Full text of "This Living World"

See other formats

CO >; 00

166479 J^

5 ;o

OSMANIA UNIVERSITY LIBRARY

54 T Accession No. & O 2-

Author

This book should be re/turned on or before the date last marked below.

THIS LIVING WORLD

This Living World

A College Course in Science

By C. C.CLARK

Associate Professor of General Science,
New York University

and R. H. HALL

Instructor in General Science,
New York University

DRAWINGS BY LOUISE WALLER GERMANN

FIRST EDITION
FOURTH IMPRESSION

McGRAW-HILL BOOK COMPANY, Inc.

NEW YORK AND LONDON
(94O

PRINTED IN THE UNITED STATES OF AMERICA

parts thereof, may not be reproduced

in any form without permission of

the publishers.

THE MAPLE PRESS COMPANY, YORK, PA.

PREFACE

THE REMARKABLE progress in the natural sciences since
the beginning of the last century has greatly increased man's
understanding of the living world. The practical application of
this knowledge has contributed many material advantages to
human welfare, providing for the maintenance of health and the
treatment of disease, markedly affecting the development of our
institutions, and even influencing our ways of thinking. Thus
the natural sciences have become a major social force. For this
reason, the essentials of a good education today require some
general knowledge of the natural laws governing the phenomena
of everyday life.

The aim of this book is to present, in a form that combines
accuracy with pleasant reading, the gist of modern knowledge
about the living world, with special reference to the physical
development of man and the structure and functioning of his
body. The volume is designed as a text for college students who
are taking a course in science for its cultural and exploratory
value. In the selection and organization of the subject matter,
the authors have been governed by considerations of what they
believe to be the best approach to satisfying the needs of students
who are not specializing in science. The presentation is an out-
growth of the senior author's long practical experience in present-
ing science courses to students who are nonscience majors.

In attempting to select appropriate reading material to ac-
company the cultural science courses offered by the authors, it
was found that while there are well-written accounts that are
suitable for many of the topics considered, these are nowhere
suitably gathered into a single volume. The existing texts that
adequately cover the separate fields of the biological sciences are
in general too detailed and specialized to meet these students'
needs and interests. It is the authors' belief that the use of these

PREFACE

special texts tends to defeat the purpose of survey courses, since
where too much emphasis is placed. upon detail the student fails
to gain the more comprehensive insight which he is seeking. One
of the primary purposes in producing this volume, therefore, has
been to treat the natural sciences on a sufficiently comprehensive
basis to give a broad understanding of the nature of living things
and of the underlying principles governing their behavior and
interrelationships. This basic requirement has been the guiding
one in the selection of subject matter. In organizing the material
the authors have kept in mind the object of presenting a logical
and connected story of life on the earth.

The physical environment which supports life and the
general characteristics of living things are discussed in the first
few chapters of the book, since these topics are thought to be
essential to understanding the more complex forms of life and
their relation to each other. The succeeding chapters give an
account of the development of life during the geologic past and
the relationship of early forms to modern creatures, including
man. Following this broad treatment of the development of
living things and their more fundamental characteristics, special
emphasis is placed on the complex physical organization and
functioning of the human body, since to the individuals for
whose use this volume is intended the human animal is of pri-
mary interest among living creatures. The text is concluded with
a chapter on the prehistoric cultural development of man, a
discussion that seemed necessary in order to complete the pic-
ture of man's early activities on the earth and to give some in-
sight into the origins of modern culture. This approach has been
found to be one that gives the most satisfactory understanding
of the world of life and man's place in it, in that it presents a
logical story with a theme or continuity running through it.

Throughout the book an attempt has been made to introduce
the different topics by reference to common knowledge and then
proceed to a discussion of pertinent material that may not be so
generally understood. Wherever possible, illustrative examples
have been selected from animal life, choosing in particular
animals that might have some human interest or familiarity.
The language of the text has been kept as nontechnical as is
consistent with clear exposition. Some of the terms of science

PREFACE vii

are included, as they must necessarily be, for clarity and definite-
ness. However, these terms have been limited to cases where
more common language could not be effectively employed, and
they have been explained in the body of the text when first used.
In order to make for interesting reading, the style of writing has
been made descriptive and narrative where the subject matter
could be clearly explained by so doing.

An annotated list of references for additional reading has
been given at the end of each chapter for those whose interests
may extend beyond the discussion of this text. In selecting the
references, some popularly written books ai^d magazines have
been included that are suitable for general reading, and some
more technical books and professional journals are listed for the
specific and detailed information which they contain.

It is a pleasure to acknowledge the help and cooperation the
authors have received from a number of persons during the
preparation of this volume. The authors' thanks are expressed to
their colleagues in the general science courses in the School of
Commerce, Accounts, and Finance, New York University, for
their valuable assistance in organizing and teaching the courses
which led to the writing of this manuscript; to Dr. William R.
Duryee, Biology Department, New York University, who helped
prepare three of the chapters in their original form; and to Louise
Waller Germann for her devoted interest and skill in making the
artistic and technical drawings. We are especially obliged to
Professor D. T. O'Connell, Geology Department, College of the
City of New York; to Professors L. G. Barth, J. H. McGregor,
A. W. Pollister, and H. Burr Steinbach, all of the Department
of Zoology, Columbia University; to Mr. James Peskin of the
Department of Biophysics, Columbia University; and to Pro-
fessor H. A. Charipper, Biology Department, New York Univer-
sity, for reading parts of the manuscript and offering many
timely and valuable suggestions for its improvement.

C. C CLARK,
R. H. HALL.

NEW YORK CITY,
August, 1940.

CONTENTS

PREFACE v

1. CHANGING CONCEPTS

Regarding the World of Life 3

2. SOLID SURFACES

Or a Consideration of Some of the Features of the Earth's Crust 25

3. LIFE'S DOMAIN

In Turbulent Oceans of Water and Air 66

4. LIVING CHEMICALS

The Nature and Physical Basis of Life 100

5. THE PATTERNS OF LIFE

Organization and Development of Living Things 1 30

6. DOWN TO THE SEA

Where Early Life Existed during the First Geologic Ages 166

7. SIZE AND CUNNING

In the Development of Vertebrate Land Life during the Latter Geologic Ages 205

8. THE LAST MILLION YEARS

Or Human Development from Early Man to Modern Races 241

9. COMPARATIVE FEATURES

Human Anatomy in Relation to That of Lower Vertebrates 273

10. THE HUMAN ORGANISM

A Study of Its Digestive and Respiratory Systems 301

11. MOVEMENTS OF MATERIALS

A Study of the Human Circulatory System and Excretory Organs 334

X CONTENTS

12. LIFE CONTINUES

The Process of Reproduction 364

13. SENSATIONS

By Which We Receive Communications from the Outside World 394

14. CORRELATING MECHANISMS

How the Body Is Integrated into a Smoothly Operating Unit 421

15. KEEPING WELL

Throu$h a Knowledge of the Nature and Treatment of Disease 450

16. THE LONG ROAD

In the Development of Human Culture 479

INDEX 511

THIS LIVING WORLD

f^ t ;1i

' '

I: CHANGING CONCEPTS

Regarding the World of Life

IN 1564 an Italian philosopher, Giovanni Maffei, wrote an ex-
tended document of one hundred and forty leaves on the
nature of the world and addressed it to the Count of Altavilla.
It was a beautifully bound manuscript, written in the quaint
old Italian script of the times. In it the author informs the count
that the world consists of fourteen parts, namely, four elements
and ten heavens. These are arranged in consecutive order from
the center outward in concentric spheres. They may be con-
sidered in the form of a great stairway, he says, and he invites
the count to ascend this progressive stairway of spheres in order
to learn what is to be encountered on each step. The various
steps include an explanation of a wide range of subjects, from
those dealing with the immortality of the soul to why man has

4 THIS LIVING WORLD

two feet instead of four, and why stones sometimes have the
forms of animals.

While the "Scala Naturale" of Maffei is considered to be
one of the minor scientific classics of medieval times, the amount
of misinformation which it contains is remarkable. It illustrates,
nevertheless, how extensively our knowledge of natural science,
particularly that relating to the phenomena of life, has increased
during the last four centuries.

Since the days of the early Greek and Egyptian civilizations,
man's knowledge of the animate world has been increasing. It is
as if mankind had been climbing an ever-expanding stairway of
understanding. It is true that his progress has not been steady;
in fact, during certain ages it has been slow or even back-
ward down the stairway. During all these centuries many
attempts have been made to get some insight into the nature
of living things and to formulate some concept of man's
place in the world of life and his relationship to the material
universe.

At the present time mankind has accumulated a large body
of knowledge regarding the make-up of the living world. He has
worked out a number of interpretations of this knowledge and
formulated a number of beliefs regarding the nature of life,
which greatly influence our thinking and behavior. The beliefs
and habits we now have in this respect are somewhat different
from those of previous centuries. They are founded upon more
adequate knowledge than earlier man possessed. In this respect
our concepts are more complete and more accurate. It would be
satisfying, indeed, if we had the perfect understanding of life;
however, this not being the case because of our lack of a complete
knowledge of living things, mankind will continue to seek after
information, and the future will no doubt see modifications of
some of our present concepts. While this treatise is primarily
concerned with getting some general understanding of man's
present information regarding the phenomena of life and his
interpretations of that information, it is of importance and
perhaps of interest first to inquire briefly into the increasing
knowledge and changing concepts of mankind in this respect
throughout past human history.

CHANGING CONCEPTS 5

Attempting to Understand the Universe

The earliest and most successful attempts at the time to
organize man's knowledge of the universe and to formulate an
explanation of man's relation to it were made by the ancient
Babylonians, perhaps two thousand years before the time of
Christ. Their early priests pictured the universe as a closed box
or chamber with the earth as the floor. In the center of this
floor were the snowy regions from which came the waters of the
Tigris and Euphrates rivers. Around the earth was a moat of
water and beyond it were celestial mountains supporting the
dome of the sky. The sun, moon, and some of the stars moved
over this dome, while most of the stars were fastened to it. The
priests kept a record of the movements of the sun, moon, and
planets and from these movements eventually learned to predict
the occurrence of the seasons, a feat that was of inestimable
practical value to a people whose very existence depended upon
knowing the time of the rising and subsidence of the rivers'
waters.

This study was the beginning of scientific astronomy. How-
ever, flushed with such success in predicting the seasons from
movements of the sun, moon, and stars, the Babylonians
erroneously reasoned that these celestial bodies exercised minute
control over human actions and affairs of life and death. A whole
fantastic scheme of magic was built up, and the astrologers
acquired very real power over the minds of men. Their rites and
ceremonies involved mimicking many acts of nature and assign-
ing to the forces of nature the properties and forms of living
creatures, which were to be worshiped and to which sacrifices
were to be made. They held that man's destiny was ruled by
the stars; hence all was governed by an inhuman and inexorable
fate. Within such an atmosphere, neither science nor any
rational philosophy of life could well be expected to develop
further. It was several centuries before any new ideas were added
to man's concept of the nature of life and the universe. This
time the advancement originated in early Greece.

