance in human metabolism:

Vitamin A: Secured from most animal fats and from yellow
and green vegetables. Prevents xerophthalmia and night blindness
and stimulates growth in children.

Vitamin D : Produced by irradiation of fatty tissues. Best
sources are fish liver oils. Essential during growth to prevent
rickets and bring about the formation of sound teeth.

Vitamin K : Produced by intestinal bacteria. Essential for the
clotting of the blood.

Vitamin B complex: Found in the germ and husks of grains,
in milk, leafy vegetables, and yeast. The following are the best-
known vitamins in this complex : Thiamine (B^ prevents beriberi,
stimulates growth, and apparently improves general health and
well-being. Riboflavin (B2) prevents cheilosis. Niacin is the most
important vitamin for the prevention of pellagra.

Vitamin C: Found chiefly in citrus fruits, also in tomatoes,
potatoes, and yellow and green vegetables. Prevents scurvy.

Improving the diet and the preparation of food, enriching com-
mon foods, and taking synthetic vitamins or concentrates are
complementary methods of insuring an adequate vitamin supply.

Excretions are of 'two kinds : the products of katabolism, and
food remnants which pass through the alimentary canal or the
blood stream but never enter into the metabolism of the cells.
Carbon dioxide is excreted from the lungs. Undigested portions
of food, products of cell breakdown, dead bodies of bacteria, and
materials from digestive juices leave the body through the anus.
Of. ...the digestive juice jmjatejj

pigments, which are formed by the breaking down of red corpus-
cles in the liver. Urea, products of the breakdown of cellular pro-
teins, salts, excess sugar, and other useless food components are
taken from the blood by the kidneys and excreted through the
urinary system. Similar products are excreted by the sweat glands.
Water leaves the body through all of the above avenues.

Urine is collected in the kidneys by branching tubules which

90 Digestion, Assimilation and Excretion

come into close contact with, blood capillaries. It is carried to the
bladder by the ureters and expelled therefrom through the urethra.

QUESTIONS

1. Describe the digestive organs and tell the function of each.

2. Tell what may happen to a bit of protein from the time it enters
the mouth until its remnants are excreted. A bit of starch. A
bit of fat.

3. Outline all the functions of the liver.

4. What things are necessary in a healthful, normal diet?

5. Outline the processes of excretion.

GLOSSARY

alimentary canal (al'i-men'ta-ri) The passage through which food

passes while being digested or absorbed.
anus (a'nus) Lower opening of the alimentary canal through which

the feces are expelled.
beriberi (ber'i-ber'i) A disease marked by inflammation of the nerves

caused by lack of vitamin B.
cardiac sphincter (kar'di-ak sfink'ter) A ring-like muscle able to

contract and shut off the opening from the esophagus to the

stomach.

chyme (kim) The semi-liquid food in the small intestine.
condiments Non-nutritive food substances eaten for the sake of their

taste.
diastase (di'a-stas) A starch-splitting enzyme. (In this chapter the

pancreatic diastase is mentioned, but the term applies to any enzyme

that splits starch.)
duct Term applied to many small tubes in the body which carry liquifk,

particularly those which carry glandular secretions.
esophagus (e-sof'a-gus) Tube which carries food from the throat

to the stomach.

•fatty acids A group of substances formed in the digestion of fats.
feces (fe'sez) pi. Waste materials expelled from the alimentary

canal.
fundus Wide, rounded portion of the stomach lying to the left of the

cardiac sphincter.
glycerol (glis'er-ol) A substance formed by the digestion of fats.

(The common term for it is glycerin.)
glycogen (gli'co-jen) Animal starch. Stored in the liver and to a

lesser extent in the muscle cells and other cells of the body.

Digestion, Assimilation and Excretion 91

lecithin (les'i-thin) Substance into which fats are transformed in the
liver to make them soluble.

lipase (lip'as) Any fat-splitting enzyme. (In this chapter, the pan-
creatic lipase is mentioned.)

parotid (pa-rot'id) Salivary gland, located below the front of the
ear.

pellagra (pe-lag'ra) Disease caused by lack of vitamin G, marked by
weakness, skin affection, and nervous disorders.

pepsin Stomach enzyme which reduces proteins to proteoses and
peptones.

peptones Substances formed in the partial digestion of proteins.

peristaltic waves (per-i-stal'tic) Ring-like contnictioas 01 the walls
of the alimentary canal which move down the canal, pushing the
food ahead of them.

proteoses (pro'te-6s-es) Substances formed in the partial digestion
of proteins.

ptyalin (ti'a-lin) Enzyme in saliva which reduces starch to a double
sugar.

pyloric sphincter (pi-lor'ic sfink'ter) A ring-like muscle able to con-
tract and shut off the opening from the stomach to the small intes-
tine.

pylorus (pi-lor'us) Tapering portion of the stomach to the right of
the cardiac sphincter.

rectum Small chamber at the end of the alimentary canal between the
anus and the large intestine.

rennin (ren'in) A stomach enzyme which curdles milk.

scurvy Disease characterized by spongy gums and bleeding from
mucous membranes ; caused by lack of vitamin C.

sublingual Salivary gland located beneath the tongue in the floor
of the mouth.

submaxillary Salivary gland located under the jaw bone.

urea (u're-a) Substance formed from the breakdown of proteins.

ureter (u-re'ter) One of the pair of ducts carrying urine from the
kidneys to the bladder.

urethra (u-re'thra) Tube carrying urine from the bladder to the
exterior.

villus (vil'lus) pi. villi (vil'li) One of the minute, finger-like struc-
tures in the wall of the small intestine into which food is absorbed.

vitamin (vi'ta-min) Any one of a number of substances whose pres-
ence in the diet in small quantities is essential to health.

xerophthdinia (ze'rof-thal'mi-a) Disease of the eyes caused by lack
of vitamin A.

CHAPTER V
MAINTENANCE SYSTEMS IN ANIMALS

The Continuity Between Man and Paramecium. — In the
previous chapters we have become familiar with the structures and
activities of single-celled organisms, in which the maintenance of
life was simplified to its most elemental form ; we have also con-
sidered the structure and activities of one of the most complex
organisms in the world — man himself. We have seen how the
human body is organized into systems of cell groups for carrying
on the functions of nutrition, circulation, excretion and respira-
tion so that cell metabolism may take place. The division of labor
among the cells of the body has resulted in a most complicated- set
of organs for the securing, absorbing, distributing and trans-
forming of the food and air necessary for cell metabolism and of
the waste products resulting from it. Between these two extremes
of animal metabolism there are many intermediate body plans,
bridging the gap which exists between the simplicity of Parame-
cium and the intricacy of the human body.

Paramecium can be taken as representative of the oldest form
of bodily organization among animals — the single cell. From this
one-celled condition all the variety of animal bodies has been
developed through the course of evolution. Since there are at
present some eight hundred thousand different species of animals
known to the zoologist, with perhaps hundreds of thousands more
that have not yet been catalogued, it would be impossible in a single
chapter to present all the bewildering variations in body plan that
are to be found in the animal world. To simplify matters, we shall
consider five different animal organisms which typify the most
important changes that have taken place between Paramecium and
Man. What we shall do is to select from the profusion of types
only a few animals which represent innovations and develop-
ments which are retained in the human body. As each modifica-

92

Maintenance Systems in Animals 93

tion of the maintenance tissues is incorporated into the body
design of the following and more advanced animal type, we eventu-
ally reach a point where we can see the summation of all these in
the human body. By considering each separate innovation, and the
animals living today which represent the persistence of that par-
ticular stage in the development of the multicellular body, we can
the better appreciate the structures present in our bodies.

First, in Hydra, we discover a multicellular organism with a
very simple digestive tract, almost a complete lack of specialized
organs, but with a certain amount of specialization of function
among its cells.

Second, in the earthworm we find several advances in complexity
over the tissue-animal type represented by Hydra. In the body
cavity, or coelom, specialized groups of organs carry on circula-
tion and excretion. The earthworm is in reality one of the first
animal types to be built up on the organ-plan, developing many
different kinds of tissues grouped in special organs for assisting
in the maintenance of metabolic activities in all the cells of the
organism.

Third, in the fish, one of the simplest vertebrate animals, the
body plan carries on the multicellular condition with the division
of labor among cells, tissues and organs previously incorporated
in the worm body plan ; with it also is repeated the coelom and the
specialized organs for circulation and excretion. In the fish these
are more like those of the human, as is the digestive system. Here
too we find a set of organs responsible for external respiration.

Fourth, in the frog, there is continued the innovations found in
the fish, but to them is added a respiratory system which is basic
in design for all air-breathing vertebrates. All of the maintenance
organ systems are now practically human in general plan.

Finally, a few minor changes, seen in any mammal, bring the
body plan to the condition found in our bodies. Not only are the
digestive, circulatory and excretory systems made more efficient,
but by becoming warm-blooded the mammal is able to have cell
metabolism go on continually regardless of the fluctuating tem-
perature of the environment. Since there is no essential difference
between the maintenance organs of man and the other mammals,
we have in this fifth stage reached the condition already described
in the previous chapters. Our chief interest now will be to de-

^4 Maintenance Systems in Animals

scribe in more detail fach of the four noteworthy advances which
ire thus intermediate between Paramecium and Man.

The Colonial Protozoa. — Paramecium and its relatives show
is that all the essential activities of animal metabolism can be car-
ded on by an organism which is merely a single cell. Among the
Protozoa, however, there is a tendency for the cell organisms to
mite in groups, forming colonies of organisms as in the case of
(See Fig. 22 A.)

FIG. 22. — Colonial protozoa. A, Vorticella individual ; B, a colonial protozoan.

Vorticella is like a goblet on a slender stalk which can coil it-
elf together whenever the animal encounters a solid object, thus
>rotecting the delicate cell. When the protozoan is stretched out
it full length, the mouth of the goblet reveals a circle of lashing
ilia which by their action create an eddy into which bits of plant
ind animal life are drawn, to be later engulfed and digested as in
3aramecium. Oxygen diffuses into the cell from the surrounding
vater. Thus the essentials for metabolism are taken care of.

Vorticella is often found singly, but sometimes six or more in-
[ividuals remain attached to each other by the ends of their stalks.
Such a formation of colonies of cells is the first step toward the
[evelopment of the multicellular body. Other Protozoa form large

Maintenance Systems in Animals 95

tree-like colonies with an animal at the end of each branch, while
still others assume the shape of flat plates or hollow spheres of
cells attached to each other. But every cell of the colony retains its
individuality as an organism, and there is no specialization of
function among the cells.

A Simple Multicellular Animal. — When groups of cells living
together begin to show a differentiation of function, they are no
longer classed as colonial Protozoa, but as multicellular animals.
Probably the simplest multicellular animals are the sponges, but
to remain more directly on the line of evolution from Protozoa to
Man, we shall describe another very low form of animal life,
the Hydra.

Hydra is found in ponds, attached to sticks and stems of aquatic
plants. It is about an eighth of an inch in length, and its slender,
translucent body can be seen swaying back and forth in the water,
searching for food. The cylindrical body is attached by a flattened
basal portion, while at the other end there is a mouth surrounded
by a circle of tentacles which aid in the capture of food and bring
it to the mouth. The body wall consists of a double row of cells
surrounding the central digestive cavity. The outer layer of cells,
known as the ectoderm, has within itself groups of cells responsible
for sensory, contractile and protective activities, corresponding in
function to our muscular, nervous and external epithelial tissues
— though differing greatly in structure. The inner layer of cells,
known as the endoderm, is chiefly concerned with the digestion
and absorption of food. The cells lining the digestive cavity are
large cells, each with one to five flagella, or whip-like extensions
of the cell wall. The flagella project into the digestive cavity, creat-
ing currents in the water and thus bringing food particles to the
individual cells.

In obtaining its food, Hydra touches its prospective prey with
one or more of its tentacles, perhaps paralyzing it with some barbs
released from the ectoderm cells of the tentacles. Then the food is
brought to the mouth by the movement of all the tentacles. The
cells surrounding the mouth opening, being ectoderm cells with
muscle components, force the food into the digestive cavity. Once
within the Hydra's body, the food is acted upon by secretions of
certain gland cells of the endoderm ; it may be churned about by
contractions of the entire body. Some of the food is engulfed by

Maintenance Systems in Animals

Stinging cell

Testis

Endoderm of
digestive cavity

Digestive cavity

Ovaiy-

Bud

FIG. 23. — Hydra.

Maintenance Systems in Animals 97

single cells of the endoderm in typical protozoan fashion, and there
digested; other portions of the food are acted upon by digestive
enzymes while in the digestive cavity, later to be absorbed into
the endoderm cells much as in higher animals. The food passes
from the endoderm cells to the rest of the Hydra's body by diffu-
sion or through the jelly-like layer between the two tissues. There
is no specialized circulatory system. Waste products likewise dif-
fuse from one cell to the other, eventually passing out of the cells
into the environment. Respiratory gases follow the same pro-
cedure.

Thus we see in Hydra a body plan designed to delegate certain
duties of anabolism to specialized endoderm cells; all the other
cells of the body are dependent upon these endoderm cells for their
food. There is little further specialization, however, every cell
getting its own oxygen and getting rid of its carbon dioxide as
well as other waste products of katabolism. The basic living ac-
tivities associated with animal metabolism are taken care of to a
limited degree by some division of labor, but there are no true
organs as in higher animals.

The Earthworm. — In the earthworm we find all the funda-
mentals of the organization of maintenance structures that are
found in the human body. In contrast with Hydra, a second open-
ing has appeared in the digestive tract, so that food moves through
it from mouth to anus. Surrounding this digestive system is a
body cavity, the coelom ; hence the earthworm body is essentially a
tube within a tube, with the edges of the outer one fastened to
those of the inner one at either end. The coelom is divided into
a large number of compartments by transverse partitions which
extend from the body wall to the digestive canal. The grooves
which run around the exterior of the worm, apparently dividing it
into a series of small rings, are each of them located over one of
these partitions. A portion of the body between two of these rings
is called a segment. Inside the coelom there are specialized organs
for circulation and excretion. The evolutionary modifications from
Hydra to the earthworm thus include the development of a two-
opening digestive cavity, the appearance of a coelom and the
presence of specialized organs.

The food of the earthworm consists of bits of vegetation and
animal matter found in the soil. This earthy material is ingested

Maintenance Systems in Animals

Mouth

Segment

Pharynx

Hearts

Dorsal blood vessel

Intestine

Ciliated opening into nephridium

Nephridial

opening to

exterior

FIG. 24. — Digestire, circulatory, and excretory system of earthworm.

Maintenance Systems in Animals 99

through the mouth at the anterior1 (or head) end of the diges-
tive tract, aided by a muscular pharynx just behind the mouth.
The remainder of the digestive canal is differentiated into various
special portions, each with a specific function. From the pharynx
the food is forced through a narrow esophagus which is without
special digestive function, though some glands lying alongside
it produce a lime secretion aiding in neutralizing food acids.
Leaving the esophagus, the food enters the enlarged thin-walled
part of the digestive canal known as the crop ; here it may be tem-
porarily stored until needed. From the crop the food passes di-
rectly into another enlarged portion of the digestive tube, the
gizzard, which is thick-walled and muscular, serving to grind
the food into smaller particles preliminary to digestion and
absorption, which occur in the remainder of the tract, known as the
intestine. After the complex proteins, carbohydrates and fats have
been acted upon by the digestive enzymes, the cells lining the in-
testinal tract absorb the food.

After absorption, the food must be brought to every living
cell of the earthworm's body. For the first time we see in the
animal body plan a special set of tissues for the purpose of dis-
tributing materials throughout the body. Absorbed food may make
its way into the liquid filling the coelom, and thus be brought
to the tissues bathed by this fluid; but most of it leaves the
digestive cells to go into the blood stream. A circulatory system
includes a closed set of blood vessels which have capillary sub-
divisions extending throughout the body wall and all the organs ;
the large dorsal blood vessel present on the upper side of the
earthworm connects with a similar ventral blood vessel by means
of five pairs of vessels known as hearts which encircle the eso-
phagus. The blood in these vessels holds hemoglobin in solution,
and has white corpuscles but no red ones. It is forced forward
in the dorsal vessel by rhythmic constrictions of the muscular
walls and passes into the hearts which also contract to send the

1 In most animal organisms, directions are indicated as follows :
toward the head or mouth the anterior

toward the tail or anus the posterior

toward the belly the ventral

toward the back the dorsal

Thus in human beings, the arms are the anterior limbs ; the legs, the posterior
limbs. The backbone is a dorsal structure and the breastbone a ventral structure.

ioo Maintenance Systems m Animals

blood to the ventral vessel. Valves in both the dorsal vessel and
the hearts prevent a backward flow of the blood. Thus a circu-
lating medium carries food from the cells in which it is absorbed
(the digestive tract) to all the cells of the body.

Only a few layers of cells on the outside of the earthworm
are close enough to the atmosphere to be able to absorb directly
the oxygen needed for katabolism or to give off the carbon
dioxide. The earthworm has no special set of tissues to take care
of external respiration; the outermost skin cells, however, are
kept moist with mucus, and have air spaces between them. As air
diffuses in and out of these cells some of the excess oxygen goes
into the small capillaries and eventually is carried about by the
circulatory system to all the body cells. Carbon dioxide in turn
is given off from the capillaries into the skin tissues and from
them to the atmosphere. In the plasma of the blood there is the
red pigment hemoglobin (contained in the red corpuscles in the
human body) which increases the gaseous carrying power of the
blood stream.

The coelom liquid has already been mentioned as a means of
distributing some of the absorbed food. More frequently, waste
products of metabolism accumulate in this fluid. These are re-
moved, together with the waste products in the blood stream, by
special excretory organs found in the coelom. These excretory
organs, known as nephridia, consist of coiled tubes which occur
in pairs in every segment except the first three and the last. Each
nephridium has a funnel-shaped opening, lined with ciliated cells,
which goes into the posterior part of the coelom of one segment.
The cilia create a current which sucks into the funnel all solid
waste particles contained in the coelomic fluid. The tube of the
nephridium leading from the funnel passes through the septum
into the coelom of the next segment, where the bulk of the ex-
cretory organ is located, consisting of a much-coiled tube in whose
walls are glands which at the same time are removing nitrogenous
material from the blood stream and eliminating it in the liquid
found in the nephridial tubes. The excretory organ terminates
in an opening in the body wall, through which the waste material
is passed out to the environment.

Thus in the body pattern of the earthworm, in so far as it is
related to the maintenance of metabolic activities, we find the basic

Maintenance Systems in Animals 101

arrangement of digestive, circulatory and excretory organs much
as they are in higher animals and man. Further advances involve
a more specialized structural division of labor among the organs
in each system, and a definite set of tissues responsible for ex-
change of gases with the environment.

The Vertebrate Body Plan. — All the animal types at the level
of complexity which we have so far described have one charac-
teristic in common: the maintenance organs are either without
any surrounding supporting tissues or else such supporting cells
act as a skeleton on the outside of the body. Collectively, organ-
isms with such characteristics are known as invertebrates. The
fish is representative of an innovation apart from the maintenance
systems, which, however, is such a basic part of the body plan
we are now to consider that a few words of explanation are
necessary. The fish is a typical primitive vertebrate in its simplest
expression. By this is meant that there is a stiffening axis running
lengthwise dorsal to the digestive tract and consisting of a series
of bony segments known as vertebrae. The whole structure is a
backbone, or vertebral column. This innovation, added to certain
improvements in the maintenance organs, has caused vertebrates
to be numbered among the most common and obvious land animals,
as attested by the amphibians, reptiles, birds and mammals.

The Fish. — The body of the fish is constructed upon the same
essential plan as that of the earthworm, in that it possesses a
coelom and a digestive tube passing through the body from mouth
to anus. It is also segmented, although the segmentation is not
as obvious as in the earthworm. The ectoderm cells have taken over
as their special duty the formation of protective tissues (such as
skin) and the nervous system. The endoderm cells have become
specialized for absorption of food (the digestive tract) and ex-
change of gases (the respiratory tract). From certain cells in an
intermediate layer there are formed the tissues responsible for
movement (muscles) circulation of materials (blood vessels and
heart), and support. From the coelom is developed a vertebrate
body cavity in which the vital organs are located, divided into
two parts by the diaphragm as in human beings. The larger cavity
contains the liver, stomach, intestines and kidneys; the smaller
contains the heart.

The food of a fish is usually smaller fish, or other aquatic ani-

IO2 Maintenance Systems in Animals

mals such as insects and mollusks. Once in the mouth, the food is
held firmly by hard projections which grow out of the walls of the
digestive tract — the teeth. In addition there is a muscular organ,
the tongue, which aids in holding and pushing the food. Both
teeth and tongue are distinct improvements over the earthworm
mouth. From the mouth the food passes through a pharynx and
esophagus into an enlarged portion of the digestive canal which

Air bladder

Gall bladder

Conads

Urinary bladder1

Gill slits and gills

Heart

Liver

Stomach

Spleen

Urogenital
opening

FIG. 25. — Maintenance organs of fish. (Redrawn from Woodruff's Foundatiotu
of Biology, The Macmillan Company.)

combines the function of a crop and gizzard — the stomach. Here
digestion is initiated with the secretion of digestive fluids from
the stomach cells. The remainder of the food tube is a slightly
coiled intestine with three short outgrowths from it which in-
crease the absorptive surface. Here the digestion of the food is
continued, and the simpler food substances resulting from the
digestive process are absorbed by the intestinal epithelial cells. Un-
digested residues pass out through the anus. Even though the twist-
ing of the digestive tract and the presence of specialized organs
along its extent make obscure the relationship with the straight
food tube of the earthworm, the fish has a digestive tract which

Maintenance Systems in Animals 103

is essentially a tube running from one opening in its body to an-
other— from mouth to anus. A new organ, the liver, has appeared ;
its secretion passes through a gall bladder to a bile duct which
empties into the intestine.

The circulatory organs consist of main blood vessels, as in the
earthworm, and a capillary system for irrigating all the tissues
of the body. There is a single large muscular organ, the heart,
which has taken over completely the function of pumping the
blood through the circulatory system. The heart, located ventrally
below the pharynx, is a two-chambered organ. The blood flows
into the first chamber, or auricle, from the large veins; passes
into the second chamber, or ventricle, where muscular contraction
forces it out into the large arteries. As the blood flows through
the capillaries in the intestinal wall it absorbs food present in the
digestive tract cells, carrying it in solution to the cells of all the
other tissues where it is made available for cell metabolism.

Excretion is the special task of the kidneys, which extract urea
and other wastes from the blood stream and pass them on to the
bladder and eventually to the exterior via an opening posterior to
the anus.

Respiratory organs function in the fish to take care of the gase-
ous exchange with the environment, replacing the slow and inef-
ficient method found in the earthworm. As the fish moves about,
water is taken in through the mouth into the pharynx, passing out
through openings in the side of the neck. During this passage, the
water — which carries oxygen in solution — passes over delicate
tissues forming the gills, and some of the air passes through the
gill cells into the capillaries with which the gills are plentifully
supplied. Gills are respiratory organs designed to carry on gaseous
exchange when the gases are dissolved in the water. Once in the
blood stream, the oxygen is transported to all the tissues of the
fish, supplying the cells with the oxygen needed for metabolism,
and removing the carbon dioxide. When the blood passes through
the gills this gas is given off as a waste product.

The Frog. — The frog exhibits certain changes in the vertebrate
body plan pioneered by the fish; these changes are the result of
adaptation to land living. The digestive tract is built on the same
basic pattern, though the tube is more coiled to provide as great
an absorptive area as possible without requiring too large a body

IO4 Maintenance Systems in Animals

surface. The mouth, esophagus and stomach have the same func-
tions as in the fish, but the intestine has become differentiated
into a small and a large intestine; and another digestive gland,
the pancreas, aids in the secretion of digestive enzymes. The cir-
culatory system is changed only by the presence of a three-cham-
bered heart. The most significant change of all is the design of
the respiratory organs, with the innovation of lungs as organs for

GonacL

Esophagus

Pancreas Spleen

Large
intestine

Bladder

Opening of'

Liver / \ ~ cloaca

Stomach Gall bladder Small intestine

FIG. 26. — Maintenance organs of frog. (Redrawn from Woodruff's Foundation?
of Biology, The Macmillan Company.)

exchanging gases with an atmospheric environment. In the fish
there is an outgrowth of the pharynx known as the air bladder;
this is a large sac filled with a gas which regulates the level at
which the fish can comfortably float. In the lungfishes this air
bladder opens into the pharynx and functions as a lung since the
blood vessels in its walls absorb the oxygen from the air in the
sac. In the frog there are two such sacs connected with the
pharynx by a short tube known as the larynx. Each sac is a simple
cavity lined with tissues rich in blood vessels, and it is here that

Cross section of frog stomach. The stomach is collapsed, so that only a
narrow, irregular cavity remains.

Maintenance Systems in Animals

105

exchange of gases concerned with cell respiration takes place. Lung
breathing is the method by which such external respiration is car-
ried on in reptiles, birds and mammals.

The Warm-blooded Organisms. — The maintenance organ
systems of a typical mammal are built upon the same plan as those
of the frog, with one major change which affects the activities
of the organism as a whole. The chemical changes which are a
part of anabolism and katabolism are, like all chemical reactions,

Diaphragm Spleen

Ki<|ney / Large intestine

kGonad

FIG. 27. — Maintenance organs of mammal. (Redrawn from Woodruff's Founda~
tions of Biology, The Macmillan Company.)

conditioned to a certain extent by the temperature. Within certain
limits, any increase in temperature results in an increase in the
rate of the chemical reaction. The frog, and all animals lower in
the scale of bodily organization, are cold-blooded organisms; the
temperature of their bodies varies with that of the environment.
When the temperature of the surrounding air or water drops, cell
metabolism begins to slow up. Cold-blooded animals usually hiber-
nate or go into a state of suspended activity under these conditions.
Mammals and birds, on the other hand, are warm-blooded. They
have a heat-regulatory mechanism whereby the body temperature
is kept relatively constant at the optimum for metabolic activity,

io6 - Maintenance Systems in Animals

irrespective of the temperature of the environment. The change
in body plan which has aided in bringing about this condition in-
cludes the addition of a skin covering which prevents heat loss
during periods of low temperatures. The feathers of birds and
the fur of mammals serve this purpose.

^\ //£

Mammal

Reptile

Amphibian

FIG. 28. — Vertebrate hearts.

The change- from the cold-blooded to the warm-blooded condi-
tion results in only one major change in the maintenance structures.
Because of the increased rate of metabolism in warm-blooded ani-
mals, a more rapid respiratory exchange has become necessary.
Consequently the smooth wall of the lining of the lung has been
greatly increased in area and thrown into many closely packed
folds to form the system of bronchioles and alveoli found not
only in man but in all other warm-blooded organisms.

Man is a typical vertebrate of the mammalian group, hence it

Maintenance Systems in Animals 107

is unnecessary to repeat here all the structural features character-
istic of his maintenance organs; these have been explained in
previous chapters. By keeping in mind the progressive stages by
which this complex body plan has become possible, we can see
in the existing colonial Protozoa, Hydra, earthworm, fishes, and
the frog representative animals which embody successive innova-
tions on a previously existing design, suggesting the origin and
relationships of the seemingly complex set of organs in the human
body, which are all essential for the carrying on of cell metabolism.

CHAPTER SUMMARY

The oldest form of bodily organization among animals is the
single cell, as represented by Paramecium. One of the most com-
plex forms is that seen in Man. The gap between the two can be
bridged by considering certain animal types which have today in
their body plan various important innovations which were essen-
tial for the evolution of the mammalian and human body organi-
zation.

The tendency toward formation of a multicellular body is seen
in various colonial Protozoa such as Vorticella ; but in such many-
celled bodies each cell retains its individuality and there is no divi-
sion of labor. Slightly more complex multicellular bodies are the
sponges, with the beginnings of cell specialization.

Hydra represents a simple multicellular animal without organs
but with some specialization of function among the cells. The
body wall, surrounding a central digestive cavity, consists of two
layers of cells: ectoderm cells responsible for sensory^ protective
an^Ncontractile activities, and endoderm cells responsible for the
digestion and absorption of food. The digestive cavity has but one
opening, a mouth, surrounded by tentacles which aid in food-
getting.

The earthworm represents an animal type much advanced over
the Hydra in that the cells are grouped into organs, and that the
maintenance activities require much more complex tissues and cell
groups. There are two openings to the digestive cavity, an anus as
well as a mouth ; and the digestive cavity has become an elongated
canal with division of labor along its length, resulting in a
pharynx, esophagus, crop, gizzard and stomach-intestine. Hydra

io8 Maintenance Systems in Animals

has no special circulatory system; the earthworm has a set of
blood vessels carrying a circulating medium to every part of the
body. The earthworm also has special excretory organs, the
nephridia. Somewhat like the Hydra, the earthworm carries on ex-
ternal respiration through the cells on the outside of the body wall.
And, finally, in the earthworm we see a body cavity, the coelom,
surrounding the digestive canal.

Protozoa, sponges, Hydra-like animals and worms are a few
of the animals which lack an internal supporting system or skele-
ton; hence they are called invertebrates. The presence of a back-
bone and other internal stiffening tissues in the vertebrates has
made possible the development of various innovations in the body
plan. A good illustration of a simple vertebrate system is found
in the fish.

The fish possesses the important organ systems which are
found in the earthworm, with added specialization of each one
of the maintenance organ systems. The coelom or body cavity
becomes divided into two ; in the smaller cavity there is the heart,
and in the larger cavity the various digestive organs and excretory
organs are located. The digestive tract itself includes a liver and
gall bladder, as well as a true stomach and coiled intestine. The
excretory organs are kidneys much like those of higher vertebrates.
Respiratory organs are gills, specialized to exchange gases with
the watery environment. The circulatory system includes a mus-
cular two-chambered heart which keeps the circulatory fluid mov-
ing.

The frog's body plan is basically that of the fish, with certain
changes necessary with the change to a land environment. Most
noticeable of these is the substitution of air sacs, known as
lungs, for the gills. Minor changes include the division of labor
between a large and a small intestine, and a three-chambered heart.
All of the maintenance organs are now practically human in
design.

With the mammals, warm-bloodedness superseded cold-blooded-
ness, making possible more continuous metabolic activities irre-
spective of the temperature of the environment. Associated with
this is a more complex respiratory system, with lungs made up of
alveoli and bronchioles.

Maintenance Systems in Animals 109

QUESTIONS

1. Why is Paramecium considered representative of the simplest
type of body plan found among animals ?

2. Is division of labor among cells a necessary step associated with
the multicellular body plan? Give reasons for your answer.

3. Why is division of labor among cells advantageous to the or-
ganism ?

4. Is such division of labor and cell specialization ever a disad-
vantage ? Explain.

5. What maintenance system do Hydra and earthworm have more
or less in common?

6. Compare the digestive canal of Hydra, earthworm and fish.

7. Compare the circulatory system of earthworm, fish and frog.

8. How is excretion carried on in the earthworm? How does this
compare with excretion in man ?

9. Which animal studied in this chapter first exhibits a coelom?
Of what significance is a coelom in the development of the ani-
mal body plan?

10. What are the warm-blooded animals? Of what advantage to
them is it to be warm-blooded?

GLOSSARY

anterior (an-te'ri-er) That part of an organism toward the head or
mouth end.

coelom (se'lom) The body cavity found between the digestive tract
and the body wall.

cold-blooded The condition found among all invertebrate animals
and some vertebrates which results in the body temperature of
the animal varying with the temperature of the environment.

colonial animal One in which the multicellular condition does not
include division of labor among the cells.

crop Thin-walled enlargement of the digestive canal where food may
be temporarily stored.

dorsal (dor'sal) That part of an organism toward the back.

ectoderm (ek'to-durm) Surface -layer of cells, as found on the out-
side of Hydra.

endoderm (en'do-durm) Inner layer of cells, as found lining the
digestive cavity of Hydra.

flagellum (fla-jel'um) A whip-like prolongation of the cytoplasm,
capable of moving about and creating a current outside of the cell.

gills Respiratory organs of aquatic animals, capable of gaseous ex-
change with water.

no Maintenance Systems in Animals

gizzard Thick-walled enlargement of the digestive canal, where food
may be ground up.

Hydra (hi'dra) Small aquatic animal, composed of two cell layers,
having a single opening to the digestive tract.

invertebrate (in-vur'te-brat) An animal without an internal skeleton,
either without any skeleton at all (earthworm) or with an external
skeleton (clam).

larynx (lar'inks) The voice box, portion of the respiratory tract in
the anterior part of the trachea.

mammal A class of vertebrates usually covered with fur and feeding
the young with milk.

multicellular The condition of bodily organization involving many
cells united together.

nephridium (ne-frid'i-um) Excretory organ of an earthworm, corre-
sponding in function to the human urinary system.

pliarynx (far'inks) Cavity posterior to the mouth, from which the
esophagus and trachea open. Throat.

posterior (pos-te'ri-er) The part of an organism toward the tail or
anus.

segment In the earthworm, a portion of the body between two of
the ring-like constrictions.

tentacles Flexible arm-like projections surrounding the mouth in
Hydra-like animals.

ventral (ven'tral) That part of an organism toward the belly.

vertebral column (vur'te-bral) The series of bony segments, or ver-
tebrae, which act as a longitudinal stiffening internal axis among
higher animals.

vertebrate (vur'te-brat) An animal with an internal, usually bony,
skeleton.

Vorticella (vor'ti-cel'a) A colonial protozoan.

warm-blooded That condition found in birds and mammals, in which
the body temperature is kept constant irrespective of environ-
mental temperature changes.

CHAPTER VI

THE BODIES OF PLANTS

The Structural Needs of the Plant Body. — In Chapter II we
became acquainted with the living organism reduced to its simplest
and most basic form ; and by studying Protococcus, Paramecium
and the bacteria we were able to single out the three fundamental
types of metabolism characteristic of living things. In the follow-
ing two chapters we considered the other extreme of organization,
focusing our attention upon the human body as the highest expres-
sion of specialization in animal metabolism. We retraced our steps,
so to speak, in Chapter V, to show how the complexity of the hu-
man body was the outcome of a progressive series of innovations
in multicellular organisms from Hydra to the mammals. This in-
creased complexity, this specialization of organs to carry on spe-
cific functions, was intimately bound up with the fact that animal
metabolism means ingestion of organic food. Hence the changes
in the animal body have been associated with the organs concerned
with securing, digesting, assimilating and transporting food mate-
rials, while the high rate of animal metabolism has made respira-
tory organs necessary.

Neither digestive nor respiratory systems have evolved in plants.
Rather, there has been a development of specialized organs and
tissues concerned with the securing of the raw materials for foods,
with the manufacture of foods, the transportation of both raw
materials and food, the storage of foods, and the provision of sup-
port and protection for the larger plant body.

The various stages in the advancing complexity of the plant
body may be summarized as follows : ( i ) thallus plants, in which
there is little or no differentiation into organs such as roots, stems,
or leaves; (2) primitive non-vascular 'land plants, without leaves
or conducting tissues ; (3) non-vascular land plants, with leaf -like
expansions, but without conductive tissues; (4) vascular land

H2 The Bodies of Plants

plants, with efficient roots, stems, and leaves, in which there is the
most advanced division of labor among the cells.

Thallus Plants. — The simplest members of the plant kingdom,
corresponding to the Protozoa and lower invertebrates among the
animals, are the thallus plants. They include all the colorless plants
as well as the simple chlorophyll-containing plants known as algae.
Two groups of thallus plants, the bacteria and the blue-green algae,
are so primitive that their cells do not have nuclei, the nuclear
protoplasm being scattered throughout the cell.

The more highly developed thallus plants include the higher
fungi and the higher algae. Of the latter there are three kinds:
the green, the brown, and the red algae. The brown and red algae
derive their names from the fact that they have brown or red pig-
ments mixed with their chlorophyll. They are the dominant vege-
tation of the oceans, while the green algae are the common sub-
merged vegetation of fresh waters.

Algae may be unicellular, or they may appear in the form of a
more or less massive plant body, known as a thallus. Among the
one-celled types are the flagellates mentioned in Chapter II, some
of which are true plants and are classed among the green algae.
Sometimes these animal-like organisms clump together to form
colonies. In one species the plant body consists of a colony of four
such flagellated cells, each similar to the other in structure and
function, all embedded in a gelatinous mass. In another species
there are sixteen such cells to the individual, the cells being ar-
ranged in a flat plate. Spherical colonies exist which consist of
thirty-two, sixty-four, up to twenty thousand cells. The latter is
the case with Volvox. Volvox is a minute plant which lives in fresh
waters. It is about the size of a pinhead, and it spins its way
through the water because the periphery of the sphere consists of
hundreds of flagellated green cells, each capable of imparting its
share of motion to the whole colony. The only cell differentiation
is that associated with reproduction, since only certain special cells
are capable of forming the eggs and sperms necessary for sexual
reproduction. These few motile plants are interesting in that they
show us an evolutionary compromise which has apparently led
nowhere. Motility and green plant metabolism are two character-
istics which do not go well together, or at least are not mutually

The Bodies of Plants

Immotile colony

Motile colony

Unbranched Massive thallus Branched

filament filament

FIG. 29. — The diversity of the plant body among the algae.

ii4 The Bodies of Plants

essential. The stationary thallus plants were able to lead to a much
larger and more efficient plant body, as we shall now see.

The simplest multicellular plant body is a thallus of cells show-
ing no division of labor; such organisms are found among the
green algae, forming irregular gelatinous masses in which are
embedded indefinite numbers of cells, each a counterpart of
Protococcus. More advanced is the multicellular condition in which
the cells are attached, end to end, in a single row or thread of
cells known as a filament. This is a very successful type of thallus,
if we judge by the number of species of green, red and brown
algae exhibiting this characteristic. Many of these filament plants
do not branch, as in the common pond scum, Spirogyra. Here
every cell is the same as the cell above and below it ; thus there is
no division of labor. Within each cell is one or more spirally coiled
chloroplasts which manufacture the food for the cell as did the
chloroplasts of Protococcus, except that here the materials for
photosynthesis are secured from the surrounding water. In other
cases the filaments develop a complex system of branches which
result in larger, bushier plants which often reach a length of several
feet.

Flat plates of cells are another type of thallus, found in all the
algae but not as common as the filamentous types. Their chief
claim to interest lies in the fact that they represent the ancestral
algal type from which land plants developed when plant life
migrated from the water to the land. In making such a transition,
the filamentous type of body was doomed to failure because of the
large cell area exposed to the fatal dryness of aerial life. The plate-
body type, with its lessened surface area and the great number of
cells within the body protected from the atmosphere, was (in the
case of the green algae) the one capable of surviving in the new
environment and thus paving the way for the higher plants.

The most complex thallus body is found in the massive structure
of many of the brown algae. Not only is the body made up of many
thousands of cells, but some of the cells have become specialized
in performing photosynthesis, others in storing food, others in
transporting it, and others in anchoring the plant to the sub-
stratum. Sargassum is one of the brown algae, found in abundance
in semitropical oceans. Although at first growing attached to the
rocks along the shore, it is often ripped away during storms and

The Bodies of Plants 115

is borne by ocean currents out into the open sea where it remains
alive for years. The plant has carried out division of labor to
such an extent that certain tissues act as basal root-like holdfasts,
Dthers form stem-like portions which support expanded areas
which function chiefly for photosynthesis, and still other tissues
form little bladders which aid in keeping the plant afloat. The giant
tcelps, which are also brown algae, generally consist of a well-
defined holdfast, stem and broad "leaf." Often these plants reach
lengths of several hundred feet, testifying to the success of this
:ype of plant body in the aquatic environment. In fact, no flower-
ing plants can compete with the algae in colonizing the oceans,
where the massive thallus type has proved its unique fitness for
survival. The longest organism known to science is a giant kelp
found off the coast of South America, specimens of which have
neasured five hundred feet in length.

At this point it might be well to say a few words about those
phallus plants which have carried on in the multicellular condition
"he type of metabolism found in the bacteria. The fungi have
specialized in colorless plant metabolism, and thus live either as
saprophytes or parasites; we shall hear more of them in the role
;hey play in the interrelations between organisms. But it might be
well to see to what extent complexity of the plant body has become
x>ssible when associated with this type of metabolism.

The higher fungi are typically many-celled thallus plants in
which the vegetative body is a branched, interwoven mass of fila-
nents, much like the filamentous algae. Some of these fungi are
iquatic, living on fish or dead aquatic organisms, but the majority
ire terrestrial. Under the general name of molds or mildews are
included a large number of fungi whose plant body is a network
>f filmy threads spreading over the nourishing medium on which
t is growing. The common bread mold appears whenever bread
s exposed to the air in damp places. The molds appear as white
nasses of threads, tipped with orange, blue, green or black masses.
These colors are due to the spores, which are the reproductive cells
)f these colorless plants*

Of the larger fungi an important group is the bracket fungi,
which live for the most part on dead trees, though a few attack
iving ones. The most conspicuous parts of these plants are the
•eproductive bodies — hard, woody structures which Appear as

u6

The Bodies of Plants

Spore -bearing

filaments

.Vegetative
filaments

Blanching filaments of Mold

Reproductive
filaments

Mushroom

Vegetative
filaments

FIG. 30. — The diversity of the plant body among the fungi.

The Bodies of Plants 117

shelves or brackets on the sides of the tree trunks. The actual
maintenance part of the thallus is a network of filaments which
penetrates through the wood and causes it to rot during the course
of the absorption of nourishment from the tree by the fungous
parasite.

The most familiar of all the fungi are the mushrooms. In
these, again, the main part of the plant is a network of threads
or filaments running through the ground in which the organic
material is found upon which the mushroom is subsisting. From
this subterranean mass of filaments, erect buds grow into the re-
productive structures, the common cap-and-stalk portion known
as the mushroom.

Primitive Land Plants.- — Before plant life could take up land-
living, the plant body by necessity had to conform to the new
environment and to adjust itself to new demands, chief of which
was the securing of the all-important water and the prevention of
drying out to a fatal degree. Neither of these was any problem at
all for the algae, living as they do submerged in the water most
of the time. In an insignificant group of plants known as the
liverworts, we can see the type of plant body similar to what
those plant pioneers must have looked like when life began its
insurgent march in the conquest of the land.

A typical liverwort, such as the Marchantia found on moist
ground, shows three basic modifications of the plant body which
are necessary for terrestrial living. First is the compact multi-
cellular body, in which only the outermost layer of cells is in con-
tact with the air and consequently liable to excessive evaporation
of water; most of the tissues are separated from the atmosphere
by one or more layers of cells. Second is the habit of flattened
growth, resulting in a prostrate plant body clinging closely to the
damp earth and making possible a contact with the substratum
over the entire lower surface of the body. This is most essential,
since the substratum is the only source of the water and minerals
needed for metabolism. And third is the beginning of division of
labor among the cells, resulting in the formation of tissues. The
uppermost layer of cells in Marchantia acts as a protective tissue
and becomes an epidermis. The several layers of cells immediately
beneath, in the well-lighted portion of the plant, are green with
chloroplasts and have as their function photosynthesis. Surround-

The Bodies of Plants

Section of a prostrate liverwort

Photosynthetic cells

Prostrate
liverwort

•Spore capsule

Leaf-like
organs

Photosynthetic cells

Ehizoida

Liverwort with
leaf -like structures

Section of 9
a moss leaf

FIG. 31.— The plant body of the bryophytes, the simplest land plants.

The Bodies of Plants 119

ing these green cells are air spaces so that the carbon dioxide and
oxygen can circulate freely, coming in through pores in the epi-
dermis. Beneath the photosynthesis layers, where the light is
poorer, the cells tend to be colorless and function for storage.
The lowermost layer of cells develops hair-like processes known
as rhizoids, which absorb the water with its dissolved minerals
Thus Marchantia exemplifies the simplest type of plant body,
lacking true organs but possessing tissues especially adapted for
terrestrial existence. Other liverworts show varying types of this
same plant body. All of them are restricted to damp and shaded sit-
uations, such as are found in swamps, along the sides of ravines
under overhanging ledges, or on the stones in streams, chiefly
because of their primitive reproductive habits which will be dis-
cussed in a later chapter.

Non-vascular Leafy Plants.— One of the most characteristic
plant organs is the leaf. Each leaf is made up of various tissues
necessary for photosynthesis to take place; and when the plant
body has leaves, usually photosynthesis is restricted to them The
leaf is therefore the last word in the evolution of that part of the
plant concerned with constructive metabolism. Thallus plants have
no leaves, neither do most of the liverworts. Marchantia was used
as an example of a non-leafy land plant of a primitive type. But
relatives of the liverworts, the mosses, represent the stage of the
development of the plant body in which leaf-like organs make their
debut.

Mosses advanced over the liverworts in several respects • they
adopted the erect habit, restricted the photosynthetic tissues mainly
to leaves arranged on a supporting stem, and developed a basal
mass of rhizoids for anchoring the plant and absorbing nutriment
from the ground. Their chief lack was the inability to develop
effective vascular (or conducting) tissues. Lack of such tissues has
predestined the mosses to be small and insignificant plants.

The common hairy cap moss of pastures and roadsides is a
typical member of the group. Each plant is about an inch in height,
and consists of three distinct parts. A tuft of filamentous rhizoids
anchors the plant to the earth rather ineffectively, and also absorbs
the water and dissolved minerals. In this respect the mosses and
liverworts are alike. There is a frail stem, consisting of a closely
packed mass of cells, of which the outermost layers may contain

120 The Bodies of Plants

chloroplasts whereas the innermost ones lack them. These colorless
stem cells show a little specialization for support but not for con-
duction of materials. The leaf-like organs are attached to the stem,
and are the greatest innovation in the plant body, adopted from this
group on as standard equipment in all the higher plants. The moss
leaf is hardly a true leaf, with the cell specialization found in the
leaves of flowering plants ; there is usually but a single layer of
chloroplast-bearing cells in the thin structure. In some cases, erect
rows of cells grow out from the surface to increase the photo-
synthetic area. In a simple way, the hairy cap moss indicates the
division into root, stem and leaf which is the pattern found in all
higher plants.

Vascular Land Plants. — At the moss stage we see a terrestrial
plant body characterized by distinct tissues for carrying on ab-
sorption, anchorage, support, photosynthesis and protection. Ad-
ditional tissues necessary in making land vegetation widespread
and successful include better supporting cells and adequate con-
ductive channels. The first plants to exhibit these in definite or-
gans (roots, stems, and leaves) are the ferns. Today, in temperate
regions, these plants make up a small and inconspicuous part of
the land vegetation, chiefly because of handicaps in reproductive
characteristics which will be considered in a later chapter. In the
geologic past, however, the ferns dominated the lands, being the
first plants to develop woody tissues; thus they formed our first
forests. However, it is in the flowering plants, the most highly
developed of the seed plants, that we find the culmination of the
vegetative as well as of the reproductive specialization possible in
the plant kingdom. Since the root, stem and leaf of the fern are
much like those of the flowering plant, it will be sufficient to con-
sider only the latter.

An oak tree, for example, displays a great amount of organiza-
tion, all with a view toward efficiently carrying on plant metab-
olism. Its organization is centered around the leaf, the place where
the major activities are carried on ; and the entire structure of the
tree may be said to serve three major purposes : (i) to bring the
leaves into maximum exposure to the sunlight, (2) to bring to
the leaf an adequate supply of raw materials, (3) to conduct the
finished products away from the leaves to the places of storage or
of secondary activities. The tree is constructed therefore in such

The Bodies of Plants 121

a way as to lift its leaves up into the air where they can get the
full benefit of the sunshine. In this position they are brought into
direct contact with the carbon dioxide. The two other supplies,
water and minerals, can be secured only from the soil, and conse-
quently a system of conveyors is required to bring these substances
to the leaf. Conveyors are also necessary to carry the finished
products from the leaves to cells where they are to be used. Each
of these systems consists of many different kinds of tissues which
perform different parts of its general function ; these are grouped
in three sets of organs — the leaves, the roots, and the stems.

The leaf is beautifully adapted for performing photosynthesis.
In the first place, it is broad and flat, so as to expose the largest
possible surface to the energy of the sun's rays. Secondly, the
arrangement of the chloroplast-containing cells within it is such
that those containing the most chloroplasts are near the top where
they will receive the most sunlight. Next, the photosynthesizing
cells are protected from an excess of sunlight, which would tend
to overheat them and dry them up, by a special layer of colorless
cells, the epidermis, the outer walls of which are coated with a
glossy layer of wax-like material. The fourth provision of these
"factory rooms" is an efficient ventilating system for the circula-
tion of gases. The epidermis is perforated by a number of open-
ings, the stomata, which communicate with a network of cavernous
passageways extending throughout the leaf. Carbon dioxide, enter-
ing through the stomata, circulates throughout these passageways,
in this way coming into contact with every one of the photo-
synthesizing cells. In the same way the oxygen given off from
the cells by photosynthesis can diffuse through them and out
through the stomata. Finally, the leaf contains a network of
branching veins, which serve as a framework to support its struc-
ture and, more important, as a distributing system connected with
the conveyors of the stem. Water, flowing up the stem and into
the leaf stalk, travels through the veins until it reaches the tips
of the smallest veinlets, from which it diffuses into the photo-
synthesizing cells. The food, manufactured by photosynthesis and
stored up in the chloroplasts during the daytime, at night passes
into these same veinlets and is transported down the leaf stalk into
the stem, which carries it to places of storage or of growth.

The only constant supply of water available for photosynthesis

122 The Bodies of Plants

is that which fills the gaps between the particles of soil in which
the roots are buried. Hence it is the roots which must receive the
water and the mineral salts dissolved in the soil for the use of the
leaf. The tips of the roots of any large plant are always in contact
with the soil water. It is in certain regions near the root tips
where most of the water absorption occurs. At the very tip of the
roots growth takes place, and just behind this region the root is
covered with a cobweb of tiny hair-like extensions of the outer
layer of its cells. These root hairs thoroughly permeate the soil
for an area of a half inch or so in diameter surrounding the root.
It is here that absorption takes place. Diffusion of water and salts
through the walls and their absorption into the cell easily occur.
Then, by a continual process of diffusion, these substances make
their way from cell to cell to the inner portion of the root where
they enter the conducting cells, which carry them up the stem of the
tree to the leaves.

Aside from its role in the absorption of water and food from
the soil, the functions of the root are quite similar to those of the
stem ; namely, to conduct materials going to and from the leaves,
to store food, and to hold the tree firmly in place. Both root and
stem, therefore, possess conducting cells, supporting cells, and
storage cells. In addition, there are protective cells and growing
cells in both. The arrangement of the tissues is slightly different
in the root than in the stem, but to avoid confusion we shall de-
scribe the stem only.

The stem includes the trunk with its branches, down to the*
smallest twigs. In any cross section of this stem we can identify
three regions which perform the important functions outlined
above. These are the wood, the inner bark or phloem, and the outer
bark or cortex. The wood, which composes the greater part of the
stem, has two important functions : supporting the tree and con-
ducting the water from the root to the leaves. The main burden of
these two functions is undertaken by two very different types of
cells, the location of which can easily be seen by a glance at an oak
board which has been cut across the grain. It is traversed by a
series of bands, the annual rings, in which light-colored wood
alternates with dark. In the light wood are many tiny pores, the
openings of long tubes known as vessels, which in the living oak
extend up and down the trunk and are the conductors of water.

The Bodies of Plants

123

Sunlight

^Oxygen

Carbon
dioxide

Water- and food-
conducting channels

Water

Nitrates and
other minerals

Root hairs
FIG. 32. — Diagram of the plant body of a seed plant (spermatophyte).

124 The Bodies of Plants

The dark wood of the annual ring consists mostly of very slender
cells with thick walls, known as fibers. These are the supporting
cells of the wood, and on the number and strength of them depend
the hardness and strength of different kinds of hard woods. Both
vessels and fibers are simply the dead skeletons of cell walls. A
third type of cells may be mentioned in passing. These are the
ray cells, seen in the wood block as thin dark bands extending
across the annual rings. They serve the purpose of food storage.

The inner bark, or phloem, contains, like the wood, two main
types of cells which have the two functions of conduction and sup-
port. But in the phloem the conducting cells conduct food rather
than water. These cells are called sieve tubes, since their end walls
are perforated like a sieve. They differ from the vessels of the
wood in that they contain protoplasm and have thinner walls.
The phloem fibers are, like those of the wood, slender and thick-
walled. The outer bark or cortex consists mostly of dead corky
cells which are well fitted to protect the inner layers.

The organization and activities of the oak tree are typical of
those found in all woody plants. These plants differ from the
smaller flowering plants mainly in the ability of their stems to
build up annual layers of woody tissue and thus make perennial
trunks. In many parts of the world most of the smaller plants are
annuals for this reason, growing from seed and reaching ma-
turity each year.

That this type of plant body is most successful on land is ob-
vious when we notice the overwhelming preponderance of seed
plants over ferns, mosses, liverworts and fungi. This culminating
type seems indeed a far cry from the minute Protococcus, existing
practically unknown upon the bark of the forest giant which is at
the other extreme of plant organization. Yet both live basically
in the same fashion ; their metabolism is identical. In between the
two we can see, living today, intermediate types of plant bodies
which demonstrate to us how the maintenance structures of the
one are logically related to those of the other.

CHAPTER SUMMARY

In Protococcus plant metabolism is carried on within the con-
fines of the single cell which makes up the body of the organism.
As we consider multicellular plants, division of labor results in

Section from a woody stem. The large cells are vessels. The dark lines are

formed by ray cells.

The Bodies of Plants 125

various aspects of metabolism being taken care of by special groups
of cells. Unlike animals, plants have no need of complex digestive,
respiratory, locomotor or nervous organs. The evolution of the
plant body into higher types has involved development of tissues
and organs concerned with photosynthesis, absorption of raw
materials, and conduction of these as well as of finished food
products from one part of the plant body to another.

The thallus plants represent the simplest type of multicellular
plant body, lacking differentiation into roots, stems or leaves. Of
these there are two types, the algae and the fungi; the former
possess chlorophyll and carry on normal green plant metabolism,
while the latter lack the chlorophyll and hence live as saprophytes
or parasites.

Some of the flagellated algae, such as Volvox, form motile col-
onies ; but the larger thallus plants, and from them, all the higher
plants, have evolved from non-motile forms. Among the latter
there are colonial plants with cells embedded in a gelatinous mass,
lacking division of labor among themselves. More advanced is the
filamentous body, with each plant being a thread-like row of cells
attached end to end ; such is the common fresh-water pond scum,
Spirogyra. In many cases, these filaments branch to form bushy
and tufted plants several feet in length. This is a highly successful
type of plant body not only among these green algae, but among
the other two groups of algae, the brown and the red algae. The
latter two are predominantly marine, known as seaweeds. The
most complex and massive plant body of all is found among the
brown algae, especially among the large kelps, which are dif-
ferentiated into holdfast, stem and flattened photosynthetic leaf-
like portion. The fungi have filamentous and branched bodies
which form a tangled mass of colorless threads in contact with the
nutrient substratum. From this the reproductive structures grow
out, conspicuous in the case of the common mushrooms.

Higher in the scale of plant complexity are the primitive land
plants which lack vascular tissues and leaf-like outgrowths. Such
are the liverworts, as typified by Marchantia, with its compact
prostrate body able to stand terrestrial drying out, its ventral
rhizoids, and special photosynthetic tissues.

The moss plants show certain advances over the preceding
groups of plants, notably in the erect habit and the division of the

126 The Bodies of Plants

body roughly into root-like rhizoids, stems, and leaf-like expan-
sions attached to the stems. This type of plant body can be seen
in the hairy cap moss.

To make plant life on land a success, special conductive, or
vascular, tissues were needed. These are lacking in the mosses.
The fern plants introduce this innovation, possessing true roots,
stems and leaves. Built on the same maintenance system plan, the
highest group of plants — the seed plants — possess definite ad-
vantages in reproductive habits which have made them practically
sole victors in the struggle of plants to inhabit the land. A typical
seed plant is the oak tree, whose structure is designed to serve
three main purposes :

1. To bring the leaf into maximum contact with the sunlight.

2. To supply the leaf with raw materials for photosynthesis.

3. To conduct the manufactured food through the plant.

The leaves, roots, and stem play the following roles in these
processes :

Leaf:

1. Exposes large surface to sunlight.

2. Provides chloroplast-containing cells most abundantly on side
nearest the light.

3. Provides a protective layer of cells, the epidermis.

4. Provides a ventilating system, consisting of stomata and
intercellular passageways, for circulation of gases.

5. Provides veins, for support, conduction of water to, and food
products away from, the leaf cells.

Roots:

1. Absorb water and mineral salts from the soil through root
hairs.

2. Conduct these substances to the stem.

Stem:

1. Conducts water from the roots to the leaves, by means of
the vessels of the wood.

2. Conducts food materials throughout the plant by means of
the sieve tubes of the phloem.

The Bodies of Plants 127

3. Supports the plant by means of fibers in both wood and
phloem.

The outer bark, or cortex, of the stem serves to protect it.

QUESTIONS

1. What organ systems are necessary in a high type of multicellular
animal which are unnecessary in a multicellular plant such as the
moss ? Why ?

2. What is a thallus plant ? Give examples.

3. What type of plant body is exemplified by Volvox?

4. Why are most of the algae aquatic plants ?

5. Name and describe the types of plant body found among the
algae.

6. What type of plant body is common among the fungi?

7. What two contrasting types of metabolism are found among the
thallus plants?

8. What three basic modifications for terrestrial living are shown
by a liverwort such as Marchantia?

9. What advances in plant body design are to be seen in the mosses ?
What important one is lacking ?

10. Name the three main purposes for which the structure of a
typical seed plant such as the oak tree is designed.

11. What division of labor is to be found among the cells of a leaf?

12. What division of labor is to be found among the cells of the
stem of a woody plant ?

GLOSSARY

algae (al'je) sing, alga (al'ga) Chlorophyll-bearing thallus plants,
mostly aquatic and known as pond scums and seaweeds.

bracket fungus A type of wood-rotting fungus, the reproductive
bodies of which are shelf -shaped and occur on the sides of trees
and dead wood.

epidermis (ep-i-durm'is) The outer layer of cells of a plant, 'best seen
on the leaf.

ferns A class of plants possessing true roots, stems and leaves ; repro-
ducing by spores, not flowers.

fiber Slender thick-walled cell found in the stem and root system
of plants, used for support.

filament Thread-like type of plant body, made up of a series of cells
attached end to end, common among the algae and fungi.

fungi (fun'ji) sing, fungus (fun'gus) Thallus plants lacking chloro-
phyll ; vegetative body a mass of filaments.

128 The Bodies of Plants

liverwort A plant belonging to a class of plants intermediate between
the algae and the mosses ; green terrestrial plants, generally pros-
trate and lacking stems and leaves.

Marchantia (mar-kan'ti-a) A liverwort.

phloem Inner bark of the stem, whose chief function is the conduc-
tion of food materials in solution.

rhizoid (rfzoid) A filamentous absorbing structure, carrying on the
functions performed by roots of higher plants ; found in the liver-
worts and mosses.

root hair Outgrowth of epidermal cell of root; function, absorption of
water and minerals in solution.

Sargassum (sar-gas'um) A type of brown alga displaying consider-
able specialization of structure.

sieve tube One of the conducting cells of the phloem.

Spirogyra (spi'roji'ra) A common pond scum. It is a filamentous
green alga.

stoma (sto'ma) pi. stomata (sto'ma-ta) Openings in the epidermis of
the higher plants, particularly in the leaves, which permit exchange
of gases.

thallus (thal'us) A plant body not differentiated into roots, stems
or leaves or similar structures.

vascular (vas'ku-lar) Pertaining to a system for conducting materials.

vessel A long tube-like structure found in wood of seed plants,
serving for conduction of water.

Volvox A spherical colony of flagellated plant cells.

CHAPTER VII

THE WEB OF LIFE

So far we have considered the living world from the individu-
alistic point of view ; but in limiting our attention to the structure
and activities of the individual we are likely to get a mistaken idea
of the self-sufficiency of any organism, thinking of it as a perfect
mechanism capable of maintaining its existence alone in a physical
environment. Such biologic isolation is very unusual. On the con-
trary, few organisms can live as independent units, for there is
an entangling web of alliances which binds together the various
species of plants and animals and which, like all alliances, is often
beneficial to one party while harmful to the other. Because of this
interdependence of living things in the balance of nature, organ-
isms must be thought of as parts of a whole rather than as entities
in themselves.

This interdependence extends through the whole realm of life.
One insect pollinates a flower, another sucks out its juice ; the grass
stem harbors the young grasshopper, but the pitcher plant drowns
and devours insects ; some birds scatter the seeds of plants, others
destroy the seedlings ; man is killed by the ravages of one micro-
organism but depends upon another for his bread ; one fungus de-
prives us of chestnuts, another makes possible the growth of
orchids. Such linkages are not isolated curiosities, they are meshes
in the limitless web of life.

Sometimes interdependence involves the assistance which one
organism gives to another in carrying out its work of reproduc-
tion. Insects, for example, often bring about the pollination of
flowers and in return receive food from them. More widespread
is the dependence of one organism upon another for the main-
tenance of life, for protection, or especially for food. It is with
these latter relationships that we shall deal in this chapter. In the
simplest type of interdependence, there is no organic association

129

130 The Web of Life

among the organisms involved. While dependent upon each other
for existence, they live separately. The most important interrela-
tionships of this type are the food linkages, or cycle of food ele-
ments, whereby the materials necessary for metabolism are kept
in circulation and constantly available for living individuals. They
include (i) the dependence of animals upon green plants as the
ultimate source of all their organic food, and (2) the dependence
of green plants upon bacteria for a constant supply of carbon
and nitrogen, with the converse dependence of saprophytes upon
green plants and animals for their organic materials.

Of a more complex nature are those interrelations which involve
a certain amount of biologic association, or living together. They
are of three types : (i) an external partnership of different species
known as commensalism, literally meaning "eating at the same
table"; (2) an internal partnership which is mutually beneficial,
neither member of the concern injuring the other, each contribut-
ing something to the general upkeep, which is known as symbi-
osis; (3) a partnership which is definitely one-sided, one member
of the firm living at the expense of the other, and contributing
little or nothing to the partnership; this is known as parasit-
ism.

The Dependence of Animals upon Green Plants. — In Chap-
ter II the importance of green plant metabolism to the whole or-
ganic world was emphasized. The abundance of green plants which
make up the vegetation of the earth, both on land and in water,
has made possible the variety of animal life as it exists today. In
the oceans, great numbers of algae synthesize carbohydrates from
the water and the carbon dioxide dissolved in it, converting these
into proteins by utilizing the dissolved salts found in the water.
Protozoa and other minute invertebrates feed upon the algae;
fishes and larger aquatic vertebrates feed upon these smaller ani-
mals, so that the food linkage may extend from microscopic algae
to whales and sharks. Without the former there can be none of
the latter. In fresh waters, too, the algae are the basic source of
all the food for fishes and other fresh-water animals.

Likewise on land, there had to be green plants before there
could be a successful migration of animals landward. These plants,
probably the ancestors of present-day liverworts, mosses and ferns,
had first to colonize the bare wastes of soil and rock Then their

The IV eb of Life 131

waste products and dead bodies formed a substratum of decaying
organic matter upon which more and larger plants could gain a
foothold. With the advent of swamps, forests and prairies, land
animals were able to secure a constant supply of food substances.
An abundance of land-dwelling reptiles, birds and mammals thus
became possible.

It is obvious that the plant-eating, or herbivorous, animals arc
dependent upon vegetation for their very existence, but this is not
so apparent for the host of carnivorous, or meat-eating, animals.

But the food chain eventually leads to some inconspicuous
plant-eating animal, often microscopic. Thus the ferocious tiger
becomes ultimately dependent upon the insignificant grasses which
he treads under foot. The large fish eats the smaller one and this
one in turn an even smaller fish, and so on, until at length we find
the plant- feeding individual who, for all his unknown existence,
is still the important link between the animal and the plant
kingdom.

The Importance of Bacteria in the Cycle of Food Ele-
ments.— Decay is a common biological phenomenon, generally
considered an unmitigated evil. It is true that decay does destroy
a small percentage of food and other articles of use to us ; but, on
the other hand, if there were no decay, life would generally slow
up, and, because of a lack of essential raw materials for plant
metabolism, eventually there would be no living organisms — at
least none like the plants and animals of today. Decay is the result
of the activity of bacteria and fungi, whereby these organisms tear
down the protoplasmic substances that other organisms have built
up, in order to get organic food for themselves.

As plants and animals, generation after generation, increase
in bulk by growth, they are continually abstracting from the en-
vironment the two very important elements, carbon and nitrogen.
Therefore it is simple arithmetic to understand that as life in-
creases the number and size of individuals, more and more of
these key elements are withdrawn from circulation and locked
up in the protoplasmic materials making up living things. Carbon
dioxide is essential for photosynthesis, yet there is not an endless
supply of it in the world. Only 0.03 per cent of the atmosphere is
carbon dioxide; this is the equivalent of about 5.84 tons of car-
bon over each acre of the earth's surface. Many crops, such as

132 The Web of Life

sugar cane, extract 15 to 20 tons of carbon per acre. Even with
diffusion of this gas from one part of the atmosphere to the
other, at the average plant consumption rate plants would use up
all the carbon dioxide in the atmosphere in thirty-five years!
With all the carbon locked up in the bodies of plants and animals,
living and dead, life would necessarily cease to be a characteristic
of the planet.

The carbon must get back to the atmosphere somehow, since
life has gone on, according to the fossil record, for over a billion
years. Some of it is returned to the atmosphere as carbon dioxide
by respiration. There is an interesting interdependence between
animals and plants in this respect, best illustrated in a "balanced"
aquarium. If just the right quantities of green plants and animals
are in the aquarium, it can be covered and left alone for months.
During green plant metabolism carbon dioxide leaves the water
to go into the plant, while during animal katabolism the carbon
dioxide goes out of the animals. Thus one uses up what the other
discards. At the same time, as a by-product of the plant metab-
olism, oxygen is given off, going out of the plant into the water ;
and, during animal respiration, the oxygen leaves the water and
goes into the animal. Without the green plants there would be
no continuous supply of oxygen to keep the fish, or other aquatic
animals, alive; nor would there be any means of removing the
carbon dioxide from the water. Conversely, without the animals,
there would not be as much carbon dioxide available for the plants.
There is the same give-and-take in the case of land plants and
animals. The vegetation removes carbon dioxide from the at-
mosphere and returns the oxygen which is essential for respiration.

Perhaps most of the carbon, however, gets back to the air
through the agency of decay bacteria and fungi. These organisms
are found everywhere, and they begin decomposing plant and
animal tissues as soon as life has left them. Thus the remains of
past generations are removed from sight, instead of becoming
encumbrances to following generations ; and the complex organic
compounds which constitute protoplasm are reduced to simpler
substances and eventually returned to circulation. For example,
wood is a common substance in which vast amounts of carbon are
locked up as cellulose. There are many wood-digesting fungi
which can excrete enzymes that change the cellulose into glucose

The Web of Life 133

and organic acids, finally into carbon dioxide and water. Thus
as the wood decays and disappears, the carbon is returned to the
atmosphere whence it originally came, ready to be used over
again by living plants. All carbohydrates in plant and animal tissue,
upon the death of the individual and in the presence of organisms
of decay, thus are broken down into carbon dioxide and water,
and these two substances are returned to the physical environment
to begin a new cycle.

GREEN PLANTS

Food for

ANIMALS

CO,

4f

o I rti

fll

I&H

COLORLESS PLANTS

FIG. 33. — Diagram of the carbon cycle.

Nitrogen is the element essential for protein-building, and there-
fore for producing protoplasm. Few plants (and no animals) can
utilize the vast store of nitrogen which exists in the atmosphere,
to the extent of four-fifths of all the gases combined. Plants syn-
thesize their proteins from the nitrates absorbed from the soil;
thus the supply of nitrates in the earth is the sole storehouse for
the nitrogen needed in making protoplasm. As in the case of the
carbon cycle, it is obvious that if nitrogen is removed by the tons
from the soil, wherever there is vegetation there must be some
way in which nitrogen can get back to the soil to make good the
loss. Otherwise, it would eventually all become locked up as pro-
teins in the dead bodies of plants and animals.

Here again the role of bacteria is an important one. The huge

134

The Web of Life

protein molecules are attacked by certain species of decay bac-
teria, and changed to ammonia. The ammonia, in turn, is used as
a source of energy for carbon synthesis by other species of bac-
teria, known as nitrite bacteria because they change the ammonia
to nitrites during the process. Still other bacteria (the nitrate bac-
teria) obtain their energy through the oxidation of these nitrites

GREEN PLANTS

Protein food for

>

ANIMALS

NITRATE
BACTERIA

NITROGEN-

FIXING
BACTERIA

t

NITRITE
BACTERIA

DECAY BACTERIA

FIG. 34. — Diagram of the nitrogen cycle.

to nitrates. And with the release of nitrates into the soil, the
nitrogen is again made available for protein synthesis on the part
of green plants — as a result of the chain of decay, nitrite and
nitrate bacteria.

But this is not all. New supplies are added to the soil by other
bacteria, the nitrogen- fixing bacteria, which are able to remove the
nitrogen from the atmosphere and leave it in the soil as nitrates.
Some of these nitrogen-fixing bacteria are free-living, utilizing the
carbohydrate materials in the soil as a source of energy for the
fixation of the nitrogen. These soil bacteria are more abundant in
light, well-aerated soils in which there is some decay in organic
material, than in the heavy, soggy ones. Other nitrogen-fixing bac-
teria live symbiotically in the roots of various plants related to
peas, clover and alfalfa, where they form little nodules. When the

The Web of Life

135

plants die and are not removed from the soil, these nodules de-
compose and add nitrates to the supply available for green plants.
Since soils can be enriched in this way, it is wise to alternate crops

FIG. 35. — Nitrogen bacteria in clover roots.

of such nodule-bearing plants with others which do nothing but
extract the nitrates.

Commensalism. — In this type of biological association there
often seems to be no obvious advantage to either organism; at

FIG. 36. — Commensalism : shark sucker attached to shark.

other times the two organisms are mutually of aid in getting to
the food supply. Some small crabs have as their homes the branch-
ing water canals of sponges; other crabs have their "shells" cov-
ered with small sea anemones. The young of some fish are always
found in company with large jellyfish, so that they can hide under

136 The Web of Life

the protective tentacles of the latter when pursued by their
enemies. The shark sucker is a fish especially adapted to fasten
itself beneath the body of a shark by means of an attachment
device on the top of its head, thus getting free transportation and
often food remnants discarded by the larger fish.

Symbiosis. — Symbiosis is a type of association between dif-
ferent species, in which both partners benefit mutually from the

Upper surface of lichen .

Rhizoids Fungus filaments

FIG. 37. — Plant symbiosis : section view of a lichen.

relationship. Sometimes both species are plants, at other times
both are animals, and in some cases one is a plant and one an
animal.

Lichens afford a striking example of the advantages of such
a partnership. Lichens are a group of thallus plants, classified with
the algae and fungi as the lowest group of plant life. They are
usually low-growing, crust-like plants of a gray-green color,
growing on bare rock or trunks of trees. One very common lichen,
which is known as reindeer moss, forms light-gray cushiony
masses on the ground in the northern woods. A very noticeable
characteristic of the lichen is its ability to grow on such inhos-
pitable substrata as bare rocks, where no other plants can live.

Lichen plants. Examples of symbiosis.

The Web of Life 137

It attaches itself firmly by means of tenacious hair-like rhizoids
on its under surface; there is no other apparent specialization —
such as stems or leaves — in these peculiar plants. If we section
a lichen, however, we discover the reason for their ability to live as
they do. Each lichen is largely made up of a mass of twisted
fungous filaments, holding in their meshes many minute spherical
single-celled algae. The green algae carry on photosynthesis, thus
manufacturing food for themselves. The fungus absorbs some of
this food from the alga, but in turn protects the little green plants
from drying out. Thus the combination of both plants makes it
possible for the species of algae and fungi to live in exposed
situations where neither of them could live alone.

Lichens are not the only examples of two plants living together
in symbiosis. The bacteria in the root nodules of peas and clover,
noted earlier in the chapter, form a combination mutually bene-
ficial. There are also many orchids which live in a symbiotic rela-
tionship with certain root fungi.

Termites are well known for their ability to destroy wood.
These wood-devouring insects have this special ability (for-
tunately lacking in most animals ) because they have, living within
their alimentary canal in a symbiotic condition, certain species of
Protozoa that are able to digest cellulose, the termites in reality
only indirectly subsisting upon the wood. The Protozoa get a
home and transportation, while in turn the termite gets a type of
food refused by most other animals and hence very plentiful.

There is a beetle which is commonly a guest in ant nests. The
beetle is blind and thus unable to get its own food easily, but the
ants take care of him and feed him. In return, the ants are allowed
to lick a tuft of hair which grows at the base of the wing covers
of the beetle. There are other examples of such insect guests in
ant nests which are fed solicitously, and which give in return
certain secretions that are evidently considered ample repayment
for the time and effort expended on the part of the ants. Other
examples of symbiosis are the crocodile bird, which removes
leeches and decaying food remnants from the mouth of the croco-
dile ; and the American cowbird which often is found on the backs
of cattle, from which it removes various parasites upon which it
subsists.

On the other hand, symbiosis may be the result of a plant and

138 The Web of Life

an animal living together amicably. Many Protozoa have green
algal cells within themselves; the minute single-celled plants pro-
vide food for the Protozoa and get shelter plus various other ad-
vantages in return. Such single-celled algae are also commonly
found in the endoderm of Hydra, giving the animal a bright green
color. Here too, the products of plant metabolism are used by the
Hydra, and in repayment the algae get protection and the materials
needed for their existence.

Parasitism. — In this type of relationship, which is much more
common than symbiosis, one "partner" receives all of the bene-
fits, and usually inflicts some damage on the other. The member of
the partnership which thus receives all the advantage at the ex-
pense of the other, becomes the parasite ; while the other organism
thus entangled in an association which he cannot escape becomes,
whether he so desires or not, the host. Most of the diseases which
afflict the human race are the result of man's being drawn into
such a relationship — a condition which furnishes the battle ground
for the thousands of scientists constantly engaged in medical re-
search. In many cases both the parasite and the host become struc-
turally changed, so that the parasite is better fitted to extract its
food from the host and the host is able to carry this extra organic
load with the least possible damage to itself. In the parasite such
organs as are not needed in the new life, mainly those concerned
with locomotion and food-getting, are much simplified; organs of
reproduction, on the other hand, become highly complex and spe-
cialized, so that enough offspring will be produced to make sure
that one reaches a proper host. Most parasites are adapted for a
specific host, making reproduction and dispersal a hazardous
undertaking.

There are six main types of parasitism, depending upon the
kind of organism which functions as host and that which is the
parasite.

Most uncommon is the combination of two green plants in such
a one-sided relationship. Since the dependent parasite in such cases
is green, it can manufacture its own carbohydrate food and thus
is partly self-supporting; often this is called hemi-parasitism. Such
is the case with the mistletoe, which grows upon woody plants,
sending absorbing roots into the host plant, from which it gets
its supply of water, and probably a small part of its food.

Indian pipes. A plant parasite.

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139

Also fairly uncommon is the combination of a flowering plant as
the parasite and a fungus as the host — the reverse of the common
condition in which the fungus is the parasite. Indian pipes are
graceful ghostly-white plants which have lost their chlorophyll,
the leaves being reduced to small clasping scales on the pale stem ;
the flower, which is borne at the tip of the stem, is also white.
Unable to carry on photosynthesis because of the absence of
chlorophyll, the plant gets its food materials from a fungus which

Hoot of parasite

Stem of host plant

FIG. 38. — Plant parasitism: Mistletoe. (Redrawn from Brown's The Plant King,
dom, Ginn and Company.)

grows in association with the roots. The fungus gets its nourish-
ment from the organic debris in the soil.

Very widespread is the parasitism upon green plants by the
thousands of species of fungi. Plant diseases cost the human race
millions of dollars yearly, as crop plants which were being relied
upon to give us our grains, fruits and vegetables become the
victims of rapidly spreading parasitic fungi. In many cases the
sporfcs of the fungus are carried by the wind and infect the leaves
of healthy plants. In the host tissue the spore germinates into a
branching mass of filaments which sap the green cells of their

140 The Web of Life

food ; greedily the fungus spreads from one part of the plant to
the other, often destroying all the leaves, with consequent death
to the plant. The white pine blister rust, the wheat rust, the ches-
nut blight, are just a few of the common fungi which have cost us
countless millions of dollars' worth of valuable plant life.

Plant diseases closely parallel the features of human diseases.
The microorganisms, in this case the spores, penetrate the healthy

Leaf tissue

Chloroplasts

Filaments of
fungus parasite

ire-producing
filaments

Spores

spores
FIG. 39. — Plant parasitism : wheat rust fungus in leaf.

plant in one of three ways : through a wound, through natural
openings such as the breathing pores in the leaves, or even through
intact surfaces, since some types of spores produce digestive fluids
capable of dissolving such surfaces. Sometimes, moreover, the
fungus spores become enclosed within the seed of their host,
and attack the seedling as it starts growth. Once the spore has
gained entrance to the host plant, it germinates into a branching
mass of filaments which grow rapidly through the plant, finding
accessible tissues where the filaments can send tiny but destruc-
tive suckers into the cells where the food supply awaits them.
Some fungi attack the leafy parts of a plant, soon destroying the

The Web of Life 141

entire photosynthetic apparatus and thus often killing the plant;
others eat away the portions of the trunk beneath the bark, leav-
ing a weakened and rotting mass of wreckage behind them. And
while this is going on, the fungus begins reproducing, sending off
from the surface of the infected plant millions of other spores
which are carried by the wind to new hosts. Little wonder that
battling plant diseases seems an almost superhuman task! To
make matters more complicated, there is often an alternation of
hosts, the fungus going from a wheat plant to a barberry, or from
a pine to a currant bush, living part of its life on one species, part
on another. When the life history of such a fungus becomes
known, however, control of the disease is simplified, because
eradication of one host usually means protection and salvation for
the other.

Green plants also become unwilling hosts to animal parasites.
This is particularly true with the great numbers of plant- feeding
insects. Aphids, scale insects and gall insects become attached to
specific host plants, which often are valuable orchard or timber
trees, much to the injury of the latter. Such parasites often com-
pletely change the tissues of the host in which they are lodged,
stimulating them to abnormal growth. Galls are abnormal swollen
portions of plants produced by many different kinds of insects,
particularly midges and gallflies. A well-known example is the
"oak apple" produced on the twigs of oak trees. The gallfly lays
an egg under the bark of the twig ; and when the egg hatches, the
larva lives on the tissue of the oak and stimulates it to produce
the round ball within which the mature caterpillar lies, well pro-
tected, during the winter. In the spring, the adult fly bores its
way out of its plant cradle and flies away. Similar insect galls
are the swellings found on the stems of asters and goldenrods,
and on various roses.

One of the best-known forms of parasitism is that involving
animals both as hosts and as parasites. When these parasites are
merely attached to the exterior of the host animal, they are known
as ectoparasites. Such an external parasite is a species of lamprey
eel, which remains attached to other fish by its sucker-like mouth
until the host is destroyed. Lice, fleas and mites often live thus as
parasites on the skin of warm-blooded animals. The ox botfly
lays its eggs on the hair of cattle ; when the larvae develop they

142 The Web of Life

bore through the skin and live there until spring. Then they bur-
row out, fall to the ground and complete their metamorphosis
into free-living flies — ready to repeat the whole process again.
Human lice cement their eggs to the hair, a new batch of lice
appear within a week, and as each new generation appears it feeds
upon the roots of the host's hair.

Internal parasites live more completely within the tissues of
their host, often in his alimentary canal. Such is the tapeworm,
a segmented invertebrate common in the digestive tract of many

Muscle fibers

Trichina of pork
Tapeworm of pig

FIG. 40. — Animal parasites.

animals as well as man. The life cycle of the human tapeworm
begins when one eats some incompletely cooked beef, pork or
fish containing little milky-white cysts, each cyst a larval tape-
worm. This becomes attached to the intestinal wall and grows to
be several feet in length at sexual maturity. Then new reproduc-
tive segments are budded off, pass out with the feces, and when
they are devoured by the proper host the cycle is complete. Other
such parasites are the hookworm and the pork roundworm, or
Trichina.

Sometimes the interrelations between the host and the parasite
become so balanced that there arc no harmful effects upon the
former. There are certain antelopes and similar mammals of Africa

The Web of Life 143

which harbor in their blood protozoan parasites known as trypano-
somes, without any discomfort to the mammals. However, if the
carriers of these parasites (certain species of flies) introduce the
trypanosomes into the blood of imported horses or cattle, fatal
consequences follow. Thus parasitism, when of long standing, does
not necessarily mean injury to the host. Often it can be assumed
that when a parasite destroys its host, the interrelationship is a
very recent one — as in the case of the infectious diseases which
are so often fatal to the human species.

Man does not stand aloof from this maze of interrelationships.
He is as much a part of the web of life as any other animal. De-
pendent upon green plants for the basic food materials of life,
man has developed this relationship to the point where agricul-
ture and husbandry have become vital to the existence of every
nation. Dependent also upon the bacteria, man has learned that
without them his soils eventually become barren; and many are
the useful species of these colorless plants which keep the human
species going. The common lot of man and his beasts of burden,
together with his domesticated animals, comes fairly close to being
commensalism. And because he acts as the host for a great num-
ber of parasites, man is at all times in danger of succumbing to the
voraciousness of these residents within himself. The various ways
in which these interrelationships of the human species with vari-
ous parasites, plant and animal, affect mankind, is the subject
matter of the following chapter.

CHAPTER SUMMARY

The interdependence of organisms results in a complicated
web of life, many different species of plants and animals being
mutually necessary to one another or affecting each other's life,
with resulting adaptation of structures and modification of activi-
ties. There are two main types of interrelationship, one involving
no organic association whatsoever, the other resulting in a cer-
tain amount of living together; the former includes (i) the de-
pendence of animals upon green plants as a source of food, and
(2) the dependence of green plants upon bacteria for their car-
bon and nitrogen materials; the latter includes (i) commensal-
ism, (2) symbiosis, and (3) parasitism.

In the food cycle, plants are found to be the ultimate source of

144 The Web of Life

all organic foods for animals. In the water it is the algae which
are the basis of all animal food, while on land food consists of
the grasses and herbage and plant products which are eaten by
herbivorous animals who may in turn be the food for the carniv-
orous species. Since green plants alone can synthesize carbo-
hydrates, proteins and fats from inorganic materials, they fur-
nish the sole source of these protoplasm-building substances for
animals.

Plants and animals are continually abstracting carbon and
nitrogen from the environment, locking up these important ele-
ments in their bodies, where they might remain forever did not
decay take place. Decay is caused by bacteria acting upon the
organic substances making up the parts of living and dead or-
ganisms. The carbohydrates are changed into carbon dioxide and
water by such bacteria ; by this means, and as a result of respira-
tion, the carbon gets back to the atmosphere where it may be
used over again by plants during photosynthesis. Other bacteria
act upon proteins, change them first to ammonia, then to nitrites
and finally to nitrates; in the latter form the nitrogen becomes
again available for plant use in making proteins. Certain plants
such as clover and alfalfa have nodules on their roots which con-
tain bacteria capable of fixing atmospheric nitrogen, transforming
it into nitrates and thus enriching the soil with this necessary
nitrogen salt.

Commensalism is a type of organic association in which two
organisms of different species associate with some advantage
usually to one or the other, or both. The shark sucker and the
shark, the crabs and the sea anemones, are examples of such an
association.

Symbiosis is a type of organic association in which both mem-
bers of the partnership definitely profit by the relationship, and
neither one is harmed., Lichens are examples of symbiosis where
both partners are plants, in this case the species being unicellular
green algae, somewhat like Protococcus, and fungus filaments.
Termites and Protozoa are examples of two animal species living
together symbiotically, as are various beetles and ants. The green
Hydra represents a type of symbiosis in which one partner is an
animal, the other a plant (unicellular green algae).

Parasitism is a type of organic association in which a one-sided

The Web of Life 145

relationship results in one member of the "partnership" living
more or less at the expense of the other, who is called the host.
For the sake of convenience, we can distinguish six types of para-
sitism : \

1. That involving two green plants. The mistletoe is a partial
parasite, obtaining its water from a host, usually a woody plant ;
being green, it can synthesize its own food. This is a rare type of
parasitism.

2. That involving a flowering plant as the parasite with a fungus
as the host. This is also uncommon; an example is the colorless
Indian pipe with its root fungi.

3. That involving a green plant as the host, with various fungi
as the parasites. This is widespread, and of great economic sig-
nificance, since it is the condition which results in most plant
diseases. Blister rust, wheat rust, and chestnut blight are a few
examples. The fungi may attack and destroy the leaves of the
host, or the stems, sometimes the flower and fruit.

4. That involving green plants as the host, and animals as the
parasite. Examples of this type of parasitism are the various insect
pests, such as the aphids and gall insects, which attack plants. The
larvae of many insects destroy leaves and fruits.

5. That involving animals both as hosts and as parasites. Para-
sitism of this type is the cause of many human diseases, such as
those produced by the tapeworm, the pork roundworm, and the
hookworm.

6. That involving colorless plants, the bacteria, as the parasites
and animals as the hosts. Under this type we find most of the
diseases of the human race, caused by microbic infection.

QUESTIONS

1. What are the two main types of interrelationships between or-
ganisms, with the various examples of each?

2. What is meant by a food linkage ?

3. In what various ways are animals dependent upon plants ?

4. In what ways, if any, are plants dependent upon animals?

5. What is the difference between a herbivorous and a carnivorous
animal ?

6. What is decay? Is it a necessary phenomenon, or could life go on
without it? Give reason for your answer.

7. How is the carbon dioxide in the atmosphere kept constant even

146 The Web of Life

though green plants are continually removing it during photo-
synthesis ?

8. What is> meant by a "balanced aquarium"?

9. Name the various ways by which bacteria replace nitrogen in
the soil in the form of nitrates.

10. Define commensalism and give an example.

11. What is the essential difference between symbiosis and parasitism ;

12. Define symbiosis and give examples of :

(a) A type in which both organisms are plants.

(b) A type in which both organisms are animals.

(c) A type in which one organism is an animal, the other 2
plant.

13. Describe a type of parasitism involving plants as the host.

14. Describe a type of parasitism involving animals as the host.

15. Give as many reasons as you can to prove that parasitism is oi
great economic importance.

GLOSSARY

carnivorous (kar-niv'o-rus) Flesh-eating.

commensalism (ko-men'sal-iz'm) An external partnership between

organisms which may or may not be of special benefit to both; i<

never harmful to either.

ectoparasite (ek'to-par'a-sit) An external parasite, such as lice or fleas
herbivorous (her-biv'6-rus) Plant-eating.
host An organism which harbors a parasite.
lichen (li'ken) One of a group of symbiotic plants, consisting oi

unicellular algae and fungi living together.
nitrate bacteria Bacteria changing nitrites to nitrates.
nitrite bacteria Bacteria changing ammonia to nitrites.
nitrogen- fixing bacteria Bacteria able to change atmospheric nitroger

to nitrates, such as those found in the root nodules of peas anc

clover.

parasitism (par'a-sit-iz'm) An internal partnership between organ-
isms in which one organism lives at the expense of the other

known as host.
symbiosis (sim'bi-6'sis) An internal partnership between organism*

mutually beneficial to both members.
termite (tur'mit) One of a group of insects capable of feeding upor

wood, because of symbiotic Protozoa in their digestive system.

CHAPTER VIII

COMMUNICABLE DISEASES

Man's Struggle for Existence. — Among other organisms, the
struggle for existence is for the most part a struggle to eat and
to avoid being eaten. Civilized human beings have fairly well
solved these two primary problems of existence. Few of the people
that we know are in danger of death from starvation, and it is
even less likely that any of them will fall prey to carnivorous ani-
mals. Aside from accidents, murder and suicide, there is just one
major cause of human death, and that is disease.

What Is Disease? — Every so often the average person goes
into a doctor's office and announces that he is not feeling so well
this morning. The doctor feels his pulse, looks down his throat,
makes a few remarks containing some long word ending in "itis,"
and the victim knows that he has a disease. If, instead of asking
whether it was serious or not, he should ask the doctor what a dis-
ease is, he might get some very learned reply, or perhaps the reply
would be, "A disease is anything that goes wrong with you that
you need me for." This is about as accurate a definition of disease
as can be given. The body machinery is so complex, and there are
so many different parts of it that can go wrong and so many ways
in which our bodily harmony may be disturbed, that it is impos-
sible to make a single definition which will cover all of the bodily
conditions which one could call disease.

Though there is no definition that would be satisfactory for
disease as a whole, one can distinguish two main types of diseases,
according to one important characteristic. Some diseases can be
carried in some way or other from a person who is sick to one
who is healthy ; these are the communicable or contagious diseases.

In all cases they are due to the presence of parasitic organisms
living somewhere in the bodily tissues and usually producing
poisons in the course of their metabolic activities. Parasitic in-

148 Communicable Diseases

vasions of the body are called infections ; hence all communicable
diseases are infectious. The functional diseases which result from
failure of some of the bodily structures to work properly or from
lack of proper materials with which to work constitute the second
type. The diseases due to vitamin deficiency fall into this group,
together with many of the most frequent and fatal diseases known
to man. In some cases failure to function is due to an infection of
the organs ; hence our functional disease may be either infectious
or non-infectious. Functional diseases will be considered in the
next chapter.

The Scourge of Pestilence. — At the time of the Civil War in
America, civilized men knew scarcely more about combating con-
tagious diseases than did the Babylonians or Egyptians who lived
six thousand years before. Ever since the human race had come
into being, disease had been the great enemy of mankind, yet men
had made very little progress in understanding its causes or its
cures.

Men would be peacefully going about their lives when sud-
denly an epidemic would appear in their midst. For weeks the
death rate would rise and rise until it became impossible to bury
the dead and it seemed inevitable that the entire population would
be wiped out. Then, as mysteriously as it had come, the pestilence
would subside. It would leave thousands dead or maimed. Families
or even towns would be utterly destroyed. And no one would know
the cause of the affliction. The people would be helpless in their
attempts to avert a recurrence.

Man had no more control over epidemic disease than over the
winds of the air. The terror-stricken people attributed it to an
avenging angel, but prayers did not check it ; they thought of it
as a legion of devils, but they could not exorcise it. They could
hate it, fear it, or become resigned to it. But they could not go
forth to conquer it because they knew nothing about it.

The Establishment of the Germ Theory. — Less than sixty
years ago scientific workers hit upon the answer to the problem
of contagion in what is known as the germ theory of disease.
Since that time remarkable progress has been made in the direc-
tion of overcoming communicable diseases, since, once men knew
what the enemy was, they could attack it intelligently and suc-
cessfully.

Communicable Diseases 149

Briefly, the germ theory holds that contagion is carried by ex-
tremely minute organisms which live in the tissues of sick plants
or animals and which poison or otherwise attack the organism in
which they live and thereby bring about the symptoms of illness.
These tiny agents of disease make their way by various means from
one organism to another, and thus the contagion is spread.

Chief credit for the establishment of the theory goes to the great
French scientist, Louis Pasteur, and to the German physician,
Robert Koch. Koch went to great pains to prove absolutely that
the contagion in the case of the disease anthrax was borne by cer-
tain microscopic organisms and by nothing else. He took anthrax
organisms from the blood of a diseased animal and grew them in
artificial cultures, completely outside the body of any animal. He
grew them in one culture until they had multiplied immensely and
then moved just a few of them to another culture. Then he moved
a few from the second culture to a third. He made several of these
transplantations until he was certain that not a trace of any disease-
causing substance taken from the sick animal could be found in
his final culture. Nothing was there but the remote descendants
of the organisms that had been in the sick animal's blood. He
inoculated mice with these bacteria from his final culture. They all
contracted anthrax and died. It could not be doubted that the or-
ganisms that Koch put into their blood had caused those mice to
develop anthrax, for nothing else had been injected that might
possibly have been the cause.

The publishing of these results in 1876 convinced scientists of
the importance of combating microorganisms in order to over-
come disease. An immense amount of research in the field of bac-
teriology was immediately started, and discoveries came thick and
fast. The organisms responsible for one disease after another were
discovered, and methods of combating them were worked out.
Death rates from numerous forms of illness began to fall off
rapidly. Pestilence no longer appeared as a wholly mysterious and
inescapable calamity. It became an enemy, or rather a horde of
enemies, quite intelligible and tangible, to be fought and con-
quered with the weapons of shrewdness and planning that are
the chief means of defense that we human beings possess.

Pathogenic Organisms. — The organisms which cause sickness
in animals, and in plants as well, may be classified as follows :

150 Communicable Diseases

A. Plants

1. Bacteria

2. Fungi (not always microscopic)

B. Animals

1. Protozoa

2. Worms (usually not microscopic)

C. Unknown

I. Filtrable viruses (most of them too small to be seen,
even through the microscope)

It should be pointed out immediately that in each one of these
groups — excepting only the filtrable viruses — only a few of the
members are agents of disease. Most of the bacteria, fungi, Pro-
tozoa, and worms are quite harmless ; indeed, many of them are
essential to our existence.

Those which do cause illness are spoken of as pathogenic or-
ganisms; and the majority of them, which are too small to be seen
without a microscope, as pathogenic microorganisms. For pur-
poses of convenience, however, they may be given the more in-
formal title of microbes.

The next few paragraphs will serve to introduce briefly the
various types.

Bacteria. — Bacteria are classified according to shape. There
are round, dot-like bacteria, called cocci; rod-shaped bacteria,
called bacilli; and spiral bacteria, known as spirilla. Frequently
they grow in bunches or chains, and they are often provided with
flagella to enable them to move about.

The cocci are responsible for boils and carbuncles, for pneu-
monia, for meningitis, and for gonorrhea. Some of the diseases
caused by various types of bacilli are diphtheria, typhoid fever,
tuberculosis, and leprosy. In addition, there is the anthrax bacil-
lus, famous because it was the first microorganism proved guilty
of causing illness in man and in the higher animals. Cholera is
the only important disease known to be caused by a spirillum.

There are organisms similar to the spirilla, which, however, have
certain animal characteristics, though they are usually classified as
bacteria. They are called spirochetes. The best-known spirochete
is the one which causes syphilis.

Fungi. — Ringworm is the best-known disease caused by a
fungus. The branching filaments of the organism become im-

Communicable Diseases

LSI

I

Tuberculosis bacilli

Leprosy bacilli

Cholera spirilla

Typhoid bacilli

1

Gonorrhea cocci

&Q

C£2> fj

£^O>€IID (/

Dysentery bacilli

Pneumonia cocci

Sore-throat streptococci

FIG. 41. — Types of pathogenic bacteria.

152 Communicable Diseases

bedded in the skin. The well-advertised "athlete's foot" is a ring-
worm disease.

Protozoa. — Only a few of the many species of Protozoa are
responsible for disease. The best-known protozoan disease is
malaria. The malarial parasite is irregular in shape, lives in the
red blood corpuscles, and has the property of breaking up into
a large number of spores, each of which can grow into a complete
new organism.

Parasitic Worms. — There is a multitude of parasitic worms
which infest man and other animals. Most of them are not micro-
scopic, and some of them, such as the tapeworm, are quite large.
The hookworm, which seems to deprive people of energy and am-
bition, belongs to this group.

Filtrable Viruses. — While the worms are probably the largest
of the pathogenic organisms, the filtrable viruses are the smallest.
Indeed, they are the smallest living things known to man. Most
of them cannot be seen through the microscope, and all are ca-
pable of passing through a porcelain filter. From this last property
they derive their name. Infinitesimal as they are, they manage to
produce in man some of the most serious diseases which attack
him. Smallpox, measles, yellow fever, and infantile paralysis are
supposed to be due to their activity.

How Microbes Attack. — One may wonder how organisms as
small as these microbes can be so effective in attacking human
beings. Three reasons for this may be pointed out.

In the first place microbes, when they enter a warm, moist place
where there is plenty of food material — the tissues of our bodies
fulfill this requirement almost ideally — are capable of multiplying
at a tremendous rate, so that they may overcome the body by sheer
weight of numbers. When Koch placed a few anthrax bacilli in
the blood of a mouse, he found that within a few hours its body
was swarming with billions of these microbes.. When actively
growing, most bacteria divide in two every twenty minutes or half
hour; hence they can multiply a thousandfold in five to six hours.

In the second place, many microbes produce, as excretions,
virulent poisons, or toxins, which, when they enter the blood,
travel throughout the body and cause the symptoms of the disease.
Diseases which come on with great suddenness, such as diphtheria,

Communicable Diseases 153

are due to the effects of these toxins, rather than to the direct
action of the bacteria.

Thirdly, there are a few microbes which can encase themselves
within hard coverings, becoming small pellets known as spores,
that may float about in the air and withstand all sorts of hard con-
ditions. These spores of bacteria can resist temperatures well above
that of boiling water, and below freezing; they can remain in
absolutely dry places for an indefinite length of time. Fortunately 3
however, spores are formed chiefly by the bacteria which cause
decay in meats and vegetables, and by very few pathogenic or-
ganisms. The organism which causes tetanus, or lockjaw, is a
spore-producer; and that is why it is so easy to pick it up if a little
dirt gets into a wound, for the tetanus spores are very likely tc
be found in the dirt.

How the Body Defends Itself Against Microbes. — Having
now acquired some notion of the nature of the enemy and the
manner in which it attacks the body, let us consider how the bodj
defends itself. Microbes are continually present all around us.
Escape from disease would be impossible if it were not for the
bodily defenses against them; and the most important methods
used in preventive and curative medicine are simply means of
building up these defenses.

The Walls. — The chief strength of a medieval castle was the
stone walls and moats that surrounded it. Similarly, we are pro-
vided with a skin — hard, dry, and impenetrable to microorgan-
isms, save when it is cut, when antiseptic precautions must be
taken to prevent the entrance of germs.

Even when microbes get into the respiratory or digestive tracts,
as they frequently do, the mucous membranes which line these
tracts offer a stout resistance to their attacks. These membranes
are covered by the sticky mucus which is secreted by the goblet
cells that form part of the membrane. In the respiratory tracts
many of the membrane cells are equipped with hair-like projec-
tions, called cilia. (See Fig. 5 in Chapter I.) The bacteria that are
breathed into the lungs become literally mired in the mucus which
covers the sides of the trachea and its branches, while the cilia
wave back and forth in such a manner as to sweep this mucus,
with the bacteria which it holds, up toward the mouth.

In the stomach and intestines, a chemical warfare is carried on

154 Communicable Diseases

against invading microorganisms. The millions of bacteria that
we swallow every day with our food are, for the most part, harm-
less, but many pathogenic ones are also introduced. Luckily for
us, when these bacteria enter the dark warm recesses of the
stomach, the acid of the gastric juice kills practically all of them.
As the food makes its way through the tortuous passages of the,
intestine, the bile that is poured in from the liver kills more of
them. Finally, it is believed by some investigators that there are
certain organisms of the filtrable virus type which have their home
in the human digestive tract and which cause deadly diseases
among the bacteria, just as the bacteria cause diseases among us.

Within the Walls. — Yet, in spite of the stout resistance they
meet, microbes are continually making their way into the tissues
of the body. But here they meet a defense quite as strong as that
encountered on the outside. The blood itself contains substances
that are harmful to most microorganisms ; and when they have
been rendered inert by these substances, the white corpuscles which
the blood contains attack them and frequently succeed in com-
pletely destroying them.

Immunity. — These defenses, nevertheless, are not impregnable;
and, in fact, their strength varies from person to person and may
be much greater or less according to our physical condition. We
have been overworking; we are tired and out of condition; our
blood is temporarily in an unbalanced chemical state. If, under
these circumstances, a very few microbes manage to make their
way into our nasal passages and lungs, they are able to establish
themselves and cause us to have a cold, even though a short time
before hundreds of them could have had no effect on us.

The resistance which one's body can offer to the germs of a
particular disease is termed one's degree of immunity to that dis-
ease; if one can successfully resist the attacks of an indefinite
number of microbes, one is completely immune to it. If one suc-
cumbs easily to the attacks of the microbes of any disease, one is
said to be susceptible to it.

Both immunity and susceptibility to particular diseases can be
inherited, although the diseases themselves cannot. For example,
it has been pretty definitely proved that susceptibility to tubercu-
losis is inherited. This does not mean that one inherits the disease
itself, but simply that one inherits a bodily structure and chemis-

Communicable Diseases 155

try that do not offer as stout a resistance to the germs of tubercu-
losis as that offered by most people.

Natural Immunity. — When a disease is brought by foreigners
into a group or nation of people that has not previously been sub-
ject to it, it may cause terrible ravages, since that population does
not possess the degree of immunity that is possessed by the
people who have suffered for centuries from it. Measles, which is
a relatively mild malady among us, was very deadly among the in-
habitants of Iceland and Greenland, and among the Indians of
America and the savages of the South Sea Islands when it was
brought to them by people of European origin. It is believed that
the Puritan settlements of New England were saved from exter-
mination at the hands of the Indians, not so much by virtue of
Puritan valor at arms as by ravages of Puritan-imported smallpox
among the Indian villages. On the other hand, many "tropical"
diseases, such as malaria, are much more deadly when they attack
the visiting white man than they are among members of the native
population.

The reason for the greater immunity of races that have long
been subject to a disease seems to be that, when a disease first
strikes a people, it kills off all those who are especially susceptible
to it, leaving only the members of the population who offer it a
strong resistance. They pass this ability to resist along to their
offspring, so that after the disease has attacked a population for
some time, only the descendants of good resisters are left, and
these individuals inherit immunity to the disease.

When a person is born with the ability to defend himself
against an illness to which other persons are susceptible, he is said
to have natural immunity to that disease. This means that his
bodily defenses against the disease are sufficiently strong to beat
off all of its attacks. In every individual natural immunity is in-
complete ; that is, he is susceptible to certain diseases. But at just
these weak places, the remarkably resourceful defense of the body
improvises emergency measures against the invading microbes
which usually result not only in bringing the patient back to health,
but in rendering him immune to future attack.

Acquired Immunity. — Men have long known that many dis-
eases can be contracted but once in a lifetime, or once in a number
of years. If one recovers from a case of smallpox, measles, or

Communicable Diseases

diphtheria, one does not usually contract it again. In other words,
one has become immune. This type of immunity, which is the re-
sult of having had a particular disease, is known as acquired
immunity. It is brought about by the development of an immunity
reaction either to the pathogenic organisms themselves or to the
toxins which those organisms secrete. Immunity reactions may
develop against substances other than those introduced into the
body by pathogenic organisms. They may be developed against
snake venoms, or against the cell substances of all sorts of plants
and animals. In general, it may be said that a wide variety of pro-
teins of the type not found in the human body may have a poison-
ous or harmful effect on the body, but that the body can "learn"1
to protect itself against them.

All proteins that can produce immunity reactions are called
antigens. When an antigen enters the body, certain tissues (just
which ones is not known) respond by producing antibodies which
act upon the antigens so as to neutralize their harmful effects. If
an individual has- a strong immunity reaction, he does not even
feel these effects; but if his immunity reaction is weak, a pro-
tracted period of illness may ensue. The longer the illness, the
stronger the immunity reaction becomes, until finally a sufficient
number of antibodies is formed to overcome the antigens, and
the individual recovers. Thereafter, whenever these same antigens
enter the body, a much stronger immunity reaction occurs, so
that no illness is produced.

The particular types of antibodies which attack pathogenic or-
ganisms themselves have many names, but we shall refer to them
all simply as antibodies. The antibodies which attack toxins are
called antitoxins. Fundamentally they do not differ from any other
antibodies.

Artificially Acquired Immunity. — In the eighteenth century
people sometimes deliberately exposed themselves to smallpox in
order to become immune to it. Smallpox was so prevalent that
they were almost certain to get it sooner or later, so they arranged
to have it over with at a time that would best suit their conven-
ience. But having a disease is a troublesome and dangerous way

1The term "learning" is not ordinarily associated with such responses as
immunity reactions. But what actually occurs is very similar to the organismic
modifications that are ordinarily spoken of as learning, namely, the strengthen-
ing of an adaptive response through practice. (See Chapter XXIII.)

Communicable Diseases *57

of acquiring immunity to it. A better method has been discovered,
namely, that of inoculating an individual with the microbes of
the disease after they have been weakened or killed.

Smallpox vaccination is the best illustration of this convenient
method of becoming artificially immunized. When the smallpox
virus attacks cattle, it produces only a mild disease, and something
happens to it that greatly decreases its virulence, that is, its capacity
for causing illness. In producing vaccine for artificial smallpox
immunity, calves are infected with the virus. They develop skin
pustules which contain the virus in high concentration. The pus-
tules are drained, and after the lymph from them has been properly
treated, it is used to inoculate persons who wish to become im-
mune to smallpox. The virus from the calf does not make people
seriously ill, but it does bring about an antibody reaction that re-
sults in immunity lasting over a period of years.

Smallpox vaccination as a practical procedure was introduced
by the physician, Jenner, at the end of the eighteenth century, but
its theoretical implications were not understood at that time be-
cause the germ theory of disease had not yet been established.
Soon after he had helped to establish the germ theory, Pasteur
showed that artificial immunity could be induced for various dis-
eases by inoculation with attenuated cultures of the antigens that
produce them. Attenuated cultures are simply cultures in which
virulence has been reduced while the capacity for producing anti-
body reactions remains. The reduction can be effected in many
ways. In some cases, as in smallpox, the antigens are passed
through the bodies of certain types of animals that have the ca-
pacity to reduce their virulence. In other cases they are grown
for several generations in an artificial culture medium, some-
times being treated with mild antiseptics. For some diseases the
bacteria can even be killed and still produce an antibody reaction
when they are inoculated. The typhoid bacillus belongs to this
latter class.

Immunity against diseases which are produced by toxins can
be secured in two ways. One is to inject some antitoxin along
with the toxin. The latter stimulates the body to produce anti-
toxin, and the antitoxin injected with it keeps the toxin from
causing illness. Another way is to attenuate the toxin by treating
it chemically. Such attenuated toxins are called toxoids. Either

158 Communicable Diseases

toxin-antitoxin or toxoid inoculations can be employed to prevent
diphtheria. The latter is the more recent and probably the prefer-
able method.

Theoretically it might be possible to immunize against all dis-
eases by means of inoculation. But in many cases practical means
of doing so have not been worked out. Tuberculosis, pneumonia,
influenza, infantile paralysis, yellow fever, malaria, syphilis, and
gonorrhea are among the more important diseases for which no
successful and established methods of immunization have been
discovered.

Smallpox and diphtheria are the diseases for which artificial
immunization methods have proved most useful in this, country.
In Asia vaccination against bubonic plague and Asiatic cholera has
proved successful. During the war outbreaks of typhoid fever
among the armed forces were probably avoided by means of inocu-
lation.

Passive Immunity. — When an individual has already acquired
a disease, it is sometimes possible to help him overcome it by intro-
ducing into his blood the antibodies that will fight that disease.
The most familiar instance of this method is the use of diphtheria
antitoxin. Previous to the introduction of antitoxin treatment,
diphtheria was an extremely fatal disease, since the bacteria fre-
quently made such a virulent attack that death ensued before the
patient could produce enough antitoxin to overcome it. Now the
practice in all cases is to inject antitoxin into the blood to tide the
patient over until his own body can produce a sufficient amount
of its own.

In preparing antitoxin, the diphtheria bacteria are grown in a
culture medium inside a flask. When they have produced a suffi-
cient amount of toxin, this toxin is injected into a healthy horse.
The horse is capable of reacting strongly against the toxin by pro-
ducing a great deal of antitoxin. Larger and larger injections of
toxin are given the horse over a period of several months, forcing
him to produce antitoxin in great quantities. Finally, the jugular
vein of the horse it cut and several quarts of blood are drained
into flasks. The antitoxin for use with diphtheria sufferers is de-
rived from this blood.

Checking the Spread of Microbes. — Immunity methods have
done much to reduce the ravages of disease, but still more valu-

Communicable Diseases 159

able have been the efforts to check its spread. One seldom realizes
the hardship under which one's enemy labors in carrying on
warfare. We are made painfully aware of the successes of the
microbes, but we frequently fail to appreciate their difficulties.
Microbes are parasites. They can grow and flourish only in the
body of some organism that is capable of affording them food and
shelter. But just as the microbes begin to succeed in exploiting
their host, that ungracious organism either dies or demonstrates
its complete lack of hospitality by attacking them with antibodies.
If their race is to continue, the microbes must get from host to
host by some means or other. Like other parasites they live in a
discontinuous environment. It is as if the earth were to become un-
inhabitable to human beings within the next hundred years, so
that the only men who could possibly survive would be those who
could transport themselves to Mars. We can check the spread of
microbes by interfering with their journeys from one host to an-
other. The movement for public health and sanitation is chiefly
concerned with this task. Some of the more effective ways of pre-
venting the spread of organisms from host to host are isolation
or quarantine, pasteurization of milk, purification of water sup-
plies, elimination of insect pests, and inculcating the habit of
cleanliness.

Isolation. — Some microbes — for example, the viruses of influ-
enza and smallpox — can make their way from host to host only
when the hosts come into immediate contact with one another,
shake hands, or sneeze into one another's faces. For diseases of
this sort, isolation or quarantine is valuable in checking their
spread.

Quarantine was practiced long before the germ theory became
established, since men early realized that disease could pass from
person to person. Its systematic and intelligent employment, how-
ever, is of rather recent date. One recalls the isolation of lepers
described in the New Testament and how they were forced to cry,
"Unclean! unclean !" whenever they were approached. This over-
severe quarantine was based more on a cruel and superstitious fear
than upon intelligent control of the disease, for leprosy is in
reality only mildly contagious, and a savagely complete isolation
of lepers is not necessary, provided intelligent precautions are
taken.

160 Communicable Diseases

One of the chief difficulties met with in applying isolation meas-
ures is the presence of immune carriers for many diseases. An
immune carrier is a person who serves as a host to pathogenic
microbes but whose antibodies are capable of preventing any harm
to himself. Nearly everyone is, in a sense, an immune carrier of
tuberculosis. That is to say, we all have a few of the germs about
us, but most of us have them under rather complete control. Quar-
antine of persons sick with epidemic influenza is not as effective
as it might be because during the epidemic it is probable that
nearly everyone carries some of the influenza virus around in his
nose and throat. It is thought that diphtheria is spread largely by
immune carriers, while the most dangerous of all immune car-
riers, perhaps, are those who carry the germs of typhoid around
in their systems.

Pasteurisation of Milk. — Bacteria can multiply wherever they
find proper nourishment. Milk is a perfect diet for some microbes,
particularly those of tuberculosis, diphtheria, typhoid, and scarlet
fever. Hence it is an extremely dangerous source of disease if
precautions are not taken to keep it free from contamination and
if it is not sterilized by the process known as pasteurization.

To pasteurize milk, it is kept at a temperature ranging between
142 and 145 degrees Fahrenheit for thirty minutes and then im-
mediately chilled to 50 degrees or lower. This amount of heat does
not kill all bacteria, but it kills the greater part of the dangerous
ones. Diphtheria bacilli are destroyed at 129 degrees, typhoid
bacilli at 136, and the bacillus of tuberculosis at 138.

It should be emphasized that pasteurization is not a substitute
for the completest sanitation around all dairies and that the strict-
est supervision over dairies is necessary, whether pasteurization
is practiced or not.

Purification of the Water Supply. — Certain microbes, notably
the; typhoid bacillus, live in the digestive tract and are expelled in
great numbers in the human excreta. Consequently, sewage is
almost certain to contain them. Many cities get their water from
rivers and lakes into which sewage has been dumped, and it is
frequently almost impossible to secure an uncontaminated water
supply for a city. By the use of proper means of purification, how-
ever, safe water can be obtained.

During the past twenty years, nearly all cities in this country

Communicable Diseases 161

which did not already have pure water supplies have tdcen meas-
ures to render their supplies safe. Jersey City offers a good ex-
ample of the manner in which the purification of water can reduce
deaths from typhoid. In 1891, while the city was using water
from the Passaic River, 101.3 persons per 100,000 population died
of typhoid. In 1898 the city got its water from the Pequanock
River, a less contaminated stream, and the death rate dropped to
40.6. In 1906 the water was still untreated, but it came from the
Rockaway River, and the death rate then stood at only 21.6. In
1913, Jersey City began to treat its water with hypochlorite of
lime. This brought the rate down to 10.3. Finally, in 1926, fol-
lowing the use of chlorine as a disinfectant, the rate fell to 1.57
for every 100,000 inhabitants.

With the pure water supplies of the present day, typhoid death
rates in cities tend toward 2.0, and nearly all cases can be traced
to immune carriers or to impure milk, ice cream, or oysters.

At the present time, water supplies in American cities are usually
so free from contamination that typhoid inoculation is scarcely
necessary; but in emergencies, when water contamination cannot
be avoided, inoculation of the entire populace is essential to pre-
vent the outbreak of typhoid epidemics. In the Louisville flood of
the spring of 1937, relief workers first administered typhoid
inoculations, then brought food to the people. It is possible that
this vigorous public hygiene procedure warded off an epidemic
that would have taken many more lives than were lost in the
flood.

Life Cycles and Secondary Hosts. — In their travels from one
host to another, microbes frequently find it convenient to change
their form almost completely in order to render themselves better
fitted to meet the hostile environment outside their hosts.

Everyone knows how a frog goes through a stage in which it
is a fish-like tadpole and later develops into an animal that lives
part of its life on land, with four legs and a breathing apparatus
of lungs rather than of gills. Similarly, the butterfly starts out in
life as a caterpillar, passes through a stage in which it is com-
pletely wrapped in its cocoon, and finally emerges in its adult
form, completely dissimilar to the worm-like creature that was its
former self. We say that the frog and butterfly pass through a life

1 62 Communicable Diseases

cycle composed of several stages. And that is exactly what the
microbes do.

One of the most important helps in preventing the spread of
microbes has been the knowledge that certain varieties spend a
part of their life cycles in one or more secondary hosts. Usually
the secondary host is an insect. The malarial parasite, for example,
lives one phase of its life cycle in a certain type of mosquito. The
parasite cannot get from one human being to another if no mos-
quitoes of that particular species are present in the vicinity. Since
the time this knowledge was gained, malaria has been stamped out
in many communities simply by killing off mosquitoes in those
regions. It can be eliminated in other places as soon as public
opinion in those places becomes sufficiently enlightened to lead to
an attack on the mosquito.

Under some circumstances, however, mosquito elimination is
a difficult and costly business. It is quite impossible in such an
immense and poverty-stricken country as India. In Italy, the gov-
ernment has found that the best way to eliminate malaria is to
distribute quinine among the people in malarial districts. Quinine
attacks the malarial parasite in the blood, and if it is taken regu-
larly will ward off the disease. Once quinine has freed the people
from the symptoms of malaria, they develop sufficient ambition to
undertake the task of mosquito elimination in their districts.

The heroic work of the American commission for the study of
yellow fever in Cuba — in which the members had to use them-
selves and other volunteers as experimental animals, since yellow
fever cannot be given to mice, guinea pigs, or any other animal
but man — led to proof that the yellow fever virus is also carried
by a mosquito; and this dread disease is also being conquered
through the elimination of mosquitoes.

In Africa the germ of sleeping sickness is carried from one
human host to another by an insect known as the tsetse fly. It is
almost impossible to get rid of this fly, and consequently sleeping
sickness is yielding only very slowly to the attacks that have been
launched against it.

Another disease that is brought to human beings by an insect
is the plague. There are two forms of this disease. The first, and
by far the most frequent, is the bubonic plague, characterized by
infection and swelling of the lymph nodes. The second, the pneti-

Communicable Diseases 163

monic plague, occurs when the same bacillus attacks the lungs.
Plague is an extremely fatal disease and has probably been re-
sponsible for the most severe epidemics of all time, including the
black death. It is in reality a disease of rats and is carried from
rat to rat by fleas. When the plague becomes so severe among
the rats that they die off in large numbers, the fleas, for want of
rats to prey upon, attack human beings, and thus spread the dis-
ease to them. It is quite impossible to rid all the rats of their fleas,
but the rat population can be kept at a minimum by killing off
rats and, better still, by keeping all stores of food in rat-proof
warehouses or rat-proof containers. All ships entering American
ports from plague-infested regions are required to kill the rats
they carry by means of fumigation. Whenever plague breaks out,
immediate war must be declared on all rats in the infected area.
By this means and by proper quarantine measures, the once ter-
rible scourge of plague can be kept under control.

Soap and Water. — When all is said and done, the modern habit
of using a comparatively large amount of soap and water, with
the tendency which goes with it toward general cleanliness in all
things, is probably as much responsible for the general decline of
the death rate as all the triumphs of medical science combined. It
is among the cleanly nations that the lowest death rates are found.
Soap and water is one of the best of mild antiseptics, and the care-
ful disposal of sewage and other wastes that is part of the habit
of cleanliness is certain to bring about the destruction of large
numbers of deadly bacteria.

Disposal of Excreta. — There are several diseases of serious
consequence to human life and health that are passed from one
host to another by way of human excrements. The hookworm,
a small animal a quarter to a half an inch in length, attaches itself
in large numbers to the wall of the intestine and sucks enough of
the victim's blood to produce an anemia that results in weakness
and general inefficiency. It is seldom fatal, but it attacks whole
populations and renders them unable to live any but the dullest and
most unproductive lives. In China and India "alone it probably
infects from three to four hundred million people. In various
parts of our southern states from twenty to seventy per cent of
the population have it. For the hookworm to be passed from host
to host, human feces must be left on the ground. There the eggs

164 Communicable Diseases

germinate to produce larvae, which enter the body through the
soles of bare feet and eventually make their way to the digestive
tract. To stamp out the disease, it is necessary to teach an entire
population to build properly constructed latrines for the disposal
of excreta.

In his An American Doctor's Odyssey, Victor Heiser tells a
fascinating story of his work with the Rockefeller Foundation in
introducing sanitary habits among the peoples of the Orient.
Teaching them how to eliminate hookworm was the first step
employed. Since the worms could actually be shown to the people,
they could understand more clearly the parasitic nature of the
disease; and the necessity for cleanliness in combating it helped
them to realize the meaning of sanitation. Once the people had
been impressed with the importance of sanitation through hook-
worm eradication, their minds were made more ready to accept
other sanitary measures urged upon them.