One of the most renowned of early Greek philosophers was
Plato, who lived during the fourth century B.C. He was a great

6 THIS LIVING WORLD

exponent of what is known as idealism and reasoned that the
universe must be perfect. Holding that the sphere was the most
perfect of forms, he pictured the heavens as composed of spheres.
The heavenly bodies were carried in cycles which had a circular
motion, and the apparent motions of the sun, moon, and stars
about the earth as the center sphere of the universe could be
explained by a combination of these cycles. He loudly condemned
experimentation and careful observation of nature as being
either impious or a base mechanical art. Plato never asked ques-
tions; he answered them with a dogmatism that was decidedly
unscientific. If he had had an experimentalist type of mind,
he probably would have discovered that the earth revolves
around the sun, and not the sun around the earth, as he taught.
Most of his science was fantastic, but in many respects it had a
profound effect upon human thinking for many centuries.
This was brought about mainly by his influence on one of his
pupils, Aristotle.

Aristotle formulated in a more definite manner a great many
of Plato's ideas regarding life and the universe. He was the
greatest collector and systematizer of knowledge the ancient
world produced. Some of his scientific work was a contribu-
tion to an understanding of nature; however, some, particularly
that relating to the nature of the physical universe, was inac-
curate and influenced by a belief in the magical. His great
importance to science was that he treated a great variety of
subjects and so systematized all knowledge that it could be
comprehended by succeeding generations. His great hindrance
to scientific advance was that for centuries his teaching became
authority and laws unto themselves, so that there was no dis-
tinction between what was correct and what was wrong. It
became the mode to accept all things upon authority from either
Aristotle or others and to disregard entirely research and
observation. Although Aristotle himself referred to his work as
the first step that should be improved upon, his ideas came to be
unreservedly accepted for fifty generations.

He extended the idea that the universe was a series of
spheres, because a sphere was the most perfect shape. In the
center were the material and perishable spheres. They were the
earth, water, air, and fire, which by their opposing and corrup-

CHANGING CONCEPTS

Aristotle was the "greatest collector and systematizer of knowledge the ancient world

produced."

tible principles of hot and cold, wet and dry, produced generation
and destruction. Beyond these were the spheres in which moved
the heavenly bodies, perfect and incorruptible. Seven of the
spheres were to account for the movement of the sun, moon and
the five planets known in those days. Altogether there were
fifty-five of these hollow, transparent balls, one within the other.
The outer and largest was by its very nature the most perfect,
Aristotle reasoned. There resided the supreme and perfect
creator of the universe, the Divine Being who ruled all. These
ideas became involved in much of the theology and religious
teachings of succeeding centuries and as such became estab-
lished as authority that could not be disputed or contradicted.

When an observing scientist discovered that some detail of
the scheme was incorrect, he was immediately ridiculed or per-
secuted out of existence because it would mean that the entire
system was imperfect. To maintain this was impious and blas-
phemous. For example, when Galileo, some eighteen centuries
after Aristotle, constructed his first crude telescope, the senators
of Rome were delighted to look through it to see ships at sea

8 THIS LIVING WORLD

which their eyes failed to reach; but when he pointed it to the
planet Jupiter at night and discovered four moons revolving
around it, the learned men of the age refused to look. By this
time the number seven had become sacred; there could be no
more than the original seven moving heavenly bodies which
were a part of the whole system of thought of the times. To
disturb a single part of the system would destroy the whole.
Yet Galileo was saying that his telescope revealed four additional
moving bodies. He was forced to deny what he had seen through
his telescope in order to escape with his life.

Thus man's knowledge and concepts of the universe had
become fixed and static. His entire reasoning and thinking
processes were based upon authority rather than upon experi-
mentation and careful observation. Then in 1543 in Nuremberg,
Germany, an old Polish astronomer by the name of Copernicus,
lying half -conscious on his deathbed, received a copy of the book
he had spent a lifetime in preparing. It was called " Revolutions
of the Heavenly Bodies" and was the result of his quiet and
careful observations over many years. Copernicus had discov-
ered, and so stated in his treatise, that the sun was the center of
the solar system and that the earth revolved around the sun as
did the other planets. Furthermore the earth was rotating on an
axis, thus producing the apparent rising and setting of the sun
each day.

So well documented a paper as this could not be brushed
lightly aside. Instead, it challenged the whole system of an
earth-centered universe and all the dogmas that had been estab-
lished upon this idea. The earth had been removed from its realm
of importance and reduced to the insignificance of a revolving
planet. Likewise, it was reasoned, man had been dethroned from
his place on the summit of creation. Thus the Copernican system
affected the human mind and human beliefs in many ways. To
accept the idea was intellectual revolution.

While Copernicus had passed beyond the realm of human
punishment, some of the contemporary philosophers who
championed the idea were dealt with severely; the most notable
of these was Giordano Bruno. He not only abandoned the idea
that the earth was the center of the universe, but taught that
the stars were scattered throughout an infinite space rather than

CHANGING CONCEPTS 9

fixed in constant and finite spheres. For his philosophy and zeal
he was burned at the stake in 1600.

The discoveries of Copernicus were slow in being accepted.
As time went on, the later studies of Tycho Brahe, the profound
reasoning of Johannes Kepler, who formulated the laws of
planetary motion, as well as the discoveries of Galileo, which
would not be silenced even though he denied his findings,
finally established the Copernican system as the true picture
of the universe. Any discussion of the many discoveries that
followed the time of Brahe, Kepler, and Galileo is not in place
here, but during the last three centuries careful scientific study
has come into favor again. Throughout that time most of our
present and much more rational understanding of the universe
was attained.

Increasing Knowledge of the Human Body

Second only to man's attempt to develop a concept of the
nature of the universe has been his attempt to understand his
physical body and to place himself in proper relationship to the
entire scheme of life. This, too, presents an ever-changing pic-
ture. The earliest beliefs and deductions that have come down
to us represent man as the highest form of animate creation.
This idea is supported at present by an immense wealth of
scientific information, but the present concept of the relationship
of man to the rest of life is quite different from that held in
ancient and medieval ages. Furthermore, as the centuries have
passed there has been an added accumulation of knowledge
regarding the physical body of man. With this greater knowledge
has come a better understanding not only of the physical struc-
ture and functioning of the human body but of the significance
and meaning of human life.

One of the first important efforts to understand the nature
of the human body was made by Hippocrates about the end of
the fifth century B.C. His chief interest was the healing of the
sick, and he has long been revered as the father of medicine.
He set for himself a high standard in obtaining the most expert
knowledge and maintained a high degree of ethical conduct in his
practice of medicine. Even today the Hippocratic oath is taken
by every person entering into the medical profession. Hippocrates

10 THIS LIVING WORLD

was familiar with a great variety of diseases and with a number
of remedies that were helpful in curing some of them or in
relieving pain. His policy was to be considerate of the patient
and to encourage him. He knew that the body itself had large
healing powers, if given proper care and attention. He appar-
ently made a number of dissectibns, had a considerable knowl-
edge of anatomy, and, it is said, performed many successful
operations. He demonstrated that disease was not dependent
upon supernatural causes and for a time liberated medicine
from the magical.

Five hundred and thirty-two years after the death of
Hippocrates, a young stranger entered Rome. He was destined
to become the most gifted physician of the second century A.D. —
and of some fifteen centuries thereafter. Having been trained at
the school of medicine at Pergamum in Asia Minor, an institu-
tion that made even the learned center of Alexandria envious,
and having a thoroughly inquiring mind, the physician Galen
(for he was the stranger) soon became well established in the
imperial city. Eventually he became physician to the Roman
emperor.

Galen was remarkably well founded in human anatomy,
having made dissections and studied human skeletons at
Alexandria. Whoever today speaks of anatomy pays tribute to
Galen. He described many of the muscles of the body and their
functions. He described most of the bones of the skeleton, and in
these descriptions he made few errors. Also, he discovered many
things about the central nervous system. He knew that the
brain was its chief organ and that the spinal cord was of next
importance. He made a series of cross sections of the spinal
cord and brain, which were some of the most important experi-
mental demonstrations of antiquity.

Galen, however, seemed to have learned little about the
nature and causes of the great plagues that spread over Europe
from time to time. It has been charged by some that he never
studied them because of the fear of becoming a victim. Whether
this be true or not he did leave Rome on one occasion for a year,
presumably to collect medicinal ores and herbs along the sea-
coast to the east and to study the drugs of the Phoenicians. It
was at this time that one of the worst plagues of ancient Europe

CHANGING CONCEPTS 11

was raging in Rome. At another time, he was requested by the
emperor Aurelius to accompany him on one of his campaigns.
On this occasion one of the plagues was not only thinning the
Roman army but also doing more to reduce the opposing peoples
than were the Roman legions. The physician informed his ruler
that he had been warned in a dream to remain in Rome and
attend the emperor's son, who would become sick. Of course,
the young boy did soon become ill of some childhood disease and
was promptly cured by Galen. For apparently saving the life of
a future emperor, the physician received the blessings and favor
of the empress as well as a substantial reward from the emperor.

In addition to practicing and teaching medicine in Rome,
Galen wrote prolifically. Some of his writings contain his best
medical information and have been exceedingly valuable to
succeeding generations. However, some were nonsensical and
speculative assumptions or vituperative tirades against his con-
temporary medical colleagues. These parts had a profoundly
adverse effect in later times, as succeeding generations made
no attempt to distinguish between established truth and
misinformation.

Galen's mind seemed to have detested doubt; he craved for
finalities. In his writings, all questions regarding the physical
structure of the body and medical practice were answered. He
solved all problems, and everything was catalogued and tabu-
lated. He sought to make medicine a closed science with his
own knowledge and speculations, an absurdity that would have
been appropriate to a much less capable mind. His effectiveness,
however, was greater than he could have hoped for. After this
last of the learned Greek scholars had passed from the earthly
scene, there was little progress in the study of human life and
human disease for many centuries.

Even during Galen's time it was generally regarded as
improper to dissect a human body. The Roman government as
well as theological authority soon established laws and canons
against such dissections, even though for centuries the whole-
sale destruction of human life in wars, games for the amusement
of the rulers, or feudal disputes rendered it cheap and uncertain.
The battlefields and arenas might be strewn with corpses, but
everyone shrank from the anatomist's knife. By the time the

12 THIS LIVING WORLD

Middle Ages had been reached there were centuries when not a
human skeleton for study was to be found in all the medical
schools of Europe. The belief became established that whoever
dissects a cadaver is guilty of sin. It is, therefore, obvious under
such conditions that little progress could be made in under-
standing the nature and functioning of the human body or in
the treatment of disease.