In addition to typhoid fever, Asiatic cholera and amoebic dysen-
tery pass from host to host through contamination of food or
drink by human excretions. Both occur only among populations
where sewage disposal is inadequate.

The Standard of Life. — The greater cleanliness of the present
day is in part a result of the greater wealth that has come to
western European peoples with the discovery of colonization of
America and the invention of labor-saving machinery. People who
must work twelve to fourteen hours a day for the bare necessities
of food and shelter have neither leisure nor facilities for keeping
clean.

The higher standard of life which we now enjoy helps in other
ways to increase the length of life. Proper nourishment for every-
one, and especially sufficient pure milk for babies, helps enor-
mously, since a high degree of natural immunity is dependent
upon a well-nourished body, fed on a properly balanced diet.

The effect of the standard of life on health is seen in the differ-
ential death rates from tuberculosis for various occupational
classes. In England, for example, persons of low economic status
are more than twice as likely to die of tuberculosis of the lungs
as are those belonging to the upper classes. Much still remains to
be done to increase the wealth of great numbers of the population

Communicable Diseases 165

sufficiently to guarantee them conditions of life that will produce
healthy and robust constitutions.

Some Triumphs of Disease Prevention. — The germ theory
was established in 1876. In 1880, in the United States, 216 per-
sons out of every 100,000 died of diphtheria. Most of them were
children. In 1941, the rate for diphtheria was i.o. In 1880 the
typhoid fever rate was 25 and the scarlet fever rate 74. In 1941
they were 0.8 and 0.3, respectively. This change seems to have
been largely due to the use of immunity methods and to better
public and private sanitation.

This marked decrease in the number of deaths caused by con-
tagious disease means that people now die later in life and that
they usually die of some functional disorder. In 1890, the ratio
of deaths caused by communicable disease to those caused by func-
tional disease was about 3 to i. In 1940, it was about i to 4. Just
four diseases or disease types were responsible for the greater part
of deaths due to contagious ailments. Their death rates were as
follows :

Pneumonia and influenza 63.8

Tuberculosis 44.5

Syphilis 13.3

Intestinal disorders 12.3

All of these disease types are now reported to be yielding to
the newest weapons of attack against microbes, the chemothera-
peutic sulfa drugs and penicillin.

Chemotherapy. — As everyone knows who understands why he
puts iodine on a cut, microbes can be killed by antiseptic chemicals.
As early as 1865, Lister, inspired by Pasteur's work, showed that
deaths from surgical infections could be greatly reduced by treat-
ing the wounds with dilute carbolic acid.

Lister's success led to a search for drugs that could be intro-
duced into the body to kill germs, thus providing a quick and
effective cure of disease. This proposed method of treatment was
termed chemotherapy, and for many years it remained more a
matter of hope than of realization, for it was soon discovered that
most substances which were capable of killing microbes were likely
to do more harm to the body than the microbes themselves could
accomplish.

As late as 1930 there were just two diseases which could be

1 66 Communicable Diseases

effectively treated by chemotherapy : malaria, treated by quinine,
a remedy that had come down from ancient times, and syphilis,
treated by arsenic compounds as a result of Paul Ehrlich's dis-
covery of salvarsan in 1910.

Then in the early 1930*8 reports came out of Germany of a dye,
prontosil, which would destroy fatal doses of streptococci in mice
with scarcely any damage to the mice. French scientists took up
the study and showed that sulfanilamide was the substance in
prontosil that was effective. Clinical work with sulfanilamide dur-
ing the next three or four years showed it to be as effective in
curing human ills as it was with mice. Other sulfa drugs or
sulfonamides, such as sulfathiazole and sulfadiazine, were de-
veloped, each being most useful with certain diseases. Soon it
began to be realized that the dream of chemotherapy had come
true. A group of really practical internal disinfectants had been
discovered.

The sulfa drugs can serve as well to protect the body against
the entry of bacteria as to overcome them once they are within
the tissues. Today when a soldier is wounded, he takes a sulfa
tablet to prevent internal blood poisoning and also dusts sulfa
powder into the wound. This use of sulfa drugs, together with
emergency blood serum injections and improvements in surgical
practice, has reduced deaths from wounds in American evacua-
tion hospitals from 18 per cent in the last war to 3 per cent in
this one.

Epidemics can be quickly controlled by giving a small dose of
sulfa drugs to everyone likely to be exposed or to expose others.
These drugs afford rapid and certain cures for diseases like gonor-
rhea which were formerly curable, but by less effective methods.
Furthermore, 'their use has greatly reduced the death rate in
diseases that were formerly highly fatal. During the last war, for
example, 37 per cent of meningitis cases in the Army proved fatal,
whereas today only 2 per cent of such cases die.

Sulfa drugs attack bacteria by preventing their use of para-
amino benzoic acid, more briefly known as P A B A. This chemi-
cal, which is a member of the vitamin B complex, is formed by
the bacteria and employed in carrying on their nutrition. The
sulfonamides thus keep the bacteria from their food, and in their
weakened state they are destroyed by white blood corpuscles.

Communicable Diseases 167

Sulfonamides have distinct limitations. They are effective
against many types of true bacteria but do not attack animal
pathogens or most of the filtrable viruses. Occasionally the bac-
teria which they attack develop the capacity to produce about'
seventy times as much P A B A as ordinarily, thus rendering
themselves resistant to the action of the drug. Another drawback
is that about three out of every hundred persons are especially
sensitive to sulfonamide poisoning and cannot be given sulfa treat-
ment.

As if in answer to theise sulfa drug problems there has been
discovered a group of therapeutic chemicals produced by molds, the
best known of which is penicillin. Although the germicidal prop-
erties of this substance were noted in 1929, its use as a chemo-
therapeutic was not developed until 1941. Penicillin promises even
more miraculous results than those obtained with the sulfa drugs.
It attacks a wide range of bacterial organisms. It is more potent
than the sulfonamides, yet it can be given in large doses without
toxic effects. It rapidly overcomes infections that have developed
special resistance to sulfa drugs.

Up to the time of the present writing, difficulties in producing
penicillin have made complete tests of its potentialities impos-
sible. Methods of quantity production have recently been devel-
oped, and its true sphere of usefulness should soon become known.

The chemotherapeutic discoveries of the past few years prob-
ably constitute the greatest medical advance since the establish-
ment of the germ theory and the discovery of the principle of
artificial immunity. New drugs and new uses for them are con-
stantly being reported. Final tests of their value await further
study, but their promise is great. Astonishingly rapid and com-
plete cures of syphilis with penicillin have been reported, but much
more experience will be needed to verify them. A new sulfa drug,
diazone, appears to effect remarkable results with 'tuberculosis, but
more verification is needed. It is known that sulfa treatment re-
duces pneumonia deaths from one in every three cases to one in
every ten, and there is hope that penicillin will improve this record.

Certain types of pneumonia, caused by filtrable viruses, together
with influenza, also caused by viruses, are as yet impervious to
chemotherapeutic attack, but it is the bacterial pneumonias that
are usually the causes of death. Influenza alone is almost never

1 68 Communicable Diseases

fatal; the complication of influenza with bacterial pneumonia is
what produces the long death lists in influenza epidemics.

Finally, infections of the intestinal tract are reported to be
especially easy to control with sulfa treatments. Thus, all the im-
portant causes of death from communicable disease seem to be
potentially under the control of the new chemotherapeutics, and
there is reasonable grounds for hope that by 1950 deaths from
communicable disease in America will be negligible in number.

Nevertheless, unless new discoveries change the picture, con-
tagion as a source of illness will continue. Children will have
measles, whooping cough, and similar ailments, and the entire
population will be plagued by that persistent nuisance, the com-
mon cold. But there is no reason to suppose that in the end scien-
tific discovery will not have relegated the microbes of illness to
the same oblivion that it seems to be rapidly preparing for the
microbes of death.

CHAPTER SUMMARY

The greatest step in mankind's conquest over contagious dis-
eases was the establishment of the germ theory, which states that
such diseases are caused and spread by the activity of micro-
organisms. The two men chiefly responsible for the establishment
of the theory were Louis Pasteur and Robert Koch. It was 'the
latter scientist who, working with cultures of anthrax, finally
proved the theory. The disease-causing microorganisms are two
types of plants, bacteria and fungi ; among animals, protozoa and
worms ; and finally a group of ultramicroscopic organisms known
as filtrable viruses. Their virulence is due ( I ) to their rapid rate of
reproduction, (2) to their production of toxins, (3) in a few cases
to the great resistance to adverse conditions which they display.

The defenses of the body against microorganisms are ( i ) the
skin; (2) the mucus and the cilia that line its various openings;
(3) chemical substances and hostile microorganisms in the stom-
ach and intestines; (4) antibodies in the blood plasma; (5) the
white blood corpuscles. Immunity is the ability to resist disease.
Four types of immunity may be distinguished: (i) natural im-
munity, with which a person is born and which is often character-
istic of races ; (2) acquired immunity, which is the result of having
had a disease; (3) artificially acquired immunity, which is the

Communicable Diseases 169

result of vaccination or inoculation; (4) passive immunity, which
is the result of the injection of an antibody to counteract an antigen
produced by bacteria. Acquired and artificially acquired immunity
are explained by 'the fact that the presence of disease-causing
microorganisms in the body stimulates it to produce a large amount
of antibodies to counteract that particular disease.

The disease-causing microorganisms may be combated also by
preventing their spread from one person to another. This is ac-
complished in the following ways :

1. Quarantining, which, however, is made more difficult by the
existence of immune carriers.

2. Pasteurization of milk.

3. Purification of the water supply.

4. Learning the life cycle of the microorganism and eliminating
the secondary host if there is one. Some examples of secondary
hosts are the mosquito which carries malaria, and the flea which
carries the bubonic plague from rats to men.

5. General cleanliness, especially adequate disposal of excreta.
Immunity measures and prevention of infection, together with

improvements in treatment, have resulted in a tremendous decrease
in contagious diseases and in deaths from contagious diseases.
The newly developed chemotherapeutics, the sulfonamides and
penicillin, give promise of well-nigh eliminating most of the re-
maining contagious sources of death, namely, pneumonia, tuber-
culosis, syphilis, and the intestinal disorders.

QUESTIONS

1. Tell about the discovery of the germ theory and its importance
to mankind.

2. What are the chief types of pathogenic organisms?

3. How are microorganisms able to cause disease?

4. Describe the defenses of the body against pathogenic organisms.

5. How is immunity acquired? How can it be artificially acquired?
What is passive immunity?

6. Describe the most effective methods that could be employed to
rid a community of each of the following diseases : typhoid fever,
malaria, smallpox.

7. What are the most frequent causes of death among the contagious
diseases ? Discuss the possibility that chemotherapeutic agents may
result in almost eliminating contagious disease as a cause of death.

170 Communicable Diseases

6. Describe various methods that are used to prevent the spread of
pathogenic organisms from one host to another.

7. Discuss the respiratory diseases.

GLOSSARY

antibodies (an'ti-bo'diz) Substances in the blood which act in antag-
onism to foreign substances such as bacteria and toxins.

antigen (an'ti-gen) A foreign protein which attacks the body and
can be overcome by the production of antibodies.

antitoxin (an-ti-tox'in) An antibody which acts in antagonism to a
toxin.

bacillus (ba-sil'us) pi. bacilli (-1) A rod-shaped bacterium.

coccus (kok'us) pi. cocci (kok'si) A spherical bacterium.

culture A group of microorganisms grown in an artificial nutritive en-
vironment for purposes of scientific study.

filtrable virus (vi'rus) A pathogenic organism so small that it will
pass through the pores of a porcelain filter. Most such organisms
are too small to be seen through the microscope.

immune carrier A person who carries contagious disease germs about
in his system and who may give the disease to others although he
himself is immune to it.

immunity Condition of being able to ward off the attacks of disease.

infection An invasion of the tissues by pathogenic organisms.

inoculation Act of introducing bacteria into tissues of a plant or ani-
mal. It is frequently done to produce artificial immunity.

microbe (mi'krob) Popular name for pathogenic microorganisms.

microorganism (mi'kro-or'gan-iz'm) An organism so small that it
cannot be seen with the naked eye.

patlw genie organisms (path-o-jen'ic) Organisms which cause dis-
ease.

secondary host An insect or other organism in which a pathogenic
organism spends part of its life cycle.

spirillum (spi-ririum) pi. spirilla A spiral-shaped bacterium.

spirochete (spi-ro-ket') A spiral-shaped bacterium having animal char-
acteristics. The best-known spirochete is that which causes syphilis.

toxin (tok'sin) An antigen in the form of a poisonous substance pro-
duced by plants or animals. In this chapter we have dealt only
with bacterial toxins.

toxoid (tok'soid) Diphtheria toxin treated to render it harmless so
that it may be used to produce artificial immunity.

vaccination (vak-si-na'shun) Act of inoculating to produce artificial
immunity.

virulence (vir'oo-lens) Power of a pathogenic organism to produce
disease or death.

CHAPTER IX

FUNCTIONAL DISEASES

THE ENDOCRINES AND THEIR HORMONES

The Ductless Glands. — We have already mentioned the vi-
tamin-deficiency diseases as belonging to the group of functional
diseases (see page 79). To this category also belong the dis-
eases that are caused by failure in functioning on the part of the
endocrine glands of the body. These glands differ widely from
one another in location and structure, but all have one character-
istic in common. They are not supplied with tubes or ducts, as are
the sweat glands, the glands of the digestive tract, and the like,
but release their secretions directly into the blood stream, and
hence they are often referred to as the ductless glands. The sub-
stances which they secrete into the blood are called hormones.
Each hormone has some special task to perform in regulating
the functioning and growth of the bodily organs. When a hor-
mone is either lacking or too abundantly present, a characteristic
pathological condition appears.

The six most important of the endocrine glands are the islands
of Lang^rjigns in the pancreas ; the thyroid and parathyroid glands
in the neck ; the adrenal glands just above the kidneys ; the pituitary
gland, attached to the base of the brain; and the cj^Qcrme^^lan^ds
in the sex organs.

Insulin and the Assimilation of Sugar. — In Chapter IV we
learned that single sugars are absorbed into the blood stream
from the small intestine and are carried throughout the body.
They are then either assimilated directly into the tissues, where
they are used in combustion, or stored in the liver as glycogen.
This process of storage and assimilation can be carried out only
with the aid of a particular hormone, known as insulin^ which is
secreted by certain ductless glands consisting of groups of cells
in the pancreas, entirely apart from the tissues which secrete pan-

171

172

Functional Diseases

creatic juice. These cells were discovered by a Dutchman named
Langerhans, and since they exist in scattered bunches, each group
completely surrounded by the other pancreatic tissues, they are
called the islands of Langerhans.

Pituitary

Parathyroids

Thyroid

•Islands of Langerhans

Adrenals

Interstitial cells
of gonads (in male)

FIG. 42. — Location of endocrine glands.

The function of insulin seems to be that of keeping each cell
of the body furnighqd with a constant supply of .carbohydrate, fuels
&Q that t it Js neces^arY^Q^bum onl^jninimutn. of proteins and
Eat&jDccasionally, usually because of failure on the part of the
islands of Langerhans to perform their work properly, the amount
of insulin in the blood falls so low that the disease known as
diabetes mellitus develops. Because of the absence of insulin, the

Functional Diseases 173

liver can no longer store carbohydrate nor can the cells of the
body assimilate and burn it, and practically all the carbohydrate
food that is eaten remains in the blood stream in the form of single
sugar. It is present in such excess that it must be carried off rapidly
by the kidneys, and hence the urine contains much sugar, and
urination is frequent. At the same time, the cells of the body are
forced to burn the protein materials of which they are composed,
and the muscles become weak and emaciated. Still more serious
results come from the burning of fats. Complete oxidation of these
substances takes place only when they are burned along with a
great deal of carbohydrate ; but since the body is now incapable
of burning carbohydrate, their oxidation goes only part way and
produces poisonous acid substances which in many cases bring
about the death of the patient.

About fifteen years ago a group of Canadian scientists evolved
a successful method of extracting insulin from the pancreatic
tissues of animals, and at the present time it is only necessary for
a diabetic patient to take insulin at regular intervals and to con-
trol his diet carefully in order to escape almost entirely the symp-
toms of diabetes. Usually, however, this regime does not bring
about a true cure, since the islands of Langerhans almost never
recover their function, and the patient must take insulin the rest
of his life.

The Thyroid Gland and the Rate of Metabolism. — Astraddle
the windpipe in the mid-region of the neck is the thyroid gland
which produces the hormone tjhyroxin^ Just as insulin creates a
preference for carbohydrate fuels over fats and proteins in the
metabolism of the body, so thyroxin i£gulat£SJhe,rate at whiduthat
metabolism goes on. The basic function of this hormone seems to
be that of helping oxygen to combine with the various body fuels ;
and hence the more thyroxin in the blood, the faster the general
rate of oxidation throughout the body. Whenever a more rapid
rate of combustion is desirable, the thyroid gland usually responds
by producing more thyroxin. For instance, the gland becomes
more active in winter so that the increased combustion may keep
.. the body warm. It also speeds up its activity whenever the organ-
ism has extra work to do and needs "pepping up," as in times of
emotional stress or during puberty, when rapid bodily changes are
taking place, or again during pregnancy, when special vitality

174 Functional Diseases

on the part of the mother is required in order to take care of her
rapidly growing child. To sum it up, the thyroid acts like the
draft of a furnace which automatically opens whenever a little
more heat is needed.

The importance of thyroxin injkeeping our bodily functions go-
ing^Lajirpper rate is vividly demonstrated whenever the gland
fails to secrete a sufficient amount of the hormone. Occasionally
along in middle life, more often among women than among men,
its activity fails, whereupon a condition known as myxedema ap-
pears in which the individual continually complains of being cold,
may even wear an overcoat on warm summer days, fatigues
readily, and becomes mentally dull and physically sluggish, while
the skin shows a peculiar puffiness, resulting from the deposit of
water in the tissues. This condition can readily be relieved by the
feeding of thyroid substance, and the cures sometimes effected are
amazing.

Still more striking are the symptoms appearing in an individual
who from birth onward is lacking in thyroid secretion, since, with
the low rate of metabolism, the growth of all parts of the body
fails to progress normally. This condition is spoken of as cretin-
ism, and individuals suffering from it are termed cretins. A graphic
description of the cretin is given in Hoskins' Tides of Life.

The skin is dry and cold to the touch. It feels thick and seems life-
less. The hair is harsh and dry and falls out readily; even the eye-
lashes may be lost. The nails are thin and brittle. The teeth are slow in
appearing and have little vitality; even with good dental care they
are frequently lost. The face is pale and puffy ; the upper eyelids are
thick, giving the child a sleepy appearance. The hands and feet arc
broad and clumsy-looking. The bones of the head and face develop
at disproportionate rates leading, among other things, to a marked
depression of the root of the nose giving it a characteristic "saddle
shape." The lips are thick and prominent, the mouth is generally
open and drooling. The eyes are dull and lustreless. The face as a
whole is completely lacking in animation, never showing the play of
emotion or interest characteristic of the normal child. The subjects
are often deaf mutes. The muscles are limp and weak. Even the
musculature of the internal organs is sluggish, leading among other
things to constipation. The higher nervous system remains unde-
veloped both structurally and functionally and the intelligence grades
from feeble-mindedness to complete idiocy.

Functional Diseases 175

Here again remarkable improvement in both bodily character-
istics and intelligence can be effected by the administration of
thyroid hormone, although advanced cases can never be com-
pletely cured.

When too much thyroid substance is secreted, another sort of
abnormal picture is presented. The individual is thin and overac-
tive, restless and nervous, sometimes even to the point of insanity.

Occasionally the gland enlarges so as to produce the marked
swelling in the neck commonly known as, goiter, There are various
causes for this condition, but the most frequent is the lack of
sufficient iodine in the diet. Iodine is the chief raw material which
the thyroid gland uses in the manufacture of thyroxin. Hence, if
a person has not enough iodine in his diet, the gland cannot create
enough thyroxin to keep the rate of basal metabolism up to nor-
mal. In an effort to do this, it becomes abnormally enlarged.

Iodine is found in salt-water fish and in any other sea food and
is usually present in very small quantities in the soil and in the
drinking water of seashore communities. Since only a trace of it
is needed in one's food, people living near the seacoast rarely have
goiter. But in the interior of many countries, where little fish is
eaten and the soil and drinking water contain little iodine, goiter
is very prevalent. There is a "goiter belt" about the Great Lakes,
one in the Rocky Mountains, and a well-known one in the Alps.

The simplest method of giving the people in the "goiter belts"
a sufficient amount of iodine is to put small amounts of it in all
the salt sold in those regions. By this method the incidence of
goiter among the school children of Detroit was reduced from
36 per cent to 1.2 per cent in the course of seven years. Along
with this reduction of goiter there probably went a considerable
reduction of the symptoms of thyroid deficiency, namely, physical
sluggishness and mental incapacity.

Probably a large number of people who are not considered to
be sick suffer to some extent from over- or under-secretion of
thyroid. Some doctors at present prescribe thyroid feeding for
people who are only mildly fatiguable and sluggish, and encourag-
ing results have been secured in many cases. It has been suggested
that one's "personality" is considerably affected by the amount
of thyroxin received into the blood, but whether this is true or not,
excepting in extreme cases, has not yet been demonstrated.

176 Functional Diseases

The Parathyroids. — When the study of the thyroid was in its
infancy, it was noted that when the thyroid gland was completely
removed in an experimental animal — or occasionally in a human
being undergoing an operation for goiter — spasms would some-
times set in within a few hours, the muscles would go into rigid
contractions, and, because of inability to breathe, death would
often ensue. It has been shown that lack of thyroxin is not re-
sponsible for this condition, but, rather, lack of parathyrin, the
hormone produced by four small parathyroid glands lying against
the under surface of the thyroids. The most important effect of

thejriood.. When the hormone is absent, these salts are deposited
in the bones, and the amount of calcium in the blood falls, with
the resulting rigid contractions of the muscles. Too much calcium
in the blood, which may possibly result from an overdose of para-
thyrin, produces nausea, vomiting, and, in extreme cases, uncon-
sciousness and death. The convulsions from which very young
children occasionally suffer are usually the result of insufficient
calcium in the blood.

The Adrenals. — The two adrenal glands, situated just above
and back of each kidney, are each shaped like a cocked hat. Each
is composed of two distinct parts, an outer rind, or cortex, and
an inner center, or medulla. The hormone produced by the medulla
will be dealt with in a later chapter. The cortex produces an en-
tirely different hormone ^or tin, whose function is not fully
understood. When it is absent or greatly diminished, a fatal ill-
ness, known as Addison's disease, develops. The victim of this
disease suffers from insomnia and nausea; his skin takes on a
peculiar brownish color; he becomes weaker and weaker and his
heart beat grows fainter and fainter until death ensues. In 1929
methods for securing adrenal cortex extract were discovered, and
since that time much has been done to alleviate Addison's disease
by the administration of this hormone. The hormone is still very
expensive, however, and it is difficult to secure it in quantities ade-
quate for complete treatment of the disease.

Interaction of the Hormones: The Pituitary. — The endo-
crine glands have been spoken of as an "interlocking directorate/'
since each gland does not carry on its functions independently,
but is in continual interaction with the others. While this is true

Section of adrenal gland of rat. The darker region on the outside is the cortex ;
the lighter region, the medulla.

Functional Diseases 177

of all of the endocrines, the one whose functions seem to be most
closely intertwined with those of the others is the pituitary. This
gland is located at the base of the brain and is divided into three
parts : the (Lgferior lotyef the posterior lobe — which is the part at
the base attached to the brain — and an intermediate pqjt between
the two lobes. Although the gland is quite small, it is known to
produce many different hormones and suspected of producing
others. The anterior lobe seems to produce the greatest number,
and we shall confine our discussion to the most important and
best known of these anterior lobe hormones.

The pituitary hormone that has been known for the longest
time is the one which jtimtilates.-jg;owth. It is probably responsible
in part for the development of our muscles when we exercise
them, but we have no way of recognizing this. The best proof of
its activity is given by people in whom the pituitary gland is ab-
normally developed. Occasionally a child starts growing rapidly
at fifteen, and reaches a height of seven feet or more by the time
he is twenty. His hands become enormous, and his head, and
particularly his lower jaw, are exceptionally large. Whenever
physicians have examined such giants, they have found an over-
growth of the pituitary gland. Sometimes, when the pituitary
does not become overactive until early adulthood, gigantism may
not result, but overgrowth of the lower jaw, hands, feet, lips and
nose may take place, producing an unsymmetrical arrangement of
the features called acromegaly^ Children and adults with an under-
developed pituitary gland are also abnormal in appearance. They
remain small in stature, their features are always small and child-
like, and they are sexually underdeveloped. While the seven-foot
prize fighter, the circus giant, and the "powerful Katrinka" prob-
ably have an overdeveloped pituitary gland, the small, effeminate
boy with a high voice, and the circus midget are victims of a lack
of pituitary secretion.

The production of this hormone, however, does not depend on
the development of the pituitary gland alone. Thyroxin is' known
to stimulate the functioning of the pituitary, and hence it is
thought that the dwarfing of cretins is fundamentally caused by
failure of the thyroid to stimulate the secretion of the growth-
promoting hormone. As if in return for the stimulation it receives
from the thyroid, the pituitary produces a separate hormone, the

178 Functional Diseases

sole function of which is to stimulate activity on the part of the
thyroid gland. In addition, it is thought to produce hormones
which stimulate the parathyroids and the adrenal cortex; some
cases of Addison's disease are believed to be due fundamentally
to failure on the part of the pituitary. It interacts with the hor-
mones of the sex glands so completely that the function of repro-
duction is as much under the control of the pituitary as of the
hormones of the sex glands.

Diabetes is sometimes produced in animals for experimental
purposes by removal of the pancreas. But if at the same time the
pituitary gland is removed, the usual signs of diabetes do not
appear. Careful study has shown that this is because insulin does
its work in opposition to two pituitary hormones, one of which
is responsible for increasing the amount of sugar in the blood,
and the other for the changes in metabolism that produce the acid
poisons resulting from incomplete oxidation of fats. Here we have
an example, not of the direct effect of one hormone upon an-
other, but of interactions between the effects produced in the
body by various hormones.

Many other examples of interaction between the endocrine
glands could be mentioned. The pituitary is by no means the only
interactor. Nearly all the hormones seem to exert influences on
sexual development. Botk_lhe__thyrpid ...... and the adrenal cortex

also during preg-

nancy, and they apparently influence the rate at which sex hor-
mones are formed. If there is an abnormal secretion of the adrenal
cortex during childhood, sexual maturity may be brought on at an
astonishingly early age, and a two-year-old boy may develop a
beard and a man's voice.

Hoskins tells of an ancient account of one such child, in which
it was reported: "The subject was an infant, a young man, a ma-
ture man, an old man, was married and begat children and all in
the space of seven years." Such precocious youngsters are said
to have a predilection for smoking cigars and discoursing on phil-
osophical subjects, but these assertions may be somewhat exag-
gerated. When the adrenal cortex is abnormal in mature women,
they lose their feminine traits and develop the beard and low voice
of the opposite sex. Most of the bearded women of the side-shows
have an overdevelQpecLadrenal cortex.

Functional Diseases 179

Other instances might be added of the mutual influence which
the hormones exert upon one another. So complex is their action
that the most careful labors of scientists during the past fifty years
have only begun to unravel the mysteries of their influence upon
human health, physical appearance, and personality. Much remains
to be done in this difficult but fascinating field of research, and
when it is done, we shall know much more about the inner work-
ings of the human organism than we know today.

THE DISEASES OF LATER LIFE

Without doubt, the efficiency with which an organism functions
is profoundly affected by the endocrine glands. Although their
activities are so complex that scientists are only beginning to un-
derstand them, it seems very likely that the glands influence the
general tone and condition of the organism in a variety of ways,
and that differences between individuals in health, strength, vigor,
disease resistance, and even such qualities of personality as aggres-
siveness and cheerfulness are partly dependent upon the balance of
hormones in their bodies.

Of the specific diseases caused by the hormones, however, only
diabetes is sufficiently frequent and serious to be one of the major
causes of death. The functional diseases .which are the real death-
deakrs .jaxe those that develop as the body grows old and its tis-
sues begin to lose their efficiency in functioning. The number of
deaths caused by these diseases of later life has increased enor-
mously during the past fifty years. The following table gives the
death rates per 100,000 in the United States for the years 1900
and 1933 for the four diseases which were most fatal at the
latter date :

1900 1933

Heart disease 132 228

Cancer 65 102

Apoplexy 71 84

Nephritis (kidney disease) 89 83

During this time the general death rate was falling rapidly, so
that even nephritis shows a relative increase in its death rate. As
Diehl, from whom we quote these figures, has put it, these dis-
eases "represent the disintegration of the individual's vital ma-
chinery before the insidious accumulation of the relatively minor

180 Functional Diseases

injuries of previous illnesses, of hereditary factors, and of per-
sonal habits, the total effect of which is too great for the indi-
vidual to withstand. Man is mortal, and though life is prolonged
by evading acute illness, death must come, then, through some
form of wearing out or degenerative process/' The very success
of the war on the microbes results in an increase in the death rate
from diseases which result chiefly from the wear and tear of
living.

Arteriosclerosis. — Arteriosclerosis, the hardening of the ar-
teries which occurs sooner or later in most human beings, together
with the conditions with which it is associated, is today the prime
cause of death. Of the four diseases shown in the table above,
three of them — heart disease, apoplexy, and nephritis — are, in
older persons, almost universally associated with arteriosclerosis.
To be sure, infections of the heart and kidneys frequently occur
in youth and may terminate fatally. If these early infections do
not result in death, they may so weaken the organ attacked by
them that when arteriosclerosis sets in, the individual succumbs
to a new attack of heart disease or nephritis.

_f the .arteries. JsL.caiised.Jqe- the, .deposit of mineral
tissues of thfiir walls, so that they .become inelastic and
brittle, likq old rubber. Such deposits are laid down whenever the
walls are weakened or subjected to strain. We do not know pre-
cisely what the most important conditions leading to this harden-
ing are, but the following probably play a part :

I- Infections, toxins, and poisons in the system, such as chronic
infections of the sinuses, tonsils and teeth, syphilis, typhoid fever,
and the disturbance of the kidney which produces gout. These
conditions destroy the tissues in the walls of the arteries. The
tissues are then replaced by scar tissue which becomes impreg-
nated with mineral salts.

2. Anything which produces constant high blood pressure, thus
putting a strain on the walls. The most definitely known cause of
high blood pressure is impairment of kidney function as a result
of infection or poisoning. Hard work and worry have also been
blamed for increasing the blood pressure, and it has been asserted
that the high nervous tension that accompanies modern living is
chiefly responsible for many arteriosclerotic deaths. Definite proof
or disproof of this theory is lacking.

Functional Diseases 181

3. Their hereditary constitution seems to predispose many
people to high blood pressure and arteriosclerosis. The age at
which the arteries grow hard differs widely, and the difference
seems to depend to a considerable extent upon the factor of
inheritance,

Death does not result 4ircctly.frpm..,artgliQ?!glg.rQgis< itself, but

xatker. from, the .damage to organs which it may produce. The

organ which succumbs is the one which has been most weakened
by the wear and tear of life and in which the arteries have suf-
fered most from hardening. In the heart, for example, the valves
may not be working properly, and the muscle tissue may have
become weakened because of some earlier infection. The organ
will be under considerable strain because of the defective valves
and the fact that it is pumping against high arterial pressure. A
sudden fit of anger or unusual muscular activity may now throw
a greater amount of work on the heart muscles; but if the arteries
running to the heart are inelastic, they may not allow enough blood
to reach the heart tissue, or they may burst. Under these circum-
stances the cells may be irreparably damaged, and the heart may
fail utterly to carry out its task.

We have already seen that kidney disorders may help to bring
on arteriosclerosis. If the hardened arteries fail to supply sufficient
nourishment to the kidney tissues, they may become so completely
atrophied as to make proper elimination through them impossible.
Again the result is fatal.

When the arteries to the brain are hardened, there may be a
slow dying out of the brain tissues because of a failure to receive
oxygen through the blood. This is probably the chief cause of the
loss of mental power which accompanies age. When, as a result
of overstrain, one of these arteries bursts, an apoplectic "stroke"
occurs. Often these strokes result only in a more or less temporary
paralysis and loss of speech. But one is likely to succeed another,
and, in the end, death may ensue.

The Relation Between Infectious and Functional Disease.
— These diseases which center about arteriosclerosis show clearly
that the distinction between contagious and functional diseases is
a rather artificial one. Properly functioning tissues and organs
have a low susceptibility to infection, while, on the other hand,
infection is probably the chief cause of lowered organ efficiency.

1 82 Functional Diseases

Although an individual may completely recover from such a dis-
ease as scarlet fever, measles, or mumps, his heart, kidneys, and
other organs may be permanently damaged. The war on the
microbes still remains man's chief problem in his struggle for
longer life. But even if pathogenic organisms were driven off the
face of the earth, arteries would still harden with advancing
age and functional diseases would remain to thwart man's desire
for health and long life. We are today as much in the dark con-
cerning how to attack many of our functional diseases as our
fathers were concerning the problem of contagious disease. Pa-
tient and intelligent scientific research may some day show the
way to the conquest of the great functional diseases. How soon,
no one can tell. At the present time there is probably no single
problem that is receiving more attention from scientific workers
than that of discovering methods of attacking the fourth great
cause of death, cancer.

Cancer. — The male section of our population is the more
susceptible to arteriosclerotic conditions, while almost twice as
many women as men die of cancer. This disease occurs when a
group of cells loses its normal function and begins multiplying
rapidly, so that a great mass of growing tissue is formed in the
body. If this mass can be completely removed by the surgeon's
knife or by radium or X-ray treatments, the cancer can be cured.
But frequently, before this can be accomplished, some of the cells
break loose and move through the lymph system to all parts of
the body. From this time on it is practically impossible to cure
cancer. Death ensues from exhaustion of the body or from inter-
ference with the functioning of a vital organ.

Cancer can be caused by long-continued irritation of the tis-
sues at the point where it begins. Cancer of the mouth, for ex-
ample, is more frequent among men than among women, pre-
sumably because in men there is more irritation from smoking.
Apparently some inherited readiness on the part of the tissues to
react in this manner to the irritation is also a causative factor.
It has been shown that certain hereditary strains of mice show a
low susceptibility to cancer, whereas in other strains cancer is
almost certain to develop. The only method now known of com-
bating cancer is to destroy it in its early stages. This means that
from thirty-five years of age onward, individuals should watch

Functional Diseases 183

themselves for signs of cancer and go immediately to a doctor to
check on any persistent sore or lump in any part of the body.
Many such conditions will, of course, prove to be harmless; but
one case of cancer detected in its early stages and cured will be
worth many false alarms, considering that thirteen per cent of all
women and seven per cent of all men over forty years of age will
die of cancer. Today vigilance on the part of its potential victims
is the chief weapon with which cancer can be combated. Tomor-
row, the scientific laboratory may find for us a more certain way.

CHAPTER SUMMARY

The endocrine glands secrete substances known as hormones
into the blood stream, where they are carried to all parts of the
body. Under- or over- functioning of these organs may result in a
variety of functional disorders. When the islands of Langerhans
in the pancreas fail to secrete their hormone, insulin, sugar
remains in the blood and is not stored by the liver or used for
fuel by the body cells. This condition produces the disease dia-
betes, the symptoms of which are:

1. Copious urination with much sugar in the urine, caused by
excess sugar in the blood.

2. Wasting away of tissues and muscular weakness, caused
by burning of proteins rather than carbohydrates.

3. Poisoning of the body, caused by partial burning of fats.

Since the discovery of methods for preparing insulin, diabetes
can be controlled by daily injections of the hormone, although this
treatment does not result in cure.

When the thyroid gland in the throat fails to produce sufficient
thyroxin, the rate of metabolism is greatly decreased. In adults,
this results in myxedema, in which the patient suffers from cold
and is physically and mentally sluggish. When the thyroid is
underactive or inactive from the time of birth, the child does not
grow properly, and a strangely shaped, feeble-minded dwarf
known as a cretin is produced. Both myxedema and cretinism
may be avoided or improved by feeding thyroid extract. People
who suffer from an oversupply of thyroxin are overactive, nerv-
ous, and irritable. Thyroxin is an iodine compound ; hence, when-
ever iodine is missing from the diet, the thyroid gland grows

184 Functional Diseases

large in order to make up for the lack of iodine. This condition
is called goiter, and it occurs frequently in regions where there is
little iodine in the soil and water.

The four parathyroid glands are located on the inner surface
of the thyroid gland. Their hormone, parathyrin, regulates the
amount of calcium in the blood. Undersecretion results in the
lowering of the calcium content, which causes spasms that may
result in death.

The adrenal glands are located just above each kidney. Each
is divided into two parts : an inner center, or medulla, and an
outer rind, or cortex. The cortex produces the hormone cortin.
When cortin is absent, a fatal condition known as Addison's
disease develops.

The endocrine glands influence one another's activity to such
an extent that they constitute what is essentially a single system,
and they have been called the "interlocking directorate" in con-
trol of bodily functioning. The pituitary gland, located at the base
of the brain, produces a hormone that stimulates growth. Giants
are produced by oversecretion of this hormone, and dwarfs by
undersecretion. Oversecretion in adulthood results in acromegaly,
a condition marked by overgrowth of the lower jaw, lips, nose,
hands, feet, and other structures. The production of this hormone
is stimulated by the thyroid gland; and, in return, the pituitary
produces another hormone which stimulates thyroid development.
It also produces hormones which stimulate the adrenal cortex and
the parathyroids, and it interacts with the hormones of the sex
glands. It produces two hormones which are responsible for the
symptoms of diabetes when insulin is absent. Among other ex-
amples of interaction among the hormones are the precocious
puberty and masculinization that result from overstimulation of
the sex glands when the adrenal cortex is overdeveloped.

With the decrease in deaths in early life as a result of the
practical elimination of many contagious diseases, the functional
diseases of old age have become the most important causey of
death. Heart disease, kidney disease, apoplexy, and cancer are
now the chief causes of death. The first three of this group are
ordinarily associated with arteriosclerosis, and death may be
caused by damage to the heart, kidneys or brain when the hardened

Functional Diseases 185

arteries burst or fail to carry enough blood to them. Positive cure
or prevention of these diseases is at present impossible.

Cancer occurs when certain cells begin to grow and multiply
rapidly. Irritation of the tissues seems to be responsible for
starting these growths. Cure can be effected by destroying the
growing tissues with X-rays or radium or by surgical removal,
provided the disease is treated before the cells have begun to move
through the lymph system to all parts of the body.

QUESTIONS

1. What is an endocrine gland? A hormone?

2. Give the location and describe the functions and medical signifi-
cance of the following endocrine glands :

(a) Islands of Langerhans

(b) Thyroid- //

(c) Pituitary

(d) Adrenal cortex

3. What is meant by saying that the glands are an "interlocking di-
rectorate" ? Illustrate.

4. What is the nature of the connection between kidney trouble
and arteriosclerosis? Between heart trouble and arteriosclerosis?

5. Why is a person who has suffered an apoplectic stroke advised
not to take heavy exercise?

6. Why is it important to be on the lookout for cancer and to report
the first symptoms to a physician ?

7. Discuss the relations between functional and contagious diseases.

GLOSSARY

acromegaly (ak'ro-meg'a-li) Disease produced by overactivity of the
pituitary gland in adulthood, characterized by overgrowth of the
jaw, hands and feet, lips and nose and other structures.

adrenal glands (ad-re'nal) Two endocrine glands located just above
each kidney.

apoplexy (ap'6-plek-si) Sudden loss of consciousness resulting from
the flooding of the brain tissues with blood from a broken artery.

arteriosclerosis (ar-te'ri-6-skle-ro'sis) Hardening of the arteries.

cancer A tumor or group of growing cells that is likely to spread
through the lymph system and continue growing until death en-
sues.

cortin (kor'tin) Hormone produced in the cortex of the adrenal
gland.

1 86 Functional Diseases

cretin (kre'tin) A person afflicted with cretinism.

cretinism (kre'tin-iz'm) A disease characterized by idiocy, malforma-
tion, and dwarfism caused by undersecretion of the thyroid during
childhood.

endocrine gland (en'do-krin) A ductless gland whigh secretes hor-
mones into the blood stream.

hormone (hor'mon) A chemical substance, usually secreted by an
endocrine gland, which makes its way through the body, usually
through the blood stream, and exerts a definite influence over the
activities of the cells, tissues and organs.

insulin (in'su-lin) A hormone produced by the islands of Langerhans.

islands of Langerhans (Lan'ger-hans) The endocrine glands em-
bedded in the pancreas.

my x edema (mik-se-de'ma) Disease characterized by mental and
physical sluggishness produced by failure of the thyroid secretion.

nephritis (ne-fri'tis) Inflammation of the kidneys.

parathyrin (par'a-thi'rin) Hormone produced by the parathyroid
glands.

parathyroid glands Four small endocrine glands located on the inner
surface of the thyroid.

pituitary gland (pi-tu'i-ta-ri) Endocrine gland located at the base of
the brain.

thyroid gland (thi'roid) Endocrine gland located in the neck.

thyroxin (thi-rok'sin) Hormone secreted by the thyroid gland.

PART II
REPRODUCTION, INHERITANCE AND DESCENT

CHAPTER X

HUMAN REPRODUCTION

General Nature of Sexual Reproduction. — Man has but one

way of reproducing his kind. That method consists, in its essen-
tials, in the union of two reproductive cells, the egg of the female

Seminal
vesicle

Epldidymis
Testis

FIG. 43. — Male reproductive organs.

and the sperm of the male, to form a single cell, and the growth of
that single cell into a new individual. All of the complicated
sexual organs of men and women, all of the complex physiological
processes which men, and particularly women, must go through,
all of the pleasures, pains, desires, and intrigues connected with
sexual reproduction exist solely for the successful bringing to-

189

190

Human Reproduction

gether of the egg and the sperm to form a new cell, and for the
normal and secure growth of that new cell into an adult human
being.

The Male Reproductive Organs. — The male reproductive
organs have two functions : the production of sperms, and the con-
duction of the sperms to the penis, whence they are injected into
the female.

Vas def erens -
Epididymis -

Interstitial cells

Sperm-forming
tubules

Sperm
Sperm mother cells

A B

FIG. 44. — A, diagram of testis ; B, cross section of testis. (Redrawn from Martin's
The Human Body, Henry Holt & Company, Inc.)

The sperms are produced within paired organs, the testes,
which are contained together in the loose fold of the scrotum.
Each testis, within its fibrous membrane, consists of two types of
structures, the interstitial cells, which produce the special male
hormones, and the sperm-bearing tubules. There are eight hun-
dred to a thousand tubules in each testis. Each tubule is very
narrow and much coiled, and at one end all of the tubules join to-
gether to form a single common duct, the epididymis.

The tubules are filled with rounded cells containing large nuclei,
known as sperm mother cells. In an adult man these cells are
constantly dividing to produce sperms. The sperms, when mature,

Human Reproduction 191

have an elliptical head, consisting entirely of nuclear material, and
a long whip-like tail of cytoplasmic substance, which lashes vio-
lently and propels the sperm along. These sperm cells, when
formed, pass down the sperm-bearing tubules and out of the
testis through the epididymis.

The structures which convey the sperms from the testis are
so closely connected with those associated with the excretion of
urine that the two together are often spoken of as the urogenital
tract. From each testis the epididymis opens into a long duct, the
vas deferens, which passes over the urinary bladder into the lower
part of the abdomen. At its end is a sac, called the seminal vesicle,
into which the sperms pass from the vas deferens. The seminal
vesicles are paired as are the testes, and lie back of and below
the bladder.

As the sperms pass from the sperm-bearing tubules through the
epididymis and vasa deferentia, liquid is added to them through
secretions from the walls of the tubes through which they pass,
and the sperm-containing liquid is stored in the seminal vesicles,
which also add to the liquid mixture.

A tube called the urethra runs from the bladder through the
penis, serving to carry the urine to the exterior. Completely sur-
rounding this tube, just beneath the place where it leaves the
bladder, is the prostate gland, which produces a liquid that is
mixed with the sperms at the time of ejaculation. The seminal
vesicles empty into the urethra through two ducts that run directly
through the substance of the prostate and enter the urethra half
an inch or so below the place where it leaves the bladder.

The penis is made up of three long cylinders of spongy tissue
which run from its base to its head, surrounding the urethra on
all sides. Under the influence of sexual excitement, blood flows
into the small open spaces which honeycomb these spongy struc-
tures, and the penis becomes so engorged with blood that it
grows erect and hard. When friction is applied to the erect penis,
the muscles in the vasa deferentia, the seminal vesicles, and the
prostate gland contract forcibly. The sperm-containing fluid from
the vesicles and the secretion from the prostate are forced into
the urethra, where they mix to form the seminal fluid or semen,
which is immediately pushed along the urethra through contrac-

192

Human Reproduction

tions of the smooth muscles of the penis and forcibly ejaculated
from the end of the duct.

The Female Reproductive Organs. — Corresponding to the
testes of the male are the paired ovaries in the female, situated be-
tween the hips. In each ovary of a woman there are, at the time she
reaches maturity, about thirty thousand eggs which are located
just under the surface. No new eggs are produced during her
lifetime. Beside the ovaries are the horn-like openings of the

Fallopian tubes >

Cervix

Vagina

FIG. 45. — Female reproductive organs.

Fallopian tubes. The Fallopian tubes lead to a hollow muscular
organ, the uterus •, or womb, which opens to the outside of the
body through a passage, the vagina. Between the end of the vagina
and the uterus is a muscular constriction, the cervix.

The external parts of the female sex organs are spoken of
collectively as the vulva. At the front there is a slight elevation
composed of fatty tissues, called the mons Veneris, from which
two folds of skin, the labia majora, extend downwards and back-
wards, coming together just back of the vaginal opening. Inside
the labia majora is a second pair of folds, the nymphae. At the
point where they join together in front is a small, sensitive organ,

Human Reproduction

193

the clitoris. During sexual excitement it becomes erect like the
male penis; in fact, it develops from the same structures in the
unborn child that in the male grow into the penis. Back of the
clitoris is the opening of the urethra, and behind it, the vagina,
the opening of which in the virgin is ordinarily closed by a thin
membrane, the hymen. During the first sexual intercourse, the
entrance of the penis into the vagina usually breaks the hymen.

Small_
intestine

Uterus

Rectum

Vagina

•Anus

Hymen

Nympha
Labium majora-

FIG. 46. — Median section of female reproductive tract.

There is a small opening in the center of this membrane, how-
ever, which sometimes stretches to permit the entrance of the
penis without breaking the membrane; and disease, accident,
exercise, or other factors may result in the destruction of the
hymen prior to intercourse.

The Production of Egg Cells and the Menstrual Cycle. —
In the course of her growth — usually sometime between her
eleventh and sixteenth birthdays — a girl arrives at the age when
mature egg cells begin to be formed in the ovary. This new de-
velopment is signalized by the onset of recurrent periods of men-
struation, during which a certain amount of blood and debris

194

Human Reproduction

from the uterus passes out of the vagina. Menstruation occurs
fairly regularly, usually about once every four weeks, except
during pregnancy, until the age of forty-five or fifty. Then the
menstrual periods become more and more infrequent and finally
stop. This period of a woman's life is known as the menopause,
and normally she cannot bear children after this time. Scattered
periods of menstruation sometimes occur after the menopause,
however, and women over sixty have been known to bear children.
The menstrual period is only a small part of a long process that
goes on every month within the female reproductive organs, and

Cytoplasm
Oil vacuoles

Yolk granules

FIG. 47. — A, diagram of human sperm; B, diagram of human egg, showing
relative size of sperm. (After McEwen.)

the events leading up to it begin two weeks or more before the
onset of menstruation. The first event is the enlarging of one of
the egg cells within the ovary. As the egg enlarges, it becomes
surrounded by a sheath of cells, which eventually form a hollow
sphere. The egg becomes embedded in a projection on the inner
wall of the sphere, which is filled with a liquid. This specialized
structure in which the egg develops is known as a Graafian follicle.
(See Fig. 48.) As the egg and follicle mature, they gradually move
up to the surface of the ovary, until they form a bump or a blister
on it. Finally the Graafian follicle breaks open and the egg is dis-
charged into the body cavity, but is almost immediately swept into
the horn-like opening of the Fallopian tube by a current of liquid
which carries it down toward the uterus. If it is not fertilized, it
continues down the tube and within a few days is discharged
through the vagina. Meanwhile, the wall of the uterus has been
growing thicker and thicker and has developed a heavy mucous
lining which, if the egg is fertilized, serves to hold it within the

Human Reproduction 195

uterus and supply it with nourishment. About two weeks after
the egg leaves the ovary, however, if fertilization does not take
place, menstruation occurs, the lining breaks down and is shed
through the vagina. The thickness of the uterine wall is reduced
to normal and in a few days the cycle begins again.

The Graafian Follicle and the Corpus Luteum. — A most in-
teresting problem is the timing of this elaborate cycle. How is it
that the uterine wall manages to enlarge just as the egg is passing
down the Fallopian tube, and is shed soon after the unfertilized
egg reaches it?

Recent experiments show that the Graafian follicle produces
a hormone, called theelin. This hormone stimulates the thicken-
ing of the uterine wall. After the follicle breaks, less of the hor-

Membrane cells
(st«tum gnmulosum)

FIG. 48. — Graafian follicle.

mone is formed, although the ovary probably manufactures a
small amount of it at all times. The cells which line the broken
follicle, however, divide rapidly to form a little ball of yellowish
tissue called the corpus luteum. This structure produces a second
hormone, progestin, which continues to stimulate the growth
of the uterine wall, producing the changes that specifically pre-
pare the uterus for pregnancy. If the egg is not fertilized, the
corpus luteum degenerates, the hormones which maintain the
growth of the uterine wall are formed in such small quantities
that the growth ceases, the wall breaks down, and menstruation
begins.

Fertilization and Pregnancy. — During sexual intercourse, the
penis is inserted into the vagina and the semen is ejaculated into
it. The sperms may be forced immediately into the uterus by mus-
cular contractions in the vagina, or they may swim upward until

196

Human Reproduction

they reach that organ. Sperm cells are so constructed that they
tend to swim against a current; and since there is a continuous,
slow stream of fluid flowing down the Fallopian tube, uterus,
and vagina, the sperm swims upward. If an egg is being trans-
ported downward by this stream after coitus has taken place,
sperm cells will swim toward it, and finally one of them will meet
it, enter, and join its substance with that of the egg. This con-
stitutes the act of fertilization. It usually occurs in the Fallopian
tube, but occasionally in the uterus.

Fro. 49. — Cleavage and growth of the zygote in Amphioxus. In the human egg,
this process is complicated by the presence of a yolk.

Collectively, the eggs and sperms are called gametes. The egg is
the female gamete, the sperm, the male gamete. A fertilized egg,
however, is not a gamete, but a zygote. In these terms, fertilization
is the fusion of two gametes to form a zygote.

Immediately after fertilization, the zygote starts to divide rap-
idly to produce a cluster of cells, which, upon reaching the uterus,
becomes embedded in its wall. In the interior of this mass of cells
a cavity forms, on one side of which the new individual, or
embryo, begins to appear. Its development is very complex because
the embryo is a parasite of the mother. Membranes grow out from
it and surround it, helping it to absorb food from the uterine
wall. At about four weeks after fertilization it begins to take

sr J1 w, 'Wjf i * ; "-'• i
:; !>*-, \,.

Section of mouse ovary. The white objects are eggs, surrounded by the cells
of undeveloped Graafian follicles.

Human Reproduction 197

form and to assume its final position in the uterus. At this age
it is only a little over an eighth of an inch long. It is somewhat
fish-like in appearance, with the beginnings of gill slits, a
segmented back, and a tail. Small bumps on the under side are the
"buds" for the arms and legs. At six weeks it is half an inch long,
the head has begun to be differentiated, and the arms and legs
have grown out, but the gill slits and tail are still present. By
eight weeks the human form is attained, even though its length
is only about one inch.

The embryo has now taken its final position in the uterus. It
is enclosed within a sac, the walls of which consist of two mem-
branes, known as the amnion and the chorion. Within the inner
of these, the amnion, and completely bathing the embryo, is a
fluid, the amniotic fluid. The membranes are therefore pressed
closely against the wall of the uterus, except in one place, where
they form a cord running into the embryo, and attached to its
abdomen. It is through this umbilical cord that the embryo re-
ceives all of its food and excretes its waste products.

Opposite the outer end of the umbilical cord, the outer mem-
brane, or chorion, has a particularly close connection with the
uterine wall over an area known as the placenta. In this placenta
the chorion is prolonged into branching projections, known as
wlli, which are embedded in the wall of the uterus. The wall is
here filled with capillaries, coming from the circulatory system
of the mother. Likewise, the villi of the chorion are filled with
capillaries which are connected to blood vessels running up to
the umbilical cord, and hence are filled with blood from the
embryo. As the mother's blood surges through the capillaries in the
uterine wall, oxygen and food materials pass from it across the
outer membrane of the wall and that of the chorion, to be taken
up by the capillaries of the embryo. Likewise, carbon dioxide and
waste materials make their way from the capillaries of the em-
bryo through the chorion and uterine membrane into those of
the blood stream of the mother. Thus it is through the placenta
that the embryo receives its food and oxygen from its mother,
and delivers its waste products to her.

When, after eight weeks, the embryo has taken on the human
form, it is no longer called an embryo, but a fetus. The further

198

Human Reproduction

8 9 10

FIG. 50. — Human embryonic development: 4-6 weeks.

Human Reproduction

190

" 15

FIG. 51. — Human embryonic development: 7-8 weeks.

200 Human Reproduction

development consists simply of the enlargement and perfection
of its parts. At four months, the fetal heart has started to beat,
and muscular movements sometimes occur. At birth the fetus is
about a foot and a half long.

Birth. — At about two hundred and eighty days, or nine months
after fertilization, the uterus, now about four hundred times as
large as when it first received the fertilized egg, begins to con-
tract. These contractions, at first slight and infrequent, are the
beginning of the period known as labor. This period is usually
Placenta , ^

Fallopian
tube

Amniotic fluid

Umbilical cord
Embryo

Uterine cavity

FIG. 52. — Diagrammatic section of uterus with embryo.

divided into three stages. During the first, which usually lasts
from twelve to sixteen hours, the contractions of the uterus,
which cause the chief pains of childbirth, force the amniotic sac,
containing its fluid and the embryo, down toward the cervix,
the muscles of which are gradually relaxing, to permit the fetus
to go farther and farther through. Finally, the amniotic sac
breaks, and its fluid pours out of the vagina. This "breaking of
the waters" usually occurs near the end of the first stage of
labor. During the second stage, which usually lasts about an
hour, the baby passes through the cervix and vagina and is born.
Finally, in the last fifteen minutes of labor, the amnion and

Human Reproduction 201

chorion pass out of the uterus and vagina, constituting the
"afterbirth."

In civilized countries, the structure and activity of most women
have apparently been modified to such an extent that bearing
children is much more difficult for them than it is for animals
and for the women of some savage races. Often the doctor,

Placenta

Chorion
Amnion

Cervix

FIG. 53. — Human fetus in uterus just prior to birth.

known as an obstetrician, who nowadays is usually present at
childbirth, must assist the process by using pairs of pincers, or
forceps, with which he takes hold of the child. Occasionally, he
increases the contractions of the uterus by administering drugs,
one of which is an extract of the pituitary gland. In some cases
the fetus is too large to pass through the set of bones, known as
the pelvic arch, through which the vagina passes. In that case the
obstetrician must cut through the front wall of the abdomen, and
take out the fetus. This is known as a Caesarian operation, and
it is commonly supposed that Julius Caesar was born in this fash-

2O2 . Human Reproduction

ion. Although it is not a dangerous operation, it leaves the ab-
dominal walls weakened. Hence, a woman who cannot safely
give birth to children in the usual way is ordinarily sterilized after
the second Caesarian delivery.

How Twins Are Produced. — Although, as a rule, but one baby
is born at a time, twins sometimes appear, as a result of devia-
tions from the normal method of conception and childbirth.
These deviations are of two very different types, and lead to the
production of two different kinds of twins, known as fraternal or
non-identical, and identical twins.

Non-identical twins are no more alike than ordinary brothers
and sisters, and have in common only their age. They are pro-
duced by the releasing of two eggs at the same time. If these are
both fertilized, they become embedded, one on either side of the
uterus, and develop there side by side. This production of two
eggs at once may be inherited as a tendency, since there are
mothers who produce more twins than single children, and whose
daughters also tend to be twin-producers.

Identical twins are produced from the same fertilized egg.
Exactly how this happens in women we are not quite sure, but
the most likely method has been discovered in the Texas armadillo,
which regularly produces identical quadruplets. Here the embryo
is split at a very early stage into four parts, each of which con-
tinues development independently and finally produces a fully
formed young armadillo. Our best evidence that this split can
also occur in the human embryo is that of Siamese twins, which
evidently are the result of such a split which has not been quite
complete. Occasionally, moreover, odd monstrosities are born
with two heads, four arms, or similar duplications of other parts.
These must be the result of the partial splitting of the early
embryo.

Identical twins are usually very much alike, both physically
and mentally, and in many cases cannot be told apart, even by their
best friends. They have often been studied in order to determine
the relative effect of heredity and environment on intelligence and
personality, since in their case the inheritance of each is exactly the
same.

One interesting peculiarity of identical twins is that they are
often the mirror image of each other. If one is right-, the other is

Human Reproduction 203

left-handed. Slight differences in the features of the two sides
of the face, which we all possess, are similarly mirrored. The
right side of the face of one twin looks more like the left side of
the other, and vice versa. This peculiarity is, of course, quite clear
to us if we consider the twins to have resulted from the splitting
of a single embryo, since the right side of one twin and the left
side of the other were originally destined to be the two sides of
the same individual.

The offspring of a multiple birth, i.e., triplets, quadruplets, etc.,
may be all fraternal, all identical, or some may be fraternal and
others identical. The famous Dionne quintuplets have been pro-
nounced all identical.

^The Hormones of the Gonads. — In both $exes the organs
which produce the gametes, that is, the sperms and eggs, are
called gonads, a term based on the Greek verb meaning "to be
born." The ovaries are the female gonads, the testes, the male
gonads, and they constitute the primary sex organs in either sex.
Hormones are produced in both. The manner in which the ovarian
hormones control the menstrual cycle has already been described.
But these hormones, especially the one formed in the Graafian f ok
licle, also regulate the development of secondary sexual charac-,
teristics, and the hormones formed in the interstitial cells of the
testes do the same thing for men.

The secondary sexual characteristics are all those characteristics
that differentiate the male from the female besides the mere pos-
session of ovaries and testes. Their development begins in the
embryo. At the earliest period of embryonic growth the structures
that are going to develop into the sex organs are no different in
men than in women. But at a fairly early stage the testes become
differentiated from the ovaries and begin to secrete their hor-
mones which stimulate the growth of the structures that develop
into the penis and prostate gland of the male. It is not known
whether the female hormones are produced in the embryonic
stage or whether the sex organs of the female develop simply
because the male hormones are missing. At birth, the chief
differences between boys and girls are the presence of the male
and female sex organs in a rudimentary stage of development, a
somewhat smaller average size among girls and slightly narrower
hips among the boys.

2O4 Human Reproduction

No further differences appear until, at the age of puberty, the
sex hormones begin to be formed in great quantities. In the girl,
the uterus and vagina increase in size and the menstrual cycle
sets in; the hips widen to make room for the bearing of children,
the breasts increase in size, and a growth of hair occurs under
the armpits and about the external sex organs. A deposit of fat
is laid down between the skin and muscles, which gives to the
woman's body its characteristic rounded contours. It is this fat
deposit which enables women to be comparatively better at swim-
ming than at any other sport, since it decreases the specific
gravity of their bodies. All these changes are brought about by
stimulation from the ovarian hormones.

The adolescent boy, under the influence of the hormones from
the interstitial cells, undergoes an entirely different development.
The penis and prostate gland enlarge and sperms begin to form,
shoulders broaden, while the hips remain narrow; hair appears
more profusely and in a different pattern than in the woman,
the outstanding difference being the growth of the beard; the
voice becomes deeper, and the muscles become firmer. The muscles
of a man are able to burn energy much more rapidly than those
of a woman, and consequently the male is capable of greater
feats of strength; but it is claimed that because men use up their
energy more rapidly than women they are less capable of going
without food and sleep.

Among domestic animals, castration, that is, removal of the
testes, is frequently practiced, and always prevents the appear-
ance of male characteristics. In the capon and in other fowls
this is done to keep the muscles of the bird soft and good for
eating. In the case of the horse, however, the operation is per-
formed in order to keep him from developing the untamable
spirit of the stallion. In various lands and times, it has been the
practice to castrate certain boys to provide eunuchs for harems,
choirs or other institutions. These eunuchs grow up lacking in
the secondary sexual characteristics of the male. They are beard-
less, have high-pitched voices and are usually fat and flabby.

The testicular hormone apparently performs its major func-
tions during the few years that a man is reaching sexual maturity.
Loss of the testes after this time may fail to produce any changes
at all in the male, except, of course, that failure to produce

Human Reproduction 205

sperms results in complete sterility. There may be no loss what-
ever in sexual desire or capacity for sexual intercourse ; and when
such loss does take place, it is probably due to the individual's
mental attitude, rather than to physiological changes.

In the female, loss of the ovaries or their failure to produce
hormones after the menopause does result in certain slight modi-
fications in the uterus and some disappearance of secondary sexual
characteristics. At the time of the menopause, ill health, irri-
tability, and emotional depression may appear. This may in part
be due to a lack of balance in hormone secretions, and it can be
controlled to some extent by administration of theelin or some-
times of thyroxin until the balance reestablishes itself. But the
menopause does not necessarily result in loss of sexual desire or
sexual capacity; and when such a change does occur, it is likely
that mental attitude plays a considerable part in producing it.

It is now known that the production of testicular and ovarian
hormones is stimulated by a hormone produced in the pituitary
gland, known as the gonado tropic hormone because it stimulates
growth and activity of the gonads. It is the hormone that really
initiates the changes that occur during puberty, although it
does not itself produce secondary sexual characteristics. The
presence of testicular or ovarian hormones in the blood inhibits
the formation of this pituitary hormone, with the result that as
these hormones are produced, the stimulation for their production
falls off. It is thought that the timing of the menstrual cycle is
dependent on this interaction between pituitary and ovarian
hormones.

L^-The Hormones in Pregnancy and Child-bearing. — The
bodily changes that take place during pregnancy, the activities of
child-bearing, and the beginning of the flow of milk, already
prepared for by the growth of the mammary glands during preg-
nancy, are all regulated by a number of interacting hormones
produced in the pituitary, the ovaries, and the placenta. The
nature of this interaction is not completely known at present, and
it is so complex that we shall make no attempt to describe it. One
interesting change in hormone secretion during pregnancy is the
appearance in the blopd and urine, within a few days after the
failure of the menstrual period, of an unprecedented amount of
the gonadotropic hormone. Evidence indicates that the increase

2o6 Human Reproduction

in the production of this hormone is a result not so much of in-
creased pituitary activity as of manufacture of the hormone in a
new structure, the placenta. Since the mere missing of a men-
strual period is never a certain sign of pregnancy — irregular
menstruation is not at all rare in women — the presence of the
gonadotropic hormone in the urine is the most certain early indi-
cation of pregnancy. In order to make a test, small amounts of
the urine are injected into female rabbits, and, if the woman is
pregnant, the animals become sexually active within a few days
through the stimulation of the gonadotropic hormone in the
urine. By this simple test doctors can make an assured diagnosis
of pregnancy much earlier than was formerly possible.

Venereal Diseases. — Gonorrhea and syphilis, the two diseases
which are usually contracted through sexual intercourse with an
infected person, constitute one of the major health problems of
the present time. Gonorrhea is caused by a coccus which attacks
the mucous membranes. These membranes in any part of the
body are subject to infection, but those of the genital tract are
the usual sites. Ordinarily it attacks the urethra in the male and
from there makes its way into the prostate, seminal vesicles, and
vasa deferentia. It may result in a closing off of the latter tubes,
producing sterility. In the female it first attacks the membranes
of the vagina and may then make its way into the uterus, the
Fallopian tubes, and even into the body cavity, where it attacks
the intestinal linings. Occasionally in both sexes it enters the
blood stream and, infecting the joints, produces a severe form of
rheumatism. In the male it makes its presence known within a
few days after infection by producing a severe pain during urina-
tion. In the female it may go undetected until it has infected al-
most the entire genital tract, when it begins to form pus pockets
that result in pain and fever. Treatment, which was formerly
somewhat difficult, has been made more certain and easy with
the development of the sulfa drugs and penicillin. It is important
that a good doctor be consulted as soon as symptoms appear.
Frequently much harm is done through attempts at self -treatment
or reliance upon quack doctors.

When a mother is infected with gonorrhea, the disease may
attack the mucous membranes of the eyes of her newborn child.
In the past this condition was one of the more frequent sources

Human Reproduction 207

of blindness. A few drops of silver nitrate solution in the eyes of
the newborn child effectively prevent gonorrheal blindness, and
in many states such antiseptic treatment is required by law for
every newborn child.

Syphilis is a disease of the blood, caused by a bacterial organ-
ism, known as a spirochete, which can enter the body through a
sore or cut, but which usually enters through the thin mem-
branes of the vagina or those at the end of the penis. When un-
treated, its symptoms appear in three stages. Two or three weeks
after infection, a small hard elevation, known as a chancre, ap-
pears at the point where the infection entered. This is the primary
stage. The chancre now disappears, and in the course of six to
twelve weeks the secondary stage begins. Its symptoms are vari-
able; there may be a swelling of the lymph glands, a rash cover-
ing the body, and fever. The secondary symptoms usually dis-
appear, whether the individual is treated or not, and then several
years may elapse before the tertiary symptoms begin to under-
mine the health of the victim. Now the spirochetes may attack
almost any organ of the body. They may produce hardening
and weakening of the arterial walls in restricted regions through-
out the circulatory system, although they do not cause general
arteriosclerosis. They may attack the heart, the stomach, kid-
neys, liver, and pancreas. One of the most ordinary types of in-
sanity is produced by syphilitic attacks upon the tissues of the
brain.

Syphilis is a very subtle enemy. Occasionally the primary and
secondary stages are so slight that the individual never knows
that he has contracted it. Even after prolonged treatment it may
not be entirely eradicated; and since it may be present in the
blood for several years without displaying any symptoms, an
individual may have it without knowing it. It is possible at any
time, however, to make a blood test, called the Wassermann test,
to determine whether the germ is present and whether treatment
should be started or continued. The treatment, which usually em-
bodies the injection of arsenic compounds into the blood to kill
the spirochete, must frequently be prolonged for two years or
more; and even after a patient is discharged, he should return
occasionally to his doctor for a Wassermann test. All too fre-
quently a patient stops going to his physician as soon as the sec-

208 Human Reproduction

ondary symptoms disappear; but the spirochete may still remain
in the blood, and treatment should never be stopped until the
Wassermann is consistently negative.

Syphilis is often called a "hereditary disease." This designa-
tion is not exact, since the term "hereditary " should apply only
to characteristics of the organism that are determined by the
material present in the zygote at the time of fertilization. The
fetus, however, can acquire the disease from an infected mother,
and many children are born with syphilis and hence are said to
have acquired it congenitally. Usually the fetus is born dead
before the normal time. When birth does take place, the symp-
toms of syphilis appear in the child, and syphilitic insanity may
develop. Congenital syphilis can be treated and cured, or it can
be entirely avoided if treatment of the mother begins at least
five months before the time of birth. To be on the safe side,
the Wassermann test should be a part of the early medical exami-
nation of every pregnant mother.

Largely because of the unwillingness of the public to face
squarely the issue of eradicating venereal disease, a great deal of
unnecessary suffering and illness has been occasioned. The fact
that the sulfa drugs are especially helpful in both preventing and
curing gonorrhea may make possible its final elimination. If peni-
cillin proves to be as effective against syphilis as it promises 'to
be, it may provide the necessary leverage for the eradication of
that disease.

Nevertheless, to stamp out venereal disease in America will re-
quire a vigorous public health campaign. Clinics must be estab-
lished where those who cannot afford to pay may secure free
examination and cure, both for their own sakes and for the pro-
tection of the entire public. Wassermann tests should be adminis-
tered as a routine measure to all individuals applying for mar-
riage licenses and to all persons working in occupations where
they might pass syphilis on to others. Barbers, cooks, and food
handlers in general fall into this latter category. Employers can
improve the efficiency of their employees by requiring medical
examinations including examination for venereal disease and at
the same time be of assistance in eradicating these diseases from
the population.

Finally, the public should be educated to seek examination for

Human Reproduction 209

and cure of venereal disease as readily as it seeks to be rid of
other dangerous ailments. Since venereal disease can be con-
tracted by other means than through sexual intercourse, there is
no implication of immorality in the request that one submit one-
self along with others to a routine examination. Public education
in avoidance ot venereal disease should accompany education
for detection and cure. The surest and best way to avoid it is to
avoid sexual intercourse with individuals of promiscuous sexual
habits. But such habits are so widespread in our population that
merely recommending abstinence will not go far enough in stamp-
ing out these diseases. It is possible to take precaution against
them both during and after sexual intercourse. Such precaution,
medically termed prophylaxis, is best effected under the direc-
tion and with the help of a competent physician, and no indi-
vidual should expose himself to infection in this way without
securing competent medical advice on prophylaxis. Public pro-
phylactic clinics are needed to encourage the employment of
prophylaxis throughout the population. At the same time, it
should be made clear to everyone that the best of prophylactic
measures may fail, and that the preferable method of prevention
is abstinence.

The Normal Sex Life. — The consequences of venereal disease
have often been so much impressed upon young people that they
develop an attitude of fear and disgust toward the entire range
of sexual life. Or feelings of anxiety about sex may develop
on other grounds. This is unfortunate, for the sexual function is
one that is peculiarly capable of bringing pleasure and happiness
to human beings, and sex is never a "problem" unless through
ignorance and fear we make it so. The sexual life needs only
to be intelligently controlled and regulated to bring to us some
of the greatest satisfactions that life can offer.

The sexual relationship should be looked upon as one of the
good things of life, to be enjoyed as fully as possible as long
as its enjoyment does not endanger one's own welfare and that
of others. By enjoyment of this relationship, we mean much more
than the purely physical satisfactions resulting from stimulation
of the sex organs. The emotional satisfactions, which to a certain
extent are built up around this physical core, are generally con-
ceded to be far greater than the purely physical ones. To be sure,

2io Human Reproduction

it is difficult to draw a hard and fast line between physical and
emotional satisfactions, and with happily married people they
blend together, the one enhancing the other. But it is possible for
physical sexual satisfaction to take place without experiencing any
of the thrill of being with an attractive person of the opposite sex
or the joy and pride of mutual love; similarly, these emotional
concomitants of sexual activity can be at least partially expe-
rienced in the absence of the physical relationship.

Considerations both of morality and of prudence make it de-
sirable for unmarried people to abstain from the physical satis-
factions of sexual intercourse. This is difficult for many because
of the great strength of the sexual appetite. But often the dif-
ficulty is doubled by a more or less conscious feeling that the
practice of chastity deprives one of the greatest of human pleas-
ures. Through misinterpretation of modern psychological find-
ings, some people have even obtained the impression that chastity
in itself may be the cause of mental and nervous breakdown.
The truth is that, at least among those who have not become
habituated to sexual intercourse, personal happiness and a good
adjustment to life do not seem to be highly dependent upon
physical sexual satisfaction. The really important sexual satis-
factions are the emotional ones, and no young man or woman
needs to miss them. The pleasures of companionship and social
relations with members of the opposite sex, the consummate
pleasures of falling in love and being in love are among the
greatest that human beings can enjoy. Older people often look
back upon them as "the happiest time of their lives/' The en-
joyment of such happy personal relationships between young
people lays the foundation for happy relationships between hus-
bands and wives, for happiness in marriage is more dependent
upon a successful emotional adjustment than upon the purely
physical side of the marital relationship.

In all but an exceptional few, however, the purely physical
sexual drive will not remain in abeyance during the years be-
tween puberty and marriage. It usually expresses itself in volup-
tuous dreams which, in the male, frequently culminate in the
ejaculation of semen. Occasionally boys or men get to worrying
about these dreams, believing that the loss of semen is "robbing
them of their manhood." This anxiety is encouraged by medical

Human Reproduction 211

quacks who offer to cure them of their ailment. Actually, it is
only a normal physical outlet, and its occurrence is practically
universal.

Another form of physical expression which occurs in the ma-
jority of both men and women at some time during their lives
is self-induced stimulation of the sex organs. Traditionally, it
has been called masturbation, but since this designation implies
a false notion of the harmfulness of the practice, we shall use
the more scientific term, auto-erotism. In the past, this practice
has been looked upon with abhorrence, and young people have
been assured that it is the cause of everything from insanity to
shifty eyes and cold, clammy hands. All reliable authorities now
hold that auto-erotism is of no harm to anyone unless he worries
about it. Perhaps it is the most common source of sexual anxiety.
Not only does the young person fear its consequences, but he is
likely to get the idea that he is lacking in moral character and
will power. However, when ninety per cent of men and seventy-
five per cent of women admit in confidential questionnaires that
they have practiced auto-erotism — and the likelihood is that most
of the others just won't admit it — the "victim" of this habit can
at least console himself that his will power and moral character
are no worse than those of the rest of the human race. When
the individual ceases to worry about auto-ero:ism, it tends to
lose any emotional interest for him, and he may simply outgrow
it. The recommended attitude is to exercise as much self-control
as possible and not to worry about lapses.

Frequently sexual daydreams lead up to auto-erotism or occur
independently of it. While they are harmless enough if they do
not occur too frequently, they occasionally come to dominate
the thoughts of the individual to an undesirable extent. This
means that he is allowing the purely sexual aspects of life, and
among them the purely physical aspects of sex, to become too im-
portant. It is as if he spent all his time thinking about food or
clothing or some other aspect of life which, important enough
in itself, should normally constitute only a subordinate part of
existence. The person who experiences continual sexual daydreams
or who thinks about sex continuously needs to take more interest
in his work, his play, his social life. Sexual interest should nor-
mally express itself chiefly in social relationships, with real mem-

212 Human Reproduction

bers of the opposite sex, not dream lovers; and as the young
man or woman falls in love and marries, it should come to be
centered in a particular individual

In marriage, the sexual aspect of the partnership is obviously
only a part of the relationship; yet it is an important aspect of
taarital happiness. The art of sexual love-making is frequently
not understood by married people, with the result that one or
both of the partners finds the sexual relation unsatisfactory. This
is almost never the result of an incapacity for complete sexual
intercourse, but usually of a lack of knowledge concerning it. At
the present time it is possible to secure a number of books which
describe proper techniques for sexual relations, and some knowl-
edge of the matter should be in the possession of every married
couple.

In conclusion, the most normal attitude toward the sexual life
is to seek to enjoy it as completely as possible within the limits
set by the moral code of the group in which you live. Do not be
afraid to think about sex or to talk about it, under circumstances
where such discussion is in good taste. At the same time, do not
be obsessed with it, or think of it as an all-important aspect of
life. If you are concerned with some personal sexual problem, dis-
cuss it confidentially with someone who has some knowledge con-
cerning sexual hygiene. The chances are that the problem will fail
to appear as important after the discussion as it now does to
you, or at least that some good method of solution can be found.
X*Tlie Control of Population. — With the advance of knowledge
concerning the reproductive function, various methods have been
developed to prevent fertilization of the egg subsequent to sexual
intercourse. Collectively they are referred to as contraception or
birth control. Contraception is usually effected by some method
of keeping the sperm cells from reaching the egg. There are
various state and national laws which make it illegal to spread in-
formation concerning the techniques of birth control, and some
states do not even permit doctors to give this information when
it might save a human life. It is safe to say that the majority of
Americans — and by far the majority of well-educated Americans
— favor birth control ; but the general indifference of the popula-
tion, combined with strong pressure from certain minority groups,
has resulted in the retention of laws prohibiting the spread of

Human Reproduction 213

contraceptive information. In recent years, judicial decisions have
tended to interpret these laws very liberally, clinics have been
established in many of our cities where people can go for medi-
cally necessary information, and thinly disguised advertisements
of contraceptive techniques are widely distributed. Unfortunately,
the safest and most convenient and effective methods are the most
difficult to disguise in advertising. In spite of all laws, contracep-
tion is very widely practiced, especially among the wealthier and
better-educated sections of the populace.

With the advance of knowledge concerning the time of ovula-
tion during the menstrual cycle, it has been suggested that birth
control be effected by limiting intercourse to times during which
the egg would presumably not be present in the Fallopian tubes or
uterus. Apparently it is not illegal to describe this method of
birth control, since pamphlets suggesting its use have been widely
and openly distributed in the United States mails. It is probably
also legal to say that it is one of the least certain of all methods.
The menstrual cycle does not work like a clock. Its timing varies
from one woman to another and from one time to another in an
individual woman. The times given in our foregoing discussion of
the cycle are only averages. Least certain of all is the period of
ovulation, and "going on time" is a very ineffective method of
avoiding fertilization and pregnancy.

The advantages of birth control are that it enables husbands
and wives to plan to have children at times when the mother is
in good health and the family budget adequate to care for the
new member of the family. Inability to control the rate at which
children are born has meant untold suffering, ill health, and pre-
mature death to millions of women. Without contraception, the
only feasible method of avoiding suffering and economic priva-
tion for millions of families in which the woman readily be-
comes pregnant is abstention from sexual intercourse; but this
method is all too likely to result in emotional strain and marital
unhappiness.

The Catholic Church, and along with it many individuals of
other faiths, holds that the employment of birth control methods
is morally wrong. Some believe that to prevent the sperm from
uniting with the egg after sexual intercourse is tantamount to
murder. Polls of public opinion, however, have shown that

2I4 Human Reproduction

most people in this country disagree with this point of view.
The writers of this book are definitely on the side of this ma-
jority, but they do feel that married people ought to consider
it both a privilege and a moral obligation to bring as many chil-
dren into the world as their health and financial opportunities
permit. To use contraceptive techniques simply to escape the
bother of raising children is a form of short-sighted selfishness
that may not only result in a loss of real happiness on the part
of the potential parents but be of grave disservice to society
as a whole.

Birth control, like so many other types of scientific knowledge,
has provided us with a power that can be of the greatest benefit
to human beings. But like all other forms of power, it can be
used for ill as well as for good. It has enabled many families to
avoid tragedy and suffering, but it has also made it possible for
individuals to avoid the responsibilities of parenthood, to decide
upon "a new car, rather than a baby/' thus impoverishing their
own lives and resulting in an undesirable decrease in the birth
rate among certain portions of the population. In every civilized
country in the world, even in countries where the most stringent
legal enactments against birth control are in effect, the spread of
its use seems eventually to result in a positive decline in the growth
of population among the classes in which birth control is widely
employed. While most people want children, most of them are
satisfied with one or two, and among a group of people in which
the two-child family is standard, the population is bound to de-
cline. In America, the wealthiest and most highly educated ele-
ments of the population are not replacing themselves, and increase
in numbers comes from the poorer and less well-educated groups
where birth control is not so widely practiced.

This phenomenon, known as the differential birth rate among
classes, commonly appears in a country in which the use of con-
traception is gradually spreading. It is well-nigh universal in
civilized countries today. As a result, the greater proportion of
the population comes from families who are unable to offer the
best cultural and educational advantages to their children. It may
be also that the hereditary capacities which the more rapidly re-
producing part of the population passes on to its children are
inferior to those of the part that practices birth control. Our

Human Reproduction 215

present knowledge of heredity is not sufficient for us to be cer-
tain of this, but it seems highly probable that it should be the
case. At any rate, the differential birth rate produces a definitely
undesirable condition of affairs.

As birth control information spreads throughout a population,
the differential birth rate tends to disappear. The birth rate of the
entire country falls so low that deaths exceed births, and the
population declines. Some decline of population would probably
be a good thing in many countries; but there is scant reason to
believe that it would be desirable in our own country, and even-
tually it would lead to the virtual annihilation of the race if it
was not checked in some manner.

"Abolishing birth control" is no solution for this problem. In-
deed, such a program would be almost impossible to carry out,
since people who have once enjoyed its advantages will offer a
very determined resistance toward giving it up. The best solution
seems to be, first, to eliminate the differential birth rate by ac-
tively disseminating birth control information among the less
well-informed members of our population, rather than by at-
tempting to keep that information from them; second, to en-
courage a higher birth rate among the more able people, thus
reversing the differential birth rate and encouraging the produc-
tion of a better, rather than a poorer, race.

Just how to encourage a high birth rate is a matter to be
worked out by trial and experience. Through taxation, financial
handicaps can be placed upon well-to-do people who do not have
children, and the funds so secured could be used to subsidize
early marriages and early births among able young men and women
whose desire for an advanced education now leads them to post-
pone marriage and children. More important still would be some
sort of propaganda that would make people in the professional
and business classes willing to undergo financial handicaps and
other sacrifices in order to have a large family. As long as a young
lawyer feels that his status in the community is raised more
through the possession of a new Buick than through having four
healthy, well-reared children, lawyers and other professional peo-
ple will buy Buicks rather than have children. We have seen
that, by proper public health propaganda, the attitudes of people
toward matters of cleanliness and sanitation can be changed. To-

216 Human Reproduction

day there is just as much call for a change in attitudes toward
the having of children.

Individuals cannot bring about these changes. They must be
brought about by governmental and social agencies. Again we
discover that the full value of scientific discovery can be secured
only through intelligent social and political action.

CHAPTER SUMMARY

Sexual reproduction is the only kind found in human beings.
In the male, sperms formed within paired organs, the testes, are
conducted through the epididymes and vasa deferentia to the semi-
nal vesicles. Here they are mixed with fluid from the prostate
gland, and ejaculated through the urethra, a duct which passes
through the penis.

In the female, eggs are liberated once a month from the ovaries.
They are surrounded by a Graafian follicle, which ruptures to
liberate the egg. The egg passes down the Fallopian tube, where
it may be fertilized. If not fertilized, it is shed through the vagina,
and later on the breakdown of the uterine wall brings about the
process of menstruation.

The development of the uterine wall, as well as menstruation,
is governed by a hormone from the Graafian follicle. Later the
Graafian follicle becomes the corpus luteum, an endocrine gland
which secretes a hormone influencing the bodily changes during
pregnancy.

If the egg is fertilized, it becomes embedded in the wall of the
uterus and develops. Three stages of development are described :

1. At two or three days, a round mass of cells.

2. At four weeks, a fish-like embryo, length 4 mm.

3. At eight weeks after fertilization, a fetus with the human
form completely developed ; length i inch.

The fetus is surrounded by two membranes, the amnion and
chorion, which are part of it, and the inner of which contains
the amniotic fluid. The membranes are connected to the fetus by
means of the umbilical cord. The chorion is closely connected to
the uterine walls by means of the placenta, through which the
fetus receives its nourishment from the mother's blood.

At the end of 280 days, birth takes place through contraction of
uterine walls and relaxation of cervix and vagina.

Human Reproduction 217

There are two types of twins : fraternal, or non-identical, and
identical twins. The former are produced by the simultaneous
fertilization of two different eggs, and the development side by
side of the resulting embryos. The latter are probably the result
of the splitting of a single embryo at an early age. Identical twins
are remarkably similar in physical and mental characteristics.

The hormones produced in the gonads, that is, the testes and
ovaries, stimulate the development of secondary sexual charac-
teristics. In the male the secondary characteristics are : the penis
and prostate gland; low voice; a characteristic male growth of
hair, including the beard; broad shoulders, narrow hips, and hard
muscles which burn oxygen more rapidly than those of the female.
In the female they are : uterus and vagina, well-developed breasts,
female pattern of hair growth, broad hips, and a deposit of fat
between the skin and the muscles.

The production of the testicular and ovarian hormones is
stimulated by the gonadotropic hormone of the pituitary. The
gonadal hormones inhibit the formation of the pituitary hormone,
so that there is a cyclical interaction between the two. A complex
system of interacting hormones governs the development of the
mother's body during pregnancy.

The venereal diseases, syphilis and gonorrhea, constitute one
of the most important public health problems of the present day.
The establishment of clinics for their prevention and cure, the
use of routine tests for their detection, and the education of the
public in means of avoiding and curing them are essential if they
are to be eradicated.

The most intelligent program for the sex life of the unmarried
adult is abstention from sexual intercourse, and enjoyment of
emotional sexual satisfactions in the form of social relationships
with members of the opposite sex. No apprehension should be felt
concerning sexual dreams or nocturnal emissions, or concerning
failure to achieve complete self-control over impulses to auto-
erotism, popularly termed masturbation.

The practice of contraception, or birth control, has been of
great benefit to human beings in that it has enabled them to avoid
having children when considerations of health or economic neces-
sity make it advisable. It has, however, resulted in a differential
birth rate between the classes, and public measures to spread birth

218 Human Reproduction

control information among the less well-educated classes and to
increase the birth rate among the better-educated classes are nec-
essary at the present time.

QUESTIONS

1. Describe the structure and activity of the reproductive organs in
man.

2. Describe the menstrual cycle in woman, naming all of the struc-
tures involved, with their functions, and a brief account of the
mechanism that causes the regularity of the cycle.

3. Give an account of the development of the human embryo and
fetus, including a description of its appearance in at least three
stages of its development.

4. What is the relation of the human embryo and fetus to the
uterus and to its mother, and how does it obtain its nourish-
ment?

5. What is the difference in relative appearance and in the mode
of origin of identical and non-identical twins?

6. What are the secondary sexual characteristics and how are they
produced ?

7. What is the basis of the "mouse test" for pregnancy?

8. Describe the causes, symptoms, cures, and possible methods of
prevention for venereal diseases.

9. Discuss the matter of an intelligent program for the guidance
of the sex life.

10. What is contraception? What are its advantages? What prob-
lems have arisen as a result of it?

GLOSSARY

amnion (am'ni-on) The inner of the two membranes surrounding
the embryo and fetus.

auto-erotism (o'to-er'6-tiz'm) Self-induced sexual stimulation. Mas-
turbation.

cervix (ser'vix) A muscular constriction at the base of the uterus,
which opens into the vagina.

chancre (shan'ker) A sore or ulcer appearing at the point where
syphilitic organisms have obtained entrance into the system.

chorion (ko'ri-on) The outer of the two membranes surrounding the
embryo and fetus.

clitoris (kli'to-ris) A small organ in the upper part of the vulva. It
develops from the same structures as the male penis.

CHAPTER XI

REPRODUCTION IN PLANTS AND ANIMALS

All Life Comes from Life. — Reproduction is a universal
characteristic of all organisms. Until about fifty years ago, many
people believed in the spontaneous generation of living things.
They thought that organisms might be formed spontaneously out
of such non-living substances as horsehair, decaying vegetation,
and dung. But with the invention of the microscope, and after
much careful research, it was definitely shown that even such
lowly forms of life as bacteria and Protozoa come into existence
only by a reproductive process. To be sure, it is still thought that
hundreds of millions of years ago life slowly evolved from in-
organic materials ; and it may be that, even today, certain almost
inconceivably primitive living things may be forming out of
non-living matter. But no organism that we know of is created in
this manner, since any organism that could be formed out of in-
organic materials would probably be so small as to be undetectable
under the most high-powered microscopes. It is only through
reproduction that the countless species that are studied by the
biologist can come into existence.

The type of reproduction found in man, however, is the result
of a long train of evolutionary development, and countless other
forms of reproduction are found. In only a few forms of animals
are the young carried in the body, attached to the uterus by means
of a placenta. Frequently the egg is fertilized entirely outside the
body of the mother, and copulation, that is, sexual intercourse,
does not occur. Indeed, sex is by no means a necessary feature of
the reproductive process, and we shall begin our study of repro-
duction among plants and animals with a description of various
forms of asexual reproduction, that is, reproduction without the
fusion of gametes to form a zygote.

221

222 Reproduction in Plants and Animals

ASEXUAL REPRODUCTION

Reproduction by Cell Division. — Among one-celled organ-
isms, a frequent type of reproduction is by simple division of one
cell into two or more. In Protococcus, the protoplasm inside the
cell wall simply divides in two parts and a cross wall is formed
between them. The daughter cells thus produced usually separate ;
and as a result of this simple reproductive process, known as fis-
sion, two new individuals have come into existence and the parent
cell has lost its identity.

Fission is also the common method of reproduction not only
among all the one-celled algae, but also among the bacteria. Each
bacterial cell merely divides into two, a cross wall is formed and
the reproductive process is finished. Such fission often takes place
several times an hour, so that the number of bacterial generations
in a day reaches unbelievable proportions. With the Protozoa, also,
reproduction by simple cell division is the rule. Among the yeast
plants there is a modified form of fission known as budding. A
small projection is formed on the parent cell ; this gradually en-
larges, and is then cut off by cell division, eventually separating as
a new generation.

Division of the Multicellular Body. — Among multicellular
organisms, special bodily structures may grow which, breaking or
being cut off from the parent, form new organisms ; or the parent
may be divided to form new organisms without the formation of
special structures. This method of asexual reproduction occurs
frequently in plants and is not unknown among the lower animals.
The following is a list of varieties of this type of reproduction:

I. Regeneration. — Among many of the lower animals, a part
of the animal that is lost may grow out again from the point at
which it was broken off. This process is called regeneration. The
regeneration of legs occurs in insects, crabs, starfish and various
other forms. One animal, known as the brittle starfish, protects
itself by breaking all its long, slender legs in pieces whenever it is
attacked, whereupon the comparatively small central part lies
under a rock and awaits the regeneration of new legs. Some ani-
mals possess so great a power of regeneration that they can
reproduce by breaking or being broken into many parts, with each
part regenerating to form a whole new animal. This method is

Reproduction in Plants and Animals 223

most common among certain simple animals called flatworms. If
a flatworm is cut into two hundred pieces, each piece will regener-
ate all the rest of the animal and become a new individual.

2. Budding. — In a few animals, a new organism may grow as
a bud upon the parent, finally breaking off and beginning an
independent existence. This is one of the ways in which Hydra
reproduces, as shown in Fig. 23.

3. Runners. — Those who have seen strawberries growing are
familiar with the fact that each strawberry plant can put out long,
horizontal, leafless stems. These stems run along the ground for
two or three feet, then root at the tips. A new strawberry plant
grows up at the tips of the runners. As soon as this plant has
reached a good size the strawberry grower can cut the runner and
separate the plants, while if left alone, the runners die away. Many
other plants can reproduce by means of runners ; and the difficulty
of getting rid of such weeds as crab grass and hawk weed is largely
due to the fact that they can quickly spread over a large field by
means of runners.

4. Rootstocks. — Rootstocks are long, scaly, underground sterns
that often look like roots, and enable the plant to spread in the
same manner that runners do. Witch or quack grass is one of the
persistent weeds that spreads itself by this method.

5. Tubers. — A farmer grows new white potato plants by plac-
ing in the ground sections of a potato. The botanist knows that the
potato is not a seed. Potato seeds are produced from the white
flowers which appear on the tops of potato plants in late summer.
The potato is, then, merely a swollen underground stem which con-
tains a large amount of stored food. Hence, when the farmer
plants a potato, he is planting about the same part of the plant as
he is when he plants a rose cutting. In the native home of the
potato, each tuber survives the winter underground, producing a
new plant the following spring.

Besides these natural methods of asexual reproduction, garden-
ers have long been accustomed to spreading cultivated plants in
still other ways, the most important of which are :

i. Separating. — Whenever plants have thick or fleshy parts —
such as roots, stems, or bulbs below ground, these may be broken
apart at the right season and both parts will grow into a new plant.
By this method, the same individual may be spread all over the

224 Reproduction in Plants and Animals

world and kept alive indefinitely. The cultivated saffron crocus,
for example, cannot be reproduced by seed, yet this variety has
been grown and propagated by separating the bulbs since the time
of the ancient Cretans 4,000 years ago.

2. Cuttings. — If a willow twig is cut and placed upright in the
ground, it will sprout roots at its base, while the buds at its tip will
grow and produce leaves. Finally, if the surroundings are favor-
able, the twig will grow into a new willow tree. Roses are also
propagated by cuttings, or slips; and sugar cane is grown by
planting sections of cane in the ground, which then grow into new
plants.

3. Grafting. — Grafting is the process of splicing a branch of a
desirable variety of tree on to the cut stem of another that is
growing. If care is taken to place the two cut ends together so that
the growing layer of one coincides with that of the other, they
will grow together, and the branch will form the whole top of the
tree, bearing its own particular variety of fruit, since its character
is not changed in the slightest by its residence on a foreign stock.
This method is extremely valuable for obtaining good fruit from
a tree of poor heritage, and must be used in propagating such
valuable fruit trees as the seedless orange.

Practically all of the organisms which may reproduce by the
above asexual methods can also reproduce sexually. Indeed, it is
not at all uncommon for organisms to be able to reproduce in sev-
eral different ways.

Parthenogenesis. — One form of asexual reproduction, known
as parthenogenesis, is peculiar in that it is obviously an evolutionary
development from sexual reproduction. The females of the species
produce eggs which develop without having been fertilized by
a sperm cell. Frequently some of the offspring in a given species
are produced sexually, and others parthenogenetically. A queen
bee, if her eggs are fertilized, produces either workers or new
queens ; but she sometimes lays eggs that have not been fertilized
which develop by parthenogenesis into males. Natural partheno-
genesis is particularly characteristic of the insects, although it is
not unknown among other forms. It occurs also in a few plants,
such as the dandelion, in which seeds may be formed without the
assistance of male structures.

The eggs of many animals can, moreover, be stimulated to

Reproduction in Plants and Animals 225

parthenogenetic development. By the action of various chemicals
sea urchin eggs can be made to divide and produce active young
animals. The eggs of starfish, as well as those of some marine
worms, have also been caused to produce young parthenogenet-
ically, the method varying with the animal. This phenomenon is
known as artificial parthenogenesis.

Formation of Spores. — The most important type of asexual
reproduction in plants is by means of spores. This method occurs
in almost all kinds of plants, from the bacteria and unicellular
algae to the largest trees, although in many forms it alternates
with sexual reproduction. Spore-formation occurs also in one
group of Protozoa, but is not found in other animals. A spore is
a single cell, frequently covered by a tough cell wall to prevent
drying out, which is capable of traveling about until it reaches a
favorable location; thereupon it germinates, growing into a new
organism. In many of the lower forms of plant life, spores may
be formed in almost any cell simply by division of the cell into
several other cells. In most of the higher fungi and filamentous
algae, as well as in the land plants, however, they grow in special
organs, known as sporangia. Among the fungi, the sporangia
usually constitute the most conspicuous part of the plant, the re-
mainder being composed of filaments embedded in the host or in
the medium on which the fungus lives.

In the case of the mushrooms, the subterranean mass of color-
less vegetative filaments develops an upward-growing stalk which
eventually expands into a cap. The under side of the cap is made
up of many radiating partitions, upon each of which thousands
of spores are produced. These spores are scattered in millions by
the wind, escaping in clouds of fine "dust" such as can be seen in
the ordinary puffball, a relative of the mushroom which bears its
spores within a tough outer coat. When a mushroom spore settles
upon a suitable substratum, it germinates into a new underground
mass of filaments. Thus the common cap and stalk of the mush-
room is but an elaborate structure to produce spores.

The formation of spores constitutes an exceptionally effective
method of reproduction. In the first place, the tiny spores may be
formed in incredible numbers. In the second place, they can be
carried about in the air until they reach some spot favorable for

226 Reproduction in Plants and Animals

the plant's growth, and thus the plant organism can take advantage
of every opportunity for life that the environment affords. The
air is at all times filled with spores of various types of fungi. A
piece of bread left in almost any exposed position will in a few
days show a covering of mold. The mold is a fungus which ger-
minates from spores that are universally present in the air. Finally,
spores are effective for reproduction because they are so resistant
to heat, cold and drying. A change in its environment may kill the
parent plant, but its spores may retain their capacity for germina-
tion over a course of years until they finally reach an environment
in which the plant can grow.

SEXUAL REPRODUCTION AND ALTERNATION OF GENERATIONS

The Evolution of Sex. — Even in the Protozoa and unicellular
algae simple types of sexual reproduction are found, usually in

Zoospores

Vegetative

filament of

UJothrix

Fusion of
gametes

S

Zygote

FIG. 54. — Reproduction in Ulothrix.

organisms that reproduce asexually as well. In all probability, the
first living things reproduced asexually, and sexual reproduction
evolved from the asexual type. It is thought that this course of
evolution took place more than once and in more than one way;
but among the algae we find certain transition steps between asex-

Reproduction in Plants and Animals

227

ual and sexual reproduction which suggest the method by which
sexual reproduction might develop from spore formation.

Many of the algae produce a type of spore that, instead of
floating in the air, swims about in the water, appearing exactly
like a unicellular flagellate plant. Because of their motility, they
are called zoospores, which means animal-like spores. For exam-
ple, in the filamentous green alga, Ulothrix, any cell of the fila-
ment can undergo division inside its cell wall to form several
zoospores, each of which has four flagella. The zoospores escape

Egg

Gamete-forming
cells

FIG. 55. — Reproduction in Oedogonium.

through a hole in the cell wall and swim about in various directions
until they have traveled some distance from the parent plant. Then
each spore comes to rest and grows by cell division into a new
Ulothrix filament.

At other times, however, the Ulothrix cell breaks up into a
larger number of smaller swimming cells, each of which has two,
rather than four, flagella. These cells escape from the cell wall and
swim about, just as the larger ones do ; but before they begin to
divide to form a new filament, each one fuses with another of the
same kind, and the new filament develops from the cell that is
formed by the fusion of two of the tiny, two-flagellated swimmers.

Certainly this is not a great change from the formation and
germination of the four-flagellated zoospores. Nevertheless, the

228 Reproduction in Plants and Animals

border line between asexual and sexual reproduction has been
crossed. The cells that fuse before germination are no longer
zoospores, they are gametes, and the cell they form is a zygote.
For any reproductive cell that must fuse with another before
growth of a new organism takes place is a gamete, the cell formed
by the fusion is a zygote, and the type of reproduction involved is
sexual.

But although Ulothrix reproduces sexually, there are no sexes.
It would be impossible to say which of the two gametes was the
sperm or the egg, since they are exactly alike. In another filamen-
tous alga, Oedogonium, however, the differentiation into egg and
sperm appears. In preparing for sexual reproduction, certain cells
of Oedogonium transform their contents into large gametes, in-
capable of movement and therefore forced to remain within the
cells in which they are formed. Each of these large gametes is
abundantly provided with stored food. Other smaller cells of the
filament produce relatively smaller gametes, in greater numbers;
each of these has a circle of cilia at one end. When released, these
gametes can swim about; eventually one finds the opening in the
cell wall surrounding a large gamete, enters and fuses with it to
form a zygote. This is destined to grow into another Oedogonium
filament. When such gametes are formed, differing in size and
capabilities, we see a stage in the evolution of sexual reproduction
wherein one large motionless gamete functions as a receptive
"egg," while a smaller, active gamete becomes a typical "sperm."
This type of sexual reproduction has not, however, reached the
condition found in higher plants, where the gametes are produced
by special organs rather than by individual cells of the body.

The latter evolutionary advance is to be seen in many other
algae, notably the brown and red seaweeds, where the plant body
has segregated the reproductive ability to certain special cells and
tissues which become sex organs, whose sole function is to pro-
duce the gametes. The remaining vegetative cells are incapable of
doing so. Male and female sex organs are at first found on the
same individual, but further specialization has resulted in the
appearance of these organs in different individuals. Thus sex
becomes the natural result of segregation of the gamete-producing
organs upon different individuals.

Reproduction in Plants and Animals 225

The Alternation of Sexual and Asexual Reproduction. —

In the more primitive thallus plants, sexual and asexual reproduc-
tion seem to occur independently of each other, often on the same

Sperm-
producing
organ

Spore
capsule

FIG. 56. — Reproductive cycle in a moss.

plant, as environmental conditions vary. But in the higher algae,
the liverworts and mosses, and all plants above them in the scale
of complexity, there is a definite alternation of a plant reproducing
sexually with one reproducing asexually by means of spores. The

230

Reproduction in Plants and Animals

plant producing these spores is called a sporophyte; likewise the
gamete-producing plant is known as the gametophyte. In the se-
quence of reproductive events, a sporophyte gives rise to a

Sperm

Spore

Adult gametophyti

Embryonic
gametophyte

FIG. 57. — Reproductive cycle of a fern.

gametophyte, a gametophyte to a sporophyte. Yet both gameto-
phyte and sporophyte can reproduce vegetatively at various times.
In the common moss we see a small erect green plant, with
leaf-like structures arranged around an upright stem. When the
plant is ready to reproduce, sex organs are developed at the tip of

Reproduction in Plants and Animals 231

the stem, surrounded by "leaves." The sperms emerge by the
hundreds from the sausage-shaped male sex organs, and in wet
weather they wriggle over the film of water that covers the moss
until they are Attracted by chemical exudations from the flask-
shaped female sex organs. They then enter the long neck of the
flask, swim down it, and fertilize the single egg found at the bot-
tom. The zygote germinates in this place. It does not grow into
another green moss plant, but into a brown leafless plant, consist-
ing of a stalk with a spore-containing capsule at the tip. Thus the
gametophyte of a moss is a green, leafy plant; and the sporophyte
is a dependent plant getting its sustenance from the gametophyte.
When the spores fall to the ground they grow into a leafy green
moss gametophyte. Thus there is a regular alternation of sexual
and asexual generations.

In a typical fern this rhythm of reproduction involves two gen-
erations which are both green. The leafy fern plant is a sporophyte,
producing spores in clusters of sporangia usually found on the
under side of the fern leaves, and looking like rusty brown spots.
The spores, after they fall to the ground, germinate into little
heart-shaped green plants about the size of a dime. These cling
to the damp earth, attached by rhizoids on their under surface.
They are the gametophytes, producing male and female sex organs
much like those of the moss, hidden among the rhizoids. The
sperms swim to the female sex organs and there fertilize the eggs.
The zygote develops into a new leafy fern plant, another sporo-
phyte generation. It is important to note that in the ferns, the
spore-producing plant is leafy and independent, in contrast to the
leafless sporophyte of the mosses. The gametophyte, on the other
hand, has become relatively less important.

The Gametophytes of the Flowering Plants. — In the group
of seed plants, which includes most of the plants familiar to us,
such as trees, grasses and flowers, the sporophyte generation is
the only one visible to the naked eye, the gametophyte having
become so reduced and inconspicuous that special microscopic study
is necessary in order to detect it at all. The visible parts of the
reproductive structures are all part of the sporophyte. In all seed
plants except the conifers and a few relatively little-known types
this reproductive structure is known scientifically as the flower,
even though in many of them, such as the grasses and various

232

Pollen e«U

Reproduction in Plants and Animals

nudeua
Egg nucleus

Enlarged
ovary

Fruit

Embryonic sporophyte
FIG. 58. — Reproductive cycle of a flowering plant.

Reproduction in Plants and Animals 233

trees, its parts have become so reduced or modified that the layman
does not ordinarily think of it as such. A typical flower consists
of four sets of parts, the sepals, petals, stamens and pistils. The
sepals and petals are accessory parts of the flower, not necessary
for the actual reproductive process. The former are usually green
and useful in protecting the flower when in bud; the latter are
frequently brightly colored and aid in attracting animals for pol-
lination. More important are the stamens and pistils, for they
produce the spores and gametes. Just as in sexual reproduction we
found a progression from organisms producing gametes that are
all alike to those whose gametes are differentiated into large and
small types, so in the evolution of asexual reproduction by spores
as a prelude to sexual reproduction, the condition where all the
spores are alike gradually changes into one in which the spores are
differentiated into large and small types. The latter condition holds
true in some of the relatives of the ferns, but is most characteristic
of the seed plants. The small spores grow into gametophytes which
produce only male organs, while the large spores grow into
gametophytes which are female.

Returning to our flowering plant, we find that the stamens
produce, in the pollen sacs (in reality sporangia), a large number
of minute pollen grains, which are developed directly from spores.1
When the flower opens, these pollen grains are shed from the
pollen sacs, and are transported, either by the wind or by animal
agencies, to the pistil of another flower. Here they land on a sticky
surface which holds them firmly, and stimulates the growth from
each pollen grain of a long slender filament of protoplasm, called
the pollen tube. These pollen tubes penetrate down the pistil to its
lower portion called the ovary which contains one or more ovules.
Each ovule encloses within a series of protective layers the vestiges
of a sporangium and, when young, one or more large spores. Each
of these large spores later develops within its coverings into a
structure consisting of several nuclei, known as the embryo sac —
in actuality, the female gametophyte. One of the embryo sac nuclei
is the female, or egg cell. The male gametophyte is, at its greatest
extent, merely the germinated pollen grain, with its long tube that

pollen grain when first formed in the young bud is a spore; but when
the flower opens each grain has become two-celled, and is therefore a young
gametophyte.

234 Reproduction in Plants and Animals

finally reaches the ovule, enters it, and penetrates to the embryo
sac. The male gamete, a single small nucleus, then passes down
this tube, emerges from it, and fertilizes the egg; and a similar
nucleus fuses with a second nucleus of the embryo sac, which gives
rise to nourishing tissue for the embryo. Fertilization in the higher
plants therefore takes place far within the sporophytic tissue of
the female reproductive organ. The only visible act of union, that
of pollination, makes the contact between the immature male
gametophyte (the pollen grain) and the sporophytic tissue of the
pistil.

Thus in the flowering plants, the sexual generation has been
reduced to the lowest possible terms, being an inconspicuous and
microscopic phase of the reproductive cycle. The sporophyte, on
the other hand, has become the dominant generation. The evolu-
tionary value of this reduction of the gametophyte for land vegeta-
tion is obvious. Reproduction by motile sperms when the parent
organisms are incapable of motion, is practically impossible for
land plants. Asexual reproduction, however, by structures capable
of aerial dissemination, is far more likely to be successful. Thus
reproduction for flowering plants is the culmination of sporophytic
reproduction, with the flower as the most complex reproductive
organ evolved by any sporophyte. The stamens and pistils, in
reality spore-producing organs, have come to be considered male
and female sex organs, since the male and female gametophytes
produced by them are generally unknown to the layman.

How Pollination Takes Place. — Although, as we have just
described, both male and female organs exist together in a typical
flower, they are not, as a rule, self-fertilized. Instead, the pollen
of one flower, after being released by the anther, is carried in some
way to the stigma of another. In land plants the two ways by which
pollen is usually carried are by wind and by insects. The oak tree,
as well as most other trees in this region and most grasses, pro-
duces great quantities of pollen which is carried long distances by
the wind until a few grains happen to reach the stigma capping the
pistil of some other plant. Plants with conspicuous flowers, how-
ever, depend on insects to carry the pollen from flower to flower.
The color and odor of flowers, as well as the fantastic forms of
many of them, such as the orchids, are simply devices which in-

Pollen grains in stamen. The small, dark objects within each grain are the

nuclei.

Reproduction in Plants and Animals 235

duce the insects to visit them, thereby transporting the pollen from
one plant to another.

As an example of the extraordinary devices that plants have
developed to insure cross pollination by insects, let us look at one
of the more complex types, that of the lady's-slipper. In this orchid,
one of the petals is modified into a large inflated sac, or lip, seamed
with creases and with an opening at the top. In the showy lady's-
slipper, which is our present model, this lip shades from white to
a delicate shell pink, and, flanked by broad sepals and petals of pure
white, forms perhaps the most beautiful and aristocratic of our
wild flowers. The bee, attracted by the color of the flower and its
faint but delicious fragrance, alights on top of the lip. Its eye is
attracted by the white creases that line the surface, and it follows
them until it drops into the hole in the center. Once in, it is pre-
vented by recurved flanges from escaping from this hole, but it
soon sees the light of two small openings at the base of the lip,
toward which it crawls. Before it reaches these exits, however, it
must push its way, first under the arched knob of the stigma,
against which it rubs, depositing pollen obtained from the flower
last visited; and secondly, against the rounded surface of one of
the two anthers, from which it picks up a sticky mass of pollen for
distribution to the next flower. Thus the bee cannot escape from
the trap of the lady's-slipper's lip until it has cross pollinated the
flower. There are hundreds of other mechanisms for cross pollina-
tion, adapted to hundreds of different insects, but always the
principle is the same. The insect must first reach the stigma to
deposit foreign pollen on it, and later the anthers, from which it
receives a new load of pollen.

The Seed. — As soon as the sperm nucleus has united with the
egg nucleus, the fertilized egg, or zygote, divides, and starts the
growth of the embryo. At the same time, another nucleus of the
gametophyte generation, which has also been fertilized by a nu-
cleus from the pollen tube, divides rapidly to produce a large
tissue of food-storing cells, known as the endosperm, which obtains
nourishment through the stalk of the ovule and supplies it to the
embryo. Meanwhile the outer coverings of the ovule grow to keep
pace with the increasing size of the embryo and endosperm; and
the whole ovary is growing larger and larger, soon becoming, as
the other parts gradually wither away, the most prominent part of

236 Reproduction in Plants and Animals

the flower. Finally, when the embryo has finished growing and
is surrounded with the densely packed food material of the endo-
sperm, the outer coverings become hard and tough, and the whole
structure, now a ripe seed, breaks away from its stalk, leaving a
scar which is quite noticeable in seeds such as the bean. The ovary
now opens up, the methods of opening varying with the plant, and
the seeds are scattered, although sometimes the whole ovary is de-
tached from the plant with the seeds.

The ripe seed, then, contains within it an embryo plant, belong-
ing to the new sporophytic generation, and consisting of a short,
rudimentary stem, two simple leaves and a small bud between the
leaves. The embryo is usually surrounded by densely packed food
materials, although these are sometimes stored within the seed
leaves themselves, and is protected by one or more tough seed
coats, remnants of the parental sporophytic generation. The food
thus stored within seeds is the most important source of our own
nourishment. All cereal grains are the seeds of grasses; hence all
flour, bread, and rice are the products of plant seeds. Other im-
portant food-producing seeds are nuts and such vegetables as beans
and peas.

Although such a seed seems quite lifeless, careful chemical tests
have shown that respiration is going on constantly, at an extremely
slow rate, within seeds. In this state of suspended animation, or
dormancy, seeds can withstand great extremes of temperature and
drought. Although some varieties sprout or germinate as soon as
ripe, most of them lie dormant for some time, and cannot germi-
nate, for one reason or another, until after a period of months.
Then, under favorable conditions of warmth, water enters, burst-
ing the seed coat, and setting into activity digestive enzymes that
are packed in with the stored food. As it is digested, the food is
absorbed by the embryo, which, by carrying on respiration rapidly,
provides energy for its growth. First the rudimentary stem elon-
gates, sending down a root into the ground, and pushing the seed
leaves upward until they break through the surface and appear
as the pair of simple, usually round or strap-shaped leaves that are
the first reward and encouragement of all gardeners. In many
plants, such as grasses, lilies, and their relatives, which have only
one seed leaf, and in some, such as the peas, which have two, the
seed leaves remain below the ground ; the part of the plant growing

Reproduction in Plants and Animals 237

upward is the shoot, which has developed from the bud beside the
seed leaf. As soon as the seedling reaches the light, it develops
chlorophyll, turns green, and begins to manufacture its own food
by photosynthesis. By this time the food within the seed is all used
up, and the empty seed coats, if they have not been borne up above
the ground by the growing seedling, shrivel away.

In the seed plants, the seed has taken over the function of dis-
persal of the species that is performed by the spore in the lower
forms. Like the spore, the seed may travel far from the parent
plant, alighting and germinating in new localities in which plants
of its species may grow. Probably this similarity between seed
and spore is what sometimes makes it hard for beginning students
to understand the fundamental biological difference between the
spore, which is a single asexual reproductive cell, and the seed,
which is a multicellular embryo, surrounded by protective and nu-
tritive tissues. Methods of seed dispersal are described in Chap-
ter XV.

Sexual Reproduction in Animals. — While alternation of gen-
erations is the predominating form of reproduction in the higher
plants, sexual reproduction predominates in the animals. It occurs
in a primitive form even in the Protozoa; and most of the many-
celled animals not only produce eggs and sperms, but, like the
human organism, form them in special organs, the ovaries and
testes. However, the unisexual condition characteristic of the
human race in which one type of individual, the male, produces
sperms, and another type, the female, produces eggs is not found
in many of the lower multicellular animals. Instead, each -indi-
vidual possesses both ovaries and testes, and hence is neither male
nor female. Such an individual is known as an hermaphrodite,
after the god Hermes and the goddess Aphrodite, who repre-
sented the male and female principles, respectively, in the my-
thology of ancient Greece. In Hydra, for example, primitive testes
and ovaries appear as swellings on the sides of the body. The
earthworm is also an hermaphrodite. After copulation takes place
between two worms, the sperm cells of each member of the pair
are mixed with the egg cells of the other, and the fertilized eggs
are left behind in a little cocoon-like case in which the embryos
develop.

238 Reproduction in Plants and Animals

Reproduction in the Vertebrates. — In the large group of
animals most closely related to man, namely, the vertebrates, her-
maphroditism does not normally occur. In this group, however, we
find an interesting course of evolutionary advance in reproductive
habits, from the most primitive group, the fishes, to the most ad-
vanced group, the mammals, which of course includes the human
species. While there are exceptions to the rule, we may say that, in
general, as one goes up the scale of vertebrate life, he finds a pro-
gressive decrease in the rate of reproduction, together with an
increase in the amount of care that is given the young. These
changes may be briefly outlined as follows :

1. Fishes: Great numbers of eggs are laid, little provision is
made for certainty of fertilization.

2. Amphibians : Fertilization occurs outside the mother's body,
but copulation between the sexes makes fertilization more certain.

3. Reptiles : Internal fertilization takes place, with nourishment
and protection being given the embryo within an egg covered by
a shell.

4. Birds : Fertilization and egg-laying occur as in the reptiles ;
but after hatching, the young are cared for in nests.

5. Mammals : The young develop within the mother's body and
after birth are fed from the mammary glands and cared for in
various other ways.

For reasons of economy of structures, in the vertebrates the
reproductive and urinary systems are combined into a urogenital
mechanism. In the bony fishes, the gonads and the urinary bladder
both open into a urogenital pore which is posterior to the anus.
When the ovaries release the eggs, they pass to the exterior
through this opening, each egg protected only by a gelatinous mass
which swells after fertilization. Similarly, the sperms are released
as a mass of "milt." Thus fertilization is external, the union of
the eggs and sperms being largely a matter of chance, aided only
by the fact that the sperms are usually released over the place in
the water where the eggs have been laid. To offset this, fishes are
by far the most prolific of all vertebrates. A female codfish lays
nine million eggs a year, and most fish lay hundreds of thousands.
However, with the exception of some of the sharks and their
relatives and a few other fish, fishes are the most negligent in

Reproduction in Plants and Animals 239

the care of their young. The small sticklebacks of our fresh-water
ponds build elaborate nests and care for their young, the father
being almost always the more solicitous parent. The marine cat-
fish father incubates the eggs in his mouth. But these are the
exceptions rather than the rule. The tiny fish hatch from the eggs
a short while after fertilization has taken place, at which time
they may be only a sixteenth of an inch in length and quite help-
less. Thus in the fishes there is little to insure union of the eggs
and sperms, the developing young are relatively unprotected, and
parental care is almost entirely lacking.

An evolutionary advance found in the amphibia is the decided
congregation of the sexes prior to the release of the gametes, thus
guaranteeing to a greater extent the meeting of the sperms and
the eggs. But even more effective is copulation. In the frogs and
toads, for example, the male clasps the female while she is laying
the eggs, and at the same time releases the sperms. Thus, though
fertilization is external, the union of egg and sperm is practically
certain. The reproductive organs of a frog represent the general
plan found in all higher vertebrates, including man. The testes
are small oval organs, found in pairs in each male frog, one under
each of the kidneys. Each testis is connected with a kidney by
many small ducts so that the sperm cells pass from the testes
through the kidneys and out through the ureter, which opens
into the common meeting place known as the cloaca, into which
the anus also opens. The ovaries are likewise beneath the kidneys,
with the coiled oviducts above them. Each oviduct has funnel-
shaped opening which is in the coelomic cavity; the other (pos-
terior) end of the oviduct is slightly enlarged to form a distensible
bag known as the uterus, which in turn connects with the cloaca
by a narrow short tube. Portions of the ureter and the uterus
become united, but their cavities remain distinct. The eggs are
released from the ovary into the body cavity; and when the male
clasps the female tightly with his fore-legs, the eggs are aided in
their forward movement to the funnel-shaped opening of the
oviduct. Once there, they pass into the oviduct and toward the
uterine end, aided by the current created by the ciliated lining of
the oviduct.

Amphibians are somewhat less prolific than fishes, although
many of them lay eggs numbering- into the thousands. They are,

240

Reproduction in Plants and Animals

Lobster

Herring

Sturgeon

Flatworm

Sea urchin

Amphioxus

Pigeon

Frog

Chameleon

Toad

Squirrel
FIG. 59. — Varieties of sperm cells.

Reproduction in Plants and Animals 241

however, more certain in their method of fertilizing the eggs, al-
though as a rule they give the young no care. The fertilized eggs

Fish

Mammal

Hydra

Crayfish

Bird

FIG. 60. — Varieties of egg cells. (The egg of the bird is proportionately much

larger than is shown.)

contain some stored food, but are protected only by an enveloping
jelly. The young hatch from the eggs long before they attain the
adult form; in fact, they are no further along in their existence

242 Reproduction in Plants and Animals

than the four-weeks' human embryo. Thereafter the young tadpole
must obtain its food by its own efforts. The eggs and developing
young are forgotten by the parents as soon as they are laid and
fertilized.

In the reptiles and birds, copulation takes place with the intro-
duction of the sperms into the body of the female. Some of the
salamanders show the transition to this condition in that the male
deposits little packets of sperms which are picked up by the female
and transferred within her body, so that internal fertilization
takes place without copulation. Reproductive progress, insuring
union of the egg and sperm, has advanced from chance external
fertilization in the fishes, more certain external fertilization by
previous copulation in the amphibia, to internal fertilization in
the reptiles, birds and mammals.