As the centuries from about A.D. 600 to 1300 rolled on, the
teachings of Hippocrates and Galen were forgotten. Treatment
of disease was first reduced to prescribing obnoxious concoctions
of such things as bitter herbs, emasculated insects, and dirt from
wagon ruts; often these were administered with the repetition
of words of magic or with supplications to some deity or demon.
Later it became the fashion to prescribe magical treatments.
One of the most famous of such formulas was the magic word
"abracadabra." This word was written, rewritten, and again
rewritten, one letter at the end being dropped with each rewriting
until only an a remained. The shortened word was written each
time immediately beneath the one above it, in such a way as to
produce an inverted cone. The piece of paper on which it was
written was tied about the neck of the patient with a flax string
and was supposed to cure or prevent disease. The Dark Ages
of the study of human life had been reached.

Of course, such practices were eventually reduced to the
ridiculous. In later centuries, the monks in the monasteries
rediscovered the writings of Hippocrates, Aristotle, and Galen.
These writings were learned and recopied so extensively that
they eventually developed into a sort of authoritative law.
Some of their teachings were widely and blindly followed to the
extent that they were accepted as true upon authority and
hence were not subject to contradiction by any man. Even at the
University of Paris, which had become the medical center of the
Middle Ages, surgical operations and bedside examinations were
outlawed. These customs along with the widespread belief in
divine healing constituted most of the medical practice of the
later Middle Ages. In these times the physical body came to be
looked upon as unworthy of high regard or understanding. It
was taught and widely believed that the body was vile clay,
imprisoning the soul.

CHANGING CONCEPTS 13

Then in 1543, a young Belgian by the name of Vesalius pub-
lished a book entitled "Fabric of the Human Body/' and it is
noteworthy that it appeared in the same year as did Copernicus'
treatise on the heavenly bodies, which changed our whole think-
ing regarding the make-up of the universe. Vesalius made a
clear and lucid description of the structure and functioning of
the human body, one that he had learned from actual experi-
mentation. It was beautifully and accurately illustrated by
excellent drawings made by a competent artist. In it Vesalius
challenged and corrected about two hundred incorrect state-
ments made by Galen some thirteen hundred years before,
which by this time had become such authority that to dispute
them was blasphemous.

For example, Galen had said that venous blood mixed with
arterial blood through pores in the heart. Every anatomist since
Galen's time had imagined that he had seen such openings.
Vesalius showed there were none. He was immediately attacked
by the professional and ecclesiastical men of his day as being
mad and dangerous. But as his thoroughness in teaching in-
creased, his fame spread. Many followers were attracted to him
in order to have revealed to them the wonders of the human
body.

Finally, a nobleman of Italy died, and Vesalius performed a
dissection on his body in the presence of many spectators. To the
surprise of Vesalius and the rest, the heart was still beating,
and the unpleasant story spread fast and far. One unauthen-
ticated report has it that Vesalius' untimely end came when his
enemies charged him with impiety and murder and condemned
him to death. Gradually, however, his teaching became widely
accepted. It produced revolutionary ideas regarding the sig-
nificance of the human body and the life that courses through it.
His anatomy and his evaluation of experiment have become
modern practice.

Another discovery, made about the middle of the nineteenth
century, revealed much about the functioning of the body,
particularly as it relates to disease. This time it was a French
chemist by the name of Louis Pasteur who was the master mind.
He established what is usually referred to as the germ theory of
disease, and this discovery constitutes one of the greatest single

14 THIS LIVING WORLD

Pasteur's discovery "constitutes one of the greatest single achievements of mankind."

achievements of mankind. Before Pasteur's time the causes of
disease were unknown, and its treatment was primarily a trial-
and-error process. Since his discoveries medicine has become an
exact science, and many of the plagues of mankind have come
under human control.

Pasteur began his epoch-making work by a study of the
fermentation of wines. He finally discovered that fermentation
was produced by microscopic organisms acting on the grape
juice. Further, he demonstrated that there were many types of
fermentation and that each type was caused by its own specific
organism. Then, in 1865, he was approached by some silkworm
raisers whose silkworms had a serious disease, called p6brine,
which threatened to destroy the silk industry of southern
France. He accepted the challenge to find the cause of the
disease and a remedy for it. In a few years, after a prodigious
amount of work by Pasteur and equally as much ridicule by
the medical profession of his day, he had found that certain
microorganisms within the bodies of the silkworms were pro-
ducing the disease. He isolated these germs and found how to
control them, and thereby saved the silk industry of France.

CHANGING CONCEPTS 15

He then reasoned that the contagious diseases of man and
other animals were caused by the presence in their bodies of
various specific types of small organisms. However, his results
and judgment were in no wise generally accepted. A few faithful
colleagues, notably Joseph Lister of England and Robert Koch
of Germany, continued investigations into the causes and con-
trol of disease. Finally, Pasteur's own work in 1877 was the
turning point in establishing the germ theory of disease. He
demonstrated that splenic fever, a devastating cattle and sheep
disease, was caused by a type of microorganism known as
anthrax bacteria and that these bacteria could be killed and the
disease cured by a vaccine he had prepared.

The demonstration had a dramatic staging. His medical and
veterinary opponents induced him to perform a public experi-
ment. They hoped that his failure, which they believed inevi-
table, would discredit him and leave them to the pursuit of old
methods that they understood. However, the plan proved to be
a veritable boomerang. A number of sheep were collected amid
a large public gathering and divided Into two groups. The first
group Pasteur inoculated with his anthrax vaccine; the other
group was left without vaccination. Fourteen days later all the
sheep were injected with a virulent culture of the anthrax
microbes which Pasteur knew caused the disease.

During the day and night following, the unvaccinated sheep
began to get sick. As the news spread, more people collected at
the demonstration farm. Those were anxious moments. The
question in everyone's mind was whether or not the vaccinated
sheep would succumb to the disease. That night Pasteur received
a staggering note to the effect that one of the vaccinated sheep
was dying with the disease. However, it proved to be erroneous.
When morning came he went to the experimental lots amid
cheering and grateful people who had already learned the
results. Not a thing had gone wrong. The sheep that had not
been vaccinated lay dead from the disease, while the vaccinated
ones browsed in their lots with perfect health. The cause and
control of splenic fever had been established.

In 1880, Koch found the bacteria causing tuberculosis and in
the following year discovered the bacteria causing cholera.
Within the. following fifteen years most of the diseases caused by

16 THIS LIVING WORLD

microorganisms had been successfully studied. The germ theory
of disease has become an established fact. We now have vaccines
and serums that prevent or cure smallpox, typhoid fever, yellow

fever, lockjaw, diphtheria, and many other contagious diseases.

•

Changing Concepts of Life's Relationships

On the earth today there are something like a million different
kinds of living creatures that are known to man. A great many
of these different forms, or species, were observed by the peoples
of ancient times; this number, however, was much smaller than
that known at present. It is now known that many of these
species are closely related to each other and that others are more
distantly related, some only remotely. Early man knew of no
such specific relationships. Today it is well established that all
living creatures have certain fundamental things in common. All
life has apparently arisen from one common source and through
the ages has separated into the great diversity of modern forms.
This concept of life is one that has been developed only in recent
times. It marks a distinct change from man's thinking of a
great many centuries past. It has come about as a result of the
accumulation of a great amount of information regarding the
structure of the physical bodies of different animals and their
life processes and development.

The first attempt at a definite classification of animal life
and a study of their relationships was made by the early Greek
natural philosophers. One of the foremost of the natural research
workers was Democritus, who lived during the fifth century
B.C. He must have studied and observed the bodily structure of
a great many creatures, both large and small. He distinguished
between the vertebrate and invertebrate animals and believed
that even the smallest creatures observable to the unaided eye
possessed many body organs. These organs, he held, increased in
complexity and number with the larger animals until the body
of man was reached, which represented a sort of world in minia-
ture. He believed that there was a decided relationship between
cause and effect both in animals and in inanimate nature and
that different animals had different body forms and functions
because of some influence that had been exerted upon them.

CHANGING CONCEPTS 17

The first great systematizer of biological knowledge was
Aristotle, who lived from 384 to 322 B.C. He had observed a large
number of species and collected in his writings all contemporary
knowledge of animal life. He held that animals may be classified
according to their way of living, their actions, their habits, and
their bodily parts; that is, they were divided into land animals
and water animals. Then, the water animals were divided into
groups that could swim, those that could only creep, and those
that were adherent to rocks or ocean bottoms. The land animals,
also, had certain similar characteristics in regard to their habits
and ways of living.

However, his most important basis of classification was the
parts of animals bodies. This, of course, is the most significant
criterion in modern biology; but, Aristotle was handicapped in
knowing little about such body structure except what he could
observe from outside appearances. Thus, he separated the ani-
mals into such groups as mammals, birds, fishes, whales, shell-
fish, and crayfish. Each of these large groups constituted a sort
of genus, according to Aristotle's scheme. Within each large
genus there were many individual forms, such as the horse, lion,
or dog in the mammal group. It is seen, therefore, that he began
a kind of system for grouping animal life, even though his
classifications were often erroneous because of a lack of definite
knowledge of body structure.

He also studied reproduction among animals and was re-
markably familiar with the embryonic development of the chick.
He was familiar with it to the extent that he was able to give
clear statements of the structure of the embryo and the develop-
ment of the body parts at different stages in embryonic growth.
In particular, the growth of the heart, blood vessels, eyes, and
legs was carefully explained. He used reproduction as a means of
differentiating between animals, including three divisions, those
that reproduce by sexual means, those that reproduce asexually,
and those that arise from spontaneous generation, for he believed
that many small forms arise out of decaying substances. Thus
his system became more and more complex. Perhaps its greatest
merit was that it was better than no system.

In addition, Aristotle drew up a scale in which the animals
were placed according to their development and pointed out

18 THIS LIVING WORLD

that those animals are highest which have a warm and moist
nature and not an earthy one. The most perfect animals were
those provided with lungs, which possessed warmth, and whose
young were born alive; of these man was held to be the highest.
Then he developed a complicated scheme in which the male was
placed in superior position to the female. Likewise, the next
lower animals were the land forms which laid "complete" eggs,
such as birds and reptiles. These were followed by the cold and
earthy animals that lay "incomplete" eggs, such as frogs and
fish. And the lowest of all were the smaller creatures of other
groups. It is seen that there was a definite and decided separa-
tion of animal life into these various groups. Later generations
added the idea that there was little or no connection between
them.

Aristotle, therefore, became the founder of systematic
biology, and his teachings were the dominant note in biological
thought for more than fifteen centuries. They became adopted
in many of the ecclesiastical laws as well as being considered
standards for intellectual discussions. In this manner they came
to be accepted upon authority to such an extent that they pro-
duced stagnation in biological work for a great many centuries.
Under such circumstances the biology of antiquity, in spite of its
splendid beginnings, never advanced beyond Aristotle's con-
ception of the phenomena of life.