Reptiles and birds are much less prolific than amphibians, al-
though the large sea turtles may lay as many as a hundred eggs
at a time. Here reproduction differs from that in lower verte-
brates and in mammals in the method of nourishing and protect-
ing the embryo and the degree of parental care. In the case of the
reptiles, the eggs are laid in the sand or mud, and development
of the embryo takes place at the mercy of the environment, the
eggs being usually neglected by the parents. Most of the reptiles,
like the amphibians and fishes, are oviparous, i.e., the eggs are
laid before fertilization or, if after fertilization, the embryos are
still within the egg membranes and cannot live outside of them.
In a few reptiles, however, the embryos are retained within the
body of the mother, as in all mammals including humans, until
they are capable of an independent existence. Such reptiles are
viviparous but do not really nourish the young through maternal
tissues as mammals do. The embryos remain in the oviducts until
they reach an advanced stage of development, but they are nour-
ished by the food stored in the egg.

In the birds, internal fertilization and the egg-laying habit
have become associated with many valuable breeding activities
such as the building of nests and the incubation of the eggs by
the mother. There is also considerable parental care, so that on
the whole reproductive progress has been t considerable as we pass
from the reptilian to the bird level.

The last great evolutionary advance in reproduction is the

Reproduction in Plants and Animals 243

viviparous condition, with the fertilized egg undergoing its de-
velopment within the maternal tissues, getting its nourishment
from the wall of the uterus. There are certain sharks in which
the yolk sac of the egg becomes attached to the wall of the uterus,
and forms a placenta-like organ through which nourishment is
received from the mother. It is in the mammals, however, that this
condition is most widespread and highly developed. Our considera-
tion of the reproductive organs and reproductive process in man
makes it unnecessary to describe those of other mammals, which
greatly resemble the human structures and functions.

In the case of mammals, the certainty of sperm meeting egg is
the result of the internal fertilization by means of copulation; the
developing embryo is protected by the maternal tissues and nour-
ished by them, thus being removed from the various hostile in-
fluences of the environment; the young, when born, are nourished
by the mother 's milk; and they are taken care of by the parents
and instructed in the activities necessary for their self-preserva-
tion. Such animals as the duckbill and the spiny anteater, both of
which lay eggs, are exceptions among mammals, as are also the
marsupials, in which group is the kangaroo, where the young are
born in an immature state and carried about in the mother's pouch
until they are able to take care of themselves.

Such are the various stages in reproductive specialization which
link the algae with the oak, the Protozoa with man. Through them
we can see a consistent progress from unspecialized reproductive
structures and activities, the transition from asexual to sexual
reproduction, the origin of sex, and the gradual improvement
in insuring that the union of the gametes will guarantee that the
zygote will develop into a mature individual.

CHAPTER SUMMARY

Reproduction is a universal characteristic of living things, but
the methods by which new individuals come into existence vary
in the complexity of the structures and activities concerned. The
simplest type of reproduction is by cell division, common among
the unicellular organisms ; this occurs in Protococcus, Paramecium
and the bacteria. The formation and separation of a new organism
by cell division is called fission. This is a form of asexual repro-
duction, without the fusion of a pair of cells (gametes) to form

244 Reproduction in Plants and Animals

the new individual. Asexual reproduction undoubtedly is the most
primitive type of reproduction.

In the multicellular organisms, there are various ways in which
asexual reproduction may take place :

1. By regeneration, where a part broken off an animal can
grow into a new animal, as in the case of the starfish.

2. By budding, where a new organism develops as a bud upon
the parent, and finally separates to lead its own existence, as in
Hydra.

3. By runners, where creeping stems form new plants, as in the
strawberry and various grasses.

4. By rootstocks, or underground stems, as in witch grass.

5. By tubers, or underground stems swollen with stored food,
as in the white potato.

6. By various artificial methods among plants, such as grafting.

Parthenogenesis is sometimes considered a form of asexual re-
production. It is the development of an egg without fertilization
by sperm, and is a common phenomenon among the insects.

The most important type of asexual reproduction is by spores,
a spore being a specialized reproductive cell which can develop
into a new organism without needing to unite with another re-
productive cell. Spores are common among plants, being produced
in special spore sacs known as sporangia. Such minute asexual
reproductive bodies are found among the algae, fungi, mosses,
liverworts and ferns. Spores are an effective means of reproduc-
tion in that they can be produced in great quantities, they are
small and light enough to be carried great distances by the wind,
and they are very resistant to desiccation and temperature changes.

Sexual reproduction involves two kinds of reproductive cells,
known as gametes. How these may have originated is suggested
by the behavior of a green alga, Ulothrix. In this plant, asexual
reproduction is by means of swimming zoospores having four
flagella. An alternate method of reproduction is the formation of
many more, smaller swimming cells with two fiagella each ; these
fuse in pairs before germination. Hence they are gametes and
form a zygote.

In Ulothrix both the gametes are alike ; there is sexual repro-
duction but no sex. In another green alga, Oedogonium, we see

Reproduction in Plants and Animals 245

a beginning of sexual differentiation of the gametes into a larger
immotile female gamete (egg) and a smaller, active male gamete
(sperm). Another step in the evolution of sexual reproduction is
the formation of these male and female gametes in special repro-
ductive organs, rather than in ordinary vegetative tissues. Still
more advanced is the condition where the male organs are segre-
gated upon one individual, the female upon another.

In plants there is an alternation of sexual and asexual repro-
duction ; the plant generation which reproduces asexually is called
the sporophyte, that which reproduces sexually is called the game-
tophyte. At the lower level of plant organization, the gametophyte
is green, often leafy, and independent, as in the moss. At the tip
of the plant the sperms and eggs are borne in special sex organs.
After fertilization the egg develops in the place of fertilization to
form a brown, dependent sporophyte reproducing by spores pro-
duced in a terminal sporangium. These spores, falling to the
ground, grow into another gametophyte.

In a common fern plant, the gametophyte is relatively smaller
and less conspicuous, though still green. The fertilized egg grows
into a sporophyte which is large, leafy and green, the common
plant known as the fern. This produces spores which fall to the
ground and repeat the cycle, growing into new gametophytes.

In the seed plants the gametophyte has become colorless, para-
sitic and (in the case of the female gametophyte) microscopic.
The leafy green plant with roots, stems and leaves is a sporophyte.
The reproductive organ is the flower with four parts :

1. Sepals, usually green, forming an outermost whorl of struc-
tures for protection of the rest of the flower.

2. Petals, usually colored, aiding in attracting insects or other
animals for the dispersal of pollen.

3. Pistil, for the production of large immotile spores.

4. Stamens, for the production of small spores capable of dis-
semination by wind or other agencies, commonly known as pollen.
A small spore grows into a small male gametophyte, at its maxi-
mum development appearing as a pollen tube with several nuclei,
the sperm cell being reduced to a nuclear mass. A large spore
grows into a microscopic female gametophyte within the pistil,
eventually producing a nucleus which functions as an egg cell.

246 Reproduction in Plants and Animals

Fertilization takes place when a male nucleus from the pollen tube
unites with the egg nucleus in the pistil.

After fertilization, the zygote develops into a small embryo
surrounded by stored food and protected by various coats; all
of this makes up the seed. The tissues surrounding the original
egg, and the bottom of the pistil, develop into the fruit which
usually surrounds the seed.

Among animals, sexual reproduction predominates ; it is found
even in the Protozoa. Among the lower invertebrates, both sex
organs are found on the same individual, resulting in a hermaph-
roditic condition. In Hydra, for example, both types of gonads
are produced on an individual ; the sperm cells fertilize the eggs
while the latter are in the ovary.

Among the vertebrates, the gonads are generally on different
individuals. As we progress from fishes to mammals, reproduc-
tive advance involves a decrease in the rate of reproduction with
an increase in the care given to the young, and the assurance that
fertilization will take place.

Among the fishes great numbers of eggs are laid, but little pro-
vision is taken to insure fertilization, since the sperms are re-
leased into the water where a female has previously released eggs,
and fertilization takes place. After fertilization, the eggs are left
relatively unprotected and there is very little parental care.

Among the amphibia there is copulation between the sexes prior
to fertilization, so that even though fertilization is external, there
is considerable guarantee that the eggs will be fertilized. The re-
productive system of a frog is basically the design for all higher
types of vertebrates including man. The reproductive and urinary
systems combine to form a urogenital system; in the male frog,
the sperms pass from the testes (located below the kidneys)
through the kidneys and out through the ureter to the external
opening ; in the female, the ovaries, likewise beneath the kidneys,
release eggs into the body cavity, where they find their way into
the funnel-shaped opening of the oviduct, and thence to the ex-
terior near the anal opening. Fewer eggs are formed than in the
fishes, but like them the frogs leave their eggs relatively unpro-
tected and there is little parental care!

In the reptiles and birds, copulation results in the introduction
of the sperm into the body of the female, with resulting guarantee

Reproduction in Plants and Animals 247

that fertilization will take place. These two groups of vertebrates
are less prolific than the amphibia or fishes; the eggs are pro-
tected by a shell, and in the case of birds there is considerable
care of the young.

The last important evolutionary advance of sexual reproduc-
tion in the higher animals is the appearance of viviparity — the
development of the embryo within the body of the mother, and
its release when developed sufficiently to adjust itself to an ex-
ternal existence. During prenatal life the embryo is nourished by
the maternal tissues. And in the case of man especially, there is
added to the certainty of fertilization by copulation and the in-
ternal growth of the embryo, the last and most important aspect of
reproduction — care and education of the young until they are
capable of taking care of themselves.

QUESTIONS

1. What is meant by spontaneous generation? Do you believe it pos-
sible today ? Why ?

2. Describe fission as a type of asexual reproduction.

3. Name five ways in which multicellular organisms can reproduce
asexually, other than by fission or spores.

4. What is a spore? Describe its formation and behavior in (a)
mushroom, (b) Ulothrix, (c) moss.

5. Define parthenogenesis. Where does it occur?

6. Describe the transition from asexual to sexual reproduction in
Ulothrix.

7. What advance in sexual reproduction is shown by (a) Oedogo-
nium, (b) moss?

8. Define alternation of generations.

9. Compare the gametophyte of (a) moss, (b) fern, (c) oak.

10. Compare the sporophyte of (a) moss, (b) fern, (c) oak.

11. Describe the reproductive organ of the sporophyte of a flowering
plant.

12. Describe sexual reproduction in a flowering plant.

13. What is a seed?

14. What is an hermaphrodite?

15. Summarize the important evolutionary changes in reproduction
found among the vertebrates.

1 6. Describe the urogenital apparatus of a male frog.

17. In what ways is sexual reproduction in amphibia superior to
that in the fishes ?

Reproduction in Plants and Animals

18. In what ways is reproduction in the birds more advanced than
that of the amphibia?

19. In what ways is reproduction in mammals more advanced than
that of the birds?

GLOSSARY

asexual reproduction Formation of new individuals without fusion of
gametes.

budding A form of asexual reproduction, found in Hydra and yeast,
during which a new organism develops as a bud upon the parent,
later separates.

cloaca (clo-a'ca) The common chamber into which the intestinal,
urinary and genital canals discharge in birds, reptiles, amphibians
and many fishes.

copulation (cop-u-la'shun) The coming together of the two sexes in
physical contact prior to release of sperms and eggs ; sexual in-
tercourse.

embryo sac The female gametophyte of a flowering plant.

endosperm (en'do-spurm) The nutritive tissue formed within the
embryo sac in the development of the seed.

fission A form of asexual reproduction, common among the uni-
cellular organisms, which results in new individuals by simple cell
division.

gametophyte (ga-me'to-fit) The plant generation reproducing sex-
ually.

germinate To begin to grow or develop, especially in the case of a
spore or seed.

grafting An artificial form of asexual reproduction used with the
higher plants to perpetuate a desired variety.

.hermaphrodite (her-maf'ro-dit) An animal with both male and fe-
male sex organs.

Oedogonium (e'do-go'ni-um) A filamentous green alga whose ga-
metes are differentiated into sperm and egg.

ovary (in flowering plants) A region at the base of the pistil in
which one or more ovules are found. After fertilization of the
eggs in the ovule, it often develops into the fruit.

oviduct (6'vi-dukt) A duct for the passage of eggs from the ovary
of an animal to the exterior.

oviparous (6-vip'a-rus) A reproductive habit involving the exclu-
sion of eggs from the body prior to their hatching.

ovule (6'vul) Sporangium of a flowering plant producing the large
spores which develop into female gametophytes.

Reproduction in Plants and Animals 249

parthenogenesis (par'the-no-gen'e-sis) Development of an egg with-
out fertilization.
petal One of the inner leaf-like structures of the flower. Usually

conspicuously colored.
pistil (pis'til) The part of a flower producing ovules and female

gametophytes.

pollen tube The male gametophyte of the flowering plant. It de-
velops from a pollen grain on the stigma, and grows down through

the tissues of the pistil to make contact with an embryo sac in

one of the ovules.
pollination The transfer of male gametophytes (pollen grains) to

the stigma of the pistil.
regeneration The restoration of lost parts by certain animals such

as the starfish ; also a form of asexual reproduction.
runner A long, creeping stem of plants, used as a means of asexual

reproduction.
seed A structure consisting of an embryo surrounded usually by

stored food material and one or more seed coats, which serves

the purpose of reproduction in the higher plants.
sepal (se'pal) One of the outermost leaf -like structures on a flower,

functioning for protection.
sporangium (spo-ran'ji-um) pi. sporangia A structure in which spores

are formed.

spore A cell specialized for asexual reproduction.
sporophyte (spor'6-fit) The plant generation reproducing asexually.
stamen (sta'men) Part of a flower producing small spores and male

gametophytes (pollen grains).
stigma (stig'ma) The surface at the top of the pistil which receives

the pollen, usually covered by a sticky substance.
tuber (tu'ber) A swollen underground stem containing stored food

and used for asexual reproduction.
Ulothrix (u'16-thriks) A filamentous green alga that reproduces by

means of spores and undifferentiated gametes.
urogenital pore (u'ro-gen'i-tal) Common opening from the genital

and urinary organs in fishes.
viviparous (vi-vip'a-rus) A reproductive habit involving internal

development of the embryo, as in mammals.
soospore (zo'6-spor) A swimming spore.

CHAPTER XII

THE REPRODUCTIVE CYCLE

Cell Division and Growth. — In the previous chapter we
learned that sexual reproduction in one form or another occurs in
all but the simplest types of organisms (and in some degenerate
ones) and that the essentials of this process are the formation of
the sex cells, or gametes, and their union to form the zygote. As
soon as fertilization is completed, the newly formed zygote splits
to form two cells ; these then divide to form four, and the process
of cell division goes on until the trillions of cells which form the
adult body are produced. Hence we may say that the growth
of an individual is essentially a process of cell division, together
with the growth of individual cells.

Mitosis. — Practically every cell division which takes place in
the formation of an individual goes on according to a certain plan
of procedure, known as mitosis, which divides the nucleus in such
a manner that each of the new cells, or daughter cells, receives the
identical sort of nuclear material that the original, or mother,
cell had. The nucleus contains several different kinds of protoplasm
organized into a very definite pattern, associated with its impor-
tant function of transmitting hereditary characteristics; and the
process of mitosis is essential to the reproduction of the same
pattern of organization in the nucleus of every cell in the body.

Before mitosis begins, the nucleus is more or less completely
filled with a number of long, slender, coiled threads, which are of
a thicker consistency than the surrounding protoplasm, and, either
throughout their length or in certain regions (depending on the
organism studied), stain more darkly when cells are prepared for
study by the usual methods (Fig. 61,1). They are known as
chromosomal threads.1

1 In most fixed and stained preparations of nuclei, and therefore in most pub-
lished drawings and descriptions of them, these threads are apparently connected
to each other by slender processes so that they seem to form a continuous net-
work throughout the nucleus. The existence of these connections in living cells,

250

The Reproductive Cycle 251

When a cell gets ready to divide, these threads can be seen to
form a definite number of bodies known as chromosomes, each of
which is believed to be composed of two — or, according to some
investigators, four — threads. At any rate, when they appear, each
chromosome is a double body which, however, behaves as a unit.
Each is composed of two identical halves lying side by side, di-
vided from each other by a fissure which runs the entire length
of the chromosome. At first they appear as long, spiral, ribbon-
like bands (Fig. 61,2), but they gradually contract, so that when
the nucleus is finally ready for division, each chromosome is a
thick, straight or curving rod, which is very dark in the usual
stained preparations (Fig. 61,3).

Although the chromosomes are clearly visible only during mi-
tosis, their presence within the nucleus in some form or other
at all times has now been proved beyond all doubt. Hence, their
number and structure are normally the same in every cell of cm
individual organism. Furthermore, the number and structure of
the chromosomes that appear in the cells of one kind or species of
organism is usually the same as that in every other individual of
the species. In other words, nearly every kind or species of organ-
ism is characterized by a definite number of chromosomes, which
appears whenever one of its cells divides. Thus the onion plant has
sixteen chromosomes, while Indian corn has twenty; cattle have
thirty-eight chromosomes, and man, forty-eight. sVx

When the chomosomes are fully contracted, the membrane sur-
rounding the nucleus disappears, and there is formed a spindle-
shaped structure which is very narrow at its two ends, or poles,
and broadest at the middle, or equator. It is composed of a firmer
type of protoplasm than the surrounding cytoplasm, and, in killed
and stained cells, appears to be traversed by a number of threads,
the spindle fibers. The chromosomes range themselves along the
equator of this spindle, so that they are spread out over a flat
surface traversing the middle of the cell (Fig. 61,4). The halves
of each chromosome then split apart and move to opposite poles
of the spindle (Fig. 61,5). Finally, the two sets of half, or
daughter, chromosomes become grouped about either pole. Grad-
ually they elongate, lose their strong staining capacity, and be-
however, is uncertain; at any rate, since they have no known function, their
presence or absence is probably of little significance.

Centrosome

Nucleolus
Chromatin

1. Resting cell

2. Spireme formed

3. Chromosomes
formed and split

'5. Chromosomes
at poles

4. Chromosomes at
equator of spindle

6. New nucleus formed,
cytoplasm dividing

7. Daughter resting cells

FIG. 61. — Diagram of cell undergoing mitosis.
252

The Reproductive Cycle 253

come reorganized into the series of coiled threads characteristic of
the resting nucleus. Meanwhile a new nuclear membrane has
formed about each of these new, or daughter, nuclei. Since each
parent chromosome has contributed half of itself to each daugh-
ter nucleus, each of the new nuclei has exactly the same number
and kind of chromosomes that the original nucleus had.

While the daughter nuclei are forming, a groove appears around
the edge of the cell on the line of the equator. This groove grows
deeper and deeper, until the cell is completely divided in two. Each
daughter cell grows until it has reached the size of the mother
cell, when it in turn repeats the process of mitosis, and so on,
throughout the growth of the individual.2

The time taken by mitosis, as determined by observations of
living cells, varies greatly in different organisms and under dif-
ferent external conditions, but in most tissues under normal con-
ditions the process occupies between one and two hours. Most of
this period is taken up by the preparation of the nucleus for divi-
sion and the reorganization of the daughter nuclei. The actual
process of division, from the disappearance of the nuclear mem-
brane until the arrival of the daughter chromosomes at the poles
of the spindle, generally takes less than half an hour, and in
rapidly dividing animal tissues at high temperatures may be
carried through in three or four minutes.

Now, one may ask, why should the cell go through this com-
plicated process whenever it divides? The end result is an equal
distribution of the chromosomal material of the mother nucleus
to the daughter nuclei. When the chromosome is ready to divide,
it is exactly symmetrical in its internal structure, so that each
daughter chromosome contains the same substances in exactly
the same arrangement as did the original chromosome. Since the
daughter chromosomes later form the daughter nuclei, the latter
bodies have the same material and the same organization as did
the mother nucleus. This equal distribution of the chromosomal
substance is of supreme importance, since this substance governs
the passing on of hereditary traits. Through mitosis every cell
comes to possess the hereditary substance necessary for every
characteristic of the organism.

* This account describes mitosis as it occurs in the animal cell. The details are
somewhat different for plant cells, but the fundamentals are the same.

254 The Reproductive Cycle

The following facts, therefore, are the important ones to be
remembered :

1. Mitosis occurs whenever cells divide (except in bacteria and
others of the lower organisms, and in a few types of de-
generate tissue).

2. It is characterized by the appearance of the chromosomes
as clearly visible structures.

3. It secures an equal division of the chromosomes, and there-
fore of the hereditary, material of the nucleus.

Since the chromosomes are the most important of the structures
found in mitosis, the following facts about them should be clearly
understood :

1. Although they are always present in some form in all nuclei,
they are almost never clearly visible except when cells di-
vide. (There is at least one exception to this rule. Chromo-
somes can be seen in the resting nuclei of the salivary glands
of flies.)

2. During cell division, they appear as rods which stain deeply
with certain dyes.

3. Their number is the same in all of the normal body cells of
an organism, and is usually the same in all normal indi-
viduals of a particular species.

4. They are the bearers of most hereditary characteristics. The
connection of the chromosomes with heredity will be dis-
cussed in the following chapter.

The Germ Cells. — As the young embryo grows, the new cells
formed by mitosis take on varying sizes, shapes, and functions,
until, in the fully developed fetus, all the great variety of tissues
that characterize the human being have developed, all from a
single cell. This development of various kinds of cells is called
differentiation. We do not wish to follow this intricate process
except briefly to trace the origin of those cells that eventually de-
velop into gametes. When the embryo is about three weeks old,
two small groups of cells, located near the kidneys, differentiate
themselves from the rest of the tissues of the body. They continue
to divide, but the cells resulting from their division do not develop
into bodily tissues. The sole function of this group of cells is
to develop into gametes, and consequently they are called germ
cells. Gradually, the gonads are formed to contain these

The Reproductive Cycle 255

cells, the testes in the male and the ovaries in the female. Within
the gonads the germ cells continue to divide, until millions of them
are produced. Finally, as the individual arrives at the age of
puberty, these cells begin to develop into gametes.
x^Meiosis and the Reduction of the Chromosome Number. —
Now, up to this point, all of the cell divisions which have oc-
curred, from the very first splitting of the zygote, have been mi-
totic divisions, so that each germ cell contains chromosomes that
are identical in structure and composition with those that were
found in the zygote. But in the formation of the gametes a process
called meiosis takes place, which consists of two mitotic cell divi-
sions that differ in interesting and important ways from ordinary
mitosis. We may illustrate how these take place by describing how,
in the sperm-bearing tubules of the testis, sperms are formed
from the sperm mother cells, which constitute the last stage in
the development of the male germ cells before they finally develop
into sperms.

If, during any mitotic division, the forty-eight chromosomes
of the human nucleus are carefully studied, it will be found that
they can be grouped into twenty-four pairs, each member of a
pair being practically identical with its mate in shape and size.
In most ordinary mitotic divisions the members of a pair are
absolutely independent of each other. Each lines up on the spindle
with no reference whatever to the position of the other, and
splits into equivalent halves, which are carried to opposite poles.

But when a sperm mother cell starts to divide, each chromosome
lines up beside its mate. The pairing takes place when the chromo-
somes are long, slender threads or bands, and is followed by their
contraction as in ordinary mitosis. When they reach the equator
of the spindle, they are firmly bound to each other, and form a
group of twenty-four pairs, rather than forty-eight single
chromosomes. (See Fig. 62, in which, for the sake of simplicity,
an organism with only six chromosomes, or three pairs, is illus-
trated.) Then, although each chromosome is split in half as in
ordinary mitosis, its two halves do not separate. Instead, one
complete member of each pair goes to each pole of the spindle,
(Fig. 62,4). Hence there are at each pole twenty-four double
chromosomes. The two daughter nuclei formed by this division
divide again almost immediately, and at this division the halves

1. Paired spireme thread

3. Chromosomes at equator

5. Chromosomes after
reduction division,
preparing for
mitotic division

2. Chromosomes lined
up beside mates

4. Chromosome
pairs separating

7. Four cells
resulting from
meiosis

FIG, 62. — Diagram of cell undergoing meiosis.

The Reproductive Cycle 257

of each chromosome separate and go to opposite poles of the
spindle. Hence each of the four nuclei formed by these two divi-
sions contains chromosomes of exactly the same form and struc-
ture as those received by the two nuclei resulting from an ordinary
mitotic division, but there are just half as many per nucleus, and
only one member of each pair is represented in any particular
nucleus. The cells which result from this second division form
themselves directly into sperms. Thus, by means of the two mei-
otic divisions, every sperm mother cell produces four sperm cells,
each of which has one member of each pair of chromosomes con-
tained in the original mother cell, and hence just half as many as
the mother cell contained.

During meiosis, the number of chromosomes in the gametes is
reduced to one-half the number in the mother cell. Since there are
two divisions but only one process of reduction, it is customary
and convenient to speak of the first division as the reduction divi-
sion, and we shall follow this custom in our references to reduc-
tion. In actuality, reduction is effected by the process of meiosis
as a whole, and it is merely an artificial convenience to speak of
one of the divisions as the reduction division.

Meiosis occurs also in the development of eggs, although in a
somewhat modified manner. The mother egg cell is very large.
When the reduction division occurs, it divides into two very un-
equal halves, to form another large cell and a polocyte. The latter
is a very small cell, containing its full complement of chromo-
somes, but only a minute portion of the cytoplasm of the mother
egg cell. The polocyte divides to form two more polocytes, while
the large cell undergoes the second meiotic division to form the
egg and another tiny polocyte. The three polocytes disintegrate
and disappear, leaving only the one large egg cell as a result of
the meiotic process. This egg cell, of course, possesses just one
member of each pair of chromosomes; and although meiosis in
the male produces four gametes, while in the female it produces
only one, the result of this process, as far as distribution of the
chromosomes is concerned, is identical in both sexes. Fig. 63 out-
lines the entire course of meiosis, comparing the formation of
aperms with that of the eggs.

The Alternating Cycle of Chromosome Numbers. — In fer-
tilization in human beings, a sperm with twenty- four chromosomes

258 The Reproductive Cycle

unites with an egg that also has twenty- four. The resulting ferti-
lized egg has a nucleus containing forty-eight chromosomes. Fur-
thermore, since every chromosome of the sperm has its counterpart

Male

Female

First (reduction)
division of meiosis

Second (mitotic)

division of

meiosis

Polocyte*

Zygote

FIG. 63. — Diagram of formation of sperm and egg, with recombination of
chromosomes in fertilization. (Redrawn from Woodruff's Foundations of Biol-
ogy f The Macmillan Company.)

among those of the egg, the fertilized egg has twenty-four pairs,
one member of each pair derived from the sperm, and one from
the egg. It is plain, then, that the pairs of chromosomes that we
know to exist in all of the body cells of an individual consist each
of one chromosome derived from the organism's mother and one
from its father.

The Reproductive Cycle 259

This fact may be put into a general law which applies to all
sexually reproducing animals. If the chromosome number in the
gametes of any organism is n, the number in the body cells of that
organism will be 2n, and will consist of n pairs, one member of
each pair derived from the individual's male parent, and one from
the female parent. This principle is obviously of great importance
when we consider chromosomes as bearers of heredity, and will be
referred to in the next chapter. The n number of chromosomes
is usually referred to as the haploid number ; the 2n number, as
the diploid number.

Although in animals the cells produced by the meiotic divisions
form themselves directly into gametes, this is not true in most
plants. In them meiosis occurs in the formation of the spores
which produce the gametophyte, so that this generation has the
haploid, or n, number of chromosomes. The gametes of plants
are produced on the gametophyte by means of a series of normal
mitoses, at all of which only the n number of chromosomes can
be counted. As a result of fertilization, the diploid, or 2n, number
is restored, and the sporophyte produced by the zygote has this
number. In the flowering plants meiosis takes place in the young
pollen sacs and the developing ovules, both processes occurring
when the buds are very small, from one to three weeks before
they open.

Comparison of Mitosis and Meiosis. — To understand the
manner in which hereditary traits are handed on from one genera-
tion to another it is absolutely necessary to understand the proc-
esses of mitosis and meiosis, since it is through these processes that
the hereditary factors contained in the chromosomes are systemati-
cally distributed from parents to child. The whole picture may be
briefly summed up in the following comparison of mitosis and
meiosis :

1. Mitosis occurs in practically every cell division that takes
place in the body, while meiosis occurs only at the final two divi-
sions which produce the eggs or sperms.

2. In mitosis, chromosomes line up singly on the spindle, and
half of each chromosome is passed on to each of the two daughter
nuclei, so that each daughter cell receives both members of each
pair of chromosomes. In meiosis, the chromosomes line up in pairs
on the spindle, and half of one member of each chromosome pair

260 The Reproductive Cycle

is passed on to each of the four daughter nuclei, so that each
daughter cell receives one member of each chromosome pair.

3. Mitosis results in the preservation of the chromosomal or-
ganization throughout all the cell divisions occurring in the de-
velopment of an individual organism. Meiosis results in the
division of the chromosomal organization into two similar halves,
so that when the gametes unite to form a new organism, half
the hereditary factors come from the father and half from the
mother.

CHAPTER SUMMARY

The two most important cells involved in sexual reproduction
are the gametes, which unite in fertilization to form the zygote.
In human beings, the gametes are the eggs and sperms, and the
zygote is the fertilized egg. A complete organism develops from
this single cell, the zygote, by means of a vast number of cell
divisions. Practically all of these divisions are of the type called
mitosis, which involves the appearance of a definite number of
chromosomes, their gathering at the equator of a spindle, their
division into halves, the passing of the half or daughter chromo-
somes to the poles of the spindle, and the formation of daughter
nuclei around the daughter chromosomes while the cell divides in
half along the equator of the spindle. Mitosis secures an equal dis-
tribution of the chromosomal, and hence of the hereditary, ma-
terial of the zygote to every cell in the body.

As the embryo develops, the germ cells, destined to produce
the gametes, are differentiated from the other body cells. The
gonads develop around these cells, and they continue to multiply
by mitotic division until, with the arrival of sexual maturity,
they begin developing into eggs and sperms.

In the formation of the gametes, there occurs a process known
as meiosis, which consists of two cell divisions. At the beginning
of the first of these, which is known as the reduction division, the
chromosomes pair, and the members of each pair split and pass as
double chromosomes to opposite poles of the spindle. At the
second division, carried through simultaneously by the two nuclei
resulting from the first, the halves of each chromosome separate,
so that at the end of this division each sperm mother cell has pro-
duced four cells, each with half as many chromosomes as the

The Reproductive Cycle 261

sperm mother cell had. Meiosis takes place in the egg mother
cell also, but in a modified form which results in the formation
of only one mature egg and three polocytes.

When a sperm cell with n chromosomes fertilizes an egg cell
with the same number, the resulting zygote has 2n, and these
chromosomes may be grouped into n pairs, one member of each
pair being derived from the egg, and one from the sperm. The
members of the pairs act independently during all the mitotic divi-
sions that produce the growth of the new individual to maturity,
but pair up in the reduction divisions that produce the sex cells
for the next generation. Thus every generation of sexual repro-
duction involves the change of the chromosome number from 2n
to n, and back to 2n. In plants, the meiotic divisions produce
the spores which give rise to the gametophyte, so that this genera-
tion has the n number of chromosomes, while the sporophyte pro-
duced by the zygote has the 2n number.

QUESTIONS

1. Define chromosomes, and tell briefly of their importance.

2. Describe mitosis, illustrating with diagrams.

3. What is the important result achieved by mitosis which would
not be achieved without this process?

4. Compare meiosis with mitosis, noting similarities and differences.

5. Define meiosis and describe it as it occurs in the production of
sperms. In the production of eggs.

6. Mr. and Mrs. Jones have a daughter Ann, and Mr. and Mrs.
Smith have a son Paul. Ann Jones marries Paul Smith and they
have a son John. Starting with the fertilized egg that gave rise
to Ann Jones, and the one that grew into Paul Smith, trace out
the chromosome conditions, and the changes in chromosome num-
ber that occurred up to the production of sperms by John Smith.

GLOSSARY

chromosome (kro'mo-som) A heavily staining rod of nuclear ma-
terial formed during cell division which carries and distributes
hereditary traits.

diploid number (dip'loid) The number of chromosomes in the body
cells of an animal. The 2n number.

haploid number (hap'loid) The number of chromosomes in the
gametes of an animal. The n number.

meiosis (ml-6'sis) The set of two cell divisions which in animals

262 The Reproductive Cycle

results in the formation of the sex cells and which reduces the
number of chromosomes from the diploid to the haploid number.

mitosis (mi-to'sis) Cell division which involves the appearance and
activity of chromosomes.

polocyte (po'16-sit) Small, non-functional cells formed in meiosis
in the female.

reduction division The one of the two meiotic divisions which, ac-
cording to conventional usage, results in the reduction of the num-
ber of chromosomes in the daughter cells.

CHAPTER XIII

THE PRINCIPLES OF HEREDITY

Early Ideas About Heredity. — Since the beginning of human
history, heredity has received as much attention from men as any
part of biology. Men have always believed that "like begets like"
and that by crossing unlike organisms new types of animals or
plants can be created. Ever since plants have been cultivated and
animals domesticated, men have tried to produce new and better
races and to keep their best breeds constant. Furthermore, the lure
of creating new forms of life has a fascination that still attracts
many into this branch of biology, known as genetics. Moreover,
people are beginning to realize more and more that careful control
of breeding in the human race will make us a better people, and
help solve many of our problems.

There have been many efforts to discover the principles under-
lying heredity, the earlier of which were almost pure speculation.
One of the most prominent ideas, which was held generally in the
seventeenth and eighteenth centuries, was that the sperm or the
egg contains a minute but completely formed organism with its
characteristics all there, and that growth is simply the unfolding
and enlarging of that organism. Some of the microscopic workers
of the day were even so bold as to declare that they had seen, and
to picture, a small, folded-up man within the human sperm. Car-
rying this idea further, they logically assumed that this little man
must contain many sperms, each of which had a smaller animal
inside, and so on. According to their view, every human being
that was ever to inhabit the earth existed already within the sperm
of some living man. In fact, a theologically minded scientist de-
clared that the ovaries of Eve contained two hundred thousand
million of these little men !

Gregor Mendel and His Discoveries. — The first man to
throw any real light on the manner in which inheritance actually

263

264 The Principles of Heredity

takes place was the Austrian monk, Gregor Mendel. Working qui-
etly and patiently in the garden of his monastery at Briinn, he
showed that many characteristics of plants are inherited according
to definite laws, and that the types of offspring which will result
from mating two parents of known pedigree can be rather ac-
curately predicted. He published the results of his experiments in
1868, but they received little attention from the world and were
soon forgotten. Then, in 1900, three biologists, quite independently
of each other, rediscovered these laws which Mendel had laid
down. In the course of their studies they also unearthed the articles
which Mendel had written back in 1868; and, realizing that he
had preceded them by more than thirty years, they generously
gave him the chief credit for his work. "Mendel's laws" im-
mediately became known throughout the world, and in the last
forty years have become the foundation of one of the most active
and progressive branches of biology.

Although Mendel knew nothing whatever of chromosomes and
their importance, recent discoveries have shown without doubt that
the explanation of his laws lies in the separation of the paired
chromosomes at the reduction division, and their coming together
in new combinations in fertilization. We know that within each
chromosome there is a large number of particles which are dif-
ferent in some way from each other. Each of these particles, known
as a gene, acts as a unit to control the inheritance of one or more
characteristics. Since the chromosomes in the body cells occur in
pairs, the genes are in pairs also ; and with the separation and the
recombination of chromosomes during reduction division and fer-
tilization, the paired genes also separate and recombine.

A Simple Mendelian Ratio: The Inheritance of a Single
Pair of Characteristics. — To demonstrate Mendel's laws and
their explanation, let us see what happens in an actual cross be-
tween two animals differing in a single characteristic: i.e., in a
cross between a pure black guinea pig and a pure brown one. The
black parent will contain, situated in a definite part of one of his
pairs of chromosomes, a pair of genes for black. These may be
denoted by the symbols BB. (See chart, Fig. 64.) Similarly, the
brown parent will contain, in the same part of the corresponding

The Principles of Heredity

PARENTS

B

Gametes

F» GENERATION
Hybrid Blacks

265

Pure Black

F2 GENERATION

B
b

Hybrid Blacks

Pure Brown

FIG. 64,— Results of a cross between two guinea pigs differing in one character,
governed by a single gene pair.

266 The Principles of Heredity

chromosome pair, a pair of genes for brown, denoted by the sym-
bols bb.

Since, at the reduction division, the members of each pair of
chromosomes separate and go to opposite poles of the spindle, each
sperm of a male black guinea pig will contain but one gene for
black, B, while each egg of a brown female will contain one gene
for brown, b. (See chart.) Hence, if we mate these two, the off-
spring will contain in their body cells, one gene for black, obtained
from their father, and one for brown, from their mother. The
appearance of these offspring is, however, quite different from
what one would expect. All of them are just as black as their1
father.1 The only explanation for this that we can give is that the
black gene, whenever present, dominates the appearance of the
guinea pig, and it is therefore called dominant. The brown gene,
apparently, can influence the animal's appearance only when it is
present doubly, without that for black, and is therefore termed
recessive. These black offspring of the first generation (denoted
by the symbol Fi, as in the chart) may be called, to distinguish
them from their pure black father, hybrid blacks, and they possess
genes denoted by the symbols Bb.

Each of these hybrid black offspring is capable of producing
gametes of two types, one containing a single black gene, B, and
the other a single brown gene, b. Furthermore, these gametes will
be produced in equal quantities, so that half of the sperms of a
male hybrid black guinea pig will contain the gene for black, and
half the gene for brown. The same will be true of the eggs of a
female. Hence, if two of these Fi offspring are mated, four com-
binations are possible in fertilization, and these will occur in equal
numbers as in Fig. 64.

As a result of this cross, therefore, three types of zygotes are
produced, which will grow up into three types of genetically dif-
ferent offspring. One-fourth of the second-generation, or F2, off-
spring are the result of the fusion of sperms containing the black
gene B, with eggs containing the same gene, and are therefore
of the constitution BB, and pure black. One-half are the result

1You should not conclude from this illustration that the male is more likely
to possess dominant traits than the female. If the female were the black one,
all the offspring would be black. A dominant gene is always dominant whether
it comes from the father or the mother.

The Principles of Heredity 267

of the fusion either of sperms containing B with eggs that have b,
or of b-containing sperms with B-containing eggs, and are there-
fore of the constitution Bb, and hybrid black, like their Fi parents.
The final fourth are the result of the fusion of eggs and sperms,
both containing the gene for brown, b, and are pure brown. Hence,
in appearance, three-fourths of the ¥2 offspring are black and
one-fourth brown. This 3-1 ratio is characteristic of the second-
generation offspring of a cross between two individuals differing
in a single characteristic, governed by a single gene pair. The
two types of black guinea pigs cannot be told apart, except by
breeding them and finding out what ratio of offspring they pro-
duce.

If we describe the offspring of a cross from the standpoint of
their appearance, we are said to be describing phenotypes, while
if we describe them in terms of gene combinations, we are de-
scribing genotypes. Thus a hybrid black is phenotypically black
and genotypically Bb. The ratio of offspring for the cross between
hybrid blacks is phenotypically 3 black to i brown. Genotypically
it is i BB to 2 Bb to i bb.

Of course, the fertilization of a given type of egg by a given
type of sperm is always a matter of chance; and consequently the
3-ijatio is a chance ratio, such as the i-i ratio between heads and
tails secured by spinning a coin. Such chance ratios hold good
only for large numbers. If you spin a coin twice, you cannot be
at all sure that it will show heads once and tails once. But if you
spin it 200 times, you can be certain that it will show heads about
100 times and tails about 100 times. Similarly, in any litter of four
guinea pigs produced by a mating of black hybrids, you cannot be
sure of finding three blacks and one brown ; but in 100 such litters
you will find approximately 300 blacks and 100 browns. This prin-
ciple holds good for all the laws of heredity. They express probable
ratios and do not predict the sort of offspring found in any par-
ticular case. These ratios are the result of the "shuffling" of the
genes that is effected when the paired genes separate in reduction
division and unite with new mates in fertilization. Such separa-
tions and unions are called genetic recombinations. The recom-
binations which took place while the guinea pigs were breeding
may be summed up as follows :

268

The Principles of Heredity

FIRST GENERATION (BLACKS x BROWNS):
Genotypes, P:» Male: BB

Gametes, P: B

Fertilizations:

Female: bb
b

Genotypes
AllBb

"\Eggs
Sperms^^

b

B

Bb

Phenotypes
All black

SECOND GENERATION (HYBRID BLACKS X HYBRID BLACKS):
Genotypes, Ft: Male: Bb

Gametes, FI: B b

Fertilizations:

Female: Bb
B b

\Eggs
Spertns\^

B

b

B

BB

Bb

b

Bb

bb

Genotypes

1 BB)

2 Bb/
i bb

Phenotypes

3 black
i brown

8 P = Original parental generation.
Fi = First generation of offspring (filial generation).
F* = Second generation of offspring.

A diagrammatic summary similar to the two above should be
employed whenever you are asked to calculate the ratio of types
in the offspring to be expected from breeding two known geno-
types. First the genetic constitution of the parents is indicated,
and from this the type of gametes that they will form is written
in the second line. A square to indicate the possible types of fertili-
zation is then constructed, with the sperms along the left side
and the eggs along the top. Since each type of fertilization has an
equal chance of occurring, the numerical ratios of the genotypes of
the offspring are readily calculated simply by counting them up in
the square. By grouping the genotypes together according to their
phenotypic characteristics, the phenotypic ratio can be indicated
in the column opposite the genotype column.

The Principles of Heredity

269

Simultaneous Inheritance of Two Pairs of Characteristics.

— Let us try another problem, somewhat more complex. Let us
cross a black, short-haired guinea pig with a brown, long-haired
one. The gene for black is dominant, as is also that for short
hair. Furthermore, the genes for long and short hair are located in
a different pair of chromosomes from those for black and brown.
The two pairs of characteristics are therefore transmitted inde-
pendently, each according to the manner just described.

With these facts in mind, let us see what the progeny of this
cross will be. The first generation should work out as follows :

Genotypes, P:
Gametes, P:

Fertilizations:

Fi:

BBSS bbss
BS bs

bs

BS

BbSs

Genotypes
All BbSs

Phenotypes
All black, short

The offspring would all be black and short haired, but would
be hybrid for both of these traits.

B

FIG. 65. — Two ways in which the chromosomes containing gene pairs Bb and
Ss could line up on the spindle at reduction division. A, one daughter cell receives
B and S ; the other, b and s. B, one daughter cell receives B and s ; the other,
b and S.

Suppose we now breed these hybrid black short-haired guinea
pigs one to the other. The first thing we must consider is that the
two pairs of genes are in different chromosome pairs. Conse-
quently, reduction division will not always result in the formation
of the same kinds of gametes. When these chromosomes pair for

270

The Principles of Heredity

reduction division, they may pair with the chromosome containing
B, and the one containing S on the same side of the spindle, as
in Fig. 65 A. In this case the gametes formed will be BS and bs.
On the other hand, the B-containing chromosome and the s-con-
taining one may chance to line up on the same side of the spindle,
as in Fig. 656. In this case the gametes formed will be Bs and bS.
Since there is an equal chance for each sort of alignment of
chromosomes in reduction division, an equal number of gametes
of each type will be formed. The results of breeding the hybrid
black short-haired guinea pigs can therefore be formulated as fol-
lows :

Genotypes, FI:
Gametes, FI:
Fertilizations:

BbSs
BS Bs bS bs

BbSs
BS Bs bS bs

BS

Bs

bS

bs

BS

BBSS

BBSs

BbSS

BbSs

Bs

BBSs

BBss

BbSs

Bbss

bS

BbSS

BbSs

bbSS

bbSs

bs

BbSs

Bbss

bbSs

bbss

P.:

Genotypes

1 BBSS

2 BBSs
2 BbSS
4 BbSs

1 BBss

2 Bbss

1 bbSS

2 bbSs
i bbss

Phenotypes
9 black short

3 black long
3 brown short
i brown long

The phenotypic ratio that such a hybrid cross gives is, therefore,
9 black short-hairs to 3 black long-hairs to 3 brown short-hairs
to i brown long-hair. This 9-3-3-1 ratio can be demonstrated ex-
perimentally whenever two pairs of hybrid characters located in
separate chromosome pairs are crossed.

Inheritance as the Result of Gene Combinations. — Now, if
the reader understands fully what occurs in the two experiments
that have just been outlined, he is acquainted with the most im-

The Principles of Heredity

PARENTS
Pure Black and Short Hair Pure Brown and Long Hair

271

Gametes

\/

GENERATION
Hybrid Black with Short Hair

BS Bs bS

BS Bs bS

Gametes

F2 GENERATION
9 -Black, Short-Hair 3-Black, Long-Hair 3-Brown, Short-Hair 1- Brown, Long-Hair

1 BBSS

2 BbSS

2 BBSs

4 BbSs

IbbSS

2 bbSs

FIG. 66. — Results of a cross between two guinea pigs differing in two characters
governed by gene pairs located on different chromosomes.

272 The Principles of Heredity

portent fundamental phenomena of heredity. We may sum up
these phenomena in the following manner :

1. Hereditary traits are passed from generation to generation
by means of genes. Although the exact nature of the genes is not
known, they may be thought of as little packets of chemicals, each
differing from the others.

2. Each chromosome contains a characteristic group of genes,
and the genes in a pair of chromosomes are paired. For example,
the gene for brown eyes may be paired with another gene for
brown eyes or with a gene for blue eyes ; or the members of the
gene pair that occupies that particular position in the paired
chromosomes may be both for blue eyes.

3. In any pair of genes, the gene for one character is usually
dominant over the one for the other character. For instance, the
brown-eye gene is dominant over the blue-eye gene, so that an
individual who has one gene for brown eyes and one for blue is
brown-eyed. One must have both genes for blue in order to be
blue-eyed.

4. Every sexual reproduction results in a recombination of
genes, because of the fact that half the genes in fertilization come
from the sperm and half come from the egg. Furthermore, each
offspring of a given pair of individuals is likely to receive a dif-
ferent combination of genes from any of its brothers and sisters,
since each sperm or egg of an individual is likely to contain a
different combination. This difference in the genetic character of
the sperms and eggs is the result of the fact that when the paired
chromosomes line up on the equator in reduction division, it is
simply a matter of chance whether a given member of the pair
lines up on one side or the other. Suppose, for example, that a
given species has four pairs of chromosomes: la, ib; 2a, 2b; 3a,
3b; and 4a, 4b. Meiosis will result in the formation of sixteen dif-
ferent kinds of gametes in equal numbers, as follows :

i: la 2a 3a 4a 9: ib 2a 3a 4a

2: la 2a 3a 4b 10: ib 2a 3a 4b

3: la 2a 3b 4a n: ib 2a 3b 4a

4: la 2a 3b 4b 12: ib 2a 3b 4b

5: la 2b 3a 4a 13: ib 2b 3a 4a

6: la 2b 3a 4b 14: ib 2b 3a 4b

7: la 2b 3b 4a 15: ib 2b 3b 4a

8: la 2b 3b 4b 16: ib 2b 3b 4b

The Principles of Heredity 273

Each one of the 16 different sperms can fertilize any of the 16
genetically different eggs; so that, as far as combinations of
chromosomes are concerned, 256 different kinds of zygotes can be
formed. In the human species, where there are 24 pairs of chromo-
somes, meiosis can result in the formation of 16,769,024 kinds
of gametes; and, as a consequence, there is a possibility of 281
trillion generically different zygotes. And the possibilities of varia-
tion are really much greater than this calculation would indicate,
since occasionally a group of genes "crosses over" from one mem-
ber of a pair of chromosomes to the other, thus causing an even
greater "mix-up" than is brought about by the chance arrange-
ment of the chromosomes in reduction division. The result of all
this shuffling and reassortment of genes is that the hereditary
constitution, that is, the assortment of genes, in one individual
is never the same as that in another. This universal variation is,
of course, just what we observe all about us. Brothers and sisters
usually show many resemblances, since their genes are all derived
from those of their parents. But, unless they are identical twins,
they are never exactly alike genetically.

Blended Inheritance. — As the reader undoubtedly knows, not
all characteristics are inherited in this comparatively simple fash-
ion. In characteristics such as height and skin color in man, and
often in the color of flowers, the offspring are intermediate be-
tween their parents. This "blended inheritance" can be produced
in various ways. The simplest is that of imperfect dominance. For
instance, if a red snapdragon is crossed with a white one, the Fi
offspring are all pink. If these are then intercrossed, they produce
reds, pinks and whites in the ratio of 1-2-1. In other words, the
pink ¥2 offspring are hybrids just like the hybrid black guinea
pigs, except that neither the red nor the white gene is dominant,
and both have their influence on the color of the flower. The same
holds true for a breed of fowl known as the Andalusian. If a
black Andalusian fowl is bred to a white one, the resulting off-
spring are a slaty blue, a highly prized color. These, of course,
never breed true to their color but, when mated with each other,
produce blacks, blues, and whites in the 1-2-1 ratio.

A similar type of inheritance determines the skin color of
crosses between Negroes and white men. Here, however, four
genes, in two pairs, are active, none of them dominant or reces-

274*

The Principles of Heredity

sive. The Fi offspring of a cross between a pure black Negro and
a white man are all mulattoes of about the same intermediate
shade. If such mulattoes are bred together, the F2 offspring are
not in a 3-1 ratio of blacks to whites, as were the guinea pigs, nor
are they in a 1-2-1 ratio of blacks to mulattoes to whites, as one
would expect if there were a single pair of genes for color, neither
of which was dominant. Instead, there is a large number of mulat-

BlAck

Dark

Medium

White

FIG. 67. — Diagram showing the proportions of the various types of offspring
as to color found in the second generation of crosses between pure-blooded
Negroes and pure whites.

toes of various shades of blackness, and only occasionally does a
pure black man or a pure white one appear.

A recent analysis of the offspring of mulattoes has, neverthe-
less, shown that they may be grouped into five classes : pure blacks,
pure whites, and three grades of mulattoes. The frequency of
these groups may be expressed by the accompanying diagram, in
which the darkness of the shading indicates the blackness of a
given type, while the number of figures in a row indicates the
frequency of a type. Out of every sixteen offspring there is one

The Principles of Heredity

275

pure black and one pure white, and the grades of mulattoes are
in the ratio of 4-6-4.

To explain this ratio, let us assume that the black grandparents
had four genes for blackness in two pairs, BB and B'B', while
the white grandparents had four genes for whiteness, bb and bT/.
The Fi mulattoes would then have the genes BbB'b'. Since they
contain an equal number of black genes and white ones, none of
which is dominant, they are exactly intermediate in color between
their parents. They produce four types of gametes, BB/, Bb', bB'
and bb'. A square showing the possible types of fertilizations
will be as follows :

BB'

Bb'

bB'

bb'

BB'

BBB'B'

BBB'b'

BbB'B'

BbB'b'

Bb'

BBB'b'

BBb'b'

BbB'b'

Bbb'b'

bB'

BbB'B'

BbB'b'

bbB'B'

bbB'b'

bb*

BbB'b'

Bbb'b'

bbB'b'

bbb'b'

By adding the ~Fz types, one can see that there is one with four
black genes (BBB'B'), four with three black genes (2BBB'b'
and 2BbB'B'), six with two black and two white (BbB'b'), four
with one black and three white, and one with four white (bbb'b').
If we assume that the blackness depends on the number of black
genes present, the five types of F2 offspring actually found, and
the ratio between them, are easily explained.

This comparatively simple example illustrates multiple factor
inheritance, which, for the more fundamental characteristics of
organisms, is more common than inheritance through a single
pair of genes. Usually, however, multiple factor inheritance in-
volves the activity of a very large number of genes, some of
which are dominant, some recessive, and some neither. This re-
sults in a type of blended inheritance in which it is frequently
extremely difficult or quite impossible to detect separation and
recombination of individual gene pairs. As a result, offspring
cannot be separated into distinct types, but may vary continuously
from one extreme of a trait to another, depending on the par-
ticular assortment of genes that each individual receives. The
transmission of height in human beings is an example of this.

276

The Principles of Heredity

The offspring of tall parents will, on the average, be tall; but
among these offspring all degrees of height may be represented,
from very short to extremely tall. Similarly, short parents may
produce offspring of all degrees of height, although their chil-
dren will average shorter than those of tall parents.

Interaction of Genes. — To add to the complexities of the
hereditary picture, we find that the effect that certain genes exert
may depend upon the presence or absence of other genes. For
instance, guinea pigs of the genotypes BB or Bb will be black
and guinea pigs of the genotype bb will be brown, only in the
presence of another gene, C, which causes some sort of pigment
to appear in the coat. Animals of the genotype cc will always be
white, no matter what other genes for coat color are present.
Hence, the ratios previously given for interbreeding of black and
brown guinea pigs and their hybrids would hold only for animals
that were pure for the presence of pigment, that is, of the geno-
type CC. If several pairs of black guinea pigs, hybrid for both
pigment and black, were mated, the ratio in the offspring would
be 9 blacks to 4 whites to 3 browns, as follows :

Genotypes, P:
Gametes, P:
Fertilizations:

CcBb
CB Cb cB cb

CcBb
CB Cb cB cb

CB

Cb

cB

cb

CB

CCBB

CCBb

CcBB

CcBb

Cb

CCBb

CCbb

CcBb

Ccbb

cB

CcBB

CcBb

ccBB

ccBb

cb

CcBb

Ccbb

ccBb

ccbb

Pi:

Genotypes

1 CCBB

2 CCBb
2 CcBB
4 CcBb

1 ccBB

2 ccBb
I ccbb

1 CCbb

2 Ccbb

Phenotypes
9 black

4 white
3 brown

The Principles of Heredity 277

In this short account, we can enter -no further into the com-
plexities of the hereditary mechanism. The reader should keep
in mind the fact that the sample ratios with which we have dealt
in this chapter represent only the simplest of the problems with
which the geneticist deals, and that most traits are governed by
a multitude of genes whose combined action results from a very
complex interrelationship among them. Underlying all these com-
plexities, however, there remains the single simple principle that
was worked out by Mendel. Hereditary traits are transmitted
from generation to generation by myriad pairs of unit factors,
and each animal or plant that reproduces sexually receives one
member of each pair from the father and one from the mother.

Inheritance in Human Beings. — Most traits that are of any
importance in human beings, such as height, weight, strength,
intelligence, and other mental traits, are governed by a multi-
plicity of gene pairs. Single gene pairs, however, govern a few
traits, most of which are abnormal. There is a single gene for
dwarfism, a gene for color blindness, and a gene for hemophilia,
that disease in which the blood fails to clot and which has recently
entered the news because it afflicts the deposed but romantic Span-
ish crown prince along with several other members of his family.
The peculiarly heavy jaw and lower lip which have been found
in the same family clear back to their famous ancestor, Charles
V, and which is therefore known as the "Hapsburg jaw/' is also
the product of a single gene. There is a single gene pair for eye
color, the gene for brown eyes being dominant over the one for
blue; but while this gene pair determines whether the eye is to
be light or dark, a number of other genes determine minor vari-
ations in color and pattern of pigmentation. A few other single
hereditary factors have been discovered in human beings, but
multiple factor inheritance is the rule, single factor inheritance
the exception.

The Application of Mendelian Principles. — The principles
laid down by Mendel have not only been the basis of a great body
of scientific knowledge which has been built up around them in
the last forty years, but they have also proved of value to prac-
tical plant and animal breeders in their efforts to produce better
cultivated plants and domestic animals. Such an age-old art as
practical breeding has not been revolutionized by these compara-

278 The Principles of Heredity

tively recent discoveries, but modern genetics has injected into it
more precise and careful methods and, above all, the ability to
make predictions and be reasonably sure that they will come true.
The fundamental principles of breeding, cross breeding followed
by the selection for desirable characteristics, have not been
changed. They have been made more efficient, however, and many
results formerly incomprehensible are now fully explained.

Progeny Selection. — One way in which genetics has been
applied to help practical breeding is in regard to the method of
selection. Formerly, breeders practiced chiefly mass selection, in
which they chose the best-appearing animals in a herd for breed-
ing, or the most vigorous-looking plants in a field as seed bearers.
This method is still practiced by the ordinary farmer, but many
of the more progressive livestock raisers and agriculturists are
finding it inadequate, since so many of the progeny of mass selec-
tion tend to "revert to type" and are no better than, if as good
as, their forbears.

The more refined method, known as progeny selection, is now
gaining ground steadily. In animal breeding, progeny selection
involves the testing of an animal's breeding qualities by the off-
spring that it gives before using it regularly as a breeding animal.
Two bulls of the same race may be equally large, strong, and
vigorous, yet one bull may have daughters with a much greater
milk-producing capacity than the other. The reason is that the
better bull is pure bred for every gene pair relating to milk-
producing capacity, while the other has recessive genes for poor
milk qualities which, although they do not affect his appearance,
yet produce the inferior offspring when paired with other reces-
sive genes from the cows with which he mates.

Progeny selection has, in several scientifically conducted ex-
periments on fowls, been particularly successful in increasing egg
production. Geneticists have found that two pairs of genes, M
and L, affect egg production, the dominant genes in each case
increasing the production. A hen may be pure bred for both domi-
nants, MMLL, or may have one recessive of each pair, MmLl;
in either case her egg production is the same. A rooster that is
pure bred for both dominant egg-producing genes, MMLL, will
produce good egg-layers when mated with either of the two types
of highly productive hens. However, a hybrid rooster, MmLl,

The Principles of Heredity 279

although he may look exactly like the pure-bred one, will, when
bred to the hybrid hen, give some poor layers. He may thus be
detected and eliminated while the pure-bred rooster is kept for
all further breeding. By this method one geneticist increased the
average annual egg production of the hens in his flock from 114
to 200 eggs in eight years.

To aid in progeny selection, pedigrees of stock are being made
more and more regularly. Nowadays only bulls of proved worth
are used in the better herds of cattle, while in horse-raising the
laurels won by a stallion's offspring increase his worth as much
as any he has acquired himself. State registration of pedigreed
stock is an outcome of the need for progeny selection, and county
fairs and stock shows help advertise the best-bred animals and
make them available for breeding purposes.

Inbreeding and Its Effects. — The practice of selection con-
ducted on a scientific basis clearly involves the mating of brothers
and sisters, as well as closely related cousins, and, in plants, self-
fertilization, since only by this method can the breeder analyze
his stock genetically. This mating of closely related animals is
known as inbreeding. There has always been much argument
among breeders as to whether this is harmful or not, although
many of the most valuable livestock are the result of continued
inbreeding. With the coming of scientific genetics, careful exper-
iments conducted on various animals and plants over a large
number of generations have given us actual knowledge about this
practice.

The chief outcome of these experiments has been to show that,
while there are many exceptions to the rule, desirable traits are
usually produced by dominant genes, and undesirable traits by
recessive genes. With continued inbreeding, more and more pairs
of recessive genes are brought together, and the characteristics
that they produce appear in a large number of individuals, with
a resulting deterioration of the stock. But the very fact that these
recessives are brought out into the open can be put to advantage
by an intelligent breeder. By selecting only the best of his stock
to breed from and rejecting all the inferior individuals, he may
completely weed out the undesirable recessives, and the offspring
of the carefully selected stock will breed true for the qualities
desired in a fashion that would be impossible in a cross-bred

280 The Principles of Heredity

stock where recessive genes may be hidden for a long time, only
to bob up occasionally and cause trouble. Inbreeding, then, is the
best way of bringing out undesirable recessives so that they can
rapidly be weeded out of a stock; and, once they are weeded out,
continued breeding within that stock is the best way of keeping
them from getting back in.

Cross Breeding and Hybrid Vigor. — While inbreeding ac-
companied by selection has no ill effects on a race, there is one
great advantage to cross breeding. In every experiment in which

Inbttd parents

1

FIG. 68. — Diagram illustrating hybrid vigor in corn.

it has been tried, cross breeding of individuals from different
constant, inbred strains has produced offspring bigger and
stronger than either parent. The same increase in vigor results
from crossing two quite different races, or even species. If the
parents are too widely different from each other, the offspring
will be sterile, for reasons that will soon be explained, but they
will be more vigorous than their parents. This phenomenon,
known as hybrid vigor, is characteristic of Fi hybrid offspring,
but always decreases in later generations unless the wide crossing
is kept up. In Fig. 68 hybrid vigor in a cross between two inbred
strains of Indian corn is shown.

The explanation of hybrid vigor lies in the fact that most domi-
nant genes tend to produce desirable characteristics. In the Fi
offspring of widely different parents, the largest possible number

The Principles of Heredity 281

of genes are in the hybrid condition, and therefore the largest
possible number of dominant genes are active, while the recessive
genes are practically all hidden. In later generations, the recessive
genes again segregate out and produce their weakening effect.

Hybrid vigor may very profitably be secured in practical breed-
ing. The importation of pedigreed sires from another line is ac-
cepted as the best way of improving a herd of cattle or a stable
of racing or show horses. If the pedigrees of the parents are
known, the nature of the offspring can at least in part be pre-
dicted, and the breed kept true by a small amount of selection.
In the case of agricultural crops, experiment stations in various
parts of the country have devised ways by which farmers can
maintain hybrid vigor. The method now recommended is that
the farmer have several seed plots in which carefully selected
races are kept constant by inbreeding, and that to produce the
seed that he plants in his field, he make crosses between two of
these races. In fact, corn breeding is now carried out in this way
on a large scale.

The Crossing of Different Species. — Often the breeder
crosses two organisms differing in many characteristics, some
governed by single and some by many gene pairs. The offspring
of such crosses are, of course, extremely variable; and when the
organisms differ so widely from each other that they are classed
as different species, the types of offspring that result from a
cross between them cannot be predicted. Those of the Fi genera-
tion are usually very much like each other and intermediate be-
tween their parents, but in the following generation the many
different characteristics segregate in different ways and form such
a number of new combinations that usually no two individuals
of this generation are alike. This is the result which to the ordi-
nary breeder is a hopeless confusion but which to the man with
foresight, great patience, and persistence, may be a gold mine
from which he picks out new races and, by careful selection over
many generations, creates some constant variety which everyone
can use.

By this method most of our valuable garden vegetables and
flowers, as well as a large number of breeds of domestic animals,
have been created. The garden strawberry, for instance, is almost
certainly a hybrid between the common wild strawberry of our

282 The Principles of Heredity

eastern states and a species from Chile. Both were introduced
into European gardens in the seventeenth century, where they
frequently grew side by side. Since only the female plants of the
Chilean species had been imported, crosses between the two nat-
urally appeared and became the source of our large-fruited culti-
vated forms. A similar origin, though much more ancient and
less clear in its details, is ascribed to our cultivated apples and
plums. They are presumably derived from crosses between certain
wild species of Europe and others native to Asia. Most of Luther
Burbank's creations are the result of hybridizing two or three
different species and of growing tremendous numbers of off-
spring, among which the "plant wizard's'1 keen eye could detect
the most valuable individuals which he caused to breed true by
means of careful selection. The loganberry, for instance, is the
result of a cross between a blackberry and a raspberry.

Among animals, poultry gives us examples of some well-known
breeds of hybrid origin. The early American colonists brought
with them to this country breeds of fowl derived from strains
domesticated in southern Europe, coming originally from India.
These were small, active birds, about the size of modern bantams,
very fertile egg layers, but not particularly good eating. More re-
cently, Yankee sea captains brought back from the Malay Islands
chickens of a different type. They were larger in every way and
rather heavy and sluggish, differing in so many characteristics
from European fowl that some scientists consider them to be
derived from different wild species. By crossing these Malay types
with the European race, poultry breeders produced our well-
known American breeds, such as the Plymouth Rock and Rhode
Island Red, which combine high egg-laying capacity with fine eat-
ing qualities.

Sterility in Species Hybrids. — One of the biggest difficulties
encountered in crossing different species is the partial or com-
plete sterility of the offspring. The mule, for instance, has been
produced for centuries by breeding a mare to a jackass, but is
always completely sterile. It is a valuable animal, since it combines
the size and energy of the horse with the hardiness, steadiness,
and persistence of the donkey, and is superior to both in strength
on account of hybrid vigor. Yet in all the centuries that they have
been produced, there are only a few authentic records of a mule

The Principles of Heredity 283

having foaled. The explanation for this lies in the difference be-
tween the chromosomes of the horse and those of the donkey.
In the zygote resulting from the fertilization of a horse's egg
with a donkey's sperm, there is a complete haploid set of chromo-
somes of the horse, together with that of the donkey. The num-
ber of each is not the same, there being slightly more chromosomes
derived from the donkey than those from the horse. During all
the mitotic divisions that build up the body of the mule, there is
no necessity for association between horse and donkey chromo-
somes. However, when the meiotic divisions begin in the testes

FIG. 69. — Left, diagram illustrating what might happen in the reduction division
in the offspring resulting from a cross between an organism possessing twenty,
and one possessing ten chromosomes in its body cells. Fifteen chromosomes are
shown, ten in white, derived from one parent, five in black from the other. Four
of the latter have found mates ; but one of them, as well as six of the former
group, are unpaired. Right, drawing of the reduction division in the mule. About
fifty chromosomes, paired and unpaired, are present, but they cannot be accu-
rately counted.

or ovaries, the chromosomes cannot pair properly, since many of
the horse chromosomes are different from those derived from
the donkey parent, and there is a number of extra donkey
chromosomes which find no mates at all. These unpaired chromo-
somes behave very irregularly, causing meiosis to be quite ab-
normal (Fig. 69). The resulting daughter cells, since they have
either too few or too many chromosomes, soon degenerate, so
that no functional sperms or eggs are produced.

Other hybrids are only partially sterile. For instance, in the
cross between domestic cattle and the wild buffalo, the resulting
"cattalo" is sterile if male, but fertile if female; that is, the fe-
male will give offspring with either a domestic bull or a buffalo
bull. Western cattle breeders have recently been experimenting
with this cross in an effort to combine the hardiness and disease

284 The Principles of Heredity

resistance of the buffalo with the beef qualities and tractability
of domestic cattle, but as yet no widely accepted breed has been
created in this way.

Still other crosses cannot be made at all. If, for instance, pollen
from a squash is put on flowers of the pumpkin, no seeds or fruit
are produced. Moreover, the physical resemblance of two types
is by no means a criterion of the ease with which they can be
crossed ; the breeder has learned that the only way to learn whether
a cross between two different species can be made and what the
offspring will be like, is to try it. The intricacies of breeding by
hybridization and selection are so numerous that they can be mas-
tered only by means of a wide practical experience combined with
a knowledge of modern genet ical theory, but the results are among
the most valuable contributions to mankind.

The Determination of Sex. — People often wonder whether
they can control the sex of their children, and how it is deter-
mined whether there will be a girl or a boy. In olden days many
absurd formulae for producing the desired sex were current ; even
today people have ways of predicting this fact which they believe
to be infallible, and many strange things are done by parents
under the belief that a boy, or perhaps a girl, will certainly result.
We know now, however, that this all-important fact is decided
for the most part, just as are the questions of brown eyes or
blue and blonde or brunette, by the chromosomes and the laws
of chance.

In human beings, there are 24 pairs of chromosomes in every
one of the body cells. In the male, however, there is a pair in
which the two chromosomes are very unequal in size, the larger
being known as the X-chromosome, the smaller as the Y-chromo-
some. In the female, the members of the corresponding pair are
both alike, and correspond to the X-chromosome of the male.
We may diagram the condition thus :

Woman Man

23 pairs of chromosomes 23 pairs of chromosomes
plus plus

XX XY

It will be readily seen that during meiosis in the male, two
kinds of sperms will be formed in equal numbers. Half the sperms

The Principles of Heredity

285

will contain the X-chromosome and half of them will contain the
Y-chromosome. All the eggs, however, will contain an X-chromo-
some. When an X-containing sperm fertilizes an egg, the XX
pair of chromosomes will be formed, and the individual usually
becomes a female. When a Y-containing sperm fertilizes an egg,
the XY pair of chromosomes will be formed, and the individual
generally will be a male. Thus is provision made for an equal
number of individuals of each sex to be born, although whether
or not a given child turns out to be a boy or a girl is largely a
matter of chance. This is the most common chromosome mech-
anism for sex determination, although a number of others exist.

This equal distribution of
the sexes which would be ex-
pected under the sex-chromo-
some mechanism has been borne
out by statistics to a certain de-
gree. For instance, the average
ratio of the sexes at birth for
the human race as a whole is
103-107 boys to 100 girls. This
ratio varies from country to
country; for instance, there are
born in Great Britain only 93
boys to every hundred girls,
but in China there are 125 male
births to every hundred female.

What determines these differences we cannot tell, but very likely
the cause is a difference between the number of males and females
that die before birth. We know, for instance, that more boys
are still-born than girls.

Sex Reversal and Intersexuality. — That chromosomes are
not the absolute arbiters of sex determination has been shown by
a number of instances of sex reversal. In one case, a hen who
had been the mother of several broods of chickens contracted a
disease of the ovaries. In a few months she had developed the
appearance of a rooster, fought with the other roosters in the
yard, and attracted the hens. Finally, when mated to a virgin hen,
she (or he, by this time) became the father of a brood of chicks.
This has been explained by the fact that the gonads of a hen

FIG. 70— Drawing of the reduction
division in the formation of human
sperms, showing several of the chromo-
some pairs, including the XY pair (in
black).

286 The Principles of Heredity

often contain a little testicular tissue among that characteristic of
the ovary. When the ovary becomes diseased and degenerates,
this tissue develops and produces sperms, as well as hormones
which give male secondary sex characteristics. Similar cases are
known in other animals; and in some bisexual plants, such as
hemp, males can be turned into females and back again, simply
by changing the environment. This is possible in plants because
the germ cells are not all differentiated in the young embryo,
but new ones are produced before each flowering period. Hence
we may say generally that the sex of any organism may be
changed if the germ cells have not all been differentiated into egg-
producing or sperm-producing- types.

A rather striking proof of this fact is the "freemartin" in
cattle. Cattle breeders have known for some time that when twins
of different sexes are born, the female is usually sterile, although
her external genitals are normal in appearance, and her udder is
that of a cow. She is known as a "freemartin," and, when ex-
amined, is found to possess rudimentary testes rather than
ovaries. This abnormality is brought about by the joining to-
gether of the blood streams of the twin embryos before the gonads
have become differentiated. In this case the gonads of the male,
as determined by his XY set of chromosomes, develop into
testes, and soon start to produce sex hormones which are carried
by the blood stream to his female twin. In this embryo, the
gonads are still comparatively undifferentiated, since the differ-
entiation into ovaries occurs later than that into testes, and so the
influence of the male sex hormones transported from the twin
embryo makes them develop in the direction of testes. The female
heredity cannot, however, be easily overridden, and the outcome of
the resulting struggle between male and female sex hormones is
the intersexual " freemartin/'

Present evidence indicates that the sex into which an undif-
ferentiated embryo will develop is determined by the rate of
metabolism in its cells at the time of differentiation. At least in
the higher animals, the male has a higher rate of cell metabolism
than the female; and if these processes, particularly combustion,
are comparatively rapid in the young embryo, it develops into a
male, and if slower, into a female. The chromosomes normally
swing the balance one way or the other by regulating this rate.

The Principles of Heredity 287

In human beings an XY organization tends to speed up metab-
olism, while cells of the XX chromosome constitution are nor-
mally less active; but in either case, if the rate is altered by some
disturbing agent, the sex is either partly or completely changed.
Using this principle, a German doctor has developed a method
in which, by regulating the diet of mothers during the early
weeks of pregnancy so as to produce a high or low rate of
metabolism in their cells, he claims to be able to produce boys
or girls at will. This method has not, as yet, been generally ac-
cepted or used.

Sex-linked Characters. — The X-chromosome contains many
genes which the Y-chromosome lacks, with the result that males
never receive more than one member of these pairs of genes.
The traits which these genes govern are called sex-linked char-
acters. Dominant sex-linked characters occur more frequently in
females than in males, while recessive sex-linked characters occur
more frequently in males. Color blindness, a sex-linked recessive,
is found in about four per cent of men, but only very infre-
quently in women. The reason is that, for a daughter to be color-
blind, both parents must have the gene for color blindness so
that it can occur in both chromosomes. But since a son receives
his only X-chromosome from his mother, he can inherit color
blindness whenever she possesses one or both genes for it. If a
color-blind man mates with a woman both of whose genes are
normal, all the sons will be normal, since they will receive their
only X-chromosomes from the mother. The daughters, however,
will be hybrid normals, since each will receive the father's
X-chromosome with its color blindness gene. If these hybrid
daughters marry men of normal color vision, all their daughters
will be normal — although half will be hybrid normals — since each
daughter will receive a normal gene from the father's X-chromo-
some. Half the sons will be color-blind, since half will receive
the mother's X-chromosome with the normal gene and half the
mother's X-chromosome with the color blindness gene, and there
will be no X-chromosome from the father to "cover" this reces-
sive gene. Thus a son always inherits his color blindness from
his mother, whether she is color-blind or not, but cannot inherit
it from his father even when his father is color-blind; while a
daughter cannot be color-blind unless her father is also color-

288 The Principles of Heredity

blind and her mother has at least one color blindness gene. It is
only for sex-linked characters, however, that the old saw that
"boys take after their mothers and girls after their fathers" is
in any sense true. For all other traits, it is a matter of chance
which parent a child most resembles.

CHAPTER SUMMARY

The science of genetics is an old one, and fantastic theories
about it once existed. The fundamental laws of heredity were
discovered by Gregor Mendel, an Austrian monk.

His laws are explained by the separation of chromosomes in
the reduction division, and their recombination in fertilization.
Each chromosome contains a number of genes, which are the units
that influence hereditary characteristics.

If two organisms differing in a characteristic that is controlled
by a single gene pair are crossed, the Fi offspring will receive
one gene from each parent. One of these genes may be dominant
over the other, so that the offspring completely resemble the
parent which possessed this gene. If these offspring are bred to-
gether, their offspring are of three types genetically; but in ap-
pearance, three- fourths show the dominant and one- fourth the
recessive trait. This is explained by the separation of the genes
in the reduction divisions of the Fi parents, and the number and
types of recombinations that are possible.

If animals differing in two characteristics, and pure bred for
each, are crossed, the Fi offspring show both dominants. If the
gene pairs are located in different chromosome pairs, they segre-
gate independently of each other, so that four types of gametes
are produced by the Fi individuals in equal quantity. In a cross
between two of them, these gametes recombine to form nine dif-
ferent genetic combinations, which give four different types as
far as external appearance is concerned, in the ratio 9-3-3-1.

The number of types of gametes formed by an individual, and
the number of recombinations possible, depends on the chromo-
some number, and in most species is very large. This explains the
large number of hereditary variations, even among brothers and
sisters.

If neither of two paired genes is dominant, the Fi offspring
are intermediate between their parents, and the case is one of

The Principles of Heredity 289

imperfect dominance. If a number of genes govern a character-
istic, the Fi offspring of a cross between the extremes of the
two types will all be intermediate between their parents, but those
of the Fa generation will show a series of gradations from one
extreme to the other. A simple example of this multiple factor
inheritance combined with incomplete dominance is given by the
results of a cross between a Negro and a white man.

If a large number of genes, some dominant, some recessive,
and some neither, affect a single characteristic, multiple factor
inheritance in which the action of single genes cannot be recog-
nized, results.

Frequently genes interact in such a way that a combination
of genes is necessary to produce a single trait, as when a gene
for pigment and another for black must both be present to pro-
duce black coat in the guinea pig. Unlike many forms of multiple
factor inheritance, however, the action of each gene pair is readily
observable.

Among human beings a few instances of single factor inherit-
ance have been discovered, but multiple factor inheritance seems
to be the rule.

In plant and animal breeding some ways in which the knowl-
edge of the laws of heredity has been useful are :

1. It has demonstrated the advantage of progeny selection, in
which the plants and animals to be used for breeding are chosen
on the basis of the offspring which they have produced, rather
than by their appearance.

2. It has explained and clarified the results of inbreeding. Since
recessive genes tend, on the whole, to produce undesirable traits
and since inbreeding tends to bring recessive genes together, many
undesirable individuals will appear in an inbred stock. When these
individuals are weeded out by selection, however, the offspring
of the desirable individuals will not only inherit the good traits
of their parents but will breed true for those traits, since "hidden
recessives" will be eliminated.

3. It has partly explained hybrid vigor, or the greater size and
strength of the offspring of a cross between two widely different
parents. This is due to the activity under these conditions of the
largest possible number of dominant genes, which produce most
strong characteristics.

290 The Principles of Heredity

The crossing of different species produces very variable off-
spring, from which careful selection must be made to produce
valuable types. Species hybrids are, moreover, often sterile, as the
result of the failure of different chromosomes to pair at the re-
duction division.

The chromosome mechanism for the inheritance of sex in
human beings is as follows : A man has in his body 23 pairs of
chromosomes + the XY pair and produces two types of sperms
in equal quantities, those containing* 23 + X, and those with
23 + Y. The woman has in her body cells 23 pairs + an XX
pair, and produces eggs containing 23 + X. If an X-containing
sperm fertilizes an egg, a female-determining zygote is pro-
duced, whereas a Y-containing sperm produces a male-determin-
ing zygote.

The sex of an organism may be reversed by an abnormal en-
vironment at any time if the germ cells have not become com-
pletely differentiated into egg- or sperm-producing types. The
sex is directly determined by the metabolic rate in the cells at the
time of differentiation.

Hereditary traits whose genes are carried in the X-chromosome
but not in the Y-chromosome are called sex-linked characters. A
recessive sex-linked character will occur more frequently in males
than in females, since it will never occur in combination with the
dominant gene. Dominant sex-linked characters will occur more
often in females than in males. Males always inherit sex-linked
characters from their mothers, since they never receive their
X-chromosomes from their father.

QUESTIONS

1. Describe the inheritance of a pair of opposing characteristics
governed by a single gene pair, showing the application of the
principle of dominance and that of segregation and recombina-
tion of genes.

2. Show how genes that are located in different chromosome pairs
may segregate independently in reduction division.

3. By means of a diagram/ demonstrate the number of genetic
recombinations and the number and proportion of different types
of offspring as to appearance that will be obtained from crossing
two individuals that are hybrid for two pairs of characteristics,
the genes for which are located in different chromosome pairs.

The Principles of Heredity 291

4. What is meant by blended inheritance? Incomplete dominance?
Multiple factor inheritance ? Interaction of genes ? Illustrate with
examples.

5. What is the advantage of progeny selection?

6. Discuss the advantages and disadvantages of inbreeding.

7. Describe and explain hybrid vigor, giving an example.

8. Of what use is the hybridization of widely different varieties or
species to the inbreeder? What are the difficulties encountered in
this process ? Give specific examples.

9. Explain the sterility of species hybrids, such as the mule.

10. Explain the manner in which chromosome combinations deter-
mine sex.

11. Explain why a color-blind rnan whose father was color-blind but
whose mother was not color-blind would not have inherited his
color blindness from his father.

GLOSSARY

dominant (as applied to genes) Expressing itself in the appearance
of an organism when present with the opposite paired gene.

gene A small particle within a chromosome which influences one or
more hereditary characteristics.

genetic recombination A change in gene constitution from one genera-
tion to another, resulting from the separation of gene pairs in reduc-
tion division and their coming together again in fertilization.

genetics (je-ne'tiks) The scientific study of inheritance.

genotype ( je'no-tip) An organism characterized in terms of the genes
it possesses.

germ cells The group of cells which eventually gives rise to the
gametes.

hybrid vigor Increase of size and vigor in the offspring of a cross
between two different varieties or species.

inbreeding The breeding together of brothers and sisters, or other
closely related individuals.

phenotype (fe'no-tip) An organism characterized in terms of its
observable hereditary traits.

Progeny selection Selection of animals for breeding purposes accord-
ing to the progeny that they have already produced.

recessive (as applied to genes) Not expressing itself in the appear*
ance of an organism in the presence of the opposite paired gene.

CHAPTER XIV

THE FACT OF EVOLUTION

The Incontrovertible Fact. — All tribes and nations of men
have some story concerning the beginning of things. Some tell
how the earth was dragged up from the bottom of the ocean like
a fish in a net; others, how some creative deity, armed with a
great wind, wrought an ordered world out of primitive chaos.
Many think of the origin of the universe as being like the birth
of a living thing, as do those who relate how the world was once
a great egg which had to be chipped open. Such a myth or theory
of the origin of things is known as a cosmogony.

Scarcely a hundred years ago the accepted cosmogony among
Christian peoples was the story in the book of Genesis, according
to which the entire world was created in the course of six days at
a date something like 4000 B.C., and man was formed out of dust
by a special creative act on the last day. Today the accepted belief
is that it is impossible to date the beginning of the universe;
indeed, that it is likely to have been always in existence, that aeons
ago the earth on which we live began to be formed of material
derived from the sun, that about two billion years ago life began
on earth in a very primitive form, that all present living forms
are the descendants of the simple forms with which life began,
and that man himself is a product of this long course of evolution
and is kin to all other living things on the earth.

Here is a revolution in thinking as drastic and perhaps as im-
portant to the life of man as any political revolution that has ever
taken place. Although the observing, experimenting, and reason-
ing of thousands of men have gone into the bringing about of
this change, it was the publishing of Charles Darwin's Origin of
Species in 1859 that really marked its beginning. Throughout
the history of human thought there have been men who have
advanced the theory of evolution as an explanation of the com-

292

The Fact of Evolution 293

ing into being of the world, but none of them offered anything
like complete proof for their theory. When Darwin published
his book in 1859, he had been working for over twenty years,
amassing a tremendous array of facts to back up his views. So
cogent were his arguments that the scientific world was forced
to investigate them. More and more evidence was unearthed that
tended to establish the theory, until, at the present time, no un-
prejudiced student can possibly reject what the authors of The
Science of Life have termed "the incontrovertible fact of evolu-
tion/' and no responsible scientist does reject it.

The universality with which scientists accept the fact of evolu-
tion needs to be emphasized, since there is a widespread popular
misconception to the effect that they are in doubt about the mat-
ter. Scientists are in doubt as to just how evolution has come
about, but they do not for a second question the fact that it has
occurred. Chapter XVI will be concerned with the theories about
the way in which evolution has taken place. At present we shall
consider the fact and the evidence for it.

First, it is necessary to point out just what the fact of evolution
is. Many people have a hazy notion that evolution means simply
that men have descended from monkeys. That is true, if we un-
derstand that our monkey ancestors were not exactly like any
of the monkeys of today, and that they were ancestral not only to
us but to all the present-day monkeys ^nd apes as well. But man's
descent from ape-like and monkey-like creatures has been only
an insignificant part of the entire process of evolution. Back of
our monkey ancestors were ancient reptiles, the common ances^
tors of man and all other mammals. Back of the reptiles, the ear-
liest amphibians, who were ancestral not only to the present-day
frogs, toads, and salamanders, but to the reptiles, birds, and the
mammals as well. The amphibians, in their turn, were descended
from fishes, the fishes from worm-like creatures, the nature of
which we can only guess at today; and still further back were
primitive microscopic protozoans, the ancestors not only of our-
selves but of all the animals the world has ever seen. And, finally,
we might carry our ancestry back to the primal, undifferentiated
bits of protoplasm, from which we believe all life, both plant and
animal, to be derived.

294 The Fact of Evolution

The History of Life. — The story of living things as the biol-
ogist now views it may be briefly summed up as follows :

Hundreds of millions of years ago — the best estimate at the
present time fixes the date between one and two billion years be-
fore the present epoch — the first tiny bits of living matter began
to appear. There is no way of knowing just what the earliest rep-
resentatives of life were like, but we may think of them as ultra-
microscopic globules of protein colloids forming themselves about
the edges of quiet, rock-rimmed pools.

Even today we find that the distinction between the living and
the non-living is vague. Certain filtrable viruses, such as that
which produces the mosaic disease of tobacco, are now known to
be only protein molecules, but yet they can reproduce themselves
in precisely the same manner as do living organisms. In fact, they
can produce alterations which may be perpetuated and which
are therefore comparable to similar changes, or mutations, which
are an important factor in the evolution of living organisms.
Although such substances as these viruses must be of more recent
origin than the complex organisms on which they live, similar
proteins, which could exist independently, were probably the in-
termediate stage between typical non-living substances and the
earliest forms of life.

These highly complex proteins, which themselves were probably
built up or evolved during long ages of purely chemical activity,
must have had two characteristics not found in other non-living
things. First, they could divide in two and then build themselves
up again, so that a single one could develop into thousands. This
was the beginning of growth, reproduction, and heredity. Second,
changes could take place within them; hence certain of these pro-
teins came to be different from others. This was the beginning
of variation. As soon as these bits of protein became organized
into cells, life as we now know it had appeared. Thus the evolu-
tion of life itself was a long, slow process, and may have taken
place in several slightly different ways on different parts of the
globe.

As these primitive bits of life became more abundant, competi-
tion arose among them for the advantages of their environment.
Those which varied in the direction of developing new methods
of exploiting their environment, of adapting themselves to the

The Fact of Evolution 295

different conditions of new areas, or of protecting themselves bet-
ter against unfavorable conditions, were the most successful. Be-
cause of the continual slight changes that went on among them,
certain types came to exploit the environment in one way, certain
types in another; and the various types came to have their own
peculiar methods of shielding themselves against destruction.

Any modifications which appeared in these early forms of life
which did not make them capable of protecting themselves or
adequately exploiting their environment resulted in the disappear-
ance of those organisms from the scene of action. Only those types
that had effective means of maintaining their existence survived.
The result was that as life went on, there was a continuous,
though extremely slow, change going on in all forms. A single
type of organism might vary in many ways to produce thousands
of new types, some of which won out in the struggle for existence,
while the great majority disappeared from the face of the earth.
Thus, through aeons of time, new phyla, new classes, new genera,
new species were formed through the natural selection of those
members of the older species that were best fitted to survive in
the environments in which they found themselves.

This process of descent with modification is the central fact
of evolution. Present-day forms are simply modifications of earlier
ones, with the complete line of their descent running back mil-
lions of years to some common group of ancestors. The modifica-
tion has gone on in such a manner as continually to produce new
adaptations to the environment; and thus have come into being
all the cunning and often weirdly intricate methods of getting
along in the world which living organisms exhibit. The modifica-
tion in the direction of adaptation has been brought about, in
part, at least, by the natural selection of those varieties best fitted
to survive. And as this endless unfolding of ever new forms of
life has gone on, there has been a continual production of more
and more complex types, capable of exploiting an ever-increasing
range of the environment. The first life must have been confined
to shallow, stagnant waters. Perhaps it was not even able to
manufacture its own food through the activity of chlorophyll, but
had to depend upon picking up energy-yielding inorganic com-
pounds. But at some time chlorophyll appeared, bringing to life
the possibility of maintaining itself wherever there was sunlight

296 The Fact of Evolution

The great plant kingdom began to spread itself abroad through
all the waters of the earth. Animals probably appeared after the
green plants, since they could not have existed unless food sub-
stances were manufactured for them by photosynthesis. They
may have descended from certain single-celled plants that lost
their chlorophyll, ceased to manufacture food, and began to live
as robbers on their more stable and industrious neighbors in the
plant kingdom.

As the long ages passed, more and more complex forms came
into being. After the appearance of organized cells, there came
the joining of those cells into colonies and then the organization
of the colonies into multicellular forms. These earliest multicellu-
lar organisms, however, have left practically no remains by which
we can tell just what they were like. The oldest known remains
of living organisms are certain spherical masses of lime laid down
in successive thin layers, resembling similar structures built up by
the secretion of very simple types of algae in ponds, streams, and
shallow seas today, and were probably made by the remote ances-
tors of these algae. Among the oldest animal remains, dated at
about five hundred million years ago, are the shells of various
shellfish, some of them nearly identical in appearance with those
of modern forms; the remains of primitive crab-like animals;
and the tracks and limy casings of certain marine worms. Life
was undoubtedly confined to the water, principally the ocean, for
long ages after it began, and all of the principal groups of ani-
mals were evolved in this medium. Fishes or fish-like forms
were the first vertebrates to appear, and they, along with the sea-
weeds, were the most highly developed forms of life for many
ages. Not until about 350 million years ago, after the greater part
of the history of life to date had been enacted, did the first plants
of any importance make their way on to the land. The first of
these were small, rush-like marsh plants which reproduced by
spores; but soon there appeared giant ferns and trees related to
them and particularly to our modern "club mosses" or "ground
pines." At nearly the same time there arose plants whose fern-
like leaves bore seeds at their tips — the earliest seed plants. These
seed ferns, along with the tree ferns and their allies, formed vast
forests which, in an age when the climate was warm and moist,
stretched from pole to pole. Most of the coal fields of the present

Early fern-like plants

Fern seeds

found on extinct

tree ferns

Later fern plants

FIG. 71. — Primitive land plants.

AMPHIBIANS

REPTILES
FIG. 72. — Earliest land vertebrates.

The Fact of Evolution 299

day are derived from those ancient forests. Then a long period
of cold and windstorms resulted in the killing off of many of
these more ancient plants and the coming into dominance of new
types of seed plants, many of them cone-bearing. The flowering
plants (Angiosperms) did not come to the fore until about a
hundred million years ago, bringing with them the types of vege-
tation we find today.

As soon as the plants had taken up their abode upon the land,
animals followed. The first land animals appear to have been fore-
runners of the insects which, however, had not yet developed
wings. As time went on, the insect forms became better and better
adjusted to land life, and all their marvelous specialized adapta-
tions were developed. Among the more important of these was the
relation that grew up between insects and flowering plants, whereby
the insects came to cross pollinate the plants and the flowers fur-
nished food for the insects. This adaptive partnership gave both
insects and flowering plants a tremendous advantage in the struggle
for survival. Consequently it has had a great influence in pro-
ducing the present-day characteristics of life, making the flower-
bearing plants our dominant plants and the insects our most nu-
merous animals.

Some three hundred million years ago our own ancestors
started to come out upon the land. At that time the fresh-water
pools and streams were rapidly drying up. Certain fishes whose
fins were thicker and more fleshy than those of most of the fish
we know began making their way from one pool to another by
crawling with the help of their muscular fins. As millions of
years went by, their fins were gradually transformed into legs,
while their air bladders — structures that are found in practically
all fish— developed into lungs. They became the first amphibians,
laying their eggs in the water and spending the larval or tadpole
stage of their lives there. As adults they were clumsy beasts, still
retaining much of the bodily form of the fishes from which they
were descended. They found their food in the water and used the
land merely to get from one pool or stream to another.

In the course of some fifty million years they gave rise to a class
of true land dwellers, the reptiles, who possessed tough, scaly skins
and laid their eggs on the land, encased in a protective shell, thus
avoiding the tadpole stage in the water. The history of life con-

CTRICERATOPS (25 ft)

PTERODACTYL (6 ft)

TY8ANNOSAURUS (47 ft)

DffLODOCUS (87 ft)

Fta 73.— Extinct reptiles. (Redrawn from Lull's Organic Evolution, The Mac-

millan Company.)

The Fact of Evolution 301

tains no more interesting chapter than the story of the reptiles.
As the first large animals capable of living continuously on the
land, they advanced rapidly into this virgin territory. Quickly they
differentiated into thousands of forms that seem utterly weird to
us, accustomed as we are to the animals of the present day. There
were tiny little plant eaters, light and lithe, running about on their
slim hind legs and displaying surprising speed and agility in escap-
ing from their fierce and dangerous flesh-eating relatives. There
were immense and ponderous feeders upon the heavier vegetation,
the largest animals ever to walk upon the earth, with long necks
and tiny heads, humped backs and clumsy, triangular tails ; there
were swift-flying pterodactyls, featherless, with parchment wings
like bats and long muzzles filled with saw-like teeth; there were
ichthyosaurs that swam like fish and plesiosaurs that navigated
over the surface of the sea, their legs transformed into paddles and
their tails into rudders. For over a hundred million years these
uncanny beasts reigned over the land ; and then they mysteriously
disappeared, leaving only a few inconsiderable remnants — the
snakes, lizards, crocodiles, aad turtles — to survive into the present,
abandoning the world to the warm-blooded offshoots of their line,
the birds and mammals.

While the race of reptiles was still comparatively young, it gave
rise to a new type of animal, a group of small, light-boned quad-
rupeds, with hair covering their bodies, with warm blood coursing
through their veins, and with larger brains than were to be found
in even the largest reptiles. The reptiles could never have been
very intelligent. Their brain cases were small; at the largest,
scarcely capacious enough to hold a man's thumb. One of the great-
est dinosaurs had a nervous ganglion located far back in its spine
that was larger than the brain in its head. It has been suggested
that

If something slipped his foremost mind
He caught it on the one behind.

But it was probably easy for slips to occur in both regions.

Some time after the appearance of the first mammals, the birds
evolved from some group of fast-running or soaring reptiles, sub-
stituting feathers for scales to protect their warm bodies from the

302 The Fact of Evolution

cold and also to increase their buoyancy in the air, and replacing
the bony reptilian tail with a clump of feathers.

For long ages of time, the birds and mammals were strictly
subordinated to the reptiles. The mammals were small creatures
living on insects. At first they laid eggs like the reptiles — as the
reader knows, there are still a few representatives of the egg-laying
mammals alive today — but they guarded over their young and

FIG. 74.— The duckbill.

suckled them. Then forms arose that gave birth to their young
alive, but in such an undeveloped state that they had to be carried
in a pouch fastened to the mother's belly as do the young of the
present-day kangaroo and opossum. Finally, at about the time the
age of the reptiles came to an end, they developed the modern
placental mode of reproduction.

The importance of the mammalian mode of reproduction in
making possible the evolution of a being like man can scarcely be
overestimated. It is not merely that the family life which has
grown out of it has formed the central core of man's existence and
has been basic to his social, political, moral, and religious develop-

The Fact of Evolution 303

rrient. In addition to that, the long period spent in the mother's
body, nourished by the mother's blood, makes possible a complexity
of development that cannot possibly go on in an egg, where the
supply of food material contained in the yolk is always limited.
More important still is the period of infancy and youth subsequent
to birth. The young reptile or the young insect, coming into a
world where it must immediately shift for itself, must be born
with a set of almost automatic responses with which to meet the
emergencies of life. It has no time to learn. But intelligence de-
pends on the ability to learn, and learning depends upon the ability
to make mistakes and then correct them. Only a young animal
blessed with parents to watch over it can afford to be born with
this dangerous, but valuable, capacity for making mistakes.1

About sixty million years ago, the placental mammals began
their course of differentiation and increase. The most primitive
of them were probably small, furry, five-toed insect eaters, appear-
ing somewhat similar to the modern woodchuck. From such an
animal have been developed all the multitudinous types of higher
mammals which we know today. In one direction there was a de-
velopment of carnivorous forms, with sharp claws, lithe bodies,
and dangerous fangs — cats, lions, tigers, dogs, wolves, bears,
otters, seals, sea lions, and their kindred. Another direction in
mammalian evolution was taken by the herbivorous animals. Since
they did not develop the weapons of the beasts of prey, speed in
running became essential to them. They began more and more to
get up on their toes to run, their claws became thickened to form
hoofs, and finally, we find them running on hoofs that have been
developed from the claw of only one or two toes. The horses,
rhinoceri, and tapirs belong to the order of odd-toed ungulates
(hoofed animals), and the deer, cattle, camels, and a number of
others, to the order of even-toed ungulates. The horses run on a
hoof that is derived from a single toe, comparable to our middle
finger. The even-toed ungulates run on a "cloven" hoof that is
derived from the toes comparable to our third and fourth fingers.

Many other mammalian orders evolved during this period. In

the bats, the front limbs were transformed into wings. Elephants

grew to ponderous proportions, although it is possible to trace

them back to an animal that stood a scant two feet high. The most

1 Cf. the description of trial and error learning, Chapter XXIV.

304 The Fact of Evolution

remarkable transformation from the small, furry ancestor of the
mammals was that undergone by an order that early took to the
water and underwent an evolution that produced the whale, the
largest animal the earth has ever seen. Early in the period of
mammalian dominance, certain rat-like forms, known as tree
shrews, started a new line of evolutionary development, involving
adaptation to life in the trees. From them arose the order Primates
— the monkeys, apes and men. This line of development will be
considered in a later chapter.

Some Principles of Evolution. — Although a detailed study of
the extinct organisms which were the ancestors of our modern
forms is the function of the paleontologist rather than the biol-
ogist, four principles have arisen from this study which are of
primary importance to all students of evolution. In the first place,
there is little doubt that evolution, although always a gradual
process, was more rapid at some times than at others, and took —
and is taking — place more rapidly in some groups of organisms
than in others. There was, for instance, a great burst of evolution
at the time when both animals and plants were beginning to con-
quer the land. Not only did many new organisms adapted to land
life appear at this time, but there was also a rapid evolution of
new kinds of fishes and other marine animals simultaneously.
Somewhat later there came a long period, at the time when the
coal beds were being formed, when evolution among both animals
and plants progressed rather slowly. This was followed by a period
of rapid evolution when the giant ferns, fern allies and seed ferns
of the coal measures, with their accompanying large amphibians,
were largely replaced by the cone-bearing seed plants and the rep-
tiles. Similar periods of rapid and slow evolution have followed
each other right down to the present. We are now in a period of
relatively rapid evolution. Man has been on the globe, for the rela-
tively short period of a million years, but within this time many
hundreds of species of both animals and plants have evolved which
are adapted only to the regions of human habitation. Rats and
mice, lice, fleas, and many other insects may be mentioned, as
well as a host of weedy species of plants. Some of these probably
have existed since before the time of man, but others are so nar-
rowly adapted to the surroundings of mankind that they must
have been evolved more recently than man. At the same time we

The Fact of Evolution 305

know of many hundreds of species, some of them formerly the
dominant forms of life on the earth, such as the mammoth,
mastodon and saber-tooth tiger, which have become extinct within
the last two or three hundred thousand years ; and a good propor-
tion of them have vanished within the infinitesimally short period
of recorded human history.

These periods of rapid evolution always occurred at times of
great change in the earth's surface. The rapid development of the
earliest life on land was accompanied by a repeated advance and
retreat of shallow seas in many parts of the world, particularly
the central and eastern United States. The extinction of the species
of the coal measures was brought about chiefly by the building up
of great mountain ranges and the inundation of a vast ice sheet;
similar conditions of mountain building and glaciation have pre-
vailed in recent times. This correlation is easy to explain. If the
environment of the earth remains stable, animals and plants be-
come perfectly adapted to the prevailing conditions; hence, any
member of a species which varies in any way from the character-
istics of the species will be less well adapted than others, and all
variations from the species pattern will tend to die out. However,
under changing conditions, variations adapted to the new environ-
ments will be favored, and the result will be a more rapid change,
or evolution.

A comparison of the rapid evolution of man with that of some
other animals will serve to illustrate the second principle, i.e.,
that evolution has progressed at very different rates in different
groups of organisms. A number of species of man have evolved
and become extinct within the last few hundred thousand years,
and one of them has completely altered its distribution and its
way of living within the last thirty thousand. On the other hand,
many insects, such as ants and termites, have apparently changed
little from their ancestors of thirty or forty million years ago, and
there is a genus of shellfish now living which has existed in prac-
tically the same form ever since the oldest of the well-preserved
series of fossil beds was laid down about five hundred million
years ago. At present there is no explanation of why evolution
should progress more rapidly in some groups of organisms than
in others ; but evidence from both living and extinct forms points
to the hypothesis that groups of organisms evolve very rapidly

306

The Fact of Evolution

BRYOPHYTES
.THALLOPHYTES I PTERIDOPHYTES SEED PLANTS (SPERMATOPHYTES)

r * > r* — > c . * "> t *

PROTEROZOIC

\ • Flagellates (?)

ARCHEOZOIC

Autotrophic bacteria (?)

FIG. 75. — The evolution of plant life.

The Fact of Evolution

307

FIG. 76. — The evolution of animal life.

308 The Fact of Evolution

when they first appear, and then their rate of evolution gradually
slows down.

A third fact that most modern evolutionists recognize is that
new groups of organisms do not evolve from the most advanced,
highly specialized representatives of the groups already existing,
but from relatively primitive, unspecialized forms. For instance,
although mammals evolved from reptiles, they did not come from
the most common and highly developed reptiles that existed at the
time when the mammals first appeared, i.e., the dinosaurs. The
ancestors of the mammals were small, unspecialized reptiles, which
occupied a rather lowly position in the life of their day. The evo-
lutionary tree of life, therefore, must be conceived not as a tall
pine which sends out new shoots from the tips of the old, but as
a much branched shrub which, when one branch is getting old or
top-heavy, sends out new branches from near its base, which
appear at first insignificant, but finally overtake and surpass the
old ones. A further complication of the evolutionary tree, which
makes the entire simile of a growing tree rather inadequate as a
representation of the true course of evolution, is the importance
of hybridization in the evolution of at least some groups of or-
ganisms. This point will be discussed more fully later, but we may
mention here that any evolutionary tree representing the true
course of development of a group of organisms may have in some
sections a network of interlocking branches. So, while recent dis-
coveries have tended more and more to confirm the fact of evolu-
tion, we know now that its course has not been nearly so simple
as it was conceived by the early evolutionists.

A fourth fact that is evident to anyone who knows the world of
living organisms, as well as the fossil remains of extinct ones, is
that evolution has not always progressed "upward," that is, from
simple forms to more complex ones. There are many cases of
regressive evolution, or "degeneration/* in both plants and animals.
For instance, the kiwi of New Zealand and other wingless, flight-
less birds are undoubtedly descended from winged ancestors. Most
modern mammals, furthermore, are simpler and more "lowly"
than their forbears of a million years ago or so. The giant deer,
tigers, and elephants now known to us only as fossils were far
superior in strength and size to their present-day relatives. In
addition, there is a whole host of parasitic animals and plants

The Fact of Evolution 309

which are extremely simple compared to their free-living ancestors.
Lice and fleas, for example, have evolved from more elaborate,
probably winged insects, and, as already pointed out, such para-
sites as the malarial protozoans are much more simple than their
non-parasitic relatives and presumably their ancestors. In fact,
most biologists picture evolution not as a steady progress of life
toward higher levels, but as a fluctuation and diversification of
organisms more or less at random, or according to the selective
activity of the environment. In some cases a more highly devel-
oped body or brain gives the organism an advantage over its
competitors or enables it to occupy a new environment, while in
others a similar advantage or opportunity is obtained by simplifi-
cation; in either case the new complexity or simplification will
survive only if it is well adapted to some part of the environment
available to it.

The Evidence for Evolution. — We may sum up the course of
evolution in a few words : Living things have varied ; those varia-
tions incapable of adapting to their environments have died out;
as a result of these and perhaps other causes the life of today has
arisen from exceedingly primitive beginnings. This is the fact
which no scientist denies. What, then, are the evidences for this
fact? The truth of the matter is that there is such a tremendous
array of evidence that a lifetime might easily be spent in finding out
what it is, criticizing it, and coming to understand it. For purposes
of briefly describing this vast array, it may be classified under the
following four headings :

1. Evidence from the fossil record of animals and plants that
lived in bygone eras.

2. Evidence from the anatomical and physiological relation-
ships between animals of the present day.

3. Evidence from the geographical distribution of present-day
forms.

4. Evidence from the science of genetics.

The Fossil Record. — For hundreds of millions of years rivers
have been flowing into the sea, bearing with them fine particles
of soil which are deposited on the ocean floor or above the water
level on river deltas. Thus layers of earth have been piled up, one
on top of another, and have gradually been turned to rock through
the agencies of pressure and chemical change. On land, similar

3io The Fact of Evolution

deposits have been laid down, both by water and by wind. Thus
the sedimentary rocks, the shales and sandstones, have been
formed.

In the deposits of soil which formed the sedimentary rocks, the
remains of plants and animals have been embedded; and, while
most of them have quickly rotted away, others have been preserved
or at least have left traces of themselves in the rocks that have
been formed as the soil deposits were buried beneath the surface
and subjected to increasing pressure. Later, through the folding
of the earth's crust, these rocks, even those laid down in the bed
of the ocean, have been upheaved to form dry land; and wind,
frost, and water have worn them down, exposing the records of
ancient life which were made in them during the period of their
deposition. These records are called fossils.

We usually think of fossils as being those great brown skeletons
of dinosaurs which look down on us from the main hall of most
of our museums, but, as a matter of fact, there are many other
types. Some of the oldest known fossils are the shells of seashore
animals and the winding tracks made by worms burrowing through
the primeval ooze. Other useful fossils are the footprints of ani-
mals, while most of our knowledge of prehistoric man has been
obtained from a study of the tools which he made and discarded.
These may in the broader sense be considered fossils.

Fossils constitute a record of the history of life, but it should
not be supposed that this record is anything like complete. In the
first place, the vast majority of plants and animals never leave
any trace of themselves in the rocks. In the second place, the most
ancient rocks have been upheaved into mountain chains and then
worn away by weathering to such an extent that the oldest fossil
deposits have well-nigh disappeared. Or, when they have remained,
the rocks have been subject to such tremendous pressures that
nearly all fossil remains have been crushed out of them. Never-
theless, we do have a fairly legible, though by no means complete,
record of life extending back over the past five hundred millions
of years.

By noting what rock layers lie on top of what others, it is pos-
sible to judge the age of the different layers. That is, we know that
the ones on the bottom were laid down before the ones on top. Of
course, we never find all the layers of rocks stacked one on top of

The Fact of Evolution 311

the other at one place. But if at one place we find layer A above
layer B and at another place layer B above layer C, it is obvious
that layer C was laid down first, layer B next, and layer A last of
all. Thus, by piecing together the relations of the layers in all parts
of the earth, it is possible to put each one in its place, from the
oldest to the most recent.

When this is done, and when the fossils in this series of layers
are studied, they produce a picture of ever-changing life forms, one
type coming after another in regular succession. And thus the
history of life can be traced from the study of the rocks as cer-
tainly as can the history of the United States be traced from the
study of written documents.

It is even possible to assign approximate dates to the rock
records by noting the percentage of uranium in a deposit that has
turned to lead in the process of radioactivity. Since the speed of
radioactivity is known, the length of time that uranium has been
turning itself into lead in that deposit can be quite exactly calcu-
lated. With certain deposits dated in this fashion, the ages of
others can be calculated by their positions relative to the dated
deposits.

The fossil record has been divided into five great eras : ( I )
the Archeozoic, extending from approximately two thousand mil-
lion years ago to approximately eleven hundred million years ago ;
(2) the Proterozoic, extending from approximately eleven hun-
dred million to about five hundred fifty million years ago; (3) the
Paleozoic, from about five hundred fifty million to something like
two hundred million years ago; (4) the Mesozoic, from two
hundred million to about sixty million years ago; and (5) the
CenozoiCj from the end of the Mesozoic to the present time.

The entire testimony of this fossil record points to continuous,
gradual change from one form of life to another. Before the
appearance of a new form of life, the fossil record usually shows
transitional forms. For example, it is possible to find in the rocks
of the early Mesozoic period the so-called dog-toothed reptiles,
having characteristics halfway between those of the true reptiles
and the mammals. There are also fossil forms which can be
classed neither as reptiles nor as amphibians which appear in the
fossil record just before the reptiles themselves come on the scene.
There is a very famous fossil of a bird, known as Archeopteryx,

312

The Fact of Evolution

that lived about a hundred million years ago, and that had feathers
like a bird's, but a tail like a reptile's. It also had reptilian teeth and
reptilian clawed fingers projecting from its wing.

In some places the fossil record is so complete that one can fol-
low the gradual evolution of the same or similar forms into widely
different types. For instance, in deposits laid down on the bottom
of an ancient lake in Germany over a period of many thousand
years, the series of shells shown in Fig. 77 was discovered. The

FIG. 77. — Evolution of snails.

four lowermost shells, found in the oldest beds, are probably
different races of the same species. From them, as higher and
higher beds are reached, is shown the gradual development of
seven distinct lines. These terminate in forms so different from
each other that, if the intermediate connecting links were not
known, they might even be considered different genera. Fig. 77
shows some of the shells found, although, for the sake of sim-
plicity, a number of the intermediate forms are omitted.

There are few forms of life for which the fossil record is as
complete as this, yet for certain of our most common mammals
fossil "pedigrees" have been worked out that are amazingly de-

The Fact of Evolution 313

tailed. For example, in the fossil beds of the early Cenozoic period
are found the remains of a small four-toed animal about the size
of a large cat that apparently trotted through the underbrush of

PLEISTOCENE AND PLIOCENE
(Equus)

Fore foot Hind foot

MIOCENE

(Protohippus)

OLIGOCENE

(Mesohippus)

EOCENE

(Protorohippus)

(Eohippus)

FIG. 78. — Evolution of the horse.

the forest, browsing off the lower leaves. From this animal we
can trace a gradual line of descent in which each change is, in
itself, very slight, but in which there is a continuous increase in

314 The Fact of Evolution

size, a continuous growth in the length and strength of the teeth,
and a continuous decrease in the size and function of all toes
except the middle one, until this line of fossils leads directly up to
the horse of today. It is obvious that the' change has come about
in order to adapt the horse to life on the plains, where speed is
necessary to escape enemies and good teeth are essential for grind-
ing the dry, flinty grass. We have similar lines of fossil ancestors
in almost as perfect detail for the elephants, the giraffes, and the
camels.

In brief, the fossil record proves positively that life has been
changing very gradually, from one form into another, over an
almost inconceivable period of time. None of the evidence points
to the conclusion that life was originally created in its present
form.

Similarities Between Organisms. — We readily recognize that
among human beings similarity points to blood relationship; the
greater the similarity, the closer the relationship. And this fact is
true throughout the world of life. Our similarity to the apes, the
dog's similarity to the wolf, and the apparently remote similarity of
the whale to other mammals, all are indicative of kinship. Not
only do organisms show likeness to one another, but many like-
nesses are not readily accounted for except on the assumption
that the organisms are related. Such similarities range all the way
from resemblance of feature to affinity in the fundamental chem-
ical basis of the blood and other tissues, but the most striking
and best known are the similarities in anatomical structure and in
embryological development.

The Evidence from Comparative Anatomy. — Anybody who
studies the form and structure of different organisms will find
remarkable resemblances between them, and many peculiarities
about individual types, which taken by themselves are merely odd
vagaries of nature or of the Creator. However, if one thinks of
these various types of organisms as being related to each other
and descended from a common ancestor, these resemblances and
peculiarities immediately become very significant, and point the
way to the roads that evolution has taken.

Although we can find resemblances which tell us about evolution
in almost any group of animals or plants, we shall here focus our
attention on the mammals since it is this group of animals that is

The Fact of Evolution 315

most familiar to us. Compare the skeletons of different mammals
and see how alike they are in the very different animals! Our
arm and hand, the dog's foot, the bat's wing, and the seal's flipper,
although each is put to an entirely different use, have almost ex-
actly the same number of bones, arranged in the same relation to
one another. On the principle of descent with modification, we
can easily explain these resemblances by assuming that the common
ancestor of these animals had a forelimb with just so many bones
arranged in a certain way, and that his various descendants have
had their forelimbs modified only enough so that they would be
suited to the purpose for which they were used. The neck of the
giraffe has just seven vertebrae in it, each vertebra almost a foot
long. The whale, which has no neck at all, and no need of it, has
seven very much flattened vertebrae in the place where its neck
should be. By this fact we can reason that the common ancestor of
mammals had just seven vertebrae in its neck. It was easier, as
mammals became adapted to very different surroundings, for the
bones to change in shape, than for a bone to be lost or added. So,
by a gradual process of change, the long neck vertebrae of the
giraffe and the flattened ones of the whale were evolved from the
medium-sized vertebrae in the neck of their common ancestor.

Striking as are these resemblances between organisms, there is
another type of evidence which we get from comparative anatomy
that makes us even more certain of evolution. This is the presence,
in almost every plant or animal, of useless organs or bits of organs
which to that organism are certainly of no use whatever. If we
think of plants and animals as having been specially created, we
can interpret these bits of organs only as the superfluous vagaries
of an over-enthusiastic and sometimes very bothersome Creator.
But if we consider that all organisms are derived from common
ancestors through a gradual process of change, we can always find
a reason for these supposedly useless afterthoughts of the Creator.
They are the relics or vestiges of organs which in more simple,
less specialized animals are well developed and functioning. These
structures are therefore called vestigial organs.

The best-known example of a vestigial organ is the vermiform
appendix of man. This little "blind alley" leading off from the
large intestine can have no function for us, since thousands of
people have had it removed and are living perfectly normal,

316

The Fact of Evolution

healthy lives without it. Yet many other animals have a well-
developed appendix which is very useful as an extra stomach for
the digestion of hard bits of food. The rabbit's appendix is a fine
example (Fig. 79). Long ago, however, our ancestors began eat-
ing foods that did not require so much digestive labor. Therefore,
the appendix has gradually dwindled in our line until it is but a
useless vestige.

Another set of vestigial organs in man is a group of muscles
around the ear. These, in many animals with large outer ears,
such as the donkey, serve to move the outer ear about toward the

^"*i*HBiHSWiSijjjgj[flpp*

FIG. 79. — Appendix of rabbit (A) and man (J5).

direction from which sound is coming, and hence to help the
animal hear more distinctly and to catch the direction of a sound.
In man, the outer ear is of very little value anyhow, and few peo-
ple are able to move it at all. Yet all of us have these muscles which
in most people are simply useless vestiges, relics from our ear-
wagging ancestors.

The number of vestigial organs which are found in other ani-
mals is very large and a few of the more striking examples de-
serve mention. Everybody who is familiar with horses knows that
the horse has, alongside of the longest bone in the lower part of
his leg, between the so-called "knee" and the "ankle," two small
thin bones, known as splint bones. These slender bones are simply
embedded in the flesh beside the main bone, do not support any
structure in particular, and have therefore no conceivable func-
tion. Yet, when we study the anatomy of the horse's leg, we find
that the part below the "knee" is simply the much elongated middle

The Fact of Evolution 317

digit and that the splint bones are the remains of what in our hands
are the longest bones of the second and fourth fingers. The horse,
which walks entirely on its middle fingers and middle toes, has no
need at all for these extra digits, but their bones still persist as
vestiges beside the large bones of the middle digit.

Many other examples might be mentioned. The vestigial hind
limbs of the whale, buried in the flesh at the beginning of its tail ;
the little hooked claws of the under surface of the python which
are all that is left of its hind legs; the vestigial wings of such
flightless birds as the New Zealand kiwi and of flightless insects
such as certain types of ants — all these show that the animals pos-
sessing them have been derived by descent through modification
from less specialized ancestors.

The Evidence from Embryology. — Still another line of evi-
dence for evolution is derived from a study of the development
of a single individual, from the time when it begins as a fertilized
egg until it reaches maturity. We have already seen how all of the
higher animals develop from a fertilized egg — even such different
ones as worms and mammals. The embryos of the various main
groups of animals follow different courses of development, but as
a rule animals as distantly related as barnacles and crabs, or fishes
and mammals, have embryos which are much more like each other
than the adult animals are.

Thus in the development of single individuals of different kinds
we see two fundamental tendencies : first, the change from sim-
plicity, as in the egg, to varying degrees of complexity, as in the
adults of various animals; and, second, the divergence from the
egg, which is much the same in all the higher animals, to very
different shapes which adult animals assume. These two trends
are essentially the same as the main trends of evolution — from
simplicity to complexity, and from similarity to a multitude of
dissimilar forms.

Thus the development of an individual is a sort of evolution
in itself and tends in part to duplicate the line of evolution which
the individual's ancestors have taken. When this fact was realized,
about seventy years ago, Ernst Haeckel made the famous statement
that "Ontogeny is a short recapitulation of Phytogeny." This re-
mark means that the development of an individual is a brief
resume of the evolution of its race.

FISH SALAMANDER TURTLE

CHICKEN

FIG. 80. — Comparison of vertebrate embryos. (Redrawn from Lull's Organic
Evolution, The Macmillan Company.)

HOG

CALF

RABBIT

MAN

Fie. 81.— Comparison of vertebrate embryos, continued. (Redrawn from Ltdl's
Organic Evolution, The Macmillan Company.)

320 The Fact of Evolution

There are two facts which should, however, be kept in mind
when one considers this law. In the first place, the embryo never
goes through the adult stages of its evolutionary ancestors, but
only tends to resemble the embryo of those ancestors more than
the adults resemble each other. The human embryo never looks like
an adult worm, fish, or reptile, but merely has characteristics in
common with the embryos of those animals. Secondly, there are
many characteristics of, and many structures in the embryo which
have nothing at all to do with the evolution of the race but are
modifications which enable it to live in its particular environment.
Thus the embryo of a mammal has many structures which are
present merely in order to enable it to get food more easily from
its mother and are not found in any of the lower animals. These
recently developed, useful characteristics are sometimes separated
with difficulty from those which reflect the evolutionary ancestry
of the organism.

A fine example of this law of recapitulation is given by the
development of the human embryo. Quite early in its development
it enters a fish-like stage, with rudimentary gill slits and several
aortic arches corresponding to the arches which pass through the
gills in a fish. At this stage the heart has but one auricle and one
ventricle, as in the fish. The backbone contains a long, flexible rod,
the notochord, found in all fishes, and in more primitive verte-
brates. Each vertebra, as in the fishes, consists of several bones.
The kidney is not the one which the adult man will use, but an
entirely different structure, corresponding to the kidney of fishes.
Many other organs resemble those of fishes rather than those of
human beings.

At a later stage the embryo loses its gill slits and develops
lungs, but it still has a tail. The bones of each vertebra fuse, the
heart develops four chambers, and a new kidney, corresponding
to that of the reptiles, develops. Finally the human embryo de-
velops the mammalian kidney, still a third structure, and has the
general form of a human being. However, even when the human
baby is born, it looks much more like the baby of an anthropoid
ape than the adults of men and apes resemble each other. In the
relative size of the head, limbs, and body, in the possession of a
fine coat of hair all over its body, and in the number of ribs pres-

The Fact of Evolution 321

ent, the human fetus before birth resembles the anthropoid apes
more than the adult man.

While, on account of its modified and abbreviated nature, the
story of the development of a human infant cannot give us the
complete history of man's ancestry, yet it is powerful evidence that
man has a common ancestry with the lower animals.

The Evidence from the Distribution of Animals and
Plants. — Everybody knows how different are the plants and ani-
mals to be found in different parts of the earth. Much of this
variation can be explained in terms of climate, temperature, and
topography. Nobody would expect to find the same fauna and
flora in the arctic as in the tropics, in a desert as in a rainy country,
or in fresh water as in salt. Yet every naturalist is familiar with
the fact that regions with similar climates do not always contain
the same animals and plants, and that other causes besides these
present-day ones must be invoked to account for the distribution
of the thousands of species of living things that populate the
globe. Thousands of otherwise inexplicable facts of distribution are
accounted for by the assumption of evolution; and when we study
the distribution of plants and animals in combination with the
geological record of what has gone on in past ages, certain really
astonishing circumstances are readily and interestingly explained.