About the middle of the eighteenth century a work on the
classification of plants and animals was completed that was of
inestimable value in reducing to order a wealth of disconnected
information regarding plant and animal life and in showing
many of the relationships that exist between them. This was the
system of classification developed by the distinguished Swedish
botanist Carl Linnaeus. While still a young man, Linnaeus de-
veloped a keen interest in botany and found little satisfaction in
the pursuit of the academic studies of his early schools. He finally
accepted the advice to study medicine and went to the university
at Upsala. His first year there was spent in dire poverty; how-
ever, he had a remarkable quality of attracting admiration and
sympathy from many of his acquaintances. Soon he acquired
friends among the faculty of the university and gained one
success after another.

CHANGING CONCEPTS 19

The following year as an undergraduate he obtained per-
mission to lecture on botany and attracted large audiences. He
received a number of grants on which he traveled to different
parts of Sweden to collect material for research on natural ob-
jects. On one t)f these trips he met his future wife, the daughter
of a wealthy physician. With the financial assistance of his
father-in-law to-be he traveled and studied in Holland, even-
tually receiving the degree of doctor of medicine. While there
and while still in his twenties he published his epoch-making
work, the "Systema Naturae," which brought him immediate
fame. He later returned to Upsala and was made professor pf
botany, where from the day of his arrival he became the foremost
member of the university. His later publications followed each
other in rapid succession. He founded many schools and sent
some of his pupils on research expeditions to remote parts of
the earth. In organizing work and reducing an enormous amount
of biological knowledge to order and system he has had few
equals.

His biggest contribution probably consisted in establishing
the species as the basis of classification of living creatures and
in giving to each species a double scientific name. In his system
a species consisted of all examples of creatures that were alike
in minute detail of body structure. During his lifetime he de-
scribed many thousands of species of plants and animals. The
species most alike were organize^ into genera, while collections
of similar genera constituted different orders. All the similar
orders were grouped into larger divisions, the classes. With a
few modifications, this system is still used in classifying plant
and animal life, and it has been of inestimable value in reflecting
the relationships that exist in organic life. The scientific name
for each creature consists of its generic and specific title. For
example, the domestic house cat is Felis domestica.

It was Linnaeus' theory, however, that each species of
creatures was created by some special act in the very beginning
as it now exists and that creatures of each species were un-
changeable. He held that one single pair, one of each sex, had
originally been created, and that each one then reproduced its
kind through the ages in all respects like the parents and thereby
accounted for all the different species now existing. There was

ZU THIS LIVING WORLD

no room for spontaneous generation in such a theory, neither
was there any possibility for the seeds of one plant to give rise
to a different kind or for any animals to produce species other
than those of their own parents.

It is not known whether Linnaeus actually formulated in any
positive fashion such a theory or merely accepted the prevailing
ideas of his time in this respect. However, these ideas appear
repeatedly and forcibly throughout a great deal of his work.
They had a profound influence in encouraging the continued
acceptance of these concepts for a century following his death.
t In 1831 a young Englishman twenty-two years of age sailed
on a five-year voyage around the world. He was the unsalaried
ship's naturalist on H.M.S. " Beagle," which was to circum-
navigate the globe, mainly in the interest of map making. The
naturalist in question was Charles Robert Darwin. The voyage
of the "Beagle" not only did much to map the oceans of the
earth, but was also the beginning of a long and prolific life's work
by Darwin, which did much to change man's concepts of the
nature and relationships of all living creatures. The three years
previous to the sailing of the "Beagle," Darwin had studied
theology at Cambridge after having given up the study of
medicine at Edinburgh because of boredom with the medical
teaching of his day. He was advised against the trip by his
parents and nearly rejected by the ship's captain because of the
shape of his nose. However, hjj father gave his consent upon the
recommendation of an uncle, and, as Darwin himself remarks,
the voyage was the most important event in his life.

During the five years of the trip, Darwin sent home copious
notes and large collections from every stopping point. After
returning to England he spent many years in working over the
material he had collected and set a great many experimental
studies for himself. At the age of thirty he married his cousin,
Hanna Wedgwood, of the family of ceramic fame, and her
considerate helpfulness and wealth enabled him to lead the quiet
life of a scholar. This became an absolute necessity because of
increasing ill-health that had been started by his almost con-
tinual seasickness on the extended exploration. It is said that his
bodily existence, so full of suffering, was compensated for
throughout his life by a freedom from passion, hate, envy, and

CHANGING CONCEPTS 21

ambition. His ideas came to be unreservedly praised or violently
attacked. He met the attacks with calm steadfastness and always
took note of and answered material objections. These qualities
won for him great personal esteem, and when he died at the age
of seventy-three he was mourned by the most distinguished
scientific and social people of his time.

His studies showed him that there are many variations within
different species of plants and animals. These variations, he
found, increased gradually as distance and isolation from the
native home of the species increased. For example, he found that
on the desolate Galapagos Islands situated off the coast of
South America was a fauna of distinctly South American genera
although of different species. Many other variations were noticed
on the voyage. One instance was that certain insect forms on
islands in mid-ocean have restricted powers of flight as compared
to the same species on the mainland. In studying the changes
that had been produced in breeding domestic animals in England
and on the Continent, Darwin observed that new species had
been developed; that is, the bulldog and greyhound, both of
which had been developed from the wild canine type, differed
from each other more than the variations he had found in many
wild life forms that were considered one species.

On the basis of such observations and as a result of long
study, he finally formulated his theory regarding the variation
and development of species, which was published in 1859 under
the title, "The Origin of Species by Means of Natural Selection."
In this work he explains that in the struggle for existence those
life forms which are less capable of adapting themselves to their
environment are destroyed, while the individuals which have
certain variations suitable to prevailing conditions survive and
reproduce themselves/ The environment itself comes to favor
the differences brought about by variations in the offspring in
relation to their parents, until a new species may arise.

Thus, the restricted powers of flight of insects on mid-ocean
islands resulted from a selection of that characteristic as best
suited to survival in that environment. The wide-flying varieties
of the species were held to have been blown out to sea by the
strong winds and to have perished. This condition of nature is
not encountered on the mainland, where the flying varieties of

22 THIS LIVING WORLD

the species are found. Similarly, the life forms on the Galdpagos
Islands developed from the South American forms that were
isolated in the islands because of the natural selection of certain
variations in the offspring that were suitable to the different
environment there, while the old forms perished in the struggle
for existence. Consequently, the struggle for existence induces
natural selection that operates to produce the origin and de-
velopment of new species. Darwin maintained that the idea of
natural selection operating in life tended to produce higher and
better forms until perfection was reached.

This was a concept of life that was of a marked difference
from the old idealistic natural philosophy, in which it was be-
lieved that each species had been produced by a special act of
creation and that the species possess an independent and
immutable existence. However, it was not only a concept that
brought order into the attempt to account for the great varieties
of living creatures, but also one that had an optimistic outlook
on life processes.

Darwin's work, for the first time in history, established the
general idea that all living things are related — that existing
forms, as well as many extinct ones, have arisen through descent
with change from preexisting forms. Many of the detailed facts
on which the original theory was based have since been shown to
be of secondary importance. Darwin's theory of the origin of
species, at least as originally stated, has long since been aban-
doned. Thus, it is now known that the principal factors con-
trolling the production and propagation of new varieties and
species of plants and animals are inherent in the hereditary
mechanism itself. The theory of natural selection is retained in
principle, but its application is known to be restricted owing to
the limits imposed upon variation by the»physical nature of the
hereditary process. The essential idea of the theory of evolution
has become well established as the central concept of natural
philosophy. Not since Newton's formulation of the gravitational
law has a scientific theory so deeply influenced man's general
conception of life as Darwin's has.

One of the important results of Darwin's work, and of the
controversies which have raged over it, has been to introduce
into biology the modern spirit of scientific research and to

CHANGING CONCEPTS 23

establish man's freedom to base his views of living processes
upon the results of that research. During the half century since
Darwin's death a wealth of information has been obtained which
gives us not only a better understanding of the processes and
relationships of life, but also a greater appreciation of life's
values and significance.

The pages which are to follow contain a general survey of
some of the conditions and characteristics relating to living
things that are well understood today. Particular emphasis is
placed upon the physical structure of the human body and the
nature of the living processes within it. The first chapters may be
regarded as a kind of introduction, in which certain physical
conditions of the earth's surface are described which make life
possible and to some extent pleasant. This is followed by an
examination of the characteristics of living things, the develop-
ment of life on the earth throughout the geologic past, and
some account of modern forms with special reference to man
himself.

REFERENCES FOR MORE EXTENDED READING

NEEDHAM, JOSEPH, and WALTER PAGEL: "Background to Modern Science,"
The Macmillan Company, New York, 1938.

This book consists of a series of lectures delivered at Cambridge by a group of noted
English scientists. The lectures are interestingly and popularly written and are a
contribution to the history of science as a great cultural subject.

LOCY, W. A.: ** Biology and Its Makers," Henry Holt & Company, New York,
1908.

This is a well- written history of biology that is developed around the lives and works
of the men who have contributed most to it.

VALLERY-RADOT, RENE: "The Life of Pasteur, " Doubleday, Doran and
Company, Inc., Garden City, 1923.

The English translation of this French biography is an interesting and detailed
account of Pasteur's life work and the contributions he made to biological and
medical science.

DAMPIER, SIR WILLIAM: "A History of Science," The Macmillan Company,
New York, 1936, Chaps. I, II, III, VI.

A concise and authoritative discussion of the development of such scientific
knowledge of the early and immediate past as is necessary to explain the philosophic
thought of those times.

24 THIS LIVING WORLD

TAYLOR, F. SHERWOOD: "The March of Mind," The Macmillan Company,
New York, 1939.

This is a short history of science by an English author who traces scientific progress
from the days of Babylon and Egypt to modern times. A considerable number of
photographs and drawings of early scientific apparatus is included. Beading the book
for a connected story of the development of the biological sciences requires consider-
able use of the table of contents.

NORDENSKIOLD, ERIK: "The History of Biology," translated by Leonard
Bucknall Eyre, Alfred A. Knopf, Inc., New York, 1928, Chaps. Ill, V,
Part One; Chap. VII, Part Two; Chaps. VIII, XVII, Part Three.

A comprehensive treatment of biological discoveries and their attendant effects
upon the thoughts and beliefs of people during the past ages. Illuminating and human
biographical sketches of the pioneers of investigation from the days of earliest Greek
civilization to modern times.

WOLF, A.: "A History of Science, Technology and Philosophy in the 16th and
17th Centuries," The Macmillan Company, New York, 1935, Chaps. I, II,
VI, XVIII.