Among many other striking phenomena of distribution is the
fact that, when the white man first entered Australia, there were
no placental mammals in that entire continent, with the exception
of a few bats and of the native human inhabitants with the mice
and dogs which they probably brought along with them. But
there were all kinds of marsupials, that is, animals that carry their
young in pouches, although the opossum is the only even fairly
abundant marsupial found outside of Australia. In addition, Aus-
tralia contained the only egg-laying mammals in the world. This
strange primitiveness of Australian mammals might receive an
intelligible creationistic explanation if Australia was not a good
place for placental mammals to live ; but the fact is that whenever
placental mammals have been introduced there, they have got
along even more successfully than in their native environments
and have tended to bring about the extermination of the less
efficient marsupials.

Geological history serves to account for this state of affairs.

322 The Fact of Evolution

During the Mesozoic era Australia was connected with the main-
land, and at that time marsupial mammals made their way into it.
But before the placentals arrived the land bridge to Asia sank into
the ocean. The result was that while the placental mammals well-
nigh exterminated the marsupials in all other parts of the world,
the latter were enabled to undergo a rather complete course of
evolution in Australia to produce many fairly complex animal
forms. Confirming this explanation is the fact that the only pla-
centals found in Australia were those that might find means of
getting across wide spaces of water, namely, the bats and man,
with his parasitic dogs and mice.

The reader will probably be surprised to learn that the flora of
the eastern United States, from New York and southern New
England southward and westward to the Mississippi, resembles
not that of the western part of our continent nor yet that of
Europe, but is most nearly related to the flora of temperate Japan
and China. Many groups of species, such as the magnolias, tulip
trees, sassafras, and the walking fern, are found only in eastern
America and eastern Asia.

Now there is no logical explanation for this strange pattern of
distribution if it is assumed that these slightly varying floras were
placed in two such widely separated regions by a special act of
creation. There are many other parts of the world in which such
plants could flourish as readily as in the eastern part of America
and Asia. Only when we view the phenomenon in the light of
known geological history does it receive an intelligible explanation.

In the early part of the Cenozoic era, both North America and
Eurasia were much flatter than they are now. They were con-
nected across Bering Strait and were probably closer to each other
across Greenland, even if no actual land bridge existed there.
Therefore, there was continuous land all around the northern
hemisphere. The climate even in Greenland was mild, so that this
whole area had a smiliar mild climate. At that time we know,
from fossil evidence, that magnolias, sassafras, and other plants
were found throughout the northern hemisphere. Later on in the
Cenozoic, Europe and the western United States changed greatly.
The Rocky Mountains were elevated to their present height and
western North America became almost desert. The Alps appeared,
and much of what is now northern Europe arose for the first time

The Fact of Evolution 323

out of the sea. Finally, the great ice sheet came down from the
north. In Europe it almost united with the ice sheet that spread out
from the Alps, while in western North America the ice sheet came
down practically to the desert and mountain areas and all but a
few of the plants which live in a mild, temperate climate were
exterminated.

But in eastern Asia and eastern America there have been no
great catastrophes to disturb and destroy living organisms. To be
sure, the ice sheet inundated the northeastern United States, but
there was plenty of flat country with a mild, moist climate to the
southward into which the plants could migrate to escape the cold.
Hence these genera and families, which once spread all through
the northern hemisphere, have persisted only in eastern Asia and
eastern America to the present day. And since each group has
gone through a different course of evolution in the two regions,
different species have been evolved, although the genera have re-
mained the same.

Another striking bit of evidence from geographical distribution
is found in the presence of distinct species of plants and animals
in small but isolated regions of the globe. The flora and fauna of
such areas as oceanic islands, solitary mountain summits, and
valleys in mountainous regions, which are cut off by natural bar-
riers to free immigration of organisms, usually contain a large
proportion of species which are found only in those areas and
which are most closely related to species found in the nearest large
region which is similar to them in climate.

For instance, on the Galapagos Islands, five hundred miles off
the coast of South America, a large proportion of species are found
only on one or two islands of the group. It was Charles Darwin
himself who, as a young man, observed this fact, and it did much
to suggest to him the theory of evolution. Thus, twenty-three out
of the twenty-six species of land birds found on the archipelago
are peculiar to it, and many are found only on one island, or on
two adjoining ones. Yet all of them are quite evidently related to
birds of South America. Similarly, there are several species of
giant lizards, all of them peculiar to the islands. There were,
originally, eleven different species of giant tortoise, each inhabit-
ing a different island. All of them were closely related to each other,

324 The Fact of Evolution

but those living on closely adjoining islands were more nearly
related than those on more distant islands.

It would seem strange that a benevolent Creator should present
each of these small islands with its own species of tortoise and
deny tortoises to many other regions that are just as well suited
to them. Yet if we think of these tortoises as having been de-
scended from a common ancestor, we can easily see how that
ancestor may have arrived on the Galapagos Islands many thou-
sands of years ago and become marooned there with his descend-
ants. These descendants have evolved, each in his own peculiar
way ; and those that, from time to time, migrated from one island
to another, became isolated in their new home and went through
their own course of evolution independently. The same principle
would apply to the land birds, lizards, and many of the plants.

Furthermore, there are no land mammals on the Galapagos
Islands. Frogs and other amphibians are absent, as they are from
all oceanic islands. Yet there are many spots on the islands which
are suitable to mammals and amphibians, and such mammals as
have been imported have thrived there. From the standpoint of
special creation, this would seem to show a sad neglect on the part
of the Creator. Yet if we think of mammals as having evolved
comparatively recently, we can see how none of them could have
crossed the ocean to reach these islands. Amphibians also cannot
cross the ocean, since they are not strong enough to cross large
bodies of water, and their eggs are very easily killed by salt water.

The Evidence from Genetics. — The final set of facts which
shows us that new species have been, and still are, evolving from
older forms is the vast amount of variation which men have pro-
duced by breeding animals and plants and which they have watched
under their own eyes.

Long before the beginning of history primitive man domesti-
cated animals and cultivated plants for his own use. He soon
learned to select the individuals best suited to his purposes and, by
breeding them, to improve the race. The final result of man's labor
is the vast number of breeds and races that almost every one of
our domestic animals and cultivated plants possesses. No natural-
ist, if he found a great Dane, a greyhound, a spaniel, a dachshund,
and a Pekingese dog running wild would think of calling them the
same, or even closely related species. Not only are they very dif-

The Fact of Evolution 325

ferent in size and shape, but their habits and their diet are quite
different as well. Yet there is no doubt that all were derived from
a few closely related species of wild dog.

Similarly, most of our cultivated plants exist in a large number
of varieties, many of which, if found wild, would be considered
distinct species. The cultivated wheats are a fine example. There
are scores of varieties: winter wheats, summer wheats, hard
wheats, and soft wheats. Many of these, when crossed with each
other, produce sterile offspring. Yet all are probably descended
from two or three species of wild wheat, of which some are still
found in the Orient. Some garden flowers, moreover, have been
developed so recently that we know their history. The garden
dahlia exists in a large number of varieties. There are single and
double types, "pompons," and "chrysanthemum" dahlias, and
flowers of almost every color. All of these varieties have been
derived, by breeding and selection, from a single species, Dahlia
varidbilis. This species, furthermore, is now known to be the
result of a cross between two simpler, comparatively constant
Mexican species of dahlia.

In recent years many wild species of animals and plants have
been brought under observation and, when bred artificially, have
produced a tremendous number of variations. Literally hundreds
of different races of the fruit fly have appeared in culture, each of
which, when isolated, breeds true. In mice many new varieties
have similarly appeared; and among plants the yellow evening
primroses and the jimson weed, a common weed with large, coarse
leaves, large, pale purple flowers, and prickly pods, are notable for
the large number of variations that they have produced when
bred under observation. There is little doubt that most species
vary under certain limits, and can at times produce new races
which breed true when isolated.

Finally, more and more crosses have been made between dif-
ferent species of animals and plants, and in many cases the hybrid
offspring have been at least partly fertile. In fact, from some of
these crosses, such as those between some different species of to-
bacco, hybrid strains have been produced which are completely
fertile among themselves but which will not intercross at all with
either of their parent species. In this way modern genetics has
entirely uprooted the old idea that species are absolutely unchange-

326 The Fact of Evolution

able entities and that each species is a unit which cannot combine
with any other species. Hence the whole conception of a species
as a separate, distinct unit breaks down, and our only alternative
is to think of species as being evolved from other species and re-
lated to each other through common ancestors. Indeed, with what
we now know about heredity and variation and the selective effect
of the struggle for existence, we should be forced to conclude
that evolution would have to take place, whether we had any other
evidence for its occurrence or not.

CHAPTER SUMMARY

That evolution has occurred is not a theory, it is a fact. There
are various theories, however, as to how evolution has occurred.
Briefly, evolution means that life began with very simple forms,
and over a period of time which may be estimated at somewhere
around two billion years it has developed to its present state
through a process of descent with modification. The fossil record
gives a fairly clear picture of the development of life through the
past five hundred million years. Among a vast number of other
records is that of vertebrate evolution, showing how amphibians
developed from fish, reptiles from amphibians, and mammals and
birds from reptiles.

Four principles of evolution are: first, that it has progressed
more rapidly at some times than at others, the periods of rapid
evolution being associated with great changes in the surface of the
earth; second, that at present some groups of organisms are
evolving more rapidly than others; third, that the evolution of
new groups of organisms is not from the most highly evolved
members of the existing ones, but from relatively primitive, un-
specialized forms ; and, finally, that evolution has not always pro-
gressed from simpler organisms to more complex ones. Regressive
evolution, or ' 'degeneration/' has occurred frequently.

There is a vast array of evidence for evolution which may be
summed up under four headings :

1. The fossil record actually carries traces showing the gradual
development of living forms from one stage in evolution to an-
other.

2. The similar anatomical plan found in organisms having
entirely different modes of life argues strongly for relationship

The Fact of Evolution 327

between these organisms. Another argument is the presence of
vestigial structures in some organisms which are apparently rem-
nants of structures that are functional in other organisms. Fur-
thermore, related organisms show similarities in embryological
development, and the development of the individual apparently
recapitulates that of the race.

3. Geographical distribution of organisms can be better ex-
plained in terms of an evolutionary geological history than in
any other way, and the presence of unique species in small isolated
regions argues strongly that these species have become different
from the relatives from which they are separated by going through
an evolutionary progress of their own subsequent to the date of
separation.

4. The actual production of new species through plant and
animal breeding shows that evolution can readily take place if
proper selective agents are at work, and the knowledge we have
gained in the laboratory of the way in which animals vary would
lead to the deduction that evolution would have to take place if
sufficient time were given for the operation of natural forces.

QUESTIONS

1. Briefly outline the history of life as recounted in this chapter.

2. Discuss, using examples, four principles concerning the progress
of evolution.

3. Give a resume of the evidence for evolution based on the fol-
lowing outline:

A. Evidence from the fossil record

1. How the fossils were formed

2. How the fossil deposits may be dated

a. Comparatively

b. Absolutely

3. Fossil links in the line of vertebrate evolution

4. Records of continuous evolutionary development

B. Evidence from similarities between related forms

1. Anatomical similarities

a. Similarities between mammals

b. Vestigial structures

2. Similarities in embryological development

C. Evidence from geographical distribution

1. Anomalies in distribution explained by geological history

2. Presence of unique species in isolated regions

328 The Fact of Evolution

D. Evidence from genetics

1. Plant and animal breeding

2. Experimental production of new species

GLOSSARY

Archeopteryx (ar'ke-op'ter-iks) A fossil bird showing marked rep-
tilian characteristics.

Archeozoic (ar'ke-6-zo'ic) The first (earliest) of the great geologi-
cal eras.

Cenozoic (se'no-zo'ic) The fifth (latest) of the great geological eras.

cosmogony (coz-mog'6-ni) A theory or myth concerning the origin
of the earth.

fossil Any record of life left in the rock strata.

ichthyosaur (ik'thi-o-sor) A type of extinct marine reptile.

marsupial (mar-su'pi-al) A type of mammal in which the young
are carried in a pouch on the mother's abdomen.

Mesozoic (mes'6-zo'ic) The fourth of the great geological eras.

notochord (no'to-kord) A long, narrow rod located just below the
spinal cord in certain fishes and in primitive relatives of the verte-
brates. It also appears in the embryos of the higher vertebrates,
but disappears before their development is completed.

paleontology (pa'le-on-tol'6-ji) The science of fossil organisms.

Paleozoic (pa'le-6-zo'ik) The third of the great geological eras.

plesiosaur (ple'si-6-sor) A type of extinct marine reptile.

Primate (pri'mat) An animal belonging to the order which includes
man, the apes, monkeys, and lemurs.

Proterozoic (pro'ter-6-zo'ik) The second of the great geological eras.

pterodactyl (ter'6-dak'til) A type of extinct flying reptile.

ungulate (un'gu-lat) A hoofed mammal.

CHAPTER XV

THE OUTCOME OF EVOLUTION

The Diversity of Living Organisms. — The age-long process
of evolution that we have described in the last two chapters has
resulted in populating the earth with innumerable organisms of
the greatest diversity. In order to comprehend this vast array of
life, the biologist must do two things. In the first place, he must
classify living organisms. This has proved an enormous task; and
although the modern system of classification of organisms has
been in use for almost two hundred years, hundreds of biologists
all over the world still are spending their lives in fitting the known
and the newly discovered animals and plants into this system.
While an understanding of this classification forms a study in
itself, and is summarized in the appendix, a conception of its
fundamental unit, the species, is essential to an understanding of
the nature and evolution of living things. Although undoubtedly
somewhat different in its genetical and physiological make-up in
different groups of animals and plants, the species may be roughly
defined as follows: It is a group of organisms which are more
or less variable among themselves, but are distinct in a number
of characteristics from the organisms composing the nearest
related species. This distinctness is due to the absence or rarity of
organisms intermediate between two groups designated as differ-
ent species, and is produced by some type of isolation which
separates them. A discussion of the different types of isolation
which are responsible for the differentiation of species is given
in the next chapter. Typical species are, for instance, the red fox,
individuals of which vary considerably in size, coat color, length
of hair, etc., but of which all are sharply distinct from any other
species of fox, such as the arctic or blue fox, not only in color, but
in body size, the proportions of the parts of the skeleton and
muscles, the habits of life, and many other characteristics. Man

329

330 The Outcome of Evolution

is a species ; although such different races as the yellow, the black,
and the white exist, all are connected with each other by many
intermediate racial types, and all are markedly distinct not only
from man's nearest living relatives, the anthropoid apes, but also
from certain extinct species of man. The number of species of
organisms is extremely large. There are about 400,000 known in
the plant kingdom, and about 800,000 of animals, and hundreds
of new species are being discovered every year. The chief object
of the study of evolution is, of course, to understand how this
multitude of species of organisms came into being; it was not
without purpose that Darwin named his classic book on evolution
The Origin of Species.

In addition to their classification, a further understanding of
the species of organisms can be obtained by studying their rela-
tionship to their environment. Since the environment of any
living thing includes not only the elements and the inanimate
objects which surround it, but also the other organisms with which
it is associated, this study includes the interrelationships of or-
ganisms, as well as their relation to the various types of inanimate
environment on the earth.

The dominant principle brought out by this study is the re-
markable adaptation of all successful organisms to their environ-
ment. When we consider the enormous range of environment that
exists on the earth — from the tropics to the arctic, from rain
forest to desert, from plain to mountain top, and from ponds
to lakes, rivers and the ocean — we can see that adaptation to this
multitude of different environments is responsible for a large
proportion of the hundreds of thousands of species that exist.
Another fact equally apparent is that the tremendous ability of
living things to reproduce their kind has caused every environment
to be filled with as many different organisms as it can support,
and that there is, therefore, a continual struggle for existence
among organisms to maintain themselves.

The Chief Cause of the Struggle— Overpopulation. — Al-
though the reader is probably fully aware of the cause for this
great struggle, a few definite examples will demonstrate just how
overpowering is the tendency for organisms to reproduce them-
selves far beyond the ability of the earth to hold the ever-growing
volume of life.

The Outcome of Evolution 331

Among plants, the trees of the forest serve as an excellent
example. In a forest of maples, for instance, each tree produces
every year about ten thousand seeds. Of these, about two per cent,
or two hundred, grow into seedlings. These are enough to cover
the dead leaves of the forest floor with a mass of leafy shoots
pushing their way upward toward the light, a sight familiar to
anyone who visits such a woods in the late spring. Of these seed-
lings, however, all but a few are doomed to grow no further.
Since, unless brought down by the lumberman's ax, not more
than one tree in a hundred of those in the forests dies and leaves
room for a young newcomer, not more than one out of every
twenty thousand seedlings can ever become a full-grown tree. A
competition among twenty thousand, of which only one can win,
must necessarily be a keen one.

Animal life gives us the same picture of excessive overproduc-
tion. Let us use as an example a pair of rabbits. They can pro-
duce a litter of twelve offspring. If each of these grows up, it can
be responsible for twelve more in a year or less, so the six pairs
of the second generation would have produced seventy-two off-
spring for the third generation at the end of two years. The
following table shows the number of offspring in the succeeding
generations :

4th generation: 432

5th generation: 2,592

6th generation: 15,552

7th generation: 93,3 12

8th generation: 559,872

9th generation: 2,359,232

Thus, at the end of eight years, if all of the offspring grew up
unmolested, the descendants of a single pair of rabbits would
number over two million. Of course the number of rabbits in the
world is not increasing; and since a rabbit can live four or five
years, only four out of the possible two million would normally
reach maturity. The others would form food for hawks, foxes,
or other enemies, or would be made into fur coats or gloves, or
would never be born on account of the early death of their pos-
sible parents.

Although the rabbit is famous for the rapidity with which it
can reproduce, many other animals can do it much more rapidly.
The case of the fruit fly, which can produce over fifty offspring

332 The Outcome of Evolution

in ten days, has already been mentioned, and it is not a very un-
usual one among insects. At this rate, if all the offspring continued
to breed and produce entire families of adult flies, it would take
only seven weeks to produce a population of over twenty million
fruit flies from a single pair !

Even the slowest-breeding animals could, moreover, quickly
populate the entire earth with their kind. The elephant is the
slowest breeder of known animals. According to Charles Dar-
win, the elephant begins to breed when thirty years old and goes
on breeding until it is ninety. During that time it produces six
young. If each of these six elephants pair to produce a third gen-
eration at the same rate, and so forth, after 750 years there would
be nearly nineteen million elephants descended from the first pair.

A practical illustration of how fast animals can reproduce their
kind is given whenever they are brought into a new country away
from their natural enemies. For example, a few pairs of rabbits
were brought into Australia by the early colonists, who thought
they would make a fine source of game for hunting and for food.
The rabbits, however, since they found no hawks, foxes, or any
other natural enemies to molest them, busied themselves with re-
producing their kind and soon overran the country, becoming the
worst pests with which the farmers had to contend. In every
Australian community great rabbit hunts were held yearly or more
often, in which all the able-bodied men would round up thousands
of rabbits in the surrounding fields and drive them into a huge
pen where they were slaughtered. Even this had no effect on
the prevalence of these animals until natural enemies were brought
in from England to cope with the prolific immigrants.

Naturalists sometimes debate the question as to where the
struggle for existence is the fiercest, and where it is less severe.
There is no satisfactory answer to this question, for the good
reason that it is everywhere about equally severe. The difference
is in the nature of the struggle. Where conditions of the inani-
mate environment are exceptionally favorable for life, the struggle
is chiefly among the hundreds of different species of organisms
which are striving to take advantage of these favorable condi-
tions ; where the environment is forbidding, organisms are waging
a continual war with the elements, and at the same time must com-
pete with their fellows to obtain the best advantage of such favor-

The Outcome of Evolution 333

able conditions as there are. This can best be understood by taking
a glance at life in various environments.

Life in a Tropical Rain Forest. — The tropical rain forest is
one of the wonders of the world. Such forests extend over hun-
dreds of square miles in regions such as the great Amazon River
basin of South America. Here the necessities for plant life and
growth — water and sunlight — are present in abundance, and the
temperature is the best possible throughout the year. Vast num-
bers of plants can grow successfully. Instead of a dozen or so
different kinds of trees, such as we are accustomed to see in our
own woodlands, four or five hundred can be counted in any tract
of this great rain forest. Consequently, there is a tremendous
struggle for a place in the brilliant tropical sunlight. Trees send
up long bare trunks two hundred feet in the air, growing hastily
to escape being shaded and thus deprived of the essential sun-
light by their competitors. Great woody vines and creepers, known
as lianas, wind their way up the tall, slender trunks, and their
foliage covers the top branches in such profusion that scarcely a
gleam of light makes its way through the mass of leaves at the
top. Because of the struggle for sunlight, the life of the forest is
lifted as if on stilts, high above the ground. Below is dimness, the
stems of trees, dead leaves, and decaying logs.

In The Sea and the Jungle, H. M. Tomlinson has given a never-
to-be-forgotten account of a trip that he took through the very
midst of the Amazonian forest and of the conflict that he saw
going on among the trees and vines. He was, of course, walking
along the ground, far below the region of teeming life.

This central forest was really the vault of the long-forgotten, dank,
mouldering, dark, abandoned to the accumulations of eld and decay.
Every tree was the support of a parasitic community, lianas swathing
it and binding it. One vine moulded itself to its host, a flat and wide
compress, as though it were plastic. We might have been witnessing
what had been a riot of manifold and insurgent life. It had been
turned to stone when in the extreme pose of striving violence. It was
all dead now.

But what if these combatants had only paused as we appeared ? It
was a thought which came to us. The pause might be but an appear-
ance for our deception. Indeed, they were all fighting as we passed
through, those still and fantastic shapes, a war ruthless but slow, in

334 The Outcome of Evolution

which the battle was ages long. They seemed but still. We were de-
ceived. If time had been accelerated, if the movements in that war
of phantoms had been speeded, we should have seen what really was
there, the greater trees running upward to starve the weak of light and
food, and heard the continuous collapse of the failures, and have seen
the lianas writhing and constricting, manifestly like serpents, throttling
and eating their hosts. We did see the dead everywhere, shells with
the worms in them.

In the top layer, far over the heads of the travelers such as he
who wrote the above description, there is another war going on
between a myriad of different animals. The roof of the forest is
a solid layer of green foliage, exposed to all the conditions most
favorable for life. Bright sunlight, a continual warmth and mois-
ture, and a rarity of violent storms make conditions ideal for plant
growth, so that the roof of the forest can be compared to a limit-
less conservatory or greenhouse of brilliant flowers including not
only those of the trees themselves, but in addition a vast number
of smaller plants, such as the orchids, which are perched in the
uppermost branches of the trees. Grasshoppers and other insects
feed on the vegetation ; bees, butterflies, and hummingbirds live on
the nectar from the flowers, while scores of birds feed on the
wealth of insect life, and monkeys clamber up and down the
branches, living on an abundance of fruit. These, in turn, are
continually sought by the birds of prey — hawks, kites and harpy
eagles — which soar overhead or glide through the treetops, always
ready to seize any bird or monkey unwary enough to expose itself
to their view.

Coloration. — In this world of excessively keen competition be-
tween organisms, every species must either be very well protected
from its enemies, or be provided with weapons of aggression or
defense. For this purpose some of the most striking adaptations
have been developed. The most widespread of these come under
the general head of coloration. The three types of coloration im-
portant as weapons for defense or aggression are concealing color-
ation, warning coloration, and mimicry.

Concealing Coloration. — The coloration of animals to resemble
their surroundings is general in all parts of the world, but nowhere
is there such a variety of devices for this purpose as in the tropics.
Tropical birds are usually protectively colored. Those living in the

The Outcome of Evolution

335

dimly lit inner recesses of the forests are colored dull brown and
gray; while the majority of them, which live among the exposed,
sunny tops of the trees, are brilliantly arrayed in green, red, yel-
low or blue to match the brilliance of their surroundings. The
tiger is striped and the leopard spotted, both of them in order to
match the patterns of light and shade found in their native jungles.

B

FIG. 82. — Mimicry of leaf and twig insects.

Insects have the most extraordinary modifications of this sort
Naturalists in the tropics all report the strange phenomena of
"leaves" that turn into butterflies and fly away; of "twigs" that
become caterpillars and "cocoons" that turn into grasshoppers.
Even the markings on the leaves of the trees, such as the spots
caused by fungus attack and the droppings of birds, are imitated
by insects and spiders. Concealing coloration serves two purposes.
It is either for protection of hunted animals against their enemies,
or for purposes of aggression, in enabling predatory animals to
stalk their prey unnoticed. In both cases, however, the resemblance

336 The Outcome of Evolution

to the environment is similar, and is equally close. A striking case
of protective coloration is that of a certain butterfly, which not
only has the outline and color of a leaf, but also imitation veins
and an imitation leaf stalk (Fig. 82). Among the remarkable
imitations for the purpose of aggression is that of a certain fly-
catching bird of Brazil, whose crest is brilliantly colored and can
be spread out in the shape of a flower. Flies are attracted by it,
and fly toward the "flower" in search of nectar, only to be de-
voured by its owner.

Warning Coloration. — This characteristic is possessed by many
(though by no means all) animals which have weapons for de-
fense in the form of poisonous fangs or stings, or are noxious
or unpalatable to the taste. The deadly poisonous coral snakes are
colored with bands of brilliant black, yellow and red; hornets and
wasps are as conspicuous as possible in their black and yellow
stripes. Ill-smelling bugs and unpalatable caterpillars and grass-
hoppers are often colored so as to contrast strikingly with their
surroundings, and walk about slowly in plain view ; a number of
different experiments have shown that they are avoided by birds
and insect-eating animals.

The advantage of warning coloration to a poisonous or noxious
animal is obvious. If a wasp were inconspicuous or similar to
other insects, it might be snapped up or crushed before it had time
to use its sting; furthermore, the repeated use of the sting is
harmful or even fatal to the insect. The safest thing for an animal
that relies on such qualities for protection is to have a sign saying
"keep away," and to display this sign in plain view.

Mimicry. — Mimicry is the imitation of the color and form of
some poisonous or noxious animal by another which is not closely
related to it. In some (but apparently rather few) cases the mimic
is harmless, defenseless, and palatable, so that its only protection
lies in being mistaken for its harmful or noxious model. In other
cases the mimic has itself a noxious quality; in this case both
mimic and model benefit from the resemblance, since predatory
animals quickly learn to recognize a certain type of color pattern
as one to be avoided ; thus all of the animals possessing this pat-
tern are automatically protected. Mimicry is frequent among
tropical insects, and certain groups serve as models for many
different, entirely unrelated insects. The fierce, stinging wasps are

The Outcome of Evolution 337

mimicked by bugs, grasshoppers, and moths, and sometimes the
resemblance is so close that even naturalists are deceived at first
sight. The usually ill-smelling, unpalatable ants are mimicked by
spiders, grasshoppers, bugs, beetles, and caterpillars. One appar-
ently harmless caterpillar of the South American tropics appears
inconspicuous when feeding, but if disturbed lifts its head and in-
flates its thorax, whereupon two raised, opalescent spots on the
thorax gleam like eyes; as its discoverer remarks, "the transfor-
mation is most impressive, and the effect when the larva is half
concealed in foliage is that of the head of a snake or lizard with
open mouth and shining eyes."

The rain forest possesses the most intricate interrelationships
between organisms of any part of the earth. Parasitism and
saprophytism are here extraordinarily well developed, and asso-
ciations for mutual benefit, both between species and within cer-
tain species, are often highly developed. A striking case of para-
sitism is that of the strangling fig. The seeds of this species
germinate high up in the branches of a living tree, but soon send
roots down to the ground. The trunk of the fig then grows like
a latticework around the host tree, completely choking it. Finally
the fig strangles its host to death, and emerges as a tall, leafy tree
on its own roots, but still embracing the dead trunk of its victim.

An interesting case of symbiosis is the relationship between
ants and a number of different species of plants. These plants
usually possess some type of nectar-secreting glands on their
stems or leaves. The glands attract certain species of stinging ants,
which regularly make their homes on these plants and protect them
from the ravages of leaf-cutting ants or of other harmful insects.
The various types of social insects, particularly termites and ants,
are best developed in the tropics. Termites, small primitive insects
somewhat related to the grasshoppers, live in highly organized
communities that often contain five or six different "castes" of
workers specialized for different functions, and entirely different
in appearance from one another. Since these insects are by them-
selves defenseless and are much sought after by hundreds of dif-
ferent enemies, their only salvation lies in the development of
large communities and the building of elaborate nests, either
hollowed out of tree trunks, built around the branches, or (in
more open forests) rising from the ground as "castles" six or

338 The Outcome of Evolution

eight feet high. These communities are always the home of many
other species of animals, some of them parasitic and feeding on
the termites, some "tolerated guests" which neither harm nor
benefit them, and some welcomed as symbionts, as in the case of
various beetles living in the nests of both termites and ants, which
are fed and tended for the sweet secretions that they produce.

The ant3 are individually more powerful than the termites, and
their communities are as a rule smaller and less highly organized.
Many of them, moreover, exist for offense as well as defense.
There are the marauding army ants, from whose voracious bands
every creature of the forest escapes with the utmost rapidity.
These have no permanent home, but at the critical time when the
larvae are developing into adults they are surrounded by "nests"
whose walls are made of the living bodies of hundreds of workers
linked together. The leaf-cutting ants march in long processions,
each worker carrying over its head a round piece of a leaf; they
are so efficient in their work that a small tree can be completely
stripped of its leaves in a few hours. Many other species, some
harmless and some stinging, exist in such profusion that to at-
tempt to climb a tree in the tropics is to invite a thousand pin-
pricks of ant stings.

Although there is little in the environment of the tropical rain
forest that is unfavorable to life, some organisms have found
protection against such unfavorable conditions necessary. For
instance, ponds are uncommon in these regions, but during the
season of heavier rainfall pools may exist for some time. To pro-
tect against the drying up of these pools, one of the pool dwellers,
the Surinam toad of South America, does not hatch its eggs in
the water, but carries both eggs and tadpoles in little pockets of
water on its back. In the case of the plants, the absence of sunlight
makes growth on the ground impossible for bushy plants and
grasses. Hence the smaller plants are mostly epiphytes, i.e.,
dwellers on other plants, mostly trees. Situated high up in the
branches of trees, the roots of these plants are unable to obtain
the steady supply of water and mineral salts necessary for con-
tinued growth. To overcome this difficulty many of them, par-
ticularly the large group of species belonging to the pineapple
family, have developed at the bases of their leaves reservoirs of
water which are always full. Digestive enzymes are apparently

WORKER

SOLDIER

COMPLEMENTAL
MALE OR FEMALE

WINGED MALE OR FEMALE GRAVID FEMALE

FIG. 83.— -Termite differentiation. (Redrawn from Lull's Organic Evolution,
The Macmillan Company.)

340 The Outcome of Evolution

secreted into these reservoirs for the digestion of the bodies of
various insects which fall into them, and from these animal pro-
teins the plant must obtain most of its nitrogen and other mineral
salts. In some forests these "reservoir plants" are so abundant
that the roof of the forest has been likened to a marsh. A powerful
indication of the ability of life to conquer new environments is the
presence in these reservoirs of scores of different species of aquatic
animals, ranging from Protozoa to worms, shrimps, scorpions,
the larvae and adults of many different types of insects, and the
tadpoles of frogs. Many of these animals live only in these reser-
voirs ; all have apparently developed the ability to resist the diges-*
tive secretions.

Life in the Desert. — Where conditions are favorable for life,
and the struggle for existence is chiefly between different organ-
isms, the various characteristics and relationships which we have
just discussed are prevalent. However, the insurgent, ever-grow-
ing swarm of life has caused many living things to spread far out
into regions where for the great majority of the time conditions
are absolutely inimical and forbidding to life. The less favorable
are the conditions for life, the more are organisms equipped, not
for competition between one another, but against their common
enemies, the elements.

Let us, for instance, look at the life of a desert. In such deserts
as those of California, Peru and the great Sahara desert of Africa,
rain falls only once every year, every four or five years, or even
less often. The rainfall, when it does come, is in the form either
of little showers, the moisture from which is evaporated before
it dampens the soil at all, or of great torrents which wash away
the soil rather than soaking it. Any organism, therefore, which
would live in the desert must be able to withstand excessive heat
and long periods of extreme dryness. Furthermore, in the absence
of a blanketing layer of moist air, the nightly temperatures of the
desert are often quite low and the changes in temperature sudden
and extreme. Finally, the desert is often swept by sudden and
fierce windstorms, against which all living things must be protected.

In spite of these hostile conditions, few deserts are totally de-
void of life. In most of them the ground is dotted here and there
with a number of different plants. These are of two general types :
those that carry on a slow, persistent activity and growth all the

The Outcome of Evolution 341

time in spite of the unfavorable conditions, and those that carry
on life only during the short intervals when conditions are favor-
able. Of the former type, the most interesting are those that store
up water for themselves and use it slowly and economically during
dry weather. The best known are the cacti, of which there are
hundreds of species in the southwestern United States, the largest
rising as great fluted columns thirty or forty feet above the desert
floor. Other cacti are in the shape of great barrels, full of water ;
these the desert Indians often cut open and by crushing the soft
pulp inside, extract its precious fluid. In every case the cactus
exposes as little of its surface as possible to the burning sun and
parching winds, and this surface is protected with a thick, waxy
coat through which little or no water can evaporate.

Other desert plants curl or fold up their leaves or drop them
altogether during dry weather and remain dormant until the rare
shower or rainstorm does come. Then they open up, spread out
their leaves, produce buds and flowers, and in a few days carry
on enough life and growth to maintain them for another long
period of inactivity. The smaller desert plants do even better than
this. During the vast majority of the time they remain as seeds
underground, containing practically no moisture and protected by
a hard, tough seed coat. When the rain comes, the seeds germinate,
grow to mature plants, produce flowers and new seeds ki a week
or two, and wither away, leaving their offspring to remain dor-
mant for another four or five years.

The slow-growing plants of the desert, although they have
comparatively few living enemies, must nevertheless be well pro-
tected against those that they have, since any damage done to
them could be repaired only at a very slow rate. For this reason,
a large majority of them possess spines or thorns, making the
"thorny wilderness" an actuality in dry countries. Spines and
thorns are found, of course, in plants of damper climates also
and are modifications of various parts of the plant. In some cases,
as in the holly, they are simply very sharp teeth on the sides of the
leaf. In others, as in the cacti, each spine is the modified remnant
of a whole leaf. The green part of the cactus is really a much thick-
ened and sometimes broadened stem, and the groups of spines
are tufts of modified leaves. Still other spines and thorns, as in the
hawthorn, are modified, reduced branches.

342 The Outcome of Evolution

The animals of the desert are likewise equipped to stand long
periods of drought, and can get along with extremely little food.
Most of them are fleet and agile, since they must roam far and
wide for their sustenance. A number of them, like plants, can
store up water within themselves. The camel has, leading from
his stomach, a number of water cavities which may be closed up
when full by means of a sphincter muscle which acts as a draw
string. When the camel drinks, he fills not only his stomach, but
the bags as well, and during a long dry march draws on this extra
supply. A camel can march for five or six days without water but
is much weakened by such a journey. Many smaller desert animals,
such as frogs, store water under their skins. Of the Australian
desert frog, one observer writes : "If you put a lean, dry, herring-
gutted Chiroleptes into a beaker with two inches of water, in two
minutes your frog resembles a somewhat knobby tennis ball."

Many desert animals keep alive over the long periods of drought
by building huge burrows underground, in which they lay up great
stores of food. The ant, to whose habits Solomon referred the
sluggard, was one of these desert dwellers; if Solomon had lived
in the American southwest he might well have chosen an even
more industrious animal and made the proverb, "Go to the kan-
garoo rat, thou sluggard." For this agile jumping creature may
amass as much as a bushel of seeds and other forage in his burrow.

Life in the Arctic Regions. — The other parts of the earth in
which conditions of life are particularly unfavorable are the arc-
tic and antarctic regions. North of the arctic circle in both the
Old World and the New are vast treeless stretches of barren
land known as the tundra. The tundra covers all of the northern
parts of Russia, Siberia, Alaska north of the Yukon, and the
shores of and islands in the Arctic Ocean, while patches of tundra
extend down the coast of Labrador and Newfoundland, recurring
on the higher mountains of eastern Quebec and New England,
such as the White Mountains. The higher summits of the Rocky
Mountains, the Cascades, and the Sierra Nevada in the western
part of our country as well as the mountains of Eurasia also con-
tain large areas of tundra. In these regions organisms must face
not only extremes of cold, but for nine or ten months of the year
a scarcity of available moisture, since all of the moisture on the
ground is during that time locked up as snow and ice, and the

The Outcome of Evolution 343

air, on account of its coldness, is physiologically dry. Further-
more, fierce winds often sweep the tundra, drying and freezing
still more the living things exposed to them, and beating down
any plant or animal which is not strong enough to withstand them,
Under these conditions both animals and plants must be equipped
with every possible adaptation against wind and drought. The
animals possess, in addition to their heavy coats of fur, thick
layers of fat inside of their skin, which serve not only as protec-
tion against the cold, but also as a reserve supply of food for
times of food scarcity. Although many of the mammals of the
temperate zone hibernate and remain dormant during the winter,
this is impossible for arctic mammals, since the winter is too long
and the summer too short for them to store up reserves of en-
ergy. The vegetarians, therefore, must all be able to feed the
year round. The larger animals, like the reindeer and caribou,
feed entirely on the evergreen lichens and mosses, which they
obtain during the winter by shoveling away the snow with their
broad, fan-shaped antlers. The most common of the smaller mam-
mals are the mouse-like animals known as lemmings, which dur-
ing most of the year live in tunnels burrowed under the snow,
feeding on the mosses, lichens, and grasses underneath. The car-
nivorous mammals, such as the ermine and the arctic fox, are
barely able to survive, although the former can maintain itself
by pursuing the lemmings through their burrows under the snow,
while the latter often stores up caches of meat during the summer
which lie well preserved in the cold storage of a snow bank for
months. Scarcely any birds exist through the arctic winter, and
the insects all remain in their pupal cases.

The plants must all have extreme adaptations for protection
against cold and drought, and in many ways they resemble desert
plants. The evergreen shrubby types all have small leaves, which
are generally hard and needle-like, or which have their edges
rolled in and their surfaces protected by heavy coats of wool.
Other woody plants shed their leaves, and are green for only a
few weeks during the summer. The smaller plants, or herbs, die
down each season, and remain alive during the winter only in
the form of roots and underground stems. Furthermore, most
of them protect themselves from vegetarian animals as do the

344 The Outcome of Evolution

desert plants. They are all hard and tough, and many are filled
with acrid or bitter substances as well.

Although life in the arctic is for nine months of the year a
grim, slow struggle against the elements, the time comes in June
when the sun shines for three- fourths or all of the time, the
snow and ice melt, and for several weeks all conditions are favor-
able for life. Then comes a mad rush on the part of both animals
and plants to take advantage of these favorable conditions, and
the struggle for existence between organisms becomes as keen
as it is in the tropics. Butterflies, bees, and mosquitoes burst from
their pupae and fill the air with their humming. Plants send up
new shoots and flowers, some of them even pushing their buds
through the snow so as to be the first to be pollinated by the
bees. Tender leaves appear on the willows and grasses, affording
ample new food for half -starved lemmings and hares, which must
bear and raise their young while this abundant supply is available.
Their appearance from under the snow banks brings new life to
the famished foxes and wolves which capture so many of the
smaller mammals that the lemmings and hares must bear unusu-
ally large litters if their kind is to survive at all. Birds arrive
from the south, to nest where there is continuous daylight for
building their nests and feeding their young, and where compe-
tition from other birds is less severe. Cooperation for mutual wel-
fare exists in the arctic summer as it does in the tropics, and is
exemplified by the arctic birds, which nest in great flocks, fly to
each other's aid against intruders, and, in the case of some aquatic
birds, join together in overturning large stones in order to pick up
the small animals hidden underneath them.

For a few weeks the moister and warmer parts of the ground
are covered with a garden of wild flowers larger and brighter
than any of their relatives farther south. The insects are so
abundant that they swarm over all of the animals that live there,
and drive the reindeer to drier feeding grounds. The flowers pro-
duce their seed, the young birds grow up and get ready to migrate
southward, the insects lay their eggs and die, their larvae feeding
and growing rapidly on the profusion of vegetation, getting ready
for their long winter's rest. Finally, in early September, the winter
storms begin again, and all life resumes its passive struggle
against the elements.

The Outcome of Evolution 345

Some Features of Life in Temperate Regions. — When
we look about us at the animal and plant life in the temperate
regions of the earth, we find that the enemies of an organism are
about equally divided between the unfavorable elements of its
inanimate environment and the other organisms which are com-
peting with it. As a result, both the adaptations highly developed
in tropical organisms and those most characteristic of the arctic
regions are found here, though in less extreme forms.

For instance, if we go to a moist swampy woodland in June, we
find tall trees shutting out the light from the forest floor, their
branches crowding together to form a veritable platform of
foliage above, while the Virginia creeper and clematis vines often
twine about their trunks like lianas. Competition between birds,
insects, and mammals is very keen, and all must be equipped with
weapons for defense or aggression. The birds are mostly of a
duller hue than those of the tropics, since there is not the same
brilliance of sunlight and foliage, and to be protected they must
blend with the dull browns and greens of their environment.
Nevertheless, many birds, such as the goldfinch, bluebird, and
scarlet tanager, are nearly as brightly colored as their tropical
cousins. There are many protectively colored insects, and some
have shapes that imitate objects of their surroundings in almost
as striking a fashion as do the tropical insects. There is a whole
series of butterflies which, when their wings are folded, bear a
striking resemblance to dried oak leaves, and there is one common
insect which resembles a stick so closely that it is not often noticed
by amateur naturalists, and is sent every once in a while to our
museums as a rare curiosity.

Warning coloration is also present among our animals. The
skunk is as conspicuous as possible in his pattern of black and
white, and the rattlesnake has perhaps the best-known warning
signal of any animal. Mimicry is found among our insects. The
most famous case of mimicry is that of two familiar butterflies,
the monarch and the viceroy. The monarch is a large, bright
orange butterfly, conspicuously marked with a network of black
lines, and notorious for its bad odor and taste. Another somewhat
smaller butterfly, belonging to a totally different family and not
at all ill-smelling or distasteful, has exactly the same color pat-

346 The Outcome of Evolution

tern; this is the viceroy. Birds, which have learned from expe-
rience to avoid the monarch, leave the viceroy alone.

Associations of all sorts are also found among organisms.
Cases of parasitism and symbiosis have been mentioned in an
earlier chapter, and the social insects, the ants and bees, are famil-
iar to all of us, though they are not so abundant and dominant as
in the tropics.

There are many spots even in our temperate climate in which
conditions are something like those in the desert and the arctic
tundra. The seashore has many of the features of the desert.
Where the shore is sandy, the soil is so porous that the rain perco-
lates through it in a few hours after each storm, making the sand
between rains almost as dry as that of the desert; the great heat
reflected by it on hot sunny days is familiar to all who seek their
summer tan on the beach. Sand beach plants, therefore, are either
succulent like the "sea rocket" or, as in the beach grass, hard and
tough, with their narrow leaves curled up like those of desert
plants. Where the shore is marshy, water is plentiful, but it is
so salty that plants have difficulty in assimilating it. Hence most
salt marsh plants are very fleshy and equipped for storing water.
The samphire or glasswort has a jointed, fleshy, leafless stem
like that of a miniature cactus, while its associates, the orache
and sea blight, are not only similar to but actually close relatives
of the salt bushes and the greasewood of our western deserts.

A habitat that in many respects resembles the arctic tundra is
the peat bog. Such bogs are frequently found in northeastern
America, within the region once covered by the great ice sheet,
and were formed by the gradual filling in of ponds by the growth
of sphagnum or peat moss. In fact, many of them still have small
ponds in their centers. The floor of these bogs is a mass of soft
peat which extends down scores or even hundreds of feet. The
great masses of decaying peat fill the water with carbonic acid,
which makes it difficult for plants to assimilate and use the water,
while there is no soil at all to provide the necessary mineral mat-
ter. Furthermore, peat moss is a poor conductor of heat, so that
even in June the water is very cold or actually frozen a few feet
below the surface. The plants growing under these conditions
are equipped to resist cold and drought as are those of the tundra,
and many of them are the same species as, or close relatives of,
arctic plants, which reach their southern limits in the«e cold, phy-

The Outcome of Evolution

347

siologically dry places. On the other hand, some of them obtain
mineral salts by means of adaptations similar to those found in
the tropical epiphytes. The pitcher plant, a common denizen of
peat bogs, gathers water in its pitchers, in which it traps and

Sundew ^

Venus's-flytrap

FIG. 84. — Insectivorous plants.

digests insects, as do the tropical pineapples and their relatives.
Furthermore, the water of these pitchers is, as is the similar
environment in the tropics, occupied by its particular set of ani-
mals. Microscopic examination of the water in the bottom of any
one of these pitchers will reveal the presence of Protozoa, wheel
animalcules or rotifers, insect larvae, and a species of water mite.
These small animals are apparently adapted to resist the digestive

348 The Outcome of Evolution

secretions of the pitcher plant, and in return for this adaptation
are able to feed on the victims that fall into the water. Another
insect-eating plant of peat bogs is the sundew, which captures
flies on its leaves by means of a surface of sticky glands. The
leaves of the Venus's-flytrap, a rare bog plant of the coast of
North Carolina, are transformed into traps. When stimulated by
flies which brush against the long bristles lining their edges, these
traps fold over their prey in a few seconds. All of these adapta-
tions for insect-catching are of value to plants of peat bogs, since
the animal proteins, when digested, supply the mineral elements
necessary for their growth.

The Seasonal Changes of Life. — The most severe change
which occurs in all of the habitats of temperate regions is the
onset of winter, and for this change many remarkable adaptations
exist. None of these are necessary in the moist parts of the
tropics, some of them are characteristic of deserts and the arctic
regions, and some are not found in either place. For instance,
there are plants which, like the cacti, carry on a slow, steady activ-
ity all the time, and others which practically suspend their life
during the winter. The evergreens, such as pines, spruces and
cedars, have leaves that are quite thick for their breadth, which
store a good deal of water and are covered with a thick, protective
layer of wax. Most other trees, like the desert shrubs, shed their
leaves during the dry, cold and windy winter, when most of the
moisture is locked up in snow, ice, and the frozen ground. The
bare twig of a tree in winter is as well equipped to stand extreme
drought and changes of temperature as is a desert plant. It is
covered with a thick, corky bark which insulates it, and effectively
keeps water from evaporating from the tissues inside. This bark
is, nevertheless, perforated with little openings which show on
the twig as dark excrescences. These admit the oxygen which the
tissues must have if they are to carry on oxidation and keep
alive. At the tip and along the sides, in what were the notches
above the leaves, are the buds. These are covered by several thick,
leathery scales, and often coated with a shining layer of varnish-
like substance. Within, often wrapped in a heavy layer of wool,
are small leaves and often flowers, already formed, and needing
only the warmth and moisture of spring to expand and burst
through the protecting scales.

The Outcome of Evolution 349

Non-woody plants, known as herbs, must in this climate die
down each autumn. They have two different ways of surviving
the winter. In some, the perennial herbs, the parts underground
remain alive and often store within them enough food to main-
tain themselves throughout the winter and start the growth of a
new shoot the following spring. These underground storage or-
gans may be either thickened roots, as in the dandelion; short

Rhizome (ginger)

Bulb (onion)

Tuber (white potato)

Corm (taro)

Taproot (carrot)

FIG. 85. — Adaptations for plant hibernation. (Redrawn from Brown's The Plant
Kingdom, Ginn and Company.)

and generally thick underground stems, or rootstocks, as in the
bloodroot, Solomon's seal, and other familiar flowers; bulbs; or
corms. A bulb consists of a broad, flat stem topped by a number
of thick, fleshy modified leaves; and a corm, as in the crocus and
trillium, is simply a much swollen, rounded base of the stem.
Bulbs and corms, with their large supply of stored food, allow
the new shoot to grow up and flower quickly, and hence are most
common in spring flowers. They, and generally rootstocks as well,
bear buds, within which are small leaves and often flower buds.
Some perennials keep a few leaves at their base alive and green

35O The Outcome of Evolution

throughout the winter. Perennials almost always spend the first
years of their lives storing up food in their underground parts
and hence do not flower until they are two, three, or more years
old.

Annual herbs flower, produce seed, and die away in a single
season, leaving their seeds as the only parts which survive the
winter. They generally produce a very large amount of seed for
the weight of the plant, and hence most of our seed crops, such
as grains, peas, and beans, are annuals.

The animals of the woods in these regions either maintain
themselves on a scant supply of food as do the deer and rabbits,
or else they hibernate in some way or another. The woodchuck,
like the kangaroo rat, builds burrows; bears lie in caves; and
frogs bury themselves in the muddy bottoms of ponds. These
long periods of suspended animation are most characteristic of
the animals of temperate regions.

In some animals that are active throughout the year in temper-
ate regions, concealing coloration has taken the form of changes
of color with the seasons. The varying hare of our northern for-
ests is brown in summer, but in winter develops a coat of white
which matches the snow over which it must wander. The
ptarmigan, quail-like birds of northern regions, have similar
changes of color.

Animal Migrations. — Some animals have adapted themselves
to changing environments by means of migrations. These are of
three types, seasonal, cyclical, and irregular or dispersal.

The seasonal migrations of birds are from winter quarters in
the south, where food is abundant, to breeding places in the north,
where the simultaneous appearance of numerous flowers and in-
sects provides a greater abundance of food and the increased
length of the days in summer gives the parents more time to
gather food for their young. Migrations are manifestations of
some of the most marvelously complex instincts in the animal
kingdom. Sometimes the distances covered are almost incredible,
as, for instance, in the case of the arctic tern, which breeds
chiefly in Greenland and Labrador and winters in southern South
America, traveling 10,000 miles, nearly from pole to pole, twice
a year.

Cyclical migrations are those taken by an animal once or twice

The Outcome of Evolution 351

in its lifetime. The most interesting are those of some species of
fish. The king salmon of the Pacific, for instance, travel up the
great rivers of western North America to spawn, combating
swift currents and leaping over falls and rapids until they reach
the quiet pools of the headwaters, in one case two thousand miles
from the sea, quite exhausted and wasted away. The young spend
their first winter in the fresh-water streams, and in the follow-
ing spring make the long journey to the sea. After two to seven
years in the ocean, they have the urge to spawn, and travel back
upstream, following exactly the same river and tributary route as
they took on their downward journey, until they spawn in prac-
tically the same spot as that in which they were hatched. Eels
migrate in exactly the reverse direction. They spend most of
their lives in fresh-water streams and pools, but when they have
the urge to spawn, they travel down to the ocean and across to
an area south of Bermuda, both the European and the American
species of eels spawning in nearly the same spot. The young eels,
as soon as they can swim, head back across the ocean. Those of
European parentage always return to Europe before reaching
maturity; the American eels head northwest, until they reach the
rivers of our coast, up which they swim, often making short
journeys overland to reach the ponds and streams where they
spend their lives. The extraordinary ability of the young eels,
which are a fraction of an inch long, to find the continent in-
habited by their ancestors is not quite so unbelievable as it seems
at first. The breeding grounds are much nearer to the American
coast than to the European coast; and the European eels are
prepared for their longer journey by taking three years to mature,
whereas their American cousins mature in one. Hence if the
young of an American eel starts toward Europe, it reaches the
stage in its development when it must transform itself into a
fresh-water fish while it is still in mid ocean, and therefore it
perishes. The young of the European eel, although they come
within 150 miles of our coast, are immature at this time, and
do not enter the shallower waters near the coast until they reach
Europe.

Irregular or dispersal migrations are due to the temporary over-
population of some region with a particular species of animal, and
are best known in the insects and mammals. The grasshopper or

352 The Outcome of Evolution

locust is an animal normally solitary in its habits; but some
species, under pressure of overpopulation and scarcity of food in
their breeding grounds, develop gregarious tendencies and swarm
across the country in droves of millions, darkening the sun and
devouring all of the vegetation in their path. These plagues of
locusts occur in open prairie regions all over the world. They have
been quite frequent in the central and western United States, but
have diminished in recent years as a result of the destruction of
the breeding grounds of the insects. Even more spectacular are
the migrations of the lemmings. These small mammals make dis-
persal migrations at irregular intervals in all parts of their range,
but they are best known in the Scandinavian peninsula, where
the high plateaus on which the animals breed are surrounded on
all sides by narrow valleys down which the lemmings must mi-
grate. When overpopulation becomes extreme, and the food scarce,
armies of lemmings rush blindly down the valleys. They are ac-
companied by crowds of birds and beasts of prey which con-
stantly devour them, but which have no effect on the persistence
of the survivors. They never end their migration, however, since
all the lowland regions are inhospitable to lemmings. Most of
them are eventually killed by animals or by man, but some of
them, in attempting to swim across the narrow fjords, are swept
out to sea and drowned. Although thousands of them have been
seen swimming together through these arms of the sea, there is
no evidence that their instinct compels them to do this; and the
frequently made statement that they head directly from the shore
toward the open sea, swimming straight ahead until they perish, is
not supported by the careful studies of biologists.

Life in the Ocean. — As we learned from the last chapter, life
existed in the ocean long before it did on the land, and even
today there is a greater wealth of animal life in the sea than there
is on any of the continents. Life has pervaded the sea as it has
the land, and has penetrated to its utmost depths. There are,
in the main, three types of environment that it has occupied : first,
the shore and the shallow waters, i.e., the littoral and sublittoral
region ; second, the surface of the open ocean, or pelagic region ;
and, finally, the depths of the ocean, or abyssal region.

The seashore and the shallow seas, down to a depth of two or
three hundred feet, are in many ways among the most favorable

The Outcome of Evolution 353

habitats for life on the earth. Here are in abundance the four
necessities for plant life — light, water, carbon dioxide, and min-
eral salts. As a result, this part of the ocean is as densely over-
grown with vegetation as is the rain forest. The plants, however,
are of fewer species, and practically all of them belong to one
group of plants, the algae. All of the four types of algae men-
tioned in Chapter VI are found along the seashore, and each
occupies a more or less definite region. The blue-green algae form
a thin scum over the surface of rocks, seaweeds, or animal shells.
The green type occurs in the shallow waters, mostly between
tide levels. The massive brown algae are the dominant vegetation
along seacoasts of temperate regions down to depths of 100-200
feet. The red algae are more common in the tropics and in deeper
waters, where they take advantage of the deeply penetrating violet
rays. There are, in addition, many different species of microscopic
algae floating in the water. These are in general similar to those
of the open ocean, as described below.

Wherever a dense growth of plant life affords abundant food,
a myriad of animals will be found competing for it, and the sea-
shore is no exception to this rule. Here only are all of the main
divisions (phyla) of the animal kingdom represented. In addition
to such relatively familiar animals as worms, mollusks, crabs, and
fishes, there is a whole host of animals that always seems weird
and strange to us land dwellers — lace-like sponges, flowery sea
anemones and corals, starfish and spiny sea urchins, squids and
octopi, as well as numerous others that resemble nothing but
themselves. All are living in, upon, or about each other, hiding
under rocks for protection or camouflaging themselves with in-
crustations of rocks, seaweed, or other animals.

Concealing coloration, as well as weapons and lures of many
kinds, is the rule. In tropical waters, the fish are vividly colored
to match the brilliance of the sunlit coral reefs and the masses
of red, green, and brown seaweeds. In temperate waters, as in
temperate forests, browns and grays prevail among both animal
and plant life. Here, however, some very striking examples of
concealing coloration can be found. The flounder and its relatives,
for instance — broad flat fish which most of the time lie quietly
on the ocean bottom — imitate perfectly their surroundings. In
fact, they are most efficient in changing their pattern of colora-

354 The Outcome of Evolution

tion to suit the particular type of bottom on which they are rest-
ing. When lying on a sandy bottom, flounders have a fine-grained
pattern, with a tawny yellow the predominant color, and are al-
most indistinguishable from the sand. If, however, a flounder
swims over to a bottom that is pebbly or rocky, its back quickly
becomes blotched with dark brown, yellow, and cream color, so
that it is again almost exactly like the surrounding ocean floor.

Powerful vises for crunching or squeezing their prey are found
not only in the well-known claws of crabs and lobsters, but in the
tentacles of sea anemones, octopi, and squids, and the slow-mov-
ing but very powerful legs of the starfish. Some shellfish have a
"tooth ribbon" which resembles sandpaper, and is used to bore
holes in the shells of crabs or other shellfish, while still others
have a boring organ equipped with acid which dissolves the
shells of their victims. These shellfish then insert a long proboscis
and suck out the insides of their prey.

Among the most remarkable fish is the angler fish, found not
uncommonly along the Atlantic coast. This fish, lying flat on the
sandy or muddy bottom a few rods from shore, is well camou-
flaged in its colors of sandy or dull brown, and bears on its dor-
sal fin a long, thread-like spine, tipped with a soft and flesh-like
flattened cap. When the fish is hungry, it lifts this spine up di-
rectly over its huge mouth and waves the writhing tip back and
forth. Small fish, mistaking this lure for a wriggling worm, make
for it, only to be snapped up by a sudden movement of the large
and powerful jaws of the angler half hidden in the mud below.

Poison darts have been adopted as a weapon of offense and
defense by a whole division of the animals of the seashore — that
including the sea anemones, corals and their relatives. These
"stinging cells" are scattered over the entire surface of the animal,
but are particularly numerous on its tentacles. The darts shoot
out like an uncoiling spring whenever the cells are stimulated,
carrying with them poison from the small sacs at their base,
which serves to paralyze the animal's prey or to ward off its
enemies.

Although many conditions at the seashore are favorable for
life, there are other adverse elements against which the animals
and plants must be thoroughly protected. One is the regular rise
and fall of the tide, depriving much of the shore of the essential,

The Outcome of Evolution 355

necessities of life twice daily. To guard against this, seaweeds are
covered with a jelly-like substance capable of storing great quan-
tities of water ; and animals either remain in the tide pools, hide
under rocks and seaweed during low tide, or, as is true of some
crabs and fish, are capable of breathing both air and water. Most
remarkable in this respect is the "walking goby" of the tropics,
which actually climbs the lower limbs of trees. It breathes air
though a lung-like extension of the gill chamber, and drowns if
kept constantly under water.

The other most inimical condition which seashore organisms
must face is the ceaseless battering of the waves. To guard against
this, the algae are pliable but exceedingly tough, so that they can
be tossed hither and thither without being broken. They are also
firmly fastened to the rocks by means of vacuum cups or "hold-
fasts" which are so powerful that the plants cannot be torn loose
without destroying either the seaweed or the rock. The animals
are either leathery and pliable, like the fishes, octopi, and — among
the smaller animals — the sea anemones, sea slugs, and sea cucum-
bers, or they are encased in thick, limy shells that are resistant to
the force of the waves. Most of the latter animals are stationary
or very slow moving, using their muscles chiefly for closing up
their shells or for squeezing and crunching their prey ; while some,
such as the barnacles and sponges, engulf microscopic organisms
by means of a vortex of current set up by myriads of beating cilia.

The floating animals of the shallow seas, the jellyfish and the
microscopic organisms, form the transition from the life of this
region to that of the next, the pelagic region. The organisms that
live there must be either floaters or swimmers, respectively termed
plankton and nekton. They must be either small or very large.
The larger plants cannot grow here, since they have no anchorage,
and plant life is confined to unicellular, microscopic algae. The
most numerous of these in northern waters are the diatoms, which
are encased in pill-box-shaped shells, often armed with horns or
prongs or ornamented with intricate and beautiful designs. They
can be caught in great quantities along the coast of New England
simply by dragging a cheesecloth net behind a rowboat. The other
group of organisms with autotrophic metabolism is a group of
flagellates, known as the dinoflagellates. These "plant-animals"
are usually equipped with brownish or yellowish plastids, and are

ANIMAL PLANKTON

FIG. 86. — Plankton life: 1-5, diatoms; 6-8, unicellular and colonial green algae;
9-11, desmids; 12, foraminifer (protozoan); 13, heliozoan (protozoan); 14-16,
radiolarians ( protozoan ) .

The Outcome of Evolution 357

encased in vase-like shells of a most elaborate design. They are
most common in tropical waters. Subsisting directly or indirectly
on the diatoms and flagellates or their remains is a whole host of
animals, small and large. There are groups of Protozoa known as
Foraminifera and Radiolaria, with elaborately carved and sculp-
tured shells ; delicate jellyfish of many types ; minute shrimp-like
animals known as copepods; "winged" snails, or pteropods; as
well as the larvae of most marine animals, including worms, star-
fish, crabs, oysters, and fishes. Most of these animals have modi-
fications of structure which make them particularly buoyant. In
tropical waters there is a striking type of jellyfish, known as the
Portuguese man-of-war, whose boat-shaped body contains air sacs
to enable it to float better, while from this body there extend
downward long, violently poisonous streamers. In the copepods,
the feet are not claw-like as in their cousins, the shrimps, crabs,
and crayfish, but are delicate fringed appendages, often brilliantly
colored; and in the "winged" snails the muscle corresponding to
that used by their terrestrial and shore-dwelling relatives for
crawling is modified into flat, wing-like structures that spread
out above the top of the shell.

This vast array of floating life known as plankton is of great
importance as food for fishes. Sometimes myriads of the micro-
scopic organisms are packed together so densely that they color
the sea a reddish brown. At night most of the plankton organ-
isms give off a tiny phosphorescent light, making the ocean spar-
kle and glow.

Besides the plankton, the open ocean supports chiefly three
forms of animal life: squids and cuttlefish, some fishes, and the
whales and their relatives, the porpoises and dolphins. The fishes
are much less numerous than they are near the shore, and consist
mostly of small types quite unfamiliar to those who know only
shore fish. They are mostly bluish or silvery in color to match
their surroundings, and since the open ocean affords them little
protection, they must be agile, fast swimmers. Among them are
the delicate flying fish, which use their enlarged fins to skim over
the surface of the water. The squids and cuttlefish are soft-bodied
animals with long tentacles equipped with disk-like suckers. They
range from small forms an inch or so long to giant monsters
which have tentacles twenty feet long, and may inflict great dam-

358 The Outcome of Evolution

age in combat with the whales which prey on them. Whales, which
are perhaps the best-known animals of the open ocean, are not
fish, but mammals. They have warm blood and suckle their young;
and under their thick coating of hide and blubber they have a
skeleton which can be matched, bone for bone, with that of land
mammals. Whales are of two main types. Some, such as the
sperm whale and the killer whale, have their jaws packed with
sharp teeth with which they kill and eat fish and seals, and even
such monsters as the giant squid and other whales. The second
group, the whalebone whales, have their teeth replaced by long,
thin, fringed plates which act as strainers. These whales feed by
gulping in huge mouthfuls of water, and by straining it out
through their whalebone plates, they extract its plankton. Here we
have the phenomenon of the largest animals in the world feeding
on millions and millions of the smallest.

Frequently the open ocean contains what may be termed "is-
lands" of shallow water life, formed by the drifting out of masses
of algae that have broken away from their anchorage on the
rocks of the shore. The commonest type of seaweed thus found is
the yellow-brown Sargassum, which has slender stems bearing
leaf-like plates of tissue and grape-like clusters of air bladders
which serve to float it. Under the shelter of the Sargassum live
shellfish, small shrimps and crabs, and many types of brightly col-
ored and strangely shaped fish. The Sargassum is carried far out
to sea by ocean currents, but does not form a true element of the
pelagic life since it lives only about two years after it is torn from
its rocks and, although it retains the power of growth, does not
reproduce during that time. The famous "Sargasso Sea" is formed
by a great eddy of the Gulf Stream and other large ocean cur-
rents which send load after load of Sargassum into this part of
the north Atlantic. It is not, however, as is sometimes thought, a
solid mass of seaweed, and ships have little difficulty in sailing
through any part of it.

As we descend from the surface of the open ocean toward the
third realm, the abyssal region, conditions for life become more
and more severe. Adequate light for plants is not found below
1,000-1,500 feet, and the greater depths are inhabited only by
animals. These must either subsist on the dead bodies of animals
and plants constantly raining down from above, or devour each

FIG. 87.— Deep-sea fish. (Redrawn from Lull's Organic Evolution, The Mac-

millan Company.)

360 The Outcome of Evolution

other. Furthermore, they must withstand tremendous pressure
amounting to several tons.

The animals living under these conditions are of the same
groups as those of the upper regions, but their shapes often seem
strange and weird to us. The plankton animals are found to depths
of 15,000 feet, and form the most abundant life of the abyssal
as well as the pelagic regions. Fishes of various types are very
abundant down to 4,000 feet, but below that are less common.
Most of them are small, from a few inches to a foot or two long,
and are of extraordinary shapes. The scarcity of the food supply
means that many of them consist of little besides an enormous
mouth and a thin ribbon of a body. There are many different
kinds of angler fish, whose lures are like those already described
in the shore form, except that they attract the smaller animals by
means of their glowing, phosphorescent tips. These fish, which
have all of the vast, dimly lighted ocean depths in which to roam,
must rarely meet a member of the opposite sex; to overcome this
difficulty the male of some species of angler fish attaches itself to
a female when it is less than an inch long, and soon becomes
completely fused to its mate, spending its entire life as a small
appendage on the head of the female. It nevertheless becomes an
adult, and, when the female on which it lives lays her eggs, is
ready to fertilize them.

Phosphorescence is the rule among most types of animals of
the medium depths, but in the deepest waters, below five or six
thousand feet, few animals have luminescent organs, and every-
thing is pitch black. In the intermediate depths many of the fish
have much enlarged eyes, but others are nearly sightless.

The bottom of the deep ocean is a soft ooze, consisting entirely
of the shells of the minute plankton organisms which have fallen
upon it in a steady rain for millions of years. Living on this bot-
tom are a number of different kinds of animals related to those
of the shore. There are spreading, fan-like corals, strange types of
thin-shelled, colorless shellfish, long-stalked "sea lilies" related to
the starfishes and sea urchins, and great spreading crabs, com-
posed almost entirely of legs which may spread over an area as
much as eleven feet in diameter. Because of the soft nature of
the ooze, these animals must have a large surface for their size.
They subsist chiefly upon the remains of plankton animals that

The Outcome of Evolution 361

•ain down on them continuously, and some are equipped for bur-
•owing through the ooze and extracting from it whatever scant
lourishment is left. One of the most striking examples of the
>ersistence of life is its penetration to these ultimate depths of
:he sea where, at depths of from 12,000 to 18,000 feet, all is
nky blackness for thousands of feet above, and the pressure is
;wo or three tons per square inch.

Adaptations to Changing Environments. — Up to now we
lave spoken of the various environments of the earth as they are
it present — apparently static entities. We know, however, that
ronditions in most parts of the earth are anything but static. In
he geologically very recent time of 25,000 years ago most of
Mew England, New York, and the North Central States were
:overed by ice or by tundra, and even in historical times much
>f the United States has been transformed from deep primeval
"orest into cultivated fields and patches of scrub pine or scrub
>ak. Obviously no organism will survive long on this earth, no
natter how well it is adapted to a particular environment, unless
t can either move to follow the changing position of its own
environments or change to adapt itself to new environments.
The former course is of no difficulty for terrestrial and oceanic
mimals ; but for many fresh-water animals and for plants it is
lot soseasy, and these organisms must have special adaptations
;or moving. In such fresh-water animals as Protozoa, wheel ani-
nalcules (rotifers), and the water fleas and shrimps this is ac-
:omplished by the formation of resistant spores which may be
)lown from one pond to another by the wind.

The Dispersal of Plants. — Plants change their location, some
>f them with considerable rapidity, by means of efficient methods
>f scattering their seeds over large areas. Excellent examples of
:his are given by two families of angiosperms which are wide-
spread and common in the north temperate regions, the aster or
:omposite family, and the rose family. Most of the former, such
is the asters, goldenrods, thistles, dandelions, and hawkweeds,
lave seeds with light, feathery plumes which may be borne long
listances by the wind. Others of them, such as the beggar's ticks
)r sticktight, the burdock, and the clotbur, have seeds bearing
>arbed or hooked spines which cling easily to the hair of animals
ind the clothing of man, and may thus be transported long dis-

362 The Outcome of Evolution

tances. The members of the rose family, which includes besides
the rose, the apple, cherry, peach and plum, as well as such berries
as the strawberry, raspberry, and blackberry, and many other
familiar plants, have most of them fleshy fruits which are avidly
eaten by animals, particularly birds. The flesh of the fruits is
digested, but the seeds are surrounded by a coat which resists
digestive enzymes, and hence are excreted whole and unimpaired
with the feces. Since the bird may fly many miles while it is
digesting the fruit, it spreads the plant quite effectively. These
three methods, hairs or plumes for dispersal by wind, and hooks
or barbs and fleshy fruits for dispersal by animals, are the most
common methods of seed dispersal in land plants.

That these methods of seed dispersal actually help the plant to
invade new regions is shown by the change of vegetation in
North America in the last few centuries. With the coming of
civilized man, the woods of this region have been largely cut
down and made into fields, pastures, or waste lots near cities.
The native woodland plants have been driven out and largely re-
placed by a host of weeds which have been brought in from
Europe and other places. There has thus been a complete change
of environment accompanied by a corresponding change in vege-
tation. Of the European invaders of this new, changed environ-
ment, among the most successful and widespread have been mem-
bers of the aster family. Dandelions, coltsfoot, hawkweeds, and
thistles are all widespread and familiar members of this invading
host of European weeds. Similarly, our native members of this
family have been among the most successful to survive the change
in conditions and occupy the new environment. Goldenrods and
asters have been increasing while woodland flowers have gradu-
ally become rarer and rarer. Our native members of the rose
family have also increased with the changing environment. Haw-
thorns have spread all over our pastures, while blackberries and
raspberries line thickets and f encerows, becoming ever more abun-
dant as the woods are cut down.

The Value of Sexual Reproduction in the Struggle for
Existence. — The alternative of migration, variation to suit a
new environment, is largely responsible for evolutionary change.
Although the mechanism of this change will be discussed in the
next chapter, one important factor in it has already been de-

*he Outcome of Evolution 363

ribed, namely, sexual reproduction. That an infinite variety of
>mbinations may be obtained by this process has been shown in
hapter XIII on heredity. Furthermore, variation is probably the
ily advantage of sexual reproduction. We have learned that in
ants there are many methods of asexual reproduction which are
>th faster and more certain than the sexual process, and that
ants may be reproduced indefinitely by these methods without
>preciably lowering their vitality. Animals, on account of their
fferent method of growth, do not as a rule possess these
ethods. Some, however, reproduce entirely by parthenogenesis,
id these are not in any way inferior in appearance or activity

> their sexual relatives. We must conclude, then, that the primary
irpose of the often long and arduous cycle of sexual reproduc-
cm is to help the organism to vary so that it may become
lapted to changing environments. Along with changes in the
>rm plasm, which will be discussed in a later chapter, the varia-
lity produced by the segregation and recombination of genes
trough sexual reproduction has been, in a changing environment,
ic prime moving factor in evolution.

Non-adaptive Differences Between Organisms. — Although
e have shown that a large proportion of the species of animals
id plants owe their individuality to their adaptation to one of
te thousands of different environments on the earth, naturalists
1 know that the differences between species often have nothing

> do with their adaptation to different environments. For in-
ance, there are many different species of warblers living in the
>rests of the eastern United States, each of them with its own
*culiar pattern of markings. Although most of them are adapted

> slightly different habitats, these adaptations can only in a small
ay account for their differences in color pattern, song, and other
laracteristics. The same thing is even more generally true of
ants. The student of trees learns to distinguish between various
>ecies of pine by observing whether the needles are in bundles
f two, three or five, the size and shape of the cones, and whether
- not the cone scales have prickles at their tips. None of these
laracteristics help the pines to become adapted to the particular
ivironments which they occupy, and yet if such differences did
Dt exist the pines would probably all belong to the same species.

364 The Outcome of Evolution

The same may be said of oaks, maples, hickories, goldenrods,
asters, and practically every other group of plants.

For this reason our understanding of evolution depends on our
knowing not only how organisms can become adapted to all sorts
of new environments, but in addition how they can evolve and
successfully maintain all sorts of variations that have little or
nothing to do with adaptation to the environment. The essentials
of our knowledge and theories on these subjects are set forth in
the next chapter.

CHAPTER SUMMARY

The world of living organisms consists of a vast number of
species, which are the end products of evolution. The chief aim
of the study of evolution is to understand how these species came
into being, and what is the cause of their close adaptation to their
environment. The adaptation of organisms to many diverse en-
vironments is partly responsible for the large number of species
that exists. This adaptation is essential to the existence of any
organism. The tendency for all organisms to reproduce their kind
at a rapid rate results in a potential overpopulation of every en-
vironment on the earth, and a consequent struggle for existence
between organisms to see which will survive. This struggle is
equally keen in all parts of the earth, but varies in its nature with
the environment.

In the tropical rain forest, where conditions are most favorable
for life, the struggle is chiefly between the multitude of organ-
isms present. Hence all organisms are equipped with devices for
protection or aggression, many of them very elaborate. Most ani-
mals possess concealing coloration, either for protection or for
aggression, and many also imitate objects of their surroundings
in shape. Poisonous or noxious animals often possess warning
coloration, which consists of a great contrast in color with their
surroundings. These animals are sometimes mimicked by harmless
animals or by other harmful ones, by which device the mimic
gains added protection. Parasitism, saprophytism, and symbiosis
are strongly developed in the tropics; and the social insects, the
termites and ants, are most highly developed there. Unfavorable
conditions against which organisms must be protected are, in the
case of aquatic animals, the drying up of rain pools, and, in the

The Outcome of Evolution 365

case of the smaller plants, the necessity of living high up in the
branches of the trees, where a constant supply of water and
mineral salts is difficult to obtain. The development by some plants
of "reservoirs" at the bases of their leaves is an adaptation to
this condition of their environment.

In the deserts, life must be adapted to protect itself against
drought, cold, and windstorms, rather than for a struggle of
organism against organism. Plants, like the cactus, have special
structures for the storage of water, expose a minimum of surface
to the air, and have a thick, waxy covering over the surface parts.
They are also covered with spines and thorns to protect them
from foraging animals. Smaller plants remain as seeds under-
ground, germinating and flowering only during the brief rainy
seasons. Animals of the desert are fleet and agile, and many of
them, like the camel, are able to store water in their bodies. Some
build huge burrows underground and store up large quantities of
food during the wet seasons when the seed plants are growing
in abundance.

In the arctic regions, the herbivorous animals must be equipped
for feeding on the sparse evergreen vegetation under the snow,
while the carnivorous animals must live on short rations for much
of the time. Birds migrate southward and insects remain dormant
for all but a few months of the year. Plants must be equipped to
resist extreme cold and drought for most of the year, but must
be able to grow and flower rapidly during the summer months.
Animals usually raise larger broods of young than their relatives
in temperate climates.

In temperate regions, adaptations are found similar to those
of tropical organisms, as well as those most characteristic of the
deserts and the arctic regions. Concealing coloration is well de-
veloped, and warning coloration and mimicry also occur. Plants
of sandy beaches and salt marshes resemble desert plants, while
many of those found in peat bogs are similar to arctic species.
The lack of available mineral salts in peat bogs has been over-
come by some insectivorous plants in the same manner that a
similar deficiency has been overcome by the epiphytic plants of
the tropics. The most important adaptations characteristic of tem-
perate regions are those which fit organisms for seasonal changes
of climate.

366 The Outcome of Evolution

One type of adaptation of animals to meet changing conditions
is migration. Migrations are of three types: (i) seasonal, (2)
cyclical, (3) irregular or dispersal. Seasonal migrations are best
exemplified by birds ; cyclical, by fish, such as the salmon and the
eel; dispersal migrations are found in insects such as the grass-
hopper and in mammals as exemplified by the lemming.

The habitats of life in the ocean are in three general regions :
(i) the littoral and sublittoral, (2) the pelagic, and (3) the abys-
sal. Conditions in the littoral and sublittoral regions are unusually
favorable for life; hence the struggle for existence between the
numerous organisms inhabiting this region is unusually keen.
Striking adaptations for protection or offense are those of the
flounder, some shellfish, and the angler fish. Unfavorable condi-
tions against which organisms in this region must contend are
the rise and fall of the tide and the battering of the waves. For
protection against the latter, plants and animals are either very
tough and flexible, or are encased in hard shells.

The majority of the organisms of the pelagic region are minute
floating forms known collectively as plankton. The most frequent
components of plankton are diatoms, flagellates, Protozoa, jelly-
fish, and various types of crustaceans, chiefly copepods. The most
important swimming animals of the pelagic region are small fish,
squids and cuttlefish, and whales and porpoises.

In the abyssal region only animals can survive. They are of
the same groups as the littoral, sublittoral, and pelagic animals,
but are often of unusual shapes. Those inhabiting the ocean floor
must cover a large surface so as not to sink through the soft
ooze, but must have slender limbs so as to resist more easily the
great pressure.

There are two means of adaptation to a changing environment,
moving away or varying.

In addition to those variations which enable organisms to be-
come adapted to their environment, a large number of non-adaptive
variations have resulted from evolution.

QUESTIONS

1. Briefly characterize the species, giving original examples.

2. Compare the tropical rain forest, the desert, and the arctic region*

The Outcome of Evolution 367

as to the factors which favor or are inimical to life and as to
the nature of the struggle for existence in the different regions.

3. Describe, with examples, concealing coloration, warning colora-
tion, and mimicry.

4. Give some ways in which conditions for life and the adaptations
of organisms in the temperate regions compare with those in the
tropics, the desert, and the arctic regions.

5. Discuss the different types of adaptations to changing seasons.

6. Describe the different types of migrations, giving examples.

/. Compare the three different regions of the ocean as habitats for
life, and briefly describe some of the adaptations possessed by or-
ganisms in each of these regions.

8. Discuss the nature and significance of plankton to life in general
and man in particular.

9. What are the ways in which animals and plants can become
adapted to a changing environment?

GLOSSARY

abyssal (a-bis'al) Pertaining to the depths (of the ocean).

bulb An underground structure for storage in plants, consisting of
a modified stem and many modified, fleshy leaves.

concealing coloration Coloration possessed by animals which blends
with their surroundings.

corin An underground structure for storage in plants, consisting of
a modified, very short, thick, and fleshy stem.

epiphyte (ep'i-fit) A plant which lives on top of, but is not para-
sitic on, another plant.

littoral (lit'o-ral) Pertaining to the shore.

mimicry The imitation in color and form of a harmful species by a
totally unrelated harmless or harmful one.

nekton The swimming life of the open ocean.

pelagic (pe-laj'ik) Pertaining to the open ocean.

plankton The floating life of the open ocean.

root stock A somewhat thickened, jointed underground stem in plants,
used largely for storage.

species (spe'shez) The unit of classification of organisms, consist-
ing of individuals which have certain characteristics in common
in which they differ in a discontinuous fashion from other related
species.

tundra The treeless areas of the high arctic and high mountain regions.

warning coloration The coloration of poisonous or otherwise noxious
animals which makes them conspicuous and therefore unmolested.

CHAPTER XVI
WHAT CAUSES EVOLUTION?

The Problem of How Evolution Takes Place. — Since the
publication of Darwin's epoch-making theory, scientists have be-
come agreed that evolution has taken place. They are agreed also
that, taking it as a whole, it has been a gradual, steady process,
and has resulted in a wonderfully precise adaptation of a tremen-
dous host of species to a great variety of environments. The great
problem at present for students of evolution is to determine just
what are its causes and what factors have guided it in the many
directions which it has taken. Much evidence from different direc-
tions has already been brought to bear on this problem, and at the
present time discoveries which may lead toward its solution are
being made at such a rate that this field of research has become
one of the most active in all biology. There are two ways of at-
tacking the problem. One is the historical and comparative method,
which involves the comparison of different organisms, living and
fossil, to determine just what directions evolution has taken and
how fast it has progressed. The other is the experimental method,
which consists of studying the variation and evolution that are
occurring at the present time. The first has the advantage of
covering the whole history of evolution in one broad field, while
the second is the more direct and gives more certain results. It is
the aim of this chapter to present the chief facts and theories that
have arisen from both of these lines of investigation and the
relative importance of each according to present-day opinion.

The Lamarckian Theory. — Half a century before the publica-
tion of Darwin's Origin of Species, Jean Baptiste Lamarck set
forth an evolutionary theory with a clear-cut suggestion as to
how evolution had taken place. Lamarck included in his somewhat
philosophical rather than scientific theory the statement that or-
ganisms adapt themselves to new environments by struggling to

368

W 'hat Causes Evolution? 369

overcome handicaps, and that these adaptations are transmitted
to their offspring. He used the example of the giraffe, an animal
with a very long neck which enables it to crop the leaves of trees.
The ancestor of the giraffe, according to Lamarck, took to reach-
ing up at the leaves of trees. In doing so it developed the muscles
and bones of its neck more than did its fellow animals which
were content to browse on grass. This ancestor then transmitted
its slightly longer and better-developed neck to its offspring who,
continuing to reach as high as possible for tender, juicy tree
leaves, developed still longer necks. Their offspring, in turn, in-
herited this added development, until finally, after generations of
neck straining, the modern giraffe was evolved.

This theory seemed to fit very well the fact of exact and com-
plete adaptation to environment that we see everywhere in the
world of life. The great difficulty with it, however, is that as yet
there is no good evidence that characteristics acquired in such a
way as Lamarck postulated can be inherited, and there is much
evidence that they cannot. Many experiments have been performed
to test out Lamarck's hypothesis, and practically all have gone
against the theory of the inheritance of acquired characters. For
instance, one zealous experimenter tried cutting off the tails of
rats for many generations; the descendants of the mutilated rats
all had just as long tails as their ancestors. A similar operation,
that of circumcision, has been performed by the Jews for cen-
turies, yet no Jewish boy has ever been born in whom it was not
necessary. That such mutilations are not inherited is, however,
not strange; more important is the fact that, in animals at least,
we have little or no evidence that more subtle, adaptive acquired
variations are inherited. Many experiments have purported to
show such inheritance, but in every case the results are either
doubtful or actually discredited.

The case of the inheritance of acquired mental characteristics
has been more difficult to decide, but here also the evidence is
mainly against such inheritance. One well-known experimenter
recently seemed to prove that the ability of rats to learn mazes
may be increased in successive generations by careful training,
but this experiment has been repeated in the same manner by two
other workers, with entirely negative results. Furthermore, the
latter have found, by keeping careful pedigrees of the rats trained

3/O What Causes Evolution?

by them, that the different qualities which help a rat to learn a
maze are numerous, and each is inherited independently in a
rather complex fashion. The experience of the human race, after
all, corresponds to that of the latter set of experiments in show-
ing that acquired knowledge or skill is not inherited. For in-
stance, English and American children have for centuries been
taught to speak English, and French children French; but if an
English child is born and brought up in France, he learns to
speak French as easily as a French child, and finds it equally
difficult to learn English.

Can Environment Influence Our Inheritance? — It is possi-
ble to look at this problem of the inheritance of acquired charac-
teristics from the point of view of what we already know about
the mechanism of heredity. In a previous chapter it was pointed
out that the hereditary characteristics of any individual are given
him through the gametes which formed the zygote that was the
start of his existence, and, in fact, are carried chiefly in the
chromosomes of those gametes. Our problem narrows down to
whether these gametes or the cells that produce them can be
changed by the environment.

If, for instance, a white man goes to live in the tropics and his
skin is constantly exposed to the burning sun, it becomes, in a
few years, as brown as that of many mulattoes. The cells of the
skin have produced brown pigment like that of Negroes, and it
is possible that changes have taken place in the chromosomes and
genes of their nuclei. But you will readily see that such changes
could not affect the hereditary nature of the race, since the skin
cells do not hand their chromosomes on to the gametes and thence
to the zygotes. The only cells from which the genes and chromo-
somes of the zygotes are derived are the germ cells in the gonads.
These germ cells, together with the gametes and zygotes, are fre-
quently spoken of as the germ plasm, to distinguish them from
the cells of the body, or somatoplasm. The germ plasm is handed
down in a continuous line from generation to generation. It is
set off at an early stage in embryonic development from the so-
matoplasm, which produces only cells that go to form a particular
individual, all of which eventually die. Hence, any changes which
take place in the cells of the somatoplasm last only as long as the
life of the individual that they form, and the only changes which

What Causes Evolution? 371

can affect the hereditary nature of the race are changes in the
germ cells. The question at issue, then, is whether the environ-
ment can change the germ plasm of a race.

The answer is that it can in certain ways, and in other ways it
cannot. It can change the germ plasm of a race by the selection
of those individuals most fitted to it, as well as by producing
mutations, which will be discussed further on. The environment
cannot, so far as we know, produce adaptive variations which
would tend directly to make an organism more fitted to it, nor
can the germ plasm be influenced to change in the direction of
changes which have been produced in the body cells. Hence we
may conclude that the inheritance of acquired characteristics is,
at least in animals, theoretically impossible. That this argument
does not hold for plants is obvious, since new germ cells are dif-
ferentiated from the cells of the growing region each season ; but
the fact remains that evidence for the inheritance of acquired
characteristics is as scanty in plants as it is in the animal kingdom.

Darwin's Theory of Natural Selection. — Lamarck's theory
of evolution was disregarded by the scientific world because he
failed to pile up sufficient evidence for the fact of evolution and
because he failed to offer a sufficiently plausible theory of how it
had occurred. When Darwin, through his observations in the
Galapagos Islands and in South America, became convinced of
the fact of evolution, he was still at a loss to explain how, genera-
tion after generation, animals and plants had gradually changed,
usually in a direction that would adapt them to their environment.
Lamarck's theory would account for such adaptive changes, but
Lamarck's theory could not be proved true. Then, suddenly, one
day, the true explanation dawned upon Darwin. It was suggested
to him by the writings of Malthus, the famous authority on
population. All forms of life are multipying at a rate that leads
to a continual struggle for survival. In each generation there is
a considerable variation in form, strength and habits among the
members of a given species. Consequently, those which vary in
such a way as to be well adapted to the environment will win out
in the struggle for survival and pass their traits on to their off-
spring. Thus over several generations there will be a natural selec-
tion of types best adapted to the environment; and as environ-
ments gradually change, species will evolve to fit them.

372 What Causes Evolution?

This theory of natural selection is so plausible, once one has
come to understand it, that it seems remarkable that Lamarck or
some of the early evolutionists did not think of it. Without doubt
it did much to bring about a favorable acceptance of Darwin's
whole theory ; and at present, while most biologists are intensely
skeptical of the inheritance of acquired characters, few doubt that
natural selection has played an important part in the evolutionary
process.

Darwin's theory, as set forth in his Origin of Species, has, how-
ever, certain limitations. He considered that any variation, how-
ever slight, could be used by natural selection in developing a
new type, and placed the most stress on fluctuations such as exist
between brothers and sisters of the same family. We know now,
however, that many of these slight variations are due to the effect
of the environment and are therefore probably not inherited. Sec-
ondly, we know from experiments that selection of such slight
variations as those which Darwin stressed often leads merely to
the reassortment of existing gene factors until a pure line is
reached, and that beyond this point selection has no effect. Other
difficulties are, thirdly, that a slight, heritable difference, if it should
appear in a single individual, would probably be swamped out by
crossing with other individuals; and, fourthly, that many of the
differences between closely related species of organisms are not all
such as would adapt them better to different environments, or to
any environment whatever. From all these facts we are now cer-
tain that not all variations can, by selection, result in evolution.
Our principal task is, then, to find out what sort of heritable
variations exist, how well they may be used by natural selection,
and, finally, what are the causes of such variations.

Mutations. — Perhaps the most important step in the modern
study of variations which are the basis of evolution was the dis-
covery made by the Dutch biologist DeVries in 1900, at the same
time as he was making known Mendel's laws of heredity. DeVries
concluded, from studies of the evening primrose, a yellow flow-
ered plant native to the United States which he found growing
in the gardens and back yards of Holland, that sudden, rather
than gradual and slight variations, are the material with which
evolution works. He found all sorts of unusual forms of this
plant ; some had exceptionally broad leaves, others reddish leaves

What Causes Evolution? 373

and stems, there were giant evening primroses, and stunted
dwarfs, all springing occasionally from a stock which otherwise
bred true to type. Such sudden changes he called mutations, and
he considered that the new forms produced were new species
which had appeared, full-fledged, before his eyes.

These sudden variations had, of course, long been recognized
by animal and plant breeders under the name of "sports." Darwin
referred to them but considered them of such rare occurrence
that they could not be of use in evolution. Since the attention of
scientists was called to them by DeVries, however, these sudden
changes have been found literally by the hundreds in both ani-
mals and plants.

An interesting mutation recorded historically is the well-known
Ancon sheep. One of the colonists of Massachusetts discovered
among his flock a sheep with much shorter legs than the others,
which appeared suddenly as the offspring of a normal, long-legged
sheep. Since it could not jump over the stone fences of his pas-
ture as easily as the rest of his flock, he decided to breed from
it, and was able, as the characteristic was inherited, to create a
new breed of short-legged sheep, known as the Ancon. Similar
sudden changes are recorded from time to time in other domestic
animals. A cat with seven toes, and a barnyard fowl with webbed
feet, are both mutations that have been recorded. In animals bred
experimentally in the laboratory, mutations appear with a striking
frequency. The most familiar of these animals, the fruit fly, has
produced more than five hundred mutations in the past twenty-
five years. These mutations include such extraordinary forms as
flies without wings, without eyes, with short, stumpy legs, and
with every conceivable shade of brown and red in their eyes, and
of gray, brown, and black in their bodies.

Types of Mutations. — At the time when DeVries published
his mutation theory little or nothing was known of the relations
between hereditary characteristics and the chromosomes. Since
then, with the development of the chromosome theory of heredity,
mutations have been shown to be associated with changes in the
chromosomes, or in the genes which they contain. On this basis,
we now can classify the sudden changes which DeVries called
mutations, and show considerable differences between various
types of them, both in the ways in which they are inherited and

374 What Causes Evolution?

in the changes of the germ plasm which are their direct cause.
The biggest distinction to be made is that between gene or point
mutations, and chromosome mutations (often called chromosome
aberrations). These two classes are very distinct, both in their
causes and in the influence that they have on evolution.

Gene mutations consist of the change in composition of a single
gene. They are much the best-known type of mutation, and, in
fact, the term mutation is often applied to them only. They may
involve a change in only one or in several characteristics, and
may have only a slight effect on the germ plasm. They are, of
course, inherited in Mendelian fashion, as described in a previous
chapter. They may be either dominant or recessive to the original
type, but as a matter of fact most of the mutations that have
appeared in experiments are recessive.

At present there is considerable difference of opinion as to the
actual importance of gene mutations in evolution. A large school
of geneticists consider that they are the "building stones" from
which nature selects to produce new and better-adapted forms.
Other workers, chiefly those in different fields of biology, main-
tain either that these mutations are changes of minor importance,
mostly abnormalities or actual defects which can be of little use
in evolution, or that their occurrence reflects an abnormal state of
the germ plasm caused by the hybrid ancestry of the mutating
species, or resulting from adverse environmental conditions. In
favor of these criticisms one must admit that most of the muta-
tions that have appeared in the laboratory have been abnormalities
and defects, and that few if any are such as would help the or-
ganism in the struggle for existence. On the other hand, we are
now certain that the germ plasm consists of chromosomes with
a highly complex but definite and regular chemical structure.
Hence the most logical method of reasoning from our present
knowledge is to assume that permanent changes of the germ plasm
are brought about by changes in the chemical structure of a
chromosome at some point along its length. Such changes would
produce the visible effect now called a gene mutation.

Chromosome mutations involve changes either in the chromo-
some number or in the gross structure of the chromosomes. The
former consist either of the addition or subtraction of a single
chromosome, or the doubling of the entire set. For instance, the

What Causes Evolution? 375

broad-leaved mutant of the evening primrose has fifteen chromo-
somes, one more than the wild type. It can be produced when, in
the meiosis of the normal form, two adjacent chromosomes stick
together and pass to the same pole of the spindle. By this means
a gamete containing an extra chromosome is formed which, unit-
ing with the normal gamete, produces the mutant. This type of
mutant cannot breed true, since the extra chromosome cannot
pair regularly at meiosis, and hence is of no importance in evolu-
tion. The doubling of the chromosome set, known as polyploidy,
occurs frequently in plants, although it is rare in animals. Plants
with this double number of chromosomes are usually larger and
more robust than, but otherwise similar to, the plants from which
they arose. The most important type of polyploidy, that accom-
panying hybridization, is discussed below.

Mutations which depend on changes in chromosome structure
are of several different types. There are mutations due to the
fragmentation of one or more chromosomes, to the translocation
of a segment of one chromosome to another, to the interchange
of the segments of two chromosomes, and many others. The great
frequency of this type of change has been recently demonstrated
most strikingly in two different ways. In the first place, these
changes have been produced artificially in great quantities by the
action of X-rays, sudden changes of temperature, the aging of
seeds in plants, and various other agencies. Secondly, by means
of hybridization experiments many changes in chromosome struc-
ture have been produced which correlate with changes in the ap-
pearance of the organisms. The fruit fly, the most important ani-
mal in modern genetic research, has proved a fine object for this
type of study. Its larvae, in common with those of other flies,
possess a number of giant cells in their salivary glands, in which
the enormous chromosomes are over 100 times as long as those
in normal cells. Furthermore, although these cells are in a perma-
nent resting condition, the chromosomes in them are not only
evident, but in addition are closely paired as at the beginning of
meiosis. Hence each chromosome can be compared with its mate
in every detail, and even the most minute structural differences
between them are clearly seen under the microscope. Many kinds
of changes in chromosome structure have been observed, and study
of them has shown that the various species of fruit fly differ from

376 What Causes Evolution?

one another as a result of structural changes in their chromosomes,
and even within certain species there are many races differing from
each other in this fashion.

The importance of chromosome changes in evolution cannot at
present be estimated. There is now at least one undoubted instance
of a visible change in the organism produced directly by the in-
version of a piece of a chromosome, and in several instances gene
mutations appear to have accompanied the breaking and rear-
rangement of chromosome segments. In addition, some of the
well-known "gene" mutations are now known to consist actually
of the rearrangement, the reduplication, or the absence, of a very
small part of a chromosome. On the other hand, some races of
the fruit fly that cannot be told apart by their external appear-
ance have been found to differ in the arrangement of their chromo-
some parts, so that we know certainly that such rearrangements
occur frequently without changing the appearance of the organ-
ism. One important result of these rearrangements, however, is
that the accumulation of a large number of them makes the pair-
ing of the chromosomes difficult, so that hybrids between two
races differing in this fashion, even though their parents look
much alike, are often more or less sterile. Hence two races may
by this means become isolated from each other genetically, so
that they can evolve independently even though they inhabit the
same region. (See below for a discussion of genetic isolation.)
There is no doubt, therefore, that chromosome mutations play a
considerable role in the multiplication and diversification of the
species of a genus, but whether they themselves can produce much
that is actually new is not known.

The Role of Mutation in Evolution. — Although the produc-
tion of hereditary changes in the organism has been observed in-
numerable times in the laboratory, evolutionists are not agreed
that these observed changes are of the same order as those which
have in nature produced new species and varieties. The skepti-
cism of many scientists is based on two facts. First, the great
majority of mutations observed in the laboratory are detrimental
to the organism, and none of them have brought a species any
nearer to one of its relatives, i.e., in no case has an experimenter
reproduced by means of mutation even the first step of some path
of evolution that has been followed in nature. Secondly, the ge-

What Causes Evolution? 377

netic differences between any two natural species, although they
are inherited in Mendelian fashion, are not quite the same as those
between a normal and a mutated race of a laboratory organism.
Most of the laboratory mutations have produced some marked
change in a single step, while most differences between natural
species and varieties are inherited according to the "multiple fac-
tor" principle, which points to their development by means of an
accumulation of slight changes. Furthermore, the observed muta-
tions have in most cases affected predominantly a single organ,
whereas most of the genetic differences between species involve
a number of organs almost equally. Typical laboratory mutations
in animals are the loss of wings, reduction in size of the eye,
shortening of the legs, and the like, while the differences between
species are such things as the average size of the body as a whole,
the proportional lengths of the various bones, and such general
characteristics as the type of food required and the average
intelligence.

This discrepancy between observed mutation and the known
course of evolution is explained partly by the fact that the genet-
icist sees and breeds those changes that are most striking to the
eye, i.e., marked changes of a particular organ. On the other
hand, the slight mutations that by their accumulation would pro-
duce the known differences between species are almost impossible
to detect, since just as great changes can be produced by the seg-
regation of genetic factors already possessed by the organism. In
some exceptionally pure genetic lines of plants, particularly snap-
dragons and tobacco, indications have been found that these small
mutations affecting several characteristics at once actually occur
more frequently than do the large, obvious changes, but this evi-
dence is as yet not definite. At present, therefore, we can merely
say that since the only known way of producing differences that
are inherited in Mendelian fashion is by mutation, the differences
between species and varieties were probably initiated in this man-
ner, although the type of mutation responsible for evolutionary
changes is not well understood.

Although a direct attack on the problem of the role of muta-
tions in evolution is at present fraught with almost insurmount-
able difficulties, a new method of indirect attack on it is being
rapidly developed with great success. This is the combining of

378 What Causes Evolution?

genetics with a study of the development of the organism, in
order to discover the relationship between genes and the charac-
teristics for which they are responsible.

As a result of studies of this sort, many evolutionists are be-
ginning to think of the organism as the end product of a long
chain of chemical reactions, each of them highly complex, but
all coordinated and following each other according to a well-
defined pattern. The functions of the genes are connected with
the regulation of these reactions and the production of the final
pattern. Some of them affect only that part of the pattern that
is concerned with the production of a single organ ; others affect
all of the reactions that are taking place at a particular time ; but
most, if not all, of them have their principal activity confined to
some particular period in the development of the organism.

For instance, there is a mutation in fowl, known as the creeper,
which results in a short-legged bird. A study of the development
of this bird has shown that a sudden retardation of all of the
metabolic processes controlling its growth occurs at one particu-
lar time, and this time is just when the leg buds are growing
most actively. There is another case, a series of mutations of a
certain gene of the fruit fly, all of which produce a greater or
less degree of reduction in the size of the wings together with a
distortion of their shape. These mutated genes stop the normal
growth of the wings at some point in their development. Those
that come into action early produce great degeneration of the
wings, while others that do not act until later produce relatively
little change from the normal fly. The effects of these various
mutations have been exactly reproduced in genetically normal flies
by subjecting them to sudden changes of temperature at particular
times in their development. By this means the experimenter can
produce a copy of any one of these mutations that he wishes,
although this change is not of course inherited, since the germ
cells are not affected.

We are now in a position to understand the underlying differ-
ence between the average laboratory mutation and most changes
of evolutionary significance. The former acts by retarding or ac-
tually inhibiting one or more of the chemical reactions necessary
for the production of the mature organism, while most evolution-
ary changes alter, apparently by gradual steps, the pattern of these

What Causes Evolution? 379

reactions. Hence scientists are looking hopefully for a solution
along these lines of the riddle of how evolution has taken place.

The Causes of Mutation. — These changes of the germ plasm
occur naturally with great frequency, but what causes them is as
yet little known. They can be produced in the laboratory by a num-
ber of agencies. The most notable of these is X-rays ; but several
other factors, such as extremes of heat, high concentrations of
ultra-violet rays, aging of seeds in plants, and the growth of plants
under unfavorable conditions of nutrition, can also produce mu-
tations. In every one of these treatments, however, mutations are
obtained only when the conditions are so severe that most of the
organisms subjected to them die. Hence the production of muta-
tions by these agencies under natural conditions is probably not
an important factor in evolution, although the fact that the muta-
tion rate as well as the amount of natural selection is increased
by adverse conditions is probably of considerable significance. An
interesting fact is that chromosomal mutations as well as gene
mutations can be produced by these agencies. This suggests that
"the natural causes of these two different types of changes in the
germ plasm are, if not actually the same, at least a good deal alike.

Perhaps the best explanation of natural mutation that can be
given at present is that the chromosome has a complex molecular
structure which, like many complex chemical compounds, is in a
more or less unstable condition. Hence a change of its structure
at any particular point can occur either spontaneously or under
the influence of external agents, and this produces some particu-
lar change in one of the chemical reactions controlling the devel-
opment of the organism. However, such postulates are at present
little more than scientific guessing, and future discoveries may
give us quite a different conception of evolutionary change.

TheTmportance of Isolation in Evolution. — Although mu-
tations of the germ plasm, acting along with natural selection,
are probably chiefly responsible for the adaptation of organisms
to their surroundings, they do not explain the enormous diversity
of the world of living organisms. Why should there be thousands
of different species of flies or fishes in the world, and scores of
them inhabiting the same small patch of land or sea?

The differentiation of a new species depends on the accumula-
tion, within a group of individuals, of a number of differences

380 What Causes Evolution?

by which the members of this group can be distinguished from
all of their relatives.

Before this accumulation of differences can take place, an in-
dividual or a group of individuals must be prevented from shar-
ing with any others outside their group the new characteristics
that appear, either by mutations or by new combinations of genes,
in their germ plasm. For this purpose another evolutionary factor
must be postulated, namely, isolation. By isolation we indicate any
influence which prevents free interbreeding between closely re-
lated organisms. There are several forms of isolation, but the
two most important are geographic and genetic isolation.

It has already been noted in Chapter XIV that geographic
isolation of certain animals on the various islands of the Gala-
pagos Archipelago has apparently resulted in the evolution of
species peculiar to each island. It is a notable fact, furthermore,
that in mountainous regions there are always many more species
of animals than in flat ones, and that such regions are generally
the centers of distribution in which many species of organisms
appear to have had their origin. The high mountain ranges effec-
tually shut off the members of a species in one valley from those
of another, with the result that they do not interbreed and hence
are likely to undergo different courses of evolutionary change.
For instance, many groups of animals and plants, such as the
pheasants and other wild fowl, as well as wheat and oats, are
believed to have originated in the mountainous country of Central
Asia. The high Andes of South America are the center of dis-
tribution for many groups of plants, including that to which to-
bacco belongs and the wild ancestors of the potato, while many
other groups center around the mountains of the western United
States. There are about fifteen species or subspecies of squirrels
and about eight of cottontail rabbits in the eastern United States,
while in the west there are about thirty forms of squirrels and
twenty-three of cottontails. In many instances valleys only a few
miles apart and having almost identical climates, but separated
by ranges and peaks where the climate is essentially arctic, have
several species of plants and animals peculiar to them ; and, simi-
larly, isolated peaks and ranges will possess species characteristic
of them alone.

Geographic isolation is seldom entirely permanent. Changes in

What Causes Evolution? 381

climate and geography or exceptional migrations will usually
bring two isolated groups into contact once more. Then they will
interbreed and the differences between them will be wiped out
unless genetic isolation, that is, sterility between the members of
the two groups or in their hybrid offspring, has developed. The
differences between the races of man are doubtless the product
of geographic isolation; but since, during the course of this isola-
tion, sterility has not developed between these racial groups, racial
lines begin to disappear as soon as two human races come into
contact with each other, although, in the human species, social
barriers against intermarriage usually retard the amalgamation
to a certain extent.

As a general thing, biologists do not consider that separate
species have been formed until genetic isolation — that is, inter-
specific sterility — has developed.

Genetic isolation may develop after geographic isolation has
produced marked differences between the two groups, or it may
develop suddenly, by means of a single mutation, without greatly
changing the other characteristics of the mutant group. For in-
stance, two races of a certain species of fly are so alike in appear-
ance that they cannot be distinguished at all on the basis of
external signs; yet when they are crossed, the hybrids are not
only sterile but have imperfectly developed ovaries and testes. In
the future course of evolution, these two races may undergo en-
tirely different courses of development, with the result that two
entirely different species may be formed, even though they live
in essentially the same environmental situations.

Genetic isolation resulting in sterility between two groups may
lead to an almost immediate differentiation into two species. A
single family may be genetically isolated from its relatives, thus
practically forcing inbreeding among the members of this family.
By this means recessive mutations which have occurred previ-
ously but have been "swamped0 by mixture with normal strains
can appear. If these are not harmful to the organism and there-
fore eliminated by natural selection, they will breed true and be
perpetuated ; hence a new species can be evolved without further
mutations.

In some instances, isolation over a long period of years has
failed to result in the formation of new species. The common

382 What Causes Evolution?

May apple, and various other plants as well, have been isolated
on separate continents for millions of years, and yet have not
evolved enough differences for botanists to be able to tell them
apart. Isolation only sets the stage for evolutionary differentia-
tion. The vicissitudes of mutation and natural selection must then
enter into actually bringing about the differentiation.

The Theory of Preadaptation. — On the basis of the knowl-
edge that mutations are rather frequent occurrences and that they
modify the organism more or less at random, evolutionists are
now considering more and more important a modification of Dar-
win's theory of natural selection. This is the theory of preadapta-
tion, or, as one zoologist has very aptly put it, "the selection of
the environment by the animal/' This theory would explain evo-
lutionary change by adaptation to a new environment somewhat
as follows : Imagine a species of fish inhabiting a large lake with
muddy shores in the days before the advent of land animals.
These fish normally can escape from their enemies by their rapid
swimming, and their swim bladder is relatively little developed,
as is true of most fishes. Among the mutations occurring in this
species of fish there appears by chance one which swims more
slowly than its fellows, but which has a larger swim bladder,
capable of holding a greater amount of air than those of its fel-
lows. This fish would, on account of its slowness, be particularly
open to attack by other carnivorous species of fish in the middle
of the lake, but, on account of the greater air-holding capacity of
its swim bladder, could gain protection by lying under the mud
at the shore of the pond and occasionally breathing air. If it
adopted this different mode of life, it would have little chance
to breed with its fellows and so perpetuate its line; but in case
two of the thousands of fish in the lake possessed similar muta-
tions, they would be drawn together by their mode of life, and
would very likely mate with each other, thus perpetuating their
peculiarities. By this means a shore-inhabiting, partly air-breath-
ing race of fish could be established which, if suitable gene or
chromosome mutations occurred in it, could become sterile in
crosses with the fish still inhabiting the center of the lake, and
therefore would be a genetically isolated, distinct species. Sup-
pose, then, that the climate of the region became drier and drier,
and the lake gradually dried up. The newer shore-inhabiting

What Causes Evolution? 383

species would increase at the expense of the older, purely aquatic
one, and would become the dominant species of fish in that re-
gion. The way would then be open for a further conquest of the
land in a similar manner.

This theory could be applied to plants as well as to animals.
Imagine the same lake filled with a species of green algae. These
plants are adapted for rapid growth, but cannot resist desiccation.
Among* the mutations occurring in the species, however, could be
plants in which the cell walls became abnormally thick as the plant
developed, thereby retarding growth, since much of the carbohy-
drate formed by photosynthesis would be built up into cellulose
rather than used as energy to promote growth. Such a mutation
is actually known to exist in the columbine, and could occur in
any plant. The spores bearing this mutation would have no chance
of developing in the middle of the lake, since the young plants
produced by them would soon be overtaken and crowded out by
the normal sporelings. However, if any spores bearing this muta-
tion should germinate near the shore of the lake, where the spore-
lings were partly exposed to the air during dry weather, the thick-
ening of the cell walls would enable the mutated plants to resist
desiccation while the normal ones perished. Their subsequent his-
tory would be much the same as that of the mutated fish.

One great asset of the theory of preadaptation is that it ex-
plains one set of facts that are very difficult to understand from
the point of view of simple natural selection, i.e., the presence
of rudimentary and vestigial organs. For instance, the sightless-
ness of cave-inhabiting fishes could be explained as follows : Imag-
ine a stream inhabited by a species of fish in which mutations
producing blindness sometimes occurred. These blind fish would
normally perish when still minnows, since they could not see to
escape from their enemies. But suppose that a tributary of the
stream flowed through a cave. This cave would not be inhabited
by normal fish, because food in it would be difficult to see and
capture. But if one of the blind mutants should chance to swim
into the cave, it would be protected by its invisibility, and there-
fore would survive by remaining in the cave. Then, if during its
lifetime another blind fish should similarly seek refuge in the
cave, the two could mate, and a new race of blind fish would
begin its existence. The theory of preadaptation 'is, of course,

384 What Causes Evolution?

only a slight modification of natural selection as conceived by
Darwin, but emphasis on it serves to answer many of the objec-
tions which have been raised to the Darwinian theory, and brings
it into better accord with the findings of modern genetics.

The Role of Hybridization in Evolution. — Aside from spon-
taneous mutation, hybridisation, or the crossing of different vari-
eties and species, is undoubtedly the most important agent in caus-
ing the variations with which natural selection works. It can act
in two different ways. In the first place, the crossing of closely
related varieties and races, which are fertile when crossed but
which differ in a number of gene factors, will produce new com-
binations of genes. If any of these combinations are particularly
well adapted to the environment in which they are found, they
may overcome their parent stocks in the struggle for existence
and thus cause a new race to supersede an old one. In other words,
the process of cross breeding and then selecting the best of the
progeny, a process which man has found the best for improving
his domestic animals and cultivated plants, has been used also by
nature in making organisms better fitted to the environment. In
fact, the whole sexual apparatus, with its carefully designed
methods of securing cross fertilization, is valuable only in that
it results in these new combinations without which a species or
race becomes stagnant and can no longer adapt itself to a chang-
ing environment. Those organisms, such as the dandelion and
some plant lice, which have given up sexual reproduction entirely,
are flourishing at the present time, but they have come to the "end
of their rope" from the evolutionary point of view, and if con-
ditions should change very much in the regions where they are
found, they would quickly succumb.

Hybridization is much more important in evolution, however,
in that it is a cause of the abnormalities in meiosis which result
in chromosome mutations. One way in which this happens has
already been described in a previous chapter in the case of the
mule. The chromosomes of the dissimilar parents cannot paft
properly and so do not go to the equator of the spindle together,
do not separate normally, and may be thrown out of the spindle
completely. The resulting gametes are almost all sterile, but may
occasionally function, even though they have not the normal

What Causes Evolution? 385

chromosome number. By this means a second generation with a
different chromosome number is produced.

The Formation of Polyploid Species. — The best-known chromo-
some change produced by hybridization is the production of a new
species with two or three times the number of chromosomes that
its parents have. Such a species is known as a polyploid. Polyploid
species are very common in the higher plants, though rare in ani-
mals. Roses, chrysanthemums, and clovers are famous examples.
For instance, there are roses with 14, 21, 28, 35, 42, and 56
chromosomes. All of the types with higher numbers are derived
from those with fourteen, chiefly through crossing. While man
has created hundreds of races by cross breeding and artificial
selection, nature has produced an almost equal number of wild
species by spontaneous crossing and natural selection.

The evidence that such polyploid species are produced chiefly
by hybridization has gradually accumulated with the creation of
one after another of new species of this type experimentally.
About forty of these experimental species of plants have been
produced, and there is one case in animals, i.e., butterflies. One
of the most famous group is that which resulted from the cross-
ing of the radish and the cabbage, since in this case the parents
were of different genera. The first-generation rado-cabbage off-
spring had 1 8 chromosomes, as did their parents, but were almost
completely sterile. They produced occasional viable gametes, many
of which, on account of the complete failure of the reduction
division, had the chromosome number of 18. From the union of
two such gametes a plant with twice the normal number of
chromosomes, or 36, resulted. This plant was quite fertile, since
the radish chromosomes could pair with each other, as could also
those derived from the cabbage. Although considerably more vari-
able than its radish or cabbage grandparents, it bred true to a
general intermediate character, and could be considered a new
species, particularly since it could not be crossed easily with either
the radish or the cabbage. By different combinations of gametes
with 9 and those with 18 chromosomes a number of different in-
termediate, fertile types were produced, all of which, except for
minor variations, bred true and could be considered distinct
species. That this same process can take place in nature has been

386 What Causes Evolution?

demonstrated by the artificial synthesis of a known wild species
from two others.

One very interesting case in which this process is known to
have taken place in nature is that of a marsh grass, known as
Spartina Townsendii. This grass was first noticed in the harbor
of Southampton, England, in 1870, where it grew alongside of
the typical European species and one characteristic of the Amer-
ican coast, which had been introduced in that locality, presumably
by transatlantic vessels. S. Townsendii soon demonstrated its
vigor, however, by spreading rapidly along the shore and out
into the harbor, where it formed clumps around which soil col-
lected and thus actually built up new land. Its value as a soil
holder and land former was quickly recognized, and it was car-
ried to many parts of the world, particularly along the dikes of
Holland. Botanists were at first puzzled by the sudden appearance
and spread of such a vigorous species, and the fact that it pos-
sessed characteristics intermediate between the two species with
which it was first found, led them to suspect a hybrid origin for
it. Recently an examination of its chromosomes has shown that
Spartina Townsendii has just as many as those of the European
marsh grass and of the American one added together. Hence there
is now no doubt that it is a polyploid form derived from the cross-
ing of these two species. Here hybridization in nature has pro-
duced a vigorous, self -perpetuating species which has, moreover,
actually changed the coast line of Europe and has been of great
value to man.

Straight-line Evolution. — The types of variation which we
have just described have all been found in experiments and, by
those who approach evolution from the experimental point of
view, are considered to be the only types of variation found in
living things. Nevertheless, students of evolution who have ob-
tained their evidence from fossils and from comparisons of dif-
ferent living forms which seem to show the course of evolution
through the larger divisions of the animal and plant kingdoms,
apparently see a different type of variation acting as the moving
force of evolution. They are not satisfied with the random varia-
tions that the experimenters describe, but believe that each one of
the main lines of evolution is guided by variations of the germ
plasm in a definite direction which is determined by th^ r^ajure of

What Causes Evolution? 387

the germ plasm of that line. Such directed, progressive evolution
is known as orthogenesis, or straight-line evolution.

The evidence for orthogenesis consists mostly of series of fos-
sils which show a gradual, continuous progression toward a cer-
tain type. The best example of such a series is found in the evolu-
tion of the horse. As far as we know, the evolution of the horse
from its diminutive ancestor has proceeded in a straight line with
regard to every characteristic. No fossil horse yet discovered
shows any features which are not intermediate between those of
the earliest horse ancestor and the modern horse. Furthermore,
horses appear to have evolved independently on both sides of the
Atlantic and both the American and the European horses fol-
lowed the same line of evolution.

A more convincing type of evidence for this mode of evolu-
tion lies in the apparent overdevelopment of many organisms,
both fossil and living, in certain characteristics. The Irish deer
evolved huge antlers which, as far as we can see, did it no good
whatever and apparently were the cause of its extinction. Of the
same nature were the huge tusks of the Columbian mammoth,
and in the present-day animals the numerous curving tusks of
some wild boars and the enormous horns of a few species of big-
horn sheep.

These latter cases have, however, been interpreted from the
point of view of random mutation and a type of natural selec-
tion. For instance, the fact that male deer fight for the possession
of the does is well known. Therefore a mutation producing larger
antlers would give a buck an advantage over his fellows, and a
particularly good opportunity for passing on this mutation. By
this means, random mutations for larger antlers could, over a
large number of generations, accumulate until the antlers became
so large that their owners could not escape from their natural
enemies, and the extinction of the race could thus be effected. In
fact, a prominent contemporary evolutionist has brought forth
this argument to refute the statement, frequently made, that the
struggle for existence is always a benefit to a race or species.

The concept of orthogenesis, if examined carefully, is not so
directly opposed to that of mutation and natural selection as one
might think. Mutation, as conceived at present, depends on the
innate ability of the germ plasm to vary; orthogenesis, on the

388 What Causes Evolution?

ability of a particular type of germ plasm to vary in a definite
direction. The orthogenetic development of certain characteristics,
such as the feet and teeth of the horse, could be accompanied by
random mutations in other characteristics which, if they were
valuable to the organism in its particular environment, would, of
course, be selected. Hence the existence of random mutations does
not preclude the possibility of other directed ones.

The great difficulty with the theory is, however, that as yet we
have never observed a series of mutations progressing in a def-
inite direction such as orthogenesis would call for. For that reason
we cannot say that orthogenesis is more than an interesting pos-
sibility and, if we are bold enough, we can consider it as a work-
ing hypothesis on which to base future experiments. Random
mutation followed by natural selection and hybridization gives us
a much simpler explanation; and it is a well-known principle in
science that one should apply the simplest explanation of any
phenomenon that will fit the facts.

Actually, the recently developed concept of mutations as changes
in the pattern of development has suggested an explanation for the
facts produced by the supporters of orthogenesis which completely
reconciles them with the hypothesis of random mutation. This is
the fact that, although many changes in the developmental pat-
tern of organisms are possible, most of these produce an "un-
workable" system that results either in death, or in the produc-
tion of an abnormal, weak, or degenerate organism. Most of the
other changes are of no value to the organism, and therefore are
very unlikely to be perpetuated. The changes most likely to lead
to success in the struggle for existence are those which exagger-
ate or modify some special adaptive structure which the organism
already has. Hence the "orthogenetic" evolution of some special
structure may be merely the result of random mutation and nat-
ural selection acting along one of the few lines open to these
processes.

CHAPTER SUMMARY

The oldest theory of evolution, that of Lamarck, stated that
evolution proceeds by the acquisition of new characteristics by
organisms through the action of their environment and the in-
heritance of these characteristics. The theory is now largely dis-

What Causes Evolution? 389

credited because of lack of evidence in favor of it. Experiments
designed to demonstrate the inheritance of mutilations, adaptive
acquired variations, and mental characteristics have been tried
many times and have with a few uncertain exceptions resulted in
failure. Furthermore, the separate condition of the germ cells in
animals makes the inheritance of most acquired characteristics
quite impossible.

Darwin's theory of natural selection is widely accepted at pres-
ent, but modern biologists restrict the types of variations which
may be the basis for natural selection. The variations accepted are
chiefly of the type known as mutations, that is, sudden random
variations that are inherited. These mutations were first brought
to the attention of evolutionists by DeVries, who believed that
species arise full-fledged in this manner.

Two types of mutations are now recognized, gene mutations
and chromosome mutations. The former consist of changes in a
single gene which are inherited in a Mendelian fashion. Most gene
mutations which have been observed in the laboratory have pro-
duced abnormalities or defects; but as the only known method
for producing a permanent change in the germ plasm, their im-
portance is great.

Chromosome mutations or chromosomal aberrations consist of
changes in either the number or the structure of the chromosomes.
The most common changes in number are either the addition or
subtraction of a single chromosome, or the doubling of the whole
set. The various changes in chromosome structure — fragmenta-
tion, translocation, interchange, and reduplication — can be pro-
duced artificially in many ways, and are known to occur frequently
in nature, being particularly evident in the salivary gland chromo-
somes of certain flies. They are of some importance in producing
directly visible alterations of form, but are probably more impor-
tant in producing genetic isolation.

The difference between the observed mutations of laboratory
organisms and the genetic differences between natural species is
that the former involve chiefly marked changes of one or two
organs, while the latter are made up of many small differences
involving several organs about equally. Expressed in terms of the
pattern of chemical-physical reactions that bring about the devel-
opment of the organism, the laboratory mutations retard or in-

3QO What Causes Evolution?

hibit one particular process, while interspecific differences involve
differences in the general pattern. Mutations may be produced in
the laboratory by means of X-rays, extremes of temperature, and
other agencies, but their natural causes are largely unknown.

In order to produce the accumulation of differences necessary
for the formation of a new species, the isolation of a group of
individuals from all related groups is necessary. The most effec-
tive types of isolation are geographic and genetic. The former is
particularly effective in mountainous and insular regions, and
partly explains the relatively large number of species present in
such regions. It does not always result in the evolution of species,
however. For this, genetic isolation, or the presence of sterility
in the hybrids between two groups, is usually necessary.

A modification of the theory of natural selection is that of pre-
adaptation. This theory states that the modification of an organ-
ism which would adapt it to a particular environment occurs be-
fore it enters that environment. This modified theory of natural
selection is helpful in explaining such problems as the apparent
reduction of structures through disuse, as exemplified by the blind
animals of caves.

Hybridization is important in evolution, first, because it pro-
duces new combinations of genes which may be selected and
perpetuated. Secondly, it may produce the abnormalities of the
reduction division which result in chromosome mutation. The
production of polyploids, or species with two or three times the
normal number of chromosomes, has often been accomplished by
hybridizing. A good example is the cross between the radish and
the cabbage, from which three distinct fertile, true-breeding
strains have been derived. Known wild species have also been
produced in this manner from other wild species. The existence
of many species and varieties with polyploid chromosome num-
bers in a large proportion of genera of plants is evidence of the
importance of this process in plant evolution.

Students of fossils and of the larger groups of living organ-
isms have developed a different concept of variation from that of
random mutation, and hold that evolution is guided by variations
af the germ plasm in a definite direction. This is the theory of
orthogenesis. Evidence for it is the existence of fossils apparently
demonstrating this process, such as the continuous series of an-

What Causes Evolution? 391

cestors of the horse and the apparent over specialization of many
fossil and living organisms. The theory, however, lacks the basis
of definite experimental evidence for it, and, furthermore, is a
complex explanation for phenomena which may eventually be ex-
plained in a simpler way.

QUESTIONS

1. Compare Lamarck's and Darwin's theories of evolution, stating
the principal evidence for and against each and the importance
of each in the modern concept of evolution.

2. Define mutation and distinguish between the two different types.

3. Using examples, describe gene mutations and give an estimate of
their probable importance in evolution.

4. Describe briefly the various types of changes in the chromosome
structure, and the method of identifying them in the fruit fly,
and discuss their importance in evolution.

5. Tell what you know of the natural and artificial causes of muta-
tions, and of their importance to evolution in general.

6. Discuss the two most important types of isolation and their im-
portance in evolution.

7. In what way does the theory of preadaptation modify that of
natural selection, and what is the value of this modification?

8. Using examples, explain two ways in which hybridization may
further evolution.

9. Define orthogenesis, and give evidence for and against this theory
of evolution.

GLOSSARY

chromosome mutation A sudden heritable change in an organism
which is caused by a change in the number or structure of its
chromosomes.

gene mutation A mutation caused by a change in the constitution
of a single gene.

mutation The production of an offspring differing from its parents
in characteristics which are heritable.

orthogenesis (or-tho-jen'e-sis) The variation of organisms progres-
sively in a definite direction.

polyploid (pol'i-ploid) An organism possessing a multiple of the
characteristic number of chromosomes for the group to which it
belongs.

somato plasm (so-ma'to-plaz'm) All the cells of the organism except
those that develop into gametes.

CHAPTER XVII

HUMAN EVOLUTION

Man's Place in the Animal Kingdom. — As has been said be-
fore, the evolution of man from his ape-like ancestors is only a
small part of evolution as a whole, but, to us, it is a very interesting
part.

First let us orient ourselves as to man's position in the animal
kingdom and learn something about his nearest living relatives.
Man belongs to the subphylum of vertebrates, or backboned ani-
mals, and to the class of mammals, the evolution of which we have
already discussed. The particular order of mammals to which man
belongs is known as the primates. The members of this order do
not possess any very marked characteristics which distinguish them
as a whole, as do such orders as the carnivorous mammals, the
ungulates, the bats, and the rodents. Instead, the primates have
gone in for a rather unspecialized bodily form, which is probably
one reason why a large, inventive brain, which could devise tools
to supplement the disadvantages of an unspecialized body, was of
particular value to them, and hence was strongly developed by
natural selection. The following characteristics are, nevertheless,
typical of them and taken together serve to distinguish them from
other orders of mammals. In the first place, primates have paws,
or rather hands fitted for grasping. Secondly, in most primates the
finger and toe nails are flat, rather than claw- or hoof-like; and
finally, most primates have just two breasts, situated on the upper,
or thoracic, part of the body.

Man's Living Relatives. — Although the living primates are all
the ends of their own particular lines of evolution, we have fossil
evidence that our ancestors resembled some of them, and that
these, as a whole, have diverged less from the common ancestor of
primates than has man. The first primates were a group of squirrel-

392

Human Evolution 393

like tree climbers which appeared about fifty million years ago.1
These animals, known as lemurs, developed from the rat-like tree
shrew that was mentioned in Chapter XIV as being the ancestor
of the primates, and resembled this animal more than they do
man. A few of these lemurs have survived until modern times on
the island of Madagascar, in parts of Africa, and in the East
Indies. One, the aye-aye, is not very different from the fossil of
the earliest known primate. It is quite hairy, about the size of a
large squirrel, and walks along the branches of trees, rather than
swinging its way, as do monkeys. Furthermore, its ears, rather
than being flattened against the side of its head, as are those of
monkeys and men, are large and stick out rather prominently, while
its eyes are set more or less on the side of its head and do not look
straight forward as do those of the higher primates. All of these
characteristics show that the lemurs are a link between the true pri-
mates and the simple ancestor of all mammals.

The next group of primates, the monkeys, is rather far removed
from our own ancestors, the modern monkeys being at the ends of
several long lines of evolution which developed independently of
that which led toward man. There are two main groups. The most
primitive are the New World monkeys, mostly small animals with
long prehensile tails, and characterized by a broad, flat nose which
is closely pressed to the face. The capuchin monkey, the familiar
organ-grinder's companion, is typical of this group. The Old
World monkeys are larger, and never have a prehensile tail.
Furthermore, their nose is narrower and more prominent, showing
a closer relationship to the apes and man. The mandrill and the
baboon are Old World monkeys that one often sees in zoos.

The Man-like Apes. — Man's closest living relatives are the
man-like, or anthropoid, apes. The four living members of this
family are all found in the tropics of the Old World and are fairly
closely related to the Old World monkeys. The smallest and most
primitive is the gibbon, found in southeastern Asia. This ape is
comparatively unspecialized, and hence probably near the earlier
common ancestor of apes and man. Its chief marks of distinction

1 It should be understood that all references to the time at which various stages
in the evolution of man and his ancestors took place are merely the estimates most
widely agreed upon at the present time. We do not possess exact knowledge of
prehistoric dates.

394 Human Evolution

are its tremendously long arms, which, although the animal is only
three feet high, have a spread of five feet or more. It is thus well
equipped for swinging its way from tree to tree through the forest,
and can easily clear spaces of twelve to fifteen feet. The orang-
utan, native of the deep forests of Sumatra and Borneo, is likewise
a tree dweller, but is much larger and stockier, with hair of a
reddish color, and rather sluggish. It is quite intelligent, however,
being exceeded among the anthropoid apes only by the chimpanzee.
This third member of the family, found in central Africa, is per-
haps the most familiar of all. Its black hair and its light face, a
grotesque caricature of that of man, are quite distinctive. Chim-
panzees flourish in captivity, and have been used for many experi-
ments to test the mentality of apes in comparison with our own.
The fourth and largest of the anthropoid apes is the gorilla, also a
native of tropical Africa. The gorilla is over five feet high, and
with its massive limbs and chest may weigh more than 400 pounds.
With its powerful arms, massive jaws, sharp teeth, and sloping
forehead, the gorilla is better equipped for fighting than for
thinking.

If we compare man with these cousins of his, we find surpris-
ingly little difference in bodily characteristics. The main differences
are in the proportions of the various parts, which in man are better
fitted to walking upright, and in the apes to tree climbing. For
instance, man has proportionately longer legs and shorter arms;
and his toes are shorter, and the joint of the large toe is changed,
so that he cannot grasp with his foot as can apes. Other differences
are in the head, which in man is balanced so that it can be carried
erect and can contain his very large brain. The jaws of man are
much smaller than those of apes, and the teeth less prominent ; and
the comparatively high forehead makes the facial angle, or angle
formed by the profile of the face, practically vertical, while in apes
it is around 45°.

The greatest difference between man and the apes is, of course,
in the size of the brain and in intelligence. Man is usually consid-
ered in a class by himself as far as intelligence is concerned; and it
was the picture of progressive, high-minded, forward-looking man
having evolved from the brutish, unthinking apes that was particu-
larly repugnant to the early opponents of evolution. Even here,
however, recent experiments have shown that the anthropoid apes

Human Evolution 395

very definitely approach man, and that, in intelligence as in physical
features, they are nearer to man than they are to the earliest
primates. Apes, as do also monkeys, display the exploratory urge
and that innate curiosity which in man has led to so many revolu-
tionary discoveries. Furthermore, they show the beginnings of
reasoning and insight. A chimpanzee can be taught to use simple
tools quite adeptly.

In regard to the emotions, which are sometimes considered
man's exclusive possessions, we may say the same thing. A chim-
panzee can feel anger, grief, jealousy, joy, affection, and sympathy
for his fellow chimpanzees. The social urges, on which our whole
moral code is based, are well developed in apes. They readily give
aid to injured companions, and are so ready to avenge the death of
one of their number that it is dangerous to injure one of a large
group of chimpanzees or gorillas.

The Evolutionary History of Man and the Apes. — Consid-
ering these resemblances between man and the other primates, we
have no reason to doubt that they have a common ancestry. The
important problems connected with this part of human evolution
are, therefore, the closeness of this relationship and the manner in
which the various characteristics peculiar to man were evolved. In
this connection, the study of fossil evidence and of comparative
anatomy has brought out the following opinions. Soon after the
monkeys became differentiated from the primitive lemuroid stock,
probably about thirty million years ago, a group of them became
adapted to climbing trees in an upright position, using their arms
for support and for swinging themselves from branch to branch,
rather than walking along the branches on all fours. This differen-
tiated the anthropoid stock from that of the monkeys and was the
beginning of the evolution of man in upright posture. Other
modifications, such as the loss of the tail, the narrowing of the
nose, and the development of a vermiform appendix, were prob-
ably evolved about the same time. One group of these early small
anthropoids developed their arms more and more, and evolved
into the gibbon. The other went in for increased bodily size and
brain capacity, becoming less adapted for tree climbing. Then,
about fifteen million years ago, some of these large anthropoids
began to develop a larger and larger brain. This enabled them to
use tools and finally fire, and to develop a social organization, so

396 Human Evolution

that arboreal life was no longer necessary for defense. Life became
possible for them in regions less bountifully supplied than the
tropical forests with easily accessible food. What caused the evolu-
tion of man's superior brain is not known, but one suggestion
seems very plausible. We know that at the time when this evolu-
tion occurred mountain ranges were being built up in many parts
of the world, particularly in central Asia, and that the climate of
the earth began to get cooler in anticipation of the Great Ice Age.
The whole history of man's divergence from the anthropoids,
therefore, is associated with conditions which favored the dis-
appearance of tropical or subtropical forests. Under these condi-
tions the struggle for existence among forest dwellers would
obviously become excessively keen, and any of them which could
become adapted to life in the open would have a greater and greater
chance for survival. The anthropoids had undoubtedly the greatest
intelligence of any animals then living, but no other characteristics
which could be modified to enable them to live in the open ; that is,
they could not develop the strength, the sharp teeth, and claws of
the tiger, the size of the elephant and rhinoceros, the fleetness of
the deer and antelope, or the burrowing ability of rodents. Hence
any mutations producing greater brain power gave their possessors
an increasing advantage in the struggle for existence outside of the
tropics. All apes which did not develop these mutations had either
to migrate to the tropics, probably already crowded with anthro-
poids, or perish. This provides a good example of the effect of a
changing climate on evolution, as mentioned in a previous chapter.

The Man Family. — The family of man, then, is, biologically
speaking, fairly closely related to that of the apes. It consists of
but one living species, as all of the various races of man are inter-
fertile and have the vast majority of physical characteristics in
common. In past ages, however, various other species of our own
genus, Homo, existed, and there were a few other genera belong-
ing to our family.

The First "Missing Link." — People often talk about the
"missing link" which connects the apes and man, and suppose that
the discovery of the remains of such an animal will clear up the
mystery that shrouds our origin. As a matter of fact, man and the
apes are not connected by a chain of known forms which will be
complete when one or two gaps are filled in ; what we actually have

Human Evolution 397

is the rusty remains of a few links of a chain that contained many
thousand. Man-like and ape-like animals were being evolved side
by side for millions of years before they developed into modern
apes and modern man, and all that we know of these animals is
what we can gather from a few fragmentary skeletons, and from
the tools that man's ancestors made and discarded.

The reasons that fossil remains of man's ancestors are so rare
are several. In the first place, these ancient primates were largely
forest dwellers, living in places where the struggle for existence
was so keen that any dead body would quickly be attacked by other
animals, and finally be completely disintegrated by bacteria and
molds. Man and his ancestors were too clever to be caught in the
quicksands, tar pits, and quagmires that caught so many of his
contemporaries in the animal kingdom and preserved their fossils
for our study. Besides, there is evidence that burial customs devel-
oped early in the prehistory of man and that bodies were either
burned or exposed in places where they easily became disinte-
grated. The only places where fossil remains of man or of apes
are found are in river sands, in which unfortunate victims of
drowning were soon buried, and in caves, which formed the dwell-
ing places of some of the earliest men.

There is still considerable doubt as to where man first appeared.
Some scientists believe that Africa is the "cradle of the human
race/' but there is more evidence pointing to Asia as the continent
which produced the first men. In south central Asia, and in no
other part of the world, are found the fossil prototypes of all four
of the modern anthropoid apes. No fossil men have been found in
just this region, but it has been comparatively little explored for
fossils.

The species which has been considered the direct ancestor of
man, the Peking man described below, lived in eastern Asia ; and
perhaps explorers in other parts of the same continent, particularly
western China and Tibet, will discover remains which will tell us
what his and our ape-like ancestors were like. The remains that we
know are all of species that appear to have migrated outward from
this center, and from time to time invaded various parts of the
globe.

The earliest fossil remnant which has any human characteristics
is the skull of a child discovered in Bechuanaland, southern Africa,

398 Human Evolution

and known as the Taungs skull. This Taungs child, who lived
about ten million years ago, was certainly a mixture of anthropoid
and human characteristics, and is called by some scientists an ape
and by others a man. The brain was small, and when fully devel-
oped would have been hardly larger than that of modern anthro-
poids. The shape of the base of the skull, however, indicates that
its possessor belonged to a tall, upright race; the forehead was
relatively high, the jaw did not protrude as do those of the apes,
and the eyeteeth for fighting, so characteristic of modern anthro-
poids, were absent. Although the evidence is as yet too scanty to
give us a real picture of the Taungs being, the discovery of this
skull provides evidence of the existence of ape-like forms which
had started to evolve in the direction of man some fifteen million
years ago.

The First True Men. — For the next nine million years, the fos-
sil record of the evolution of man is a complete blank; but at a
period which may be placed at about a million years ago, traces of
the man family again appear, and there can be no doubt that they
were left by beings that were truly men, although they were so
primitive and ape-like in many of the features of their anatomy
that they are not classed as belonging to our species. One piece of
evidence that makes it certain that they were true men is that they
possessed a culture. Culture is the thing that most definitely marks
the human species off from all other animals. It may be defined as
the sum total of all the traditional ways of behaving and thinking
that have been handed down to us by our ancestors. These tradi-
tional ways of behaving and thinking include the making of tools;
the carrying on of agriculture, industry, and commerce; the
observance of religious rites and moral laws; and all the behavior
and thought that are involved in art, science, and the maintenance
of political and legal institutions.

Whatever an animal may learn during its lifetime about adjust-
ing to its environment dies with it. The animal cannot pass its
discoveries on to its descendants. But what an individual man
learns may be imparted to others, and thus become a part of the
traditional manner of getting along in the world that is followed
by all the members of a human society. Language is the instrument
that men use to pass cultural tradition on from generation to gen-
eration. It is the chief basis for the difference between human and

Human Evolution 399

animal life; and when we study the brains of apes and men, we
find that the part that in man is specially concerned with speaking
shows scarcely any development in the ape.

If our classification of man as an animal has seemed to be a
derogation of man's true dignity, this feeling is to a certain extent
justified. To be sure, any organism that ingests its food before
digesting it is an animal, and therefore man belongs in that class
of organisms. But man is a very unique animal, differing greatly
from the "dumb" brutes, who have no language. For his language
has enabled him to build up a cultural life which, as it has evolved,
has become civilization. All the things that we feel most proud of,
that seem to us to mark us off from the beasts, are aspects of
culture.

We now believe that life has evolved from inorganic beginnings.
But when life did come into being, something with entirely new
properties arose. A definite boundary line was passed. When-
ever something possessing entirely new qualities develops in the
course of evolution, it is called an emergent. Just as life has
emerged out of the inorganic world, culture has emerged out of
the organic world, producing the human quality of living which
differs radically from the sort of life that animals lead.

The first indications of true men that we find in the fossil record
are the remains of their culture in the form of crude stone imple-
ments. At first these implements are so crude that we cannot be
entirely certain that they were formed by men rather than the
forces of nature. But in the rocks that were laid down approxi-
mately a million years ago, we find pieces of stone that show
unmistakable evidence of having been formed into implements by
human hands. And in four parts of the world, remains of the men
who used these implements have been discovered. Three of these
finds — one in Java, one in England, and one in east Africa — have
revealed only a few skeletal fragments of creatures who were dis-
tinctly of the human type, but who were much more like the apes
than are modern men. The human beings who left these remains
are referred to as the Java Ape Man, the Piltdown Man, and the
Kanam Man. The fourth of these earliest known human groups is
by far the most important, since it may have been directly ancestral
to present-day man.

This species, represented by parts of five skulls discovered ir a

4OO Human Evolution

cave thirty miles from the city of Peking, is known as the Chinese
Man of Peking. Since the various skulls were found embedded at
different levels in the sediments composing the floor of the cave,
this type of man must have lived there for a long period of time,
and the species of animals associated with him indicate that he
lived about eight hundred thousand to a million years ago. The
skulls show a brain capacity considerably lower than that of our
species, but the relatively narrow, high brain case is strikingly sug-
gestive of a line of development toward modern man. The lower
jaw is definitely man-like, except that the chin is not well devel-
oped. The teeth, although very unequal in size, are like those of
modern men in shape, and in fact have certain characteristics
which have a definite relationship to the modern yellow, or Mongol,
race. This, along with certain peculiarities of the jaw, has led cer-
tain scientists to the belief that the Peking Man was the direct
ancestor of the present-day inhabitants of China.

The cultural remains in the cave show that the Peking Man
knew the use of fire, and was able to chip crude ax heads and
scrapers out of stones. Furthermore, the complete absence of any
bones except those of the skull has shown that the actual inhabi-
tants of the cave buried their dead outside. The skulls found in the
cave are believed to be the spoils of head-hunters, indicating that
even at the earliest known time of their evolution man's ancestors
had learned to kill each other.

Neanderthal Man. — We often hear primitive men spoken of
as "cave men." Actually, only the primitive men who left fossil
remains were cave men, for human bodies left outside the protec-
tive shelter of a cave were almost certain to disintegrate rapidly.
The men who lived for hundreds of thousands of years after the
time of the Chinese Man of Peking were apparently not cave
dwellers, for they are represented by only three fossil finds, a jaw-
bone in Germany, a skull in Rhodesia, and eleven skulls in Java,
found close to the spot where the Java Ape Man was discovered.
Our chief evidences of human life during this time are stone imple-
ments, of which many specimens have been found. It was not
until about a hundred and fifty thousand years ago that there ap-
peared a race of cave men in western and southern Europe and
eastern Asia to leave a fairly large collection of fossils for the
enlightenment of the modern scientist. These forerunners of our

RHODESIAN

Fi€. 88.— Fossil men. (Redrawn from Lull's Organic Evolution, The Macmillan

Company.)

402 Human Evolution

species are known as the Neanderthal men. They were dwarf -like
in stature, but very strong and stocky, with stooped shoulders and
receding chins and foreheads. Their brains were as large as those
of modern man, but it is improbable that they were as highly devel-
oped in the direction of intellectual capacity. Their stone imple-
ments were considerably superior to those of their predecessors,
and they were successful hunters of the large bison, cave bears,
horses, reindeer, and mammoths that lived in Europe at that
period. There is even some indication that they believed in an after-
life, for they buried their dead with implements and food at hand.

Cro-Magnon Man. — The Neanderthal men ruled Europe for
a hundred thousand years, and then, apparently, they were driven
out or exterminated by a new sort of human, tall, with a high fore-
head and prominent chin, armed with superior weapons and sup-
ported by a generally superior culture. This new cave dweller,
known today as Cro-Magnon Man, was the first member of our
species of whom we have any record. He appears to have lived in
Europe up to about twenty thousand years ago, and it is highly
probable that he is numbered among our own ancestors.

Not only did the Cro-Magnons have relatively fine weapons and
tools ; they seem to have been a people with a real love of beauty,
for they dressed in furs, wore ornaments of shell and ivory, and
painted the walls of their caves with colored pictures of the animals
that they hunted. Their art is considered to be of high excellence.
They were very tall, and from the physical standpoint they seem
to have been one of the finest stocks the human race has ever
produced.

Cultural Evolution. — The appearance of the Cro-Magnons
marked the virtual end of the biological evolution of the human
race. These prehistoric men differed no more from present-day
men than the races of man differ among themselves, and these
differences between human races are, biologically considered, so
slight as to be negligible. Nor can any one of the human races be
considered more advanced biologically than any other. For in-
stance, the shape of the skull and jaw of the white man is less like
that of the ape than are the skull and jaw proportions of the
Negro. But the white man is more like the ape in that his legs are
shorter, his lips thinner, and his body more profusely covered with
hair. Not, as we shall see in Chapter XXVI, has it ever been

Human Evolution 403

established that one human race is innately superior to any other
in intelligence or capacity for developing a civilization.

To many people it may come as something of a shock to learn
that, from the biological point of view, they are no whit superior to
the Cro-Magnon cave man or the African barbarian. What of our
civilization? Isn't that a product of evolution? Yes; but not of
biological evolution.

The reader will recall that culture is the product of the discov-
eries made by individuals which become a part of the heritage of
the entire race. Early men could not possess as high a culture as
our own, since fewer inventions could have been made up to their
time. In the course of human history, one discovery builds upon
another, and thus a cultural evolution takes place that greatly im-
proves the conditions of man's life. This is particularly true of the
material aspects of human culture, the tools and machines, and the
things which they produce. Not only is each invention built upon
its predecessors, but the more inventions there are present at a
given time, the greater is the opportunity for and stimulus to new
inventions. Because of this, material progress becomes more and
more rapid as time goes on.

A few thousand years after the time of the Cro-Magnons,
man discovered the advantages of agriculture over hunting as a
means of getting a living. The wealth produced through this new
mode of life made possible the building of cities, which in turn
facilitated intellectual stimulation between man and man, giving a
new impulse to discovery and invention. The use of metals was
discovered, making possible a vastly higher development of tools
and machinery than could ever have been achieved with stone and
wood. The art of thinking in mathematical terms gradually im-
proved, the alphabet was developed, and thus it was made easier
for man to think and to transmit his thoughts. Thus, with each
important advance in invention, new advances were facilitated
until, about five hundred years ago in western Europe, a tre-
mendous acceleration of scientific discovery and invention began.
Since that time, one invention has followed another with breath-
taking speed relative to the rate at which 'culture developed in
earlier years.

It is this rapidly accelerating cultural evolution, rather than any
biological changes, such as modifications in our brains, that places

404 Human Evolution

us at a higher level of life than was attained by the CroMagnons.
As for savage and barbarian tribes who have failed to attain our
level of civilization, here again there is no reason to believe that
biological evolution accounts for the difference between our so-
ciety and theirs. Rather, failure to be provided with opportunity
and stimulation for cultural development seems to account for their
relative retardation. And culture has not only provided us with
the material benefits which we enjoy. Our art and literature, our
political and legal institutions, our religion, and our moral ideas are
all a product of cultural development.

The Future of Evolution. — The study of evolution is bound
to raise in our minds a final question. What is the future of our
race and of the whole world of life? Of course, it is impossible to
prophesy ; we can only examine the possibilities and consider what
might happen. For the next few thousand years at least, the bio-
logical future is bound up almost entirely with the future of
human culture. The first possibility is that the human race is only
sporadically capable of building up and maintaining a complex
civilization such as the one we now live in. Many thoughtful per-
sons see signs of decay in our present culture, and it is possible that
we shall soon revert to a barbarism in which the great scientific
tradition that has been built up in the past few hundred years will
be totally destroyed. But if the scientific tradition does not die out,
if it continues to maintain itself and grow as it has in the past, we
may expect it to wield its great power over nature to speed up the
processes of evolution in a thousand directions. New kinds of
plants and animals may be brought into being by methods that we
cannot even imagine. The human race, through scientific control of
its own evolution, may utterly transform itself, producing a popu-
lation of "men like gods," or, perhaps, a race that, from our
present point of view, would appear freakish and monstrous. Evo-
lution has been going on for hundreds of millions of years ; the
scientific movement has arisen only during the last five hundred
years. What science may do to modify life in the millions of years
to come, no one can imagine. Mother Nature gave birth to culture
when the organism Homo sapiens was evolved, but human culture
is becoming a very lusty infant, and may in the future shape the
destinies of its parent in marvelous and unpredictable ways.

On the other hand, the human race may die out within a few

Human Evolution 405

thousand years; but biological evolution will continue, and per-
haps some new organism will arise with sufficient intelligence and
talent to produce a culture. Perhaps life, either through human or
other agencies, will discover means of moving from one planet to
another, or even to distant regions outside the solar system, so that
it may become immortal, escaping the doom that threatens it when
in the long course of stellar evolution our planet becomes cold, and
the light of our sun fades into darkness.

Still another possibility exists that, while other forms of life are
evolving or being brought into being by cultural interference with
natural processes, the human race may persist relatively un-
changed for hundreds of millions of years, as a few other forms
have done after reaching what seems to be their final stage in evo-
lution.

We can only guess at the future ; for, while it may well be that
all the events occurring in the great world of life are as subject to
the laws of mechanical causation as are the revolutions of the
planets, our science of today possesses no means of predicting the
course of life processes. Of one thing we may be as certain as of
the continuation of day and night. Life will change. In the millions
or even billions of years which may remain for life upon this
planet, new forms will shape themselves and vanish as living
forms have done since life began. But what the nature of those
transformations will be, no one can surmise; and having dimly
glimpsed the strangely shifting spectacle of the past, we move to-
ward a future full of exciting and unpredictable possibilities.

CHAPTER SUMMARY

Man belongs to the class of mammals and the order of primates.
The members of this order have few marked characteristics that
distinguish them, the most important being hands fitted for grasp-
ing, a collar bone, flat finger and toe nails, and usually two breasts
situated on the upper part of the body.

The chief groups of primates are the lemurs, primitive, squirrel-
like animals that form a link between the primates and their an-
cestors; the New World monkeys, with flat noses and often a
prehensile tail ; the Old World monkeys, with narrower noses and
a non-prehensile tail, the anthropoid apes, which lack tails, and
approach man in intelligence; and man. There are four living

406 Human Evolution

types of the anthropoid apes: the gibbon, the orang-utan, the
chimpanzee, and the gorilla. The physical differences between the
anthropoid apes and man are chiefly in the proportions of the
various parts. The greatest difference is the greater intelligence of
man, although apes are nearer to man in this respect than they are
to the most primitive primates.

The differentiation of the human family from the other primi-
tive stocks is believed to have occurred at a time when the tropical
forests were retreating; thus an exceptionally intelligent primate
might be greatly favored in the sharp struggle for existence in
the comparatively cold and treeless regions that were appearing.
The first fossil link between man and the anthropoids is the Taungs
skull, which apparently belonged to a child possessing such a mix-
ture of human and anthropoid characteristics that its classification
is uncertain.

True men are marked off from all other animals by the posses-
sion of a cultural tradition which may be considered an evolution-
ary emergent, since it imparts a quality to human life that is not
found in other animals. Stone implements, the first indications of
human culture, are found in rock laid down a million years ago or
more. There are a few fossil human remains which are dated
shortly after the appearance of the first implements, of which the
most important are those of the Chinese Man of Peking.

The next important human remains are those of Neanderthal
Man, which are found throughout western and southern Europe.
While this race had developed a considerable culture, it was not
sufficiently advanced to be accounted one of our species.

The first men belonging to our species were the Cro-Magnons.
Since their time, there has been no significant biological change in
the human race, and human advancement has been entirely a matter
of cultural evolution.

If our scientific tradition does not disappear, it is probable that
human culture will exert a greater and greater effect upon the
course of evolution, bringing about changes, not only in other
species, but in the human race itself. It seems probable that we are
transitory as most other forms of life have been, and that in the
long course of evolution we will either disappear or develop into
some form of life quite unlike ourselves.

On the other hand, our race may remain stationary, from the

Human Evolution 407

evolutionary standpoint, for millions of years to come. The only
prediction we can make concerning the future of evolution is that
the process of living transformation will continue as long as life
continues.

QUESTIONS

1. Describe the order of primates.

2. What factors led to the evolution of man from the primate stock?

3. What emergent came into being with the evolution of man?

4. Describe several of the fossil species that link jnan with the apes.

5. Outline the course of evolution which differentiates us from Cro-
Magnon man.

6. Discuss the future of evolution.

GLOSSARY

anthropoid ape (an'thro-poid) A man-like ape. Applied to the four
groups of primates most similar to man.

Cro-Magnon (cro-man'yon) Earliest known race of men belonging to
our species.

culture The sum total of the traditional ways of behaving and thinking.
Also applied to a particular tradition, as "the Cro-Magnon culture/'
"American culture," "the culture of the western European peoples/'
etc.

Neanderthal (ne-an'der-tal) The most advanced race of man not be-
longing to our species.

PART III
BEHAVIOR AND MENTAL ACTIVITY

CHAPTER XVIII

THE RESPONSE SYSTEM: THE EFFECTORS

The most remarkable things that happen in this world are the
everyday occurrences which we accept as a matter of course. To
most people there is something of a fascinating mystery about the
manner of digestion of our food or the passage of blood through
the arteries and veins of the body, since these things are hidden
from view. But when all about us we see people busily engaged in
various activities and pastimes, walking, running, carrying on
conversations, writing, using tools, and performing the most re-
markably delicate operations, it never occurs to us to inquire
what goes on in the human body to make such truly amazing
phenomena possible. If we should observe a machine capable of
walking, talking, manipulating instruments, and directing its activ-
ities entirely without outside assistance we should immediately
want to inquire into how it was made to work that way. But we
are so accustomed to human behavior that we take it as a matter
of course and seldom feel the least curiosity concerning the
mechanism that makes it possible.

The next few chapters are devoted to describing, as well as pos-
sible in our present state of knowledge, what happens in the human
body to produce what we call human behavior. Already we have
seen that the living part of the human organism is essentially a
highly complex system of protein colloids, organized into cells
which themselves are organized to form organs and tissues that
are fitted into an intricate pattern which constitutes the organism
as a whole. It is this patterned protoplasmic system that must carry
on the activities that we call behavior; and it carries them on be-
cause of a fundamental property of protoplasmic colloids — their
ability to respond to stimulation.

Let us take a simple illustration of human behavior. A squad of
soldiers is lined up on a parade ground. The corporal in command

411

412 The Response System: The Effectors

of the squad shouts, "Forward, march I'9 The soldiers walk for-
ward and, keeping perfect step and alignment, continue down the
field until the order to halt is given.

Now, to give a simple explanation of what has happened, we
may say that the soldiers are organisms that are "wound up" to
perform certain activities. All that is needed to get one of those
activities into progress is something to "pull the string" that sets
them off. The thing that pulls the string and sets the soldiers
marching is a pattern of sound waves which passes through the air
from the lips of the corporal and finally sets into vibration certain
small sensitive cells in the ears of the soldiers. Just the small dis-
turbance produced by these sound waves is sufficient to set the
soldiers into vigorous activity. We call the sound waves a stimulus
and the marching a response.

A stimulus, therefore, may be defined as a small physical or
chemical disturbance which touches an organism in a sensitive
spot and causes it to begin some particular activity. A response, on
the other hand, is an activity which is set into progress by a
stimulus.

The things that human beings do are in all cases responses to
stimuli; and the way to an explanation of human behavior lies in
discovering what stimuli are acting upon a human being and in
what manner they produce the responses which are observed.

Ability to respond is one of the fundamental characteristics of
protoplasm, and we find responses taking place throughout the liv-
ing world. The antibody reactions which were described in Chapter
VIII are responses which the cells of the body make to stimuli
offered by the presence of bacteria or toxins. When a man chops
wood, the skin of his hands responds to the rubbing of the ax upon
it by growing thick and hard.

The most important responses of plants are growth responses.
The roots of a tree respond to the force of gravity by growing
downward, the branches by growing upward. The roots also re-
spond to moisture and rich soil by growing toward them. The
stems of the leaves respond to sunlight by growing in such a
fashion as to give the leaf a maximum amount of light.

The most important responses which animals make are re-
sponses of movement. This is because most animals have to move
around to get food. Movement responses may be observed in the

The Response System: The Effectors 413

simplest animals, such as the small, one-celled creature, Parame-
cium, which may be found in a drop of water from the scum
around the edge of a pond. If, while swimming about, it chances
to bump into an obstruction, it will back away, turn slightly to the
side, and swim forward again. Thus it responds to the obstruction
by swimming around it.

You may have noticed a certain peculiar thing about all the
organismic responses that we have described : they usually are of
advantage to the organism in its task of maintaining and protect-
ing itself. They help it to survive. We say that by means of re-
sponses an organism adjusts to its environment.

But at this point an interesting question arises; how does it
happen that the right response — that is, a response that adjusts an
organism to its environment — is usually the response that is made
to the stimuli which that environment affords? Why should the
soldiers go forward when the corporal calls "March" and stop
when he calls "Halt," rather than doing something entirely differ-
ent ? Why should a tree be stimulated by sunlight to grow toward
the light, rather than away from it or off to the side ? Why should
the Paramecium be stimulated by an obstruction to do just the
thing that is necessary to get it around the obstruction ? One an-
swer that might be given is that only those organisms that are
"wound up" to make adaptive responses have survived in the
struggle for existence and are alive to be studied by the scientist.
But this is something like explaining how an automobile runs 'by
saying that an automobile that didn't run wouldn't be used to carry
people about. Actually, we know that the running of an automo-
bile can be explained by describing its various parts and how they
are put together — in short, by describing the pattern of its struc-
ture. Everything that we know about organisms leads us to believe
that the specific direction their behavior takes in response to stimu-
lation is accounted for in a similar way — by the pattern of their
protoplasmic structure. The Paramecium, the tree, the skin on a
man's hands, the marching soldiers, all possess a protoplasmic pat-
tern which causes them to respond to definite stimuli in definite
ways. Our science has not yet advanced to the stage where it is
possible to point cut the exact nature of these patterns or to show
just why they result in the particular behavior that they do pro-
duce. Probably only very slight changes in the protoplasmic pattern

414 The Response System: The Effectors

of the outer tissue of their brains would lead the soldiers to go
forward at the cry "Halt" and to stop when the sound waves of
"March" vibrate in their ears. But that the nature of a response is
dependent upon some protoplasmic pattern in the brain is some-
thing that every scientific student of human nature believes. And
although we cannot trace these patterns in detail, we can show that,
in a general way, organisms are "hooked up" to produce just the
sort of behavior that we can observe in them.

Receptors, Conductors, and Effectors. — Just as we have a
digestive system to prepare our food for assimilation, a circulatory
system to carry materials from one part of the body to another,
and various other systems for the performance of particular or-
ganismic functions, so we possess a system which underlies our
behavior, one which enables us to adjust to the environment
through response to stimulation. It is called the response system
and is divided into three types of structures, classified according to
the functions they perform: the receptors, or sense organs; the
conduct orSy or nervous tissues, comprised of the brain, spinal
cord, and the nerve trunks which branch from them; and the
effectors, or muscles and glands.

The receptors are organs which contain cells that specialize in
sensitivity. Each receptor is sensitive to some particular form of
energy. Certain cells in the eyes are stimulated by the energy of
light waves ; other cells in the ears specialize in sensitivity to those
mechanical vibrations known as sound waves; and the sensitive
cells for taste and smell are stimulated by the presence of certain
chemical substances.

When any receptor cell is stimulated, its response is to stimulate
a conductor cell. Every receptor cell is in contact with one or more
of these nerve cells and is able to stimulate them. A nerve cell is
a long, thin structure, stretching through the body for a con-
siderable distance. The response that such a cell makes to the stim-
ulation coming from a receptor cell is to conduct a nervous impulse.
The impulse in the first nerve cell thereupon stimulates the other
cells to conduct impulses. These impulses are carried through the
nervous system to still other cells which pass them along. Finally,
the impulses are carried by the last of the long chain of nerve cells
to the cells of one or more muscles or glands, whereupon they
stimulate these muscle and gland cells to respond by contracting or

The Response System: The Effectors 415

secreting. It is the responses of the muscles and glands which
finally adjust the organism to its environment, and those struc-
tures, therefore, are called the effectors. But the total response of
the organism begins in the sense organ, continues through the
nervous system, and is merely completed in the muscles and glands.
Of course, this whole process of the passage of a nervous impulse
from a receptor to an effector and the contraction or secretion on
the part of the effector may occupy only a small fraction of a
second.

The usefulness of this arrangement of the response system is
obvious. First, it makes for specialization. One set of cells concen-
trates on responding to light, another on responding to sound. The
nerve cells have nothing to do but to conduct stimulation, while
the muscle cells can be made as efficient for the business of con-
traction as it is possible for a cell to be, since contraction is their
sole duty. Second, it enables muscles and glands in one part of the
body to adjust the organism to stimuli falling on an entirely
different part. And, finally, it binds the organism together, en-
abling it to respond as a unit to its environment, rather than as a
mere aggregate of cells, with each cell responding in its own inde-
pendent way. This last advantage is probably the most important
of all. Just how the organization of the response system produces
this advantage may not appear obvious at first, but it will be made
clearer as we go along.

The remainder of this chapter will be devoted to a description
of the effectors, and the arrangement of the conductors and re-
ceptors will be described in the next two chapters.

The Effectors. — In human beings and in most animals, there
are two kinds of effectors, the muscles and the glands. Already the
reader has become acquainted with a number of glands : the sweat
glands, the glands which secrete digestive juices, and a few of the
endocrine glands. Since most glands are set into action by the
stimulation they receive from the nerves, they form an integral
part of the whole response system. Frequently glands are activated
by hormones as well as by nerves. Indeed, nerves and hormones
may cooperate in activating not only our glands but our muscles
as well.

Of the muscle effectors, there are three kinds — the skeletal mus-
cles, the heart muscles, and the smooth muscles.

4i6

The Response System: The Effectors

Skeletal Muscles. — The skeletal muscles are those which are at-
tached to the bones of our skeleton and which serve by their con-
traction to move our bodies about. They are large, powerful and
quick-acting. They are made up of long, thin cells which, when
viewed under the microscope, appear to have tiny transverse stripes
running across them. For this reason the skeletal muscles are often
spoken of as striped muscles.

Fig. 89 shows the extreme tip of three skeletal muscle cells. On
the scale shown in the drawing, these cells would reach something
like twenty-five feet from tip to tip. In actuality, they are about
an inch long. Compared to other cells they are very large, and,
because they are so large, each cell contains many nuclei.

FIG. 89. — Terminal portions of skeletal muscle cells. (Redrawn from Martin's
The Human Body, Henry Holt & Company, Inc.)

Skeletal muscle cells are packed close together and are bound by
sheaths of connective tissue into small bundles. These bundles are
bound into larger bundles which are themselves bound together to
form the muscle as a whole. The sheaths of connective tissue in
which the muscle cells are enmeshed come together at the ends to
form the tendons which attach the muscles to the bones.

Fig. 90 shows a typical striped muscle, the biceps muscle of the
arm. It has a thick, soft central part, called the belly, which is com-
posed of muscle cells held within the meshwork of connective
tissue, and it tapers at either end to pass into the tough, cord-like
tendons which attach it to the bones. The biceps, as the figure
shows, is attached by a single tendon to a bone in the forearm1 and
by two tendons to the shoulder bones. When the muscle thickens

1 There is also a thin extension of this tendon, not shown in the drawing, which
holds the lower end of the muscle close to the elbow.

The Response System: The Effectors 417

and shortens in contraction, it pulls on the forearm bone, causing
it to move upward toward the shoulder.

Anyone at all acquainted with the physical laws of the lever can
see that the forearm bone acts as a lever with its fulcrum at the
elbow and the force applied at the point of insertion of the muscle.
The muscle, therefore, works at a considerable mechanical disad-
vantage and must contract very powerfully to lift a heavy weight
held in the hand. At the same time, it is thus enabled to move the
forearm rapidly and to carry the hand through a wide-sweeping*
arc while the muscle itself is shortening by only a few inches.

FIG. 90. — Biceps muscle and arm bones.

Nearly all our skeletal muscles are thus attached to the bones so
that a short contraction of the muscle can produce a wide and
rapid movement of limbs or trunk. Although the elbow and knee
joints are arranged to bend in only one direction, many of the
bones of the body, such as those of the upper arm and thigh, have
joint and muscle arrangements that enable them to be moved in
all directions. Thus provision is made for the rapid, vigorous, and
versatile system of movements which is so essential to animal or-
ganisms for securing prey, escaping enemies, and quickly adjust-
ing to all the emergencies in their rapidly shifting environments.
Heart Muscles and Smooth Muscles. — The skeletal muscles take
care of our adjustments to the external environment, and heart
and smooth muscles take care of those movements which must take

418 The Response System: The Effectors

place inside our bodies. Muscle cells with transverse stripes are
capable of contracting rapidly; those without them are slow in
their action. Since the heart must beat rapidly, while other internal
movements may take a more leisurely pace, heart muscle is the
only internal muscle that displays these stripes. (See Fig. 92.)
Heart muscle cells are smaller than those of the skeletal muscles.
The heart muscles differ from those of the skeleton largely by
virtue of the fact that they will continue to contract rhythmically
without any nervous stimulation. Every contraction of the skeletal
muscles is brought about by stimulation from the nerves which
run to them, and they can be kept in contraction only by continual
volleys of such stimulation, without which they immediately relax.

FIG. 91. — Smooth muscle cells.

But the heart may be taken from the body and completely severed
from all nervous connections and yet continue to beat for several
hours. Indeed, the heart of a cold-blooded animal, such as a turtle,
if put into a proper sort of solution, may continue to beat for sev-
eral weeks. Nervous impulses may, however, speed up or facilitate
the heart beat or else slow it down or inhibit it ; and to provide for
both effects the heart is controlled by two sets of nerves, one of
which facilitates and one of which inhibits its action.

The smooth muscles are so called because they do not show the
stripes that are characteristic of the skeletal and heart muscles. In
Fig. 91 it can be seen that their cells are simple in structure; and,
while they vary greatly in size, the largest types are scarcely one-
hundredth the length of an average skeletal muscle cell. They are
bound closely together, usually in flat sheaths, to form layers of
tissue in the walls of the alimentary canal, the arterioles, the blad-
der, and various other internal organs. Contractions of the smooth
muscles run in waves down the walls of the stomach and along
the long intestinal tubes, churning the food, mixing it with diges-
tive enzymes, and pushing it slowly through the digestive tract. In

The Response System: The Effectors 419

this fashion, and in many other ways, they help to carry on the
vital functions of the body.

The characteristic slow action of the smooth muscles can seldom
be observed, since they are located inside the body. But those in the
iris of the eye which contract and expand the pupil are located
within a transparent tissue. If you stand before a mirror in the
dark and then suddenly turn on the light, you can watch the rela-
tively slow, steady movement of the iris as it contracts to make the
pupil smaller.

Glands. — Glands are usually formed from epithelial tissue.
Fig. 5 D in Chapter I shows the simplest form of gland structure,
namely, a few secreting cells interspersed among
the other cells of an epithelial lining. Glandular
cells of this sort merely serve the purpose of
keeping the lining moist. When a copious secre-
tion is required, the lining folds inward to pro-
duce a considerable area of secreting cells, just
as the many folds of the lung surface produce
a large area for exchange of gases. The simplest FIG. 92.— Heart
sort of glandular folding is shown in Fig. 93 A. muscle. (Redrawn
-T-U • t ± u j j u -11 • from Martin's The

I here is a single tube, surrounded by capillaries Human Body

in which the cells lining the tube manufacture Henry Holt &
the secretion and pour it into the tube, whence Company, Inc.)
it makes its way to the surface. Sweat glands
are of this type, except that they are long and coiled. Another
simple type of gland, a small sac, is shown in Fig. 93 B. The more
important glands of the body are usually of a more complex type
than this. They are composed either of branching tubules or of
branching ducts which lead to chambers surrounded by sacs (Fig.
93 C and D). The latter construction reminds us very much of
the lungs, and emphasizes the fact that both types of structure have
the same function, the securing of a considerable surface for
physiological activity. In both cases the structure has probably
developed from a smooth epithelial surface in the course of
evolution.

A thick capillary network usually surrounds the cells which line
the tubules or sacs, and these cells have the power to absorb mate-
rials from the blood and manufacture them into secretory sub-
stances. Many of them are connected with the nerve cells; and

420 The Response System: The Effectors

when the nervous impulses stimulate them, they pour these sub-
stances into the sacs or tubules, whence they are carried to the
point at which the main duct of the gland empties itself.

In the endocrine, or ductless, glands, the secretory cells are usu-
ally embedded in a thick capillary meshwork. They absorb sub-

m

::;.^

."..-.»,.

FIG. 93. — Four types of glands. A, simple tubular; B, simple sac (racemose) ;
C, compound tubular; D, compound sac.

stances from the blood, manufacture their secretions, and, when
properly stimulated, simply pour them into the tissue fluid, whence
they dialyze into the blood and are carried to all parts of the body.
Sometimes the secretory cells of the endocrines are arranged
around small sacs into which they empty their secretions, but these
sacs have no ducts which lead the secretion away, and hence the
only place it can go is into the blood stream.

The Response System: The Effectors 421

CHAPTER SUMMARY

The things human beings do are in all cases responses to stimuli.
A stimulus is a small physical or chemical disturbance which
touches an organism in a sensitive spot and causes it to begin some
particular activity. A response is an activity which is set into
progress by a stimulus.

Ability to respond is a fundamental characteristic of protoplasm,
and responses take place in all organisms. Growth responses are
especially characteristic of plants, and movement responses of ani-
mals, although both types of response occur in all forms of life.
Through response to stimulation, organisms adjust to their en-
vironments.

In human beings there is a highly developed response system
composed of three parts :

1. The receptors, or sense organs, which specialize in sensi-
tivity to stimulation.

2. The conductors, or nerves, which specialize in conducting
stimulation from sense organs to effectors.

3. The effectors, or muscles and glands, which carry out the
responses of the organism.

This arrangement of the response system has the following
advantages :

1. It makes for specialization of function.

2. It enables responses to take place in a different part of the
organism from the part that is stimulated.

3. It makes for organization of response, so that the organism
acts as a unit.

There are four kinds of effectors :

1. Skeletal muscle, composed of long, thin, striped cells,
quick acting. This type of muscle is attached to the bones of the
skeleton and is used for movements of the trunk, limbs, head and
face.

2. Heart muscle, composed of striped, quick-acting cells which
will contract automatically without nervous stimulation.

3. Smooth muscle, found in internal organs and blood vessels,
composed of small, simple cells, slow acting.

4. Glands, which are modified epithelial tissues.

422 The Response System: The Effectors

QUESTIONS

1. What is meant by stimulus and response? Discuss response as a
general attribute of all organisms. What responses are especially
characteristic of plants ? Of animals ?

2. Describe the human response system. What are the functions of
each division? What are the advantages of such a system?

3. Classify and describe the effectors.

GLOSSARY

conductor A structure (usually nervous) which carries stimulation
from receptors to effectors in an organism.

effector A structure (usually a muscle or gland) which actually per-
forms the activities involved in an organic response.

receptor A structure (usually a sense organ) which is specialized
for sensitivity to some particular form of stimulation.

response An activity in an organism set into progress by a stimulus.

skeletal (skel'e-tal) Pertaining to the skeleton. Applied to muscles
that attach to the skeleton.

stimulus A small physical or chemical disturbance which touches an
organism in a sensitive spot and causes it to begin some particular
activity.

CHAPTER XIX

THE NERVOUS SYSTEM

Conduction and Integration. — The nervous system has two
major functions. The first of these is to carry nervous impulses
from the receptors to the effectors. The exact nature of the nerv-
ous impulse is not known, but it may be described as an electro-
physical disturbance which travels from one end to the other of the
very long, thin cells which make up the nervous system. The rate
at which the nervous impulse travels varies, but a representative
figure is 120 yards per second. The conduction of a nervous im-
pulse is the way a nerve cell has of responding to stimulation
received from either a sense organ or another nerve cell, and it
is by the relaying of nervous impulses from one nerve cell to
another that stimulation is carried from the sense organs to the
muscles and glands.

But the nervous system does more than simply to conduct these
impulses. It arranges them so that a definite pattern of stimulation
is sent out to the effectors, and, consequently, our response to the
environment is not a haphazard, unorganized affair, but one in
which each muscle and gland responds in relation to a unified plan
of action. Even when a man carries on as simple an act as walk-
ing, every muscle in the body falls in line with the general course
of his activity. Not only does each leg muscle contract and relax at
just the right moment, but the muscles of the trunk tip him from
side to side in such a way as to maintain balance, and his arms
swing backward and forward in the rhythm of his stride. The re-
sponses involved in walking are made by the entire organism
acting as a unit, not simply by certain parts responding without
any relation to the whole. This organization of our responses into
a unified plan of action is called integration, and it constitutes the
second function of the nervous system.

423

424

The Nervous System

Cerebrum
Cerebellum

— Spinal cordl

Nerve

The General Contours of the Nervous System. — In order
to understand how these all-important functions of conduction
and integration are carried on, it is necessary to know something
of how the nervous system is put together. It is generally con-
sidered to be composed of two
divisions : the centred nervous
system, which includes the brain
and the spinal cord; and the
peripheral nervous system, com-
posed chiefly of nerve trunks,
which branch from the brain
and spinal cord to all parts of
the body. ( See Fig. 94. )

Fig. 95 A shows a view of
the brain from the side, and
Fig. 95 B is a similar view of
the surface formed when the
brain is cut longitudinally in
half. It will be seen that there
are three chief regions : the
brain stem, which is located at
the base; the cerebrum, which
fills the greater part of the skull
cavity; and the cerebellum,
which is located at the back,
above the brain stem, but cov-
ered over almost completely by
the cerebrum. The functions of
these three regions will be
pointed out as we go along.

The spinal cord is simply a
continuation of the brain stem
which passes through an open-
ing at the base of the brain case and continues down inside the
"back bone" or vertebral column to a point about two-thirds
of the way down the back. The vertebral column is made up of
a series of ring-like bones, called vertebrae, placed one on top
of the other; and each one of the rings encircles the spinal cord;
thus it makes its way down the back completely encased within
them. (See Fig. 96.)

FIG. 94. — General diagram of the nerv-
ous system.

The Nervous System 425

As the long, thin nerve trunks pass outward from the spinal
cord and also from the brain stem, they branch so profusely that
they finally make contact with nearly every receptor or effector
cell in the body. They leave the brain stem and spinal cord in pairs,
one member of the pair going to the left, the other to the right.

•Cerebrum

Brain stem

Cerebrum

Brain item'

Cerebellum

-Spinal cord

FIG. 95. — The brain. A, side view; B, longitudinal section view. (A redrawn
from Martin's The Human Body, Henry Holt & Company, Inc.)

one pairs A^^ spinal ^ojxf, making a totaTof

eighty-six nerve trunks in^all. „ """

The function of the nerve trunks is to conduct stimulation
from the sense organs to the central nervous s^sjem jnd^Trbm
thjL.COTtraLsxstem jto the effectors. The chief function of the cen-
tral nervous system is integration. It organizes the stimulation
coming in over the nerve trunks from the sense organs into a

426

The Nervous System

definite pattern to be carried by the nerve trunks out to the muscles

and glands.

Neurons. — Nerve cells are generally called neurons. Since their

function is to carry nervous impulses from place to place, they

are usually long, thin structures
which reach tremendous dis-
tances— considering that they
are single cells — from one part
of the nervous system to an-
other. The nerve cells which
carry impulses from the sense
organs over the nerve trunks to
the central nervous system are
called sensory neurons. Those
which transmit impulses from
one point to another within the
central nervous system are called
connector neurons, and those
which carry impulses over the
nerve trunks from the central
nervous system to the effectors
are called motor neurons.

Every neuron is made up of
three parts, a cell body, a den-
drite (or dendrites), and an
axon. Nervous impulses always
enter a neuron over the den-
drites, run along them to the
cell body, and then pass out over
the axon, leaving the cell at the
tips (or end brushes) of the
axon. The cell body plays only a
minor part in the conduction of

Cerebrum

Cerebellum

Spinal cord

-Vertebrae

FIG. 96.~Central nervous system.
(Redrawn from Woodruff's Founda-
tions of Biology, The Macmillan Com-
pany.)

nervous impulses. It is, how-
ever, the part of the neuron which contains the nucleus, and it
carries on the nutritional activities which keep the cell alive.

While the shapes and sizes of neurons in various regions of the
nervous system differ considerably, there are only two fundamen-
tal structural plans, one characterizing the sensory neurons, and

' ' "' '' " '; "

:rf," ''W-'--''~ri^'

.^'-.^^^y,1

- ,'-IM ^liA^S^, ',",,-- •"
r^^irf'f'T""'1 ''""

«;!'

Motor neurons from spinal cord of ox. The large, dark structures are the
cell bodies. The dendrite and axon processes branching from them show rather
dimly.

The Nervous System

427

the other the connector and motor neurons. ( See Fig. 97. ) In the
latter, the dendrites form a bush of thin protoplasmk strands,
branching out from the cell body on all sides ; the axon is an ex-
tremely long, thin strand which leaves the cell body and may

Sense

Muscle

Dendrites
Cell body

•White insulating
material

-Axon

End brush

B

FIG. 97. — Types of neurons. A, motor neuron ; B, sensory neuron ; C, connec-
tor neuron of a type found in the cerebral cortex, called an association neuron;
D, connector or motor type of neuron, showing the white insulating material
surrounding the axon.

extend out from it for a few inches or even several feet. Usually
an axon branches, and at the end of each branch there is an end
brushy where the strand divides in several directions at once.
When one considers that a neuron is a cell, microscopic in size,
the length of these axons relative to their thickness is nothing
short of astonishing.

Sensory neurons do not have a thick bush of dendrites sur-
rounding the cell body. Instead, they have a single long dendrite,
similar in structure to the motor and connector axons. The cell

428

The Nervous System

body is located a little to the side of the dendrite and axon, being
connected to them by a short strand of protoplasm at the point
where the dendrite ends and the axon begins. Hence, nervous
impulses coming in over the dendrite probably do not pass through
the cell body at all, but are simply carried along the axon, the
structure of which is not very different from that of the dendrite

or of the central and motor axons.
The Synapse. — An impulse
will first enter the nervous sys-
tem over the dendrite of a sensory
neuron, pass on into the axon of
that neuron, and then be relayed
along to the dendrites of a con-
nector neuron. It passes through
the dendrites of the connector
neuron and along its axon and on
to a second connector neuron, or
perhaps a motor neuron, until it
finally reaches an effector. At the
point where the impulse passes
from one neuron to another, the
strands which form the end brush
of the axon in the first neuron run
parallel with the dendrite strands
of the second neuron, coming into
close contact with them; and it is
across this surface of contact,
called the synapse, that the impulse

passes from axon to dendrite. (See Fig. 98.) What actually
happens is that the nervous impulse in the axon of the first neuron
stimulates the second neuron to conduct an impulse of its own, but
it is common parlance to say that the impulse crosses the synapse.
It is important to realize that most axons branch and make
synaptic contact with more than one neuron. Furthermore, an im-
pulse may either pass a synapse or fail to pass it, depending on
the conditions that hold at the time it reaches the synapse. Hence,
the rours? fhfl,r an impulse takes thrrmgh the nervous, system de-
nprm wfricb n.f tfoe sy ftajit jfi r-OfltflCtS Hla4fi .by. the. clXQH along

FIG. 98. — Diagram of a synapse.
Solid black: End brush of axon of
first neuron. Stippled: Dendrites
and cell body of second neuron.
The synapse is the surface of con-
tact between the end brush and
dendrites. (Redrawn from Her-
rick's An Introduction to Neurol-
ogy, W. B. Saunders Company.)

The Nervous System 429

Neural Connections in the Spinal Cord. — Fig. 99 shows a
very simple series of connections between neurons occurring in
the spinal cord. The cord is shown in cross section, with* a nerve
trunk entering on either side. As each nerve trunk approaches the
cord, at a point immediately outside the vertebrae, it thickens to
form a small mass of nervous tissue, called a sensory ganglion.
These sensory ganglia of the nerve trunks contain the cell bodies
of the sensory neurons. Just beyond the sensory ganglion, the
nerve trunk branches to form the dorsal root entering the cord
toward the back, and the ventral root entering it toward the front.

Dorsal nerve root

Sensory
neuron

Central neuron

White
matter

Ventral
nerve

ro°t Motor neuron
FIG. 99. — Diagram of a simple reflex arc in the spinal cord.

The axons of the sensory neurons enter the cord over the dorsal
branch, and those of the motor neurons leave it over the ventral
branch. Each dendrite of each sensory neuron runs all the way
from a sense organ to a sensory ganglion, where its cell body is
located. Its axon continues from the ganglion on into the spinal
cord. All motor neurons have their cell bodies in the spinal cord
or brain stenj, and — with certain exceptions to be mentioned later
— their axons pass from the cell bodies all the way out to the
muscles. The connector neurons are always located entirely within
the central nervous system.

As is shown in Fig. 99, the spinal cord is divided into two types
of tissue, white matter on the outside and a butterfly-shaped core

430 The Nervous System

of gray matter on the inside. The axon of the sensory neuron
entering the cord passes into the gray matter, where it makes syn-
aptic contact with the dendrites of the connector neuron. The axon
of the connector neuron passes into the gray matter in the ventral
side of the cord, and makes a synaptic contact with the motor
neuron.

When you unwittingly touch something painful — a hot stove,
for example — your arm jerks the hand away from it automatically,
even before you become conscious of the pain. Such a response
is called a spinal reflex, and it is brought about by a set of neural
connections similar to that in Fig. 99. The sensory neuron carries
the nervous impulse from a pain receptor in the hand into the
spinal cord, where the connector neuron passes it on to a motor
neuron, which conducts it out to a muscle in the arm, setting the
muscle into action. Such a neural hook-up is called a simple spinal
reflex arc. Actually, of course, a whole group of neurons would be
necessary to set an entire muscle into action. Furthermore, the
sensory axon would probably branch, making several synaptic
contacts, while the motor -neuron would also be in contact with
more axons than the one coming from the connector neuron. A
more realistic picture of the relationships between connector neu-
rons and the sensory and motor neurons in the cord is shown in
Fig. 1 02.

Incomplete as our picture of a simple reflex arc may be, it
nevertheless represents relationships between neurons which hold
for every response we make. Sensory neurons carry impulses in
over the nerve trunks from the sense organs to the central nervous
system. Connector neurons relay them through the central nerv-
ous system to the motor neurons which carry them out to the
effectors. Furthermore, synapses between neurons are in all cases
located UlJhe gray matter of the nervous system.

White and Gray Matter.— Wfien""tHcT tissues of the central
nervous system are studied, they are found to be partly whitish
in color and partly gray. We have already shown how, in the
spinal cord, the white matter is on the outside, while the gray
matter forms a butterfly-shaped core within. This relationship
holds throughout the entire length of the cord. In the brain stem,
the gray matter does not form a single core, as in the cord, but is
embedded throughout the white matter in masses of varying

The Nervous System 431

shapes and sizes. In the cerebrum and cerebellum, however, the
matter is on the inside, and the gray matter forms a rather

Gray matter

•White matter

FIG. 100. — Longitudinal section through cerebellum.

.thin covering all over the outside. These coverings of gray matter
are called the cerebral and cerebellar cortexes, respectively. Fig.

Fissure

Left lobe of
cerebrum

Right lobe of
cerebrum

Gray matter

White
matter

FIG. 101. — Diagram of transverse section of brain.

100 shows the cerebellum cut in half to show the relation between
the white matter and the gray matter. Fig. 101 shows the brain

432 The Nervous System

cut in half along a line that would run approximately from ear
to ear. The cerebrum is divided by a deep fissure into two lobes
and the gray matter almost completely covers each lobe. In addi-
tion, there are bunches of gray matter located in the inner part
of each lobe of the cerebrum.

The surface of the cerebrum is thrown into a number of folds
or convolutions which greatly increase the area of the cortex and
hence the amount of gray matter. In human brains these convolu-
tions are deeper and more numerous than in any other animal,
and we therefore have more gray matter in proportion to the size
of our brains than any other organism.

Thejwhite^

nectorjieurgns running in closely packed bundles from one gray-
matter region to another. Each axon has an insulating layer of
white fatty material wrapped around it which causes the entire
tissue to appear white. The term "white matter" applies to the
tissue as a whole, not to the insulating substance. Since both the
motor axons and the sensory dendrites are wrapped with this sub-
stance, the nerve trunks may also be said to be composed of white
matter.

The gray-matter regions are places where the cell bodies and
dendrites of neurons are bunched together. The tips of the axons
enter these regions to make synaptic contact with the dendrites,
and consequently all the synapses are located there. Since the cell
bodies and dendrites of the connector and motor neurons are not
wrapped with the white insulating material, these regions appear
gray.

From the standpoint of the two major functions of the nervous
system, the white-maJ^jK^on^ carry-

iiyy .impulses from one grajjmatter_j§g.ion.<.to anQth^^and the
gray-matter regions =_ jjgr form the^ Junction ^jof jntegratipii. Just
how this integration is effected will be made clearer as we progress.

Possible Courses of a Nervous Impulse. — An impulse com-
ing in over a sensory neuron may be carried to almost any part of
the central nervous system. The sensory axons branch when they
enter the spinal cord and make synaptic contact with several con-
nector neurons. The axons of the connector neurons also branch,
making it possible for the impulse to spread to more and more
neurons as it passes through the central nervous system. It may go

The Nervous System

433

directly from the points where it enters the spinal cord to almost
any other gray-matter region of the cord and there be passed on
to motor neurons running to almost any part of the body. Or it
may be carried up through the white matter of the cord to some
of the little bunches of gray matter in the brain stem, where it may
activate still other motor neurons, or else be relayed through the
white matter of the cerebrum to the cerebral cortex. Once it enters

Cferebral
tortex"

White
matter

of
cerebrum

Spinal cord

^ » •"•-^
FIG. 102. — Diagram of nervous connections between spinal cord and cerebrum.

the cortex, there is no limit to the directions in which the impulse
may spread. The cortex is a pgrf <** network, .of billions of syn-
jigses, and each cell over which the impulse travels may carry it to
dozens of other cells, with the result that it can be carried back
from the various regions of the cerebral cortex through the brain
stem and spinal cord to practically every motor neuron in the body.
Fig. 1 02 attempts to portray in simple diagrammatic fashion
the various possible pathways that an impulse can take. The
synapses made by most of the branches of the connector axons
are not indicated, since if all these branches were followed out,

434 The Nervous System

practically every one of the billions of neurons in the nervous
system would have to be shown.

An illustration will serve to suggest more clearly the multitude
of paths which a nervous impulse can take in passing through the
spinal cord and brain. Let us take the case of a man who is enjoy-
ing his vacation in his cabin in the woods and is engaged in the
unfamiliar task of frying his morning flapjacks on the hot camp
stove. In the act of turning one of the flapjacks, he awkwardly
allows his little finger to touch the top of the stove. Instantly,
nervous impulses flash from the receptors in his finger up his arm
and into certain connector neurons which relay them across the
spinal cord to the motor neurons which run to the muscles of the
man's arm. Certain of these muscles are stimulated to contract
vigorously, and the arm leaps upward, drawing the finger away
from the stove. This, the retraction reflex, is the first complete
response made to the stimulus, but it is far from being the only
one. The various branches of the sensory neurons coming from
the finger make contact with connector neurons, along which the
impulses speed up and down to motor neurons in every part of
the cord and brain stem. Passing out over these motor neurons,
these impulses produce a "start of pain and surprise," that is, a
sudden muscular rigidity all over the body. Meanwhile, other
branchings of the sensory or connector neurons direct impulses
up to the cerebral cortex, and the response which we call "feeling
the pain" is made. We do not know just what the nature of this
response is in terms of nervous and muscular activity, but it is
known that neurons in the cortex are necessary to carry it out.
Now the stimulation from the finger has started pouring through
the cortex, and it produces a great variety of responses. The man
shakes his hand up and down and puts it in his mouth. The muscles
of his lungs, throat, lips, and tongue combine to produce sounds
that any pious person would shudder to hear. The man writhes and
groans and curses, and finally goes to the medicine kit, picks out
a soothing salve, puts it on the burn, and wraps his finger with
gauze. Before he has finished, practically every muscle in his body
has responded to nervous impulses that had their start in his
burned finger, and in addition the secretions of the digestive
glands and of certain of the endocrine glands have been affected.
To be sure, stimulation from other receptors combined or o>-

The Nervous System 435

operated with the stimulation from the pain receptors in the finger
to produce many of these responses, but impulses from the finger
played a part in the stimulation of every muscle and gland that
was set into activity. Hence, we may lay it down as a general prin-
ciple which is true with only a few exceptions : An imjnilse starting
in any receptor can make its way through the central nervous sys-
tem to every effector in the body.

Inhibition, Reinforcement and Integration. — But if it can,
why doesn't it ? Our sense organs are being stimulated every min-
ute of the day and night. If the stimulation of a single sensory
neuron has within it the possibility of setting every effector in our
bodies into action, why are our muscles not in a continuous state
of rigid contraction ? Why doesn't every gland secrete as copiously
as possible without ever stopping to rest ?

The answer is found in the fact that Qm. .nervous impulse is
capdhle^.of canceling out the effect of another so that an impulse
does not cross every synapse it comes to. The canceling-out effect
is called inhibition^ and it is one of the fundamental processes
whereby integration is effected.

As nearly everyone knows, the biceps muscle, which lifts the
forearm up toward the shoulder, is opposed in its action by the
triceps muscle in the back of the arm, which straightens the arm
out. When one stands with his arm hanging normally at his side,
both the biceps and triceps muscles are slightly contracted. This
slight contraction, characteristic of all muscles in the resting posi-
tion, is called muscle tonus; and if we did not have it, we would
be as limp and formless as jellyfishes. Now when an individual
whose arm is hanging at his side wishes to lift the forearm, the
slight contraction of the biceps muscle must be greatly increased
and the muscle must contract vigorously. At the same time, some-
thing else must happen: the triceps must relax. If it did not, the
slight tonus contraction would pull against the biceps, and the
movement would be greatly impeded.

The manner in which this happens is as follows : While the
muscles are maintaining a normal tonus, small volleys of nervous
impulses are continually passing from certain connector neurons
in the spinal cord into motor neurons running to the two muscles.
When the arm is about to be contracted, a strong volley of nervous
impulses starts down from the cerebral cortex. When it reaches

436 The Nervous System

the spinal cord, it reinforces the impulses that are going to the
biceps muscle, making them stronger or more numerous. At the
same time just the opposite effect is exerted on the impulses run-
ning to the triceps muscle. They fail to cross the synapses into
the motor neurons ; that is, they are inhibited.

The principles involved in the performance of this simple act
hold good throughout the nervous system and for every act of our
lives. Mi^tuall^_compatible responses reinforce one another and
mutually LJncompatible responses inhibit one another, with the
result that a unitary pattern of response is formed for the organ-
isili^&A..'>YhpAe.%The various part responses of this pattern rein-
force one another, while all responses incompatible with the general
pattern are inhibited. Consequently, the organism responds as an
organized unit to the multitude of stimuli falling in haphazard
fashion upon its sense organs; and thus we see that inhibition
and xdMQJcej™ responsible for what we

Inhibition and reinforcement can take place only at the synapses.
Once a nervous impulse starts along a neuron, nothing can either
stop it or help it along; but whether or not an impulse crosses a
synapse depends upon whether or not it is inhibited or reinforced
by the action of other impulses. For this reason, integration can
gQ on only in the gray matter where the synapses are located.

Integration in the Spinal Cord and Brain Stem : Reflexes. —
The gray matter in the spinal cord and brain stem is responsible
for only the simpler forms of integration. If the spinal cord of a
dog is cut in the neck region, so that there is no connection be-
tween the brain and the dog's legs, the legs can still carry out a
considerable amount of integrated movement. If, for example,
pressure is exerted against the left paw, the paw is pushed down-
ward while the right paw is lifted upward. This is obviously a
pattern of response that is used in walking. It is integrated in that

lifting of one leg

and the pushing downward of the other are responses which go
together for the performance of a useful act, just as the relaxation
of the triceps goes together with the contraction of the biceps in
lifting the arm.

But only the more mechanical part of the walking response is
integrated in the spinal cord. The dog whose cord has been severed

The Nervous System 437

from the brain never walks toward anything, because the move-
ments of its legs cannot be influenced by stimulation coming from
its nose and eyes.

The legs of this dog can perform various other mechanical
movements, such as scratching the flank or withdrawing from a
prick, a burn, or an electric shock. Simi4£»Xe$p9Jas^
which can be integrated in the brain stem or spinal r^jjaire^ called
reflexes. The winking of the eyelid when something suddenly
approaches the eye, the sneeze, the "knee jerk," the movements
of breathing are other examples of reflexes. They can be per-
formed without any activity on the part of the cerebrum.

Integration in the Cerebellar Cortex. — The cortex of the
cerebellum has a very special integrative function. In order for us
to make even the simplest movements, the activities of many
muscles must be delicately coordinated. Each must contract and
relax at just the proper instant of time. Furthermore, nearly every
movement we make throws us off balance, so that if we did not
automatically "catch ourselves" we would fall over. The cere-
bellum acts as. a, center for the, coordination oi muscular move-
ments and for producing the slight compensatory movements
which are continually necessary to keep us balanced and "on our
feet/* We might sum it up by saying that the cerebellar cortex has
nothing to _. say .. about what we shall^ jdo, but simplj^segs^to it that
the performance runs off smoothly.

Integration in the Cerebral Cortex. — The cerebral cortex in-
tegrates those, responses which adjust the organism as a whole
to .the environment, as^a^whole. .Working, playing, reading, talk-
ing— such are the activities which depend upon the cerebral cortex
for their organization.

The neurons which lead to the cortex bring to it nervous im-
pulses from every sense organ in the body. And in passing through
the cortex, these impulses mutually reinforce and inhibit one an-
other, until they become organized into a complex pattern which
is sent down through the brain stem and spinal cord and out to
all parts of the body, producing a complicated but well-organized
response which is appropriate to the situation at hand. Thus by the
action of his cerebral cortex a man faced with a difficult situation
is able to take everything into consideration and act intelligently.

438 The Nervous System

And that is why having a great deal of gray matter in the cerebral
cortex is a synonym for intelligence.

There are stories told of soldiers who were so thoroughly disci-
plined that an officer had only to give the word of command to
march a whole company over a high cliff. If such a thing ever
happened — and we seriously doubt it — it would mean that the
nervous impulses from the ears of the soldiers somehow managed
to inhibit the effects of the impulses from their eyes, so that they
responded to the "Forward, march !" in total disregard of their
future health and happiness. In most cases, we are sure it would
be the other way round. The sight of the cliff would inhibit the
usual effect of hearing the command to march, and the response
of the company would be really adequate to the entire situation,
rather than to a single auditory stimulus.

The entire science of psychology is given over to the study of
the responses which are integrated by the cerebral cortex, and
until we progress to the study of that science it will be possible
only to hint at the complex nature of its activities. It is sometimes
said that the cortex is the "seat of consciousness. " Whether this is
true or not is a matter for debate. At any rate, when a blow on
the head puts the cortex out of commission, we immediately lose
any consciousness that we may have; hence it seems fair to say
that we become conscious by making certain responses that are
integrated by the cortex.

It is also said that we think with the cortex, or, more popularly,
that we think with our brains. As a matter of fact, we probably
think with our entire receptor, conductor, effector apparatus, for,
as we shall point out later, thinking. Js_ just a special form of
response. But the cortex is the place where thinking responses are
integrated, and it is the structure which makes thinking possible.
When we think, we adjust not only to the situation at hand, but
to the past, the future, and to objects far out of the range of our
sense organs; and we may therefore say that the cortex enables
Usjoj^us^ot^pnly__to.the immediate environment as a whole,
but to the entire universe in which we live.

There is a story of an old Roman who was sent as an emissary
to foes who were attacking his city. In the course of negotia-
tions it became necessary to impress the enemy with the courage
and valor of the Romans. The old Roman stepped up to a torch

The Nervous System 439

that was burning nearby, thrust his hand into the flame, and held
it there until it burned off.

In describing this act in terms of the nervous system, we should
say that the activity of the cerebral cortex inhibited the retraction
reflex. The retraction reflex could respond only to the stimulation
of pain receptors in the hand. The cortex was adjusting to the
complex political situation with which the old hero was faced, to
the danger that threatened the city he loved, to the enemies who
might be defeated if they could be made afraid of the Romans,
and to the fact that he was the only Roman there to make them
afraid.

Thus, the cortex dominates human behavior, .because it inte-
grates those responses .Yrfiidbuena^ .beings to cope
with the intricacies of the world about us,

CHAPTER SUMMARY

The nervous system has two functions : ( I ) the conduction of
nervous impulses from receptors to effectors, and (2) the integra-
tion of those impulses into patterns which enable the organism to
respond according to a unified plan of action.

The system is composed of the following general divisions:

A. The central nervous system, which includes :

I. The spinal cord

2.' The brain, which includes :

a. The brain stem

b. The cerebellum

c. The cerebrum

B. The nerve trunks, which branch from the spinal cord and
brain stem and run to all parts of the body.

The central nervous system contains the regions where neurons
come into synaptic contact, and it is therefore the place where
nervous impulses are integrated. The nerve trunks contain the
dendrites of sensory neurons, carrying impulses from the sense
organs to the central nervous system, and the axons of motor
neurons, carrying impulses from the central nervous system to
the effectors.

Nerve cells are called neurons. They are composed of three
parts : ( i ) the dendrite or dendrites, through which nervous im-

44° The Nervous System

pulses enter the neuron; (2) the cell body, which is the center for
nutrition; and (3) the axon, over which the impulses travel to
the point where they leave the neuron. The end brushes of the
axons of the sensory and connector neurons make contact with
the dendrites of the connector or motor neurons. The surfaces
of contact are called synapses, and nervous impulses make their
way across them from one neuron to another.
There are three kinds of neurons, as follows:

1. Motor neurons, which have thick, bushy dendrites cluster-
ing around their cell bodies, which are located in the brain stem
or spinal cord, and long axons which run from the central nervous
system out over the nerve trunks to the effectors. The dendrites
of a motor neuron are in synaptic contact with the end brushes
of several connector neurons.

2. Connector neurons, whose general structure is similar to
that of the motor neurons. They are located wholly within the
central nervous system and carry impulses from one part of the
central system to other parts. The dendrites of a connector neuron
are in synaptic contact with several sensory or connector neurons,
while the axon makes contact with several connector or motor
neurons.

3. Sensory neurons, each of which has a single long, thin den-
drite that carries nervous impulses from a sense organ to a point
just outside the spinal cord where the cell body is located. From
this point the impulses continue into the spinal cord over the
axon, which branches to form a synaptic contact with several con-
nector neurons.

In the central nervous system are found two different kinds of
regions: (i) the white-matter regions of conduction, which are
composed of axons bundled tightly together, carrying impulses
from one gray-matter region to another, and (2) the gray-matter
regions of integration, composed of cell bodies with their sur-
rounding dendrites plus the synapses between the dendrites and
the end brushes of the axons. Gray matter is found in the follow-
ing regions :

1. In an H-shaped core running up through the spinal cord.

2. In little spherical bundles, called nuclei, located in the brain
stem.

3. In the cortexes of the cerebrum and the cerebellum.

The Nervous System 441

White matter is found in all other regions of the central nervous
system.

Because of the fact that nearly every neuron makes synaptic
contacts with several other neurons, it is possible for nervous
impulses to spread from almost any receptor to almost every ef-
fector in the body. The directions in which impulses actually do
pass are determined by the mutual inhibiting and reinforcing in-
fluences they exert upon each other.

Because of this mutual inhibition and reinforcement, responses
become organized into unitary patterns, with the various part re-
sponses reinforcing one another and inhibiting all responses not
compatible with the pattern. In other words, through the reinforc-
ing and inhibiting influences which they exert upon each other,
nervous impulses become integrated and produce integrated re-
sponses. Since inhibition and reinforcement take place at the
synapses, the gray-matter regions are the centers for integration.

The gray matter of the spinal cord and brain stem integrates
the simple response patterns known as reflexes.

The gray matter of the cerebellar cortex is the center for the
coordination of muscular movement and for producing the com-
pensatory movements which enable us to keep our balance.

The gray matter of the cerebral cortex integrates those activities
through which the organism as a whole adjusts to the environment
as a whole. These activities include our conscious responses and
frequently involve thought and the exercise of intelligence.

QUESTIONS

1. What are the two primary functions of the nervous system?

2. Describe the general contours of the nervous system. What are
the functions of the nerve trunks and of the central nervous system ?

3. Describe the structure, location and functioning of the three dif-
ferent kinds of neurons.

4. What is a synapse ? What part do synapses play in integration ?

5. What are white matter and gray matter? In what parts of the
central nervous system is each located?

6. Discuss and illustrate the possible course of a nervous impulse
through the central nervous system.

7. What is meant by (a) reinforcement, (b) inhibition, (c) in-
tegration ?

442 The Nervous System

& What sort of responses are integrated in (a) the spinal cord,
(b) the brain stem, (c) the cerebellum, (d) the cerebrum?

GLOSSARY

ax on (ak'son) Long, thin part of a neuron which carries nervous
impulses from the cell body to points of synaptic contact with other
neurons.

brain stem Lower part of the brain, lying under the cerebrum and
cerebellum and continuing directly into the spinal cord.

cell body Roughly spherical part of a neuron, lying between the
dendrites and the axon. It carries on the nutritive functions of the
cell.

cerebellar cortex (ser-e-bel'ar) The layer of gray matter which ex-
tends over the entire surface of the cerebellum.

cerebellum (ser-e-bel'um) Portion of the brain lying at the back
below the cerebrum and above the brain stem.

cerebral cortex (ser'e-bral) The layer of gray matter which extends
over the entire surface of the cerebrum.

cerebrum (ser'e-brum) The largest part of the brain, filling the en-
tire upper portion of the skull.

connector neuron Neuron located entirely within the central nervous
system, carrying impulses from one part of the central nervous
system to another.

convolutions (con-vo-lu'shuns) Folds such as those on the surface of
the cerebrum and cerebellum.

dendrites (den'drits) The bushy processes extending out from the
cell body of a neuron over which impulses enter the neuron. (Sen-
sory neurons, however, have a single long, thin dendrite, resembling
the axons of other neurons, except that stimulation enters through
it as it does through the dendrites of other neurons.)

*nd brush A group of thickly branching processes at the end of an
axon which make synaptic contact with the dendrites of another
neuron.

ganglion (gan'gli-on) pi. ganglia Any small clump of gray matter.

gray matter Name applied to those grayish-colored portions of the
nervous system which contain cell bodies, dendrites and synapses.
They are the portions where integration takes place.

inhibition (in-hi-bish'un) Process whereby the nervous impulses
underlying one response check or prevent the occurrence of another
response that is incompatible with it.

integration (in-te-gra'shun) Process whereby the responses of an
organism are organized into unitary patterns, so that the whole
organism responds as a unit to the many stimuli falling upon its

The Nervous System 443

sense organs. Inhibition and reinforcement are the processes

whereby integration is achieved.
motor neuron Neuron with cell body and dendrites located in spinal

cord or brain stem and an axon which carries impulses out over

the nerve trunks to the effectors.

muscle tonus (to'nus) A continuous contraction of the muscles.
nerve trunks Bundles of motor axons and sensory dendrites which

branch in pairs from the spinal cord and brain stem and run to all

parts of the body.
nervous impulse An electrophysical disturbance running from one end

of a neuron to the other, where it usually crosses one or more

synapses and continues through other neurons.
neuron (nu'ron) A nerve cell.

peripheral nervous system (pe-rif'er-al) The part of the nervous sys-
tem outside the brain and spinal cord. Composed chiefly of the

nerve trunks.
reflex (re'fleks) A simple response which can be integrated in the

spinal cord or brain stem.
reinforcement (xe-in-fors'ment) Process whereby nervous impulses

producing the same response or compatible responses combine to

strengthen one another.
sensory neuron Neuron which carries impulses from a sense organ

into the gray matter of the spinal cord and brain stem.
synapse (sin'aps) Surface of contact between the end brush of one

neuron and the dendrites of another across which nervous impulses

pass from the first neuron into the second.
white matter Name applied to white-colored portions of the nervous

system which are composed of bundles of axons and sensory den-
drites. They are the portions in which conduction takes place

CHAPTER XX

THE SENSE ORGANS

Specialized Irritability. — The receptors or sense organs of any
organism are cells or arrangements of cells that specialize in the
protoplasmic attribute of irritability. Primitive protoplasm, such
as that found in one-celled organisms, is sensitive to all sorts of
stimulation, but the specialized sense organs are usually sensitive
to only a very limited range of stimuli. Our eyes are sensitive to
light waves, our ears to sound waves, the sense organs in our skin
to mechanical pressures and to changes in temperature, the sense
organs of taste to certain chemicals that are dissolved in the saliva
of the mouth, and the sense organs of smell to chemicals dis-
solved in the mucous membrane of the nose.

A highly developed sense organ usually is composed of two sets
of tissues, the sensitive tissues, which are the actual receptors
since they are the ones which really respond to stimulation, and
the auxiliary tissues, which are not especially irritable but which
are arranged to bring the stimuli into proper contact with the
sensitive tissues.

These outstanding characteristics .of .sense organs — first, limi-
tation in the range of stimuli which act upon them and, second,
the possession of auxiliary as well as sensitive tissues — are well
illustrated in our most highly developed sense organs, the eyes.

The Structure of the Eye. — The eyeball is composed chiefly
of certain jelly-like substances held within a tough membranous
sheath known as the sclerotic coat. It is constructed on the same
principle as a camera. Inside the eye at the back is a membrane,
called the retina, which contains the cells that are sensitive to
light. It corresponds to the sensitive plate of the camera; and just
as in the camera chemical changes taking place in the sensitive
plate result in producing a record of the image that was thrown
on the plate, so in the eye chemical changes produced by light in

444

The Sense Organs

445

the sensitive cells of the retina result in the sending of nervous
impulses to the brain that enable the organism to react to the
situation that is represented by the image on the retina. Like a
camera, also, the interior of the eye is darkened by a coat of black

Lens

Muscle

Iris

Vitreous humor

Neurons

•WKTB fibers

Rod

FIG. 103. — Diagram of visual structures. A, eye; B, retina.

pigment, the choroid coat, placed between the retina and the scle-
rotic coat, so that no light rays can be reflected from its sides.

In the front part of the eye is a lens system which throws an
image of the world outside upon the retina. The first element in
the system is the cornea, a hard, transparent sheath that is a con-
tinuation of the outer coat. It is placed in front of the pupil and

446 The Sense Organs

iris and bulges forward in such a way as to bend the light rays
which pass through it. The iris (the colored part of the eye) is a
membrane that is open in the center to form the pupil, which is
the peep hole that lets light through to the lens. The lens is a
tough, transparent body, shaped like an ordinary magnifying
glass. It completes the work of bending the light rays so that they
will form an image on the retina. The large inner cavity of the
eye, between the lens and the retina, is filled with a soft jelly called
the vitreous humor. The space between the cornea and the iris con-
tains a liquid, the aqueous humor.

The eye shows further resemblance to a good camera "in that
it can be adjusted for the distance of the object that is being
brought to focus and also for the brightness of the light entering
it. Surrounding the lens is the ciliary muscle, the contraction of
which causes the lens to thicken and thus shortens its focus. In
addition, the iris, which surrounds the pupil, may contract when
the light is bright to produce a very small pupil, or open wide
when the light is dim to let in as much light as possible. These
adjustments are simple reflex responses. When a great deal of
light enters the eye, neural impulses are carried to the muscles of
the iris, stimulating them to contract. When the light becomes
dim, the muscles are stimulated to relax. Similar reflexes serve to
bring the lens into focus whenever the objects of regard change
from near to far or far to near.

It will be seen that all the structures of the eye except the sensi-
tive cells of the retina are merely auxiliary. Their purpose is to
make it possible to throw an image of the outside world upon the
retina. The light makes a pattern on the retina which clearly rep-
resents the objects that are in the environment. Organisms such as
worms that do not possess a lens system to throw definite patterns
of light on their visually sensitive cells cannot react to objects,
but merely to degrees of brightness or darkness. The only differ-
ence for such an organism between a printed and a blank page
would be that the blank page would appear a little brighter than
the printed one. The forms of the letters could never be discrimi-
nated.

The Retina. — The retina is a membrane about one six-thou-
sandth of an inch in thickness. It is made up of connective tissue
cells in which sensitive cells and nerve cells are thickly embedded,

The Sense Organs 447

The sensitive structures are actually modified dendrites of sensory
neurons. They are of two kinds, known as rods, and- cones-Jje-
cause of their characteristic shape as shown under the microscope,
and they are located in the back layer of the retina. (See Fig.
103 B.)

There are millions of these rods and cones in the retina. Around
the edges the rods are most numerous, and at the very center
there is a slight depression, called the jovea, which is composed
entirely of cones. (See Fig. 103 A.) Any object that we look at
directly is focused by the lens on the fovea. The rods are sensitive
only to light and darkness, while the cones are sensitive to color.
If we had only rods for sensitive cells, the world would be quite
colorless to us, with only whites, grays and blacks, as in a photo-
graph. The rods, however, have one advantage over the cones
in that they can increase their sensitivity when the intensity of
light decreases. As night falls, the cones become quite blind, be-
ing incapable of stimulation from the dim light; and the rods,
which have greatly increased their sensitiveness, take over the job
of seeing almost completely. That is why we do not see colors at
night. When one is looking for a rather dim star, it fades if
looked at directly, but as soon as the eyes are shifted a little to the
side it reappears. The reason is that the fovea,, jvvhich is the center
of vision, is nearly blind at night, since it contains no rods.

The Stimulus for Vision. — It is well known that light is due
to certain waves in the ether. But many people fail to realize thai
these waves are identical in kind with radio waves, heat radiations
utra-violet rays, and certain radium rays. All these ether waves
are called electromagnetic waves. They are all alike in that they
travel through the ether at the rate of 186,000 miles per second;
in fact, they differ from one another only in wave length and rate
of vibration.

The longest waves, naturally, have the lowest rates of vibration.
Certain radio waves which are about twelve miles long vibrate
about 15,000 times per second, while certain radium emanations,
about one-millionth of a centimeter in length, vibrate three hun-
dred million trillion times per second. Between these two extremes
are all gradations of length and vibration rate. Light rays are
simply those electromagnetic waves to which the rods and cones

448 The Sense Organs

happen to be sensitive. They range in vibration frequency from
380 trillion to 800 trillion vibrations per second.

The hue or color of any visual sensation is dependent upon
the frequencies of the light waves. Red has the lowest frequency,
violet the highest. A typical frequency for red is 460 trillion vibra-
tions per second, for yellow 520 trillion, for blue 630 trillion. A
frequency halfway between red and yellow gives orange, but mix-
ture of yellow and red waves also gives orange.

The brightness of light depends upon the energy of the vibra-
tions. The more energetic the vibrations, the greater is their am-
plitude. But brightness also depends upon the sensitiveness of the
retina to particular wave frequencies. It is most sensitive to yellow
light; and hence, in the daytime, the yellows look bright and the
reds and blues dark in proportion to their wave amplitude.

The Structure of the Ear. — The auxiliary tissues of the ear
are arranged to transmit sound waves from the air outside to
the sensory cells buried in the skull on either side of the head. By
following the diagram in Fig. 104 A, the course of these waves
may be made out. They enter through the passage that opens to
the outside, travel down it and set into vibration the membrane
known as the eardrum which is stretched across the end of the
tube. Beyond the eardrum is a chamber, known as the middle ear,
which opens into the throat by way of the Eustachian tube. The
vibrations on the eardrum are carried across the middle-ear cham-
ber by three minute bones or ossicles, the hammer, anvil, and
stirrup. The hammer is attached to the eardrum, and the stirrup
to another membrane that covers an opening to a second chamber,
the inner ear. The anvil is located between the hammer and
stirrup and joins them. The three bones together act like a rod
which is pushed back and forth with the vibrations of the ear-
drum and which in turn pushes the membrane over the inner
ear back and forth, thus transmitting the vibrations from one
membrane to the other.

The inner ear is a very small cavity in the bone, filled with a
watery fluid. It resembles a miniature limestone cavern, with a
number of winding passages leading off from the main chamber
which is called the vestibule. Three of these passages make half-
circle turns out of the chamber and back into it; they are called
the semicircular canals. A fourth passage, called the cochlea, leaves

The Sense Organs

449

Bones of the

middle ear '

(ossicles)

Tympanic
membrane
(eardrum)

Opening into throat

Tectorial membrane

Spiral lamina
Basilar membrane

B

Nerves

Sensitive hair cells

Basilar membrane

FIG. 104. — Diagram of auditory structures. A, diagrammatic section view
through right ear; B, longitudinal section through the entire cochlea; C, cross
section of the cochlear passage. (A and B redrawn from Martin's The Human
Body, Henry Holt & Company, Inc. C, after Gray.)

450 The Sense Organs

the lower part of the vestibule from a single opening and winds
about in a spiral course, taking about three complete turns to a
blind ending in its tip. If the bone were whittled down around
the cochlea until it was a mere fraction of an inch thick on all
sides of the passage, the structure formed would resemble noth-
ing so much as a tiny snail shell.

The semicircular canals and the vestibule contain receptor cells
that have to do with maintaining the balance of the body. JThe
sensitive cells for hearing are located in the cochlea. They stand
upright on a spiral membrane known as the basilar membrane,
which is stretched across the cochlear passage. The liquid which
fills the inner ear is set into vibration by the movement of the
stirrup against the membrane which separates the inner ear from
the middle ear. The vibrations of the liquid set the basilar mem-
brane into vibration. This vibration causes the sensory cells to
rub against the tectorial membrane, which hangs over them from
above, and in this way they are stimulated. The lower ends of
the sensory cells are in direct contact with nerve dendrites which
carry the stimulation from them to the brain. (See Fig. 104 C.)

The Stimulus for Sound. — Sound waves are mechanical vi-
brations which may pass through almost any sort of body, but
which are usually brought to our ears through the air. The pitch
up.Qn,tkaxate of vibration of sound waves,

just as color depends upon the rate of vibration of light waves
in the ether. The lower the tone, the slower the rate of vibration.
The lowest tones that can be heard by human beings vibrate about
20 times per second; the highest, about 20,000 times. Middle C
on the piano vibrates 256 times per second. Going up an octave
doubles the rate of vibration; coming down an octave divides it
in half.

The loudness of sound depends upon the amplitude of sound
waves, just as brightness of light depends upon the amplitude of
the ether vibrations ; but here again the relationship is not per-
fect, because of the fact that the ear is more sensitive to certain
vibration rates than to others. The region of greatest sensitivity
lies between 500 and 5,000 vibrations per second.

The Chemical Senses. — Smell and taste differ from the other
senses by virtue of the fact that the stimuli which arouse them are
chemicals in solution.

The Sense Organs 451

In the tongue are numerous small cavities, each of which con-
tains a few sensitive cells for taste. (See Fig. 105 A.) Each
sensitive cell is in contact with the dendrite of a senory neuron
which transmits stimulation from it to the brain. The sensitive
cells themselves are stimulated by various chemical substances in
solution in the saliva.

In spite of the fact that we attribute a different taste to nearly
every different substance we take into our mouths, there are really
only four hpsfir. tasfp s^nsafipns. They are sweet, sour, salt, and

Supporting Supporting:

cells cell

//
Sensory cells

A

FIG. 105. — Diagram of the chemical sense organs. A, taste ; Bt smell.

bitter. What we ordinarily call the taste of a substance is really
a combination of its true taste with touch and temperature sensa-
tions aroused by its contact against the tongue and the sides of
the mouth and, most important of all, the sensations of smell that
are aroused by the vapors which pass into the nose through the
place where the nasal passages enter the back of the mouth. Things
taste so flat when one has a cold because the cold cuts off the
smell sensations that are the most important part of the taste.
If our food is spicy or peppery, there is usually a component of
pain added to its taste.

The sensitive cells of smell are squeezed in between epithelial
cells in the lining of the upper part of the nasal passages. Chemical

452

The Sense Organs

substances which enter the nostrils as gases are dissolved in the
mucus which covers the lining of the passages and are thus made
capable of stimulating the sensory cells. (See Fig. 105 B.)

The sense of smell can be aroused by astoundingly small
amounts of an odoriferous substance. In the case of some sub-
stances, one part in thirty billion parts of air is sufficient for a
man to detect it. And as everyone knows, smell is vastly more

Fid 106. — Sense organs of tendons and muscles. A, muscle cells with nerve
endings; B, tendon with nerve endings. (Redrawn from Herrick's An Introduc-
tion to Neurology, W. B. Saunders Company.)

highly developed in many of the lower animals than in man.
Probably many animals get most of their information concern-
ing the outside world by means of the sense of smell, just as we
get our most valuable information through sight. But although
sjpdl may not serve us to any great extent as a bringer of in-
formation, it probably plays a more important role than is or-
dinarily suspected in the life of feeling and emotion. Nearly
everyone has experienced the manner in which an odor will bring
back a forgotten scene and with it a very vivid sense of the feel-
ings he had at the time it was enacted. And it is probable that

The Sense Organs 453

we do not realize to what extent our reactions to people or things
are favorable or unfavorable because of the presence of pleasant
or unpleasant odors.

The Somesthetic Organs. — Scattered throughout the body,
in the skin, the smooth muscles, the skeletal muscles, the tendons,
joints, and elsewhere, are numerous very simple sense organs.
Some are merely free endings of dendrites, without any special
sensitive or auxiliary structures connected with them. In others,

ABC

FIG. 107. — Sense organs of the skin. A, sensory dendrite wrapped around the
base of a hair ; B, Meissner corpuscle in a papillus of the finger ; C, end bulb of
Krause, from the conjunction of the eye. (Redrawn from Herrick's An Intro-
duction to Neurology, W. B. Saunders Company.)

the dendrite endings are enclosed in small capsules of tissue which
are auxiliary and possibly sensitive in function. Figs. 106 and
107 show a number of these receptors. The free nerve endings are
believed to be the sense organs for pain. The receptors at the roots
of the hairs are sensitive to pressure. The functions of some of
the others are indicated below, but in the case of many of these
organs little is known about the nature of their sensitivity.

At any rate, the functions of all these receptors are so com-
pletely interlocked that it seems best to treat them as a single
sense-organ group, calling them the organs of bodily sensation, or
somesthetic sense organs. In spite of their interlocking function,
we find that these organs enable us to respond and adjust to three
different types of facts: organic, having to do with the con-

454 The Sense Organs

dition of our bodies; kinesthetic, having to do with the position
and movement of the limbs and trunk and the weight of objects
being lifted; and tactual, having to do with the qualities of ob-
jects that are touched or handled or that in other ways come
into contact with the surface of the body.

If one touches a thin, cold piece of wire to his wrist at a num-
ber of points, he will discover that the sensation of cold is aroused
only at certain points in the skin. Now if the wire is heated, par-
ticular spots of sensitivity to warmth may also be located, and
these spots are not found in the places where the cold spots are
placed. By similar means, numerous spots that give a sensation
of pain and less numerous spots that produce a feeling of pres-
sure can be found.

These three sensations — pressure, pain, and temperature — seem
to be the only kinds provided us by the somesthetic sense organs,
although different types of pressure, pain, and temperature are
produced by different kinds of stimulation. Thus, we find that
a very warm wire will produce not only a feeling of warmth at
the warm spots, but a feeling of cold at the cold spots, and that,
to a lesser degree, the warm spots are stimulated by a very cold
wire. Still greater heat or cold will stimulate the pain spots. We
therefore have at least six kinds of temperature sensations :
warmth, produced by stimulation of the warm spots; heat, by
stimulation of the warm and cold spots ; burning heat, by stimula-
tion of the warm, cold, and pain spots; cool, by simple stimula-
tion of the cold spots; cold, by stimulation of the cold and warm
spots; and painful cold, by stimulation of the cold, warm, and
pain spots. The difference between extreme heat and extreme
cold is probably a difference in the balance between cold- and
warm-spot stimulation. Those who have participated in the old
trick of making some poor blindfolded victim believe he is being
branded with a hot iron while touching him with a piece of ice
will realize that the difference in feeling between hot and cold is
not very great, after all.

Similarly, there are different kinds of pain. Itching is the result
of slight stimulation of the pain receptors, pricking is produced
by somewhat stronger stimulation, while what is called "clear
pain" results from cutting the skin. The pain receptors in the
muscles produce aching pains, and those under the finger nails

The Sense Organs 455

yield a lively and most disagreeable sensation called "quick pain."
Pressure sensations also are of all types, from a light tickle on the
skin to the strain that is felt in our joints when we lift something
heavy.

Organic Sensitivity. — We are made aware of the internal con-
dition of our bodies chiefly by sense organs located in the smooth
muscles of the digestive, circulatory, and excretory tracts, al-
though feelings of bodily heat or cold are probably mediated by
the temperature receptors in the skin, and feelings of fatigue
result largely from stimulation of the sense organs in the striped
muscles. Four kinds of organic feelings which are probably pro-
duced by complex patterns of pain and pressure stimulation may
here be noted.

1. The feeling of hunger is apparently produced by stimulation
of receptors in the walls of the stomach. It will be remembered
that during the time we are digesting a meal, waves of peristaltic
contraction move down the stomach. These contractions apparently
arouse no sensations; but as soon as the stomach becomes fairly
empty, certain stronger, slower contractions set in, which arouse
the sensations that we call hunger. When we are hungry, the
pangs become very intense for a few minutes and then fade away,
only to return again ; and it has been shown that the most intense
pangs come just at the time that the stomach is contracting most
vigorously.

2. The feeling of thirst is mediated by receptors in the mucous
lining of the throat. Whenever the throat dries out, usually as a
result of lack of water in the body, the thirst receptors are stimu-
lated. If, for any reason, the flow of saliva is stopped, we feel
thirsty even though our bodies contain sufficient moisture.

3. It is known that sensations of nausea are accompanied by
waves of contraction that move up the alimentary canal, rather
than down it as peristaltic waves do. It seems probable, therefore,
though it is not known for certain, that these "antiperistaltic"
contractions provide the stimuli for the sensations.

4. Whenever the mucous membranes are stretched, whether in
certain regions of the alimentary canal or in the bladder, sensa-
tions of strain and fullness are produced which may become quite
painful.

In addition, we experience many bodily sensations in connection

456 The Sense Organs

with sexual activity, emotion, illness, and other conditions the
stimulation for which is imperfectly understood.

Kinesthetic Sensitivity. — Embedded in our skeletal muscles
and in the tendons which attach them to the bones, and occupying
the surfaces between our joints are numerous somesthetic organs
which secure information as to what is going on in our limbs
and muscles. They are the sense organs for the kinesthetic sense,
the sense of movement. It is a remarkable fact that, although this
sense is probably the most essential one we possess, most people
are not even aware of it, and it was not called to the attention
of the scientific world until the middle of the last century.

Anyone who stops to notice it can sense a feeling of strain in
his muscles when they are strongly contracted. Closer attention
will reveal the fact that even the slightest movement produces
some sense of strain in muscles and joints. Even though we habit-
ually disregard our kinesthetic sensations, every movement we
make is guided by kinesthetic nervous impulses. It would be im-
possible to perform any sort of skilled action without them, since
these impulses coming into the central nervous system exert in-
hibiting and reinforcing influences which serve to coordinate the
activity of the muscles concerned. As might be expected, most of
the impulses entering the cerebellum come from kinesthetic
receptors.

You may readily demonstrate how your own movements are
guided by kinesthetic sensations. Close your eyes and hold out
your left hand with the fingers spread apart. Then touch the tips
of each finger with the index finger on your right hand. No mat-
ter how much you move the left hand about, you will have little
difficulty in finding the tips of the fingers, although the kinesthetic
sense is the only one that can guide you to them.

When, in the disorder known as locomotor ataxia, the nerve
fibers of the spinal cord which carry kinesthetic impulses to the
brain are destroyed by the spirochete of syphilis, it is no longer
possible to control the movements of the legs properly, and the
gait becomes jerky and irregular. The patient has to watch his
legs when he walks to find out what they are doing !

Tactual Sensitivity. — Whenever anything touches our skin in
almost any part of the body we can be made aware of it by the
tactual organs located in the skin tissues. In addition to warning

The Sense Organs 457

us of danger and preventing our coming into contact with noxious
substances, the tactual sense gives us information about the
smoothness, temperature, and other qualities of the objects we
touch. In combination with the kinesthetic sense it enables us to
adjust to the shape, size, and weight of objects. This combined
action of skin and kinesthetic organs is so complete that it is
impossible to draw a line between the point at which skin sensa-
tion leaves off and kinesthetic sensation begins.

The Maintenance of Equilibrium. — In addition to having the
kinesthetic sense to guide our movements, we possess two sets of
organs, the otolith organs and the semicircular canals, located in
the inner ear, which help us to maintain our balance. The otolith
organs are found in two small sacs, known as the utricle and
saccule, located inside the vestibule. (See Fig. 104 A.) Each organ
is composed of a clump of "hair cells," that is, cells with tiny hair-
like strands extending from them. They are receptor cells and are
in direct communication with sensory nerve fibers. Scattered
among the hairs are small granules called otoliths \ and as the
head tips one way or another, the otoliths press from various
directions against the hairs, stimulating the cells so that they re-
spond to the relation between the force of gravity and the posi-
tion of the head.

The semicircular canals are located so that they open into the
utricle. There are three of them in each ear, and they lie in the
three planes of space, that is, in positions that would be parallel
to the back, side, and bottom of a cubical box.

At one of the openings of each canal into the utricle, there is a
clump of hair cells similar to those in the otolith organs. These
cells, however, are stimulated not by otoliths, but by the move-
ment of the liquid which fills the canals. If one twirls a bucket
full of water back and forth by means of quick turns of the wrist,
the water may remain quite stationary while the sides of the bucket
move rapidly over it. A similar thing occurs in the canals. When
the head is moved, the liquid in the canals remains relatively sta-
tionary and as the hair cells move over it they are stimulated.

The location of the canals in the three planes of space is now
readily understood. Let us, for the purpose of making the point
clear, imagine that a man's head is square, or, rather, cubical in
shape. If he wags his head from side to side, the hair cells in the

458 The Sense Organs

canals parallel to the back and front surfaces of the head will
be stimulated. If he nods his head forward and backward, the
canals parallel to the right and left sides will go into action. And
if he turns his head, the canals parallel to the top and bottom
surfaces will be involved. The receptors in the semicircular canals,
therefore, are arranged to respond to motion of the

We are never conscious of sensations from the otolith organs
and the semicircular canals; but with their ability to respond to
motion and to changes in position with respect to gravity, those
organs act as receptors for a complex group of reflexes, of equilih.-
rium, or righting reflexes. The nervous impulses for these re-
flexes pass through the cerebellum and brain stem, and it is largely
as a center for righting reflexes that the cerebellum acts to co-
ordinate the movements of an organism. When the individual is
standing still, he always sways slightly, but the instant he begins
to tip too far in one direction there is a reflex contraction of the
muscles that will pull him back into position. As one writer has
put it: "The act of standing is a continual process of falling and
righting oneself. " The minute one begins to walk or move about,
the reflexes of equilibrium are called upon to an even greater ex-
tent. Walking is essentially a tipping from side to side from one
leg to another while the legs are swung back and forth. Practically
every movement we make— of arms, trunk or legs — throws us
off balance and makes necessary some righting reflex to keep us
from tipping over completely.

Although the otolith organs and the semicircular canals are
specialized receptors for the righting reflexes, they are, fortunately,
not the only receptors that can be used. Both our eyes and the
kinesthetic receptors can and do cooperate in stimulating the right-
ing reflexes. In certain deaf persons all the receptors of the inner
ear have been destroyed, yet such individuals successfully main-
tain their equilibrium on the basis of reflexes from other recep-
tors. It is said, however, that when they dive into water they are
as likely to swim downward as not, since when they are under
water neither their eyes nor their kinesthetic receptors can tell
them " which way is up."

When the semicircular canals are ovcr§timulatedy they~.prpduce
djzziness. The tendency to fall down is due to exaggeration or

The Sense Organs 459

improper direction of the righting reflexes. The impression that
the world is spinning about one is caused by reflexes from the
canals that make the eyes jerk back and forth, and the nausea
which accompanies dizziness is due to other reflexes that affect
the muscles of the alimentary tract.

CHAPTER SUMMARY

The most highly developed receptors or sense organs are com-
posed of auxiliary tissues which serve to bring stimuli in contact
with sensitive tissues. In each of our sense organs the receptor
cells specialize in being sensitive to certain types of stimuli.

The auxiliary tissues of the eye serve as a camera to focus a
pattern of light coming from the world outside on to the rods and
cones (the sensitive cells) of the retina. The rods are sensitive to
white, gray and black and are capable of adapting to dim light.
The cones are sensitive to colors as well as brightnesses, but they
cannot adapt to dim light.

The stimuli for vision are light waves, that is, electromagnetic
vibrations in the ether. The hue of a visual sensation depends
upon the frequency of the light waves, red having the lowest
frequency and violet the highest. Brightness is dependent on the
energy or amplitude of the waves and also upon the degree of
sensitivity of the rods and cones for certain hues.

The auxiliary tissues of the ears exist for the purpose of con-
ducting sound waves to where they can cause the sensitive hair
cells of the basilar membrane in the cochlea to vibrate.

The stimuli for hearing are sound waves, that is, mechanical
vibrations that usually come to us through the air. The pitch of a
sound depends upon the frequency of the sound waves ; the loud-
ness, upon the energy or amplitude of the waves and upon the
special sensitivity of the ear to certain wave frequencies.

The receptors for taste are found in little pits in the tongue.
There are four basic taste sensations : sweet, sour, salt and bitter.
What we call the taste of foods depends also on smell, tempera-
tures, pressure and pain.

The receptors for smell are in the mucous lining of the nose.
They are sensitive to extremely minute amounts of chemical sub-
stances which become dissolved in the mucus.

In the skin, muscles, and mucous linings of the body are sim-

460 The Sense Organs

pie sense organs of different types, known as somesthetic recep-
tors. Their stimulation results in various types of pressure, pain,
and temperature sensations. Their functions overlap considerably,
but they furnish us three types of sensitivity: (i) Organic sensi-
tivity to physiological changes in the body, which produces feel-
ings of thirst, hunger, nausea, feelings of strain and fullness,
and many other bodily sensations; (2) kinesthetic sensitivity to
muscular movements and to the movement and position of the
limbs; (3) tactual sensitivity to objects touching the skin.

In the otolith organs and semicircular canals of the ear there
are receptors that never produce sensations. Their stimulation re-
sults in certain righting reflexes which help the body to maintain
its equilibrium. The kinesthetic receptors and the eyes cooperate
with them in producing righting reflexes.

QUESTIONS

1. What are the functions of the sensitive and auxiliary sense organ
tissues ?

2. Describe the eye, comparing it to a camera.

3. What are the rods and cones? What is the function of each?

4. Describe the relations between the auxiliary and the sensitive struc-
tures for hearing.

5. Compare the stimuli for vision and hearing (a) as to their nature
and (b) as to the effects they produce.

6. Discuss the chemical senses.

7. Discuss somesthetic sensitivity from the following points of view :
(a) the receptors, (b) the kinds of sensation, (c) the kinds of facts
to which it adjusts us.

8. Describe all the factors of sensitivity which enable us to keep our
balance and integrate our movements.

GLOSSARY

aqueous humor (alcwe-us) The liquid substance between the cornea

and lens of the eye.
auxiliary tissues Parts of a sense organ which serve to bring stimuli

into proper contact with the sensitive tissues.
bastlar membrane (bas'i-lar) Spiral membrane in the cochlea on which

the sensitive cells for hearing are placed.
choroid coat (kor'oid) A black pigmented membrane located between

the sclerotic coat and the retina of the eyeball.
ciliary muscle (sil'i-a-ri) A smooth muscle surrounding the lens of

The Sense Organs 461

the eye which contracts to thicken the lens and relaxes to allow the

lens to flatten.

cochlea (kok'le-a) A spiral passage which winds out from the ves-
tibule in the inner ear and across which the basilar membrane is

stretched.
tones Modified sensory dendrites in the retina which act as sensitive

cells for vision. They are sensitive to hues as well as brightnesses.
cornea (kor'ne-a) The transparent part of the coat of the eyeball,

covering the iris and pupil.
electromagnetic waves Vibrations in the ether. Light waves are the

electromagnetic waves that range from 380 trillion to 800 trillion

vibrations per second. Other electromagnetic waves are radio waves,

heat rays, ultra-violet rays, and X-rays.
Eustachian tube (u-staTd-an) Tube running from the middle ear to

the throat.
jovea (fo've-a) Slight depression in the retina which is the center of

focus in the eye. It contains only cones.
iris (I'ris) A contractile membrane, perforated by the pupil. It is the

colored part of the eye.
kinesthetic sensitivity (kin'es-thet'ik) Sensitivity to movements of the

muscles and changes in position of the muscles and trunk.
organic sensitivity Sensitivity to internal physiological states and

changes, such as sensitivity to hunger, thirst, and nausea.
ossicles (os'i-k'ls) Three small bones in the middle ear (the hammer,

anvil and stirrup) which transmit sound vibrations from the ear-
drum to the membrane covering the inner ear.
otolith organs (6'to-lith) Small clumps of hair cells, located in the

utricle and saccule, which have small hard granules, called otoliths,

scattered among them. They are receptors for the righting reflexes.
retina (ret'i-na) Sensitive membrane in the eye upon which an image

of the outside world is focused. It contains the rods and cones.
rods Modified sensory dendrites in the retina which act as sensitive

cells for vision. They are not sensitive to colors.
saccule (sak'ul) Small sac in the vestibule of the inner ear which

contains an otolith organ.

sclerotic coat (skle-rot'ik) The tough external membrane of the eye-
ball.
semicircular canals Semicircular passages running out of and back to

the vestibule of the inner ear. They act as receptor organs for the

righting reflexes.
sensitive tissues The tissues in the sense organs which possess special

sensitivity to definite types of stimuli.

462 The Sense Organs

somesthetic sensitivity (so'mes-thet'ik) Sensitivity to stimulation of
the skin and internal bodily tissues.

tactual sensitivity (tak'tu-al) Sensitivity to stimulation of the skin.
The sense of touch.

tectorial membrane (tek-to'ri-al) The membrane lying over the sensi-
tive cells on the basilar membrane. Vibrations of the basilar mem-
brane cause the cells to be stimulated by rubbing against the tec-
torial membrane.

utricle (u'tri-k'l) Small sac in the vestibule of the inner ear at the
base of the semicircular canals. It contains an otolith organ.

vestibule (ves'ti-bul) Cavity of the inner ear out of which the cochlea
and semicircular canals run.

vitreous humor (vit're-us) A jelly-like substance which fills the eye-
ball and through which light waves pass from the lens to the retina.

CHAPTER XXI
INTERNAL ADJUSTMENTS

The Vital Reflexes. — It is usually said that the function of the
response system is to adjust the organism to its environment. This
it does by causing the organism to move about in a manner that,
as a rule, protects it from harm and secures for it the necessities
of life. But there are other movements that go on within the body
that may or may not have to do with its relationship to its en-
vironment, but that are quite as essential to its existence as its
external behavior. These internal movements are also functions of
the response system, although, as we shall see, some of them are
to a certain degree independent of nervous control. They are
usually integrated in certain specific gray-matter regions in the
brain stem, known as the vital centers, and they are spoken of as
the vital reflexes. For the most part, they are not under voluntary
control, or are only incompletely so; which means that impulses
from the cerebral cortex — the center for the integration of our
so-called voluntary responses — are not always able to inhibit or
reinforce them.

The Nervous Control of Breathing. — There is no known in-
stance of a man's committing suicide by the simple expedient of
holding his breath. Nor does one have to keep his attention on
breathing in order to insure its progress. This is because there is
a center in the brain stem which insures the sending out of
impulses to the muscles of the chest and diaphragm, whether the
cerebral cortex enters into the picture or not. To be sure, cortical
action upon this center can cause the movements of breathing to
be hastened or retarded. But if one attempts to hold his breath
for more than a minute or two, sooner or later the cortical in-
hibition will be overcome, and breathing will recommence. What
happens is that, as carbon dioxide piles up in the blood, the breath-
ing center becomes more and more acid — since carbon dioxide

463

464 Internal Adjustments

combines with water to make carbonic acid — and this acidity acts
as a strong stimulus to the neurons that innervate the breathing
muscles. The acid stimulus finally becomes too strong for the
cortical inhibition.

The vigor of our breathing depends upon the amount of acid
in the breathing center. When we exercise severely and the blood's
acidity increases steeply, breathing becomes deeper and more
rapid, until that acidity is reduced. On the other hand, if one
forces oneself to breathe deeply and rapidly for a minute or two,
carbon dioxide is washed out of the blood so completely that, for
a short time, breathing may come almost to a standstill.

The regular rhythm of breathing is dependent not upon the
direct stimulation of the center by acid, but upon reflexes from
the receptors located in the lung tissue. There are two sets of
these receptors, one stimulating inspiration as the lungs collapse,
and the other stimulating expiration as they become filled. Thus
a regular inflow and outflow of air is automatically maintained.
At the same time, the center may receive reflex stimulation from
many other sources. A sudden dash of cold water on the skin
will cause us to catch our breath, and various emotional states
may produce rapid breathing, deep breathing, holding the breath,
or sighing. Among the most delicate indications of emotional dis-
turbance are changes in the ratio between the time taken for
inspiration and the time for expiration. A successful card shark
once boasted that he could always tell when his opponent was
bluffing by watching his breathing movements.

All this means that neurons from many regions of the nervous
system must make synaptic contact with the neurons of the breath-
ing center, making possible all sorts of reflex or intentional con-
trol of this most important vital function, and thus integrating
it with the other activities of the organism.

The Autonomic Nervous System. — Although breathing
movements themselves do not take place inside the body where they
cannot be seen, they belong properly among the responses of in-
ternal adjustment, since the important change which they effect is
in the internal condition of the body, not in the relation of the
body to its environment. Breathing differs from most of our
internal adjustments, however, in being carried out by skeletal
muscles. Ordinarily the effectors for internal adjustment are either

Internal Adjustments

465

heart muscles, smooth muscles, or glands, all of them buried in
the tissues where their activities are seldom noticed. The motor
neurons which run to these three groups of effectors are different
from those that have so far been described, in that the axons
leaving the spinal cord do not run all the way to the effectors,
but relay their impulses to a second set of neurons which carry
them to their destination. This double set of motor neurons is
generally referred to as the autonomic nerve fibers.

Scattered throughout the body cavity are found small bunches
of nervous tissue called autonomic ganglia. The autonomic nerve
fibers which leave the spinal cord run out to these ganglia, where

inal cord Autonomic ganglion

Preganglionic
fibers

Postganglionic
fibers

Effectors
FIG. 1 08. — Diagram of preganglionic and postganglionic neurons.

each one makes synaptic contact with several neurons whose cell
bodies are located in the ganglia. The axons of these latter neurons
carry the impulses to the effectors. (See Fig. 108.) The neurons
which run from the spinal cord to the ganglia are spoken of as
preganglionic fibers ; those which run from the ganglia to the ef-
fectors are called postganglionic fibers. The arrangement permits
a nervous impulse starting from the central nervous system over
a single preganglionic fiber to be carried to several different ef-
fectors by the postganglionic fibers with which the preganglionic
fibers make contact.

The autonomic ganglia and the pre- and post-ganglionic nerve
fibers constitute what is known as the autonomic nervous system.
It is sometimes mistakenly supposed that this system is quite
independent of the nervous system as a whole, but that is untrue.
It is simply that particular part of the system which carries motor

466 Internal Adjustments

impulses to the smooth muscles, heart muscle and glands, rather
than to the skeletal muscles.1

The autonomic nervous system is divided into three parts. The
first division is called the cranial because its preganglionic fibers
leave the central nervous system from the brain stem. The pre-
ganglionic fibers of the second division, the sympathetic, arise in
the spinal cord in the region back of the chest cavity and stomach;
while in the third division, the sacral, the preganglionic fibers
come from the lowermost region of the spinal cord. The cranial
and sacral divisions are sometimes classified together as the para-
sympathetic system, because their activity is always opposed to
that of the sympathetic system.