These chapters include a comprehensive treatment of the advances made in astron-
omy and biology during the sixteenth and seventeenth centuries. Most of these
chapters is written around the work and discoveries of the leading scientists of these
two centuries.

WOLF, A.: "A History of Science, Technology and Philosophy in the 18th
Century," The Macmillan Company, New York, 1939, Chaps. I, XV, XVII,
XVIII, XIX.

Included in these chapters is an account of advances made in the study of geology,
botany, zoology, and medicine during the eighteenth century. A companion volume
to the earlier publication.

2: SOLID SURFACES

Or a Consideration of Some of the Features of the Earth's Crust

SOME understanding of the nature of the earth's crust has
been of great advantage and importance to man. Upon its
surface he lives and exerts his activities. From it he digs his fuels,
building materials, water, salt, and minerals. The plants, which
get their substance from the soil and air, provide him with food.
This thin surface, including the air above it, provides a home
for life on the earth. So far as we know, it is the only place in the
universe where such life exists.

While we have some knowledge of the structure and density
of the interior of the earth, it is chiefly the relatively thin outer
layer known as the earth's crust that has been extensively
studied. It is the only part of the earth that is directly accessible
to man and, therefore, the only part that can be thoroughly
investigated. This outermost layer is a shell of rock about
twenty to one hundred miles thick. It is a small covering, indeed,

26 THIS LIVING WORLD

compared to the eight thousand miles of expanse through the
entire earth. However, it contains the continents and ocean
beds and because of this it is the most important part of the
earth to us. Many remarkable changes have occurred in the
earth's crust during past geologic ages. These have not only
brought about the present geographic features of land and sea,
but also made for conditions that have profoundly influenced
life here.

Earth's Relief

To a person traveling over the earth, its surface seems very
irregular. The land part has valleys, hills, and mountains. The
oceans seem very deep. These irregularities of the surface, or the
relief form of the earth, are great or small only by comparison. A
person on foot finds them very great, one in an automobile less so;
a person flying in an airplane finds them much less. As Howard
Hughes and his four flying companions recently winged their
way around the earth in less than four days, they probably
noticed little of its up-and-down surface beneath them.

Compared to the total size of the earth, its relief is very small.
In this sense it may be considered a smooth ball, much smoother
relatively than the surface of an orange. The highest elevation of
the land, Mount Everest in The Himalaya, is approximately
29,000 feet above sea level; the deepest known point in the
ocean, in the Pacific, is about 35,000 feet below sea level. This
makes a total difference in elevation of some 64,000 feet, or
about 12 miles. Between these extremes exist all the relief ele-
vations. However, about ninety per cent of the earth's people
live in a narrow elevation extending from the level of the sea to
about one-fifth of a mile above sea level. The mean height of all
land areas above sea level is approximately 2,900 feet. In North
America the average is less, about 2,400 feet. The average depth
of the oceans is 13,000 feet, it being greater for the Pacific than
for the Atlantic.

The main features of the land consist of plains, such as the
Atlantic Coastal Plain; plateaus, such as the Colorado Plateau;
and mountains, such as the Appalachians. One thing that is
relatively characteristic of the relief form of continents is that
they tend to have mountain chains as coastal rims with interior

SOLID SURFACES 27

plains or basins. The North American continent is especially
typical of this condition. The major relief divisions of this con-

fterra Mir, Rocky Mtr. Appalachian

General relief divisions of North America found in the United States. Vertical scale
greater than horizontal scale.

tinent, all of which extend through the United States, are the
Atlantic Coastal Plain, Appalachian Highlands, the Interior
Plains, Rocky Mountain System, Intermountain Plateaus, and
Pacific Mountain System.

Within these relief divisions are found most of the varieties
of rocks, soil, and geographic features that constitute the land
surface of the United States.

Rock Materials

There are three principal classes of rocks which make up the
earth's surface, including the continents and ocean beds. They
are igneous, sedimentary, and metamorphic rocks. These con-
stitute the bedrock that forms much of the outer portions of the
earth's solid masses. In addition to the bedrock, there is spread
over most of the land part of the earth's surface a sort of rock
debris called mantle rock. This debris is in various stages of
disintegration and decomposition. It is not a separate class of
rodks from those mentioned above, but it consists of broken
fragments of them.

Mantle, rock consists of sand, soil, pebbles, and boulders,
together with decaying remains of organic tissue. At most it
consists of a very thin skin over the rocks beneath and in some
areas it does not exist at all. However, it is this exceedingly thin
skin that supports most of life on the land, including man him-
self. Without it, life here would be quite a difficult process.
Mantle rock is continually being shifted about by winds and
water and being used up to form some of the more permanent
rocks mentioned above. However, its supply is ever replenished
by the same forces which produced it originally.

Great areas in the United States are covered with mantle
that has been produced by weathering of older rocks. In regions

28 THIS LIVING WORLD

of plentiful rainfall this rock debris usually becomes covered
with a layer of soil. The soil and mantle often show a gradual
change downward into the bedrock beneath, and this gradation
indicates that the mantle rock has been slowly built up from the
rock it covers. Such a formation is referred to as residual mantle.
In other regions the weathered debris has been deposited by
running water, wind, or other agents of transportation. Old
river valleys and flood plains are likely to contain layers of this
mantle. The same is true of regions that were once covered by
glaciers. This kind of rock debris is known as transported
mantle.

In the northern part of North America and Europe millions
of acres are covered with drifts of mantle rock which have been
deposited by glaciers in past ages. They consist of sand, pebbles,
and boulders spread out in rolling sheets and covered with soil.
This is the unsorted debris that was carried along with the
glacier, then dropped as the ice began to melt and the glacier
to recede, the soil being formed during later times. What is
often found is a great ridge of unbedded material piled up by the
melting glacier. Extending out from such a ridge will usually be
a series of outwash plains of sand and gravel that have been
carried away by the running water. The ridges are called terminal
moraines. Such moraines have greatly added to the fertility of
the soil, and particularly to the features of the landscape. Many
of the low-lying hills of New England and the Middle West
were formed by glacial deposits. The many small lakes of Wis-
consin, Michigan, and Minnesota that make those states
popular vacation lands had their origin in the same process.

Igneous Rocks

One of the great divisions of rocks making 'up the earth's
surface and much of its subsurface are the igneous rocks. These
are the so-called fire-formed rocks; at least, that is the meaning
implied by the term "igneous." Those formed on the earth's
surface were produced by the solidification of molten masses,
called "magma/' that have come up from within the earth.
The magma may be discharged from volcanoes or flow out of
fissures in the earth's crust as streams of lava. While volcanoes
are the more spectacular and better known examples of lava

SOLID SURFACES 29

flow, other greater movements of this molten rock have occurred
to form large areas of the earth's surface bedrock.

The formation of igneous rocks has occurred in two general
ways. In one instance the magma rose from the depths of the
earth to higher levels but was stopped before it reached the sur-
face. By a slow process of cooling, the magma gradually hardened
into masses known as igneous intrusions. These rocks may be
seen on the surface only where erosion or other forces have
removed the overlying formations and exposed the intrusions to
view. In the other case the rising magma has reached the sur-
face, where the lava flowed out, either violently or smoothly, to
form deposits that cooled more rapidly than the intrusions.
Such rocks are called igneous extrusions. They form the well-
known volcanic cones and great lava beds or thick deposits of
basaltic rock that cover large areas of land.

Igneous Intrusives

On the relatively flat landscape of western New Mexico just
south of the Mesa Verde National Park is a curious elevation
known as Shiprock. It was so named because it resembles a sail-
ship on a flat ocean surface. An examination of its structure
shows it to be a volcanic neck standing some 1,300 feet above a
wide expanse of sedimentary sandstone formation, and extending
out from it for several miles is a low-lying, narrow, igneous dike.
By reconstructing the picture of the formation of this unusual
feature of the flat country, it is known that it originated from a
mass of lava that was forced up along a fracture in the sand-
stone, which at the time had a much greater thickness than at
present. The flow of the lava was arrested before it reached the
surface, and slowly it solidified into igneous rock. A later erosion
of the softer sandstone through many hundreds of feet has left
exposed the more enduring igneous rock, and its contour is
evidence of the general shape of the vertical fracture into which
the lava was forced.

This formation and many others similar to it are examples of
smaller intrusions of once molten lava into fractures in the over-
lying rocks, the flow of which was arrested before the lava
reached the surface. Perhaps the best known example in

THIS LIVING WORLD

Airplane view or bhiprock near rarmington, IN. rvi., a voicanic necK projecting
1,300 feet above the surrounding country. (Photograph by Barnum Brown, American
Museum of Natural History.)

America of a lava intrusion between horizontal layers of over-
lying rock are the Palisades along the west side of the Hudson
River.

Examples of other intrusive actions on a larger scale are seen
in certain dome-like mountains known to the geologist as lacco-
liths, the classic example of which are the Henry Mountains in
Utah. The Henry Mountains are elevations that project some
5,000 feet or more above the surrounding, deeply eroded, sand-
stone formations and highly inaccessible country. The moun-
tains are covered with strata of sandstone, and it is only along
the sides, where a long period of erosion has enabled running
water to cut through the overlying layers, that the igneous core
beneath them is revealed.

The Henry Mountains, and others similar to them, are really
great "blisters" on the earth that were formed by lava intruding
itself between the layers of strata and arching it up into great
domes. Without breaking through to the surface, the lava hard-

SOLID SURFACES

ened and formed the igneous cores of the mountains. In such
cases the lava has apparently come up from below through a
relatively narrow fissure and ^^^^^^^^^^^^^^^^^
then spread itself out hori-
zontally as it pushed up the
strata above it.

The formation of igneous
intrusives on a grand scale is
seen in some of the great
mountain belts of North
America and other continents.
They have become visible
through exposure by erosion,
or elevated to the surfaces by
processes of mountain mak-
ing. These intrusives are enor-

The Henry Mountains and others similar to
them/ known as laccoliths, were produced by
lava intruding beneath layers of overlying
strata, and arching it up into great domes.
Without breaking through to the surface, the
lava hardened and formed the igneous cores
of the mountains.

mous bodies of igneous rocks that seem to extend downward
indefinitely into the earth's crust and are the main units in the
structure of great areas of its surface. They are technically
known as batholiths, and they consist chiefly of granite, this
being an igneous rock formed by magma cooling beneath the
surface.

The largest batholith in the United States is the one in
Idaho, which is exposed over an area of about 16,000 square
miles. The Sierra Nevada Mountains is another example of such
a formation on a large scale. Here elevation of the mountains has
exposed great areas of granite. In some places high mountain
peaks have been formed, one of which is Mount Whitney, the
highest point in the United States. Just to the east of Mount
Whitney the Sierra elevation drops rapidly into Owens Valley,
and it is revealed that the granite layer is at least 8,000 feet
thick. From this peak extend in all directions for many miles,
even hundreds of miles in some directions, great ranges of
granite which are silent evidences of former great movements of
magma.