Heart muscle, smooth muscles and glands are usually governed
by two sets of nerves, one of which sets them into action and
one of which causes the muscles to relax and the glands to stop
their work of secreting. In other words, one set of nerves is ex-
citatory and the other inhibitory. In all cases of this double in-
nervation, one set of nerves comes from the sympathetic system
and the other from the parasympathetic. Sometimes the sym-
pathetic system has the exciting influence and the parasympathetic
is the inhibitor. At other times these relations are reversed. For
example, stimulation from sympathetic neurons speeds up the
heart beat, while that from certain cranial fibers slows it down.
On the other hand, activity of the cranial system causes muscles
in the iris of the eye to contract, thus narrowing the pupil, while
sympathetic stimulation causes the iris to relax, thus dilating the
pupil.

The activities of the sympathetic system are especially interest-
ing in that this system employs the services of a hormone to back
up its action on the body. Sympathetic nerve fibers run to the
medullary part of the adrenal glands, and whenever the sympa-
thetic system goes into action, they stimulate the glands to secrete
their hormone, adrenin. Whatever responses are stimulated by
the sympathetic system are also stimulated by the adrenin, and
whatever responses are inhibited by the action of the sympathetic
system are also inhibited by adrenin. The sympathetic nervous

1 Some authorities classify the sensory neurons to the blood vessels, digestive
organs, heart, lungs, etc., as parts of the autonomic system. Here we include only
Jtt pre- and post-ganglionic motor neurons.

Internal Adjustments 467

system and the adrenal glands work together in such harmony
that they are now looked upon as a single system for the regula-

MTO-BRAIN
BRAIN STEM

THORACIC

REGION OP

SPINAL CORD

LUMBAR
REGION

SACRAL
REGION

Eye

Lacrymal
glands

Nose, palate
Salivary
glands

Mouth

Salivary

elands

Heart

Respiratory
tract

Sympathetic
Parasympathetic

Bladder

Sex
organs

FIG. 109. — Diagram of the autonomic system. (Modified from Meyer and Gott-
lieb's Experimental Pharmacology, Urban and Schwarzenberg and J. B. Lippin-
cott Company; by permission of the publishers.)

tion pf internal responses, and are referred to as the sympathico-
adrencd system.

Adrenin is not the only chemical mediator of autonomic re-

468 Internal Adjustments

sponses. When impulses are sent out over sympathetic fibers to
the various effectors which they innervate, certain adrenin-like
substances, known as sympathins, are formed at the junctions be-
tween the nerve fibers and the effectors. It is thought that the
sympathetic nerves stimulate or inhibit the effectors through the
mediation of these chemical substances. Like adrenin, sympathins
may be carried by the blood stream to all parts of the body, where
they have the same effect on the muscles and glands that is pro-
duced by adrenin or sympathetic stimulation.

Parasympathetic stimulation or inhibition of the effectors re-
sults in the formation of another sort of chemical, acetylcholine,
which, however, is not carried through the circulatory system,
since there is an enzyme in the blood which causes its disintegra-
tion almost as soon as it enters the blood stream. Acetylcholine is
formed in both the sympathetic and parasympathetic ganglia
whenever impulses are passed from the preganglionic to the post-
ganglionic neurons, and there is evidence that it is also formed in
the stimulation of skeletal muscles. It is possible that chemicals
play an important part in the passage of impulses across synapses
in the central nervous system, and it may be that further study
of these chemical mediators of nervous and muscular response will
throw much light on the nature of inhibition, facilitation, and
learning.

The Regulation of Circulation. — We have already seen how,
when the muscles of the body become active, the rate of breath-
ing is automatically increased to take care of the rapid rate of cell
respiration. It is just as important that blood be carried to the
muscles at a rapid rate to bring large amounts of oxygen to each
muscle cell.

When a muscle is at rest, only a few of its capillaries are open
at a given moment. This can be demonstrated by making the
skin semi-transparent by rubbing oil on it. Then, if a powerful
light is focused upon a certain spot, the capillaries in the under-
lying muscles can be seen by means of a microscope. Only a few
of them are visible at a time, but there is a continual closing and
disappearance of the visible capillaries and opening of others.
Now, if the muscle is contracted, nearly all the capillaries will
open out and remain open as long as the muscle cells are at work.

Internal Adjustments 469

The number of open capillaries may be a hundred times greater
than the number in the resting muscle.

It is believed that the opening and closing of the capillaries is
regulated by the presence or absence of certain products of me-
tabolism which cause the muscle fibers in their walls to relax, with
the result that they open up. As soon as the blood carries these
products away, the capillaries close. In the resting muscle these
products are formed so slowly that most of the capillaries remain
closed; but when the muscle is active they form rapidly enough
to keep nearly every capillary open continously, and the muscle
actually swells and grows larger with the added volume of blood.
Not only is there more blood in the muscle, but it flows through
at a more rapid pace. Obviously, changes must take place in the
entire circulatory system if enough blood is to be brought to the
muscles during periods of activity. The following is an outline
of the nervous connections which serve to bring about these
changes :

1. A center in the brain stem for inhibiting the heart beat,
sending out parasympathetic fibers to the heart.

2. A center in the brain stem for augmenting the heart beat,
sending out sympathetic fibers to the heart.

3. A vasoconstrictor center in the brain stem, sending sympa-
thetic fibers out to the arterioles and capillaries in the skin and
abdominal organs. Impulses from this center cause the muscles in
the walls of the arterioles to contract, thus causing less blood to
flow through them to the tissues of those regions.

4. Certain vasodilator fibers, running to the arterioles and
capillaries of the skeletal muscles and causing these vessels to
dilate through relaxation of their muscular walls. The nature of
these fibers and their reflex connections is not well understood at
present.

5. Fibers running to the large veins. Presumably they are sym-
pathetic fibers which cause the veins to contract.

During the waking hours of the day, at times when the body is
not very active, all these fibers constantly carry a small amount of
stimulation to their effectors, just enough to keep the heart beating
regularly and to maintain a certain degree of tonus in the blood
vessels. Such would be the condition of a man working at his desk
in the office. Now suppose a sudden stimulus, such as a fire alarm,

47° Internal Adjustments

causes him to jump to his feet and run for his life. The first thing
that happens is that the flow of nervous impulses going from the
cerebrum to the muscles inhibits the action of the center for slow-
ing the heart beat, resulting in an immediate acceleration of the
beat. An instant later, the sympathetic fibers go into action. The
heart begins to beat even more rapidly, and as the veins contract,
forcing more blood into it, its contractions become stronger and
stronger. The arterioles and capillaries in the skin and abdominal
regions contract, greatly reducing the amount of blood flowing to
those parts ; and, with the expansion of the capillaries and arterioles
of the muscles through stimulation by metabolic products as well
as by the vasodilator fibers, nearly all the blood in the body is
forced to flow rapidly through these latter vessels. Finally, there
is a stimulation of the adrenal gland by other sympathetic fibers,
and the added secretion of adrenin reinforces all the sympathetic
responses that are being made.

In brief, when the muscles are active, the inhibition of para-
sympathetic impulses running to the heart, together with a wide
range of activity on the part of the sympathico-adrenal system, re-
sults in an increase in the rate of blood flow and a shunting of the
major volume of blood from the skin and abdominal organs into
the muscles.

The Regulation of Digestion. — There are three typical kind?
of effector stimulation that serve to bring about our internal ad
justments. The first is local stimulation, as illustrated by the self
stimulation of the heart referred to in Chapter XVIII, and the
stimulation of the capillaries by metabolic products. The second
is stimulation from autonomic nerve fibers, and the third is stimu-
lation from hormones, as instanced by the action of adrenin. in
the control of digestive responses, all three play a part.

Digestive responses are of two kinds : the movements of the
digestive tract, of which the peristaltic movements are the chief
type, and the secretion of digestive juices. The peristaltic move-
ments can go on quite automatically, being controlled by a net of
nerves located in the walls of the stomach and intestines and hav-
ing no connection with the central nervous system. But the vigor
of these movements is controlled by autonomic nerve fibers. The
parasympathetic group stimulates the contractions; the sympa-
thetic, with the adrenal gland rooperating, inhibits them.

Internal Adjustments 471

Similarly, the secretion of saliva and of the gastric and pan-
creatic juices is effected by the parasympathetic nervous system,
while the sympathico-adrenal system inhibits their secretion. The
sight, taste, or smell of food stimulates reflexes through the
cranial autonomic neurons which cause the digestive glands to
secrete. Once the food begins to be digested in the stomach, how-
ever, hormones are formed in the partially digested food which
stimulate both the gastric and the pancreatic secretions. Indeed,
secretion of the pancreatic and intestinal juices is almost entirely
dependent upon the formation of a hormone which is made by the
action of acid from the gastric juice upon certain substances in the
walls of the small intestine. Nevertheless, stimulation from the
parasympathetic nervous system is necessary to start the digestive
secretions ; and without this start, the formation of the hormones
is impossible. Hence, the sympathico-adrenal system, by inhibiting
both the movements and the secretions that are essential to diges-
tion, may put a stop to the entire process.

The General Function of the Sympathico-adrenal Sys-
tem.— But why should the digestive organs be subject to the
stimulation of a nerve-gland system whose whole function is to
ruin digestion? The answer is found if we consider how this action
cooperates with the sympathico-adrenal control of circulation.
Since the blood supply to these organs is shut off, they are de-
prived of the materials needed for active metabolism, and it is
therefore essential for contraction and secretion to stop.

The general function of the sympathico-adrenal system is. to
prepare the body for gftfpflg; piuscular effort. It not only causes the
blood to run more rapidly through the skeletal muscles but in-
creases the amount of sugar in the blood by stimulating the liver
to secrete sugar from its glycogen stores. It goes into action not
only when the muscles are actually busy, but whenever we experi-
ence fright, anger, worry, or any other exciting emotion. Nor-
mally such emotions occur when we face emergencies, and the
sympathico-adrenal system is getting us ready to fight or run for
our lives. In civilized life, of course, fear and anger are not always
followed by extreme muscular activity, and the internal adjust-
ments which prepare us for activity may have no result other than
to spoil our digestion.

It is characteristic for every part of the sympathetic system to

472 Internal Adjustments

go into action at once, whereas parasympathetic reflexes usually
occur in only one part of the body at a time to meet special local
conditions. An anatomical difference between the two systems
underlies this difference in manner of functioning. (See fig. 109.)
The sympathetic ganglia form two long chains on either side of
the spinal cord plus three ganglia located in front of the cord.
Any sympathetic impulse leaving the spinal cord is likely to spread
through all these ganglia, setting the whole system into activity.
Furthermore, the transportation of adrenin and sympathins
through the blood stream tends to produce characteristic sympa-
thetic activities throughout the body whenever any part of the
sympathetic system is active. The parasympathetic ganglia, on
the other hand, are placed close to the particular organs to which
they relay stimulation — the eye, the heart, and so on — and are
not interconnected. We therefore find the parasympathetic system
regulating thg eyei^day work of each organ of the body, while
the sympathetic, with its ally, the adrenal gland, sounds the alarm
at the approach of an emergency, and prepares each organ to play
its part in the total plan for adapting to this external situation.

Heat Control. — As you will recall, most animals are cold-
blooded, while birds and mammals are warm-blooded. "Cold-
blooded" and "warm-blooded" are not the best terms to use, since
the real difference between the two groups is that the warm-
blooded animals maintain an approximately constant temperature
throughout the year, whereas the body temperature of a cold-
blooded animal is usually only a few degrees above the tempera-
ture of its environment. On a hot summer day, the temperature of
a "cold-blooded" animal may be even higher than that of a "warm-
blooded" one. Since the rate of oxidation automatically decreases
when temperature falls, the movements of cold-blooded animals
slow down in cold weather, and these organisms usually die or
become dormant with the approach of winter. You have doubtless
noticed how sluggish house flies become on cold days. Respira-
tion in frogs is so slow during the winter that if their lungs are
removed they can still secure enough oxygen through the skin to
keep alive. In warm weather, however, they are quickly asphyx-
iated upon removal of the lungs.

lli^heaLJwh^ the temperature of warm-blooded

animals above that of their environment isLjkrived.lrQm the oxi-

Internal Adjustments 473

dative reactions in the cells. We become warm with exercise
because so much oxidation is taking place in the muscle cells.
Since the rate of oxidation does not necessarily follow fluctuations
in external temperature, the rate at which heat is lost from the
body must be controlled by certain vital reflexes. There is a heat
center in the brain stem which is connected with sympathetic
nerve fibers running to the sweat glands, and parasympathetic
fibers running to the arterioles and capillaries of the skin. When
the temperature in this center is increased because of the flow
of warm blood through it, it sends out impulses over both these
sets of fibers, with the result that the capillaries in the skin dilate
and sweat breaks out. The evaporation of the sweat helps to cool
the skin; the blood flowing just below this cool surface is itself
cooled and, passing to all parts of the body, cools all the tissues.

This is the main device for heat regulation. In addition, we
lose heat through all the excretory processes and especially through
breathing. In animals with thick fur or hair, the latter may be the
chief avenue of heat loss, which explains the constant panting of
dogs on hot days.

In cold weather, the first sign that heat is being lost too rapidly
is the formation of "goose-pimples" on the skin. This is really a
vestige of a method of resisting cold that was employed by our
animal ancestors. The goose-pimples are formed by the contraction
ol .smooth muscles at the base of the hairs. In animals with~long
hair this causes the coat to become fluffier and hence a poor con-
ductor of heat. A second warmth-conserving reflex is the.j£nsing
of the ...skeJetaLmugcles which we speak of as "resisting the cold."
Finally, the muscles begin the rapid alternate tension and relaxa-
tion that we call shivering. This reflex muscular activity warms
the body by increasing the rate of oxidation in the muscle cells.
If the shivering reflex fails to warm us sufficiently, we may begin
to run around and stamp our feet to warm up.

So efficient are the temperature-regulating mechanisms of the
body — especially the flushing and sweating reflexes — that, unless
the heat center is affected by the toxins of disease, causing us to
have chills or fever, our temperature seldom varies much from
the normal 98.6 degrees Fahrenheit. When, however, the air
surrounding us is warm and at the same time so humid that
evaporation of sweat cannot take place, we may develop a slight

474 Internal Adjustments

fever and actually begin to feel ill. It used to be thought that the
ill effects of poor ventilation were due to the accumulation of
carbon dioxide in the air. It is now known that carbon dioxide
has nothing whatever to do with the matter. In a hot, crowded
room, the evaporation of sweat fills the air with moisture,
while the heat escaping from the bodies of the crowd increases
the temperature. Soon it is no longer possible for heat loss
to take place, and every person in the room begins to run a
temperature. For good ventilation, the air in the room need not
be changed as long as it is kept cool and set slightly in motion to
facilitate evaporation.

The human body, when it is covered by an ordinary amount of
clothing, seems to thrive best in an environment where the tem-
perature is .between 65 and 70 degrees Fahrenheit and there is a
slow current of moderately humid air to allow for some evapora-
tion of sweat and yet prevent the drying out of mucous membranes
in the respiratory tract ; and modern air-conditioning systems or-
dinarily aim to provide an environment of this sort. At the same
time, health seems to be best maintained when there are occasional
changes in temperature and humidity, possibly because these
changes afford needed exercise to the temperature-regulating
mechanisms.

CHAPTER SUMMARY

Physiological equilibrium within the body is maintained by mus-
cular and glandular responses produced by certain vital reflexes
integrated in certain vital centers within the brain stem. The
breathing movements are caused by stimulation of the breathing
center by the acid formed in the blood when it is carrying carbon
dioxide. The rhythm of the breathing movements, however, is
produced by reflexes that are stimulated by the movements of
inspiration and expiration.

Unlike the breathing movements, most responses of internal
adjustment are performed by the heart muscle, smooth muscles
and glands, which are innervated by the autonomic nervous sys-
tem. This system is composed of a special group of motor neurons
which run to the heart muscle, smooth muscles and glands. There
are two groups of neurons, the preganglionic and the postgan-
glionic. The preganglionic neurons run from the spinal cord to the

Internal Adjustments 475

autonomic ganglia, which are small bunches of nervous tissue
scattered throughout the body cavity. Here they make synaptic
contact with one or more postganglionic neurons which carry the
stimuli to the effectors.

The autonomic nervous system is made up of three divisions :

1. The cranial division, in which the preganglionic fibers leave
the brain stem.

2. The sympathetic division, in which the preganglionic fibers
leave the middle region of the spinal cord.

3. The sacral division, in which the preganglionic fibers leave
the lower region of the spinal cord.

The sympathetic system sets the medullary portion of the
adrenal glands into action ; and since the hormone, adrenin, which
is thus produced has the same effect on the muscles and glands as
the sympathetic nervous stimulation, the nerves and the glands
together are called the sympathico-adrenal system. The cranial and
sacral systems combined constitute the parasympathetic system,
which is opposed to the sympathico-adrenal system in its action on
the effectors.

Adjustment of the blood flow during muscular activity is
brought about by (i) dilation of the capillaries in the muscles
through direct response to certain katabolic products in the blood
stream, (2) inhibition of the parasympathetic innervation of the
heart, resulting in a speeding up of the heart beat, (3) a wide
range of activity on the part of the sympathico-adrenal system
which results in acceleration of the heart beat, relaxation of the
arterioles in the skeletal muscles, contraction of the arterioles in
the skin and digestive tract, and contraction of the large veins.
In this way the amount of blood carried through the muscles
greatly increases.

Digestive activities are facilitated by ( I ) local reflexes brought
about by the nerve net in the digestive tract, (2) parasympathetic
stimulation which reinforces the activity of the muscles of the
digestive tract and starts the flow of saliva and gastric juice, (3)
stimulation by hormones formed in the food that is being di-
gested which reinforces the flow of gastric juice and is chiefly
responsible for the pancreatic and intestinal secretions. The sym-
pathico-adrenal system inhibits all the digestive activities which
are stimulated by the parasympathetic system. In so doing, it con-

Internal Adjustments

serves the energies that might go into digestion for the work of
the skeletal muscles. The general function of this system is to
prepare the body for muscular activity, and it goes into action not
only when we are really Active, but also whenever we experience
emotional excitement.

The vital reflexes having to do with heat control are as follows :
Cooling reflexes: Expansion of the blood vessels in the skin, sweat-
ing, and (chiefly in furry animals) panting. Warming reflexes:
Constriction of blood vessels in the skin, fluffing out of the hair
(represented by "goose-pimples" in man), tensing of the muscles,
shivering.

QUESTIONS

1. Explain in physiological terms why deep-sea divers take a few
deep, rapid breaths before diving.

2. Why aren't the breathing movements set in action by stimulation
from the autonomic fibers?

3. What is the difference between the arrangement of the autonomic
fibers and ordinary motor neurons?

4. What happens within the body when we become emotionally ex-
cited or active ? Describe in detail.

5. Describe the responses whereby digestion is carried on.

6. What is the importance of heat regulation in the body, and how
is it maintained ?

GLOSSARY

acetylcholine (as'e-til-ko'len) Chemical formed when parasympathetic

fibers act upon their effectors.
adrenin (ad-ren'in) Hormone secreted by the medullary part of the

adrenal glands. It reinforces the activity of the sympathetic nervous

system.
autonomic ganglia (o-to-nom'ik gan'gli-a) Small bunches of nervous

tissue scattered throughout the body cavity where preganglionic

neurons make synaptic contact with postgangl ionic neurons.
cranial (kra'ni-al) Pertaining to the cranium or brain case. Applied

to the division of the autonomic nervous system whose fibers arise

in the brain stem.
parasympathetic system (par'a-sim-pa-thet'ik) A system for internal

adjustment composed of the cranial and sacral divisions of the

autonomic nervous system, which fiauiie&JMLihs life-sustaining vital

fiaictions. Its action is opposed to that of the sympathico-adrenal

system.

Internal Adjustments 477

peristaltic waves (per-i-stal'tik) Ring-like contractions of the walls of
the alimentary canal which move down the canal, pushing the food
along ahead of them.

postganglionic fibers (post'gan-gli-on'ik) The neurons which run
from the autonomic ganglia to the heart muscle, smooth muscles,
and glands.

preganglionic fibers (pre'gan-gli-on'ik) The neurons which run from
the spinal cord to the autonomic ganglia.

sacral (sa'kral) Pertaining to the sacrum (a long bone near the base
of the spine). Pertaining to the division of the autonomic nervous
system whose fibers arise in the lower part of the spinal cord.

sympathetic Applied to the division of the autonomic nervous system
whose fibers arise in the middle section of the spinal cord.

sympathico-adrenal system (sim-path'i-co-ad-ren'al) A system for in-
ternal adjustment composed of the sympathetic division of the
autonomic nervous system and the adrenal glands, which prepares
the body for_activity. Its action is opposed to that of the parasym-
pathetic system.

sympathin Chemical formed when sympathetic fibers act upon their
effectors.

vasoconstrictor (vas'6-con-strik'tor) Applied to nervous structures
which cause the arterioles and capillaries to constrict.

vasodilator (-di'la-tor) Applied to nervous structures which cause the
arterioles and capillaries to dilate.

vital centers Gray-matter regions in the brain stem that integrate the
vital reflexes.

vital reflexes Reflexes which produce muscular and glandular re-
sponses that carry on the internal adjustments of the body.

CHAPTER XXII
BEHAVIOR AND MENTAL ACTIVITY

,The Level of Cortical Integration. — The preceding chapter
has described the responses going on within the body which en-
able the vital organs to do their work properly. The present one
deals with those responses which adjust the organism as a whole
to its environment. These are the functions of the organism
which are from day to day of greatest concern to the average
human being. In this class of adjustments fall all the activities
of work and play, of companionship and achievement that a man
thinks of when he speaks of "his life" or tells about when he
writes his autobiography. They are the responses that are in-
tegrated in the brain — more specifically, in the cerebral cortex.
Popularly, it is said that the brain is the organ with which we
think. More accurately, it is the organ in which the thinking re-
sponses are integrated. But it is more than that. When a football
player takes the ball and runs through the open field, dodging
here and there, changing his course to suit every change in posi-
tion of his interference and of the opposing tacklers, he must
react far too rapidly to have time for anything that might properly
be termed thought ; yet a good open-field runner may display much
cleverness in adjusting to the situation that presents itself to him.
Such adjustments could not be made without integration in the
cerebral cortex. Only the myriad of synapses present there could
make possible the infinite variety of response that must be made
to this constantly shifting situation. Indeed, the amount of in-
tegrative interplay that enables you to perform such a simple act
as rising from your seat in the classroom and finding your way
out through the door requires activity on the part of the cerebral
cortex. In brief, the brain integrates immediate adjustments to
the environment as well as thinking responses.

Now, when we begin to study responses at this level of in-

478

Behavior and Mental Activity 479

tegrative complexity, we discover that new phenomena appear
which do not have to be taken into consideration when we are
dealing with mere reflexes. Three things especially must be taken
into consideration : consciousness, motivation, and thinking. This
chapter will be devoted to a brief consideration of such phenom-
ena, with emphasis on the fact that they are all aspects of th*
general process of response to stimulation.

CONSCIOUSNESS

Consciousness is one of the most remarkable properties that
organisms possess. The problem of why an organism should be
conscious is one that has puzzled philosophers from time im-
memorial. Some have come to the conclusion that consciousness
is a fundamental property of matter which reaches its highest
level in animal organisms. According to these thinkers, even
atoms and molecules possess a dim sort of consciousness ; and as
matter becomes more and more highly organized in living things,
consciousness becomes clearer and more definite, until it reaches
its apex in man. Others have taken the view that matter could
never become conscious at all and that an organism to be con-
scious must possess an immaterial soul which interacts with its
response system in some fashion. Still others hold consciousness
to be an emergent — in the sense that life and culture are emer-
gents — which suddenly appears when the response system in a
species reaches a certain level of complexity. The psychological
scientist finds that these problems are quite insoluble. What
he does discover is that whatever it may be that we refer to when
we speak of consciousness, it is certainly something that is cor-
related with the activity of the response system. By arranging
definite stimulus situations and getting his human subjects to
respond to them by saying "I see this" or "I hear that" he can
learn what they become conscious of from moment to moment
and how their consciousness varies with each stimulating situa-
tion, with their past history, and with the responses that they are
capable of making in a given situation. This sort of verbal re-
sponse to situations arranged by the psychologist is called intro-
spection. Note that its meaning is different from the ordinary
meaning of the term. Ordinarily it means mulling over your

480 Behavior and Mental Activity

thoughts and feelings, thinking about yourself, about the things
you want and why you want them. Here it means simply telling
a psychologist what you see, hear, taste, smell, feel or in other
ways become conscious of in a given situation.

Because of the philosophical controversies that have raged
around the term ' 'consciousness" some psychologists have thought
it best to drop the term entirely and speak only about what we
actually observe, namely, verbal responses ; while others have em-
ployed the word "experience" to indicate the thing they are study-
ing when they get introspective verbal reports. But "experience"
also has its philosophical difficulties. Here we shall use both "con-
sciousness" and "experience" interchangeably, and, disregarding
philosophical problems, will mean by them "whatever it is that
we study when we get introspective verbal responses."

The Analysis of Consciousness: Sensory Consciousness. —
Consciousness may be analyzed into three types : sensory, imaginal,
and emotional, although, as we shall see, the latter may actually
be a form of sensory consciousness. Sensory consciousness is the
experience of things that stimulate our sense organs, either by
directly touching them or by sending stimuli to them. When I
touch a polished table top, for example, I have a sensory con-
sciousness of its smoothness and hardness. When I look at a red
lantern, the light waves coming from it give me a sensory con-
sciousness of its redness and brightness. Each of our sense or-
gans provides us with a form of consciousness that is entirely
different from that of any other sense organ. We express this
fact by saying that there is a modality of consciousness which
corresponds to each of our senses. Conscious modalities may be
classified as follows:

1. Vision, or the visual modality, referring to sights.

2. Audition, or the auditory modality, referring to sounds.

3. Olf action, or the olfactory modality, referring to odors.

4. Gustation, or the gustatory modality, referring to tastes.

5. Somesthesis, or the somesthetic modality, referring to the
feelings evoked by the sense organs in our bodily tissues.

a. The tactual submodality, referring to the feelings evoked
by substances touching the skin.

b. The kinesthetic submodality, referring to feelings of po-

Behavior and Mental Activity 481

sition, movement, or strain in the muscles, tendons, and
joints.

c. The organic submodality, referring to internal feelings
other than the kinesthetic.

In the primitive organisms from which we evolved, there was
little or no specialization of the senses. Somesthesis probably rep-
resents the general modality out of which the senses have been
specialized. The three submodalities under somesthesis may be
thought of as not sufficiently specialized to merit a rating as in-
dependent modalities in their own right. We have seen that their
sense organs show only a low degree of specialization with respect
to both structure and position. They are all pretty much alike
and are scattered throughout the body. We speak of the senses or
modalities that are more specialized as "higher." In the above
table, the modalities and submodalities are arranged in order of
their "height."

Imaginal Consciousness. — We are not conscious merely of ob-
jects that are immediately present to our sense organs. We can
be conscious of the sound of a bell when no bell is ringing, and
of the visual appearance of a hat when no hat is in the room.
We are said to imagine these things, and this type of conscious-
ness is called imaginal. Imaginal consciousness exists in exactly
the same modalities as sensory consciousness. One can have a
visual image of a house, an auditory image of the sound of a
friend's voice, an olfactory image of the smell of ham and eggs,
a gustatory image of the taste of salt or sugar, a tactual image
of the footsteps of a fly walking across the cheek, a kinesthetic
image of the movement and strain in one's legs in climbing a
flight of stairs, or an organic image of the distress of nausea.
Some readers may doubt this, since most people pay little atten-
tion to their images. They are merely aware of the fact that they
are "thinking of" certain objects and they never realize that they
"think of" them in terms of images belonging to one or more
of the sensory modalities. When reading, nearly everyone experi-
ences a series of auditory images of the words, yet few are aware
of this fact until their attention is called to it. Then they can easily
introspect the auditory imagery, usually upon the first trial.

Individuals differ greatly in the types 6f images that they ex-

482 Behavior and Mental Activity

perience. In some, imaginal consciousness is almost entirely au-
ditory; in others, it is visual; while with a few people, both
auditory and visual imagery is almost completely absent, and
kinesthetic-tactual imagery takes its place. The writer was once
acquainted with a girl who said she could imagine neither the
sight nor the sound of a church bell, but could picture how it
felt to touch it and the vibrations set up by its ringing. This, of
course, is the only way a blind and deaf person could become
aware of a bell; but this girl was not blind or deaf, she was
merely curiously lacking in visual and auditory imagery. As a
rule, the higher sense modalities furnish the clearest and most
easily introspected images. Kinesthetic imagery is probably the
hardest to introspect, which is not surprising when we recall that
most people do not distinguish even their kinesthetic sensations.

Imaginal consciousness is seldom as definite, clear, and strong
as sensory consciousness. Picture the visual appearance of a
printed word of about three syllables. You may believe that you
have a rather clear image of the word, but now try to read the
letters backward! If you can read them with anywhere near the
same fluency with which you read the letters of a word that is
actually present to your senses, your visual imagery is exception-
ally clear. To be sure, some people have this very clear imagery.
They are able to look for a moment or two at the silhouette of an
animal and later project the image of this silhouette so clearly on
a piece of paper as to be able to draw an outline of it. Such people
are said to have eidetic imagery. Imagery of this degree of defi-
niteness occurs rather frequently in children, but usually disap-
pears by the time they are grown up. Actually, it seems to be of
little practical value.

The Sensory Areas in the Cortex . — The mere stimu-
lation of a sense organ is not enough to awaken sensory conscious-
ness. Certain regions of the cortex must also be active. Nervous
impulses may pass from the ear through the brain stem and out
to an effector without producing any consciousness of sound. In
order for sounds to be heard, the impulses must at least reach
certain parts of the cerebral cortex located on either side of the
brain, and known as the auditory areas. At the back of the brain
there are similar areas for vision ; at the top, for somesthesis ; and
at the base of the brain, between the two halves of the cerebrum,

Behavior and Mental Activity

483

are areas for olfaction and possibly gustation. Each half of the
cerebrum has one of each of these areas in the same location as
the other half. (See Fig. no.)

There are fairly direct neural pathways from each sense organ
to its appropriate sensory areas, and impulses from the sense or-
gans make their way into these regions of the cortex before being

Somesthetic area

Auditory tret

Somtsthetic area

Olfactory ire*

FIG. 1 10. — Sensory areas of cerebral cortex. A, external view; B, section
view. (Redrawn from Dashiell's Fundamentals of General Psychology, Hough-
ton Mifflin Company.)

carried to other regions. In operations on the brain under local
anesthetics, it has been found that direct electrical stimulation of
these areas produces a diffuse and unorganized sensory conscious-
ness without any stimulation of the sense organs. When the audi-
tory area is stimulated, the individual hears a mass of tones and
noises. Stimulation of the visual area produces flashes of light
and color that may have scarcely any localization in space.

It has been thought by some that the functioning of the neurons
in these regions is what produces sensory and imaginal conscious-
ness. Others hold that the impulses must not only enter these re-

484 Behavior and Mental Activity

gions but must pass through them and undergo further integration
in other parts of the cortex, finally being carried out to the
effectors before consciousness occurs. We have no final proof of
the correctness of either of these theories, but it may safely be
assumed that for sensory or imaginal consciousness to arise, im-
pulses must pass through these regions. In the case of sensory con-
sciousness, the impulses come from the appropriate sense organ,
while the impulses which produce imaginal consciousness may
originate in other sense organs and make their way less directly
into the sensory area corresponding with their modality.

Emotional Consciousness. — At the present time there is a de-
bate as to whether emotional consciousness is only a special form
of sensory consciousness or whether it is something unique. When
we are angry, the sympathetic nervous system goes into action,
producing smooth muscle responses that stimulate the organic
receptors in those muscles ; we flush or pale, thus stimulating tem-
perature receptors in the skin. At the same time, we tremble with
rage and set our muscles for combat, so that various kinesthetic
receptors are stimulated. Some students of the subject believe
that our emotional consciousness, that is, the "angry feeling"
that we have, is produced by all this sensory stimulation sending
impulses to the somesthetic area, while in other emotions other
combinations of organic and kinesthetic sensations constitute the
emotional consciousness. This is the famous James-Lange theory
of emotion, first promulgated by the renowned American psychol-
ogist, William James, and the Danish physiologist, Carl Lange.
According to these men and their followers, the only difference
between emotional consciousness and other forms of sensory con-
sciousness is that emotional sensations are not analyzed and local-
ized as visual and auditory sensations are, but come to us as a
shapeless, spaceless mass of somesthetic feeling. Some individ-
uals, indeed, seem to localize their emotional sensations more
accurately than others. When they are afraid, they have a "sink-
ing feeling" in the pit of the stomach; when they are thrilled,
they feel shivers down the spine ; hot anger rages in their breasts ;
and they are made sick with disgust.

There is no doubt that sensations produced by the responses
of our smooth and skeletal muscles are present at times of emo-
tion, but many who have investigated the question doubt that they

Behavior and Mental Activity

485

are the central thing in emotional consciousness. It has been found
that a certain region in the upper part of the brain stem, the
thalamus, is the center of integration of emotional responses, and
some investigators believe that impulses passing from the thalamus
to the cortex are responsible for emotional feelings.

Perception. — When we possess sensory consciousness of a
thing, we are said to perceive it. We distinguish it from other
parts of the environment and have some knowledge of its position,
size, shape or other qualities. Practically all clear sensory con-
sciousness is perceptual, which means that it gives us some knowl-
edge of our bodies or of the world about us. But what is meant
by "having knowledge" ? A little thought on the subject will lead
us to realize that when we know about things we are thereby made
capable of responding appropriately to them. This is clear enough
when we make mistakes in perception. Nearly everyone has ex-
perienced the embarrassment of having acted inappropriately in
cordially greeting a stranger whom he has mistakenly perceived
to be a friend.

A B

FIG. in. — How much can you see?

Usually our perceptions are accurate enough for us to "get by,"
but nearly all perceiving is slightly inaccurate. Sometimes a given
stimulus situation may be perceived in more than one way, yet the
various ways seem equally correct. Is Fig. mA a picture of a
goblet or of two identical twins gazing into each other's eyes?

Probably the most important error in our perceptions is the

486 Behavior and Mental Activity

failure to see everything that is present in a situation. Some things
we do not immediately distinguish, and there are usually many
aspects of a situation that we never distinguish. In Fig. mB
you doubtless recognize the brain immediately, but probably a little
time will elapse before you distinguish the brain child. Going back
to Fig. inA, you probably saw the goblet and the twins, but did
you notice the little square-headed man with the under-sized hat ?
One of the best illustrations of this failure on the part of our
perceptual processes to get everything present in a situation is the
difference between what the musically untrained person and the
one with training in music can hear when a complex musical selec-
tion is rendered. The writer once attended a concert at which a
pianist played one tune with his left hand and a complementary tune
with his right, in the manner of a fugue. Then he asked the audi-
ence what tune he had played with the left hand. Only a half-dozen
out of the two hundred or so present had recognized it. It was
"Yankee Doodle."

We have emphasized this incompleteness of perceptual response
and its occasional ambiguity in order to make clear the fact that
what we see, hear, or perceive in other ways does not depend
simply on what is there, but on how we respond to what is there.
Different people may respond differently, or the same person may
respond differently at different times, and thus we get different
perceptions of the same situation.

IMPLICIT RESPONSES

Perceiving is as much a matter of responding to stimulation as
moving an arm or leg. Perceptual responses belong to a class which
we call implicit responses. An implicit response is one which in-
volves the activity of the nervous system and in some cases — if
not all — of the muscles. But the muscles do not contract strongly
enough for any movement to be readily observed ; hence the re-
sponse is not overt, but hidden, or implicit. To demonstrate, sup-
pose you say in a loud, firm tone of voice, "Implicit responses can-
not be observed, overt responses can be observed." In doing so
you will have performed an overt verbal response, one that anyone
present could have observed, either by hearing the sound or watch-
ing the movements of your mouth, throat, and chest. Now sup-
pose you make the same response, but make it less vigorously.

Behavior and Mental Activity 48?

You repeat the statement in a lower voice, which means that
you use fewer muscle cells in making the response, that the
activity of certain muscle cells has been inhibited. — Now try
it again, but still less vigorously. — You whisper. — Still less vig-
orously!— Your lips scarcely move, and no sound comes out of
them. — Make it less vigorous than that. — And this time it is
impossible to see any movement, although you are conscious of
saying the sentence to yourself. The response has become implicit.
Yet you will notice that its becoming implicit is merely a matter
of the gradual inhibition of more and more of the muscular ac-
tivity involved, until it becomes so slight as to be unobservable.
The response is reduced to a mere vestige of its former self.
Whether in these vestigial responses there is ever a complete
elimination of muscular activity or not is a question that remains
undecided. By using a radio amplifier to pick up minute electrical
disturbances in the muscles, it has been shown that when we form
an image of lifting our arms, there is a slight activity in the arm
muscles, even when no movement can be observed. In this par-
ticular implicit response, therefore, unobservable muscular activity
still remains; but it is not impossible that in some implicit re-
sponses, muscular activity may be completely inhibited and the
response may take place entirely in the brain.

But what is the good of making motionless responses that fail
to effect any adjustment to the environment? The function of
implicit responses is to act as stimuli to inhibit or reinforce overt
responses. A small child starts toward the cupboard to get candy
when she thinks, "Mother spanked me last time." And this implicit
response inhibits the overt one of reaching for the candy. Or you
are asked to multiply 46 by 59 mentally. You cannot immediately
respond with the answer, but by going through a series of implicit
responses, you finally say to yourself "2714," whereupon your im-
plicit response may stimulate ' the overt response of saying the
answer aloud.

At about this point, you are doubtless saying to yourself, "But
what this writer is calling 'implicit response' is just what I call
thinking or mental activity." Precisely. Perceiving, thinking, imag-
ining, remembering are the things which the mind does. But they
are also implicit responses. "The mind" is simply the everyday
term for "the process of implicit response."

488 Behavior and Mental Activity

The Nature of Perceptual Responses. — The function of these
implicit, "mental" responses is to prepare the organism for overt
activity. Instead of a simple, direct conduction of nervous impulses
from receptor to effector, there are intermediate processes which
adjust the organism to the environment by getting it ready to
make adequate overt responses.

These preparatory adjustments are not always completely im-
plicit. Perceptual responses involve two types of preparation : first,
turning the attention toward some specific part of the environment ;
second, getting ready to make a great variety of responses to the
objects to which attention is given. A part of the former prepara-
tion is the turning of the sense organs toward the object, and this
is an overt response. However, we can look at a thing without
attending to it, or attend to something that we see out of the
corner of the eye; hence the implicit part of attending is quite as
important as the overt adjustment of the sense organs.

The second aspect of the perceptual response is somewhat more
difficult to understand. How can this implicit response get us
ready to make all the responses we might possibly make toward
an object? One theory is that the perceptual response is composed
of the implicit vestiges of the responses the individual has been
accustomed to make with respect to the object or to similar ob-
jects. An apple, for example, belongs to a class of objects that
have been eaten. Perceiving the apple involves making a vesti-
gial response of eating. Now, if we happen to be hungry, that
response will be reinforced strongly enough for it to be-
come overt. But the vestigial eating response will not be the
only one involved in the perception of the apple. A multitude of
other responses that we have habitually made to objects similar
to the apple will also be vestigially present. For instance, the
apple belongs to a class of small, round objects of the sort that we
often throw, and in perceiving it, we may make a vestigial throw-
ing response. The sight of a suitable target might reinforce this
implicit throwing response, so that we might respond in that man-
ner to the apple.

Whether or not this theory of vestigial response in perception
is correct, the function of the perceptual adjustment is to make
us ready to respond to a situation in whatever manner is appro-
priate relative to what the organism is motivated to do.

Behavior and Mental Activity 489

Conceptual Adjustments. — In everday language, we say
that, through perceptual responses, we know how to respond to the
situation in which we are placed. Such knowing responses are
called cognitive, from the Latin verb cognosce, to know. The
term perception is confined to cognitions of the environment im-
mediately present to the senses. But we may know about many
things that we can never see or hear, and we may at any time
be planning action with respect to objects that are far outside
our range of perception. These cognitions of things absent are
implicit preparatory adjustments, just as perceptions are. They
are generally referred to as ideas, or concepts. As you have doubt-
less guessed, they involve imaginal consciousness of the absent
situation. More important, however, than imagery in making us
aware of absent environments is symbolism. Words are the most
important symbols. The perception of a sentence, written or
spoken, and referring to something outside the range of a man's
immediate environment, makes him ready to act relative to the
situation described. You want to find a friend. "Where's John?"
you ask. "He's over in George's room." With this brief exchange,
you are prepared to find John much more easily than you could
if you could not respond to the words as symbols.

By the use of word symbols, we can come to know about things
we have never perceived, even things which no one has ever per-
ceived. In this book we have talked about molecules and atoms,
yet no one has ever seen them; about things that happened on
the earth hundreds of millions of years before any man was
present to view their occurrence. Man is distinguished from his
animal relatives by the tremendous range of his cognitive adjust-
ments. Even for the higher apes, cognition of anything outside
the immediate range of the sense organs must be a very dim and
fleeting affair. The difference between man and the apes in this
respect is not so much a difference in inborn intelligence as it is
in the possession by man of that incomparable tool of knowl-
edge, spoken and written language.

MOTIVATION

In summary, we may say that the implicit responses called cog-
nitions make us ready to respond appropriately to an object or
situation and that this readiness is what we ordinarily term "know-

490 Behavior and Mental Activity

ing" about the object or situation. But though we are thus pre-
pared to respond in an almost infinite number of ways, only a
few of the responses for which we are made ready actually occur
at a given time. We usually respond according to some definite
plan of action, selecting from the great variety of possible re-
sponses only those which fit in with the plan. We say that we are
motivated to respond in one way or another. But what does moti-
vation mean in terms of stimulus and response? An example of
motivated behavior may help to make this clear.

A man is sitting in his study late at night, reading a detective
story. For hours he reads steadily, hardly moving, but at about
eleven o'clock he begins to show signs of restlessness ; he crosses
and uncrosses his legs, wriggles about in his chair, pulls at his
collar, unbuttons and buttons his vest. Once he even gets up and,
reading his book all the while, walks over to the mantel, leans
against it, reads and returns to his chair, still reading. The restless
movements cease for a few minutes, but soon begin again. The
man gnaws at the back of his thumb. Finally, he throws his book
down, goes to the door of his study and opens it, switches on
the light in the next room, crosses this room and then another
and makes his way into the kitchen, fumbles about for several
minutes to find the cord to the kitchen light, finally gets the light
on, goes to the icebox, opens it and takes out a bottle of milk,
goes to the dish cupboard and gets a glass, pours the milk into
the glass, sets glass and bottle on the table, goes to another cup-
board and gets a piece of pie, draws a chair up to the table, and,
holding the pie in one hand and the milk in the other, begins to
enjoy a midnight supper.

Here is a whole series of responses that fit together into a pat-
tern of activity which finally ends with the arrival at a definite
goal, namely, the filling of the man's stomach with food. Out of
all the possible responses to the situations in which the man found
himself, all the way from the study out to the kitchen table, only
those were selected that would be of some service in bringing
him to his goal. Furthermore, this line of activity had to put an
end to or inhibit another line of activity which the man had been
engaged in up to the moment he began to search for food. We
could even see a sort of struggle between the reading activity and
the food-getting activity in the man's restlessness just before he

Behavior and Mental Activity 491

finally began his search. In ordinary terms, we would say that the
man behaved in this way because he was hungry. — But what does
"being hungry" mean in terms of stimulus and response?

Let us suppose that before he started to read that night we had
persuaded the man to swallow a small rubber balloon with a long
tube attached 'to it. We would keep hold of one end of the tube,
so that after the balloon had reached his stomach we could blow
it up to make it fit tightly against the stomach walls. We would
then attach the tube to a little rubber bulb, or tambour, which
would be slighly expanded every time the muscular walls of the
stomach squeezed against the balloon. By attaching a recording
device to the bulb, we could make a record of every movement
of the man's stomach. At first there would be only the steady
little ripples caused by the peristaltic movements. Hours would
pass by as the man's supper left his stomach, and the peristaltic
contractions would gradually die down. Suddenly certain stronger,
slower contractions would set in. Simultaneously, the man's rest-
lessness would begin. The stomach movements would cease for
a few moments, and the man would become less restless ; then they
would begin again, stronger than ever. Finally, as if in response
to certain especially strong stomach movements, the man would
rise and start for the kitchen; whereupon, no doubt, we would
neatly and considerately remove the balloon to give him a chance
to enjoy his meal.

But we would have discovered the stimulation which caused
him to seek and eat food. We know that the receptors in the walls
of the man's stomach would be stimulated by the stomach move-
ments and that they would initiate nervous impulses passing into
the central nervous system. These impulses would, then, exercise
inhibiting and reinforcing influences over the responses which
the man's cognitive adjustments had prepared him to make. They
would also affect the direction of these cognitive adjustments. In
other words, they would determine to what he would pay atten-
tion. Before tte hunger stimulus, his cognitive adjustments were
directed toward the book. He was probably scarcely conscious of
the room in which he was sitting. Then his attention began to
be distracted, he began to look around the room, then to think of
the kitchen and the icebox. And then these newly adjusted cogni-

492 Behavior and Mental Activity

tive preparations were integrated with the hunger stimuli in such
a manner as to produce movements in the direction of food.

Such interactions as this between cognitive and motivating
factors account for the greater part of our behavior. Very little
of our activity — in fact, only the reflex part of it — is simple,
direct response to a single stimulus. Most of it is organized into
long chains of responses leading up to more or less definite goals
in which both cognitive and motivating factors play a part.

We may call any stimulus which causes us to move toward
a definite goal a motivator. When we are responding to a moti-
vator, we are said to have a motive, a desire, a want, a need, a
wish, an aim, an objective, an urge, or a drive. All these words
are used to designate goal-directed activity; and whenever they
are employed, the fact that they signify activity under the stimulus
of a motivator should be kept in mind.

Physiological Motivators. — The hunger stimulus belongs to
a class which we may term the physiological motivators. Any
condition which produces a need on the part of our tissues that
can be provided for only by some adjustment to the environment
may produce a stimulus of this sort. Thus, lack of water in the
tissues will stimulate the thirst receptors and cause us to seek
water. Excessive heat and cold which cannot be relieved by the
internal warming and cooling mechanisms will produce move-
ments toward cooler or warmer places, or, in the case of cold,
considerable muscular activity. Tension in the bladder or rectum
may produce activities leading toward urination and defecation.
The changes that the body undergoes during puberty produce
physiological conditions that direct the organism toward sexual
activity. Finally, certain bodily conditions, as yet incompletely
understood, produce the stimuli which cause us to become fatigued
and to seek rest or sleep.

These internal changes which act as physiological motivators
may either stimulate the somesthetic sense organs, as in the case
of the hunger contractions, or, like the chemical changes that
stimulate the respiratory reflexes, they may act directly on the
nervous system.

EJxternal Motivators. — Internal stimuli, however, are not the
only ones that have a directive effect on our activities. Tactual
stimuli, such as those that produce itches and other uncomforta-

Behavior and Mental Activity 493

ble sensations, will cause us to behave in various ways in order
to remove them. A child will work and struggle to reach and han-
dle some brightly colored toy or other outstanding object that
causes his eyes to be stimulated. Fearsome objects may produce
a series of movements that are integrated in the direction of escape
from them. In short, the external world may be as motivating as
the internal condition of the body.

Sets. — It is characteristic of a motivating stimulus that it per-
sists until the goal toward which it directs the organism is
reached. Frequently there is no persistent stimulus, of either the
physiological or the external group, to account for a persistent
line of behavior. For example, if I tell a child that I have hidden
a piece of candy in a certain room and that if he finds it he can
have it, he may search persistently for several minutes until he
reaches his goal, even though he has had a good meal and hun-
ger can in no way account for his behavior.

To explain this persistence, we must assume that an implicit
response has taken place which maintains itself over a period of
time until the goal is reached. Such implicit adjustments are
called sets. Because of their persistence, they are believed to be
related to overt postures. Overtly a man may set his muscles to
run, as when a starter in a race says, "Ready, get set!" or his
muscular posture may prepare him to fight, or to greet a com-
panion. According to what we may call the vestigial theory of
implicit response, motivating sets are implicit postural responses.

The most remarkable thing about sets is the way they persist
until the goal is reached. An incompleted task may produce a sense
of restlessness and frustration lasting for days, because the set
to complete the task keeps driving us back to it. In one experi-
ment, subjects were given a number of tasks to perform. Some
they were allowed to complete, while in others they were inter-
rupted before the completion. The next day the subjects were
asked to describe all the tasks that they remembered. The ones
they remembered best were those which were not completed. Ap-
parently the persistence of the sets to complete these tasks aided
in their recall.

Another characteristic of a set toward the completion of a task
is that it may grow stronger as the work on the task proceeds.
When a student first sits down to his work, his attention is easily

494 Behavior and Mental Activity

distracted, since the set toward that line of activity is not strong
enough to keep his cognitive adjustments channeled along the line
of study. In a quarter of an hour or so, if he is a good student,
his concentration will be much improved. If he is distracted for a
few minutes, however, he may find it difficult to get back to work
again. Many students who spend much time on their work to little
avail probably have never learned to develop this deeper concen-
tration. They allow themselves to be so continually interrupted by
small distractions or by daydreams that they never really get
warmed up to the task. These individuals need to set out con-
sciously to learn the habit of becoming absorbed in their work.

Cognitive Sets. — Some sets function more nearly like cognitive
adjustments than motivating stimuli. They do not aim toward
any goal, but merely prepare us to respond in certain ways. For
instance, after a baseball batter has watched several fast ones go
by, the pitcher can often fool him by throwing a slow curve, be-
cause the batter has become set to hit a fast ball. This set is not a
determination to reach some goal, but an assumption concerning
the nature of the environment to which he must adjust. Its func-
tion is cognitive rather than motivational ; but unlike the cognitive
adjustments that we have already become acquainted with, namely,
perceptions and ideas, it is not correlated with any form of sensory
or imaginal consciousness. The batter does not see a fast ball
coming, he does not have an image of a fast ball, nor does he
say to himself, 'This will probably be another fast one." He is
merely ready for that kind of ball. His muscles and brain are
set for it. But this set functions in exactly the way that a per-
ception or an idea would function.

Unclassifiable Sets. — Our implicit responses may be classified
under two headings : motivating sets and cognitive adjustments,
with perceptions, ideas, and cognitive sets all fitting into the lat-
ter category. The classification is based on the way the implicit
responses function as stimuli. If they cause us to persist in at-
tempts to reach a goal, we call them motivational; if their func-
tion is to guide us toward the attainment of the goal, we call them
cognitive. But anyone who has ever had a job that involved the
filing of papers has probably discovered that, with the best of
classificatory systems, he is continually being confronted with
papers that seem to fit as well into one pigeonhole as another.

Behavior and Mental Activity 495

Similarly, there is no sharply drawn line between a desire and an
idea, or, in general, between cognition and motivation. We would
be hard put to say whether some sets are cognitive or motivating.
For instance, if an experimenter says to a subject, "I am going
to read you a list of words, and I want you to respond to each
word by giving its opposite/' the individual will respond to these
instructions by developing a set to say the word whose meaning
is opposite to that of each word he hears. Now, if we thought
of this set as a desire to answer with the opposite, we would be
classifying it as a motivational set. If we thought of it as knowing
what the experimenter wanted, we would be putting it in the
cognitive class. Actually, the set seems to function in both ways.
It is on the border line between cognition and motivation.

Indeed, we separate the implicit activities of the response sys-
tem into individual motivating and cognitive responses chiefly for
convenience in thinking about them. If we could actually study all
such activities by means of some super-X-ray-microscopic device,
it is very unlikely that we should discover a number of individual
implicit responses, each separate from the other and each perform-
ing some definite motivational or cognitive task. Rather we should
observe a single continuous flux of implicit preparation for ex-
ternal activity, which, as a unified whole, would be performing
both cognitive and motivational functions. Our separation of
certain aspects of all this fluctuating implicit activity into indi-
vidual cognitive and motivating responses or adjustments helps
us to grasp the nature of mental processes and to understand
them in terms of stimulus and response. But if we find at times
that it is difficult to fit everything into our neat but artificial
system of classification, there is no reason to feel surprised.

THINKING

Often a man is placed in a situation where he is motivated
toward a certain goal but lacks the cognitive preparation to reach
it. Under such circumstances, he may either move around at ran-
dom, trying one response after another until he reaches the goal
almost by accident, or he may hunt around, implicitly, for the
proper cognitive adjustment. The former activity we call trial and
error\ the latter, thinking. But thinking of this sort is essentially
implicit cognitive trial and error.

496 Behavior and Mental Activity

Here is an illustration. The superintendent of a large public
school discovered one night that in order to be prepared for a
conference that he was to have the following morning he must
get some papers that were on his desk in the office. He got into
his car and drove three miles to the school, only to find that the
building was locked and he had forgotten his keys. Not wishing
to return all the way home, he went around to the side of the
building where his office was; its window was about twelve feet
above the ground. First he tried to climb up to the window by
holding on to projecting surfaces, but found this impossible.
Then he went around to the back of the building and found an
ash can. He carried it over and placed it under his window, but
found that standing on it didn't enable him to reach the window
sill. Then he noticed a rather small slide on which the younger
children played during recess. He found that it was light enough
to drag up to the window and that by climbing up it he could
reach the sill. But all his efforts were of no avail ; the window
was locked.

Up to this time he had been trying to solve his problem by
overt trial and error. Now, after putting things to rights, he
went around and sat down in his car and tried to think of a solu-
tion to the problem. First he analyzed the situation. There were
only two keys to the building and two to his office. He had one
set and the janitor the other. The janitor lived in the school
building, but the window to his room faced on an inner court.
The superintendent knew that at this time of night he would
probably be in his room playing solitaire. He thought of getting
as close to the room as possible and shouting, but decided that it
would be impossible to make the janitor hear, and anyhow it
might disturb others and place him in an undignified light. "Darn
it," he meditated, "there ought to be a doorbell connected with
his room. — If he only had a telephone, I could call him up from
the corner drug store. — Ah, but wait a minute, the junior high
school principal has a phone in his office and that's just below
the janitor's room. If it keeps ringing long enough, he might
go down to find out what it's all about." . . . Five minutes later,
the superintendent was on his way home with the papers.

In overt trial and error, the superintendent moved about in his
environment physically. In thinking, he explored it mentally, try-

Behavior and Mental Activity 497

ing one idea after another until he finally hit upon one that would
work. Thinking is a course of implicit cognitive response, where
one implicit response serves as a stimulus to another, under the
direction of some motivating factor. It often involves going
through some course of action in an anticipatory fashion, so that
full preparation is made for overt action. Trial and error in
thinking appears only when the thinker cannot immediately see
what series of actions would lead him to his goal. Much of our
everyday thinking does not involve trial and error at all but
merely implicit anticipation of the course of action. For instance,
the following situation offers no problem at all to a person with
a fair degree of intelligence :

Jones and Smith have an eight-gallon cask of wine which they
want to divide between them with absolute equality. They have
an empty two-gallon and an empty five-gallon cask, but nothing
else to measure with. How will they go about dividing the wine
into two four-gallon lots?

The anticipation of action is so easy that the whole course of
thought occurs in a fraction of a second. A somewhat more
complex situation, however, will require trial and error for its
solution. Try yourself on this one :

Jones and Smith have an eight-gallon cask of wine and also
an empty three-gallon and an empty five-gallon cask. How would
they go about dividing the wine into two equal lots ?

Here you will find yourself trying out one plan of action, find-
ing that it doesn't work, attempting another and another, and so
on until you manage to hit on the right one. Or, if you happen to
be clever or lucky enough to solve the above problem without
trial and error, here is one for which a direct solution is well-
nigh impossible :

Jones, Smith, and Robinson had two nine-gallon casks of wine
and also an empty two-gallon and an empty five-gallon cask. How
could they divide the wine into three equal lots?

If you introspect at all carefully on your method of solving
these problems, you will find that you do it by talking to yourself.
You think in terms of verbal symbols, accompanied, perhaps, by
some visual imagery of the situation; but the symbols are the
essential part of the thought process, and the visual imagery is
hardly more than an accompaniment. Most thinking goes on in

498 Behavior and Mental Activity

this symbolic form, especially thinking that must be exact, since
symbols represent a situation more conveniently and exactly than
images do. Often we find it convenient, when thinking through
a complex situation, to jot our symbols down on paper, where they
remain fixed, so that we can go back and check over the entire
course of mental exploration to make sure there is no flaw in it.
The first great advance in thinking was made when language
was developed; the second, when written verbal and mathemati-
cal symbols began to be employed.

Thinking and Investigation. — More often than not in prac-
tical life, thinking alone will not solve our problems. A business
man, faced with the problem of selling his goods, can seldom
solve it by sitting down and cogitating. He must discover where
his potential customers are, what their wants are, and their habits
of buying. When he has his information, he can begin to think
out a plan. Even then, he may find that, as his thinking proceeds,
he must investigate further to make sure that his conclusions are
correct. Modern science is based on this combination of thinking
and investigation. On the basis of facts that are already known,
thought leads to certain conclusions, which are never certain, since
other facts must be ascertained before they can be fully estab-
lished. Investigation ensues, and the conclusions must be modified
somewhat. New ideas are suggested, and they must be checked
on by further investigation. This investigation leads to a new
train of thought, and so science advances. An interesting thing
about this scientific advance is that it never seems to lead to final
conclusions. It is not that the conclusions already arrived at are
false, but that they are incomplete. They fail to tell the whole
story. What the layman is interested in as far as science is con-
cerned is the vast body of knowledge that has already been built
up, but the mind of the scientist is occupied with the problems
that yet remain to be solved. For him, science is not so much a
body of knowledge as a process of acquiring knowledge. And the
fundamental secret of this process is the combination of thinking
with investigation.

Working out the solution of practical problems and scientific
activity both belong to the type of thinking which we call realistic.
They commonly combine thinking with investigation, and their
purpose is to discover what the world is really like. The result is

Behavior and Mental Activity 499

knowledge which enables one to make successful overt adjust-
ments to the real world. The difference between scientific and
practical thinking is that in the latter we always have some specific
problem of overt adjustment confronting us, and we are simply
trying to learn enough to solve it. Scientists, on the other hand,
think and investigate simply for the sake of discovering truth,
regardless of any practical problem with which they happen to be
faced. But since scientists think realistically, as practical men do,
their conclusions frequently have important practical applications.
In fact, their practical results may, in the end, be much greater
than the results of purely practical investigation and thinking,
because specialization in the business of searching for truth leads
to a more complete picture of the real world than that attained by
the man who stops to apply his knowledge as soon as he gains
a little.

Wishful Thinking. — There is another type of thinking which
is quite different in its aims from scientific or practical thinking.
Its goal is not knowledge which will enable us to arrive at other
goals through overt behavior, but the direct satisfaction of desire
through thinking alone. It is called wishful thinking. In spite of
all our search for truth, we seldom learn enough to reach many
of our goals. Our real environment is one in which urges are
thwarted and hopes blasted. But by means of thought responses
we can picture to ourselves worlds much nearer to the heart's de-
sire than the one which stimulates our sense organs. And so our
urges, avid for satisfactions that are denied them, spur us on to
build up these unreal worlds or to form distorted pictures of the
one in which we live. The following little jingle portrays fairly
accurately the general trend of most daydreams :

If I were an Angel Bright,

If I were a Child of Light,

Resplendent, borne

On wings of the morn

I'd soar to heaven's height,

And the gaping throng on the earth below

Would wonder at the sight!

Alone, in the splendid night,

Like a flashing meteorite,

Toward the bright maroon

SOO Behavior and Mental Activity

Of the rising moon

Would I direct my flight.

Where the Mystic Mountains of the moon,

Majestic in their might,

Surround the vale where the moon maids croon

An eerie, winsome, wilesome tune

In the Land of Pure Delight !

Although he puts it all quite ornamentally, one can plainly see
what sort of a world this young fellow wants. He wants one in
which he secures distinction, admiration, power, and sexual satis-
faction, and all without too much effort ; let the wings of the morn
do the work! Furthermore, he wants to be an angel bright and
he wants his delight to be pure; in other words, he doesn't want to
feel that he has done wrong in getting wtyat he wants, nor does he
want to be criticized for it. Such is the burden of most wishful
fhinking, for the urges to secure admiration, approval, mastery,
sexual satisfaction and rest are the ones that are most often
thwarted in our civilization. As a general rule, we are sufficiently
well fed so that we seldom daydream of food ; but let a man go on
a diet or fast, and he will find images of things to eat literally
forcing themselves on his mind.

When our wishful thinking does not take the form of day-
dreaming, it distorts the real world, causing us to believe untruths
about it. Some people, for instance, thoroughly enjoy believing
that others bear an unjust grudge against them, for that explains
why they don't get better pay, higher grades, or more commenda-
tory smiles. Children occasionally convince themselves they have
been adopted and that their ostensible parents are not really theirs.
There is no other way of accounting for the fact that such excep-
tional persons as themselves should be brought up in such mediocre
home surroundings. Flat-chested Phi Beta Kappas believe that all
football players are morons, and the mediocre student is certain
that all Phi Beta Kappas are flat-chested.

Occasionally an individual is carried away by his daydreams to
the point where his beliefs become so utterly outlandish and ri-
diculous that we call them delusions and send him to a hospital
for the insane. Such people are not very different from the rest of
us ; they have only gone a little farther than we all do in protecting

Behavior and Mental Activity 501

our egos from the pitiless light of reality by shrouding them in a
rosy haze of wishful believing.

Wishful Perceiving. — Our urges can even come to dominate
our picture of the world so fully that we do not perceive what is
present to our senses, but what we want to perceive. The writer
has noticed that when he becomes thirsty while hiking in a forest
it is quite impossible for him not to interpret the sound of the wind
in the trees as running water.

Sometimes the thing which conditions an illusion is not &o much
a wish as an expectant set. A superstitious individual in a reputedly
haunted house will see a ghost in every fluttering rag and hear a
shriek in every gust of wind. People who tell you they have seen
miraculous phenomena with their own eyes may very well be tell-
ing the truth, but what they saw was an illusion or hallucination.
The term "hallucination" is popularly used to mean a false belief %
although the proper term, of course, is "delusion." Both illusions
and hallucinations are false perceptions, the difference between
them being that there are no perceptible sensory stimuli for an
hallucination, while an illusion is a misinterpretation of stimuli
that are actually present. Thus, if a patient on the ward looks at
the doctor's hat and sees a black cat, he is experiencing an illusion ;
but if, when there is nothing whatever on the floor of his room, he
shrieks and runs away from what he describes as a large green
alligator, he is subject to an hallucination.

Illusions and hallucinations are very characteristic of the insane.
Usually they serve to confirm their delusions. The man who has a
delusion of persecution will hear voices deriding and threatening
him or feel sharp pains shooting through his body which he says
are caused by the poisons that are put in his food. Or the patient
who believes he is God — as many do — will see the other patients as
angels, while the doctor, like as not, will be seen wearing a pair of
horns and a forked tail.

Dreaming. — Hallucinations are much like images, the chief
differences between them being that the hallucinated objects are
much more vivid, and the individual feels that they are actually
present. In dreams we experience a train of thought which is made
up of a long series of hallucinations. Stimuli falling on the sense
organs may act as cues for these hallucinations, as when an indi-
vidual who is covered too warmly dreams that he is being cooked

Behavior and Mental Activity

in an oven. But the manner in which the cues are interpreted de-
pends upon the urges and sets of the individual ; in other words,
dreaming is a form of wishful thinking.

Many people will protest that they do not dream of the sort of
world that they desire; quite the contrary, they have the most
horrid nightmares. It is possible that both our sleeping dreams and
our daydreams may be controlled to some extent by fearful and
apprehensive sets, as well as by our wishes. According to the great
Viennese doctor, Sigmund Freud, however, a dream always sig-
nifies a wish, although the wish may be somewhat disguised. Ac-
cording to him, we refuse to recognize some of our wishes because
they would make us feel too ashamed of ourselves. But in our
dreams, these wishes secure a certain amount of satisfaction by
expressing themselves in a disguised and symbolical form. Thus,
a daughter who nas an unrecognized desire to have her mother out
of the way will dream of her mother's death. Although she may
feel the greatest sorrow and loss in the dream, thus disguising the
fact that she really wants to be rid of her mother, still the dream
is a form of wishful thinking, for the unrecognized desire receives
a certain amount of satisfaction.

CHAPTER SUMMARY

At the level of cortical integration of response the phenomenon
of consciousness appears. Philosophers have been unable to decide
what consciousness is or why it should be correlated with highly
integrated nervous activity. As psychologists, we go no further
than to say that it is the thing that we study when we get intro-
spective verbal reports from our subjects.

Consciousness may be analyzed into sensory, imaginal and emo-
tional consciousness. Sensory consciousness occurs when the sense
organs are stimulated, and may be divided into the following
modalities or sense departments : vision, audition, ol faction, gusta-
tion, and somesthesis. The last is divided into three submodalities,
the tactual, the kinesthetic, and the organic. Imaginal conscious-
ness is similar to sensory consciousness except that it occurs when
the object of which we are conscious does not stimulate the sense
organs. It may occur in any of the modalities, although it is most
easily introspected in the visual and auditory modalities. It is
seldom as clear and definite as sensory consciousness, although it

Behavior and Mental Activity 503

approaches the clarity of sensation in eidetic imagery. Each of the
sense modalities is correlated with an area or areas in the cerebral
cortex which must be activated if sensation or imagery in that
modality is to be experienced.

According to the James-Lange theory, emotional consciousness
is merely the mass of somesthetic sensation aroused when we
make an emotional response, but others believe that it is a special
type of consciousness aroused by impulses passing from the thal-
amus in the upper brain stem up to the cortex.

What we ordinarily call "mental activity" is actually implicit
response, that is, response which cannot be observed since it in-
volves little if any muscular activity, but which can function in
our behavior by stimulating overt (observable) responses.

An important type of implicit response is the cognitive response,
or cognition. In making such responses, we come to know about
objects or situations, which is to say that we are prepared to re-
spond overtly to them in a variety of ways. The actual overt re-
sponse depends upon how we are motivated. Two types of cog-
nitive response may be distinguished : the perceptual response,
whereby we come to know about things actually present to the
sense organs, and the ideational or conceptual response, whereby
we come to know about things not present to the sense organs. In
perceiving, we actually see, hear, smell, taste, or feel an object. In
conceiving, we experience an image of an object or see, hear, or
"say to ourselves'' a word or other symbol of the object.

Motivation is produced by stimuli known as motivators which
cause us to make responses that carry us toward a goal. The
motivating stimuli are integrated with our cognitive responses act-
ing as stimuli in such a fashion as to cause us to move in a direc-
tion that is likely to bring us to the goal.

Motivators may be physiological conditions, external situations,
or implicit responses, known as sets. Sets may also be of a cog-
nitive nature, or they may display the characteristics of both
motivation and cognition.

Thinking is a series of cognitive responses in which one response
acts as a stimulus for the next one. Realistic thinking is a trial-
and-error attempt to arrive at a new cognitive adjustment to a
situation. It aims at the discovery of truth.

Wishful thinking is a course of thought aimed toward the pic-

504 Behavior and Mental Activity

turing of a more desirable world than the real one. Sometimes it
leads to wishful believing that this more desirable world actually
exists. This is called delusion. Delusions are sometimes accom-
panied by wishfully motivated perceptions, called illusions and
hallucinations. Dreaming is essentially a series of wishfully mo-
tivated thoughts or hallucinations.

QUESTIONS

1. Into what departments is consciousness analyzed? What is the
relation of emotional to sensory consciousness, according to the
James-Lange theory?

2. What is an implicit response?

3. Discuss cognition and motivation in terms of stimulus and re-
sponse.

4. Criticize: "If I can see and feel a thing, I know it actually exists. "

5. Describe thought and tell what its functions are in terms of stim-
ulus and response.

6. What is an hallucination ? An illusion ? A delusion ?

GLOSSARY

audition The sense of hearing.

cognition The act of coming to know about objects or situations.
Cognition is effected by cognitive responses, which are for the
most part implicit.

concept A cognitive adjustment to objects or situations not present
to the senses. A thought or idea.

delusion A false belief.

eidetic imagery (i-de'tik) Very clear and sharply defined imagery,
closely approaching the clarity of perception.

goal The end condition toward which an organism's activity is di-
rected when it is acted upon by a motivator.

gustation The sense of taste.

hallucination (ha-lu'si-na'shun) A false perception occurring when
there are no apparent stimuli to be misinterpreted.

idea Approximately synonymous with concept. The term concept is
sometimes reserved for the more abstract ideas.

illusion A false perception resulting from misinterpretation of stimuli.

implicit response A response that involves little or no muscular ac-
tivity; hence it is unobservable.

modality (mo-dal'i-ti) A sense department, such as vision, audition,
somesthesis, and the like.

Behavior and Mental Activity 505

motivator A stimulus or group of stimuli which causes an organism's
activities to be directed toward a goal.

ol faction (61-fak'shun) The sense of smell.

overt response (6'vert) A response which can be readily observed.

perception Cognition of objects or situations through stimulation of
the sense organs by those objects or situations.

sensory area A region of the cortex which is especially concerned
with a certain modality of sensory or imaginal experience.

set An implicit or overt response which prepares an organism for
action or produces a consistent course of action.

symbol Any object which stands for another object or situation. By
experiencing symbols, we make conceptual adjustments to the ob-
jects for which they stand. Words are the most important types of
symbols.

thalamus (thal'a-mus) Region in the upper part of the brain stem
which integrates emotional responses.

trial and error A course of action in the pursuit of a goal which
involves the attempting of many possible responses directed toward
the goal until finally the successful responses occur. Trial and
error occurs when an organism's cognitive adjustments are not
adequate to bring it directly to the goal.

vision The sense of sight.

CHAPTER XXIII

GROWTH RESPONSES IN PLANTS AND
ANIMALS

Why Organisms Must Respond. — About twenty years ago,
during the difficult times of the World War, a certain wistful bit
of verse seemed to catch the public fancy and was reprinted again
and again in newspapers and magazines. It went something as
follows :

I wish I was a little rock
A-settin' on a hill,
A-doin' nothin' all day long
But just a-settin' still.

I wouldn't work, I wouldn't eat,
I wouldn't even wash,
But just set still a thousand years
And rest myself, by gosh!

But the restful life of a little rock can never be the fate of any
organism. The little rock just sits still, and hence needs no nourish-
ment to supply energy for a restless round of activities. Through
the thousand years of its life, it is slowly worn to dust by the
ceaseless action of wind, water, and frost, yet it does nothing to
prevent its gradual extermination. And when at last it has gone,
it leaves no descendants to carry on for another long but indolent
millennium. Time and change bring an end to all things. Rocks
wear away, continents sink below the sea, stars grow dark and cold.
But these inorganic objects do nothing to ward off their slow
oblivion. What permanence they possess is entirely a result of a
tough immunity to the onslaughts of their environment. The or-
ganism, on the other hand, is a delicate affair, and helplessly de-
pendent on its environment for the maintenance of its existence.
The slightest change in the relationship between itself and the

506

Growth Responses in Plants and Animals 507

world around it may mean its end, while at the same time it must
continuously wrest from its surroundings the substances necessary
for the carrying on of life. Hence, an organism must be able to
change itself to meet each new problem that its environment pre-
sents. In other words, it must be able to respond. Through this
capacity to respond, the organic world has resisted the all-pervad-
ing destructiveness of the universe and maintained its existence
throughout millions of centuries.

Two Kinds of Responses. — If a young tree is tipped so that
it lies horizontally, either by accident or through the act of an
experimenting scientist, its tip soon turns and grows upward,

FIG. 112. — Centrifugal force and gravity as a stimulus. (Redrawn from Smith,
Overton et al., Textbook of General Botany, The Macmillan Company.)

carrying its leaves up into the light, while at the same time its main
roots bend downward so that they grow into the ground, thus
holding the tree firmly in place. This is a definite response to the
force of gravity, as can be demonstrated by placing a seedling
plant on the surface of a revolving wheel, whereupon the shoot no
longer grows upward and the root downward, but the former
slants inward and the latter outward in the direction of the resolu-
tion of the centrifugal force wTith that of gravity. (See Fig. 112.)
If a shoot or root, before being made to respond in the above
manner, is marked with a series of horizontal, parallel lines placed
close together, these lines can be seen to spread farther and farther
apart on the outside of the bend produced by the response, while
maintaining about the same distance from each other on the inside.
This shows that the bending is produced by the faster growth of
the shoot on the lower, and of the root on the upper side. Thus it is
an entirely different sort of response from the movement responses

508

Growth Responses in Plants and Animals

of human beings with which we have dealt so far. Growth and
movement constitute the most important types of organic response,
although other types, including glandular responses and antibody
reactions, exist ; the implicit responses with which the last chapter
dealt have developed out of movement responses. The most im-
portant type of response in the plant kingdom is the growth
response, since it is by proper growth that the plant adjusts itself
to the environment, whereas animals feature the movement re-
sponse. To be sure, there are many plants that respond through
movements, and growth response forms an important aspect of the
development of all animals,

FIG. 113. — Diagram showing differential rates of growth in geotropism. (Re-
drawn from Smith, Overtoil, et al., Textbook of General Botany, The Macmillan
Company.)

The Mechanism of Plant Responses. — In addition to grav-
ity, a number of other stimuli can produce growth responses in
plants. Of these, light is one of the most important. Light in gen-
eral inhibits the growth in length of plant shoots. If two pea
seedlings of equal size are placed, one in strong light, and the
other in a dark room for a few days, the latter will become very
long and spindly, while the former will increase relatively little in
length. On the other hand, light from a definite direction causes
shoots to bend toward it, just as they bend away from the force of
gravity. The adjustment of a plant part by bending to a stimulus
from a definite direction is known as a tropism. Two of the best-
known tropisms are those already described, geotropism, or the
response to gravity, and phototropism, the response to light.

Extensive studies of these two tropisms have given us much
information on the mechanism of growth response in plants. In
both cases the response occurs in a different region from that
which receives the stimulus. The bending always take place about

Growth Responses in Plants and Animals

509

halfway down the shoot of a grass seedling; but if the tip of a
shoot is protected from the light by a cap of tinfoil, that shoot is
prevented from bending, while removal of the tip destroys the
response of the shoot to both gravity and light. The tip cells are
therefore receptors, while the cells whose changes in rate of growth
actually bring about the bending are effectors. Obviously, there
must be some method of conducting the effect of the light stimulus
from the point of reception to the effector cells located halfway

Light
source

FIG. 114. — Diagram illustrating phototropism. When the tip of a plant is
shielded from the light, the plant does not bend. (Redrawn from Smith, Overton,
et al, Textbook of General Botany, The Macmillan Company.)

down the stem. A little experiment can be performed which demon-
strates pretty clearly how this comes about.

An oat seedling is exposed to light for a short time and then the
tip is cut off. The cut surface of the tip is then pressed against a
small disk of gelatin and left there for some time. Now the tip is
removed from a second seedling which has never been exposed to
light and the gelatin disk is taken from the stimulated tip and
placed against the cut surface of this unstimulated seedling. The
seedling will proceed to bend in the usual manner, as if its own tip
had been exposed to light. (See Fig. 115.)

This experiment demonstrates definitely that the response of the
shoot is produced by the activity of a growth substance, similar irf

Growth Responses in Plants and Animals

effect to the growth-regulating hormones produced by the endo-
crine glands of animals/Another experiment demonstrates that
this growth substance is the same one that regulates normal growth
in plants. Three seedlings are decapitated and placed in the dark.
The substance from two of them is collected in agar disks, as in
the previous experiment. One of the disks is replaced on the mid-

Gelatin disk

FIG. 115. — Experiments on hormone control of phototropism. A i, Tip removed
from stimulated seedling and placed on gelatin disk. A 2, Gelatin disk placed on
cut end of unstimulated seedling. A 3, Unstimulated seedling bends as stimulated
seedling would have bent if its tip had not been removed.

B I, Tip in dark without auxin-containing gelatin fails to grow. B 2, Tip with
gelatin on one side grows most rapidly on that side. B 3, Tip with gelatin on
top grows straight upward.

die of the cut surface and causes the shoot to grow upward nor-
mally; the second is placed on the side of the second shoot, causing
this shoot to bend away from the treated side ; while the third shoot
is left decapitated, and ceases to grow.

By means of experiments such as these, the amounts of growth
substance in different plant organs have been carefully measured,
and the extracted substances have been isolated and studied chem-
ically. Three slightly different chemical substances, known as
auxins, have been found to be active as growth substances. These
auxins can be extracted not only from the tips of roots and shoots.

Growth Responses in Plants and Animals 511

but from many other plant parts as well, and from some animal
substances, such as urine.

In the small concentrations in which they occur in the bending
region of the shoot or root, these auxins produce only one type of
growth, i.e., the elongation of the cells. This is brought about by
the activity of the auxins in making the walls more elastic, thereby
enabling the osmotic pressure within the cell to stretch them. On
the other hand, the same substances when applied in high concen-
trations to the cut ends of stems or other plant parts produce an
increased cell division, resulting in the formation either of a bump
or callus, or of roots. Apparently the entire process of growth and
differentiation, as well as response, is in part regulated by the
activity of auxins or similar substances. The effect of light on
growth is explained largely by the fact that, whereas light in-
creases the production of growth substances by tissues, it inhibits
the activity of these substances in producing cell elongation.

The method of conduction of the growth substance from where
it is produced in the shoot or root tip to where the bending re-
sponse takes place gives us an important insight into conduction
in general in plants. If one looks through the microscope at the
cells in the leaf of a type of water plant, Elodea, or in certain
epidermal hairs of plants, one can see currents of protoplasm
streaming rather rapidly around the cell. Similar currents of
protoplasm exist in many types of plant tissues, and experiments
have produced strong evidence that the growth substances are
carried through the plants by means of these currents, diffusing
from one cell to another through minute openings in the cell walls.
Hence the method of transport is similar to that of animal hor-
mones, except that in animals there is a special conducting system,
the blood stream, while in plants the conduction is carried out by
ordinary living cells.

Varieties of Tropisms. — Four other types of tropisms are
those in response to the stimulation of chemical substances, water,
temperature, and touch or pressure. The first, known as chemo-
tropism, is most common in saprophytic plants such as fungi,
enabling them to reach the substratum on which they feed. Hy-
drotropism is seen in the tendency of roots to bend toward a source
of water, and thermotropism produces the bending of shoots away
from an area of too high or too low temperature.

512 Growth Responses in Plants and Animals

Thigmotropism, or the response to touch or pressure, is well
exemplified by the growth of tendrils of vines. Sweet peas, wild
cucumbers, and similar plants, as everyone at all familiar with
them knows, attach themselves to their supports by means of ten-
drils that grow out from the stem and wind around the wires,
strings, or twigs with which they come in contact. As the tendril
begins to grow out from the stem it behaves very much as if it
were groping about in search of something to which it might
fasten itself. First the cells on one side of the tendril — say the left
side — grow more rapidly than those on the right, thus causing the
tip to bend toward the right. Then the cells at the top may begin
to grow most rapidly, bending the little shoot downward. Next the
tendril will be bent to the left by the rapid growth of the cells on
the right side ; and so it grows, bending back and forth and up and
down until it finally comes in contact with a support around which
it may twine. This contact acts as a stimulus to a new sort of ac-
tivity. Growth becomes greatly accelerated, particularly on the side
opposite the point of contact, so that the tendril bends toward the
support and wraps itself around and around it in a tight spiral.

Another example of this type of response is the reaction of
trees to the force of the wind. Anyone who has had an opportunity
to observe trees growing along the seashore or in a high mountain
pass in regions where winds are strong and where they almost
always blow from the same direction, will have noticed that the
limbs on the side which is buffeted by the prevailing wind are short
and relatively leafless, while those on the opposite side are con-
siderably longer, and the tree seems to lean over as if it had been
pushed back by the wind. This impression of leaning is produced
partly by the disproportionate length of the branches on the shel-
tered side of the tree. The fact is that the wind has not pushed the
tree back ; it has simply stimulated the branches to grow longer on
the lee and to grow shorter on the side that faces the wind. The
continual force of the wind acts just as would the pressure of a
solid object ; it inhibits the growth of the tree. Similarly the pull of
the wind on the sheltered side facilitates the growth of these
branches. It is a general law of plant growth that pressure against
the tip of either a root or a stem retards or partially inhibits growth,
while a pull or strain upon either a root or stem usually speeds it up
or facilitates it.

Growth Responses in Plants and Animals 513

It should be emphasized that it is not the force of the wind
which pulls the branches of the tree out so that they become longer,
as one may lengthen a mass of taffy by pulling at it. The force of
the wind merely stimulates the branches on the lee side to grow
rapidly. The energy for that growth comes not from the wind but
from the tree itself.

Here we have an excellent example of the manner in which
plants may adjust themselves to their surroundings by means of
growth responses. In a normal situation these same trees would
grow with their branches and roots approximately the same length
on all sides. But where a hard wind blows almost constantly from
one side, long, strong roots grow to grip the soil at just the place
where they are most needed and the foliage and branches become
"streamlined" and offer a minimum of resistance to the prevailing
air currents.

The Developmental Reactions of Animals. — There are two
ways in which the environment can affect the growth of an
organism, whether plant or animal. First, it may furnish stimuli to
growth responses ; second, it may furnish or withhold certain nu-
tritive elements essential for growth, such as organic foods, oxy-
gen, water, or mineral salts. It is not always possible to say whether
a given condition affecting growth is a stimulus to a growth re-
sponse or an essential condition for nutrition. Thyroxin, for ex-
ample, is frequently spoken of as a stimulus to growth in men and
animals; but apparently the function of thyroxin is to enable cells
to use a maximum amount of oxygen, and hence it is probably
more correct to speak of thyroxin as a furnisher of the nutrition
necessary for growth. Actually, it is hard to draw a hard-and-fast
line between stimuli to growth and necessary conditions for
growth; and this difficulty is especially marked in the study of the
development of animals. We can get around the difficulty by
speaking only of the environmental conditions which produce de-
velopmental reactions in the growing structures, remembering that
some of these conditions refer merely to essential supplies of nutri-
tion, while others refer to true stimuli for growth.

The development of an organism from the zygote up to the
adult condition embodies three types of changes in the cells :

i. Growth in size of each individual cell, which may go on more

514 Growth Responses in Plants and Animals

or less rapidly from the time the cell is formed until it divides or
reaches an adult stage.

2. Division of cells, so that a vast multitude of them are formed
from the single zygote in which the life of the organjsm has its
beginning.

3. Differentiation of cells, so that each cell comes to possess a
structure and function of its own, fitting into the structural and
functional pattern of the entire organism.

The manner in which each cell will grow, divide, and differen-
tiate depends upon two factors : first, what is in the cell, including
the way its cytoplasm is organized and the equipment of genes in
its nucleus ; second, the conditions surrounding it. It is because the
genes enter into the development of each cell that they exert con-
trol over the organism ; and, since they are handed on intact with
each cell division and are passed on from generation to generation,
they influence the organism to develop its hereditary characteris-
tics. But they are not the sole influence; for if they were, each cell
would develop in the same fashion, and there would be no differ-
entiation. The conditions surrounding the cells, however, espe-
cially their relationship to other cells of the growing organism,
interact with the genes ; and thus the developmental environment
is as important as the genetic constitution in determining what the
organism shall become. The formation of identical twins is a good
illustration of this fact. If at a very early period, two halves of the
growing organism become separated in some fashion, two or-
ganisms develop, one from each half. This means that the cells of
the left half of the organism, which, if they were under the nor-
mal influence of the cells of the right half, would develop into only
half an organism, now, being freed from that influence and de-
veloping entirely by themselves, react to this different environ-
ment by growing into an entire organism. In some animal or-
ganisms this capacity of cells to react to a different environment
by undergoing a different course of development is so pronounced
that even when the zygote has divided into sixteen cells, it is possi-
ble to separate the cells and have a complete — though rather
small — adult develop from each one of them.

Another good example of how environmental conditions can
change the development of an organism has been demonstrated in
certain minnows. If, at a certain time in their development, mag-

Growth Responses in Plants and Animals

515

nesium chloride is added to the water in which they are growing,
it changes the structures that are growing into eyes so as to pro-
duce a single eye in the middle of the head, rather than two eyes on
either side. The chemical does not have to be present throughout
the period of growth, but only at an early stage, whereupon certain
cells are changed in such a fashion as to direct all the succeeding
development toward the formation of the single eye.

Many of the "freaks" that we see in circus side shows are
people in whom some slight change in their embryonic environ-
ment has resulted in a strange course of development comparable

A B

FIG. 116. — A, normal and one-eyed minnow, B, twinning in minnows.

to that of the one-eyed fish. Siamese twins are merely identical
twins who became separated from each other at a later stage in
development than is ordinarily the case, so that the separation was
not as complete as usual. One investigator, by varying the envi-
ronment at different stages of growth, has produced fish that show
all degrees of "twinning," from complete identical twins to indi-
viduals with only slightly separated heads on the same body. The
two-headed giants of our fairy tales are not biological impossi-
bilities. Indeed, children are occasionally born with two heads,
with single eyes, or with various other marked abnormalities
brought about by accidents in embryological development ; but in
most cases they die soon after birth.

A striking characteristic of animal growth is the manner in
which one part of the animal body may act as the condition which
regulates the growth of other parts. An instance of this is the*

Growth Responses in Plants and Animals

manner in which hormone secretions govern the growth of the
body. We have seen how the appearance of secondary sexual char-
acteristics is dependent upon the secretions of the gonads. The
development of the gonads, in turn, is dependent upon one of the
pituitary secretions, while the type of gonad developed, whether
male or female, depends not only upon the organism's equipment
of chromosomes, but also on environmental conditions in the
earliest stages of life which govern the rate of metabolism in the
embryo. Thus the development of the organism results from com-

FIG. 117. — Effect of transplanted organizer on salamander embryo. Left: dor-
sal view, with changes produced by organizer shown on left side. Right : left side,
showing how organizer produces a groove similar to the normal groove on the
dorsal side.

plex interactions between the genes and the environment, and also
from interactions among various parts of the growing body.

An important instance of the interaction of bodily parts is the
development in the embryo of various groups of cells which in
some manner control the growth and differentiation of the cells
around them. Such a cell group is called an organiser. Early in the
growth of vertebrate animals, a certain region in the unformed
body of the organism becomes the organizer for the formation of
the embryonic spinal cord, together with the regions surround-
ing it.

It is possible to take cells from this organizing region in an
embryo salamander and transplant them on to the side of a sala-
mander of an entirely different species in which the spinal cord has
already started to form along the back, whereupon they will stimu-
late the cells along the side to begin the formation of a second
embryonic spinal cord, together with various other structures, so

Growth Responses in Plants and Animals 517

that the embryo almost develops into two individuals, one attached
to the side of the other. Without stimulation from this foreign
organizer, the cells that develop into the second group of nervous
structures would normally develop into the skin of the side. Simi-
larly, the part of the eye that develops into the retina acts as the
organizer for the entire eye. If the cells that would normally de-
velop into the lens and other structures of the eye are removed, and
other cells, say from the skin of the back, are put in their place,
this second group of cells will form all the necessary eye structures
under the stimulation coming from the organizer. On the other
hand, if the organizer is removed, the rest of the eye may be very
defective in its development.

CHAPTER SUMMARY

The two most important types of response are growth response
and movement response. Growth response in plants may involve
merely a general rapidity or slowness of growth, as when plant
stems grow more rapidly in the dark than in the light, or it may
involve bending toward or away from stimuli coming from a defi-
nite direction. Such a response is called a tropism. Stimuli that can
evoke tropisms are : gravity, light, water and other chemical sub-
stances, heat or cold, and pressure.

For many tropisms, definite receptor and effector cells can be
found, as in a plant reacting to light, where the receptor cells are in
the tip and the cells that elongate, causing the plant to bend, are
slightly lower down on the stem. The impulses to grow are con-
ducted from the receptors to the effectors by chemical substances
known as auxins. Auxins are carried by means of streaming cur-
rents of protoplasm. By means of growth responses plants adjust
themselves to special features of their environment, as when a tree
becomes "streamlined" when a hard wind blows upon it always
from one direction.

The growth of animals is essentially a reaction to the conditions
of the environment, although it is not always possible to determine
whether the condition affecting growth is to be considered a true
stimulus or an essential condition for nutrition.

Changes in the environment in which an embryo develops may
greatly change the course of development, as when one-eyed min-
nows are produced simply by adding magnesium chloride to their

518 Growth Responses in Plants and Animals

water. Furthermore, the tissues in the developing embryo affect
one another's development, as when two individuals develop from
a zygote that has been divided in half during the earliest stages of
development, whereas each of them would have developed into
half an individual if it had not been separated from the other half.
During the course of development, certain groups of cells, known
as organizers, act to control the growth of all the cells surrounding
them, so that under experimental conditions they will cause certain
structures to form out of cells that normally would have formed
entirely different structures.

QUESTIONS

1. Show how it can be proved that chemical substances make their
way from receptor to effector cells in producing plant responses.

2. Mention some of the ways in which growth responses adjust plants
to their environments.

3. Criticize: "A man is bound to become whatever his hereditary
nature determines that he shall become."

4. What similarity in principle is there between the formation of
identical twins and the action of organizers?

GLOSSARY

auxins Chemical substances that stimulate growth in plants.

chemotropism (kem-ot'ro-piz'm) Tropistic response to chemicals.

geotropism (je-ot'rd-piz'm) Tropistic response to gravity.

hydrotropism (hi-drot'ro-piz'm) Tronistic response to water.

organiser A group of embryonic cells which controls the develop-
ment of the cells around it.

phototropism (fo-tot'ro-piz'm) Tropistic response to light.

thermotropism (ther-mot'ro-piz'm) Tropistic response to heat or cold.

thigmotropism (thig-mot'ro-piz'm) Tropistic response to pressure.

tropism (tro'piz'm) A response to stimulation in terms of the direc-
tion from which the stimulation comes. Tropisms in plants involve
a bending of growing structures toward or away from a stimulus.
Tropisms in animals are movements of the entire body toward or
away from a source of stimulation.

CHAPTER XXIV

MOVEMENT RESPONSES IN PLANTS AND
ANIMALS

Movement Responses in Plants. — Growth responses in ani-
mals differ significantly from those in plants in that they are not
much concerned with adjusting the organism to its environment,
but rather, with bringing about its completed development, and
hence they involve chiefly the response of one part of the animal
body to another. Plants, on the other hand, effect most of their
adjustment to the environment through growth and find little
need for movement responses. To be sure, swimming movements
similar to those in the Protozoa are found among the flagellated
algae and the sperm cells of all except the seed plants and the
terrestrial fungi, but only a few of the higher plants display move-
ment responses at all. What movement response does occur among
the higher plants is exemplified by the folding of the leaves of
various plants in response to light or to touch. The leaves of
the common sorrel are spread out flat in moderate light, so as
to expose the greatest possible surface to the activity of photo-
synthesis; but either in excessive light and heat or in darkness
the leaflets are folded down closely against the leaf stalk, thereby
reducing water loss from evaporation. In the "sensitive plant"
of the tropics, a similar folding occurs whenever the plant is
touched or shaken. In this case the leaves fold up so rapidly that
their movement is easily watched, and strong stimulation of one
leaflet will cause an impulse to travel through the plant in a few
seconds, making the leaves fold up in succession.

The mechanism for this type of response consists in a group
of large cells at the base of each leaflet, which normally are
filled with wrater at high osmotic pressure, by means of which
they hold the leaf up. Upon stimulation, they lose this water
rapidly and collapse, causing the leaflet to drop. The mechanism

5-9

520 Movement Responses in Plants and Animals

by which the impulse is carried through the plant is not clearly
understood.

Similar folding movements occur in insectivorous plants, as
described in Chapter XV. The mechanism for these is apparently
the same. In other plants, such movements are found in the
stamens or the stigma, and help in pollination.

Movement Responses in Animals. — Although the funda-
mental distinction between the plant and the animal is that the

Unstiaukted lorf

Stimulated leaf

FIG. 118. — Movement response in sensitive plant. (Redrawn from Smith, Overton,
et a/., Textbook of General Botany, The Macmillan Company.)

latter ingests its food, the most noticeable difference between the
two kingdoms is the immobility of plants and the mobility that
animals have developed in their constant search for material to
ingest. No one who has looked through a microscope at the
teeming, restless world of the Protozoa can doubt that, from its
lowest forms upward, the animal world is a world of movement.
We have already mentioned the major forms of animal move-
ment— the rapid jerk of the striped muscles, the powerful, slug-
gish contraction of smooth muscles, the waving of minute cilia,

Movement Responses in Plants and Animals

521

the snapping of flagella, and the queer, formless flow of ameboid
pseudopodia.

The most interesting aspect of animal movements is the man-
ner in which the impulses which set them into action are carried
from receptors to effectors, being integrated on the way so as
to produce patterns of movement that result in the most delicate
adjustments of the animal to its environment. In a few simple
forms, such as Ameba, there are no ascertainable structures spe-
cialized for conduction and integration.
But even in the lowly Paramecium, tiny,
threadlike pathways of conduction exist,
stretching from the anterior end of the
organism, where the protoplasm is most
sensitive, to the cilia in all parts of the
body. These conductors are invisible in
the living animal, but when the cell is
stained, they appear in the form of mi-
nute fibrils. Near the fore end, these
fibrils come together to form a network,
known as the motorium ; and it has been
shown in Protozoa that are similar to
Paramecium, that when the motorium is
removed, the activities of the cilia lose
coordination. Thus, even in this simple
one-celled organism, there is an organ
of coordination similar to our brain.

The Nerve Net of Hydra. — As the
multicellular animal body evolved from
its precursor, the protozoan colony, the

development of conducting and integrating structures had to begin
all over again ; and it is not surprising to find that in the lowest
form of multicellular animal, the sponges, there are no true nerv-
ous structures whatever. In Hydra and the other coelenterates,
however, there is a complete network of nerves which extends
throughout the entire body of the animal. (See Fig. 119.) There
is no separation between cells in this nerve net. Thin fibers ex-
tend all the way from one cell body to another, and there are
no synapses at which impulses may be held up or passed along,
depending upon the dominant activity of the animal at the time.

FIG. 119. — Nerve net in
Hydra.

522 Movement Responses in Plants and Animals

But while the selective action of the synapses is absent in Hydra,
a certain degree of coordination of response is effected simply
by the fact that the nerve net stretches throughout its entire body.
Thus, if the tentacles of the animal are touched by the point of
a needle, sensitive nerve fibers in the ectoderm will be stimulated
and will start impulses which move throughout the network to
contracting cells in nearly every part of the body. The result is
that both the tentacles and trunk contract, and the tiny animal
draws itself up into a ball. The distance that a stimulus will
travel through the network from the point of stimulation seems
to depend upon the strength of the stimulus, so that a light touch
upon one of the tentacles may result in the retraction of the
tentacle only, rather than of the entire body.

In the human body, the impulses for the peristaltic movements
of the digestive tract are carried down the tract by a nerve net
unconnected with the central nervous system and spreading
throughout the muscles of the walls, much as Hydra's nerve net
spreads throughout its tissues. And these simple, unvarying peris-
taltic movements, together with the few other types of move-
ments of the alimentary canal, suggest the simplicity of behavior
that must characterize organisms possessing no synaptic nervous
system. Hydra can wiggle and squirm about, take in food, con-
tract its tentacles and its body to escape harm, and make a few
other responses, but its repertoire of behavior is very limited.

The Nervous System in the Earthworm. — In the earthworm
we discover a nervous system that in many fundamental ways is
like that of man and of the other higher animals. Instead of a
spinal cord running from head to tail on the dorsal side of the
body, the worm has a nerve cord running the length of its body
just inside the lower, or ventral surface. In each segment of the
worm, the cord enlarges to form a ganglion which contains a
number of synapses. At the anterior end of the body, the cord
divides into two branches which pass on either side of the pharynx
and come together in a ganglion which serves as a sort of brain.
Nerve trunks, composed of the fibers of sensory and motor neu-
rons, run out from each ganglion to the various parts of the body;
and connector neurons run from ganglion to ganglion throughout
the entire system.

rThe nervous system of the earthworm is like ours in that it

Movement Responses in Plants and Animals

523

is made up of neurons joined to one another by synapses, thus
making co'mplex integrations possible and allowing for consider-
able variety of responses. It differs from ours in that the nerve
cord runs along the ventral rather than the dorsal side of the
animal, and integration is carried out in a chain of ganglia, one
in each segment of the worm's body, rather than being centered
in a single great anterior ganglion, or brain. Hence, the activity
of the organism as a whole is not as completely unified, nor can
the organism respond appropriately to as complex an environ-
mental situation. To a large extent, this is dependent upon the
fact that the chief sensory regions of the worm are not as com-
pletely centered about the head as in the vertebrates. When we

Nerve
trunk

Brain ganglion

Nerve trunks

Segmental ganglion Ventral nerve cord

FIG. 120. — Anterior portion of nervous system of earthworm.

consider that sight, hearing, taste, and smelling in human beings
are all located in the head region, we can realize why this region
has become such an important center of integration in man. In
the worm, sense organs are found in somewhat greater numbers
in the head and tail regions, but there is no such tremendous
concentration as occurs in human beings. The animal is sensitive
to vibrations and pressures, to chemicals, and to light; but the
entire surface of the body is sensitive to all these stimuli.

The Vertebrate Nervous System. — The nervous system of
the earthworm contains all the fundamental features of the human
nervous system. There are neurons with long axons or dendrites,
each neuron joined to others by means of synapses. The synapses

5^4

Movement Responses in Plants and Animals

are located within a central nervous system in definite regions of
integration (the ganglia) and these regions of integration are
connected by regions of conduction through which the axons or

Eyespot

Notochord

Dorsal nerve cord

FIG, I2i. — Amphioxus.

dendrites carry impulses from one part of the central nervous
system to another or between the central system and the sense
organs and muscles. But only in the vertebrates, or in their rela-

PRIMITIVE FISH

AMPHIBIAN
FIG. 122. — Vertebrate brains.

lives among the chordates, do we find a nervous system that is

built upon the same general pattern that we find in human beings.

The basic features of this pattern are to be found in Amphi-

QXUS, a transparent, fish-like creature, about two inches in length,

Movement Responses in Plants and Animals 525

which swims about close to the shore and burrows in the sand
at night. It is not itself a vertebrate, but it probably represents
better than any other living form what the immediate ancestors
of the vertebrates were like. Like the earthworm, its central nerv-
ous system is composed of a long cord of nervous tissue running

REPTILE

BIRD

MAMMAL
FIG. 123. — Vertebrate brains (continued).

from front to back, with nerve trunks branching from it. Unlike
the earthworm, however, the cord runs along the dorsal side of
the animal, above the alimentary canal rather than below it, and
there are no segmental ganglia. The only appearance of anything
like a ganglion is a slight enlargement of the cord at its anterior
tip. This enlargement is the beginning of the vertebrate brain

526 Movement Responses in Plants and Animals

which, as one ascends the vertebrate series from the fishes up
through the amphibians and reptiles to the birds and mammals,
becomes larger and larger until, in the human species, it reaches
its apex in size relative to the size of the animal and contains by
far the greater portion of the nervous tissue in the body.

Figs. 122 and 123 show five types of vertebrate brains. The
brain of the fish is little more than a brain stem with a cere-
bellum added. In each of the succeeding higher forms, the cere-
brum increases in size relative to the brain stem, until in the mam-
mals it is larger than all the rest of the brain put together.

The Evolution of Behavior. — When we leave the study of
the nervous systems of animals and begin to consider their be-
havior, we find that they fall roughly into three groups, as fol-
lows :

1. The lower organisms, including Paramecium, Hydra, the
earthworm, and many other forms. They are characterized by a
relatively simple repertoire of responses to the stimuli which af-
fect their sensitive structures, and they possess a limited capacity
to vary these responses in case their first reactions to a situation
fail to satisfy the motives which the situation, together with the
internal conditions of their bodies, arouses in them. When Ameba,
for example, is moving through a dimly illuminated region and
comes upon a beam of strong light, it withdraws the first pseudo-
podium that enters the beam, but immediately puts forth another
one, which, on entering the beam, is likewise withdrawn. After
a few of these protrusions and withdrawals, it changes its be-
havior completely, begins to .put forth pseudopodia on the side
opposite the light, and thus moves away from the beam. Its first
responses fail to adjust it to the situation, so it modifies or varies
its behavior until it does become adjusted. Obviously, this is a
simple form of trial and error activity. Another, more permanent
way of modifying behavior is for an organism to learn to respond
in a new way to a situation with which it has had previous ex-
perience. In the lower organisms learning, if it takes place at all,
occurs very slowly, and probably quite infrequently. Nevertheless,
the modifiability of behavior as seen in the lower organisms marks
the evolutionary beginnings of human intelligence.

2. The insects, together with their relatives, such as the spiders,
crabs, and other arthropods. They display a marked advance over

Movement Responses in Plants and Animals 527

the lower organisms with respect to the complexity and variety of
their behavior patterns, but their capacity for modifying these
patterns is only a little further advanced than that of the lower
forms. They adjust to the environment almost entirely by in-
stinct.

3. The vertebrates. The vertebrates display a complexity of be-
havior as great as or greater than that of the insects, but, in

FIG. 124. — Nervous system of fly. (Redrawn from Herrick's An Introduction to
Neurology, W. B. Saunders Company.)

addition, they show a marked capacity for modifying their re-
sponses which has increased tremendously throughout the course
of vertebrate evolution, until it has reached its greatest develop-
ment in the behavior of man.

The difference between vertebrates and insects in this respect
seems to be related to a difference in the arrangement of gray
matter in their nervous systems. The nervous system of the insect
is arranged on the same basic pattern as that of the earthworm.
There is a ventral nervous cord with a chain of ganglia, and a
large brain ganglion located above the alimentary canal. The
chief differences are that the ganglia are larger, there are not so
many of them, and the brain ganglion forms a more important

528 Movement Responses in Plants and Animals

part of the nervous system. In such a nervous system as this, the
synapses are located in little clumps within the ganglia. There is
no widely spread surface of gray matter, as in the human cerebral
cortex. Apparently clumps of gray matter are best for integrating
unmodifiable, instinctive behavior, while sheets of gray matter
are essential for modifiable, intelligent behavior. Thus, in our
own bodies, the relatively unmodifiable reflexes are integrated in
the spinal cord and brain stem, while the cortex is the seat of
intelligence. Even among the higher vertebrates, we find that the
cerebrum of the bird, although it is fairly large and contains a
good deal of gray matter in solid clumps inside the white matter,
has a very poorly developed cortex, relative to that of the mam-
mal. Correlated with this is a very high development of bird
instincts, as seen in the intricate architecture of their nests and
their long seasonal migrations, together with a development of
intelligence considerably inferior to that of the mammal, although,
to be sure, it is much superior to that of the insects or even of
the lower vertebrates.

The Unchangeability of Insect Behavior. — A good exam-
ple of the complex but unmodifiable behavior of the insect is
found in the nest-building activity which characterizes many kinds
of wasps. The mason wasp of India builds a dome-shaped cham-
ber of mud, leaving a hole at the summit of the dome. She then
pushes her abdomen into the hole and fixes an egg near the top
of the dome, whereupon she makes many flights through the
surrounding countryside, capturing live caterpillars, one after an-
other, and depositing them in the chamber. When it is full she
closes the hole and flies off, soon to die. The wasp larva, after it
has hatched from the egg, feeds on the live caterpillars which
its mother has provided for it.

Here is a cycle of behavior which provides for the preserva-
tion of the species of the mason wasp with an exquisite neatness.
Compared with the limited complexity of response in such an
animal as Hydra, it is astonishingly elaborate. Each step in the
process fits into the preceding and succeeding ones in such a way
as to make the most complete preparation for the welfare of the
offspring which the mother wasp herself will never see. From
casual inspection it would appear that the wasp is anxiously look-
ing forward to the birth of her child and leaving nothing undone

Movement Responses in Plants and Animals 529

to insure that it will be well cared for until it is able to shift for
itself. Actually, the wasp knows nothing about what she is doing.
She goes about her task quite blindly, and is unable to make the
slightest change in her preordained cycle of behavior if some ob-
stacle is raised in the path of its successful completion. A striking
instance of this incapacity to modify behavior is related by Major
Kingston, who has carefully studied the behavior of many in-
sects.1 He writes :

I cut away the top of a cell before the wasp fixes her egg. The
breach involves that spot in the dome to which the egg is always at-
tached. What will happen now? The wasp will be unable to satisfy
her instinct. For the one spot of attachment is gone. We wait until
the time for egg-laying arrives. The wasp comes, puts her abdomen
into the cell and brings it to the correct spot. She feels for the surface
against which to lay. The surface is gone so she withdraws her abdo-
men. She gets very agitated. There is something amiss. Again she tries
it. Again failure. She gets more and more impatient, evidently burst-
ing with the impulse to lay. Now we see the unyielding rigidity of
her forethought. There is plenty of space within the dome. She might
fix her egg anywhere, to the sides, to the floor. Just the slightest
deviation to the right or to the left and the wasp will find plenty of
space. But she will not do this on any account. Her instinct permits
of no such deviation. It has been ordained that the egg shall be
anchored only at the very top of the cell. The wasp makes more
efforts then more withdrawals. A time comes at last when she can
wait no longer. She must get rid of her egg. Were does she lay
it? Exactly in the place where it should be laid, that is, in the very
top of the cell. But of course there is nothing to which she can fix it.
Hence it is shot into the air and tumbles down to the bottom of. the
cell. Here we see instinctive foresight carried to its extreme degree.
If the wasp would deviate a fraction she would find plenty of spots
for anchorage. But she stubbornly refuses to make any deviation.
Instinctive foresight demands one spot. No other spot is of any
account.

The behavior of insects has always struck the human beings
who have studied it as remarkable and almost uncanny. It is so
different from our own. Our own activity, as described in Chap-
ter XXII, is goal-directed. Consciously or unconsciously, we are

1 Kingston, R. W. G., Problems of Instinct and Intelligence, chap. iii. By
permission of The Macmillan Company, Publishers.

530 Movement Responses in Plants and Animals

always aiming to attain certain ends; and if some unforeseen
circumstance interrupts our progress toward a goal, we begin to
try new ways of approaching our destination. We proceed by trial
and error, either overt or implicit, until we succeed. Insect be-
havior seems to be directed toward certain goals; but when we
study it, we discover that it is simply a chain of responses, one
following another, without any real directedness toward the end
results of the chain, and if a single link is broken, the entire chain
falls to pieces as far as attaining results is concerned. Thus the
5pider, which, while it is not a true insect, is so closely related
to insects in structure and behavior that it is usually mistaken
for one, constructs a web of the most perfect geometrical design,
simply by making one response after another. If a single thread
in the net is cut, the spider can never go back to mend it, but must
continue its chain of responses, although the result it then achieves
is simply a formless tangle of threads.

But stupid as the behavior of insects may appear to be, it still
arouses our wonder because the insect can do so much without
ever learning how. No human being could create a net as perfect
as the spider's web without much training, yet the spider builds
its first net as perfectly as it does any other. The entire cycle
of activity whereby the wasp provides for the future of its off-
spring occurs only once during its life, and, barring accidents, is
carried on quite successfully that first and only time. Such com-
plex, unlearned chains of behavior are called instincts. Human
beings are very poorly endowed with instincts. It is doubtful that
even such simple performances as the sucking response 'of the
infant or the activity of walking are wholly instinctive in human
beings. Even these universal human patterns of behavior may
require a certain amount of learning to bring them to perfection.

How Learning Occurs. — Learning usually occurs as a result
of motivated or goal-directed activity. This may be illustrated by
what happens when a cat is placed in a cage from which it must
learn to escape. Fig. 125 shows such a cage. In order to open
the door, the cat must pull out one latch by stepping on a lever
attached to a string ; a second latch must be removed by clawing
at the string to which it is attached, and, finally, one of the
wooden bars in front of the door must be pushed upward. A
hungry cat is placed in the cage, and a bit of fish is put outside.

Movement Responses in Plants and Animals 531

Of course, the cat is not endowed with any instinct which enables
it immediately to perform the three acts that permit it to get out
of the box. But both the motive to escape from confinement and
the motive to get the fish cause it to respond in a variety of
ways that seem to be directed toward the goals of the motives.
"It tries to squeeze through any opening; it claws and bites at
the bars or wires, it thrusts its paws out through any opening
and claws at everything it reaches; it continues its efforts when

FIG. 125. — Animal puzzle box. (Redrawn from Thorndike's Animal Intelligence,
The Macmillan Company.)

it strikes anything loose and shaky; it may claw at things in the
box."2

Finally, almost by chance, it happens to perform all three of the
acts that liberate it. The next time it is placed in the cage, the
successful acts are likely to be performed sooner. Gradually the
unsuccessful acts come to be inhibited, and the successful acts are
reinforced whenever the cat is placed in the box, until finally it
escapes almost immediately whenever it is placed in the situation.

Practically all learning is of this nature. An animal is sub-
jected to a motive which it cannot immediately satisfy. It varies
its behavior until it finally arrives at its goal. With repeated ex-
periences in the situation, the responses which bring it to its goal

2 Thorndike, E. L., Animal Intelligence, The Macmillan Company, New York,
1, p. 35-

532 Movement Responses in Plants and Animals

are strengthened and those which fail to bring it to its goal are
weakened, until it learns to arrive at the goal directly.

Sometimes, when the situation is simple, the animal learns al-
most immediately to make the goal-directed responses. In one type
of experiment, for instance, a hungry rat is put into a small box
which contains a lever and a receptacle for food. If the rat de-
presses the lever, a small pellet of food drops into the receptacle
and the rat's hunger drive is momentarily satisfied. When the
rat is first put into the box, a considerable time may intervene
before the lever is depressed. The rat responds to the situation
by moving around restlessly, smelling of this and that, grooming
its coat, and so forth. Then, in the course of these random ac-
tivities, it strikes the lever and receives the food. A much shorter
time will now usually intervene before the lever is pressed again,
and soon the rat is pressing the lever almost as rapidly as it can
eat the pellets. The fundamental principles governing this learning
are the same as those which govern learning in the cat. The re-
sponses which satisfy a motive are strengthened, those which fail
to satisfy it are weakened, so that when the same motive is active
in a similar situation, the responses which formerly satisfied the
motive occur immediately, rather than at the end of a train of trial
and error.

Behavior Patterns. — Just as every species has its own special
pattern of bodily structure, so each species displays a certain pat-
tern of behavior which distinguishes it from others. In closely re-
lated species, to be sure, we find behavior patterns which show a
general relationship to one another, but there is something unique
in the form of behavior of each species. Thus, nearly all birds
build nests, but each species has its own particular pattern of nest
building. Reproduction in all species of placental mammals fol-
lows a single general pattern, involving copulation between male
and female, carrying of the young in the mother's body, birth,
suckling, and other care for the young; but each species mates
and cares for its young in a manner of its own, which is correlated
with the social life or the lack of it in that species. Wild horses,
for example, live in groups known as harems, in which a single
stallion is followed by several mares, and the young grow up
under the protection of this group. Domesticated horses, turned
loose upon the range, will revert to this form of family life.

Movement Responses in Plants and Animals 533

Among certain wild cattle, on the other hand, males and females
ordinarily live in separate herds which join each other only dur-
ing the mating period, and the young grow up within the female
herd. Among many species of mammals and birds, a monogamous
family life develops; but, depending upon the species, this may
last for only a few days, the female being charged with all the
care of the young, or it may last for a season, with the male as-
sisting in the care of the young, or it may last over a period of
years.

In more specific ways behavior patterns vary from species to
species. A horse rises to its feet fore legs first ; a cow, hind legs
first. A rabbit stands beside its food and nibbles; a rat may do
this, or it may hold its food in its fore paws while eating. A
chicken takes a sip of water and throws back its head to swallow;
a dove dips its bill into the water and sucks it up, swallowing with
its head down. This cataloguing of the behavior patterns of species
could be endless. However, enough has been said to show that
species are differentiated by their behavior as much as by their
anatomy.

The Maturation of Behavior Patterns. — In the preceding
chapter it has been pointed out that the development of animal
structures takes place through a complex interaction between the
organism and its environment. Part of this growth produces the
structures which carry on our responses, namely, the muscles,
glands, sense organs, and nerves. As the response system de-
velops, certain capacities for response develop. For instance, the
young bird cannot fly until its muscles and the nerves connected
with them reach a certain stage of development; the young kitten
shows no tendency to hunt for prey until a certain stage of physi-
cal growth is reached; and complete sexual behavior does not
appear in most organisms until their bodies have grown almost
to the adult stage. This development of behavior which parallels
and is dependent upon the growth of the response system is called
maturation.

When the capacity to make a response is dependent upon the
development of the sense organs — as when young kittens must
wait to respond to visual stimuli until their eyes are open — or
upon the development of the muscles — as in the newborn child,
whose muscular development is not sufficient to enable him to

534 Movement Responses in Plants and Animals

walk — the fact that the development of behavior depends upon
physical growth is easily apparent. It is probable, however, that
the most significant type of physical growth for maturation of
behavior is the development of synaptic connections in the nervous
system. The nervous system grows in size faster than any other
part of the vertebrate embryo, and at a very early stage of de-
velopment, all or nearly all the cells that will ever be formed will
have appeared. But the complete growth of axons and dendrites
to make synaptic contacts proceeds slowly, and probably is not
completed until adulthood is reached.

This development of synaptic relationships is a difficult thing
to observe, but through the study of the tadpoles of a certain
salamander it has been shown that synaptic development exactly
parallels the maturation of certain behavior patterns. When the
tadpoles first begin to develop from the eggs, they lie motionless
in the water, getting nourishment from the yolk of the egg, which
remains attached to their bellies. Gradually, the yolk grows smaller
and the tadpole larger, and soon the little animal must be able
to swim around and secure its own food. The first sign of a swim-
ming movement is a turning of the head to the right or left
when the skin of the head is touched. The animal always moves
its head away from the side on which it is stimulated. This move-
ment appears quite suddenly. Before its appearance, you may
stimulate the skin of the head as much as you please, but there
is no response. An hour later, you may go back and touch the
head, and the animal twists to the side. What has happened during
that hour? Studies of the response system before and after the
appearance of the turning movements show that previous to the
movement both the sense organs and the muscles involved are
completely developed. There are sensory neurons passing from
the sense organs to the brain stem, and motor neurons from the
brain stem out to the muscles. There are also connector neurons
which are in synaptic contact with the motor neurons, but whose
dendrites have not yet developed. Immediately after the twisting
response occurs, it is discovered that the dendrite-s of these neurons
have grown out and made contact with the sensory neurons from
the skin, thus enabling impulses to pass from the skin on one
side of the head across to the muscles in the fore part of the

Movement Responses in Plants and Animals

535

body on the other side. The result is that a response having a very
definite pattern suddenly appears. (See Fig. 126.)

This twisting response is only the beginning of the total swim-
ming response, which involves the waving back and forth of the
head and tail, with the tail always moving toward the side op-
posite to the direction of movement of the head. Progress toward

•Brain

Muscle
Segment

FIG. 126. — Maturation of response in salamander. The figure on the left shows
the bending response that is made possible by the maturing neuron shown in the
figure on the right. (Redrawn from Coghill's Anatomy and the Problem of Be-
haviour, Cambridge University Press.)

the complete swimming response occurs in stages, and each stage
is marked by a definite new development of nervous connections.
We can say that the swimming response pattern grows with the
growth of the nervous system.

The Role of Learning in the Development of Behavior. —
To the casual observer, the development of swimming behavior
in the tadpole might not appear to be very different from the
development of skill in a human being who is learning to swim.
To begin with, the movements are very imperfect, and they grad-
ually improve until the act can be carried on successfully.

536 Movement Responses in Plants and Animals

perimental study has shown, however, that the two types of
behavioral development are far different with respect to their
causes. One is produced by maturation, the other by learning.
Learning is always brought about by exercise of the function of
response — what we commonly refer to as practice. The animal,
or human being, usually strives to attain a certain goal, and
sooner or later its behavior is modified so as to enable it to arrive
at the goal more directly and easily. But the maturation of the
swimming response in the tadpole occurs as readily without ex-
ercise of function as with it. This has been demonstrated by an
experimenter who placed tadpoles in water containing a drug
which rendered them completely incapable of movement through-
out the period of maturation of the swimming response. All dur-
ing this time they did not so much as wiggle the tips of their
tails; but when they were placed in pure water and the effects
of the narcotic had worn off, they began to swim just as well
as tadpoles that had apparently been practicing all the time. Thus
it was shown that learning has nothing to do with the establish-
ment of the swimming pattern of response in tadpoles. It is de-
pendent upon neural growth that has not been stimulated by
exercise of function.

We find that among birds and mammals learning and mat-
uration usually work together to produce the behavior of the
organism. Maturation seems to provide the organism with cer-
tain necessary motor skills (swimming, walking, seizing food
etc.) and certain goal-directed motives. Learning may result in
improvement of the skills, and determines the way in which the
motives will be satisfied. Chicks, for example, begin to peck for
food as soon as they are born. At first, however, they will peck
at almost any small object on the floor. But soon they begin to
avoid inedible objects — that is, objects which do not satisfy the
hunger drive — and to direct their efforts entirely toward objects
that are good to eat. This selection of means of satisfying the
hunger motive results from learning. It is an illustration of how
learning occurs through the dropping out of responses that fail
to bring the organism to its goals and the firm establishment of
responses that do bring the organism to its goals. Furthermore,
research indicates that the accuracy and skill of the pecking re-
cponse are improved by learning as well as by maturation. If the

Movement Responses in Plants and Animals 537

chick is kept from pecking for four or five days after birth and
fed by forcing food into its mouth, it does not immediately begin
to peck as accurately as chicks that have practiced throughout the
five days. To be sure, maturation has progressed during this time,
since the chick learns accurate pecking more rapidly at the end
of the five days than do chicks who begin to practice at birth.
But some practice is required in order to perfect this skilled act.