Igneous Extrusives

One of the most spectacular scenic views in Northwestern
United States is the Seven Devils Canyon in the Columbia

THIS LIVING WORLD

s „

_ J j. -_ _. ___ )

Many great mountains of Western North America and other continents consist of
enormous bodies of intrusive igneous rocks which seem to extend downward indefinitely
into the earth's crust. They are known as batholiths. The illustration represented above is
about 20 miles wide. (Redrawn from Longwell, "Physical Geology.")

Plateau. Here the Snake River has cut a winding trail more than
three thousand feet deep in an enormous bed of lava flow that
covers an area of more than 200,000 square miles. The structure
of the igneous rock as well as the sedimentary deposits now
covering it, except where they have been removed by erosion,
reveal that this area was once covered with great seas of molten
lava that flowed to the surface, where it cooled relatively fast.
The lava that once deluged this wide expanse of country must
have flowed over the land without any explosive activity, as it
has formed broad plains, the beds of which often show horizontal
layers. The chief rock of this formation is basalt, which is a hard
and enduring type of igneous rock that is often formed by lava
cooling on the surface.

Another example of mass eruption of lava is to be found in
western India. There an area equally as large as the Columbia
Plateau is covered with basalt which reaches a maximum thick-
ness of about six thousand feet. Likewise, the bedrock of the
northern part of the British Isles as well as the islands to the
north is remnant of a basalt plateau which has been deeply
eroded by the sea since its formation. It may be that this plateau
once extended as far north as Iceland,

SOLID SURFACES 33

Volcanoes are a form of magma flow that is more generally
known than any other process of igneous rock formation. This is
so primarily because of their quickness of action, spectacular
displays, and visible evidences of damages done. One of the more
recent volcanic eruptions that received wide notice was the
activity of Mauna Loa in Hawaii in 1935. During this year there
was an outpouring of lava which exceeded in volume any other
similar activity witnessed by modern man. A sea of molten rock
flowed down the side of the great cone from an elevation of about
nine thousand feet, where it erupted, and threatened to destroy
the town and harbor of Hilo at the seacoast near its base. A
series of bombings by the U. S. Army Air Service diverted the
flow from its course toward the town, and some of it finally
spilled into the sea at a safe distance before the eruption ceased.
The volcano itself is a mass of igneous rock that has been built
up to an elevation of nearly fourteen thousand feet by former
eruptions, and it is the largest of all modern volcanoes. These
recent eruptions have added another layer of lava rock to its
expanding sides and base.

Perhaps the most violent volcanic eruption of modern times
is the one that occurred in 1883 at a volcano in the Indian
Ocean near Java, known as Krakatao. The volcanic cone ex-
ploded and the entire island was blown to pieces. Dust from the
explosion was thrown upward for many miles and carried by the
winds over the entire earth, producing in many places such
brilliant sunsets as had never been seen before. The force of the
explosion caused waves in the sea some fifty feet high which
swept away the coastal villages in near-by Java and Sumatra.

It is not to be implied by these two examples that all vol-
canoes either pour forth great volumes of lava or erupt with
explosive violence. In many cases volcanic activity may consist
primarily of the escape of water vapor and carbon dioxide gases,
accompanied by smaller quantities of other gases, such as
hydrogen chloride and hydrogen. In all cases, it is generally
believed that such gases come from the magma itself, being dis-
charged when the pressure on the magma is released as it moves
through the vent to the volcanic cone.

Many different-shaped volcanic cones may be built up, the
size and shape depending upon the nature and materials of the

THIS LIVING WORLD

The flow of lava from Mauna Loa, Hawaii, in 1935 destroyed the forests in its path
and threatened the town of Hilo on the nearby coast. A river of the lava is shown in the
upper right corner of the picture. (Science Service photograph.)

Recent airplane view of Krakatao, showing the stump of the neck that remains today.
The volcano blew up in 1 883, producing the loudest noise modern man has ever h$ard.
(Life Magazine photograph, courtesy of N.E.I., Army Air Corps.)

SOLID SURFACES

eruptions. These vary from the relatively symmetrical cones
with a small vent or opening at the top, to the large and irregular-
shaped ones, some of which
may have many vents of
different ages. Crater Lake
in Oregon is on the summit
of a volcanic mountain of
basalt that has long since
ceased to be active. Appar-
ently the entire top of the
old volcano "caved in" to
form a great pit some five
miles in diameter and about
four thousand feet deep. It
is now about half filled with
water of a beautifully blue
hue, making this lake one of
the most spectacular in all
America.

Volcanic cones are built up in various
shapes and may consist of a variety of rocks.
The one illustrated above has two vents that
have been active. The cone is built up of strata
of solidified magma, represented by the black
lines, and strata of other materials, such as
volcanic ash and wind-blown sediments. The
magma producing volcanoes rises through a
vent in the strata below the cone from some
reservoir at a greater depth.

The source from which such great quantities of magmas
come which produced the volcanoes and other greater flows of
lava is still somewhat undetermined, especially as regards
magmas of different types and different chemical composition.
However, it is likely that the parent lava from which all diverse
forms are produced is basalt and that the other forms result
from chemical changes which occur in the basalt as the magma
is pushed upward. The ample source of basaltic magma is be-
lieved to be a deep layer of basalt that underlies all the earth's
crust. This basalt is probably in a rather solid state at the pres-
sures exerted upon it, although it is very hot. When this pressure
is lessened by some weakening in the crust above, the basalt
becomes somewhat liquid and begins to flow toward the surface
through some vent, fissure, or crack in the overlying rocks.

Sedimentary Rocks

The Grand Canyon of the Colorado River is widely known to
the peoples of the earth because of its ever-changing beauty and
gigantic size. Here the river has cut its way into the earth to a
depth of about one mile and to a width of twelve to fourteen

THIS LIVING WORLD

Crater Lake, Oregon. The lake's surface is six miles Ions, four miles wide, and 6,293 feet
above sea level. (Photograph by Ewing Galloway.)

miles. In addition to the scenic grandeur of the mighty chasm,
its eroded walls have exposed in one great picture geologic
processes throughout many millions of years. The upper four
thousand feet of these walls consists of nearly horizontal layers,
or strata, of sedimentary rock that have been cut through and
laid bare by the eroding river. These rocks have been formed as
a result of thick beds of sediments that were deposited in past
geologic ages. Some of these sediments were laid down when the
area was beneath an inland sea, and others were deposited at
times when it was elevated above the water. A slow elevation in
relatively recent geologic ages has caused the river to cut deeper
and deeper as the surrounding country has been raised.

The lowest formation shown in the accompanying picture of
the canyon wall consists of shale and sandstone more than one
thousand feet thick. Above it is a five hundred-foot layer of
Redwall limestone whose massiveness causes it to form many
steep cliffs and some of the most interesting scenery of the
canyon. The next overlying layer is a thick series o* «1™1^« «nrJ

SOLID SURFACES

South wall oF the Grand Canyon of the Colorado River near El Tovar, showing six sedi-
mentary formations. They are lettered to correspond with explanations in the text.
(Photograph by N. W. Carkhuff, U. S. Geological Survey.)

sandstones. Above these is a layer consisting entirely of shale.
On top of this is a layer of white sandstone known as Coconino
sandstone. Above this and forming the top layer is the Kaibab
limestone that was deposited under a sea that flooded this country.
The formations that are shown on such a large scale at Grand
Canyon are examples of sedimentary rocks. These rocks underly
large areas of the entire land part of the earth. In some places
these sedimentary rocks extend to depths of forty thousand to
fifty thousand feet. In other places they may be very thin and,
of course, in still others they do not exist at all. These are forma-
tions that were once sediments which have since changed into
rocks. They constitute a second group of the earth's bedrocks.

38 THIS LIVING WORLD

One of the most noticeable features of sedimentary rocks
is that they usually consist of layers or strata. These layers may
vary greatly in thickness from a fraction of an inch to many
hundreds of feet. In most cases the strata differ from each other
in some manner, such as a difference in color, texture, composi-
tion, or all of them; and the rock may split easily along the place
where the strata join each other. In many places the strata may
be horizontal or nearly so, as is the case at Grand Canyon. In
other places they may be vertical or inclined at a large angle
from the horizontal. Since all strata were deposited originally as
horizontal layers of sediments, it is concluded that present
stratified rocks which are inclined at some significant angle or
warped into twisted designs have had their positions altered
by some geologic forces acting upon them.

The strata, when present, have been produced by the manner
in which sedimentary rocks are formed and by the composition
of the sediments laid down. The ceaseless actions of erosive
forces continually produce fragments of the older rocks, or reduce
such rocks to chemicals that are held in solution in the water,
draining from the land. For the sake of clarity we may think
of the materials that eventually form the sediments as divided
into two classes; first, those that consist of broken bits of rocks
which are carried along by moving water, winds, or glaciers
and, second, those that are dissolved out of soil and rock and
held in solution in water to be later deposited by some process
of separation from the water. The fragmental materials consist
of mud, silt, sand, and gravels while those in solution are mainly
calcium carbonate and silicates.

The separation of the fragmental materials into mud, silt,
sand, and gravels is primarily a division of such materials
according to size. Mud and silt consist of the finest particles of
the insoluble sediments, usually so small as to be distinguishable
only under a high-power microscope. They are usually flaky
minerals that have a great tendency to float and therefore re-
main suspended in the water for long periods of time. They
settle out only after long standing in relatively quiet water.
They are likely to be deposited over flood plains during times of
decreased flow of water and on oceans and lake beds at some
distance from the shore, where the water is relatively quiet.

SOLID SURFACES

One of the most noticeable features of sedimentary rocks is that they usually consist
of layers or strata. This is remarkably well shown in the Teapot Dome Rock in Wyoming,
famous in oil and legal history. (Science Service photograph.)

Thus, in time, layers of fine-grained clay material are deposited
which upon long standing may produce strata of sedimentary
rock of the characteristics of the finer sediments.

The fragmentary materials consisting of particles about the
size of refined table salt are considered as sand. These grains are
likely to become more or less rounded in their process of trans-
portation by water and wind. Wind-blown sands become rounded
most, and those in desert areas may become almost perfect
spheres. The sands of oceans and lakes will be less rounded than
the wind-blown ones, and river sands are usually the most
angular of all. Since the sand grains are larger than silt particles
they will settle out of the water or air somewhat sooner than
the silt, and beds of these deposits are formed at the mouths of
rivers, relatively near ocean and lake shores of moderately quiet
waters, and in land areas of high sand content where prevailing
winds blow. Eventually these sediments may form strata of
sedimentary sandstone.