Instincts and Species Habits. — The behavior patterns of in-
sects are doubtless almost entirely the product of maturation with-
out the intervention of learning. In other words, they are true
instincts. But it seems highly improbable that the behavior of
mammals, their hunting, mating, and other activities which are
frequently spoken of as instinctive, are truly so. The instinctive
responses — that is, those resulting solely from maturation — prob-
ably appear as imperfect parts of the behavior patterns that are
finally developed, and the perfecting of each pattern is dependent
upon learning. For this reason, it is better to speak of the dis-
tinctive behavior patterns in animals as species habits rather than
as instincts.

To be sure, they are not like the habits of a trained dog that
has been intentionally taught to perform tricks, or of a human
being whose whole scheme of behavior is determined by the in-
tentional or unintentional education he receives from others.
Species habits are not taught to an animal; it learns them nat-
urally through its interaction with its natural environment. As a
general thing, neither its parents nor other members of its species
"show it how" to do things; it simply learns how by trial and
error, as the cat learns to escape from its cage or the rat learns to
get food by depressing a lever. Indeed, the learning of species
habits may be looked upon as merely a continuation of the natural
process of development which begins with the formation of the
zygote, and continues always as an interaction between the or-
ganism and its environment. Learning is a developmental reaction
to exercise of function under the spur of a motive, just as the
growth of the eye is a developmental reaction to the organizing
influence of the embryo retina within the type of environment
that permits the growth to take place. For we must suppose that
the change that takes place in the organism when it learns some-
thing is basically a change in the structural pattern of the neurons

538 Movement Responses in Plants and Animals

or of the protoplasm within the neurons. To be sure, no such
changes have ever actually been observed. What we observe in
learning is a change in the responses made to certain stimuli; but
if such a change is to take place, there must be back of it an in-
crease of resistance at certain synapses and a decrease at others,
and such changes in synaptic resistance must be fundamentally
dependent upon changes in the structure or chemistry of the
neurons. Learning, then, is really a mere part of the total process
of growth and differentiation in the organism.

In using the term species habit, rather than the older term
instinct, to describe a type of behavior characteristic of a species,
it is essential to guard against the underestimation of the great
part that maturation plays in the production of most species habits.
Studies of the rat show that both sexual behavior and nest building
are carried out in practically the same way upon their first ap-
pearance as after practice. Of course, both these forms of be-
havior appear at a time of life when the animal has had an
opportunity to practice most of the individual responses that go
to make up the total pattern. The truth is that in many cases it is
extremely difficult to discover whether a certain sort of behavior
is the product of maturation alone or of maturation plus learning.
For this reason it is desirable to use the term species habit for
any characteristic form of behavior unless it has been definitely
proved that it is not the result of learning, in which case the term
instinct may properly be applied.

Do Animals Think? — Nearly everyone has at some time met
the belligerent gentleman who proudly asserts that his dog "has
got more sense than most humans." He is likely to give you the
discouraging impression that he would have a great deal more
respect for you if you could only manage to be as smart as his dog.
As a matter of fact, it is usually impossible and quite unjust to
compare one type of animal with another with respect to mental
ability. It is highly improbable that any human being could equal
the skill of the spider in constructing a web upon the first attempt.
When it comes to making friends and influencing people — even
to influencing their estimates of one's mental endowment — few
men or women possess a talent equivalent to the cordial and flat-
tering tail-wagging of the canine species. And psychologists have
discovered that in the task of learning how to thread a maze of

Movement Responses in Plants and Animals 539

passageways, the lowly white rat is quite as capable as most hu-
man beings.

Each species seems to have its own special abilities which ade-
quately adjust it to its environment, and, within the range of these
abilities, it usually shows a great deal of "good sense." Neverthe-
less, there seems to be good reason for believing that the human
species far transcends any group of animals in certain aspects
of intelligence. The old philosophers called man "the reasoning
animal"; and it is true that, in his ability to think, man differs
tremendously from any of the lower animals. To be sure, many
people will dispute this assertion and insist that animals are able
to reason as well as human beings, but this point of view is the
result of jumping to conclusions on the basis of an inadequate
observation of animal behavior.

For instance, one close student of animal behavior came to the
conclusion that cats could reason because he saw a cat open a
door by jumping up and catching hold of the handle of the latch
with one paw while depressing the button on the latch with the
other, at the same time kicking and scratching with its hind paws
against the door post so as to push the door open. This observer
concluded that the cat must have sat down and looked the situa-
tion over and reasoned in advance that this method would open
the door for it. But actual observations of cats in situations simi-
lar to this one show that in solving such a complex problem they
learn by overt trial and error, rather than by the implicit trial
and error that we call thought. The observer did not see how
the cat learned to open the door, and hence jumped to the false
conclusion that reasoning was responsible for it.

The cognitive adjustments of animals seem to be almost en-
tirely of a perceptual nature. There is some evidence that they
may have rather dim ideas of objects outside the range of their
senses ; but that they can manipulate these ideas in a trial-and-error
fashion to develop new ideas, as we do in thinking, has never been
proved. The nearest thing to thinking that has ever been observed
in animals is the implicit problem-solving that is displayed by
chimpanzees. For instance, a chimpanzee is placed in a cage with
a banana or orange hanging from the roof. A box is also placed
in the cage about eight feet from the point where the food hangs.
The ape first tries to reach the fruit by jumping for it, but finds

54O Movement Responses in Plants and Animals

it impossible to do it. It paces up and down the cage, and then
suddenly goes over to the box and, picking it up, places it near
enough to the fruit to enable it to jump from the box and secure
this food. It is clear that the animal does not at first perceive the
box as something that may be used to get the fruit. At the same
time, once it does so perceive the box, it uses it immediately, with-
out any overt trial and error. Whatever trial and error has taken
place must have been of an implicit sort. But even here the animal
is only rearranging its perceptual adjustments. It is not required
to use anything outside the immediate range of its senses. Animals
may be clever enough on the instinctive level of adjustment. They
may learn rather rapidly by means of overt trial and error, but
careful study indicates that they show only the barest rudiments
of ability to think.

Animals Have No Language. — Animals are not inferior to
human beings in what is commonly called the "sense of direction,"
as indicated by the ability of rats to learn mazes and by the re-
markable abilities shown by both birds and mammals in finding
their way about to and from their homes. They are definitely in-
ferior, however, in their capacity to appreciate spatial relation-
ships and the possibilities of using tools. Most of all, they are
incapable of learning a language and using that language as a
tool of thought. Animals do have certain sounds which they make
whereby they stimulate one another to various forms of activity.
There are mating calls, growls and snarls which signify a readiness
to attack, and cries of warning which, uttered by a single mem-
ber, may send an entire herd of animals into a stampede. But
they lack words that stand for or signify objects. They have only
a system of signs, not a true language. Thus they can have only
the haziest sort of concepts. Words can make ideas and concepts
nearly as definite and as capable of being dealt with as objects
that are placed before our eyes or in our hands; and human be-
ings are able to think efficiently chiefly because they are able to
talk to themselves, while animals, lacking this capacity for im-
plicit verbal behavior, can make fine adjustments only to things
that directly affect their sense organs.

Furthermore, words can stand for abstractions and generaliza-
tions. An animal may learn to respond differently to live animals
than to dead ones, but only by the use of the words "life" and

Movement Responses in Plants and Animals 541

"death" can one have a clear-cut concept of the difference be-
tween the two classes of objects. And only by a gradual extension
of the meaning of the word "life" to things which grow and re-
produce as well as to things which move about, can the real dif-
ferences between the organic and the inorganic world finally come
to be recognized. Thus, by his tool of language, man can, as it
were, "get a grip" on the world in which he lives. He is no longer
condemned to adjust only to situations immediately present, nor
to concrete objects and relationships. He can come to know the
world in an entirely different way from that in which an animal
knows it. Gradually, by adding one word to another, or by en-
larging the meanings of the words he possesses, he builds up a
body of knowledge and wisdom which can be passed on to his
offspring. He is no longer a "dumb" brute but a "beast that rea-
sons."

CHAPTER SUMMARY

Movement responses in plants occur chiefly in the flagellates and
the sperm cells of the higher plants, although a spreading or fold-
ing of the leaves occurs in a few of the higher plants in response
to light or pressure.

In animals movement response is almost universal, and even
in the Protozoa the structures are arranged so as to integrate
movement responses. In Hydra and its relatives, the nervous sys-
tem is in the form of a nerve net which extends throughout the
body. In the earthworm, there is a ventral nerve cord running
the length of the body. Integration takes place in ganglia which
are located in each segment along the cord. In the vertebrates,
the central nervous system is composed of a dorsal cord with a
single ganglion at the anterior end, known as the brain. The size
of the brain relative to the cord increases tremendously in the
evolutionary series from fish to man. The cerebrum and especially
the cerebral cortex increase out of all proportion to the increase
in the rest of the system.

Instinct is the capacity to respond adaptively to a situation by
a pattern of response that has not been modified by learning.

Intelligence is the ability to respond adaptively by varying re-
sponses to a given situation and by learning to perform the suc-
cessful variations.

542 Movement Responses in Plants and Animals

The lower animal organisms display slight complexity of in-
stinctive response and a very low degree of intelligence.

The insects have very complex instinctive responses, but only
a low degree of intelligence.

The vertebrates display a relatively high degree of intelligence
which comes to preponderate more and more over their instinc-
tive capacities with each advance in the evolutionary series from
fish to man.

Learning usually occurs in connection with trial and error at-
tempts to reach a goal. The trial responses that successfully bring
the organism to its goal are strengthened so that they occur more
readily the next time the organism is placed in a similar situation.

Instinctive patterns of behavior develop through growth of the
response system — which, as pointed out in the preceding chapter,
results from the interaction between the organism and its environ-
ment. The gradual development of these patterns is called mat-
uration.

Subsequent to the development of a pattern through matura-
tion, it may be improved or otherwise modified through exercise
of the function of response, that is, through learning.

By the combination of maturation and learning, animals de-
velop characteristic species habits which are the product of the
interaction between the normal environment of the species and its
inherent capacities for development. Since it is difficult to de-
termine whether or not a given pattern of behavior results from
maturation alone, it is better to apply the term species habit to a
characteristic behavior pattern whether it is thought to be purely
instinctive or not.

The capacity for thinking in animals — if it exists at all — is
greatly inferior to man's, chiefly because cultural tradition sup-
plies man with a language that enables him to work with precise
conceptions of objects and situations that are not present to the
senses.

QUESTIONS

1. Describe the movement responses that occur in plants.

2. Outline the evolutionary development of integrating structures in
. animals.

Movement Responses in Plants and Animals 543

3. Illustrate the difference between purely instinctive and intelligent
behavior.

4. Illustrate the difference between the development of a species habit
by maturation alone and by maturation plus learning.

5. What evidence do we have as to whether animals can think ? From
what handicap do they suffer in thinking ?

GLOSSARY

Amphioxus (am'fi-ok'sus) A small, fish-like animal resembling the

supposed ancestors of the vertebrates.
instinct A pattern of response that has not been modified by learning.

The capacity to make such responses.
intelligence The capacity to vary responses in the direction of arriving

at a goal, together with the capacity to learn successful responses.
maturation The development of capacity for response that parallels

the growth of the response system.

motorium Center for integration of responses in certain protozoans.
species habit A pattern or type of response characteristic of a species.

CHAPTER XXV

THE DEVELOPMENT OF HUMAN BEHAVIOR

Species Habits and Culture Habits. — Man, like other mam-
mals, builds upon his maturated responses by learning. The suck-
ing response, which, since it enables the child to sustain life, is
without doubt one of the most important behavior patterns of
our earliest days, is developed in this way. In a child born sev-
eral weeks before its time, this response may be so incompletely
maturated that the young infant must be artificially fed, as the
Dionne quintuplets were fed by means of a medicine dropper. But
usually, before the time that it would normally be born, it will
have learned to carry out the response successfully. In the child
born at the proper date, the response will be much more com-
pletely maturated ; but even then, for the first few times that it is
fed, there is likely to be a certain amount of trial and error be-
fore the response is carried out properly. And although matura-
tion seems to be the chief factor at work in the development of
this pattern of behavior, it reaches perfection only through learn-
ing.

The behavior of the child in picking up objects shows a beauti-
ful series of changes as one pattern of activity develops into an-
other in the course of maturation. At first it merely squeezes
objects between the "heel" of its hand and its fingers; then it
begins to place the thumb opposite the fingers, and gradually it
ceases to use all its fingers, picking the object up between the
thumb and two fingers. Then it ceases to put the flat of its hand
against the object, but picks it up between the. tip of the thumb
and one or two opposed fingers.

It seems fairly certain that these changes in the patterns of
manipulative behavior result from a gradual growth of nerves and
muscles, with probably some degree of perfection being added
through practice. Both the sucking response and the manipulative

544

The Development of Human Behavior 545

behavior of the infant are essentially species habits, which would
probably develop in any child placed in almost any kind of en-
vironment. In this respect, they are unlike most of the habits
which human beings acquire. For unlike animals, we do not grow
up in a natural environment, but in one that is produced in large
degree by the traditional ways of acting that characterize the cul-
ture of our group. Without that cultural tradition, our behavior
would undergo an entirely different train of development from
that which actually takes place. Most of the habits which we
acquire are not species habits, but culture habits \ and they vary
from one culture to another, although some of them, such as
speaking some sort of language, using fire, and possibly the habit
of walking on the hind legs (it is not certain whether the latter
should be classified as a species habit or a culture habit) are char-
acteristic of all human cultures, are not found among the animals,
and would in all probability fail to develop in human children
brought up entirely out of contact with human culture.

The Development of Language. — The most fundamental of
all culture habits is language. Language and culture must have
grown up together, since language is the chief means by which
culture is transmitted from one generation to another. The fact
that human beings develop language and other mammals do not,
seems to be due to the fact that human babies maturate a form
of behavior which can only be described as "playing at making
noises. " They babble and crow from morning till night, and, to
add to the sport, will produce more or less adequate imitations
of the sounds they hear. Certain birds show this form of be-
havior, and doubtless it is their tendency to play with sounds and
imitate them that enables such birds as parrots and crows to
learn to croak a few creditable imitations of human words. Birds,
however, are too lacking in intelligence to grasp the significance
of words, and their speech is a mimicry which, to them, is entirely
meaningless.

Among the higher mammals, on the other hand, some capacity
to understand words seems to be present. A few years ago, the
experiment was tried of rearing a young chimpanzee in the com-
pany of a human child, treating it in every respect like a true
member of the family. The experiment extended over a period of
nine months, from the time the little ape was eight months ar\ji

546 The Development of Hitman Behavior

Hie child eleven months of age. During this time, the ape learned
the meanings of many words that were spoken to it. But it never
learned to utter a single word. Unlike the little boy, the ape sel-
dom made sounds just for the fun of it. It would make a special
noise when it wanted something to eat, a sort of bark when it
was angry, a screech or scream when it was afraid, and a whim-
pering "oo-oo" sound that seemed to take the place of the fretting
of a human infant. Always the sounds seemed to be the result
of special external or internal stimuli. The ape never vocalized
just for the sake of hearing its own voice. Furthermore, while the
child learned to make noises like the ape, the compliment of imita-
tion was never returned. Strong efforts were made to get the ape
to repeat words, but there was never any success. At the same
'ime, in most activities requiring some degree of intelligence, the
chimpanzee was approximately equal to the child, the difference
in their ages being taken into account.

The babbling of articulated sounds, which begins in the average
child at seven or eight months of age, seems to be the maturated
behavior pattern on the basis of which language behavior de-
velops. Imitation is probably acquired as a habit. The child hap-
pens by chance to make the same sound more than once, falls
into the habit of imitating itself and then gradually into the
habit of imitating others. Having learned to imitate, it can begin
to learn to apply words to specific objects or situations. In learn-
ing to speak words, it may be motivated in three ways. First, the
use of words may enable it to signal its wants to others or to
attract their attention. Second, the child discovers that words go
with things, and it may find naming things an entertaining pas-
time, just as it enjoys handling them and throwing them around.
Finally, the use of words may secure the coveted approval of the
child's parents.

From the very beginning of language development, children will
use words as stimuli to themselves as well as to others. The writer
has seen a little girl of eleven months, who, upon approaching
a dangerous situation, would say "No-no," and then draw back,
whether she was conscious of the presence of others or not. The
two-year-old child will babble to himself as he plays with his toys,
throwing down his engine with the remark, "Naughty train !"
a,rid in other ways dramatizing all his activities to himself. Three-

The Development of Human Behavior 547

and four-year-olds will talk aloud to themselves after being left
alone in bed at night, sometimes continuing to do so for an hour
or more. They are indulging in the "overt daydreams" of child-
hood. Gradually the child learns only to murmur the words, and
then merely to "think them to himself/' And at this point, the
system of implicit self -stimulation which we call thought has com-
pletely developed.

The Development of Motives. — A few years ago it was the
fashion among psychologists to ascribe the motives of men to
so-called "hereditary instincts." The term instinct was used to
describe not a maturated pattern of behavior, such as the swim-
ming of tadpoles or the nest building of wasps, but an "inborn"
desire to reach certain goals. Every desire or impulse on the part
of the human being was attributed to the activity of one of these
instincts.

There have been few careful experimental studies of the de-
velopment of the major human motives. However, our knowledge
of the way behavior develops in mammals, and in human beings
especially, gives us good reason to believe that, aside from physio-
logical drives, such as hunger, thirst, sexual desire, and the like,
together with a general tendency to be active and to explore the
world, and a few not very highly differentiated emotional tend-
encies, our motives do not maturate, but are developed by learn-
ing through the interaction between the environment in which we
live and the few motives that are supplied by maturation.

There is little doubt that the most important of all motives in
the social life of human beings is the desire to appear well in the
eyes of others, to secure their approval and admiration. It is im-
probable that such a motive is truly instinctive. Rather, the infant
is placed in a situation where, to satisfy his physical needs, to
avoid punishments, to secure the emotional satisfactions that are
derived from petting and other displays of affection, he must cause
others to react favorably toward him. Day in and day out, situa-
tions wherein he is dependent upon the good will of others recur,
until he comes to feel a reassurance whenever others approve of
him and a sense of anxiety when they disapprove; and pleasing
others comes to be an end in itself, rather than a means of secur-
ing other satisfactions.

A more easily observable motivational development is tfce

548 The Development of Human Behavior

growth of possessiveness in young children. One of the outstand-
ing characteristics of young children is their tendency to play
with a great variety of objects present in their environment. An
eighteen-months-old child placed in a roomful of toys will pick
up one toy after another, look at it, handle it, put it down, and
go on to a new object of interest. If two such children are placed
in the room together, an interesting development takes place.
Without paying much attention to each other, they begin their
process of exploration. Any toy picked up by one child, however,
is thereby likely to be called to the attention of the other, who
reaches for it, only to be thwarted in his attempt by the first child
who is intent himself on exploring this object. Conflict ensues,
and, for a while, that one toy becomes the chief center of interest.
Let this go on for a few days, and a complete change in the be-
havior of the children can be noticed. They are no longer in-
terested in playing with the toys, but only in monopolizing them,
and they spend all their time trying to take toys away from each
other. Possession, which was at first only a means to the end of
enjoying the toy as a plaything, comes, in the course of learning,
to be an end in itself.

Conditioned Emotions. — Without doubt, many human mo-
tives develop through this process of the transformation of a
means of arriving at a goal into a goal in its own right. Another
form of motivational learning, and one which has been demon-
strated by carefully controlled experiments, is the conditioning
of emotions. The classification of emotions, on the basis of either
outside observation or personal introspection, is really an ex-
tremely difficult task. In an approximate fashion, however, it may
be said that the emotions of fear, anger, and love are the ones
that maturate in human beings. But the objects toward which we
display these emotions depend to a great extent upon learning.

The emotion of fear, for example, is aroused, prior to learn-
ing, by unexpected, violent stimuli, such as a sudden loss of sup-
port, a loud noise, and the like. Believers in the existence of
elaborate human instincts have claimed that children are instinc-
tively afraid of furry animals. This has been attributed to the fact
that animals constituted a real and ever-present danger to our
primitive ancestors ; hence we have inherited a fear of them. Ac-
tually, a child brought for the first time into the presence of an

The Development of Human Behavior 549

animal, whether furry or not, usually displays interest and even
delight, rather than fear. It has been shown experimentally, how-
ever, that if a child is given a white rat to play with and at the
same moment a sudden loud noise is made, causing the child to
be startled and frightened, it soon learns to fear the animal. Here
we probably have an explanation of the fact that a large number
of children actually do show fear of animals. A child goes up to
an animal and starts playing with it. The animal makes a sudden
move, jumps against the child, barks, squeals, or frightens it in
some other manner, and the child learns to be afraid of animals.

This type of learning is somewhat different from the type de-
scribed in the foregoing chapter. In the first type there is a
strengthening of responses that lead to goals and a weakening
of responses that fail to lead to goals. In this second type — called
conditioning — a stimulus, known as the unconditioned stimulus
(the loud sound, for example), is capable of arousing a certain
response (the fear response). The unconditioned stimulus is pre-
sented along with a second stimulus (the animal) in such a way
as to secure the response. After one or more such presentations,
the response is made to the second stimulus as well as to the un-
conditioned stimulus. The second stimulus is then referred to as
the "conditioned stimulus," and the response as a "conditioned
response."

Probably many of the emotional reactions and prejudices of
human beings are attributable to emotional conditioning of this
sort. A man of thirty-five was puzzled to understand why the
name "Stella" aroused in him a feeling of aversion or disgust.
Finally he managed to remember that when he was only four or
five years of age, his family had engaged a servant girl of none
too pleasing appearance who was discovered to be afflicted with
lice. Her name was Stella.

Conditioning occurs in both animals and men. Simple reflexes,
especially, seem to be particularly susceptible to it. If a bell is
rung, for example, just before a dog is fed, he will soon begin
to salivate at the sound of a bell. The food is the unconditioned
stimulus for the secretion of saliva, the bell becomes the condi-
tioned stimulus. Perhaps the reason emotions are conditioned so
readily is that they are essentially reflex responses.

Because conditioning occurs in simple reflexes, many psycholq-

550 The Development of Human Behavior

gists have come to believe that it is the basic unit of learning, that
the most complex forms of learning are merely combinations of
many conditioned reflexes. The assumption underlying this theory
is that our more complex respons.es are actually combinations of
many simple reflexes. This assumption is itself dubious, and the
theory that the conditioned reflex is the unit of learning has never
been established.

As a matter of fact, conditioning takes place under some cir-
cumstances and fails to take place under others, and we do not yet
understand why. For instance, if a child is given blocks to play
with, and a loud, frightening noise is made at the same time, the
youngster develops no conditioned fear of the blocks. Indeed, it
is perfectly obvious that conditioning cannot take place every time
two stimuli are presented together, and hence the problem of the
fundamental cause of learning remains as much a problem as ever.

The Cultural Determination of Motives. — Because of the
possibility of modifying motives through learning, our motives
are determined to a great extent by the culture in which we live.
Let us consider, for example, the possessive or acquisitive mo-
tive as it develops in our culture. To go back to the two children
who learn to monopolize rather than to play with their toys, it
is unlikely that their monopolistic struggle would proceed very
far before the cultural tradition would begin to interfere with it
in the form of elders who would make such remarks as, "No,
Freddy, you mustn't take the ball from Johnny, that's Johnny's
ball," or older children who would shout, "You can't either of
you have that, that's mine." Soon "mine" would come to be a
magical word to both children. It would be a sign whereby they
might expect to establish monopoly over an object by means of
a simple vocalization, rather than a bitter struggle. Many children
learn to say "mine" before they learn to say "mamma" and
"dada," or any other word. In acquiring this verbal sign, they
are developing a concept that has been handed down by genera-
tions of cultural habit, the concept of the right to personal mo-
nopoly of objects so that others can make use of them only upon
the sufferance of the owner. Gradually the child learns that he is
allowed to apply this magic sign only to certain objects ; that he
is expected to take good care of these objects, not to lose them
qr let them be destroyed; and that the more of such objects that

The Development of Human Behavior 551

he possesses, the greater is the respect and admiration that he
may expect from others. He learns also that the key to the mo-
nopoly of all sorts of objects, together with the power to control
the actions of others and to win their respect, is the right to say
"mine" with reference to certain metal disks and pieces of paper —
in short, that the possession of money is the open sesame to the
satisfaction of nearly all his desires. No wonder that, under these
circumstances, the acquisition of personal possessions and their
display before the envying eyes of others become the chief pre-
occupation of a very large proportion of the individuals in our
society, and that in a few, known as misers, the possession of
money becomes such a monomania that they fail to seek any other
satisfaction in life.

It is cultural tradition, not inborn nature, that provides people
with the strong possessive motives that we observe in ourselves
or in our friends. In other societies, the desire for ownership
does not develop nearly as strongly as in ours. Property is in the
hands of a family or tribe, and there are few things that the in-
dividual feels to be his own. Suppose that, as young Johnny and
Freddy began to develop their habit of attempting to monopolize
toys, no one had said to them "mine" or "yours." Or suppose
these magic words had applied only to the momentary possession
of a toy. It is yours as long as you are using it, then you have
no more right to control what happens to it. The two boys would
gradually learn not to quarrel over the possession of their toys,
just as we learn to respect the property rights of others. But the
whole idea of permanent ownership would be absent, and hence
they would never develop the desire to own things that charac-
terizes ourselves and our friends. Possessiveness, as it develops
in our society, is neither an instinct nor a species habit; it is a
culture habit or a social attitude. A social attitude may be defined
as an habitual way of thinking and feeling about things, together
with the motives that such thoughts and feelings imply, that is
developed in human beings as a result of the traditional attitudes
they find in their social environment. In brief, it is a socially
determined motive.

Language constitutes an important tool in the development of
social attitudes. The words "mine," "yours," "his" provide us
with concrete symbols that define a certain way of feeling and

The Development of Human Behavior

thinking about objects; and by learning their meaning through
experience with the way other people react toward them, we are
provided with a set of motives, namely, to seek to add to our
own property, to protect the things we do possess, and to respect
the property rights of others. Our attitudes toward property con-
stitute only a part, though it is an important part, of the great
mass of socially conditioned attitudes that make life what it is.
Our moral laws, our political institutions, our religious and our
family life — in short, the entire pattern of human behavior — is
determined by the attitudes that we develop in the course of con-
tact with human society.

The behavior of a wild animal is pretty much a function of its
genetic constitution. Given a certain assortment of genes, it is
practically predestined to develop a certain set of behavior pat-
terns, whether through maturation or learning. The behavior of
a human being, on the other hand, is determined chiefly by the
culture in which he is reared. Given exactly the same sort of
hereditary materials to begin with, a human being reared among
the Australian bushmen would develop an entirely different set
of behavior patterns, and an entirely different set of attitudes,
than one reared in a large city. And if there was ever a human
society developed where the people crawled on all fours, spent
most of the time in the water diving for fish, and lived in bur-
rows along the banks of streams, the young human beings born
into it would doubtless take to that life quite readily; standing on
the hind legs would be viewed as an inexcusable breach of taste,
and human nature would come to resemble the nature of the otter
more closely than it resembles the nature of present-day human
beings.

The Modifiability of Human Nature. — Of course, no na-
tion or tribe of "otter men" has ever existed to anyone's knowl-
edge, nor is such a society likely to develop. Men can find much
more interesting and comfortable modes of life than that of the
otter. But the contemplation of the possibility of such a society
is worth while, since it emphasizes the tremendous extent to which
culture can modify human behavior. A large number of people,
when faced with a proposal for any social change more far-
reaching than an increase in the tariff on cucumbers, will im-
mediately assert that such a modification is impossible because

The Development of Human Behavior 553

•'you can't change human nature." The belief that the motives
and ways of behaving which characterize the group in which
one lives are universal and eternal characteristics of human nature
is a fallacy which seems to be very easy to develop.

The following are excerpts from a speech made by an Indian
medicine man at a time when a missionary was attempting to
persuade his people to abandon the custom of cannibalism :

In all ages, as far back as the memory of the oldest man can reach,
enemies killed in battle have been eaten and prisoners fattened into
proper condition for killing. When a custom is so ancient, it is not
dependent upon the will of men. It is not an accident of their history,
but a law of their nature, instituted by the gods themselves. Hearts
too tender may deplore it, but against natural fatalities it is vain and
puerile to wish to fight. . . .

Repudiate, then, Oyampis, these new ideas. Anticannibalism is a
doctrine essentially chimerical. Men have always eaten one another;
they will continue to do so in the future as they have in the past. . . .

Similarly, many people are convinced that, since war has al-
ways been the method of settling disputes among tribes and na-
tions, it is contrary to human nature to settle them in any other
fashion. But psychologists who have devoted their lives to the
study of human nature are of a different opinion. A few years
ago the following question was sent to all the members of the
American Psychological Association : "Do you as a psychologist
hold that there are present in human nature ineradicable instinc-
tive factors that make war between nations inevitable ?" Out of
nearly four hundred answers there were only ten which assented
to the proposition.

This does not mean that there are no instinctive factors at work
in the production of human nature; it merely means that human
culture builds upon those instinctive factors, so that the pattern
of human behavior becomes something that is almost entirely
a product of that culture. For good or ill, we become whatever our
society makes of us; and when the cultural pattern of our society
changes, as it surely will, the "human nature" of our descendants
will change to fit it.

Maturation of Capacity to Learn. — Heretofore we have
spoken of maturation as if it applied only to the development of
specific instinctive patterns of behavior which might be improved

554 The Development of Human Behavioi

upon by learning or used as a basis for other learning. But along
with the development of the nervous system there goes a gradual
maturation of capacity for learning without any apparent matura-
tion of new behavior patterns. In learning things requiring both
memorization and logical understanding, the capacity to learn in-
creases rapidly up to about sixteen, when the rate of increase
falls off gradually until about the age of thirty, at which time the
capacity has reached its peak. There is then a gradual falling off
up to the age of forty-five, when learning capacity is about equal
to that of the sixteen-year-old. This is, of course, contrary to the
old saw that "you can't teach an old dog new tricks' ' and the idea
that children learn more rapidly than adults, although there is
reason to believe that the curve of learning ability falls off rather
rapidly from fifty onward. The causes for the changes in the
curve after the years of adulthood are reached are not certain;
but there can be little doubt that the rapid rise up to the age of
sixteen is due to development of the neurons in the brain, and,
hence, that it represents a true maturation.

Because of this factor of maturation, it is frequently useless to
attempt to "push" children too rapidly in the acquisition of
knowledge and skills. For instance, at a time when a certain pair
of identical twins was about twenty months old, twin C was sep-
arated from the other and given an intensive vocabulary drill.
Twin T was isolated from all contact with other children, and no
words were spoken in her presence. Then at the end of five weeks
twin T was given training exactly like that of twin C, but at
this more advanced stage of maturation she learned much more
rapidly. The same principle holds for the learning of motor skills.
When the twins were forty-six months old, twin T was given an
intensive six weeks' course in stair-climbing. At the end of that
time the training of twin C was begun. Within two weeks C had
learned more than T did in the entire previous six weeks.

There can be little doubt that much time and effort are wasted
in our educational system in the attempt to teach children things
which are too difficult for them, but which they could learn readily
enough at a later stage in their development. Most children are
very poor at writing during the first few grades of school. Rec-
ognizing that this is probably due not to lack of practice, but
rather to incomplete maturation of the capacity to learn, many

The Development of Human Behavior 555

schools do not encourage children to write in script during the
early years of schooling, but allow them to print their letters until
they have developed to the stage where script writing is fairly
easy for them to master.

The Learning of Skills. — Among the important things we
gain from our social heritage is a considerable equipment of skills.
The basic skill, of course, is the ability to speak and understand
language. There are also motor skills, such as writing, handling
tools, playing various games, typing, telegraphy, and the playing
of musical instruments, together with such non-motor skills as
reading, receiving telegraphic messages, and the like.

In addition to the maturation of the capacity for learning, an
important /actor in the mastery of skills is the presence of a
strong motive to learn. It is utterly untrue that "practice makes
perfect," unless practice is accompanied by a motive of some
sort. For example, in order to graduate from a business school,
a stenographer may be required to learn to typewrite sixty word?
per minute. In the course of six months' training, she may arrive
at this level of skill. Now if she goes to work in an ordinary
business office, she may continue to practice typing for several
years without any increase in speed. In spite of continual prac-
tice, she learns nothing. But suppose at the end of all these years
she becomes ambitious to secure a job which requires her to type
at the rate of ninety words a minute. As she does her work she
will continually try to increase her speed, and in a relatively short
period she will have reached the goal of ninety words per minute,
although the amount of time spent per day in typing may not
have changed.

The type of motivation that is active here is different .from
that which produces learning in animals. The goal in animal learn-
ing is usually food or the satisfaction of some other physiological
motive. In much of human learning, the immediate goal is simply
to learn, although there is usually some motive back of this,
whether it is to get a better job or simply to prove to oneself how
capable one is at learning. Here again, the possession of language
produces a difference between human and animal behavior. It is
doubtful that animals ever realize that they are learning, since
they have no word for it. And without a word for it, the concept
of learning must be nebulous indeed. The obvious convenience of

556 The Development of Human Behavior

getting an organism to learn by simply telling. it to do so, rather
than being put to the necessity of arranging some external mo-
tive to persuade it to learn, doubtless helps to account for the
fact that men are able to learn so much more during their lives
than are animals.

When the goal of learning is simply to learn, information con-
cerning progress helps considerably to speed the rate of learning;
and if an individual who is engaged in acquiring an act of skill
keeps close account of his progress, always striving to improve,
his learning will proceed most efficiently.

Few human beings ever arrive at the level of skill of which
they are capable, simply because they fail to note whether they
are improving or not, have a comfortable feeling th#t "practice
will make perfect," and do not struggle to improve, once they
have attained a level of achievement that enables them to "get by."
Most students could considerably increase their efficiency in read-
ing and thus save themselves much wasted time, if they would
maintain a continual effort to improve in this direction. When
reading fairly easy material, read as rapidly as possible, keeping
track of how many pages per hour you are able to read. Your
progress may not be amazingly rapid, but if you keep a record
of it, you will probably have the satisfaction of seeing your read-
ing efficiency gradually increase to the point where you will be
making a material saving in the time required for you to study
a lesson assignment.

Acquiring skills of this sort is always a gradual procedure.
Improvement takes place rather slowly, and there are many fluc-
tuations, so that on one day an individual may be less skilled
than he was the day before; hence progress can be noted only
in terms of weeks or even months. Fig. 127 shows curves of
progress in learning to send and receive telegraphic messages.
These curves show that, even though records were kept in terms
of weeks, there were some weeks in which achievement was lower
than in the week preceding. If the record were kept in terms of
days, the fluctuation would be even greater. This figure also shows
that on the curve for receiving very little gain was made between
the twelfth and the twenty-fourth week. Such a portion of a
learning curve is known as a plateau. Plateaus have an uncom-
fortable way of appearing in learning curves. Weeks may pass

The Development of Human Behavior

557

while the individual makes little progress and usually feels very
bored and discouraged. Then suddenly he begins to forge ahead
again at a rapid rate. As the individual approaches his physiologi-
cal limit, that is, the utmost of his capacity, in a certain skill, the
rate of improvement gradually decreases, until finally he can im-
prove no further. The sending curve in Fig. 127 apparently ap-
proaches the physiological limit of the subject. Actually he stopped

140

130

120

110

100

90

80

70

60

50

40

20
10

z

140

130

120

110

100

90 |

80 I

70 §

60 |

60 3

40

30

20

10

8

12

28 32

36

40

16 20 24
Weeks of practice

FIG. 127. — Learning curve for telegraph operating. (Redrawn from Ruch's
Psychology and Life, Scott, Foresman and Company.)

his practice on a long plateau. An operator on a "fast wire" must
send much more rapidly than this.

The Principles of Efficient Study. — A most important part
of our cultural tradition is the body of knowledge which has been
built up through man's age-long search for understanding of him-
self and of the world about him. Since the readers of this book
are at present chiefly engaged in the task of acquiring some small
part of this vast store of learning, we shall devote the remainder
of this chapter to a practical consideration of how to study effi-
ciently.

Study is the name we give to intentional learning on a con-
ceptual level. Efficiency in study is dependent, first, upon apply-

558 The Development of Human Behavior

ing the principles that hold good for all sorts of mental work,
and, second, upon applying principles that are especially concerned
with conceptual learning. Among the more important principles
belonging to the first group are the following:

1. Work according to a plan. This principle involves more than
simply having a schedule of hours for work and play. When you
sit down to work, you should know as completely as possible just
how long you are going to work and what you plan to accomplish
in that time. Then you should do everything in your power to
accomplish that amount during the time you have allotted for it.
You should have a schedule of what is to be done each week,
and allot ample time for doing it. Life is so constituted that the
best of plans must be continually modified, but that does not alter
the fact that people who work according to plan and struggle
against having to alter their plans get the most done.

2. Dont plan to work too many hours in a day or a week.
Most people when they plan their work get into a very heroic
mood. Usually they have fallen behind in what they were doing
and wish to catch up. A college student will plan to study ten
or twelve hours a day, but the one who can actually stick to such
a schedule is one in ten thousand. Even if he does, he would
probably get more done if he worked only seven hours a day
and really worked during those hours. It has been shown that,
when the hours in a factory are reduced from twelve per day
to eight or nine, more is actually produced in a week on the
latter schedule than on the former. On the whole, concentrated
effort over a short period of time is more efficient than work
that is dragged out over so long a time that one has no leisure
to enjoy life. Individuals differ greatly in the amount of time
during which they can work effectively; but a college student
who will schedule forty hours a week, including time spent in
classes, and then actually works during those forty hours, .is
headed for Phi Beta Kappa if he has even a moderate amount of
intelligence.

3. Schedule study periods that are neither too long nor too
short. Studying is a task for which we "warm up" slowly. Most
people probably do not reach their peak of efficiency for half an
hour or so. Little is really known about it, but the writer's own
experience suggests that fatigue from study begins to appear after

The Development of Human Behavior 559

about three hours of concentrated effort. This applies only to
those who really "warm up" to their work. If you continually feel
that you would like to quit while you are studying, you are not
warming up at all. Many students never warm up and never gel
to studying efficiently. A definite plan to get a certain amount of
work done during a given period of time is of great assistance
in warming up.

4. Don't plan to study at times when you are fatigued. The
converse is to get enough rest so that you won't be fatigued when
you study. In a group of college students, it was found that those
who said they did not study when they were tired made better
grades than those Spartans who studied in spite of fatigue.

5. Study at times and places where you will be free from dis^
traction. This is of special importance in enabling you to warm
up and stay warmed up. It also eliminates fatigue in study. Fre-
quently a student finds it impossible to be free of distractions
while studying in his own room, and it is necessary to find some
other place. It is possible, however, to allow oneself to be too
easily disturbed by distractions. One should form the habit of
being able to concentrate when there is a certain amount of noise
about, rather than using it as an excuse for failure to concentrate.
The added effort required to shut out distracting noises, while
it is doubtless somewhat fatiguing, may actually help you to
maintain a vigorous, active attitude toward your work.

6. Maintain an active, attentive attitude toward your work.
It is believed by many that the fatigue which accompanies mental
work is almost entirely the product of the slight muscular tension
which seems to be necessary if one is to maintain attention. At-
tention is essentially a set of the type described in Chapter XXIL
It is an implicit posture, but, like other implicit postures, it is
likely to become at least partly overt, and it may in all cases in-
volve some muscular activity. Specifically, close attention seems
to involve a rather general contraction of the muscles throughout
the body. It has been shown experimentally that subjects given
something to grip and hold tightly in their hands learn more
rapidly than those whose hands are relaxed. Muscular tension
seems to be essential to really active mental effort. You will study
best if you sit up straight at your desk, with your whole bodj*
expressive of alertness and determination.

560 The Development of Human Behavior

So much for the necessary conditions for studious work. Now
for the principles of learning that should be applied.

1. Distribute your learning over a considerable period of time.
Everyone knows that a thing that has been recently learned is
remembered better than a thing that has been learned some time
previous — in short, that we tend to forget things as time goes
on. On the basis of this fact, many students jump to the conclu-
sion that the most efficient method of studying for an examina-
tion is to do it all just before the examination is given. This as-
sumption is absolutely false. There is no psychological principle
better established than the fact that distributed learning is more
effective than massed learning. This means, for example, that if
you were given a poem to memorize, told to spend just five hours
in learning it and to have it ready for recitation in three weeks,
the most efficient method of going about it would not be to put
in the entire five hours just before you were to be called on to
recite, but to spend half an hour on it every two or three days,
with perhaps a half hour of practice just before you started to
recite. The amount of time required to learn a thing by distributed
learning is frequently not more than half that required to learn
it by massed learning. In the first extensive experimental study
of memory that was ever made, it was found that a series of
nonsense syllables could be learned as well in the course of 38
repetitions distributed over three days as in the course of 68 repe-
titions at a single sitting. One of the best methods for a college
student to get good grades without working very much is to
schedule a half -hour review period every other day for each one
of his courses. Such a system will make a long period of cram-
ming before examinations unnecessary, and yet will insure a better
command of the subject matter of the course.

2. Practice what you are expected to learn, and always study
with the definite aim of learning that thing. Many students seem
to feel that if they read over an assignment, their brains will
somehow absorb the knowledge that is contained therein. This
is an utterly mistaken conception of the learning process. You
are not being asked to learn to read, you are being asked to learn
to tell about the subject matter with which an assignment deals.
Therefore, you must practice telling about it. Most studying
should be in the form of taking an imaginary examination, re-

The Development of Human Behavior 561

citing to yourself what you have read about, then going back to
the book or to your notes to see if you have succeeded in covering
all the points which have been dealt with.

You should always have a definite picture in mind of what
you are trying to learn. Things are seldom learned incidentally,
but usually under the spur of a motivating factor; and the most
reliable motivating factor for study is a definite intention to learn.

An illustration of the intention to learn is provided by the fol-
lowing experiment. The subject is instructed to read several times
through a list of words, such as that given below, learning to
respond to each word in the first column by saying the word
opposite it in the second column.

Chicago Philadelphia

London Liverpool

Berlin Munich

Paris Bordeaux

Madrid Barcelona

Rome Naples

Calcutta Delhi

Sydney Melbourne

It will not take long for this learning task to be completed.
But suppose, after testing the individual on his ability to say
"Philadelphia" in response to "Chicago," we try to see if he can
respond to "Philadelphia" with the word "London." Hardly any
learning of this sort will have taken place, although the subject
will have read "London" after "Philadelphia" just as often as
he read "Philadelphia" after "Chicago."

3. Learn in terms of general concepts; seek for the meaning
of what you are learning. Without doubt, this is the most im-
portant principle of all, since the real purpose of a college edu-
cation is to acquire general concepts, not to memorize a battery
of unintelligible facts. A single concept can be of tremendous
help in remembering a great range of facts that are related to
that concept. Thoughtless critics often conclude that, since an
individual five years out of college will have forgotten most of
the individual facts that he learned there, his education can have
been of but little use to him. What they fail to notice is that in
the course of a college education a man develops a series of con-

562 The Development of Human Behavior

cepts about the world in which he lives which never leave him
entirely. Five years from now, you will have forgotten most of
the facts you have learned in the reading of this book; but, unless
you are completely uneducable, you will remember the concept
of the cell, of protoplasm, of the gene, of evolution, of stimulus
and response, of implicit response as being equivalent to mental
activity; and these concepts will be ready to function for you
whenever you need to deal with the facts to which they apply.
The reason for including many facts in a college course is that
worth-while concepts are always related to facts. If you try to
learn a concept without at the same time learning the facts which
illustrate it, your concept will be so indefinite and inaccurate that
it will be of no use to you.

A good illustration of the manner in which concepts can act
to relate individual facts is found in the list of cities just above.
The moment the individual realizes that all the words in the list
are the names of cities, he possesses a concept which narrows
down the range of possible responses, and thus facilitates think-
ing of the right response. If then he notices that both the cities
in each pair are located in the same country, the problem of
memorization will be greatly simplified, since he will know that
neither Munich nor Philadelphia can follow London, and he will
have only to remember that Liverpool was the other English city
included in the list. Finally, if he were learning to recite the
entire list, he might be helped by noticing that the countries are
listed in the order of a possible trip around the world; and by
developing a visual image of the itinerary in terms of a world
map, he would be assisted in finding his way from country to
country in the proper order.

An important example of the superior efficiency of learning
that is subject to conceptual guidance is the fact that meaningful
words — words about which one can have conceptions — are much
more readily memorized than words that have no meaning. The
following sentence has twenty-six syllables, but you will probably
memorize it easily in a single reading :

"The quarterback tore around left end and made a beautiful
run down the field for a gain of fifteen yards/'

But see how many times you have to read this twenty-six syl-
lable list in order to remember it •

The Development of Human Behavior 563

* 'Lemon horse lawn brick till city hat forty water put gone blue
never apple cry golf prime classic mint child."

And now count the number of times you have to go over these
twenty-six syllables:

"Gos nof taf bek lal tef zaf dir nug kiz nar miz fod ref pog
gif puz rak paf zik pel gep ror dop jur ket."

Memorize completely all three of these groups of twenty-six
syllables; then, after two days during which you completely avoid
thinking about them, try to repeat them again. The chances are
that you will remember most about the first, although you will
have spent a much shorter time in committing it to memory.

The use of this principle in carrying on efficient study is ob-
vious. The big thing is to understand thoroughly the material that
you are reading and to organize it, in so far as possible, into a
logical, meaningful idea or set of ideas. There are many ways in
which this can be done. One of them is to try to summarize each
chapter you read in a single sentence, then to make sub-summaries,
and so on down until you have covered the chief details of the
chapter. Another way is to make a set of diagrams which express
in terms of lines and drawings the outstanding ideas in the chap-
ter, while a third method is the good old formal outline. Many
very successful students do not bother with writing out their or-
ganization of a chapter, but you will nearly always find that they
carry a good outline in their heads. An excellent habit for making
sure that you are getting the meaning of a book is to attempt to
think of illustrations of all the principles laid down, and, if the
author gives illustrations, to think up others of your own. Another
excellent idea is to talk your lesson assignments over with fellow
students to compare your understanding with theirs. This practice
also helps to motivate study and make it more interesting and
somehow more "real."

A special application of the principle of learning things in a
meaningful way is the advantage in learning a foreign language
vocabulary in terms of sentences or phrases, rather than by mem-
orizing the definition of isolated words. Everyone of course rec-
ognizes the value of a special vocabulary list which can be
systematically reviewed. The best way to form such a list is to
place each word to be remembered in a short sentence or phrase,
with an English translation just below. Then in going over th*

564 The Development of Human Behavior

list, repeat each sentence or phrase a few times. Thus you learn
how the word fits into the language in a meaningful way, and you
are much more likely to grasp its meaning when you see it in
another sentence than you would be if you had simply memorized
an English equivalent. An additional advantage in this method is
that it hastens the process of learning to think in the new language,
rather than having to translate into English in order to get the
meaning.

CHAPTER SUMMARY

Human behavior develops through learning on the basis of mat-
uration, but since culture exerts a great influence on what we
learn, most of our habits are culture habits rather than species
habits. Speech is the most basic culture habit. It develops out of a
maturated "playing with sounds." Words are learned because of
the advantages their use brings to the child. Thinking is de-
veloped through the use of words to stimulate the self and through
their gradually becoming implicit.

New motives are learned when a means to the attainment of
a goal becomes a goal in itself. Emotions are sometimes condi-
tioned when a stimulus originally incapable of arousing the emo-
tion is presented along with a stimulus capable of arousing it, with
the result that the emotion comes to be aroused by the foimer
stimulus.

Motives are culturally determined through the development of
social attitudes. The definition of situations through language is
an important aspect of this development. So great is the influence
of culture in producing motives that changes in culture may bring
about very great changes in "human nature."

The capacity to learn, as well as definite behavior patterns, de-
velops through maturation.

The goal involved in the acquisition of skills is frequently the
conscious desire to learn. Unless this or some other motive for
learning is present, mere practice of the skilled act may fail to
result in improvement. Improvement in skills is usually gradual
and fluctuating.

Efficiency in study — i.e., intentional learning of concepts — can
be attained by intelligent planning o* one's work, together with

The Development of Human Behavior 565

attention to the following rules which apply especially to the learn-
ing process :

1. Distribute learning over a considerable period of time, al-
lowing intervals between practice sessions.

2. Practice the thing you wish to learn — not some other ac-
tivity— and always study with the definite aim of learning that
thing.

3. Learn in terms of general concepts; seek for the meaning
of what you are learning.

QUESTIONS

1. Discuss as completely as possible the role of culture in determining
the behavior patterns of a human being.

2. Discuss the problem of efficiency in learning.

GLOSSARY

conditioned response A response made to a conditioned stimulus.
conditioned stimulus A stimulus which conies to arouse a response

through being presented along with an unconditioned stimulus for

the response.
conditioning A form of learning in which a response comes to be

elicited by a conditioned stimulus.
unconditioned stimulus A stimulus which arouses a response prior to

conditioning.

CHAPTER XXVI

THE BEHAVIOR OF THE INDIVIDUAL

Emphasis upon the Individual. — In the last chapter, we have
presented the picture of the great, impersonal force of cultural
tradition seizing upon man's gradually maturating capacities for
learning, to create the pattern of human behavior. In thus doing,
we have overlooked the fact that, in reality, there is a special pat-
tern of behavior for every human individual. Nor have we viewed
the process of cultural acquisition from the point of view of the
individual. The average man does not look upon his life as an
arena for the interaction between two such abstract entities as
maturated capacity and social tradition. Rather, he sees it as a
struggle to make good, to attain to the standards that he has set
up for himself, to please his friends and, if possible, his im-
mediate family and his relatives. In this chapter, therefore, we
shall deal with the differences between human beings, and with
the problems that face the individual man in his attempts to lead
a life that is interesting and satisfying.

The Mental Test. — Each human personality is such an amaz-
ingly unique affair that to study the differences between individuals
would seem an almost hopeless task. Yet this is the task to which
psychologists have been increasingly addressing themselves since
the day in 1905 when the Frenchman, Alfred Binet, introduced the
first intelligence test. To be sure, they have not as yet succeeded
in seizing upon and subjecting to measurement all the myriad
facets of human individuality. Nevertheless, by employing the
device of the standardized mental test, they have made real prog-
ress in that direction.

The mental test is simply a systematic and carefully constructed
device for sizing people up. Anyone called upon to make an esti-
mate concerning a man's personality will attempt to put him to

566

The Behavior of the Individual 567

some sort of testing procedure. He will ask him questions and
seek from his answers to make a guess as to what sort of man
he has to deal with. The standardized test does this, but it does
it systematically, on the basis of carefully recorded experience,
and hence it arrives at a more dependable estimate of a man than
can be secured in any other manner, except, perhaps, by the study
of the individual's lifetime achievements.

Men have always felt a need for some method of arriving at
an evaluation of the qualities of themselves and of others. So
great has been this need that astrologers have come forward to
satisfy it by reading men's fate in the stars, and phrenologists
have attempted to estimate character by the contour of the skull.
All sorts of magical means of diagnosing personality have been
developed, and many cults of character-reading have attracted a
wide following. Careful scientific investigation, however, has
shown that a man cannot be judged by the stars under which
he was born, or by the bumps on his head, or by his complexion
or the shape of his nose. The only dependable device for measur-
ing individual differences is a properly constructed mental test.

To be sure, a mental test is nowhere near as precise an instru-
ment of measurement as a yardstick, a spring balance, or a ther-
mometer. Practical decisions on the basis of mental measurement
should always be made under the guidance of a psychologist who
knows enough about the inaccuracy of the device to make the
proper allowance for it. Yet, in spite of their imperfections, mental
tests are being employed more and more widely because they have
been found to be of definite use in practical situations.

For example, a few years ago a great electrical company called
upon a psychologist to see if he could do anything to reduce acci-
dents occurring in their switching stations. Errors in switching
current loads from one line to another sometimes resulted in loss
of life and frequently caused economic losses running into many
thousands of dollars. The psychologist spent several months care-
fully constructing a test to measure proneness to accidents of the
specific kind that were occurring in this company's substations.
When the test was finally completed, all applicants for jobs who'
fell below a certain score were rejected, and low-scoring men
already on the job were shifted to other types of work. As a result.

568 The Behavior of the Individual

the incidence of accidents fell from about thirty per month to
only three or four.

Such "tailor-made" tests as this, aimed at measuring fitness
for a certain job, can be of widespread use throughout the world
of business and industry. Other more general tests are of help in
directing young people toward the types of occupations for which
they are likely to be most fitted — although it should be emphasized
that interpretation of the tests for this purpose should be in the
hands of a trained and experienced vocational counselor who will
always take other factors than a mere set of test scores into con-
sideration in giving his advice. Psychological tests have come into
widest use in the field of education, where they can be employed
to help make decisions as to the proper educational program for
the individual student, as well as being put to use for many other
purposes.

There are scores of different kinds of mental tests : tests of
mechanical, musical, and artistic abilities ; tests of the individual's
attitudes, interests, and feelings; and tests of traits of personality.
But without doubt, the most widely used and successful of all
have been the tests of intelligence. It is for this reason that the
real beginning of the science of mental testing dates from Binet's
first intelligence test, although many other tests had been intro-
duced prior to that time. And because of the outstanding impor-
tance of the trait of intelligence, together with the fact that a fairly
adequate means of measuring it has been worked out, we shall
confine our discussion of individual differences to differences in
intelligence.

What Intelligence Is. — In Chapter XXIV, intelligence has
been described as the capacity to adjust to new situations and to
learn. Among human beings this capacity shows itself most clearly
in the ability to deal with symbolic situations, with words and
with mathematical symbols, and the ability to perceive or conceive
of spatial relationships. For instance, it takes a certain degree of
intelligence to state adequately the difference between "justice"
and "mercy"; here one is dealing with words. Or it takes intelli-
gence to complete the following series of numbers, placing in the
two blanks the numbers that would appropriately follow to com-
plete the sequence :

4 16 8 64 12 144

The Behavior of the Individual

569

Here one is dealing with mathematical symbols. A problem in
spatial relationships is shown in Fig. 128.

The outstanding ability of a small proportion of the population
to deal with complex symbolic situations has provided us with the
achievements in literature, philosophy, and science which underlie
the difference between our civilization and the primitive cultures
of savage tribes. The average man could never have made these
essential contributions, and it may be truly said that civilization
depends upon the work of a small but highly intelligent minority
of human beings.

Furthermore, the degree of
intelligence he possesses is of
prime importance to the indi-
vidual. The idiot is so lacking
in intelligence that he is in-
capable of learning to speak a
language and is condemned to
live as a sort of animal parasite
upon the society in which he
is born. The imbecile must
always be cared for like a
child of six or eight. He can
be taught to perform various
simple tasks — washing dishes,
pitching hay, and the like —
but he can never be trusted to

FIG. 128. — Show by drawing in lines
how you would divide the given figure
into four pieces of equal size and identi-
cal shape.

make decisions for himself or

to find his way around in strange surroundings. The moron can
learn an unskilled trade and may be able to take care of himself,
though always in a rather inadequate fashion. But he is unable to
plan for his future, and, unless he is very unadventurous, is likely
to be continually getting into scrapes through his lack of in
telligence and foresight. The person of average intelligence can
carry on a small business, or succeed as a farmer, a clerk, a sales-
man, or a skilled mechanic ; but those who fit into the leading roles
in our society, who enter such professions as law, medicine, or
engineering, who run the large businesses or achieve the major
political offices are for the most part considerably above the average
in intelligence.

57° The Behavior of the Individual

This doesn't mean that one can always judge a man's intelli-
gence by his position in life, for the latter depends not only upon
intelligence, but upon opportunity and upon the non-intellectual
traits of the personality as well. Nevertheless, intelligence does set
limits beyond which certain types of achievement are impossible.

How Intelligence Is Measured. — If you were asked to esti-
mate the intelligence of a person you had never met before, you
would probably begin by giving him problems to solve not un-
like those described a few paragraphs above. If you knew the per-
son, you would probably try to judge his intelligence by the range
of information you knew him to possess, by the ability he had
shown in understanding topics of conversation, or by the way he
had solved various problems that you had presented to him. It is
precisely in this common-sense fashion that the psychologist has
gone about the business of measuring intelligence. The difference
between an intelligence test and a shrewd person's estimate of in-
telligence is simply that the test eliminates certain tendencies to-
ward error and standardizes the whole procedure. In estimating
intelligence, we are likely to be led astray by the social effective-
ness of an individual, by his ability to make a good impression. A
cheerful, active, talkative person or one who is able to look very
profound while agreeing with the opinions of his listeners usually
receives credit for more intelligence than he possesses. Even with
this error out of the way, it is impossible, without investigation,
to know whether the performance of a given task is a sign of in-
telligence or not. Suppose a person is able to solve the problem in
Fig. 128 within five minutes. Just how intelligent does that make
him, relative to the rest of the population? Actually, you have no
way of knowing. Psychologists have worked out exact ways of
measuring intelligence simply by trying out a great many individ-
ual tests — known as items in an intelligence test — to discover
just what percentage of individuals at a given age might be ex-
pected to pass them. They then pick the items that seem to work
best and put them together in a standardized test. Each item is
always presented to each person tested in exactly the same way,
and all answers are scored according to the same plan.

Intelligence cannot be measured in such units as inches and
pounds, starting from a definite zero point. In mental measure-
ments it is only possible to compare one individual with another.

The Behavior of the Individual 571

One way is to measure the intelligence of children in terms of the
average intellectual capacity found at various ages. A child of
six who can succeed on items that are just within the range of the
average child of nine is obviously very intelligent. Only a frac-
tion of one per cent of all children can do this. Another child of
six who fails on all the items that are ordinarily passed by four-
year-olds is obviously lacking in intelligence; in fact, he is defi-
nitely feeble-minded, and even when he is grown he will not be
any brighter than the average child of seven or eight. He will al-
ways have to be taken care of in either a public institution or a
private home.

Intelligence is a product of maturation and learning. It grows
as the individual increases in age, and we express. the degree of
growth in terms of mental age. A child who can just pass the
items that the average seven-year-old passes is said to have a
mental age of seven, whether his age in years (chronological age)
is fourteen or four. The rate of growth — which is a measure of
how bright he is compared to other individuals — is called the
intelligence quotient, or I.Q., and is measured by dividing the
mental age by the chronological age and multiplying by 100.
(Formula: I.Q. = M.A./C.A. X 100.) A six-year-old with a
mental age of six has an I.Q. of 100; one with a mental age of
seven years six months has an I.Q. of 125. One with a mental
age of five has an I.Q. of 83. Unless some special factor enters
in to change his rate of mental growth, an individual's I.Q. re-
mains practically the same throughout the entire period of mental
growth.

An I.Q. of 100 indicates average intelligence. According to
the most recent findings, the distribution of I.Q.'s in the population
as a whole is as follows :

Below I.Q. 68 2 per cent of the population

I.Q. 68 to 83 14 " " " *

I.Q. 84 to 116 68 " » " "

I.Q. 117 to 132 14 " " n " "

Above I.Q. 132 2 " * * *

It will be seen that most individuals have I.Q.'s falling close
to the average, and that very high and very low I.Q.'s are excep-
tional.

Most of the real leaders of society come from the upper 2 jfer

572 The Behavior of the Individual

cent, and most of the inmates of institutions for the feeble-
minded from the lower 2 per cent. Roughly speaking, idiots range
below I.Q. 25; imbeciles, between 25 and 50; and morons, be-
tween 50 and 70. The highest I.Q.'s on record range between 180
and 210. At this level we find the true intellectual genius. A few
cases are on record of children whose I.Q.'s measured 180 or above
and who have now grown to adulthood. In every instance these in-
dividuals have been credited with outstanding creative or scholarly
accomplishments during their late teens or early twenties.

Incidentally, there seems to be no reason for accepting the an-
cient superstition that "precocious" children are likely to come
to no good end. Of course, if one means by precocity that the
child's parents are forever urging it to display its intellectual gifts
to the astonished gaze of the public or that the child is kept so
busy acquiring inappropriate bits of knowledge and skill that it
has no time for play and normal social intercourse with other
children, then, to be sure, precocity may result in tragedy in later
life, not because the child "burns himself out" — as if intelligence
were a sort of fuel which could never be replaced when once it had
been put to use — but because he fails to make a normal emo-
tional adjustment to other people. If by precocity is meant noth-
ing more than that a child is very bright for his age, then the
outlook for his future not only is as good as that for the average
child, it is better. Indeed, it has never been shown that intellec-
tually brilliant children are, on the average, inferior to the run of
the population in any respect. In the most extensive study of the
question that has yet been made, a group of about 600 children
whose I.Q.'s ranged between 135 and 175 were compared with
a group of ordinary intelligence. The former group averaged
somewhat taller, heavier, and stronger than the latter, and they
showed themselves to be superior in tests of moral judgment and
emotional stability.

Is Intelligence Inherited? — For thousands of years, argu-
ment has raged around the question as to whether a man's ability
is fixed forever by his hereditary constitution or whether educa-
tional and other environmental factors are responsible for the
differences between individuals. Believers in democracy have in-
sisted that the deplorable intellectual incompetence displayed by
tHe greater part of the human race is the result of lack of op-

The Behavior of the Individual 573

portunity for mental development, and that, in so far as their in-
nate capacity is concerned, the common people are the equal of
their masters. Aristocrats, on the other hand, have retorted that
the special privileges enjoyed by the upper classes are no more
than right, since they are the people who are born with the ability
to put these privileges to good use. It is clear that personal preju-
dice has entered into the points of view that have been expressed
on this problem. But with the development of intelligence tests
it has been possible to approach it in a more objective fashion,
and more has been learned during the past fifteen years of scien-
tific investigation than throughout the long centuries of philo-
sophical debate.

In the first place, it is definitely certain that intelligence runs
in families. While the offspring of any pair of parents may vary
considerably in intelligence — as would be expected on the basis of
our knowledge of Mendelian inheritance — and while intelligent
parents may occasionally produce very stupid children and unin-
telligent parents very bright children, nevertheless, the children of
intelligent parents are on the average more intelligent than those
of unintelligent people. But this does not prove that intelligence
is inherited in a biological fashion. The children of intelligent
parents are brought up in homes where they receive more intel-
lectual stimulation. Frequently they are accorded better educational
facilities. And if we know no more than the mere fact that intelli-
gence runs in families, we cannot be certain that the superior cul-
tural environment provided by intelligent parents is not solely re-
sponsible for the intellectual superiority of their offspring.

In determining an individual's intelligence, there are three possi-
ble factors at work : ( i ) The genetic factor, the genes that the
individual receives from his parents. (2) The physiological factor,
the conditions of nutrition, hormone supply from the mother dur-
ing embryonic development, and all other factors that affect the
growth and physiological functioning of the nervous system. (3)
The cultural factor, the degree of intellectual stimulus and oppor-
tunity for learning presented to the individual by his social en-
vironment. If we are to know anything about the inheritance of
intelligence, we must isolate these factors.

One excellent way of securing this isolation is to compare the
intelligence of identical and fraternal twins. Of course, twins,

574 The Behavior of the Individual

whether identical or fraternal, are reared in a very similar en-
vironment. Both the cultural and physiological factors will affect
each twin to about the same degree. On the basis of these factors
alone; we should expect both types of twins to resemble each other
very closely. But with respect to the genetic factor, identical twins
are exactly alike, while fraternal twins are different from each
other genetically because, as was pointed out in Chapter XIII, no
one zygote is at all likely to receive the same combination of
genes that is found in any other zygote. If, therefore, pairs of
identical twins are more nearly alike in intelligence than pairs of
fraternal twins, we could explain this greater likeness only on the
supposition that the genetic factor has produced it; hence we
would know that the genetic factor does affect intelligence.

The fact is that identical twins are more nearly alike in intelli-
gence than fraternal twins. They are so much alike that the dif-
ferences in their I.Q.'s are almost entirely attributable to the
inaccuracy of the tests. Even when identical twins are separated
in infancy and reared in entirely different homes, they remain, on
the average, as much like each other as fraternal twins. But they
do not remain as much like each other as identical twins reared in
the same family. The cultural factor, and possibly the physiologi-
cal factor, have definitely affected their intellectual ability ; and the
more unlike their home environments are, the more unlike the
twins become.

Other lines of investigation entirely support these results. All
three factors — the genetic, the cultural, and the physiological —
affect intelligence, although we do not know as much about the
physiological factor as we do about the other two. Conditions of
nutrition and hormone stimulation during the embryonic stage,
accidents at the time of birth, and severe illnesses in early life
can affect the individual adversely. On the other hand, the common
opinion that defective tonsils and adenoids, or even the presence
of hookworm may retard intellectual development in children has
been found to be false. The intellectual standing of most people
seems to be chiefly a product of the interaction of the genetic
and the cultural factors.

Education appears to be the chief cultural factor affecting the
I.Q. In England there is a group of people who spend their en-
tire lives on canal boats, scarcely mixing at all with the populations

The Behavior of the Individual 575

on shore. Their children never attend school, with the result that
between the ages of six and twelve the I.Q.'s of these children
fall from 90 to 60.

It has been shown also that beginning one's education at an
early age has a stimulating effect on mental growth. Children
placed in pre-school at the age of two or three years sometimes
undergo a considerable increase in I.Q. during the time they are
in school. For some time it was doubted that these improved
I.Q.'s would be maintained throughout life. It was thought that
in later school years their I.Q.'s would fall to the level of those
of other children with equally maturated capacities for learning.
But the most recent information indicates that the stimulation
received in pre-school leaves a permanent effect on intellectual
ability.

The next most important cultural factor is the home environ-
ment. Children adopted into good homes show a definite increase
in I.Q. and are brighter than might be expected on the basis of
the intelligence of their true parents. It has also been shown that
life in the city has a stimulating effect upon intelligence. Children
whose parents take them from a rural to an urban environment
experience an average increase in I.Q. of about five points.

Scientific investigation of the factors affecting intelligence fails
to uphold the extreme views of either the environmentalists or
the hereditarians. Although a stimulating environment will make
for increased intelligence, individuals receiving equal environ-
mental opportunities will display the most extreme differences in
intellectual achievement. Children reared in orphan asylums, where
opportunity is practically equal for all, are as different from one
another in I.Q. as those in the general population. There can be
no doubt that some are born gifted and others born handicapped,
but that in a good social environment mental handicaps may be
at least partially overcome, and gifts may receive adequate op-
portunities for expression.

Differences Between Races. — One of the most persistent of
human ideas is the belief that one's own race is inherently su-
perior to all others. Our own race has been especially prone to
this point of view, and we have frequently used the argument
of our own inherent superiority to justify our conquest and ex-
ploitation of darker peoples. A specific instance is the widespread

576 The Behavior of the Individual

belief in the inferiority of the black race. To a considerable extent,
this is held to be an inferiority in intelligence. Even so strong an
advocate of democracy as Thomas Jefferson believed that all Ne-
groes are essentially uneducable, and the great southern statesman
Calhoun said that they were so profoundly unintelligent that they
should not be classed as human beings. Even today many people
will confidently assert that no Negro could possibly be as intelli-
gent as any but the most feeble-minded of the whites.

The development of intelligence tests has made it possible to
secure definite information on this matter. On the basis of a large
number of studies of Negro intelligence, it appears that the aver-
age I.Q. for American Negroes is somewhere between 75 and 85.
At the same time these studies emphasize clearly that the indi-
vidual Negro should not be judged according to the low average
of his race. Approximately twenty-five per cent of Negroes pos-
sess an I.Q. higher than that of the average white man. In the
Chicago schools a full-blooded Negro girl has been discovered
with an I.Q. of 200, one of the highest ever measured. It is per-
fectly clear that belonging to the Negro race does not condemn
an individual to inescapable feeble-mindedness.

A more difficult question concerns the extent to which the dif-
ference in intelligence between Negroes and whites in America is
produced by the genetic factor or by the cultural or possibly the
physiological factors. It cannot be doubted that the cultural fac-
tor is definitely responsible in part for the low average mentality
of the Negro. The educational advantages offered him, especially
in the southern states of this country, are so poor as definitely to
handicap him. Even in the North, where Negroes usually attend
the same schools as the white children, their entire life outside
the school is spent with a group of people who have for centuries
lacked even the most meager cultural advantages. In spite of the
intellectually unstimulating home life of the average northern
Negro, the test given to army recruits during the World War
found the Negroes of Pennsylvania, New York, Illinois, and
Ohio superior to the whites of Mississippi, Kentucky, Arkansas,
and Georgia. The whites in the northern states, however, were
superior to their Negro neighbors, and it is likely that the northern
Negroes had received somewhat better schooling than the southern
whites. A study of 500 Negro children in the schools of Los

The Behavior of the Individual 577

Angeles showed them to have an average I.Q. of 104.7, some-
what higher than that of the white children with whom they
were compared; and in New York City also, a group of Negroes
has been found which tested as high as a comparable group of
whites.

It may be that these children in New York and Los Angeles
represent what the Negro race as a whole can do when it has
freed itself of the cultural handicaps which centuries of slavery
and deprivation of social and economic privileges have forced
upon it. On the other hand, it may be that they constitute a
specially selected group of Negroes inherently superior to the
bulk of the Negro race. Many psychologists have held that the
intellectual superiority of the northern Negro is the result of
selective migration, that is, that the Negroes migrating to the
North are on the average genetically superior to those that remain
in the South. This assumption, however, has never been proved;
and what little evidence we have indicates that the Negroes who
remain in the cities of the South are equal in intelligence to those
who move northward.

The above discussion has been able to provide merely a glimpse
of the difficult problems that are encountered in any attempt to
determine the inborn or genetically determined intellectual ca-
pacities of racial groups. On the basis of the entire mass of studies,
it seems fair to draw the following conclusions :

1. It has never been proved that any race of people is inherently
inferior in intelligence to any other great race.

2. The differences in intelligence between races as they exist
today are due in large part, if not entirely, to differences in cul-
tural opportunity.

3. Genetically conditioned differences between races, if they
exist, are very slight relative to the difference between individuals
within a single race; and all races are capable of producing men
and women of genius.

Differences Between Social Classes. — Social status in
America depends chiefly upon the occupation in which the family
breadwinner is engaged. Many studies have shown that the chil-
dren of men in occupations of high social status are more intelli-
gent than the children of those in occupations of low social status.

578 The Behavior of the Individual

The following table shows the approximate average I.Q.'s for
offspring at various occupational levels :

Professional men and big business men I.Q. 115

Clerical workers and small business men I.Q. 105

Skilled laborers I.Q. 100

Semi-skilled laborers I.Q. 95

Unskilled laborers I.Q. 90

It is impossible to determine to what extent these differences
are due to the genetic or to the cultural factors. In a competitive
society such as ours, which provides opportunity for men of out-
standing ability to rise to the top, one would expect the genetic
factor to be partially responsible for the differences. At the same
time, it should be remembered that a large proportion of the un-
skilled and semi-skilled laborers in this country are recent immi-
grants who have not had the opportunity to rise to a social status
commensurate with their inherent abilities, and that even in Amer-
ica, the mere fact that one is born into a laboring-class home de-
prives one of many opportunities for intellectual and social
advancement. At any rate, we may conclude that the inherent
capacities of the "lower classes" are not as far below those of
the upper occupational levels as the actual differences in I.Q.
might seem to indicate.

Eugenics and Euthenics. — As might be expected, the dif-
.ferential birth rate, described in Chapter X, applies to differences
in intelligence as well as to differences in social standing. Children
from large families average lower in intelligence than children
from small families. There are two opposed schools of thought
concerning the effect that this differential in the rate of reproduc-
tion is likely to have upon the human race. Believers in eugenics
hold that the chief problem which the human race faces in its
attempts to improve the quality of our civilization is to improve the
genetic constitution of the race. What we need, they say, is not
more hospitals, but fewer genes that produce susceptibility to dis-
ease; not bigger and better schools, but more genes for intelli-
gence. Advocates of euthenics hold that better economic and
cultural advantages for the entire population are essential to raise
civilization above its present level.

Hence, the euthenists are inclined to scoff at the alarm with
which the eugenists greet the differential birth rate for intelli-

The Behavior of the Individual 579

gence. They insist that if the same privileges were accorded the
numerous offspring of the unskilled and semi-skilled laborer as
are the birthright of the less numerous progeny of the profes-
sional man, the former would become quite as adept as the latter
in satisfying the demands of the psychological tester. The radical
advocate of eugenics, on the other hand, cries aloud that the
biological heritage of the race is being destroyed by the dying out
of the better racial strains and the multiplication of the poorer
ones. He insists that giving social and economic advantages to
these poorer strains merely encourages them to breed more rapidly
and at the same time lowers the death rate among them, so that
the increase in numbers of the inferior stocks is greatly accelerated.
A survey of the actual facts of the situation leads to the con-
clusion that both these schools of thought are right with respect
to their positive programs of action, and wrong in criticizing the
proposals of their opponents. The truth of the matter is probably
fairly well summed up as follows :

1. Improvement in the social and economic status of the "lower
classes" would doubtless lower the death rate among them. At the
same time, however, it would probably result in a lowering of
their birth rate, as a result of wider employment of birth control
methods, so that their net rate of increase would be actually low-
ered. The higher the social, economic, and educational standing
of a group of people, the more likely it is that they will practice
contraception.

2. Improvement in social and economic status, together with
improvement in educational opportunities, would actually increase
the measurable intelligence of our population, regardless of any
improvement through selection of genes for superior intelligence.

3. Decrease in the birth rate among members of the laboring
class would enable them to make better provision for their off-
spring and thus raise their intellectual standing, regardless of any
improvement in the genes.

4. The present differential birth rate probably does result in a
more rapid increase among genetically inferior stocks than among
genetically superior stocks, although this proposition has never
been definitely proved.

5. Even if there is no genetic difference between the "upper"
and "lower" classes, a reversal of the present differential birth

580 The Behavior of the Individual

rate to produce a higher rate in the upper levels than in the lower
levels would be desirable on purely euthenic grounds. If the pro-
fessional man is able, simply through providing a better cultural
environment for his offspring, to develop in them an I.Q. 25 points
higher than the I.Q. of the day-laborer's children, it is most
desirable that an increasing proportion of our population be
reared in the type of home the professional man provides.

Thus, eugenics and euthenics are not really antagonistic pro-
grams for human betterment. On the contrary, a successful pro-
gram of eugenics would improve the social environment, while
improvement in the lot of the less fortunate classes would proba-
bly have a definite eugenic influence on the development of the
race.

Intelligence Is Not All-important. — Because we have se-
lected the trait of intelligence for special discussion in dealing
with the differences between individuals, we may have given the
impression that intelligence is the one important trait that an in-
dividual can possess, and that, lacking it, he is automatically
doomed to failure and ignominious inferiority. No point of view
could be more untrue. To be sure, a high degree of intelligence
is requisite for the performance of certain important social func-
tions. The work of the scientist, the doctor, the lawyer demands
a degree of intelligence above the average. But as far as the in-
dividual is concerned, his success or failure should be judged in
terms of the use to which he has put his mental gifts, rather than
by the fortune he has enjoyed in receiving them. None but the
most hopelessly feeble-minded are so lacking in intelligence that
they cannot become happy and useful members of society. In-
deed, there is no relationship at all between personal happiness
and intelligence. Some of the most brilliant people are miserable
failures in life, and many individuals of the most humble mental
capacity are happily successful in the really essential aspects of
living.

If we accept that rather crude standard of success, the capacity
to make money, we find that intelligence is by no means the only
important personal quality which makes for wealth. It has been
found that the income of engineers is not closely related to in-
telligence, but is closely related to "personality." Another study
ha& shown that among large business executives there is no ap-

The Behavior of the Individual 581

parent relationship between intelligence and success. This does
not mean that intelligence is not necessary for the work of an
engineer or business executive. Both groups are selected from
the most intelligent elements of the population. But if one has
enough intelligence to do the work at all, his money-making ca-
pacity in either field seems to depend more upon "personality"
than upon his intellectual standing relative to others in the field.

But what is this mysterious thing called ' 'personality' '? Many
answers can be given, but there are two ways of answering it that
seem to have considerable psychological importance.

The definition of a "good personality" that is probably most
widely accepted in everyday life is that it is an effective person-
ality. It is the capacity to secure favorable responses from others,
to make them like you and to influence their actions. Dale
Carnegie has summed up the characteristics of this type of per-
sonality in the title of his best-seller, How to Make Friends and
Influence People. Without doubt, there is no trait more sought
and yearned after by men than the possession of an effective per-
sonality. Mr. Carnegie believes that the secret lies in liking and
being interested in other people, in always considering their feel-
ings first, and, above all, in trying to see their point of view and
presenting things to them from that point of view. Without doubt,
these qualities are essential aspects of the effective personality.
Other qualities, quite as necessary, are courage, enthusiasm, en-
erSY> and aggressiveness. But, as Mr. Carnegie points out, we
must learn to be aggressive in a pleasant way and to call attention
to ourselves by talking about the other fellow and his interests,
rather than about ourselves and our own interests. Actually, we
have no carefully worked-out scientific knowledge of what con-
stitutes the effective personality. What we do have is a knowledge
of the methods of making friends and influencing people which
men have worked out by trial and error in everyday life. And,
to a considerable extent, an effective personality is a gift which
descends upon the individual whose genetic constitution and ex-
perience in life have developed in him qualities of self-confidence,
cheerfulness, aggressiveness, and friendly interest in people.

The second definition of the good personality is the one that
has been worked out by psychologists and psychiatrists (medical
men who specialize in nervous and mental disorders) in their

582 The Behavior of the Individual

dealings with all sorts of personalities which seem somehow to
have gone astray and fallen into difficulties. According to this
definition, the good personality is the well-adjusted personality,
the personality of one who is fundamentally at peace with himself
and with the part that he is called upon to play in life. Ordinarily
we do not recognize the warfare that is going on within the
mind of the maladjusted personality, but we do recognize such
individuals as being "unbalanced" or "queer."

On the whole, the well-adjusted personality is the effective
personality, although this is not always true. Sometimes people
with ill-balanced personalities develop a considerable degree of
social effectiveness and are able to exert great influence over others,
especially other maladjusted personalities. The leader of the ec-
centric religious cult usually furnishes a good example of a mal-
adjusted but highly effective personality. On the other hand, there
are many quiet, unambitious people who are content with a few
friends and little personal influence who possess well-balanced
but not particularly effective personalities. But the person who
strongly desires personal effectiveness but who is unable to attain
it is almost always handicapped by some degree of personal mal-
adjustment.

Information relative to the extent to which personality is af-
fected by the genetic or the cultural factors has not yet progressed
as far as it has relative to the trait of intelligence, chiefly because
it has proved difficult to develop good tests of personality. But
the little that we do know suggests that an individual's personality
is much less dependent upon his heredity and more dependent upon
his environment than is his intelligence ; and we do have a con-
siderable amount of information concerning the manner in which
a maladjusted personality is developed through the contacts of the
individual with others. Maladjustment grows out of the sense of
hopeless anxiety which the individual develops when he is faced
with a situation where he feels himself unable to come up to the
standards demanded of him by his parents, friends, or other mem-
bers of society, and fears that they will therefore turn against
him. In the preceding chapter, we pointed out how the desire to
please others becomes the most important social attitude de-
veloped in the child, since he is entirely dependent upon others
fqr his welfare. This attitude becomes so completely a part of

The Behavior of the Individual 583

our nature that we never wholly outgrow it, although it may
fall into the background of our consciousness so that we scarcely
realize it is there. Indeed, it usually undergoes a complex develop-
ment, similar to the development of the possessive urge which
we noted in the preceding chapter. While this development has
never been worked out in full experimental detail, it appears to
proceed about as follows : First the child learns that he is depend-
ent upon adults for the care and protection that he needs. Grad-
ually the pleasing of adults, which was at first chiefly a means
to get what he wanted, becomes an end in itself. Now, if he fails
to please others, whether or not this results in punishment or
deprivation, he will feel the anxiety that would naturally go with
the threat of punishment or deprivation. Using this motive to
gain their ends with the child, others will now commend him for
desirable actions and show anger or contempt when he fails to
come up to their standards, Gradually, the desire to come up to
standards becomes in itself a desirable thing, although it was
originally merely a means to an end. The child develops an ego
ideal, namely, a conception of the sort of person he would like
to be; and now, if he fails to come up to this ideal, he develops
the feelings of anxiety that formerly were characteristic of his
failure to please others. These feelings of anxiety which result
from failure to live up to the ego ideal are ordinarily referred
to as feelings of guilt and inferiority. That they originate from
our need to please others is suggested by the fact that we usually
feel much more guilty when we are caught doing the wrong thing
than when we only know about it ourselves.

Taking advantage of these universal desires to please others
and to attain an ideal character, society is able to impose its stand-
ards of behavior upon the individual. But in a complex society
like our own, standards may come into conflict. For instance, a
young man who has grown up in a home in which the doctrine of
evolution is held to be irreligious and false comes to college and
learns that all his professors and most of his fellow students be-
lieve that it is true. If he holds to his anti-evolutionary point of
view, he fears that he will be looked upon with contempt by the
people in his immediate surroundings. At the same time he feels
a strong compulsion to retain his loyalty to his parents and, as
he believes, to his religion. Out of the conflict between these two

584 The Behavior of the Individual

incompatible motives, he develops an evergrowing feeling of
anxiety.

Or a young woman may have been given the impression that
she is not attractive to men, and at the same time have been led
to believe that being attractive to men is the one important quality
that a girl can possess. Wihout it she feels that her life is doomed
to failure. The anxiety which she experiences will be vastly greater
than that developed by the young man with a religious conflict.
To her way of looking at it, her whole existence as an acceptable
member of society is threatened.

Thousands of instances of similar situations with which human
life is beset could be cited. We are all of us certain to meet with
conflict and frustration, and in those circumstances we develop
feelings of anxiety. There is almost always some solution to the
conflict, some method of overcoming the frustration. The young
man may come to realize that he is no longer a child who must
take his opinions ready-made from either parents or teachers, and
so he may be able to come to conclusions concerning evolution
on a basis of an objective consideration of the evidence. The
young woman may learn that the girl does not exist who cannot
make herself attractive to men by providing them with companion-
ship and by taking intelligent care of her personal appearance.
Those who are fortunate enough to discover the way out of their
difficulties develop well-adjusted personalities. But frequently, be-
fore we find the way, our minds, anxious to escape the strain of
anxiety, play tricks upon us and hide the anxiety from us before
we succeed in overcoming the situation which has produced it.
These tricks of the mind are called escape mechanisms. They are
means of concealing anxiety without actually ridding us of it,
and thus they leave us fundamentally at war with ourselves and
with the parts that we are called upon to play in life. In short,
they cause our personalities to be maladjusted.

In the remainder of this chapter, we shall describe certain of
these escape mechanisms, namely, repression, dissociation, fixation,
regression, projection, compensation, and rationalisation.

Repression. — Repression is the inhibition of the conscious real-
ization of anything that produces anxiety. If, for instance, we
have a desire to do something that we know is wrong, we refuse
to admit to ourselves that the desire exists ; thus we get rid of any

The Behavior of the Individual 585

conscious anxiety about the matter. Sometimes people speak of
"repressed desires" when they simply mean that the full expression
of the desire is inhibited. Such a use of the term is incorrect, since
repression means inhibition of conscious recognition and recall.
For example, if you wish to kill a man and are perfectly aware
of your wish but do not carry it into fulfillment because you con-
sider such an action to be wrong, you are not repressing your
wish, you are simply inhibiting its fulfillment; but if you wish
that someone was dead but inhibit the recognition of your wish,
so that you are not aware of it, then you possess a "repressed
desire."

Frequently we repress the memory of an entire incident in
order to escape unpleasantness in remembering it. The following
story illustrates what may happen in such cases :

A seven-year-old girl went for a picnic with her mother and
aunt. When it came time for the mother to return home, the little
girl begged to stay with her aunt, who was planning a walk
through the woods. She was allowed to stay upon promising that
she would obey the aunt, but she forgot this promise and ran off
to play about a small waterfall. Somehow, as she was climbing
about, her foot got wedged between two rocks and she was held
helpless in a position where the water came tumbling down and
rushing past her on all sides. When her aunt found her there some-
time later, she was badly frightened and fearful that she would
be punished when she got home. The aunt promised her that she
would never tell about the incident and shortly thereafter went
away and did not return for several years.

The little girl, however, had been considerably upset. Her sense
of security had been shaken and she had been made to feel
ashamed of herself and fearful of punishment. Perhaps she felt
that she ought to be punished for her misdemeanor and that she
was dishonest not to tell about it, and the recall of it aroused
in her a gnawing sense of guilt. The whole affair was so dis-
tasteful that she put it completely away from her, forgot about it.

But she did not get rid of the emotional feelings connected
with it. Every time she came near running water, she would sud-
denly be struck by a horrid fear which she could neither under-
stand nor conquer. So great was her terror that it sometimes took
three members of her family to give her a bath. Once when^she

586 The Behavior of the Individual

heard the sound of a drinking fountain, she fainted away. Her
horror of water continued until she was twenty years of age,
when her aunt returned to visit the family. Upon hearing of her
strange fear, the aunt told the story of her accident at the water-
fall. As soon as the incident was recalled to the young woman,
her strange symptom disappeared.

The story illustrates the danger of attempting to escape the
anxiety by repression. One is faced with a problem, yet one re-
fuses to deal with the problem consciously and intelligently, and
the result is that the problem is never adequately settled. The little
girl repressed any intelligent response to the situation at the water-
fall, so that only her childish emotional reaction remained. This
reaction asserted itself, even though the memory of the incident
which produced it was gone. As soon as her aunt's story enabled
her to react to the incident from an adult point of view, her
troubles were over. She could laugh it off as a childish peccadillo,
whereas at the time it occurred it had seemed to threaten her
moral integrity.

Dissociation and Hypnotism. — When the young woman was
made to recall the incident at the waterfall, the emotional attitudes
aroused there could be reacted to by her entire personality. Previ-
ously, they had been leading a life of their own, a strange life,
separated from the everyday, common-sense world in which the
young woman lived. They were living in a world in which a ghastly
retribution falls upon little girls who disobey their parents and
in which running water is the means of bringing about the retribu-
tion. Somehow, because of the repression exerted upon them, these
childish, superstitious emotional attitudes could not come into con-
tact with the real personality of the girl, a personality that could
dispose of such superstitions immediately, once it made contact
with them. In other words, the fear of water and the emotional
attitudes that lay back of it constituted a dissociated system of
response. Dissociation is the separation of a system of responses
from the main personality, so that they carry on activities quite
apart from those that are a product of the main personality.

An excellent illustration of dissociation is the phenomenon of
"automatic writing/' that is, writing without being aware of the
fact that one is doing so or having any knowledge of the words
put down. A student, enrolled in a course in which, despite almost

The Behavior of the Individual 587

desperate efforts on his part, he seemed almost certain to fail, was
seated one day with his pen in hand, doing his best to grasp the
meaning of a difficult lecture. He had long since given up the at-
tempt to take notes and had fastened all his attention upon the
lecturer in hopes of getting some inkling of his meaning. Sud-
denly his hand began to trace lines on the sheet of paper that
lay on the arm of his chair. After some preliminary scrawling, the
following words were slowly spelled out : "I can't stand it ! Let me
go, let me go !" Subsequent questioning showed that he had been
entirely unaware of writing anything at all.

Automatic writing can frequently be produced by hypnotic
methods. Indeed, hypnosis is an artificially produced state of dis-
sociation, in which all the symptoms of natural dissociation can
be secured. It differs from natural dissociation in that it usually
results, not from a desire to escape the anxiety, but rather from
a simple willingness to follow the suggestions of the "hypnotist"
or hypnotic operator. A study of hypnotism will therefore give us
some idea of what dissociation is like.

To begin with, the following statements may serve to contra-
dict some of the more widespread fallacies concerning the nature
of hypnotism :

1. Hypnotism is not "just a fake" ; it actually occurs. Probably
over ninety per cent of the population can be lightly hypnotized.
A much smaller number of individuals, however, is susceptible to
really deep hypnosis.

2. Hypnosis is not brought about through the operator's over-
coming the "will power" of the subject. It is a response which the
subject makes to the operator, and in practically all cases the re-
sponse is a voluntary one. The subject is by no means forced to
make it.

3. To be susceptible to hypnosis is not a sign of "weak will"
or any other sort of inferiority. In fact, the only really marked
difference between a person susceptible to hypnosis and one who is
insusceptible is that the former is a good subject for a hypnotist
to work with and the latter is not

4. A good hypnotic operator does not necessarily have a "strong
will," a glittering eye, or a dominating personality. He only needs
to be able to win the confidence of his subjects.

5. While the few medical men and psychologists who employ

588 The Behavior of the Individual

hypnotism in their work do not encourage its practice among
irresponsible persons who are merely doing it for entertainment,
they are pretty well agreed that in the hands of competent and
responsible operators it is not dangerous.

There are numerous methods of putting a subject into a hyp-
notic state. One of these methods is to have him lie quietly on a
couch while the operator repeats over and over some such words
as these: "You are falling sound asleep! You are falling into a
deep sleep." The subject simply fixes his attention on the opera-
tor's words and thinks of nothing else. Gradually, his attention
becomes more completely fixed on the operator's suggestions.
Everything else is being shut out. Now the operator tells the sub-
ject that his eyes are tight shut and that he cannot open them.
Soon, even though the subject tries, he cannot open his eyes. All
the everyday world of common sense has been shut out for him,
He no longer responds to it, but only to the suggestions of the
operator. His true personality is in the background, and a disso-
ciated remnant of it has control over all his actions.

Now, if the subject has been deeply hypnotized, he will accept
almost any suggestion which the operator makes. If the operator
tells him that he has no feeling in his right arm, a needle stuck
into the arm will apparently arouse none. If he is told that the
arm is paralyzed, he will be unable to move it. It is possible to
make him see things that are not present and to fail to see things
that are present. All his responses are narrowed down to one
channel : accepting the suggestions of the operator. If he is awak-
ened, he will usually forget everything that occurred during the
trance, especially if it is suggested to him that he is going to
forget it. This forgetting as a result of suggestion very closely
resembles repression; and it can be shown that the system of re-
sponses which was set into action during hypnosis is still alive,
just as repressed responses are active.

While the subject is still in the trance, the operator may in-
struct him to perform some action after he awakens. Such an
instruction is called a post-hypnotic suggestion. Let us say that,
fifteen minutes after he comes out of the trance, he is to walk
over to a window and open it. Almost exactly at the correct time
he will perform the action and he will be forced to perform it,
although his overwhelming desire to do so may be quite as mys-

The Behavior of the Individual 589

terious to him as the young woman's fear of water was to her.
He remembers nothing of the instructions he received during the
trance and simply feels an unaccountable impulse to open the
window. The story is told of a man who bet that he could resist
a post-hypnotic suggestion. When the impulse came to carry out
the act, he recognized it and resisted it. But at midnight that night
he returned to the spot and performed the act. He lost the bet but
he wanted to get some sleep !

Although an individual loses all memory of what has happened
to him in a hypnotic trance, if he is hypnotized again he will be
able to recall accurately everything that occurred. The hypnotic
system of responses is not destroyed by awakening; it is merely
inhibited from direct conscious contact with the system of re-
sponses characteristic of the waking state. In other words, it is
dissociated.

Multiple Personality and Fugues. — One of the most dra-
matic forms of dissociation is that known as multiple personality.
Personality may be defined as the total organization of one's re-
sponses. In a few people two or more such total organizations
have been found which alternate in their control over the in-
dividual's actions just as the hypnotic and waking states may
alternate. The most famous case of multiple personality was that
of a certain Miss Beauchamp, in whom three different personali-
ties were discovered. At one time she would be a retiring, over-
conscientious, saintly individual; at another time she would be
ambitious, selfish and realistic. She would change from one state
to another quite abruptly, and neither "The Saint" nor "The
Realist" could remember what the other had done. Indeed, when
they learned about the doings of each other through other people,
or through the notes that they wrote to one another, they usually
thoroughly disapproved of each other's actions. In the course of
hypnotic treatment whereby Miss Beauchamp's two personalities
were eventually synthesized into one, a third personality made
its appearance. This newcomer was a childish, prankish individual
called Sally, who claimed to have been buried behind Miss Beau-
champ's maturer personality (or personalities) ever since Miss
Beauchamp began to learn to walk.

Somewhat less startling than the phenomenon of multiple per-
sonality is the fugue, during which the individual forgets all about

590 The Behavior of the Individual

his past life and wanders away from his former surroundings.
Such a person is reported as an "amnesia victim" by the papers.

A young bond salesman had got himself into difficulty through
certain dishonest business deals. When he saw trouble ahead, he
got into his automobile and, with a bottle of whiskey to help ease
his conscience, drove away. The next morning, he woke up in a
ditch a few hundred miles away from his home. He had a large
bump on his head, no automobile, and no memory for any part of
his past life. He was taken to a hospital, and his description was
published in the papers throughout the country. After two or
three different women had appeared, claiming to be his wife, his
father and his real wife arrived and were able to identify him.
They brought him home, and his father arranged matters so that
he could keep his job and would not be prosecuted for his dis-
honesty. Still he remembered nothing of what had happened to
him previous to his finding himself in the ditch the morning after
his disappearance. He did not recognize his former home or the
members of his family, although on one occasion he reached up
and turned on the cellar light, the button for which was located
in a place where no stranger would be likely to find it without
much search. He was taken to a psychopathic hospital where the
doctors tried in vain to help him remember his former self; only
flashes of memory could be aroused. Whenever his family visited
him, they made plans for the great Christmas celebration they
were going to have, and he began to look forward to it with keen
anticipation. Then, just five days before Christmas, he was told
that he could not leave the hospital until his memory returned.
By the next morning he was completely cured.

The doctors who worked with his case did not believe that the
young man was "faking." Nor did they believe that the blow on
his head accounted for his loss of memory. Rather they believed
that, because of his desire to escape the uncomfortable situation
that faced him, he unconsciously brought about a dissociation of
all the memories of his early life and maintained that dissociation
until it became more convenient to remember.

Fixation and Regression. — The anxieties that one suffers as
a child are believed to be the most important ones for the develop-
ment of the personality. These anxieties seem to be centered
a child's relationship with his parents, and they are most

The Behavior of the Individual 591

likely to develop in children whose parents are themselves badly
maladjusted. In subtle ways the child senses the parent's anxiety,
fears that the parent is unable to care for him or to feel real
affection for him. The child's anxiety is repressed, but it exhibits
itself in an added feeling of dependence upon the parent. The
parent may unconsciously take advantage of this situation by em-
phasizing to the child his dependence upon the parent, by keep-
ing him from making normal social contacts in the guise of
protecting him from the sins and dangers of the world, until he
grows up with no confidence in his own ability to take care of him-
self and completely dependent upon the parent's love and care.
Sentimentalists often speak of a child-parent relationship of this
sort as a "beautiful love/' but love has little to do with it. The
young man or woman is motivated by fear and lack of self-
confidence to be a "mamma's boy" or a "daddy's girl." The par-
ent's hidden anxiety is appeased somewhat by the opportunity to
dominate his child's life and monopolize his affection. The son or
daughter is said to be "fixated" upon the parent.

Fixation upon parents is one of the major causes for failure
to get along with a husband or wife. There is the husband whose
wife fails to please him because she cannot cook like his mother,
and the wife who cannot even learn to cook because mother al-
ways took care of everything at home. These and a thousand
other marital difficulties arise because people fail to make a suf-
ficiently complete transfer of their loyalty and affection from their
parents to their husbands and wives, and, furthermore, fail to
meet the responsibilities of adulthood because of failure to out-
grow their childish dependence upon their parents.

Frequently a person who is fixated manages to get along fairly
well until he faces some difficulty. Then he regresses to childish
modes of behavior and begins to seek for someone to care for
him and "baby" him. A few striking examples of this sort of
behavior appeared during the war among soldiers who had been
greatly frightened by shell fire. One such individual lost all pow-
ers of speech, began to toddle about like a one-year-old, played
with various objects as if they were toys, and would cry if his
toys were taken away from him. It is reported that "he quickly
made friends and became a universal pet in the ward."

Less striking instances of regression occur every day.

592 The Behavior of the Individual

people, for instance, become very babyish when they are even
slightly ill, while others begin to pout and stamp their feet when-
ever they fail to get their way.

Projection. — Many of our escape mechanisms consist of dis-
guised expressions of feelings or desires that have been more or
less repressed. Let us suppose, for example, that a four-year-old
girl is severely scolded and punished for exhibiting some sort of
sexual curiosity. The result may be a strongly repressed sense of
guilt concerning anything sexual. As she grows up, this repressed
guilty feeling may keep her from having a normal interest in the
opposite sex. She becomes a "man-hater." She is particularly im-
pressed by the "wickedness" of men, and soon she begins to take
a strong interest in the misdoings of other people. Thus her
repressed sexual curiosity receives a roundabout satisfaction and
at the same time her feelings of guilt are turned outward toward
other people, rather than toward herself. She punishes others by
indignantly spreading the news of their misdoings, but this is
really a roundabout way of punishing herself for a childish curi-
osity concerning which she felt so guilty. In short, she becomes
a gossiping old maid through the influence which her repressed
attitudes of curiosity and guilt exert on her personality.

The gossip has adjusted to the hidden, gnawing sense of sin-
fulness which, unknown to herself, continually assails her, by
projecting her guilt. She no longer feels horrified at herself, but
at other people, and the latter feeling is much more comfortable
than the former. Projection means believing that other people
have the traits or attitudes which one does not wish to recognize
in oneself. Some individuals project, not their own guilt, but
their own accusing sense of guilt into other people. They believe
that others are accusing them of crimes and misdemeanors, al-
though those others may have no such thoughts. It is their round-
about way of accusing themselves.

Compensation. — A little boy is sternly treated by his father
and develops a feeling of fear and inferiority. He goes off to
school, gets into a fight and wins. This victory brings an exag-
gerated exaltation to him, since it helps to relieve his feeling of
inferiority. He is so pleased at winning that he immediately seeks
other battles and soon he becomes the best fighter in the school.
Bvtf still, hidden deeply away, is his incurable sense of inferiority,

The Behavior of the Individual 593

driving him on to other victories. Life comes to mean for him
nothing but competition, getting ahead of the other fellow. As
he grows older, he works sixteen hours a day to make money,
since money is the symbol of victory over competitors. He is
ruthless in his business dealings. He amasses millions of dollars
and is finally stricken with indigestion because he cannot find
time to eat his meals properly. He ends his days in Florida, play-
ing winning, if not always sportsmanlike, golf against men half
his age and making himself hated because of his bragging.

The "hard-boiled" millionaire has adjusted to his repressed
feeling of inferiority by overcompensation. Compensation is the
process of making up for a felt weakness by a conspicuous suc-
cess. If not carried too far, it is one of the best methods of ad-
justment to feelings of guilt and inferiority. The saintliest man
is the one who is compensating for a deep sense of guilt, and the
greatest genius is one who is reacting to repressed feelings of
inferiority.

Unfortunately, few people are able to compensate in wholly
desirable ways. Compensation for the feeling of guilt too often
takes the form of a nagging, puritanical conscience, concerned
with keeping the individual from committing the slightest moral
error, and it has in it nothing of the generosity and unselfishness
which characterize the truly good man. The sense of guilt makes
Pharisees. Again, compensation for inferiority usually produces
only competitive, domineering individuals who must demonstrate
their superiority over others at all costs.

Rationalization. — One of our favorite methods of deceiving
ourselves concerning ourselves is to invent good reasons for our
acts, beliefs, or misfortunes, and thus hide from ourselves the
real reasons. This universal method of self-deception is called
rationalization. For instance, a student who has failed a certain
course explains his failure to himself and others by asserting that
the subject matter of the course was not worth acquiring. At the
same time, the professor who teaches the course may be ready
with a thousand proofs that such a course must form an essential
part of any adequate program of education, although his real
reason for wanting the course to stay in the curriculum is that
he finds studying it and teaching it a pleasant sort of a job, and
pleasant or unpleasant jobs are not to be sneezed at nowacj^ys.

594 The Behavior of the Individual

A "true believer" in almost any sect or creed can find many rea-
sons for his belief, although the real reason is usually a senti-
mental attitude that he has developed which would make it painful
for him to abandon his faith.

It would be interesting to make a catalogue of the reasons given
by people for buying new automobiles before the old ones are
worn out. In nine cases out of ten, the fundamental motive is
the same one that impels a Mexican harvest hand to spend half
his week's pay for a green silk shirt. Possibly the fun of display-
ing one's magnificence and feeling superior is worth the money,
to both the driver of the car and the wearer of the shirt, although
the owner of the new car seldom explains it that way, since such
expensive methods of "showing off1' are usually considered some-
what foolish and vulgar.

Escape Mechanisms and the Personality. — The individual
who has perused the above account without finding some of his
own weaknesses described therein may be assured that he pos-
sesses a set of escape mechanisms that work with such efficiency
that they never allow him to catch the slightest glimpse of the
truth about himself. All personalities are maladjusted to some
extent, and all men employ escape mechanisms to hide their anxi-
eties from themselves. It is the intricacy and ingenuity of the
escape mechanisms that impart a unique flavor to each human
personality, that make it so difficult to measure or even to ap-
preciate the individuality of each man and woman.

But if the anxieties that have been buried away by repression,
half satisfied by fixation, regression, projection and compensation,
and carefully glossed over by rationalization are especially severe,
the outcome for the personality may be most unfortunate. The
individual will be markedly unhappy, though his lot viewed from
the outside may be a most fortunate one. He seems nervous and
queer, and eventually he may develop the symptoms of a definite
mental disease.

For the symptoms of mental disease are merely escape mech-
anisms— either the ones described above or others that we have
not had space to describe — carried to a point where the indi-
vidual's efficiency is markedly handicapped, or still further to the
point where he can no longer be trusted to take care of himsdf
an<* must be confined to a hospital. For this reason, perfectly

The Behavior of the Individual 595

normal people may be led to see their own traits exemplified in
any description of the symptoms of the mentally diseased. If in
your reading of the next chapter, therefore, you begin to wonder
if there isn't something the matter with yourself, you may be as-
sured that many other people completely free from mental dis-
order have had the same experience. Indeed, you may accept any
apprehensions you feel as indications of your own sanity — for if
you were really crazy, you wouldn't know it.

CHAPTER SUMMARY

Mental tests have been found useful in arriving at judgments
of the manner in which one individual differs from another.
Their advantage over mere estimates lies in their thorough stand-
ardization. Intelligence tests — which measure ability to deal with
symbolical situations and spatial relationships — are the best known
of the many types of mental tests. Intelligence is measured in
terms of I.Q. (intelligence quotient) which is secured by dividing
the mental age by the chronological age and multiplying by 100.

An individual's intelligence is the resultant of three factors :
the genetic, the physiological, and the cultural; and neither he-
redity nor environment is solely responsible for differences in
intelligence.

Races display differences in average intelligence, although there
is usually much overlapping in intelligence between two races;
that is, a considerable proportion of the individuals of the in-
ferior race are superior to the average of the superior race. It
has been shown that at least a part of the intellectual inferiority
of the Negro race in America is a product of the cultural factor.
Whether Negroes are on the average genetically inferior to whites
has not been determined.

The intelligence of children is correlated positively with the
socio-economic status of their parents, but it is uncertain whether
this relationship is due chiefly to the cultural or to the genetic
factor.

The eugenist stresses the importance of the genetic factor in
producing superior human beings; the euthenist, the importance
of the cultural and physiological factors. Each appears to be right
as far as his positive program is concerned, but wrong in his
attack upon the program of the other.

596 The Behavior of the Individual

In terms of individual welfare, a good personality is probably
more important than high intelligence. The type of personality
one develops is probably dependent more upon the environment
and less upon biological heredity than is the degree of one's in-
telligence. The term "good personality" may be applied in two
senses to mean either the effective personality or the well-adjusted
personality. The former characterizes the individual who is capa-
ble of "making friends and influencing people" ; the latter belongs
to the man who is fundamentally at peace with himself and with
the part that he is called upon to play in life.

Maladjustment of the personality occurs through the develop-
ment of feelings of anxiety, guilt, and inferiority, and is usually
associated with conflicts between opposing motives. If these diffi-
culties cannot be overcome, we take refuge from them in escape
mechanisms such as repression, dissociation, fixation, regression,
projection, compensation, and rationalization.

Repression is the inhibition of conscious recollection or recog-
nition of our failure to meet cultural standards and personal
ideals. Repressed desires frequently express themselves through
a dissociated system of responses, that is, a system separated
from the main personality so that it carries on activities quite
apart from those of the main personality. Automatic writing,
multiple personality, and fugues are instances of dissociation.
Hypnotism is a form of artificial dissociation in which the indi-
vidual's attention is centered completely upon what the operator
suggests, and the system of responses formed during the hypnotic
state becomes cut off from the waking personality and is not re-
membered when waking occurs.

The failure to outgrow childish emotional attitudes is called
fixation, and the return to childish modes of behavior in the
face of difficulties is called regression.

Projection is the belief that other people have the traits and
attitudes that one does not wish to recognize in oneself.

Compensation is the process of making up for a felt weakness
by a conspicuous success. This is a good method of adjustment
unless it is carried too far and unless the individual fails to recog-
nize the reason for his compensatory conduct.

Rationalization is a process of self-deception whereby we in-

The Behavior of the Individual 597 <

vent good reasons for our acts, beliefs, or misfortunes, and thus
hide from ourselves the real reasons.

QUESTIONS

1. What is the cause of the superiority of mental tests over other
estimates of individual differences?

2. What is intelligence? Why is it important to the individual? To
society ?

3. Discuss differences between races and classes in intelligence.

4. Discuss eugenics and euthenics.

5. What is meant by a "good personality" ?

6. Define and illustrate each of the following escape mechanisms :
repression, dissociation, fixation, regression, projection, compen-
sation, rationalization.

GLOSSARY

fugite (fug) A form of dissociation in which an individual forgets
his identity and wanders away.

hypnosis (hip-no'sis) Process of putting an individual in a dissociated
state in which he readily accepts all the suggestions of the operator.

post-hypnotic suggestion Suggestion made during the period of hyp-
nosis which is followed after awaking from the hypnotic trance.

CHAPTER XXVII

MENTAL ILLNESS AND MENTAL HEALTH

What Is Mental Disease? — Many people view the study of
nental disease with a sort of unreasoning aversion. They feel
that it is a morbid preoccupation. This attitude has probably
descended from the ancient superstitious belief that insane people
were "possessed of demons." The idea was that a mysterious and
malignant spiritual being had made his way into the afflicted
man's body and was using it for a dwelling place. You will prob-
ably recall the story in the New Testament in which a "legion of
devils" was forced to escape from a certain madman and take up
their abode in a herd of swine, whereupon the swine rushed down
the mountain and drowned themselves in the Sea of Galilee, while
the man went home in his right mind. During the Middle Ages,
one of the accepted methods of curing mental disease was to tie
the unfortunate sufferer up to a post and whip him in hopes of
driving out the demon that possessed him. Another method was
to read to the devil inside the insane man a solemn proclamation,
or exorcism, warning him to depart immediately and calling him
all sorts of thunderous and vile-sounding names, whereupon he
was expected to leave the body of his victim and creep off to hell
in sheer terror and humiliation. With this theory of demoniacal
possession current, it is no wonder that people felt a horror of
the "madman." One could never be sure that the devil would not
leave his victim and take up his residence in oneself. And, while
people no longer believe in demoniacal possession, some of this
superstitious fear of insanity persists even to our own times.

The scientific study of mental diseases which has gone on dur-
ing the past hundred years, however, has shown that they are
really only manifestations of normal tendencies in the growth of
the personality which have somehow become exaggerated or
warped out of their normal line of development. They are like

598

Mental Illness and Mental Health 599

the gnarls that form in a tree trunk when the tree has been bruised
or wind-blown during its youth. They are kinks in the process
of mental growth. Every one of us has such kinks, and, indeed,
we would probably be exceedingly dull fellows if we didn't. The
deranged personality is simply somewhat kinkier, and the kinks
are of a more exaggerated kind.

The psychiatrist, who studies and tries to help the interesting,
if somewhat bizarre, individuals who live in our psychopathic hos-
pitals, finds most of them to be extremely human persons, strug-
gling as best they know how with the problems that face every
human being in adjusting to cultural standards and finding
strange, but often fascinating, solutions to those problems.

Kinds of Mental Disease. — Most people have very vague
notions of the nature of mental diseases. The term insanity
usually calls up in their minds pictures of "raving maniacs'* or
of persons who consider themselves to be Napoleon Bonaparte.
Although the excited, overactive individual and the man with
delusions of grandeur are found in hospitals for the insane, a
much more common type is the rather dull, apathetic patient
who sits in his chair all day and mumbles to himself. The ma-
jority of persons suffering from mental diseases do not need to
be put in hospitals at all; their ailment is termed a neurosis, or
minor mental disorder, and many of them are capable of con-
ducting their business and social affairs as well as anyone else.
The more severe mental diseases are called psychoses, and the
majority of people who have them are definitely insane and need
to be treated in a special hospital for the insane where they will
be kept from doing damage to themselves or others. The psy-
choses are divided into two main groups : the functional psychoses.
for which no definitely causal physical defects have yet been dem-
onstrated; and the organic psychoses, in which the disease can
be shown to be due, partly at least, to some actual damage to
the nervous tissue of the brain.

In the next few pages, we shall outline briefly the character-
istics of the more important types of mental disease. It should
be understood, however, that few actual cases of neurosis or
psychosis conform exactly to any classificatory types and that we
are only describing what is typical, not what occurs in every case.

The Neuroses. — It is most usual to classify the neuroses into

6oo Mental Illness and Mental Health

three major disease types, namely, neurasthenia, psychasthenia
and hysteria.

Neurasthenia might be considered the basic form of mental ill
health. It is characteristic of many persons who show other forms
of mental disorder, and, on the other hand, in its milder forms
it is found in a large percentage of the population. The neuras-
thenic's personality seems to be one which is dominated by un-
pleasant and conflicting emotions, of which he is often not fully
aware, and these more or less repressed emotions seem to set his
autonomic nervous system into complete disorder. He has strange
palpitations of the heart, headaches, stomach troubles ; his circula-
tion doesn't perform properly; his hands become cold and yet
sweaty, while his face may be burning; he has spells of dizziness,
and he is very readily fatigued. At the same time he experiences
feelings of anxiety; he fears that something dreadful is going to
happen, although he hasn't the slightest idea of what it is going
to be. He is troubled with insomnia. In brief, he is a puzzled,
unhappy person who has become "all upset" because, buried deep
where he can't get at them to deal with them intelligently, are
impulses which he fears to satisfy and tormenting feelings of
inferiority and guilt.

The neurasthenic has usually compensated for his feelings of
inferiority by setting up for himself an imaginary goal of great
superiority. He feels that he ought to be — indeed, that he must
be — the winner in all contests. But unlike the "hard-boiled" mil-
lionaire described in the previous chapter, he frequently fails to
make an active compensatory adjustment. When an opportunity
to compete with others presents itself, he backs out for fear that
he might not win, since not winning would be for him a tragedy.
Then he frequently begins to rationalize his lack of success. Wish-
ing to explain to himself that it is not fear of failure that makes
him unwilling to compete, he begins paying close attention to
his stomach troubles, or heart flutterings, or insomnia. Usually
he is not in particularly robust physical condition, but he greatly
exaggerates his illness. He goes to the doctor and tries to get
him to discover some fatal disease that is attacking him. He is
disgusted when he is told that there is nothing much the matter
with him, and starts looking for a better, less encouraging doctor.
Frequently he is able to find a quack, or even several quacks,

Mental Illness and Mental Health 60 1

willing to fuss over him to his heart's content. He becomes al-
most happy, treating his symptoms and developing new ones. His
rationalization is complete; and he has a perfect alibi for never
doing anything worth while.

Any description of the neuroses or psychoses can only give
what is typical of them, and very few cases actually conform to
type. Many neurasthenic people never become hypochondriacal,
that is, engrossed in their symptoms; indeed, they often fight
against them. Charles Darwin, for example, could seldom write
or study for more than two hours at a time without being as-
sailed by nausea. Apparently a part of his personality was doing
its best to give him an excuse for not working, and, since he
had an independent income, he could hardly have been blamed
for accepting such an excuse. Yet for twenty years he struggled
to secure complete proof for his theory of evolution, although
on many days he was unable to spend more than half an hour at
his work and at times he was totally incapacitated. In the end,
in spite of the difficulties which his own repressed anxieties set
in his way, he made one of the greatest contributions to human
knowledge that it has been the privilege of a man to make
throughout the history of human life.

Psychasthenia is the name given to neurotic ailments that are
characterized by obsessions, phobias, compulsions, or doubts and
scruples.

An obsession is a useless thought which comes to an individual
over and over again, which the individual recognizes as useless,
abnormal, and unpleasant, but which he cannot get rid of. Nearly
everyone has had the experience of being unable to forget a tune
which keeps running through his head. A man suffering from an
obsession has a similar difficulty. Some thought, for example, "I
am going to kill myself," may keep repeating itself to him; and,
though he has no intention of committing suicide, he is quite
incapable of getting rid of the idea.

A phobia is an irrational, uncontrollable fear of some thing or
situation. The reader will recall the phobia for water which was
described in the last chapter. Other individuals show abnormal
fears of open places or closed places, of the dark, of certain ani-
mals or certain kinds of people, or of thousands of other things.
Many people whom we would class as perfectly normal show

602 Mental Illness and Mental Health

irrational fears of rats, snakes or insects, or of blood, or of the
dark, or of thunder and lightning; and if the reader does not
have one or more slight phobias — fears which he realizes are
foolish — he is a rather exceptional person. Indeed, from the stand-
point of the steeple jack or construction man, nearly everyone
has a phobia for high places. But in the psychasthenic, a phobia
may be strong enough to dominate the individual's life, so that
he is unable to go out on the street for fear of meeting a dog,
or must stay away from all public gatherings because of his fear
of being in a crowd.

A compulsion is an uncontrollable impulse to perform some
act which the individual may recognize as foolish or wrong, yet
which he cannot avoid doing. Kleptomania, the uncontrollable de-
sire to steal, belongs among the compulsions. Some years ago
Dr. William Healy, working with children in the Chicago Ju-
venile Court, found that many of them experienced desires to steal
because of a repressed sexual interest. For example, a child might
have been taught certain obscene words and also led into stealing
by an older companion. Having already secured the impression
that any interest in sex is the most reprehensible of crimes, the
child would try to forget, or repress, the obscene words. At the
same time there would be a strong impulse to think about them
or use them. This latter impulse, being repressed, would seek
roundabout satisfaction through the performance of another
wrong act which had become associated with the unrecognizable
wickedness of using obscene language. The child would experi-
ence an overwhelming impulse to steal. Without any recognition
of the fact on the part of the child, stealing would be substituted
for what, to him, was a much graver misdeed.

It seems possible that all the obsessions, phobias, compulsions,
and scruples of the psychasthenic are substitutes or symbols for
repressed desires or for the repressed feelings of guilt which the
psychasthenic harbors. A psychasthenic symptom reminds one of
the children's game of walking along a sidewalk, being careful
to avoid stepping on the cracks where the stones are joined to-
gether and chanting in unison :

"If I step on a crack
Til break my mother's back!"

Mental Illness and Mental Health 603

After enjoying the pastime for a while, the youngster begins
to be cautious about stepping on cracks even when he is not play-
ing the game. He has developed a mild scruple, that is, a hesitancy
at performing a certain act. Stepping on the crack has come to
symbolize for him doing harm to his mother. Indeed, the writer
remembers that once when parental discipline had irked him
mightily, he went out and stepped on all the cracks he could find
in the sidewalk ; as a matter of fact, he jumped up and down on
them. A psychasthenic compulsion may have a similar symbolic
meaning of defiance of one's parents, but the psychasthenic has
a better conscience, he hides his hatred from himself, and his
symptom seems utterly unaccountable to him.

Hysteria is a neurosis especially characterized by the operation
of dissociated systems of response. Both the phenomena of multi-
ple personality and of the fugue belong to the hysterical group
of disorders. Hysterical people are subject to fits in which they
lose all control of themselves, apparently being under the control
of a dissociated system of responses ; for this reason when some-
one begins to laugh or cry in an abandoned fashion, we fre-
quently say that he or she is "hysterical."

Very frequent among hysterical ailments are anesthesias and
paralyses. Hysterical anesthesia is a loss of sensitivity in the eyes,
ears, or other receptors which is due not to any impairment of
the sense organs themselves or of the nerves running to the brain,
but rather to the dissociation of sensations coming from that re-
gion. It is usually called "functional" anesthesia since it is brought
about by a defect in the working of the nervous system rather
than by a defect in its structure. We have already mentioned how
such anesthesia may be produced by hypnotic suggestion. Func-
tional anesthesia (as well as other hysterical symptoms) is fre-
quently also brought about by suggestion. For instance, a laborer
is working at a job which he dislikes when a hot oily rag catches
on a belt, is whirled along and thrown into his face. When the
oil is wiped away he finds that he is blind ; but the accident has
not caused the blindness, it has only suggested that method of
escape from his job. Functional paralysis is the loss of ability to
move some part of the body, although no damage has been done
to the nerves or muscles concerned. It occurs frequently after ac-
cidents. Various sorts of muscular twitches and tremblings and

604 Mental Illness and Mental Health

likewise many different kinds of fits also belong among th'e hys-
terical disorders.

The "shell shock" suffered by men during the war was usually
some sort of functional disability brought on to enable the indi-
vidual to escape from the horrors of trench warfare. It should be
understood that in the majority of cases there was no intentional
malingering, that is, "faking." Nevertheless, there was a marked
increase in the rate of cures immediately after the signing of the
armistice.

Mild disorders of the hysterical or functional type are not at
all uncommon in everyday life. From the little boy who wakes
up with a headache which persists until it is too late to go to
school, to the college student who develops writer's cramp in the
midst of an examination, we are all inclined to escape difficulties
by the route of the functional disorder. Some doctors claim that in
practically every case of illness there is a persistence of symptoms
after the physical causes of the symptoms have been removed,
such "hangover" symptoms being the product of suggestion.

The majority of cures wrought by "faith healers" are cures
of functional ailments, and anyone who has any knowledge of
the work of a faith healer must have noticed the high proportion
of cures of blindness, deafness, or paralysis that are effected by
faith.

The Functional Psychoses. — We shall deal rather briefly with
those types of mental illness which regularly result in commit-
ment to a hospital for the insane. The important thing to un-
derstand about them is that they are all methods of adjusting
to the difficulties (whether consciously recognized or repressed)
with which the individual is faced. Just as the hysteric adjusts
to his troubles by dissociation, so the individual with manic-
depressive psychosis adjusts by emotional excess; the man with
paranoia, by compensatory ideas; and the person with dementia
praecox, by shrinking away and daydreaming.

Manic-depressive psychosis is characterized by two opposite
emotional conditions. An individual may come to the hospital in
a greatly elated state of mind. His whole mental life is colored
by his joyous mood. He talks rapidly, although somewhat inco-
herently, is extremely active, sleeps little. He has "big ideas"
about himself and, if not brought to the hospital quickly enough,

Mental Illness and Mental Health 605

may spend all his money in some extravagant business adventure.
He is domineering, and may fight with anyone who opposes him.
He is, to himself, the joyous conquering hero. As he becomes
more and more elated, his beliefs and his very perceptions may
be completely colored by his mood. He entertains delusions of
being the greatest, most powerful man in the world. He suffers
illusions, being very likely to mistake one person for another, and
will hail the doctor as his long-lost brother whom he is going to
seat on the right hand of his throne as vice emperor of the uni-
verse. He may even experience hallucinations and see and hear
angel choirs coming down from heaven to worship at his feet.

A few days later, this same man may be in the depths of de-
spair. He hardly moves, except to wring his hands. When spoken
to he does not answer for a long time, then his voice is low and
despondent. Questioning, however, may reveal the fact that he
now has new delusions and hallucinations in harmony with his
new mood. He believes that he has committed the unpardonable
sin, that he has ruined his family, and he hears voices of friends,
threatening him with punishment for his wrongdoing.

Among manic-depressive patients, there exist all degrees of ela-
tion and depression. Some go only into the manic (elated) con-
dition, others only into the depressive state, but most of them
alternate between mania and depression. Usually they remain in
these states for comparatively short periods of time (from a few
days to a few months) and then swing into the opposite state or
into a normal mood. Most manic-depressives are normal for the
greater part of their lives, but suffer from recurrent episodes of
insanity. For the most part they are sociable, likable people whose
only defect seems to be their tendency to go on emotional "sprees."

The individual with paranoia possesses an entirely different per-
sonality from that of the manic-depressive. He appears to be one
who has experienced an intolerable sense of inferiority, but in-
stead of rationalizing his failure by attributing it to ill health, as
does the neurasthenic, he blames it on other people who have
schemed against him and given him a "dirty deal." The more
he broods on the matter, the more he becomes sure that a large
society of some sort is plotting his destruction. He interprets
everything that happens to him in this light, and builds up strong
delusions of persecution.

606 Mental Illness and Mental Health

Now he begins to compensate for as well as to rationalize his
inferiority. If people are plotting against him he must be a very
important person. He discovers that he is a great inventor whom
the General Electric Company is trying to get out of the way
because his inventions would run all their products off the market.
Or he is a heaven-sent reformer, a veritable prophet, whom those
agents of the devil, the Masons, are trying to destroy. Or, again,
he is secretly beloved by a certain rich heiress whose father plots
to have him killed. He is building up delusions of grandeur along
with his delusions of persecution. He writes letters to the Presi-
dent of the United States, telling him of the wrongs that are
being done him and demanding redress, or he brings legal suits
against his enemies, or he gets a pistol and begins to put them
out of his way. These activities bring him to the psychopathic
hospital, where he usually spends the rest of his days, since it is
almost impossible to convince him of the falsity of his delusions.
At the same time, he may remain an intelligent, capable indi-
vidual, and occasionally he may talk a visitor at the hospital into
believing that his delusions are true; he is so sure of himself and
so rational !

Dementia praecox (frequently called schizophrenia) is the psy-
chosis that is developed by individuals who shrink from the pains
of adjusting to reality and build up in daydreams a world that is
"nearer to the heart's desire." The individual who suffers from
this malady cannot get along with people; they hurt his feelings
because, being very much interested in himself, he cannot bear
to have that self treated with any but the utmost respect. Because
the social world pains him, he withdraws all emotional interest
from it. He seems cold and unfeeling, but he is only cold and
unfeeling toward others; for himself and his daydreams he enter-
tains the tenderest regard. Everyone has daydreams, but the
dementia praecox patient believes in his. Nothing else, to him, is
important, and he makes no effort to criticize them in the light
of reality. As a consequence his entire stream of thought is a
figment of dreamlike delusions and fantasies. His delusions are
not well thought out and rationalized, as are those of the paranoid,
nor are they products of an overpowering mood, as are those of
the manic-depressive. They are dreams, and they often possess
all the incoherence and inconsequentiality of a dream. Occasion-

Mental Illness and Mental Health 607

ally one may hear a schizophrenic uttering a strange meaningless
gibberish, called a "word salad." The following is an illustration:

The invention of the electric steam locomotive with rubber wheels
while they were chopping up the kindling wood went to a place called
St. Paul, Minnesota, because if you listen to a parrot, it will pick
your insides out because when you are full of electricity and electric
wires run the steam locomotive which went a hundred miles an hour
on rubber wheels, because it set fire to your insides although the
kindling wood was outside when McKinley ran for president between
Wall Street and St. Paul, Minnesota, you have to blow your nose
after the Mexicans had chopped the wood the electric steam locomo-
tive ran on rubber wheels and they ate so many bananas after Mc-
Kinley ran for President over my dead body.

Here the man's dream has "gone to pieces" and become quite
incoherent.

Dementia praecox appears utterly bizarre to us because the
sufferer is not in contact with us ; he is living in a different world.
Sometimes he stands rigidly peering out the window all day long,
or walks up and down, performing incomprehensible movements,
perhaps repeating the same set of movements over and over.
These apparently meaningless performances are in reality "dream
movements," having some inner significance to the patient which
cannot be deciphered by outsiders; indeed, for all we know, the
patient himself may not be conscious of their meaning. Yet some-
how they symbolize his innermost wishes. They are his way of
getting what he wants, of escaping his anxieties.

It would be impossible here to catalogue all the symptoms of
this psychosis. They are all variations on a fundamental pattern
of life, that is, shrinking from reality and obtaining satisfaction
from daydreams.

It has been found that these dreams are usually regressive and
are symbolic of childhood situations when the patient had no dif-
ficulties to meet and was cared for by loving parents.

Like the paranoid, the dementia praecox patient is a hard man
to cure, since he could get well only by a return to reality, and
he finds his dreams much more attractive. Perhaps in the majority
of cases he is happy because, although he may have bad dreams
as well as good ones, he has abandoned the host of responsibilities
that worry normal people. As his psychosis continues, his mind,

608 Mental Illness and Mental Health

divorced from the need of concerning itself with real problems,
deteriorates. His thoughts become more and more incoherent. Yet
he may continue to live in his hospital environment for years, fed
and sheltered by the "cruel world" of men which he, emotionally
at least, has entirely abandoned.

The Organic Psychoses. — It would take too long to describe
the symptomatology of the organic psychoses, and we shall con-
fine ourselves largely to listing the more important agents which,
acting upon the nervous system, can cause organic psychoses to
appear.

1. Bacterial Infections. — Various kinds of microbes, attacking
the brain, can bring about abnormal mental conditions, by far
the most frequent of such attacks being made by the spirochete
of syphilis. Syphilitic diseases of the brain are responsible for
about ten per cent of patients admitted to mental hospitals. The
outstanding syphilitic disorder is known as paresis.

The spirochetes attack the brain some five to ten years after
the time of the original infection of the blood stream; and if a
cure is not effected, their ravages bring about a gradual deteriora-
tion of the patient until he becomes quite empty-minded and help-
less and finally dies through the sheer incapacity of his nervous
system to take care of his bodily functions. In recent years it
has been found that many cases of paresis can be improved or
cured by giving the patient a case of malaria. (After the malaria
has done its work, of course, it is cured by administration of
quinine.) An even more recent method of attacking paresis is
the use of diathermy, a treatment employing certain electromag-
netic waves to produce a high artificial fever.

There is an organism which occasionally makes its way from
the nasal passages into the brain, and produces a type of brain
infection called encephalitis lethargica or, sometimes, "sleeping
sickness," although it is entirely different from the well-known
African sleeping sickness produced by the tsetse fly. Many un-
fortunate symptoms appear both during and after the attack,
among the outstanding ones being uncontrollable impulses to at-
tack and destroy. At present no means of curing this disease
is known.

2. Toxins produced by bacteria can bring on psychotic symp-
toms. It has been found that some cases of dementia praecox and

Mental Illness and Mental Health 609

manic-depressive insanity can be cleared up by removing infected
teeth, tonsils, £nd the like.

3. Narcotic drugs, when taken continuously in excessive quan-
tities, poison the tissue and bring about various psychoses. The
most frequent of such mental diseases are those caused by alco-
hol, of which the best known is delirium tremens. Alcohol ac-
counts for about as many hospital admissions as does syphilis.

4. Physical injuries to the brain tissues, caused by blows or by
brain tumors, may produce many different kinds of mental
disease.

5. Dying out of the brain cells in old age produces the forget-
fulness, mind-wandering, and egocentricity of senility.

6. Hardening of the arteries going to the brain, usually in old
age, deprives the brain cells of sufficient nourishment from the
blood and produces a variety of symptoms.

7. Disorders of the endocrine glands are coming to be recog-
nized as causes of mental disturbance. The mental difficulties at-
tendant upon over- or under-secretion of the thyroid have already
been mentioned. At the time of the menopause, or shortly after,
many women suffer from a depression that is called involutional
melancholia. Treatment with ovarian hormones or with ovarian
and thyroid hormones has been found to relieve this condition in
many cases. Apparently some, but by no means all, cases of de-
mentia praecox and manic-depressive insanity are brought on by
endocrine disorders of various kinds. The former especially seems
often to result from inadequate functioning of the gonads.

The Causes of Mental Disease. — The symptoms of all men-
tal diseases, with the exception of a few that are due to damage
done to circumscribed regions of the brain and a few others that
involve only the loss of intelligence, show some failure to adjust
to the strain of socialization and a falling back upon escape mech-
anisms of an exaggerated sort. In the organic psychoses, the
failure to adjust seems to be due to some damage done to the
nervous system which makes the individual incapable of carrying
the burden of adjustment to cultural standards without a resort
to escape mechanisms. These diseases are therefore best attacked
by attempts to remove or prevent any attack upon the nervous
system.

With respect to the functional psychoses and the neuroses, two

6io Mental Illness and Mental Health

points of view are held. One school, called the organicists, holds
that these are also really organic psychoses, but that the organic
causes have not yet been discovered. They point out that such
so-called functional diseases as dementia praecox and manic-de-
pressive insanity have in some cases been cured by gland therapy
and by clearing up focal infections. They claim that these diseases
are really a number of different diseases, each having its own
special cause, but that most of these causes have not yet been dis-
covered.

During the past year or two, cures of dementia praecox have
been effected through daily injections of insulin in such quantities
as to drive the sugar out of the blood and into storage in the
tissues. This change in blood chemistry has a marked effect upon
the nervous system, producing mental confusion and trembling of
the limbs. The condition is called "insulin shock," and it some-
times occurs in diabetics who take an overdose of insulin. A series
of insulin shocks does not always bring about a cure in cases
of dementia praecox, but, according to present reports, insulin
treatment is remarkably effective with patients who have not had
the disease for more than a year or two. As yet it is impossible
to say whether these cures are permanent, nor is the cause of the
cure understood. But if a change in the chemistry of the blood
can cure this disease, there is some reason to believe that blood
chemistry may have something to do with its cause. In short, the
insulin cure of dementia praecox apparently supports the conten-
tions of the organicists.

The other school, that of the functionalists, holds that the func-
tional psychoses and neuroses occur in individuals who have en-
countered special difficulties in adjusting to cultural standards and
have become habituated to the employment of extreme escape
mechanisms in attempting to adjust. This school therefore advo-
cates the employment of psychotherapy in treating such disorders.

The best-known method of psychotherapy is the one originated
by the Viennese neurologist, Sigmund Freud. It is called psycho-
analysis, and it consists essentially of a series of long interviews
between the doctor and patient in which the patient is encouraged
to tell the doctor everything that comes into his mind, no matter
how silly, meaningless or indecent it may sound. The series of
treatments may continue for two or three years, and the patient's

Mental Illness and Mental Health 611

repressions are gradually broken down, so that he is able to face
and conquer the anxieties that have been hidden away in his per-
sonality since childhood. There is much dispute at the present time
concerning the effectiveness of psychoanalysis, and much argu-
ment among psychoanalysts over the precise course of develop-
ment which the human personality undergoes. Indeed, some of
their theories appear outlandish, and they have been roundly at-
tacked by more conservative scientists. Nevertheless, Freud has
profoundly influenced the thinking of all psychologists, for his
method does have the virtue of exploring deeply into the hidden
recesses of human nature. Furthermore, both the practitioners of
psychoanalysis and their patients are profoundly convinced that it
produces cures.

Other methods of psychotherapy do not involve going all the
way back to the childhood anxieties on the basis of which the
later anxieties develop, but believe in educating the patient to face
the difficulties of life without too much recourse to escape mech-
anisms. Frequently a change in occupation or in the family
situation in which he lives will enable a patient to adjust adequately
to life, provided his mental disorder is not of too serious a nature.
These less thorough methods of treatment possess the virtue of
not being as expensive as psychoanalysis, which requires the serv-
ices of a highly paid physician over a long period of time.

Psychotherapy has been rather successful in dealing with the
neuroses. Occasional cures of the functional psychoses by psy-
chotherapy are reported, just as cures by organic therapies oc-
casionally occur; but most of these cases either get well of their
own accord or else never get well. Medical science does not yet
know how to deal with them effectively.

Since both psychotherapy and organic therapy are useful in
curing mental disease, it seems probable that both one's habits
of adjustment and the health of one's nervous tissues combine to
determine whether or not one shall show symptoms of mental dis-
ease. It is probable that a well-adjusted personality can stand
much more damage to the nervous tissues than a poorly adjusted
one before signs of psychosis or neurosis appear, and it seems
possible that a very poorly adjusted personality may develop
mental disease without any damage to the nervous system at all

6i2 Mental Illness and Mental Health

Both psychotherapy and organic therapy are indicated in the treat-
ment of mental disease.

It is frequently asserted that mental disease is inherited, and
this belief has doubtless caused much anguish and apprehension
to those whose parents or other near relatives have succumbed
to some psychosis. There is one rather infrequent malady, known
as Huntington's chorea, which develops in early middle age and
which is the product of a single dominant gene ; but with the ex-
ception of certain types of feeble-mindedness, we know of no
other mental disease which an individual is certainly fated to de-
velop on account of the characteristics of his genes.

It is true, however, that certain gene combinations make one
susceptible to such diseases as manic-depressive insanity and de-
mentia praecox, just as certain combinations make one susceptible
to tuberculosis. Probably a number of genes contribute to produce
this susceptibility; and just as the individuals of a given family
vary considerably in height, so they vary considerably in suscepti-
bility to mental disease. But for all we know at present, the most
susceptible person may completely avoid mental disease if he is
brought up in a mentally hygienic environment.

Mental Hygiene. — Few people have any conception of the
extent to which mental ill health afflicts our population. It is said
that half the hospital beds in this country are in hospitals for the
mentally abnormal. Furthermore, it has been estimated — or at least
guessed — that half the people who come to doctor s' offices are suf-
fering from ailments of a functional nature, that is, they are
mildly neurasthenic or hysterical. Mental ill health is nearly as
widespread as other forms of sickness, and it probably causes quite
as much loss of efficiency and happiness.

Very few people are perfectly strong and well physically, and,
similarly, very few people are perfectly adjusted mentally. In ad-
dition to those that have easily recognized mental diseases there
are unhappy people, overenthusiastic and unreliable people, grouchy
people, timid people, spendthrifts, misers, drunkards, prudes, sex-
ually frigid people, people who are obsessed with sexual thoughts,
liars, swindlers and thieves, all of whom display these undesirable
traits because of maladjustments they have developed and methods
of escape that they have learned to use. All are mentally unhealthy

Mental Illness and Mental Health 613

to some extent; and when you take all such people out of the
population, how many do you have left?

The problem of mental hygiene, then, is an extremely broad
one, since it involves not only the cure of obviously diseased per-
sons, but the better mental adjustment of well-nigh the entire
population. Practically all the "social problems" with which we
are faced, such as criminalism, poverty, divorce and other failures
in marriage, are in part problems of mental hygiene; while the
problems and difficulties which individuals find in their own lives
are very frequently problems which hinge around the failure of
themselves, their friends, or the members of their families to ad-
just adequately to the problems of life.

The remainder of this chapter will deal with the ways in which
the general mental health of the population may be improved.

Need for Improvement of Medical Service. — Up to the
present time the medical profession has been poorly prepared to
deal with mental maladjustments. The unintelligent horror of mad-
ness that has survived since the days of demoniacal possession has
kept most medical men away from the study and treatment of
such diseases. Psychiatry has often been looked upon as a not very
respectable field of work, and the psychiatrist has frequently been
considered as not much better than his patients. This unfortunate
and fundamentally superstitious attitude has retarded research in
the field, so that only in recent years has much knowledge con-
cerning mental ill health been acquired. As a result, there are not
nearly enough well-trained specialists in psychiatry, and almost
none of our general practitioners have received the training that
would enable them to deal intelligently with the many malad-
justed people who come into their offices. Gradually, as knowledge
increases, the medical profession as a whole is becoming better
informed, and skilled psychiatrists are becoming more numerous.
Furthermore, a new profession, that of clinical psychology, is
developing for the treatment or reeducation of cases of malad-
justment in which no organic disease is present. The clinical psy-
chologist does much of his work with children who, while not
mentally diseased, fail to get along well in school, with their play-
mates, their parents or with the officers of the law.

Parent Education. — The clinical psychologist has discovered
that when something is wrong with a child, the fundamental dif-

614 Mental Illness and Mental Health

ficulty almost always lies in his relationships with his parents.
And the psychiatrist has found that when he traces a mental
maladjustment back to its beginning, that beginning is almost
always in the home. Mental maladjustment begins in childhood,
2nd preventive mental hygiene can be applied only to children and
young people. One of the strange anomalies of our civilization is
that, while we demand special training for teachers who do noth-
ing more difficult and important than to instruct the young in
reading, writing, history, and foreign languages, the education
that is of profoundest importance to the individual, namely, the
acquisition of those emotional attitudes and sentiments which
constitute what we call his character and personality, is left in the
hands of people (i.e., parents) who are not required to know
anything at all about what they are doing. What we learn at
school is of necessity the most superficial part of our education;
it is what we learn at home that counts the most. This education
at home is rendered no less fundamental by the fact that fre-
quently neither parent nor child is aware of the fact that it is
taking place; often the child accepts the attitudes of his parents
without either being capable of formulating those attitudes. For
instance, if a mother conducts all her affairs in such a way that
she sacrifices every other interest to securing the admiration and
envy of her friends and neighbors, her children will usually ac-
quire the attitude, "social standing is the most important thing in
the world," although neither they nor their parent would ever
think of stating it in such terms. Or if a parent is continually
suspicious of the motives of others, the child will quite uncon-
sciously develop the attitude, "People are not to be trusted."

If a parent has consciously or unconsciously developed a sys-
tem of attitudes which adjust him adequately ct> the culture in
which he lives, his children will learn these attitudes and will
grow up to be mentally healthy. If the parent's attitudes are un-
hygienic, those of the child will also become unhygienic. Occa-
sionally, of course, young people react negatively to their parents'
way of life, but such negative reactions are likely to be exag-
gerated through overcompensation, and a maladjustment develops
that is simply the reverse of the parental "kink." For this reason,
the education of parents in a knowledge of what attitudes make
for hygienic mental and emotional development and what make

Mental Illness and Mental Health 615

for the reverse constitutes the greatest hope we now possess for
the development of mental health throughout the population. Here
we can only briefly outline some of the things that every parent
should know.

1. The child's first strong emotional attachment is to his parents
and many maladjustments result from failure on the part of par-
ents to return warmly the child's affection. Indifferent or hostile
parents develop in a child a feeling of insecurity which he is fre-
quently quite unable to overcome in later years. It is normal for
parents to love their children, and if a parent fails to do so it is
probably the result of some failure of his own to adjust properly.

2. The child needs to grow away from his first emotional at-
tachment, to find many friends outside his family, and finally to
find a mate. Many parents, usually those whose married life is
not quite satisfactory to them, attempt to hold too. much of their
child's affections. When a young man's affections remain too
closely attached to his mother, any adequate marital adjustment
on his part is almost impossible.

Friendships with youngsters of his own age and with adulti
other than the parents should be encouraged from earliest child-
hood upward, and the parent should avoid jealous attempts to
monopolize the child's affections.

3. When a parent feels somewhat insecure himself, he is likely
to protect and care for his children too assiduously. The children
never learn to care for themselves, and usually develop a strong
feeling of helplessness and inferiority. Children should be en-
couraged from babyhood upward to achieve independence and
self-confidence, to dress themselves, to take care of their posses-
sions, and to fend for themselves in rivalry with their playmates.
The way they can learn to do these things is to be given oppor-
tunity to practice.

4. Parents who are anxious to compensate for their own feel-
ings of inferiority are often too anxious about their children's
achievements. They try to force them to do things that they are
incapable of doing and are never satisfied unless their child is at:
the head of the procession in everything. Such an attitude in-
evitably develops a feeling of inferiority in the child.

5. Parents who have a thwarted urge for mastery, or who have
developed a strong sense of guilt, are likely to regulate their chil

616 Mental Illness and Mental Health

dren's lives to the point of tyranny. This makes it impossible for
the children to develop moral responsibility of their own, and they
react either by going explosively to the bad as soon as they get
out from under the parents' thumb or by developing such a set
of repressions and inhibitions that life becomes utterly painful to
them.

6. Ignorance concerning the development of the sexual impulse
and the feeling that sexual relations are nasty and shameful (but
at the same time horribly enticing) are probably responsible for
more mental ill health and personal maladjustment than all other
causes combined. Parents who possess these attitudes and enjoy
such ignorance are likely to pass on both to their children. To
very young children, every aspect of the world is an object of
curiosity. But the instant this curiosity is turned toward the sex
organs, a horrified parent is all too likely to convey to the child
his own feelings of fear and shame toward such objects, with
the result that a feeling of anxiety concerning sexual matters is
developed almost before the child has learned to talk. As the
child grows up, this attitude develops even more strongly, and
the more or less repressed feelings of guilt inhibit a normal de-
velopment of the emotions of sexual love. The result is that the
sex drive seeks roundabout means of expression. Many of the
symptoms of the psychoses and neuroses are expressions either of
repressed sexual impulses or of feelings of guilt in relation to sex.

It should be thoroughly understood that we do not mean that
normal inhibition and control of the sex urge result in mental ill
health. The individual who frankly recognizes his sexual impulses
but who, through prudence or moral idealism, inhibits expression
of them, is obviously subject to some strain, but he is meeting this
strain in an intelligent manner. It is the unintelligent horror of
the sexual, rather than its intelligent control, that results in mal-
adjustment; and individuals most often learn this unintelligent
horror from their parents.

The Education of the Individual. — Obviously, if the parent
is to be a good teacher of hygienic attitudes, he must himself be-
come well adjusted ; and, indeed, every educated individual should
make it his business to know something about the principles of
mental hygiene, for even if he himself is well adjusted, he needs
to know how to sympathize and deal with people who are not.

Mental Illness and Mental Health 617

It is impossible here to give anything like an adequate outline
of the knowledge that everyone should have, but one who is in-
terested in securing that knowledge may find it in books, in maga-
zines, in lectures, and in a few high school and college courses;
and it is hoped that the past two chapters may serve as a preface
for further study on the part of the readers of this book.

Briefly, good mental hygiene requires that we frankly face
whatever difficulties we experience in adjusting to cultural stand-
ards and social realities, and avoid resorting to escape mechanisms.
It involves the building up of a conscience that is responsive to the
welfare of others and an ambition to take a worth-while, but not
necessarily exalted, part in the work of the world, while avoiding
as completely as possible useless fears of doing wrong or of failing.
It involves the ability to make friends and the overcoming of the
egocentricity and "touchiness" that make friendship difficult;
and, finally, it involves training and practice in those habits that
make for successful adjustment to culture and society.

CHAPTER SUMMARY

Mental diseases are merely extreme forms of the maladjust-
ments common to many people. They are divided into two groups :
the neuroses, or minor mental ailments, and the psychoses, or
major mental ailments. The psychoses are divided into two groups :
the functional psychoses, for which no definitely causal physical
defects have yet been demonstrated, and the organic psychoses,
of which actual damage to the nervous system is known to be
a cause.

Three types of neurosis are distinguished, namely, ( I ) neuras-
thenia, which is characterized by physical ailments that are due
to maladjustment of the autonomic nervous system, anxiety, and
hypochondria; (2) psychasthenia, characterized by obsessions,
phobias, compulsions, doubts, and scruples; and (3) hysteria,
which is a disease of dissociation, being characterized by fugues,
various types of fits, anesthesias, and paralyses.

Three types of functional psychoses are distinguished, namely,
( I ) manic-depressive psychosis, characterized by spells of elation
and mania alternating with depression and retardation of thought
and activity; (2) paranoia, characterized by delusions of persecu-

618 Mental Illness and Mental Health

tion and of grandeur; and (3) dementia praecox, characterized by
withdrawal from reality and regression.

Seven causes of organic psychoses are listed, namely, ( I ) bac-
terial infections, especially those of syphilis which cause paresis
and other diseases, and those which cause encephalitis; (2) bac-
terial toxins, which may produce certain cases that are diagnosed
as dementia praecox and manic-depressive insanity; (3) narcotic
drugs, especially alcohol; (4) physical injuries to the brain, caused
by blows or by tumors ; (5) dying out of the brain cells in old age
which produces the senile psychoses; (6) hardening of the ar-
teries going to the brain; and (7) disorders of the endocrine
glands, which bring about several abnormal mental states, includ-
ing involutional melancholia and certain cases diagnosed as de-
mentia praecox and manic-depressive insanity.

There is considerable dispute concerning the causes of mental
disease. The organicists believe that the failure to adjust to the
strain of socialization is in all cases due to some actual damage
done the nervous tissues, while the functionalists believe that many
cases of neurosis and functional psychosis can be produced by poor
habits of adjustment alone. One's hereditary allotment of genes
can determine one's susceptibility to mental disease, but in only a
few exceptional cases are the genes known to be the crucial de-
terminers of a mental disorder.

Not only because of the great prevalence of mental diseases, but
because of the still greater prevalence of minor maladjustments,
a program of mental hygiene is highly desirable. Such a program
involves, first, the better training of medical practitioners and
specialists and the development of the profession of clinical psy-
chology; second, the proper training of parents, since mental mal-
adjustment is usually handed on from parent to child ; and, finally,
considerably greater knowledge of mental hygiene throughout the
entire population.

QUESTIONS

-i. Outline the various types of mental disease.

2. What may be said concerning the causes of mental disease?

3. What are some of the things needed to produce more hygienic
mental conditions in our civilization?

Mental Illness and Mental Health 619

GLOSSARY

anesthesia (an'es-the'zhi-a) Loss of sensitivity.

compulsion A morbid, uncontrollable impulse to perform an act which
is usually recognized by the individual to be wrong or foolish.

dementia praecox (de-men'sha pre'coks) Name for disorders charac-
terized by extreme withdrawal from reality.

encephalitis lethargica A type of nervous and mental disease caused
by brain infection.

hypochondria (hl'po-kon'dri-a) Morbid concern about one's usually
imaginary illnesses.

hysteria (his-te'ri-a) Name for disorders characterized by loss of
emotional control, fugues, functional anesthesias and paralyses, and
many other symptoms whereby the patient escapes difficulties
through dissociation.

manic-depressive psychosis Name for disorders characterized by ex-
cessive elation and depression.

neurasthenia (nu'ras-the'ni-a) Name for disorders characterized by
malfunctioning of the autonomic nervous system, fatigue, anxiety,
and hypochondria.

neurosis (nu-ro'sis) Minor mental disorder.

obsession A morbid idea which continually recurs against the indi-
vidual's will.

paranoia (par'a-noi'a) Name for disorders characterized by compen-
satory delusions of persecution and grandeur.

paresis (par'e-sis) Insanity caused by syphilitic attack upon the cere-
bral cortex.

phobia (fo'bi-a) A morbid, uncontrollable fear.

psychasthenia (sik'es-the'ni-a) Name for disorders characterized by
obsessions, phobias, compulsions, doubts and scruples.

psychoanalysis (si'ko-a-nal'i-sis) A method of treating mental dis-
eases by causing the patient to talk about his troubles until he be-
comes conscious of the repressed anxieties that produce his symp-
toms.

psychosis (si-ko'sis) A major mental disorder.

CONCLUSION

We have completed our survey of the role of the human organ-
ism in the world of life. We find that man, by virtue of his biologi-
cal heritage, is essentially an animal, and yet that he has become
somehow uniquely human by virtue of his cultural heritage. We
realize at the same time that culture itself is not something quite
apart from and above other biological phenomena, but rather a
special aspect of the total activity of the world of life, being
made possible by the special anatomy and physiology that de-
veloped in the human response system during the course of evolu-
tion.

In conclusion, we shall turn our attention to the sciences that
have provided us with this picture of the human race, to discover
what their contributions have been to the cultural tradition which
we have found to be of such importance in human life.

In the first place, the scientific study of life has provided us
with an entirely different picture of man and of the world in which
he lives than we possessed prior to the development of biology.
Just as the science of astronomy has greatly enlarged our pic-
ture of the physical universe, so the discoveries of Darwin and
of other biologists who preceded and followed him have revolu-
tionized our picture of the origin of the human race. We find that
our history goes back for hundreds of millions of years, involving
a gradual emergence of living beings from inorganic materials and
the slow modification of these beings to produce millions of dif-
fering patterns of organic structure, of which the human pattern
is only one.

Biological science, especially in the fields of physiology and
psychology, has provided us with a different view of our own
make-up than we previously had. The traditional view held that
we are composed of two parts, a physical body, and a non-physical
or spiritual mind. It cannot be said that this idea has been dis-
proved by biological research, but merely that no evidence in favor

620

Conclusion 621

of it has ever been unearthed. Moreover, the very attitude of
the scientist tends to lead him to think of the human organism
as a unitary affair in which the traditional mental qualities of
consciousness, purpose, and capacity for reasoning are looked
upon as the outcome of the special kinds of physical and chemical
activities that go on in the organism.

It would be impossible and out of place here to go into a dis-
cussion of the relationship of these new points of view to tradi-
tional religious belief. The picture of the world and of man with
which science provides us is indubitably different from the pic-
ture provided by our religious tradition, but that does not mean
that the really essential truths of religion have been overthrown
by science. Many of the greatest religious thinkers of the present
era believe that the changes in outlook which scientific evidence
has forced upon us involve only the non-essential parts of the
Christian religion.

Religion is concerned with the relationship between God and
man. The most essential change in our point of view concerning
that relationship which acceptance of the scientific point of view
entails is that it seems to make man a less important part of God's
universe than he formerly considered himself to be. But religion
is the worship of God, not of man; and when the human race
has had time to assimilate scientific knowledge and make it an in-
tegral part of its religious thought, the most important change
may be merely that man will walk much more humbly before his
God than he ever has in the past.

6ut although the biological sciences, along with other branches
of science, may have struck blows at man's self-conceit, they have,
by increasing his control over the forces of nature, given him
grounds for greater self-confidence than ever before. By means
of biological knowledge, man has begun the conquest of disease
and has been able to increase his store of wealth through plant
and animal breeding, through the elimination of plant and animal
diseases, and through knowledge of how to provide proper condi-
tions of nutrition for the organisms upon which he depends for
his livelihood.

At the same time, there is reason to wonder whether the civiliza-
tion that has been built up on the basis of science during the past
few hundred years has actually produced a better and happier so-

622 Conclusion

ciety of men than have the numerous prescientific cultures that
have preceded it. The mere fact that the weapons forged by science
can be employed to wage warfare on a more destructive scale
than ever before should free us of complacency concerning the
value of our scientific civilization. Up to the present, the biological
sciences have done more to make war humane than to make it
more terrible. But already the physiological properties of poison
gas constitute an interesting problem for those who are engaged
in getting us ready to defend our rights against the assertions
of right which other nations hold equally dear; and already men
are beginning to talk of attacking their enemies through the dis-
semination of the germs of disease.

Furthermore, in spite of the opportunities for wealth which
scientific inventions have opened up to .us, millions of the earth's
population still live in direst poverty, and we seem unable to
organize our economic life in such a manner as to assure even a
modest degree of material welfare for all our people. And while
we have done much to overcome the ravages of pathogenic or-
ganisms, human ignorance, prejudice, and superstition still block
the way to accomplishing as much as we could in that direction.
Only a few nights before the writing of this conclusion, a radio
commentator was prevented from talking on the campaign to
eliminate syphilis because it was against the policy of the authori-
ties to allow such things to be discussed in public.

There are so many human problems to which science as yet can
offer no solution. If anything, our civilization has produced an
increase in the number of people who live out long lives wracked
by the subtle torture of mental maladjustment. The mental hy-
giene that we know may offer some alleviation for this condition,
but only further knowledge can enable us to eliminate it.

It is well to recognize these problems, these failures on the
part of our scientific civilization. But they should not cause us
to lose our faith in science. Indeed, only modern men imbued
with the modern tradition, which is essentially a scientific tradi-
tion, could think of any other attitude toward these conditions
than fatalistic acceptance. Today, because we have solved certain
problems in the past, we feel that we can solve others in the fu-
ture. Much of the failure of science to benefit mankind may be
laid to what we call ' 'human nature. " But already we know that

Conclusion 623

if we can only find out how to go about it we can change human
nature. There is only one cure for the ills of a scientific civiliza-
tion, and that is more science, better directed, and more intelli-
gently assimilated into the program of human life.

It can be so assimilated only if educated human beings come
to understand science and the part that it can play in everyday
life. To begin your education in that direction has been the major
purpose of this book. As for the better direction of scientific en-
deavor, there are two points on which we believe most well-in-
formed persons would agree. First, without decreasing the amount
of work going on in the physical sciences, there is need for a great
increase in the scientific study of man himself, within the range
of both the biological and the social sciences. Second, without
decreasing the amount of study of practical scientific problems,
there should be a considerable increase in the study of purely
theoretical problems which at the moment seem to have no bear-
ing on human welfare. This is a principle which is at first diffi-
cult to understand, but a careful consideration of what you have
learned in the reading of this book should enable you to see that
scientific knowledge constitutes a unified whole, in which the great
general principles direct us toward the information we need to
solve specific problems. The discovery that all organisms are com-
posed of cells was at first of purely theoretical interest, but every
student of pathology employs this information in his efforts to find
how our tissues may be made resistant to the attacks of patho-
genic organisms. Mendel's fundamental laws of heredity would
never have been discovered by an individual who was interested
only in how to breed better race horses ; but they have provided us
with the basic key to the control of the inheritable potentialities
of all organisms. The most important thing for the non-scientific
public to learn about science is that purely theoretical studies con-
stitute the most important aspect of scientific research. For it is
the public which supports scientific work and makes possible scien-
tific activity on a large scale, and the direction of scientific en-
deavor lies in its hands.

The aim of this brief conclusion has been to discuss the signifi-
cance of science, especially of the biological sciences. The point
of view expressed comes from no higher authority than the authors
themselves ; you need not agree with it. But if the discussion has

624 Conclusion

suggested to you that scientific knowledge should become an in-
tegral part of your life, as science itself is an integral part of the
civilization in which you live, and that both you and your civiliza-
tion need to consider how science may best be fitted into the whole
scheme of human life, the chief end of this conclusion — and, in-
deed, of this entire book — will have been attained.

Appendix I
THE CLASSIFICATION OF ORGANISMS

The world of life contains a vast number of species, the naming
and classification of which is a study in itself. In order to orient
the reader to the methods of classification, and to the relationships
among animals and plants, an outline of this system of classification
is given here.

The basic unit of the system is the species, which is defined in
Chapter XV. Subdivisions of the species which have different geo-
graphic ranges but which intergrade with each other in their morpho-
logical characteristics and are infertile are known as subspecies or
varieties. Two or more species that are alike in many characteristics,
as are the various species of foxes, are grouped together in the same
genus. The scientific name of any species is made up of the Latin
name for the genus, followed by the Latin for the species itself.
Thus the red fox is called Vulpes fulvus; Vulpes to indicate that
it belongs to the fox genus, and fulvus to show that it is of the species
red fox. Similarly the blue fox is called Vulpes lagopus.

The genus Vulpes is classed, along with other genera having dog-
like characteristics, in the dog family, or family Canidae. The
Canidae are similar to several other families, such as the bear family
and the cat family, in that they have teeth especially developed for
killing and eating other animals ; and on the basis of that similarity
all these families are placed in the order Carnivora. The carnivores
show resemblance to horses, rats, bats, monkeys, men, and various
other animals in that they have hair and suckle their young. All of
these belong to the class Mammalia. The mammals in turn are like
birds, reptiles, fishes, and certain other classes in that they all have a
cord of nervous tissue running down their backs, similar to our spinal
cord, and are therefore classified in the phylum Chordata. Finally,
the chordates, along with all other animal phyla, belong to the animal
kingdom, rather than to the plant kingdom. In addition to the above
categories, subkingdoms, subphyla, subclasses, suborders, subfamilies,
and subspecies — or varieties or races — are used whenever they are

625

626 Appendix I

necessary for classificatory purposes. The chordate phylum, for exam-
ple, is divided into four subphyla, of which the subphylum containing
the animals with backbones, the Vertebrata — including, as it does,
the fishes, reptiles, birds, mammals, and two other classes — is the
most important.

This scheme of classification is used for all living forms. As an
illustration of how it works out, we may give the biological classifi-
cation of a human being:

KINGDOM: Animalia
PHYLUM : Chordata

SUBPHYLUM : Vertebrata
CLASS: Mammalia
ORDER : Primates
FAMILY: Hominidae
GENUS : Homo

SPECIES : Homo sapiens

The reader will doubtless be startled to learn that the biologist has
him filed away under such a complicated system of headings and sub-
headings. As a matter of fact, the biologists look upon the species
as the fundamental unit of classification, and for ordinary purposes
a member of the human race would be just another Homo sapiens.

The following is a list of the major phyla of the plant and animal
kingdoms. Several minor phyla are omitted, the list being confined
chiefly to the phyla containing members that have been introduced
in the body of this book. In some cases the classes included in a
phylum are mentioned.

THE MAJOR PLANT PHYLA

SCHIZOPHYTA — Includes the bacteria and blue-green algae, which re-
semble each other in that they have no well-defined nucleus and
never reproduce sexually.

CHLOROPHYTA — The green algae.

CHRYSOPHYTA — A group of yellow-green algae, including the uni-
cellular diatoms which constitute an important element of the
plankton.

PHAEOPHYTA — The brown algae.

RHODOPHYTA — The red algae.

FUNGI — All the colorless thallus plants except the bacteria. They in-
clude yeasts, molds, mildews, blights, rusts, smuts, mushrooms, and
bracket fungi. The lichens are classed as fungi, although they al-
ways have green algae living with them symbiotically.

The Classification of Organisms 627

BRYOPHYTA — Non- vascular seed plants. The phylum includes two
classes: (i) Hepaticae, or liverworts. (2) Musci, or mosses.

PTERIDOPHYTA — Vascular land plants without seeds. The most impor-
tant plants of this phylum are the ferns. Others, less well known,
are the club mosses and horsetails.

SPERMATOPHYTA — The seed-bearing plants. The phylum includes two
classes: (i) Gymnospermae, or cone-bearing plants, composed
chiefly of such evergreen trees as the pines, hemlocks, and spruces.
(2) Angiospermae, the flowering plants, including the plants we
ordinarily term flowers and grasses, together with the non-cone-
bearing trees and shrubs. Nearly all our domesticated plants belong
in this class.

MAJOR ANIMAL PHYLA

PROTOZOA — The unicellular animals. There are four classes: (i)
Rhizopoda, which display ameboid movement. (2) Flagellata, which
move by means of flagellae. (3) Infusoria, which possess cilia. (4)
Sporozoa, which reproduce by means of spores.

PORIFERA — The sponges. These are non-motile animals which ingest
their food by means of ciliary movements that drive currents of
water through thousands of pores placed in their sides.

COELENTERATA — Animals with a body composed of two fundamental
cell layers, with a mouth, but no anus. In the addition to Hydra,
this phylum includes jellyfish, sea anemones, and corals.

ANNELIDA — Segmented worms, including many marine worms, the
earthworm, and the leech.

MOLLUSCA — Sessile or slow-moving animals, usually having a shell.
The phylum includes snails, slugs, clams, oysters, octopi, and
squids.

ARTHROPODA — Animals with jointed legs and an external skeleton.
The phylum includes insects, spiders, centipedes, crabs, lobsters,
and many similar forms. It is the most numerous of all the phyla,
and the class Insecta alone contains far more than half o%f all the
known animal species.

CHORDATA — Animals with a dorsal nerve cord. It includes four sub-
phyla, of which the subphylum Vertebrata is the most important.
Of all the forms in the other subphyla, Amphioxus alone has been
mentioned in this book. The subphylum Vertebrata is divided into
seven classes : ( i ) Cyclostomata, long, thin, fish-like creatures with-
out jaws or lateral fins, including the lampreys and hagfish. (2)
Elasmobranchii, fish with cartilaginous 'skeletons, of which the
sharks are most widely known. (3) Pisces, the bony fishes, includ-
ing the greater proportion of all fishes. (4) Amphibia, frogs, toads,

628 Appendix I

salamanders, and newts. (5) Reptilia, snakes, turtles, lizards, alli-
gators. (6) Aves, or birds. All members of this class are warm-
blooded and have feathers. (7) Mammalia. The members of this
class are warm-blooded, have hair, and suckle their young. The
class is divided into three subclasses: (i) Prototheria, or egg-
laying mammals, commonly called monotremes, from the name of
their order. They include the duckbill, shown in Fig. 74. (2) Meta-
theria, or animals that carry their young in a pouch. They include
the kangaroo and opossum, and they are often spoken of as mar-
supials, from the name of their order. (3) Eutheria, or placental
mammals, having a placenta to nourish their unborn offspring. The
following is a list of the most important orders among the placental
mammals :

Insectivora — the insect eaters, small brained and very primitive —
shrews, moles, hedgehogs.

Chiroptera — winged mammals — bats.

Primates — lemurs, monkeys, apes, men — distinguished chiefly by their
large brains and prehensile digits, with thumb opposed to finger.

Carnivora (suborder Fissipedia) — flesh eaters, with strongly de-
veloped canine teeth — lions, tigers, cats, hyenas, raccoons, bears,
otters, weasels, skunks, foxes, dogs.

Carnivora (suborder Pinnipedia} — aquatic, carnivorous mammals
with fin-like limbs — seals, sea lions, walruses.

Rodentia — small, clawed, with strongly developed front teeth for
gnawing — squirrels, woodchucks, beavers, rats, mice, gophers, rab-
bits.

Edentata — slow moving, teeth absent from front jaw — anteaters,
sloths, armadillos.

Ungulata — herbivorous, hoofed, molars large and broad — elephants,
horses, camels, deer, cattle, sheep, swine.

Sirenia — aquatic, herbivorous, front limbs fin-like, hind limbs absent,
hair almost lacking — includes the dugong and manatee.

Cetacea — aquatic, fish-like in shape, front limbs many-jointed fins,
hind limbs absent, hair almost lacking — whales, dolphins, porpoises.

Appendix II
THE BRANCHES OF BIOLOGICAL SCIENCE

The study of living organisms has grown into such a vast, complex
field of science that it is impossible for one man to possess all of the
knowledge that constitutes biology. As a result, the science is divided
into a number of branches, which differ chiefly in their methods of
approaching the study of life. None of these branches, however, is in-
dependent of the others, and it is impossible for a scientist really to
grasp any one of them without knowing something about the rest. For
a long time biologists, as well as scientists in other fields, tended to
become more and more specialized and to "know more and more
about less and less," but this attitude is gradually changing, and
biologists in all branches of the science are realizing more and more
what contributions the other branches can make to their own.

Each of the branches is itself a "pure" science, i.e., the scientists
working in it are concerned chiefly with discovering new facts and
theories in it, thereby adding to the world's knowledge. Much of this
knowledge, however, has a practical application to the needs of man,
and there is consequently a series of "applied" branches of biology,
which are concerned with the application of science to the needs of
mankind. In the following the applied branches are mentioned after
the pure branches to which they correspond most closely.

Botany is the study of the plant kingdom, and zoology that of ani-
mals. This division more or less separates biologists into two groups,
although many branches, such as physiology and genetics, include
parts of both botany and zoology.

Systematic biology (botany or zoology) or taxonomy is concerned
with the classification of animals and plants. When studied by itself,
it centers about the interrelationships and the evolution of species (as
well as genera, families, and orders), but it is a valuable tool for the
other branches, since one cannot understand the significance of the
form or functions of an organism unless one knows its name and its
relationships to other organisms. This branch of biology is divided
into a number of subbranches concerned with the different divisions
of the animal and plant kingdoms, such as protozoology, entomology

629

<>3° Appendix II

(the study of insects), icthyology (the study of fishes), ornithology
(birds), mammalology, bacteriology, algology, mycology (fungi),
bryology (mosses), etc. The application of systematic zoology is
general, but is, of course, most important when concerned with or-
ganisms that are either useful or harmful to man. Thus icthyology
and the systematic study of plankton organisms are very important in.
studies of fisheries, economic entomology is of great importance in
studying the characteristics and the identification of insect pests, and
systematic bacteriology and mycology are of obvious value in con-
quering the diseases of animals and plants. Systematic botany of the
higher plants is of particular economic value when concerned with
trees (dendrology) or with the various crop plants (economic botany
in general).

Ecology is the study of organisms in relation to their environment.
Many of the problems in this field are discussed in Chapter XV.
Particularly in the form of animal ecology, it is largely a more exact,
scientific study of the same problems taken up formeily by students
of natural history. Plant ecology is largely the application of the
principles of plant physiology to the study of natural communities of
plants in the field, and forms the scientific background of forestry.
The applications of animal ecology are chiefly in the study of our
fish and game resources, as well as in the management of cattle and
other stock ranges.

Pathology is the study of diseases of animals and plants. It is, oi
course, largely an applied science, animal pathology being one of the
major divisions of medicine, and plant pathology is essential to agri-
culture. Medical pathology is, of course, closely linked with bac-
teriology, while plant pathology is similarly associated with the study
of fungi, or mycology, and with economic entomology.

Morphology and anatomy are the study of the form and structure
of organisms, the former emphasizing the external, and the latter the
internal features. The men who study these branches of biology use
their knowledge to help explain the evolutionary history of the larger
divisions of the animal and plant kingdoms, i.e., the orders, classes,
and phyla, using criteria such as those described in Chapter XIV
under "The Evidence from Comparative Anatomy."

The application of the study of human anatomy is obvious, as it is
the basis of all surgery, while mammalian anatomy in general is
equally important to veterinary science. The anatomy of woody
plants is valuable to forestry and the lumber industry; and in eco-
nomic botany the anatomy of the fiber-producing plants, such as
cotton, hemp, and flax, is of great practical value, as is also that of
the plants which produce valuable secretions, such as rubber.

The Branches of Biological Science 631

Embryology is the study of the early development of organisms, and
as a separate branch of biology is practically confined to animals,
since the embryology of plants is too simple to be a study in itself.
The embryologist is occupied with the relation of his subject to evolu-
tion, but more particularly to the problem of the differentiation of
organs and tissues. The application of mammalian embryology to
medicine and veterinary science is obvious.

Histology is the study of tissues, and, like embryology, is confined
as a separate branch to zoology, since plant tissues are simple enough
so that their study comes within the field of the plant anatomist. The
histologist, like the embryologist, is concerned with the differentiation
of cells and tissues, and also with their function, including such prob-
lems as glandular secretions, pigmentation, etc. The application of
histology to medicine is great, since most disease bacteria confine
their activities to particular tissues.

Cytology is the study of the cells of organisms. Although many
cytologists are concerned with the nature of protoplasm in general,
and thereby join hands with the physiologists, the importance of the
chromosomes in heredity has attracted so much attention to these
bodies that the bulk of modern cytology is the study of the structure
and behavior of the chromosomes, as well as their importance in
heredity and evolution. The application of cytology is chiefly to plant
and animal breeding.

Genetics, or the study of heredity, is only thirty-seven years old as
a separate branch, dating from the rediscovery of Mendel's laws.
(See Chapter XIII.) But it has become one of the largest and most
complex of the branches of biology. Geneticists are occupied primarily
with the nature and mode of operation of the factors controlling in-
leritance, chiefly the gene, but are also interested in the study of mu-
tations as applied to evolution. The application of genetics to the
improvement of domestic animals and cultivated plants has already
been discussed, and comprises the applied sciences of animal breeding
and plant breeding; the study of human genetics with a particular
view toward bettering the inheritance of mankind is known as eu-
genics.

Physiology is the study of the organism from the point of view of
its activity and functions. There are three main aspects of this most
important branch of biology. General physiology aims to study the
nature of protoplasm and of the essential processes which keep it
alive. Animal physiology studies in particular those processes most
essential to animals, such as circulation of the blood, muscular and
nervous activity, and the digestion and absorption of food. Plant
physiology studies such problems as the manufacture of food by

632 Appendix II

photosynthesis, the intake and outgo of water, the absorption of
mineral salts, -and the translocation of substances through the plant.

The applications of physiology are very numerous and varied. Ani-
mal, or rather human, physiology is, of course, the basis of a very
large proportion of the science of medicine. Furthermore, it forms
the background of animal husbandry, an applied science dealing with
the problems connected with domestic animals, such as milk produc-
tion in cows and egg production in poultry. Plant physiology is equally
important in its applications to agriculture. Efficient production in
agriculture depends on a scientific knowledge of the factors affecting
ihe growth of plants. The problem of the fertilization of plants is
within the field of plant physiology, as is that of irrigation in dry
regions and of hardiness and frost resistance in regions with cold
winters.

Psychology is the study of the behavior and experience of animals.
Plant behavior is so slightly developed that a specialized science is
not required for its study. The chief interest in psychology has
centered in the human organism ; but, while it is difficult to study the
experience of animals, the branch of psychology known as animal be-
havior is now developing rapidly. Certain branches of psychology are
closely allied in interest to the physiology and anatomy of the sense
organs and the nervous system, but the psychologist studies the be-
havior of the intact organism, rather than the responses that take
place in isolated cells, tissues, and organs. Scientific psychology came
into being about sixty years ago through a fusing of the interests of
physiologists and philosophers. Those who view it as chiefly descended
from philosophy are inclined to class it with the social sciences ; and,
indeed, it is basic to social science, since social science is very largely
a study of the behavior of human beings. However, much of the most
important part of psychology has little social significance, and other
biological disciplines, such as genetics and the study of evolution, are
also important to social science. Hence, we feel that psychology is
properly a branch of biology, although it is doubtless near the border
line between the biological and social sciences; and we feel certain
that most psychologists, in this country at least, would concur in
that classification. Psychology is being applied in industry, mer-
chandising, education and child care, and in dealing with criminals
and delinquents as well as with others who are mentally ill or socially
maladjusted. Psychiatry is the medical specialty which has to do with
the care of the insane and neurotic. Since, in the past, psychology
has not been sufficiently advanced to furnish an adequate set of
principles for dealing with the complex problems of mental abnormal-
ity, psychiatry has been developed largely by practical medical men,

The Branches of Biological Science 633

without much contact with the main body of psychological research;
but at the present time psychologists and psychiatrists are taking
more and more interest in one another's work ; and in due time psy-
chiatry will doubtless take its place as that part of applied psychology
which has to do with the cure of mental disease.

THE RELATIONSHIPS BETWEEN THE BIOLOGICAL, SOCIAL,
AND PHYSICAL SCIENCES

There is a close connection between biology and physics and chem-
istry, since an organism is essentially a highly complex system of
physico-chemical processes. Biophysics and biochemistry study the
physics and chemistry of protoplasm and its activities. By doing this
they link physiology closely with physics and chemistry. Mathematics
is an integral part of the science of biometry, in which biological
problems, chiefly those involving large populations of organisms, are
treated statistically. Statistical methods also play a large part in
genetics and psychology. The study of extinct, fossil forms of life, i.e.,
paleontology, links biology with geology. Another connection between
biology and geology, as well as geography, is through plant and animal
geography, i.e., the study of the present distribution of plants and
animals. Since the distribution of the species of organisms often is
intimately bound up with the history of the region in which they occur,
this branch of biology has made contributions to geology and geogra-
phy. Through human biology, connections are established between
biology and the various social sciences. Archaeology, or the study of
ancient cultures and civilizations, and anthropology and ethnology9
the study of human races and cultures, connect the biological aspects
of human evolution with history and sociology.

SUGGESTED READING

The following are a few of the many books that deal with the
matters discussed in this text. Some of them, especially the textbooks,
have been chosen because they give a comprehensive account of
certain fields of biological science. Others are included because they
embody interesting and valuable treatments of special topics.

GENERAL

Wells, H. G., Huxley, J. S., and Wells, G. P. The Science of Life.
Doubleday, Doran, 1951- This book deals with nearly all the
subjects covered in this text, but in much greater detail. It is
interestingly written for the general reader.

INTRODUCTORY TEXTBOOKS

Jean, F. C, Harrah, E. C, and Herman, F. L. Man and the Nature
of His Biological World. Ginn, 1934. An elementary text stress-
ing the importance of biological knowledge to human life.

Brown, W. H. The Plant Kingdom. Ginn, 1935. An excellent ele-
mentary botany.

Guyer, Michael F. Animal Biology. Harpers, revised edition, 1937.
An outstanding text, stressing the functional aspects of animal
life.

Hegner, R. W. College Zoology. Macmillan, revised edition, 1936.
The standard introductory text in structural zoology and animal
taxonomy.

Crandall, Lathan A. An Introduction to Human Physiology. Saun-
ders, 1934. A brief elementary text.

Williams, J. F. A Text-book of Anatomy and Physiology. Saunders,
1935. This short text is especially good for its anatomical charts.

Martin, H. N. Human Body. Holt, I2th edition, 1934. A somewhat
more advanced, but clearly and interestingly written text in
anatomy and physiology.

Dashiell, J. F. Fundamentals of General Psychology. Houghton Mif-
flin, 1937. A leading textbook stressing the psychology of be-
havior.

Boring, E. G., Langfeld, H. S., and Weld, H. P. Psychology. Wiley,
1935. The best text for the study of the psychology of con-
sciousness.

634

Suggested Reading 635

Ruch, Floyd L. Psychology and Life. Scott, Foresman, 1957. An
elementary text dealing especially with the aspects of psychology
that are of greatest interest and practical value to the college
student.

PLANTS, ANIMALS, AND THEIR EVOLUTION

Romer, A. S. Man and the Vertebrates. University of Chicago Press,

1933. An interesting account of the evolution of the vertebrates.

About half the book is devoted to human evolution and the

development of the human body.
Coulter, Merle C. The Story of the Plant Kingdom. University of

Chicago Press, 1935. A description of plants and their evolution.
Mason, Frances, ed. Creation by Evolution. Norton, 1935. Points of

view on evolution by several authorities.
Lull, Richard S. Organic Evolution. Macmillan, 1927. The classic

textbook in the field of evolution.
Morgan, Thomas H. The Scientific Basis of Evolution. Norton, 1935.

A discussion of the causes of evolution by a leading geneticist.

HEREDITY AND DEVELOPMENT

Snyder, Lawrence H. The Principles of Heredity. Heath, 1935. A

standard textbook on genetics.
Holmes, S. J. Human Genetics and Its Social Import. McGraw-Hill,

1936. A clearly written discussion of heredity and eugenics.
Davenport, Charles B. How We Came by Our Bodies. Holt, 1936.

A description of the mechanisms of heredity and the course of

development.

Mohr, Otto H. Heredity and Disease. Norton, 1934.
Sinnott, E. W., and Dunn, L. C. Principles of Genetics. McGraw-Hill.

2nd edition, 1952.

THE HUMAN BODY AND ITS HYGIENE

Clendening, Logan. The Human Body. Knopf, revised edition, 1937.
A popular and fascinating account of human physiology and
anatomy.

Haggard, Howard Wilcox. The Science of Health and Disease.
Harpers, revised edition, 1938. A popular account of human
ills and how to avoid them.

Diehl, Harold S. Healthful Living. McGraw-Hill, 1935. A good book
dealing with the everyday hygiene of the normal individual.

Hoskins, R. G. The Tides of Life. Norton, 1933. A popular, authori-
tative treatment of the endocrine glands.

Cannon, Walter B. The Wisdom of the Body. Norton, 1932. A de-
scription of research on the functions of the vital reflexes.

636 Suggested Reading

Parran, Thomas. Shadow on the Land: Syphilis. Reynal & Hitchcock,

1937. An appraisal of a serious public health problem by an out-
standing leader in the present campaign against venereal disease.

THE HUMAN MIND AND ITS HYGIENE

Anastasi, Anne. Differential Psychology. Macmillan, 1937. A well-
written book on the differences between individuls and groups.

Guthrie, E. R. The Psychology of Learning. Harpers, 1935. An under-
standable discussion of how learning takes place.

Warden, Carl J. The Emergence of Human Culture. Macmillan,

1936. Culture is shown to be an emergent appearing in the course
of biological evolution.

Kellogg, W. W. and L. A. The Ape and the Child. McGraw-Hill,
1933. The story of an interesting experiment to discover how
human an ape would become if reared like a normal child.

Mead, Margaret. Sex and Temperament in Three Primitive Societies.
Morrow, 1935. How culture molds human nature. A description
of the life of three primitive tribes.

Hepner, Harry W. Finding Yourself in Your Work. Appleton-Cen-
tury, 1937. A stimulating discussion of the problem of choosing
a vocation and succeeding in it.

Menninger, Karl. The Human Mind. Knopf, revised edition, 1937.
A very popular book on the mind as the psychiatrist sees it.

Hart, Bernard. The Psychology of Insanity. Macmillan, 1934. The
best book for acquiring an elementary understanding of func-
tional insanity and its causes.

Guthrie, Edwin R. The Psychology of Human Conflict. Harpers,

1938. Maladjustments are viewed as resulting from the wrong
sort of learning.

Homey, Karen. The Neurotic Personality of Our Time. Norton,

1937. An interesting account of human maladjustments and their
relation to our particular form of culture.

SCIENTISTS AND THEIR WORK

Jaffe, Bernard. Outposts of Science. Simon & Schuster, 1935. Tells
of the work of many outstanding modern biologists.

Locy, W. A. Biology and Its Makers. Holt, 1908. A history of biology
in terms of the men who have contributed most to it.

de Kruif, Paul. Microbe Hunters. Harcourt, Brace, 1926. The work
of the bacteriologists.

de Kruif, Paul. Hunger Fighters. Harcourt, Brace, 1928. Many kinds
of biologists who have contributed to the increase and improve-
ment of our diet.

Suggested Reading 637

Keller, F. S. The Definition of Psychology. Appleton-Century, 1937.

Tells of the men who have laid the basis for the development

of present-day psychological thought.
Garrett, Henry E. Great Experiments in Psychology. Appleton-Cen-

tury, 1930.
Darwin, Charles Robert. Life and Letters. D. Appleton, 1888, 2 vols.

Notable for a single chapter which contains the simple, modest

autobiography which Darwin wrote for his children.
Vallery-Radot, R. The Life of Pasteur. The Garden City Publishing

Company.
Pruette, Lorine, E. G. Stanley Hall. Appleton-Century, 1926. An

analysis of the mind and character of an early leader of American

psychology.
Curie, E. Madame Curie. Doubleday, Doran, 1937.

INDEX

(References to figures are in italics.)

Abdominal cavity, 47

Absorption, 30, 34

Abstractions, 540-541

Abyssal region, life in, 358-361

Accidents, reduction of, through test-
ing, 567-568

Acetylcholine, 468

Acromegaly, 177

Adaptation, 330

Addison's disease, 176

Adenoids, 52-53

Adjustment of plants by means of
response, 513

Adrenal cortex, 176; and precocious
sexual development, 178

Adrenal glands, 171, 172, 176; activity
in exercise, 470

Adrenin, 472 ; and sympathetic system,
466

Afterbirth, 201

Agriculture, cultural evolution of, 403

Air, percentage of gases in, 55

Air bladder in fish, 102, 104

Air sacs in lungs, 53, 60

Alcohol, as a condiment, 77 ; as cause
of psychosis, 609; as constituent of
urine, 86 ; produced in fermentation,

33

Algae, 112; blue-green, 112; brown,
112, 114, 115; colonial, 112; filamen-
tous, 114; green, 112; in lichens,
137; red, 112; types of, 113

Alimentary canal, 66

Alternation of generations, 229-234

Alveoli, 53, 54, 60

Ameba, 44-45

Ameboid movement, 44

Amino acids, 27, 71 ; absorption of,
76; splitting of, 76; variation of, 76

Ammonia, 32, 76, 84; as product of
decay, 134

Amnion, 197, 200, 201

Amniotic fluid, 197, 200

Amoebic dysentery, 164

Amphibians, primitive, 298

Amplitude of light waves, relation to
brightness, 448

Anabolism, 24

Ancon sheep, mutation in, 373

Anesthesias, functional, 603

Annual herbs, 350

Anterior defined, 99

Anthrax, 149, 150

Antibodies, 156

Antibody reactions, 156-157; as re-
sponse to stimulation, 412, 508

Antigens, 156

Antitoxins, 156-158

Ants, social organization of, 338 ; sym-
biotic relationships of, 137

Anus, human, 66, 67, 72, 83-84, 193;
in earthworm, 97; in fish, 102, 103

Anxiety, as cause of maladjustment.
582-584; concerning sex, 209, 616;
in neurasthenia, 600

Aorta, 41, 48, 49, 54

Apes, anthropoid, 393-395; failure to
vocalize and imitate words, 546;
similarity of human infant to, 320

Aphids as parasites, 141

Apoplexy, 1 80, 181

Appendix, 67; as a vestigial organ,
315; of rabbit and man, 516

Aqueous humor, 445, 446

Archeopteryx, 311-312

Archeozoic era, 311

Arctic regions, life in, 342-344

Arteries, 41

Arterioles, 41 ; contraction and expan-
sion in exercise, 470

Arteriosclerosis, 180-181; as cause of
psychosis, 609

639

640

Assimilation, 73-83

Attending, 488

Attitudes, social, 551-552; transmis-
sion from parent to child, 614

Audition, 480

Auditory area of cortex, 483

Auditory nerve, 449

Auditory structures, 449

Auricle, in fish, 103; left, 47, 48, 49;
right, 47, 4*

Australia, fauna of, 321

Auto-erotism, 210-211

Automatic writing, 586-587

Autotrophic organisms, 34

Auxiliary tissues of sense organs, 444

Auxins, 510-511

Axons, 426, 427; branching of, 432-
433; of motor neurons, 429; of sen-
sory neurons, 429

Aye-aye, 393

Bacilli, 150; dysentery, 151; leprosy,
157; tuberculosis, 751; typhoid,
151, 157, 160

Bacteria, 31, 33; as pathogenic organ-
isms, 150, 157; as thallus plants,
112; autotrophic, 32; contained in
feces, 84; decay, 32; decay, role in
carbon cycle, 132; decay, role in ni-
trogen cycle, 134; disease, 32; het-
erotrophic, 32; in the cycle of food
elements, 131-135; iron, 31; nitrate,
32; nitrate, role in nitrogen cycle,
134; nitrite, 32; nitrite, role in ni-
trogen, 134; nitrogen, 135; nitro-
gen-fixing, role in nitrogen cycle,
134

Balanced aquarium, 132

Basilar membrane, 449, 450

Behavior in lower organisms, 526

Behavior patterns, 532-537; human,
552; maturation of, 533-535

Beriberi, 81, 83

Biceps muscle, 416, 417, 435

Bile, 71

Bile pigments, 84

Bile salts, 71, 73

Binet, Alfred, 566, 568

Birds, arctic, 344; mimicry in, 545

Birth, 200-202

Birth control, 212-215

Index

Bladder, human, 84, 85, 86, 189, roj;

in fish, 102, 103; in frog, 104; in

mammal, 105
Blight, 31; chestnut, 140
Blindness, gonorrheal, 207
Blood, 5, 14, 18, 42-45 ; circuit of, 47 ;

clotting of, 45-46
Blood flow, rate of, 50 ; in earthworm,

99

Blood pressure, 50-51 ; and arterioscle-
rosis, 180

Blood stream, 14

Blood sugar, concentration of, 75

Blood vessels, 41-42; in earthworm,
98, 99; in fish, 101, 103

Bracket fungi, 31

Brain, 425; cause of its evolution,
396; difference between human and
ape, 399; in amphibian, 524; in
bird, 525; in earthworm, 522, 523;
in fish, 524; in fly, 527; in mammal,
525; vertebrate, 5^-5/5, 526

Brain stem, 424, 425; integration in,
436; upper, 433; white and gray
matter in, 430, 431

Breathing, and heat control, 473 ; nerv-
ous control of, 463-464

Breathing center, 463

Brightness of light, 448

Bronchi, 53, 54

Bronchioles, 53

Bryophytes, body of, 118

Bud, in Hydra, 96; in seeds, 236

Bulbs, 349

C.A., 571

Cacti, 341

Caesarian operation, 201

Caffeine, as a condiment, 77; as con-
stituent of urine, 86

Calhoun, 576

Calories, 76, 77

Camera, similarity in structure to eye,
444-446

Canal boat children, 575

Cancer, 182-185

Capillaries, 41 ; chemical control of
the opening and closing of, 468-469 ;
contraction and expansion of, in
exercise, 470; in fish, 103; lymph,
52

Carbohydrate digestion, 72

Index

Carbohydrates, 10, n, 26, 32, 33;
and insulin, 173 ; conversion into fats,

75

Carbon cycle, 131-132, 133
Carbon dioxide, 18, 26, 28; effect on

breathing, 463
Care of young in fishes, 239
Carnegie, Dale, 581
Carnivorous animals, 131
Castration, 204
Cat, learning in, 53O-532
Catalysts, 29

Cattle, behavior patterns in, 533
Cell, 9-13, 10 ; animal, 12; plant, 13
Cell bodies, of motor neurons, location

of, 429 ; of sensory neurons, location

of, 429

Cell body of neuron, 426, 427
Cell division, 250-260
Cell wall, 13, 25

Cells, various types of, 14-16, 17, '18
Cellulose, 13 ; transformation to sugar,

32, 132

Cenozoic era, 311
Central neuron, 429
Centrifugal force as a stimulus, 507
Cerebellar cortex, 431; integration in,

437
Cerebellum, 424, 425, 426; white and

gray matter in, 431
Cerebral cortex, 431, 433; integration

in, 437-439, 478
Cerebrum, 424, 425, 426; white and

gray matter in, 431; white matter

of, 433

Cervix, 192, 201
Chain responses in insects, 530
Chancre, syphilitic, 207
Changing environments, adaptations

to, 361-362
Cheilosis, 81
Chemical regulation of plant growth,

500^511

Chemical senses, 450-453
Chemotherapy, 165
Chemotropism, 511
Chicks, development of pecking in,

536
Chimpanzee, 394; intelligence and

emotions in, 395; problem-solving

in, 539-540; reared with child, 545
Chinese Man, 399-400

641

Chlorophyll, 26; importance in evolu-
tion, 295-296

Chloroplasts, 12, 13, 25, 26; in Mar-
chantia, 117

Cholera, 150; Asiatic, 158, 164

Chordates, nervous system in, 524-526

Chorion, 197, 200, 201

Choroid coat, 445

Chromosomal threads, 250

Chromosome mutations, 373-376

Chromosome number, reduction of,
255-257

Chromosome numbers, alternating cy-
cle of, 257-259; in various species,

251

Chromosome pairs, derivation of, from
father and mother, 25 -258

Chromosomes, 251-261, 252, 256, 258,
269; as carriers of genes, 272;
pairing in meiosis, 255 ; XY and sex
determination, 284-287

Chronological age, 571

Chyme, 70, 71

Cilia, 1 6, i/, 29; as defense against
pathogenic organisms, 153; in ne-
phridia, 98, 100; in Vorticella, 94

Ciliary muscle, 445, 446

Circulation, 40-53 ; pulmonary, 50 ; reg-
ulation of, 468-470; systemic, 50

Circulatory organs in fish, 103

Circulatory system, human, 40-50; of
earthworm, 98, 99

Class differences in intelligence, 577-
580

Clinical psychology, 613

Gitoris, 193

Cloaca in frog, 104, 239

Cocci, 150; gonorrhea, 151; pneu-
monia, 151

Cochlea, 448-450, 449

Coelom, 93 ; in earthworm, 97 ; in fish,
101

Cognition, 489; and motivation, inter-
action between, 491-492

Cognitive adjustments of animals, 539-

540
- Cold-blooded animals, 105, 472

Colds, 80, 168

Colloidal system, 6-8, 7

Colonies, first appearance of, 296

Color-blindness, the inheritance of,
287-288

642

Coloration, concealing, 334-336, 345,
353; in the struggle for existence,
334-337; warning, 336, 345

Colorless plants, 31-33, 33; auto-
trophic, 31-32; heterotrophic, 32-33

Commensalism, 130, 135-136

Comparative anatomy, evidence for
evolution from, 314-317

Compensation, 592-593; in paranoia,
604, 606; in parents, 615

Compulsions, 602

Concepts, 489 ; and language, 540-541 ;
in learning, 561-564

Conceptual adjustments, 489

Condiments, 77

Conditioned reflexes as basic units of
learning, 550

Conditioned response, 549

Conditioned stimulus, 549

Conditioning of emotions and reflexes,
548-550

Conducting cells in roots and stems,

122

Conducting channels of seed plant,

123

Conduction, as function of white mat-
ter regions, 432; nervous, 423; of
auxins, 511

Conductors, 414

Cones, 445, 447

Conflict as a cause of anxiety, 584

Connective tissues, 16, 17, 18; fat
storage in, 75

Consciousness, 479-486; analysis of,
480-485 ; emotional, 484-485 ; imagi-
nal, 481-482; in atoms and mole-
cules, 479; relation to cerebral cor-
tex, 438; sensory, 480-481

Contraception, 212-215

Convolutions of cerebral cortex, 432

Copepods, 357

Copulation, 221; in amphibians, 239;
in mammals, 243; in reptiles and
birds, 242

Corals, 34

Corms, 349

Cornea, 445

Corpus luteum, 195

Corpuscles, red, 42-44, 60; white, 43-
44; white, in earthworm, 99

Index

Cortex of kidney, 85

See also Adrenal cortex; Cere-
bral cortex.

Cortical integration, behavior at the
level of, 478-479

Cortin, 176

Cosmogony, 292

Cranial division of autonomic system,
466

Cretinism, 174-175

Cro-Magnon Man, 402

Crop in earthworm, 98, 99

Cross breeding, 280-284

Crossing of species, 281-284

Cultivated plants, evolution of, through
natural selection, 324-325

Cultural factor, and class differences
in intelligence, 578; and individual
differences in intelligence, 573-575 ;
and personality, 582; and race dif-
ferences, 576

Culture, as determiner of behavior
patterns, 552-553 ; as distinctive char-
acteristic of human species, 398-399

Culture habits, 545; development of,
545-553

Cultures, attenuated, 157; bacterial,
149

Cytoplasm, iot n, 12, 13, 15, 25, 29

Dahlias, evolution of, through artifi-
cial selection, 325

Darwin, Charles, 292, 330, 332, 371,
373, 601 ; observations on Galapa-
gos Islands, 323-324

Daydreaming, 499-500; in dementia
praecox, 604, 606-607

Daydreams, overt, 547; sexual, 211

Death rates, apoplexy, 179; cancer,
179; heart disease, 179; nephritis,
179; pneumonia, 165; tuberculosis,
165; typhoid, 161

Decay, 131-134

Deep-sea fish, 359

Defenses against pathogenic organ-
isms, 153

Deficiency diseases, 79-82

Delirium tremens, 609

Delusions, 500-501 ; in the functional
psychoses, 604-607; of grandeur,
599, 606; of persecution, 605

Index

Dementia praecox, 604, 606; in endo-
crine disorder and focal infections,
608-609

See also Insulin in the treatment
of dementia praecox.

Demoniacal possession, 598

Dendrites, 426, 427

Dependence of animals on green plants,
130-131

Depression in manic-depressive psy-
chosis, 605

Descent with modification, 295

Desert, life in, 34O-342

Desire for approval, development of,
547, 582-584

Desmids, 356

Detroit, reduction of goiter in, 175

Development, of human behavior, 544-
564; of synaptic relationships, 534-

535
Developmental reactions of animals,

513-517

De Vries, 372, 373

Diabetes mellitus, 172-173

Dialysis, 57-59

Diaphragm, human, 47, 54; in fish,
101 ; in mammal, 105

Diastase, pancreatic, 71, 72

Diathermy in treatment of paresis, 608

Diatoms, 357

Diazone, 167

Diehl, 179

Diet, 79-83

Differential birth rate, 214; and eu-
genics, 577-58o

Differentiation of cells, 254, 514

Diffusion, 57-58

Digestion, human, 66-73; in Parame-
cium, 29; of wood by decay bac-
teria, 32; regulation of, 470-471

Digestive juices, 68

Digestive system, human, 66-68; of
earthworm, 98

Digestive tract, human, 67; in fish,
1 01, JO-?; in frog, 103, 104

Digestive tube, in earthworm, 98, 99;
in fish, 101, 102

Diphtheria, 150, 152, 158, 165

Diploid number, 259

Disease defined, 147

Diseases, communicable, 147-168;
functional, 148, 177-183; functional,

643

relation to infection, 181-182; of la-
ter life, 179-183; venereal, 206-209

Dispersal of plants by means of seeds,
237, 261-262

Dissociation, 586-500

Distraction and study, 559

Dizziness, through overstimulation of
semicircular canals, 458-459

Dog-toothed reptiles, 311

Domestic animals, evolution of,
through natural selection, 324-325

Dominance, 266, 272

Dominant genes and hybrid vigor, 280

Dorsal defined, 99

Dorsal nerve cord, in amphibians, 525 ;
in Amphioxus, 524

Dorsal root of nerve trunk, 429

Double sugars, digestion of, 71

Dreaming, 501-502

Drugs, as causes of psychoses, 609;
as condiments, 77

Ducts, glandular, 419, 420; lymph, 52 ;
salivary, 68

Ear, structure of, 448, 449

Eardrum, 448, 449

Earthworm, hermaphroditism in, 237;
maintenance system in, 97-101 ; nerv-
ous system, 522, 523

Eastern Asia, flora of, 322-323

Eastern United States, flora of, 322-

323

Ectoderm cells in fish, 101
Ectoderm in Hydra, 95, 96
Ectoparasites, 141
Education, effect on I.Q., 574-575; for

mental hygiene, 616-617
Eels, migration of, 351
Effectors, 414-421 ; in plants, 509
Egg, formation of, in meiosis, 258;

human, 189, 194; in fern, 230, 231 ;

in flowering plants, 233; in moss,

229, 231 ; in Oedogonium, 228
Egg cells, production of, 193-195
Egg nucleus in flowering plants, 232
Eggs, care of, by birds, 242; in fishes,

238; in frogs, 239; in reptiles, 242;

varieties of, 241
Ego ideal, 583
Eidetic imagery, 482
Elation in manic-depressive psychosis,

605

644

Electromagnetic waves, 447
Elephants, overpopulation in, 332
Embryo, human, 196-197, 200; in flow-
ering plants, 235, 236; in reptiles,

2/J2

Embryo sac in flowering plants, 232,

233-234
Embryology, evidence for evolution

from, 317-321
Embryonic development, human, 198,

199
Embryos, comparison of vertebrate,

3i8, 319

Emergent, culture as, 399
Emotion, and breathing, 464; during

sympathico-adrenal activity, 471 ;

James-Lange theory of, 484-485 ;

part played by smell, 452-453
Emotions, conditioned, 548-550; mat-
uration of, 548
Encephalitis lethargica, 608
End brush of axon, 427
Endocrine glands, disorders of, as

causes of psychosis, 609

See also Glands, endocrine.
Endoderm cells in fish, 101
Endoderm in Hydra, 95, 96
Energy, kinetic, 27; of light waves,

relation to brightness, 448 ; potential,

27

Environment, developmental, 514

Enzymes, 29, 32, 33, 68

Epidermis in plants, 117, 121

Epididymis, 189, 190

Epiphytes, 338-340

Equilibrium, maintenance of, 457-459

Eras, geological, 311

Ergosterol, 80

Escape mechanisms, 584-595; and the
personality, 594

Esophagus, human, 66, 67, 68-69; in
earthworm, 98, 99; in fish, 102; in
frog, 104; in mammal, ^05

Eugenics, 578-580

Eustachian tubes, 448, 449

Euthenics, 578-580

Evening primrose, mutation in, 372,

375

Evolution, causes of, 368-388 ; cultural,
402-404; different rates of, 304-308;
evidence for, 309-326; future of,
404; human, 392-405; of amphibians,

Index

299 ; of animal life, 307; of behavior,
526-528; of birds, 301-302; of flow-
ering plants, 299; of insects, 299':
of land plants, 296-299; of mam-
mals, 301-304; of man and the apes,
395-396; of plant life, 306; of pri-
mates, 304; of reptiles, 299-301; of
snails, 312; of the horse, 313-314,
387; outcome of, 329-364; princi-
ples of, 304-309; regressive, 308-
309; straight-line, 386-388; the fact
of, 292-326

Evolutionary tree, 308

Excreta, disposal of, 163-164

Excretion, 28, 83-90

Excretory system, 83-90; of earth-
worm, 98, 100

Exercise of function of response and
learning, 536

Exhalation, 56

Exorcism, 598

Experience, 480

Eye, 445; structure of, 444-447

Eyes and righting reflexes, 458

Ft generation defined, 266
Facilitation, and acetylcholine, 468; of

heart beat, 418
Faith healers, 604
Fallopian tubes, 192, 194, 200
Fat storage, 75
Fatigue, in neurasthenia, 600; in study,

558-559

Fat-like substances, 8, 13
Fats, 10, n, 26; absorption of, 75;

digestion of, 71, 72; in the blood, 75;

oxidation of, in diabetes mellitus,

173

Fatty acids, 71 ; absorption of, 75
Fear, conditioning of, 549-550
Feces, 66, 72, 84
Fermentation, 33
Fern plants, primitive, 297
Fern-like plants, primitive, 297
Ferns, 120
Fertilisation, 195; external, 242; in

flowering plants, 234; internal, 242-

243; recombination of chromosomes

in, 257, 258

Fetus, 197-198; in uterus, 201
Fibers of stem, 124

Index

Fibrils, conducting, in Protozoa, 521

Fibrin, 45-46

Filtration, 57

Fish, maintenance organs in, 101-103,
102

Fishes as first vertebrates, 296

Fixation, 590-592

Flagella, 34; in Hydra, 95, 96

Flagellates, 34, 112, 355-357; move-
ments in, 519

Fleas, as ectoparasites, 141; as sec-
ondary hosts, 163

Flower, 231-234, 232

Flushing, and heat control, 473

Food, in oak tree, 121 ; in seeds, 236

Food cycles, 131-135

Food linkage, 130-131

Food manufacture, 25-27

Foods, absorption and use of, 73-83;
enriched, 82-83; inorganic, 11; or-
ganic, ii

Foraminifera, 356, 357

Forelimbs of mammals, 315

Forests, ancient, 296-299

Fossil implements, 399; of Neander-
thal Man, 402; of Peking Man, 400

Fossil men, 401

Fossil record, as evidence for evolu-
tion, 309-314; incompleteness of,
310; succession of strata, 311

Fossils, defined, 310; human, reason
for rarity, 397

Fovea, 445, 447

Freaks as products of abnormal em-
bryonic environment, 515

Freemartin, 286

Frequency, of light waves, relation to
hue, 448; of sound waves, relation
to pitch, 450

Freud, Sigmund, 502, 610

Frog, maintenance organs in, 103-105

Fruit, 232

Fruit flies, chromosomes in salivary
glands of, 375-376; mutations in,
373; overpopulation in, 331-332

Frustration as cause of anxiety, 584

Fugues, 589-590, 603

Functionalists, 610-611

Fundus, 67, 69, 70, 72

Fungi, 31-33; as parasites, 139-141;
as pathogenic organisms, 150-152;
body types in, 115-117, 116; bracket,

645

US, JJrf; filamentous, 115, 116; in
lichens, 137; methods of attacking
hosts, 140-141

Galapagos Islands, fauna of, 323
Gall bladder, human, 67, 71; in fish,

102, 103; in frog, 104
Gall insects, 141
Galls, 141

Gamete, male, in flowering plant, 234
Gametes, 196; as part of germ plasm,

370; formation of, in meiosis, 255-

257 ; in filamentous algae, 228
Gametophyte, female, in flowering

plants, 232, 233; in fern, 230, 231;

in moss, 229, 231 ; male, in flower-

ing plants, 232, 233-234
Ganglia, autonomic, 465, 467; in

earthworm, 523; in insects, 527, 528 ;

parasympathetic, 472; sensory, 429;

sympathetic, 472
Gastric juice, 69, 72; as defense against

pathogenic organisms, 154
Gene defined, 264
Generalizations, 540-541
Genes, as determiners of behavior pat-

terns, 552 ; as determiners of growth

and differentiation, 514; functions

of, in development of organisms,

378; interaction of, 276-277; nature

of, 272
Genetic factor, 573; and class differ-

ences in intelligence, 578; and per-

sonality, 582; and race differences,

576

Genetic recombinations defined, 267
Genetics, 263; evidence for evolution

from, 324-326
Genotypes defined, 267
Geographic distribution, evidence for

evolution from, 321-324
Geotropism, 507-509
Germ cells, 254-259, 258, 370
Germ plasm, 370-371
Germ theory, establishment of, 148-149
Germination in seeds, 236
Gibbon, 393-394
Gigantism, 177
Gill slits, 102; in human embryo, 197,

320

Gills, 102, 103
Gizzard in earthworm, o£, 99

646

Glands, 419-421; as effectors, 415;
connection with autonomic system,
465; endocrine, 171-179, 172; gas-
tric, 68 ; lymph, see Lymph nodes ;
parotid, 68; salivary, 68; sublingual,
68; submaxillary, 68; types of, 420

Glandular responses, 508

Glycerol, 71 ; absorption of, 75

Glycogen, 74-75

Goal-directed response, 529-530

Goals, 490; and development of mo-
tives, 548; in learning, 555-556

Goiter, 175

Goiter belts, 175

Gonadotropic hormone, 205

Gonads, 203, 254-255 ; hormones of,
203; in fish, 102; in frog, 104; in
mammal, 105

Gonorrhea, 150, 158, 166, 206-207, 208

"Goose pimples/' and heat control, 473

Gorilla, 394

Gout, 1 80

Graafian follicle, 194, 195

Gray matter, 430-432 ; in brain, 43 1;
in insects and vertebrates, 527-528;
in spinal cord, 429; regions, 432

Green plant metabolism, 25-28

Growth, i, 250; and maturation, 534;
and pituitary, 177; in earliest forms
of life, 294

Growth responses, 506-517

Guilt, feelings of, 583

Gustation, 480

Hair cells, of basilar membrane, 449;
of semicircular canals, 457

Hallucinations, 501, 605

Haploid number, 259

Heart, human, 46-47, 48, 54; in amphib-
ian, 106; in fish, 101, 102, 103,
106; in frog, 104; in mammals, 105,
106; in reptile, 106; vertebrate, ro6

Heartbeat, 418; acceleration in exer-
cise, 470; center for augmentation
of, 469; center for inhibition of, 469

Heart disease, 179-181

Heart muscle, connection with auto-
nomic system, 416-418, 419

Hearts in earthworm, 98, 99

Heat, formation of, in metabolism, 77

Heat center, 473

Heat control, 472-474

Index

Heiser, Victor, 164

Hemoglobin, 43, 55, 59 ; in earthworm,

99

Hemophilia, 46; inheritance of, 277

Hepatic vein, 48

Herbivorous animals, 131

Heredity, and arteriosclerosis, 181 ;
and cancer, 182; and syphilis, 208;
early ideas about, 263 ; in earliest
forms of life, 294; in guinea pigs,
264-270, 265, 271, 276 ; principles of,
263-290
See also Inheritance.

Hermaphrodite, 237

Heterotrophic organisms, 34.

Hibernation, animal, 350; plant, 349,
350

High blood pressure, 180

History of life, 294-304

Home environment, effect on intelli-
gence, 575

Hookworm, 152, 163, 164

Hormones, 171-179; and stimulation
of digestive secretions, 471 ; interac-
tion of, 176-179

Horses, behavior patterns in, 532

Hoskins, 174

Hue, 448

Human body, substance and structure
of, 5-19

Human nature, modifiability of, 552-
553

Hunger, 455

Hunger contractions, 491

Huntington's chorea, 612

Hybrid vigor, 280-281

Hybridization, importance in evolution,
308, 384-386

Hydra, hermaphroditism in, 237 ; main-
tenance system in, 95-97, 96; nerve
net of, 521, $22

Hydrotropism, 511

Hymen, ipj

Hypnotism, 587-589

Hypochondria, Col

Hysteria, 603-604

I.Q., 571

Ichthyosaurs, 301
Ideas, 489
Idiot, 569
Illusions, 501. 605

Index

Image, retinal, 444-445

Imbecile, 569

Imitation of words, 545-546

Immune carriers, 160

Immunity, 154-158; acquired, 155-158;
natural, 155; passive, 158

Immunity reaction, 156

Imperfect dominance, 273

Implicit preparation, 495

Implicit responses, 486-487 ; develop-
ment of, from movement responses,
508 ; function of, 487-488

Inbreeding, 279-280

Indian pipes, 139

Individual, behavior of, 566-595

Individual differences, 566-575 ; in
imaginal consciousness, 482

Infantile paralysis, 152, 158

Infections, as causes of psychosis, 608 ;
defined, 148

Inferiority feeling, 583

Influenza, 158, 160, 167

Ingestion, 24, 34

Inhalation, 56

Inheritance, blended, 273-276; in hu-
man beings, 277; influence of en-
vironment on, 370-371 ; multiple fac-
tor, 275; multiple factor and inter-
specific differences, 377; of acquired
characteristics, 369-371 ; of coat col-
ors, 264-270, 265, 271, 276-277; of
intelligence, 572-575; of mental dis-
ease, 612; of skin color, 273-275;
of two pairs of characteristics, 267-
270, 271
See also Heredity.

Inhibition, 435-436; and acetylcholine,
468; in production of implicit re-
sponse, 487; of heartbeat, 418

Inner ear, 448-450

Inoculation, 157-158; typhoid, 161

Insect behavior, unchangeability, 528-
530

Insectivorous plants, 347

Insects, behavior in, 326-530; role in
pollination, 234-235; social, 337-338

Insomnia in neurasthenia, 600

Instinctive behavior and location of
gray matter, 528

Instincts, 537, 547; and war, 553; as
chain responses, 530; in birds, 528;
in insects, 527; of fear, 548

647

Insulin, 171-173; in treatment of de-
mentia praecox, 610

Insulin shock, 610

Integration, 423, 435-436; as function
of gray matter regions, 432

Intelligence, and achievement, 569-570 ;
and cretinism, 174-175 ; and success,
580-581 ; dependence on cortical gray
matter, 438; measurement of, 570-
572; nature of, 568-570

Intelligence quotient, 571-572

Intelligence tests, 566, 568; standardi-
zation of, 570-572

Intelligent behavior and location of
gray matter, 528

Intention to learn, 561

Interdependence of living organisms,
129-131

Internal adjustments, 463-474

Intersexuality, 285-287

Interstitial cells, 172, ipo, 203

Intestinal juice, 68, 71, 72

Intestine, in earthworm, ?£, 99; in
fish, 102

See also Large intestine; Small
intestine.

Introspection, 479-480

Invertebrates, 101

Investigation and thinking, 498-499

Involutional melancholia, 609

Iodine and goiter, 175

Iris, 445, 446

Iron, in hemoglobin, 44

Irritability as an attribute of proto-
plasm, 444

Isolation, defined, 380; genetic, 381-
382; geographic, 380-381 ; importance
of, in evolution, 379-382

James, William, 484
Java Ape Man, 399
Jefferson, Thomas, 576
Jersey City, purification of water in,
161

Kanam Man, 399

Kangaroo rat, 342

Katabolic metabolism, 24, 28, 32, 33

Katabolism, 27

Kelps, 115

648

Kidneys, disease of, 105; human, 84,
85; in fish, 103; in frog, 104, 239;
in mammal, 105; sugar elimination
in, 75

Kinesthetic receptors and righting re-
flexes, 458

Kinesthetic sensitivity, 454, 456

Kinesthetic submodality, 480

Knowledge, effect on learning, 556;
maturation of capacity for acquiring,

554
Koch, Robert, 149, 152

Labia major a, 192, 193
Lady's-slipper, pollination in, 235
Lamarck, Jean Baptiste, 368
Lamarckian theory of evolution, 368-

371

Land plants, primitive, -297

Langerhans, 172

Language, absence of, in animals, 540-
541 ; and cognition, 489 ; and de-
velopment of social attitudes, 551-
552; and establishment of intention
to learn, 555; as instrument for
passing on cultural tradition, 398;
as tool of thought, 540-541 ; devel-
opment of, 545-547; foreign, learn-
ing of, 563

Large intestine, human, 66, 67

Larynx, human, 53, 54; in frog, 104

Leaf insect, 33$

Leaf-like organs in moss, 118, 119-120

Learning, 530-531 ; and acetylcholine,
468; and immunity reactions, 156;
as characteristic of mammals, 303;
as determiner of behavior patterns,
552; as developmental reaction, 537-
538; distributed, 560; incidental, 561 ;
massed, 560; of human species hab-
its, 544-545; principles of, applied
in study, 560-564; role of, in de-
velopment of behavior, 535-537

Learning curves, 556-557

Leaves, in ferns, 120; in seed plants,
123; in seeds, 236

Lecithin, 75

Lemmings, migrations of, 352

Lemurs, 393

Lens of eye, 445, 44*

Leprosy, 150, 159

Lichens, 136, 137

Index

Life cycles in bacteria, 161-163
Light rays, 447-448; amplitude and

frequency of, 448; bending of, by

cornea and lens, 446
Lipase, 71, 73
Littoral region of ocean, life in, 352*

355

Liver, 44, 45, 68; formation of bile,
71 ; formation of lecithin in, 75 ; in
fish, 102, 103 ; in frog, 104; in mam-
mal, 105; splitting of amino acids in,
76; storage of glycogen, 74

Liverworts, 117-119, 118

Lobes of cerebrum, 431, 432

Lockjaw, 153

Locomotor ataxia, destruction of kin-
esthetic sensation in, 456

Loudness of sound, 450

Lungs, 53-57, 541 artery to, 49; as
excretory organs, 83-84; in frog,
104; in mammal, 105; in warm-
blooded animals, 106

Lymph, 52

Lymph capillaries in villi, 75

Lymph nodes, 45, 52, 162

Lymph system, 52-53

M.A., 571

Machinery, cultural evolution of, 403

Magnesium chloride, effects on devel-
opment of minnows, 514-515

Maintenance organs, of fish, 102; of
frog, 104; of mammal, 105

Maintenance systems in animals, 92-

107
Malaria, 152, 155, 158, 162, 166; in

treatment of paresis, 608
Malthus, 371
Mammals, egg-laying, 243, 302, 321 ;

placental, differentiation of, 303;

similarity of skeletons, 315
Manic-depressive psychosis, 604-605 ;

and endocrine disorders, 609; and

focal infections, 608-610
Manipulative^ behavior, maturation of,

544-545

Maples, overpopulation in, 331

Marchantia, 117-119

Marriage, 211

Marsupials, 243; confinement to Aus-
tralia, $?i

Mass selection, 275

Index

Masturbation, 210

Maturation, as determiner of behavior
patterns, 552; defined, 533; impor-
tance in formation of species habits,
538 ; of capacity to learn, 553-555

Maturation of response in salamander,

534, 535

Meaning and learning, 561-564

Measles, 152, 155

Medulla, adrenal, 176

Meiosis, 255-260, <5<5; comparison with
mitosis, 259-260; in plants, 259

Membrane, cell, 10, 12, 13; nuclear,
10 ; plasma, 13; vacuolar, ij

Membranes, dialyzing, 59; protoplas-
mic, 12 ; semipermeable, 58-59

Mendel, Gregor, 263-264

Mendelian principles, application of,
277-284

Meningitis, 150

Menopause, 194, 205

Menstrual cycle, 193-195; and con-
traception, 213

Mental activity as implicit response,
487

Mental diseases, 598-617; causes of,
609 ; incidence of, 612 ; kinds of, 599

Mental hygiene, 612-617

Mental tests, 566-572; "tailor-made,"
568

Mesozoic era, 311

Metabolic activity, 29, 33

Metabolic processes, 28

Metabolic rates, 76-77

Metabolism, 23-35; animal, 28; auto-
trophic, 24-25, 3i; basal, 76, 77;
colorless plant, 31 ; heterotrophic,
24; rate of, 173

Microbes, 150

Microdissection needle, 5

Microorganisms, pathogenic, defined,
150

Microtome, 16

Middle ear, 448

Migrations, animal, 350-352; cyclical,
350-351; dispersal, 351-352; sea-
sonal, 350

Mildews, 31

Mimicry, 335, 336-337, 345-34$

Mind as implicit response, 487
Mineral salts and arteriosclerosis, 180
Missing links, 396-398

649

Mistletoe, 138, 139

Mitosis, 250-254, 252; comparison with
meiosis, 259-260; time taken by, 253

Modalities, of consciousness, 480; of
imaginal consciousness, 481

Molds, 31

Monkeys, 393

Mons Veneris, 192

Monsters, 202

Moron, 569

Mosquito as secondary host, 162

Moss, 118, 119-120

Mother egg cell, 257

Motivation, 489-495 J and learning,
53I-532; and mastery of skills, 555;
for learning to speak, 546 ; in think-
ing, 497-502

Motivators, 492-493; external, 493;
physiological, 492

Motives, cultural determination of,
550; development of, 547-553; pro-
duced by maturation, 536

Motor skills, learning of, 555-557;
produced by maturation, 536

Motorium, 521

Mouse test for pregnancy, 206

Mouth, human, 67, 72; in earthworm,
97, 98, 99 ; in fish, 102, 103 ; in frog,
104; in Hydra, 96; in mammal, 105

Movement of substances through the
body, 57-61

Movement responses, 519-541 ; in
plants, 519

Mulattoes and heredity, 275

Multicellular organisms, first appear-
ance of, 296

Multiple personality, 589, 603

Muscle cells, skeletal, 416; smooth,
418

Muscles, as effectors, 415 ; heart, 417-
418; of ear, as vestigial organs,
316; skeletal, 416-417; smooth, 417-
419; striped, 416

Muscular movements, coordination of,
in cerebellar cortex, 437

Muscular tension and mental activity,
559

Mushrooms, 31, 116, 117

Mutations, 37^-379; causes of, 379;
chromosome, 374; chromosome, in
genetic isolation, 376; effect of en-
vironment on, 371; gene, 374; in

650

filtrable viruses, 294; role, 376-379
Myxedema, 174

Nasal cavity, human, 67

Natural selection, in earliest forms of
life, 295 ; theory of, 371-372

Neanderthal Man, 400-402

Negroes, intelligence of, 576-577

Nekton, 355

Nephridia, 98, 100

Nephritis, 179, 180

Nerve fibers, autonomic, 465

Nerve net, in digestive tract, 470; in
Hydra, 521, 522

Nerve trunks, 424; function of, 425;
in earthworm, 522, 523

Nervous connection between spinal
cord and cerebrum, 433

Nervous impulse, 414; crossing of
synapse, 428; nature of, 423; pos-
sible courses of, 432-435

Nervous system, 423-439, 424; auto-
nomic, 464-468, 467; autonomic, in
neurasthenia, 600; central, 424, 426;
central, function of, 425-426 ; in Am-
phioxus, 524; in chorda tes, 524-525 ;
in earthworm, 522, 523; in Hydra,
521, 522 ;oi fly, 527; peripheral, 424

Neurasthenia, 600-601

Neurons, 426-428; connector, 426, 427;
connector, functions of, 430 ; connec-
tor, location of, 429; motor, 426,
427, 429; motor, functions of, 430;
sensory, 426, 427, 429; sensory, func-
tions of, 430; types of, 427

Niacin, 81, 83

Night blindness, 79

Nitrates and protein synthesis, 27

Nitrogen cycle, 133-135, *34

Nocturnal emissions, 210

Nocturnal vision, 447

Nodules, root, 134

Non-adaptive differences between or-
ganisms, 363-364

Non-vascular land plants, 116-120

Notochord, in Amphioxus, 524; in hu-
man embryo, 320

Nucleus, 9, jo, 12, 13; in mitosis,
250-254, 252

Nymphae, 192, 193

Index

Oak tree, maintenance structures in,

120-124

Obsessions, 601
Ocean, life in, 352-361
Oedogonium, 228
Olfaction, 480

Olfactory area of cortex, 483
One-eyed fish, 515 *
Optic nerve, 445
Orang-utan, 394

Orchid, symbiotic relationships of, 137
Organic sensitivity, 455-456
Organic submodality, 481
Organic therapy in mental diseases,

611-612

Organism defined, I
Organismic activities, I
Organizers, 517; effects of, 516
Organs, 17-18

Origin of Species, The, 292, 330, 372
Orthogenesis, 386-389
Osmosis, 60-61
Osmotic pressure, 6l
Ossicles, 448, 449
Otolith organs, 457
Otoliths, 457
Ovarian hormones, 203
Ovaries, human, 192; in frogs, 139
Ovary, in formation of seeds, 235; in

Hydra, 96

Overcompensation, 593
Overdevelopment of organisms as

evidence of orthogenesis, 387
Overpopulation, 330-333
Overt responses, 486
Oviduct in frogs, 239
Oviparous organisms, 242
Ovules, 232, 233; in formation of

seeds, 235

Oxidation, 24, 27; of sulphur, 32
Oxygen, II, 26, 27

Pain, sensation of, 454-455

Paleozoic era, 311

Pancreas, endocrine functions of, 171-

173; human, 67, 68, 71; in frog,

104; in mammal, 105
Pancreatic juice, 71, 73
Paralysis, functional, 603
Paramecium, 28-31, 29^ conductors in

521 ; response in, 413
Paranoia, 604-606

Index

Parasite, 33

Parasites, animal, 142; internal, 142

Parasitic organisms as evidence of re-
gressive evolution, 308-309

Parasitism, 33, 129, 138-143; in tem-
perate regions, 346; in tropical rain
forest, 337

Parasympathetic impulses, inhibition
of, in exercise, 470

Parasympathetic system, 466; stimu-
lation of digestive secretions, 471 ;
stimulation of peristaltic movements,
470

Parathyrin, 176

Parathyroid glands, 177, 176

Parent education, 613-616

Parents, emotional attachment of child
to, 615

Paresis, 608

Parotid, 68

Pasteur, Louis, 149

Pasteurization, 160

Pathogenic organisms, 149; methods
of attack, 152-153

Peat bog, life in, 346-348

Peking Man, 397

Pelagic region, life in, 355-358

Pellagra, 81, 83

Penicillin, 167, 206, 208

Penis, 189, 191

Pepsin, 69, 72

Peptones, 69

Perception, 485-486

Perceptual adjustments in chimpan-
zees, 540

Perceptual responses, 486; nature of,
488

Perennial herbs, 349-350

Peristaltic movements, 69, 70-72, 470

Permeability, selective, 58-59

Personality, and success, 580-581 ; ef-
fective, 581 ; well-adjusted, 582

Petal, 232, 233

Pharynx, in earthworm, 98, 99; in
fish, 102

Phenotype defined, 267

Phenotypic ratio, 268

Phloem, 122, 124

Phobias, 601-602

Phosphorescence in ocean animals, 360

Photosynthesis, 26-27, 28; in leaf, 121

Phototropism, 508-511, 509

Physiological drives, maturation of,

547
Physiological factor, 573, 5741 and

race differences, 576
Physiological limit in learning, 557
Piltdown Man, 399
Pitch of sound, 450
Pitcher plant, 347
Pituitary gland, 171, 172, 176-178;

anterior lobe, 177; posterior lobe,

177; stimulation of adrenal cortex,

177 ; stimulation of parathyroid, 177 ;

stimulation of thyroid, 177
Pituitary hormone, administration in

childbirth, 201
Plague, bubonic, 158, 162; pneumonic,

162-163

Plankton, 355-357, 35*, 36o
Plant responses, mechanism of, 508-

5ii

Plants, bodies of, 111-127
Plasma, 42, 52

Plate-body type in plants, 114
Plateau, appearance of, in learning

curve, 556-557
Platelets, 46
Plesiosaurs, 301

Pleura, parietal, 56; visceral, 55-56
Pleurisy, 56

Pneumonia, 150, 158,. 165, 167-168
Pollen cell, 232
Pollen grains, 233
Pollen sacs, 232, 233
Pollen tube, 232, 233
Pollination, 234-235
Polocyte, 257, 258
Polyploid species, formation of, 385-

386

Polyploidy, 375

Population, control of, 212-215
Portal vein, 42, 48, 74
Portuguese man-of-war, 357
Possession, concept of, 551
Possessiveness, cultural determination

of, 550-55 1 ; development of, 548
Posterior defined, 99
Postganglionic fibers, 465
Post-hypnotic suggestion, 588
Postural responses and sets, 493
Posture and study, 559
Preadaptation, theory of, 382-384
Precocious children, 572

652

Preganglionic fibers, 465

Pregnancy, 195-200

Pre-school attendance, effect on intel-
ligence, 575

Pressure, feeling of, 454*455

Primates, characteristics of, 392; liv-
ing, 392-393

Progeny selection in animal breeding,
278-279

Progestin, 195

Projection, 592

Prontosil, 166

Prophylaxis, venereal, 209

Prostate gland, 189, 191

Protein consumption and metabolic
rate, 76

Protein molecules as first forms of
life, 294

Protein synthesis, 27, 28

Proteins, 6-8, n, 27, 28; digestion of,
69-72; foreign, 7; types of, 76

Proteoses, 69

Proterozoic era, 311

Protococcus, 25, 26-28

Protoplasm, 5-9

Protoplasmic pattern as determiner of
response, 413-414

Protozoa, 28; as pathogenic organism,
150, 152 ; colonial, 94, 95

Pseudopodia, 44

Psychology as the study of response
integrated in the cerebral cortex,
438

Pterodactyls, 301

Pteropods, 357

Ptyalin, 68, 72

Pulse, 51

Pupil of eye, 445-446*

Puzzle box, 531

Pylorus, 67, 69-71, 7*

Quadruplets, 203
Quarantine, 159
Quinine, 162
Quintuplets, Dionne, 203

Rabbits, overpopulation in, 331-332
Race differences in intelligence, 575-

579

Radiolarians, 356, 357
Rado-cabbage. 385

Index

Rate of metabolism and sex differen-
tiation, 286-287

Ratio, 3-1, in single gene pair crosses,
267; 9-3-3-1, in inheritance of two
pairs of hybrid characters, 270

Rationalization, 593-594; in neuras-
thenia, 600-601 ; in paranoia, 605

Rats and bubonic plague, 163

Ray cells, 124

Reading, increase in efficiency in, 556

Realistic thinking, 498

Recapitulation, 317-321

Receptors, 414; in plants, 509

Recessive genes, 266

Recessive traits, elimination of, in in-
breeding, 279

Recombination of genes, 272

Rectum, 66, 67, 72, 83-84, 193

Reduction division, 257, 258} and
separation of genes, 266, 269, 270;
in hybrids, 283

Reflex, spinal, 430

Reflex arc, spinal, 429, 430

Reflex visual adjustment, 446

Reflexes, conditioning of, 549-550;
heat regulating, 473; integrated in
spinal cord and brain stem, 436-437 ;
righting, 458-549; vital, 463

Regression, 590-592; in schizophrenia,
607

Reindeer moss, 136

Reinforcement, 435-436

Rennin, 69, 72

Repression, 584-586; in neurasthenia,
600

Reproduction, i; asexual, 221-226;
human, 189-215; in amphibia, 239-
240; in earliest forms of life, 294;
in fishes, 238-239; in plants and
animals, 221-244; in vertebrates,
238-243; placental, evolution of, 302
See also Sexual reproduction.

Reproductive cycle, the, 250-260; in
fern, 230; in moss, 229; of flower-
ing plant, 232

Reproductive filaments in various
fungi, 116

Reproductive organs, female, 192, 193;
male, 189, 190-192; of fish, 102; of
frog, 104, 239
See also Sex organs.

Index

Reptiles, extinct, 300; period of dom-
inance, 301 ; primitive, 298

Respiration, 24, 27, 28; external, 30,
53; human, 53-57; in earthworm,
i oo ; in fish, 103; in seeds, 236; in-
ternal, 30

Respiratory organs, in fish, 101, 103;
in frog, 104

Respiratory system in human, 54

Response, I ; as adjustment to environ-
ment, 413; defined, 412; growth,
412; movement, 412; necessity for,
506-507

Response system, 4ii-4I5

Retina, 444, 445, 446-447

Retraction reflex, 434

Rheumatism, gonorrheal, 206

Rhizoids, in lichens, 136, 137 ; in liver-
worts, 118, 119; in moss, 119

Rhizomes, 349

Riboflavin, 8r, 83

Rickets, 80

Ringworm, 150-151

Rockefeller Foundation, 164

Rods, 445, 447

Root hairs, 122, 123

Root system, 123

Roots, 120-122

Roughage, 77

Rust, wheat, 140; white pine blister,
140

Saccule, 449; 457

Sacral division, autonomic system, 466

Saliva, 68, 72

Salivary glands, 67, 68

Salts, as constituents of urine, 86; as

food components, 78-79 ; calcium, and

parathyrin, 176; mineral, 8
Salvarsan, 166
Saprophytes, 32
Saprophytism, 337
Sargassum, 114-115
Scarlet fever, 165
Schizophrenia, 606-608
Science as investigation and thinking,

498

Science of Life, The, 293
Sclerotic coat, 444, 445
Scrotum, 196

Scruples, psychasthenic, 603
Scurvy, 82

653

Seashore, life on, 346

Seasonal changes of life, 348-350

Secondary sexual characteristics, 203-
204

Seeds, 232, 235-237; and plant dis-
persal, 361-362

Segmental interchange, 375

Segments in earthworm, 97, 98

Selective migration, 577

Self-maintenance, I

Semen, 191

Semicircular canals, 448, 449, 450, 457-
458

Seminal vesicles, i£o, 191

Sense organs, 444-459; in earthworm,
523; of muscles, 452; of smell, 451;
of taste, 451; of tendons, 452; of the
skin, 453; somesthetic, 452-457;
specialization of irritability in, 444

Sensitive cells, for hearing, 450; for
smell, 451; for taste, 451; of retina,
444, 445, 447

Sensitive plant, 519; movement re-
sponse in, 520

Sensitive tissues of sense organs, 444

Sensory areas of the cortex, 482-484,
483

Sepals, 232, 233

Set and attention, 559

Sets, 493-495; cognitive, 494; persist-
ence of, 493-494; unclassifiable, 494

Sex, determination of, 284-285

Sex life, normal, 209-212

Sex-linked characters, 287-288

Sex organs, in fern, 230, 231 ; in moss,
229, 231 ; segregation on different in-
dividuals, 228
See also Reproductive organs.

Sex reversal, 285-287

Sexual development and hormones, 178

Sexual reproduction, evolution of, 226-
229; general', nature of, 189; in ani-
mals, 237-243; value of, in struggle
for existence, 362-363

Sharksucker, 135, 136

Shell shock, 604

Shivering and heat control, 473

Shrinking from reality in dementia
praecox, 607

Sieve tubes, 124

Similarities between organisms as evi-
dence of evolution. 314-321

654

Simple Mendelian ratio, 264-268, 26$

Single sugars, absorption of, 74

Sinusoids, 42

Skills, learning of, 555*557; matura-
tion of capacity for acquiring, 554;
motor, 555; non-motor, 555

Sleeping sickness, 162

Small intestine, human, 66, 67, 71, 73,
193

Smallpox, I, 152, 155, 157, 158

Smell, sense of, 452

Smooth muscles, connection with au-
tonomic system, 465-466

Somatoplasm, 370

Somesthesis, 480; and emotion, 484

Somesthetic area of cortex, 483

Sound, stimulus for, 450

Sound waves, amplitude and frequency
of, 450; transmission in ear, 448-449

Spartina Townsendii, 386

Spatial relationships and intelligence,
568

Species, nature of, 329-330

Species habits, 537-538; human, 544-

545

Speech, 2

Sperm-bearing tubules, 190

Sperm mother cells, 190, 255

Sperm nucleus in flowering plant, 232

Sperms, 189, 190, 191, 194; formation
of, in meiosis, 258; in fern, 230, 231 ;
in fishes, 238 ; in frogs, 239 ; in moss,
229, 231 ; in Oedogonium, 228 ; of
plants, movement in, 519; varieties
of, 240

Sphincter, cardiac, 69; pyloric, 69, 70

Spinal cord, 424, 426, 429, 433; in-
tegration in, 436; neural connections
in, 429-430; white and gray matter
in, 429, 430

Spindle, equator of, 251 ; poles of, 251

Spindle fibers, 251

Spireme, 252

Spireme thread in meiosis, 256

Spirilla, 150; cholera, 151

Spirochetes, 150

Spirogyra, 114

Spleen, human, 45, 67; in fish, 102; in
frog, 104; in mammal, 105

Splint bones of horse as vestigial or-
gans, 316

Sponges, 34, 95

Index

Spontaneous generation, 221

Sporangium in fern, 230, 231

Spore, in fern, 230, 231 ; in moss, 229 ,
231

Spore capsule in moss, 118, 229

Spores, 153, 225-226; in malarial para-
site, 152; large, 233; small, 233

Sporophyte, in fern, 230; in flowering
plant, 232; in moss, 229, 231

Sports, 373

Stamens, 232, 233, 234

Standard of life, and disease, 164-165

Starches, n, 26; digestion of, 68

Stem, 118, 120, 122, 123, 124

Sterility, 205 ; and genetic isolation,
381-382 ; in gonorrhea, 206 ; in species
hybrids, 282-284

Sterilization after Caesarian delivery,
202

Stigma, 232

Stimulation, local, 470

Stimulus defined, 412

Stomach, human, 66, 67, 69, 70, 71, 72 ;
in fish, 102; in frog, 104; in mammal,
J05

Stomata, 121

Storage cells in root and stem, 122

Streptococci, sore-throat, 151

Struggle for existence, 330, 371 ; in
man, 147; nature of, in various en-
vironments, 332-333

Study, efficiency in, 557-564

Sugar, 26-27 ; as constituent of urine,
86; assimilation of, 171; secretion
under sympathico-adrenal stimula-
tion, 471

Sugars, double, n ; single, 10, 27

Sulfa drugs, 166, 206, 208

Sulfonamides, 166-167

Sulphates in protein synthesis, 27

Sulphur bacteria, 32

Sundew, 347

Sunlight and photosynthesis, 26

Supporting cells in root and stem, 122

Susceptibility to disease, 154

Swallowing, 68, 69

Sweat glands as excretory organs, 83,

84

Sweating and heat control, 473
Symbiosis, 129, 136-138; in temperate

regions, 346 ; in tropical rain forests,

337

Index

Symbolism, 489; and thinking, 408; in
schizophrenia, 607

Symbols, and intelligence, 568; psy-
chasthenic symptoms as, 602

Sympathetic nervous system, 466-468,
467

Sympathico-adrenal system, 467; ac-
tivity of, in exercise, 470; general
function of, 471-472; inhibition of
peristaltic movements by, 470

Sympathins, 468, 472

Synapse, 428; as seat of inhibition and
reinforcement, 436

Synapses in earthworm, 523

Syphilis, 150, 158, 165, 166, 167, 206-
209; and arteriosclerosis, 180; as
cause of locomotor ataxia, 456; as
cause of psychosis, 608; congenital,
208

Systems, 18-19

Tactual sensitivity, 454, 456-457

Tactual submodality, 480

Tadpoles, maturation of swimming in,
534-536

Tail in human embryo, 320

Tapeworms, 142, 152

Taste sensations, 451

Taungs skull, 398

Tectorial membrane, 449, 450

Temperate regions, life in, 345-352

Temperature sensations of, 454

Tentacles in Hydra, 96

Termites, differentiation of, 339; social
organization of, 337-338

Testis, human, 189, 190; in frogs, 239;
in Hydra, 96

Tetanus, 153

Thalamus and emotion, 485

Thallus, 112, 114

Thallus plants, in, 112

Theelin, 195 ; in menopause, 205

Thermotropism, 511

Thiamine, 81, 83

Thinking, 495-501 ; as implicit re-
sponse, 487 ; as implicit cognitive re-
sponse, 497; as implicit trial and
error, 495-497 ; cultural evolution of,
403; development of, from overt
speech, 547; in animals, 538-540; re-
lation to cerebral cortex, 438

Thirst, 455

Throat, 66

Thrombokinase, 45-46
Thyroid gland, 171, 172, 173-176
Thyroxin, 173 ; administration in meno-
pause, 205; stimulation of pituitary,

177
Tissue, connective and supporting, 15,

18; epithelial, 15, I/; glandular, 15 ;

muscular, 15 ; nervous, 15
Tissue fluid, 40, 52 .
Tissues, 14-16
Tomlinson, H. M., 333
Tongue, in frog, 104; in mammal, 105
Tonsils, 52
Tonus, in blood vessels, 469; muscular,

435

Tools, cultural evolution of, 403

Tourniquet, 46

Toxins, 33, 152, 156; as causes of psy-
chosis, 608

Toxoids, 157

Trachea, human, 53; in mammal, 105

Transitional forms in the fossil record,

3H

Translocation, 375
Trial and error, in learning, 532; overt

and implicit, 495-497
Triceps muscle, 435
Trichina, 142
Triplets, 203

Tropical rain forest, life in, 333-340
Tropisms, plant, 508-513; varieties of,

5U-5I2

Tsetse fly as a secondary host, 162
Tuberculosis, 56, 150, 158, 160, 165, 167
Tubers, 349

Tubules in kidneys, 85, 86'
Tundra, 342
Twig insect, 335
Twinning in minnows, 5/5
Twins, fraternal, 202; identical, 202-

203, 514, 554; intelligence of, 574;

Siamese, 202, 515
Typhoid fever, 150, 158, 165; and

arteriosclerosis, 180

Ultra-violet rays as causes of muta-
tions, 379

Umbilical cowl, 197, 200, 201
Unconditioned stimulus, 549
Ungulates, 303

656

Uranium and dating of rock layers,

3H

Urban environment, effect on intelli-
gence, 575

Urea, 76, 84, 86

Ureters, human, 84, 85, 86; in frogs,

239

Urethra, 85, 86, 189, 191, IQ3
Urinary system, 83, £5
Urine, 86; in diabetes mellitus, 173
Urogenital mechanism in vertebrates,

238
Urogenital opening in fish, 102

Urogenital pore, 238
Urogenital tract in man, 191
Uterus, 192, 193, 194; in frog, 239
Utricle, 449, 457

Vaccination, smallpox, 157

Vaccine, 157

Vacuole, 12, 13, 44 ; contractile, 29, 30 ;

food, 29, 31 ; water, 12
Vagina, 192, 193, 194
Valves of a vein, 42
Variation, as a cause of evolution, 371-

372; in the earliest forms of life,

294

Vas deferens, 189, 190, 191
Vascular land plants, in, 119-124
Vasoconstrictor center, 469
Vasodilator fibers, 469
Vegetative filaments in various fungi,

116
Veins, 41; contraction of, in exercise,

470 ; in leaf, 121
Vena cava, inferior, 48, 50; superior,

48

Ventilation, 474
Ventral defined, 99
Ventral nerve cord, in earthworm,

522, 523; in insects, 527
Ventral root of nerve trunk, 429
Ventricle, in fish, 103; left (human),

47-49, 48; right (human), 47-49,

48

Venules, 41
Venus's fly* trap, 347
Vertebrae, 424; in necks of mammals,

315
Vertebral column, 101, 424

Index

Vertebrate body plan, 101

Vertebrate brains, $24-52$

Vertebrate nervous system, 523-526

Vertebrates, 93, 101; behavior in,
527

Vessels of stem, 122

Vestibule of inner ear, 448, 449, 450

Vestigial organs, 315-316

Vestigial response, 487; in perception,
488

Villi, of intestine, 74, 75-76; of pla-
centa, 197

Virulence, 157

Viruses, filtrable, 150

Vision, 481 ; stimulus for, 447-448

Visual area of cortex, 483

Visual structures, 445

Vital centers, 463

Vitamin A, 79; and cold prevention,
80

Vitamin B complex, 79, 8 1

Vitamin C, 79, 82

Vitamin D, 79, 80

Vitamin K, 79, 80-81

Vitamins, 79 ff. ; as food components,
78; sources of, in foods, 79 ff.

Vitreous humor, 445, 446

Viviparous animals, 242-243

Vocalization in infants, birds, apes,
545-546

Vocational guidance, 568

Volvox, 112

Vorticella, 94

Vulva, 192

Warm-blooded animals, 105-106, 472
Warming up in study, 558-559
Wasps, nest-building in, 528-529
Wassermann test, 207-208
Water, as a food component, 78; in

oak tree, 121 ; role of, in human body,

78
Water supply, purification of, 160-

161

Web of life, 129-143
Wheats, evolution of, through artificial

selection, 325
White insulating material of nerve

fibers, 4*7 > 432

Index

White matter, 430-432; in spinal cord,
429; regions, composition of, 432

Wilts, 31

Wishful perceiving, 501

Wishful thinking, 499-501; dreaming
as form of, 501-502

Word salad, 607

Worms as pathogenic organisms, 150,

151
Writing, maturation of capacity for,

554-555

657

X-rays as causes of mutations, 375, 379
Xerophthalmia, 79

Yeasts, 31, 33

Yellow fever, 152, 158, 162

Zygote, cleavage and growth of, 196;

in fern, 238; in filamentous algae,

228; in moss, 231
Zygotes, 370