Fragmentary materials of larger particles constitute the
gravels. These coarser materials also become somewhat rounded

40 THIS LIVING WORLD

in transportation by water and glaciers, the amount being
roughly a measure of the distance carried. Such larger particles,
of course, are dropped first by the transporting agents, and they
may form thick layers of deposits. This has particularly been
true at the ends of melting glaciers, where low-lying hills or
moraines have been built up. Gravel deposits often contain
finer-grain materials that have been caught by the gravel and
prevented from removal unless the transporting water has a
fairly rapid flow. These smaller particles as a rule are sands or
similar materials that do not easily go into solution or suffer
chemical decay.

Of the second type of materials forming sediments, that is,
materials in solution, calcium carbonate and the silicates are
the most abundant. These materials are dissolved out of land
areas and transported to the oceans or inland seas of no outlet,
such as Great Salt Lake or the Dead Sea. After long accumulation
in sea water, parts of these materials are removed by various
processes and form sediments at the sea bottoms. Continued
evaporation of the inland, or "dead," seas results in concentra-
tion of the solution to the point where sediments begin to form.
However, most of the sediments from these materials in solution
are accounted for in other ways than by evaporation; partic-
ularly is this true of calcium carbonate.

Calcium carbonate in solution is particularly sensitive to
the amount of carbon dioxide dissolved in the water. Where
the temperatures of the sea water are highest more carbon
dioxide will be driven off by the heat and consequently less
remains in the water. These areas, of course, are near the surface
of the sea, and especially in shallow portions of the oceans along
continental margins and in shallow submarine banks in tropical
regions. As the content of carbon dioxide becomes less, the
calcium carbonate becomes more concentrated, and it will be
precipitated out when saturation is reached. Often in such places,
great beds of calcium carbonate are deposited. These beds
eventually form strata of limestone rocks, and the thickness of
the strata will be determined by the length of time elapsing
before some change of conditions occurs and by the concentra-
tion of calcium carbonate in solution. In some places, beds of

SOLID SURFACES 41

limestone thousands of feet thick have been formed by processes
of this character.

Marine animals and plants are also active agents in removing
calcium carbonate, as well as other chemicals, from sea water.
These materials are withdrawn from the water in the course of
the life processes of the organisms to f orn^ shells or other mineral
parts of the organic body. At death of the organism these hard
parts drop to the bottom to become a part of the sediments.
Shells of marine animals may be deposited in such quantity
as to form beds of limestone rock consisting almost entirely of
such shells. Limestone beds of great thickness that contain
shells or other calcium carbonate remains of once-living marine
creatures are not uncommon. Limestones containing such re-
mains, known as fossils, were used in building King Solomon's
Temple, as well as the Pyramids of Egypt, and they are found
extensively over the earth in regions that once were the bottoms
of shallow seas.

Sediments of the various types mentioned above become
converted into rocks by the action of numerous forces upon
them. One of the most important of these forces is pressure from
overlying deposits. The sediments may become cemented by
the actions of certain minerals that are present in them or
percolate through them in ground water, which harden when
this water is removed. Upon hardening they form a sort of
mineral "gel" in the spaces between the sediment particles
which holds these particles tightly together, thereby binding
them into sedimentary rocks. As a result of these forces and
other less important ones not mentioned here, the various
sediments eventually form rocks. Thus, calcium carbonate pro-
duces limestone, silt forms shale and mudstones, sand results in
sandstone, and gravel produces conglomerate rock and glacial
till.

Metamorphic Rocks

There still is a third general type of rocks included in the
earth's surface that must claim our attention, even in such a
brief discussion as this. These are a highly distinctive group of
rocks that are neither igneous nor sedimentary and are known

42 THIS LIVING WORLD

Contact-metamorphosed rocks, shown in dotted areas, may be produced by igneous
intrusions into older sedimentary rocks.

as metamorphic. The term itself means changed in form, and
rocks so named are those that have been transformed from older
rocks, both igneous and sedimentary, so that their original
character is partly or completely altered. This transformation
is in reality the result of the original rocks adjusting themselves
to a new and different environment from that in which they
were formed. Rocks are subject to change when different con-
ditions are imposed upon them. When they have adjusted them-
selves to a given set of conditions, they are said to be stable for
that environment. Upon a change of these conditions they
become unstable, and a new equilibrium demands that an
alteration in the original minerals be made. During the process of
becoming stable in the new environment metamorphic rocks
are produced.

Let us consider a simple example. A body of limestone that
is deeply embedded beneath the surface is subjected to magma
penetrating the cracks and fissures of the strata. The heat of the
magma will raise the temperature of the adjacent limestone.
The limestone is unstable under such conditions and profound
changes take place in it. It may become somewhat fluid in char-
acter, and when it has resolidified new types of crystals will be
formed. The physical appearance will be changed. Any fossils

SOLID SURFACES 43

that may be present are likely to be entirely destroyed. When
the limestone has reached a stable state under the new condi-
tions, metamorphism has converted it into marble. Should there
also be vapors escaping from the intrusive magma, these vapors
and some silica of the magma will penetrate the pores of the
sedimentary rock. They are likely to combine chemically with
the calcium carbonate of the limestone, producing calcium
silicates and many other compounds. As a rule they become
beautifully crystallized and may form many new minerals.

Heat and a swarm of chemical mineralizers from magma
constitute, then, agents that may bring about alteration of the
rock into which the magma intrudes. Other forces bring about
metamorphism of rocks on a much wider scale than the contact
with intrusive magmas just mentioned. One of these is pressure,
and its accompanying heat, that is produced by a movement of
great areas of existing rocks. Such movements come about as a
result of folding of the earth's crust or elevation of mountains
through long periods of time. The changes that take place in
rocks under such pressures are quite complex, and only in general
can they be considered here.

Rocks ordinarily break into fragments when they are sub-
jected to sudden pressure or blows. However, when they are
confined on all sides and the pressures applied over long periods
of time, quite different effects are produced. The rocks become
plastic, and may even be made to flow, just as a block of ice can
be made to flow through an opening in its container when a
great and continuous pressure is exerted upon it. It is in this
manner that the ice on the underside of a glacier actually flows
down a mountain valley.

Rocks that have been subjected to mountain-building pres-
sures may be metamorphosed into new structures and new
chemical combinations. It is generally true that when rocks
become metamorphosed they take on forms to suit the new
environment, in which both the crystal structure and the chem-
ical composition become denser and the new rocks usually
harder.

Without mentioning any of the details of metamorphism, a
genera] type of summary may be stated. Considering first the
sedimentary rocks, sandstone is metamorphosed into quartzite,

THIS LIVING WORLD

This injection gneiss, in the inner gorge of the Grand Canyon, near Bright Angel Creek,
shows the white intrusive masses and the dark masses of metamorphosed sedimentary rock
between. Both the metamorphic and igneous materials have been twisted and bent by
the motion of the deep strata in which ^they were once contained. (Science Service
photograph.)

shale into mica schists, and limestone into marble. Among the
important igneous rocks, granite is metamorphosed into gneiss,
while lava and basalt may produce serpentine and soaps tones.
If we remember that metamorphic rocks include a great variety
and complexity of forms, the above simplified grouping may be
of value in clarifying our thinking without misleading us.

Within the narrow confines of Manhattan Island there are
two interesting illustrations of metamorphic rocks. One is mica
schist, the other is marble. At Morningside Heights is a high
elevation overlooking the Hudson River to the west. The rocks
are mica schist and they were metamorphosed from a former
shale. They have resisted much of the weathering of past geologic
ages. As typifying their endurance man has built at this location
some of his most permanent structures — the Cathedral of Saint
John the Divine and Columbia University. Just to the east and

SOLID SURFACES 45

at the foot of a steep cliff are the Harlem flats, eroded almost to
sea level. The underlying rocks here are the less resistant marbles
that were metamorphosed from sedimentary limestones. Even
these less enduring rocks seem to have exerted an influence on
man. The Harlem section is covered almost entirely with low-
type tenements, in marked contrast to the lofty spires of the
cathedral towering above them.

Earth's Shifting Surface

One general opinion man has of the earth is that it is solid,
stable, and enduring. The notion of a terra firma that has been
so long and universally held would seem to fortify our ideas of
its rigidity and permanence. However, the earth's surface is
continually being shifted about, a fact that is evident enough to
the inhabitants of an earthquake country. The former idea of an
" unshakable earth" is now outmoded. The earth, even with its
immensely long history, has not yet reached a static and stable
condition. It probably never will.

Earth-crust movements vary from those that are quick and
violent, as manifested in the most pronounced earthquakes, to
those that are so slow as to be imperceptible. These movements
may effect only small, local areas, or they may represent uplifts
or downwarps of a large portion of a continent or ocean basin.
Sudden movements are more impressive and significant to the
popular mind than the slow movements because of the destruc-
tion they produce and the evident shifting of local land areas.
However, the crustal movements which take place more slowly
and on a larger scale are of much greater importance in producing
profound geographic changes in which large areas may sink
beneath the sea or similar ones be elevated into mountain ranges
over long periods of time. A single word has been invented to
denote all the diverse and complex movements of the solid parts
of the earth. It is well worth knowing, and we pause to note it
here. This word is diastrophism. There are many evidences that
diastrophism has occurred on the earth in recent times as well
as in past geologic ages.

One of the points of interest to many tourists in Naples is
the ruins of the temple to Jupiter Serapis on the seacoast near
by. This temple was built before the time of Christ on the shore

46 THIS LIVING WORLD

overlooking the blue waters of the sea. By A.D. 1200, the land
had sunk and the floor of the temple was beneath the sea to a
depth of twenty feet. During the eighteenth century the land
was elevated again. Three of the forty-six old columns were still
standing. These shafts were pitted over the twenty feet that
had been beneath the water by the borings of marine mollusks.
Today the floor of the old court is again slightly under sea
water, and the three columns still stand as monuments to a
glorious past, and as a recorded history of the rising and sinking
of the land in that area.

There is direct evidence that a part of the southern coast of
California has been elevated from below sea level. This consists
of terraces along the shore that were formed when part of the
present land was below water. At San Pedro Hills, near Los
Angeles, is a succession of such terraces cut by the waves of the
Pacific that are plainly marked. The highest and oldest of these
terraces are over a thousand feet above sea level at present,
while the lowest, which contains many seashells, is about one
hundred feet above the water.

On the eastern coast of North America are many "sub-
merged" river valleys and small lakes near the shore which
indicate that the eastern coast of this continent has been sinking.
In some places this subsidence has occurred to the extent of
hundreds of feet. For example, the Hudson Valley of New York
is a good illustration of a "drowned" river. A regional subsidence
of hundreds of feet now allows tide waters to flow up this river
for a hundred miles above its mouth. Soundings of the sea
bottom to the east of New York City seem to show that in
former geologic times the Atlantic Coast was much higher than
at present and that the Hudson River flowed a hundred miles
farther out to sea and probably emptied into the ocean over a
waterfall.

Diastrophism is in no sense confined to shore lines, as might
be inferred from the illustrations just given. There are many
examples of earth-crustal shiftings in the wide expanse of the
continents themselves. Most of these have occurred during the
geologic past; however, there are evidences that some are going
on at present. One example of a change in land levels that is
evidently in progress at present is in the Great Lakes region;

SOLID SURFACES 47

and this has had its repercussions in human affairs within recent
years.

Many people still remember the highly publicized contro-
versy a few years ago regarding the Chicago Drainage Canal,
which permits some of the waters of Lake Michigan to drain into
the Mississippi rather than follow the present natural outlet
through the other lakes and the St. Lawrence River. It was
argued pro and con that the Drainage Canal would produce a
disastrous lowering of the water levels of the Great Lakes. How-
ever, it appears likely that in future ages Lake Michigan will
drain through the Mississippi, regardless of man's opinions or
efforts. A far greater force than any which he controls is at work
to change the level of the shores of the Great Lakes. The land
to the northeast of this region is slowly being elevated, as is
evidenced by the fairly rugged shore lines to the north of the
lakes. The area to the southwest of the lakes is sinking, the low-
lying coast and submerged valleys being witnesses to this fact.
The net effect of this is to tip the lakes toward the south. If
nothing happens to change the present tendency of this tipping,
Chicago is eventually destined to find itself marooned on a low
island or completely submerged as the waters of Lake Michigan
and Lake Superior find their outlets to the Gulf of Mexico.

Mountain Building

However, much more pronounced examples of diastrophism
on a large scale are to be found within the mountainous regions
of North America and other continents. These are great eleva-
tions of land that have been uplifted, for the most part in the
remote past. In certain parts of the Rocky Mountains are strata
of sedimentary rock that now exist at elevations from one to two
miles above sea level. Many of these strata contain shells of sea
animals. These sediments were unmistakably laid down beneath
the surface of the sea, and a later upheaval raised the sedi-
mentary rocks to their present levels. In The Himalaya sedi-
mentary strata containing marine fossils have been found up to
heights of four miles. These two examples and others similar
to them are definite evidences of broad uplifts of land areas
which have produced in some cases extensive mountain
ranges.

THIS LIVING WORLD

An action photograph showing the collapse of a wall of "Sinking Canyon1* near Buhl,
Idaho. The slump of the canyon along a geologic fault began in 1937, and parts of the
canyon wall give way from time to time. (Photograph by Ewing Galloway.)

Airplane view of Death Valley, California, and the block mountains on each side that
were formed by faulting. (Photograph by Stephen H. Willard.)

SOLID SURFACES 49

The movements of the crustal rocks have taken place in
various ways, as indicated by the structure and form of the
mountains produced. One significant way has been by faulting
on a large scale, or breaking and displacement of rocks along a
line for a considerable distance. Another has been by a deforming
of the rocks into great folds, giving rise to what are called fold
mountains. In many places these two types of movements have
taken place concurrently, producing mountains that are complex
in their structure.

Many of the north-south ranges of eastern California,
Nevada, and Utah are mountains that have been formed by
faulting of great blocks of rock. The magnificent Sierra Nevada
range is a single, somewhat tilted, fault block of granite over
500 miles long and about 75 miles wide. It has been sharply
upthrusted on the eastern face to elevations of 14,000 feet in
some places. The western slope, however, decends gradually for
distances of about 75 miles into the Great Valley of California
only a few hundred feet above sea level, thus indicating the tilt
of the entire mountain block.

These mountains evidently were formed by a granite mass
of large dimensions slipping upward along a fracture several
thousand feet deep that extends in a general north-and-south
direction. This fracture no doubt developed from some weak-
nesses in the bedrocks which yielded when the forces exerted
upon them exceeded their breaking point. Later forces have pro-
duced upward movements along this old fault to form the present
mountains.

Many other mountains as well as valleys are the results of
upward or downward movements of the earth's crust along
faults in the rocks. An idealized situation is represented in the
drawing on the following page to illustrate how forces that act
largely in a vertical direction may elevate some areas into
mountain heights and depress others into valleys, the movements
taking place along great faults in the earth's crust.

Mountains in which the rocks are strongly folded and broken
are also common examples of crustal rock movements. Where
these folds, or their eroded remnants, consist of sedimentary
strata it is possible to determine the processes whereby the
mountains have come into existence, even though the complete

THIS LIVING WORLD

history of the mountains may have been quite complex. Such
is the case with the Appalachians of Eastern United States. A

brief consideration of these
mountains may give a gen-
eral insight into certain pro-
cesses of diastrophism that
have extended over many
geologic periods.

Even a casual trip across
the Appalachians would re-
veal that most of the exposed
rocks are the commonest

Many mountains as well as valleys are the
results of upward or downward movements
of the earth's crust along fractures or faults in
the rocks. An idealized mountain range with
a deep valley on each side is represented
above. There was a relative moment of the
bed rocks along two faults, as indicated by the
arrows.

kinds of sedimentary strata,
such as sandstones, shales,
and limestones. Many of
these rocks contain marine
fossils of the type which
indicate that the strata were laid down on the floor of a relatively
shallow sea. Careful examinations have shown that these strata
were deposited to thicknesses of 25,000 to 35,000 feet. Since the
strata are of shallow-water origin, it is evident that the sea
bottom must have been sinking during times when the sediments
were deposited. Such a slow sinking can be accounted for by the
great pressures exerted on the lower strata by the weight of
the accumulating sediments above.

These deep deposits are now known to have been laid down
gradually in a large subsiding trough, extending in a general
north-and-south direction, about 100 miles wide and several
hundred miles long. Such an elongated trough is known as a
geosyncline. The Appalachian geosyncline was filled in during
much of its history by an inland sea which at times extended to
the west probably as far as is the Mississippi River, and at other
times was reduced to great swamp areas.

The geosyncline was bordered on the east by a belt of old
rock near the present Atlantic Coast. This ancient land of
unknown extent must have reached far to the east and out into
the present ocean. Much of the sediments that were deposited
in the geosyncline are known to have been eroded and trans-
ported from this eastern belt of land. A study of the Appalachian

SOLID SURFACES 51

strata shows that the coarser materials are on the east, with finer
materials grading westward into marine shales and limestones.
Only a general drainage from the east would sort the sediments
in this fashion on the bottom of the inland sea.

This high plateau to the east and its western geosyncline is
known to have existed for six geologic periods of about fifty
million years each in order to account for the types and depths
of the strata formed in the subsiding trough. As this enormous
volume of sediments was being delivered into the geosyncline,
the bordering eastern belt must have been rising either con-
tinually or intermittently to cause erosion in this fashion to
continue until such great deposits were made. This evident
rising of the eastern belt was certainly the result of some vertical
upward force, and it may have been aided by the belt's becoming
lighter as the eroded materials were removed.

As these processes of plateau erosion and deposition of the
materials in the geosyncline to the west were going on, we have
evidences of one of the remarkable procedures of nature. Tre-
mendous horizontal compressive forces were brought to bear
on the sedimentary strata and their eastward coast line. The
strata were warped into folds, and a mountain chain was literally
raised out of the inland sea. The structural features of the
Appalachians indicate that just such folding took place in several
stages to produce the mountains from the old geosyncline. These
folds were pushed up to great heights. In some places the folds
were severely compressed, and in others there were great frac-
tures and a faulting of strata over the original overlying beds.
A later erosion has reduced these great folds and fault thrusts
to the feeble old mountains of the present that present little
more than a reminder of their former glory.

The folding took place entirely in the strata of the geo-
syncline. This trough was the area of the greatest weakness in
the rocks, and it would be, therefore, the one that would be
affected most by the horizontal pressures applied in the earth's
surface. Furthermore, extensive folds would take place in deep-
lying strata when horizontal forces were applied, since the great
pressures from overlying beds would render the rocks somewhat
plastic. It is only where the folding has become extreme, or less
plastic conditions exist, that fractures and faulting take place.

THIS LIVING WORLD

During the first stage of the formation of the Appalachian Mountains, deposits of
sedimentary rocks were formed in a subsiding basin, known as a geosyncline. These
sediments were eroded and transported from an ancient highland of unknown extent that
reached far to the east.

At a later geologic time great compressive forces were brought to bear on the sedimen-
tary strata and their eastern coast line, producing a folding and fault thrusting of the rocks
in the geosyncline. Later erosions produced the feeble old mountains of the present from
these folds and the Atlantic Coastal Plain from the eastern highlands. (Redrawn from
Longwell, "Physical Geology.1')

It may be stated as a general rule that in mountains pro-
duced by folding, one side of the range will consist of much older
rocks which were the source of the sediments that formed the
strata of the mountains. Bordering these older rocks on one side

SOLID SURFACES

was a geosyncline and shallow sea which received the sediments
that were later warped into the mountain range. This has been
the history not only of the
Appalachian range, but also of
much of the Rocky Mountains
at a vastly later period. The
Rocky Mountains developed
from a great geosyncline that
stretched north from the Gulf of
Mexico to the Arctic, with high-
lands to the west.

A shrinking of the earth would set up
horizontal forces in the earth's crust that
would produce folds in the weaker
rocks.

Why These Earth-crust Movements?

To explain why and how such movements of the earth's
crust take place is much more difficult than to ask the question.
There are several theories to account for the enormous forces
involved in diastrophism. While there are many lines of evidence
that point to these general conclusions, it should be kept in mind
that the detailed causes of crustal movements are not known.
One of the ideas that is generally agreed to is that the earth is
shrinking, and that this shrinking probably brings about a
wrinkling and fracturing of its crust. Another idea held by some
is that a sort of mechanical balancing of heavier and lighter
areas brings about crustal adjustments.

It is fairly well known that the earth's interior, although
rigid, is very hot. This being true, slow cooling would cause a
shrinking below the comparatively shallow depth of the cooler
crust. This condition would bring about a buckling of the outer
surface. In general a reduced size of the earth sphere would pro-
duce forces applied horizontally in the buckling crust. Thus the
rocks, particularly the weaker or more plastic sedimentary
strata, would be pushed up into folds, somewhat the same as
one may produce folds in a sheet of paper by properly pushing
on its sides.

It was suggested about 1890 that some of the movements of
the earth's crust may have occurred through a general process
of mechanical balancing. Let us see what this means. All parts of
the crust are pulled toward the earth's center by the force of

THIS LIVING WORLD

gravity. This force will be in direct proportion to the masses
of different areas. Some parts of the earth's crust are heavier than

others. The oceanic portions of the
globe are denser than the continents ;
volume for volume they are heavier.
Especially is it true that there is a
considerable difference in density
between ocean beds near the margin
of the continents and the continental
areas. The ocean beds tend, there-
fore, to be pulled more strongly
toward the earth's center than the
continents, thus setting up strains
and stresses.

The tendency will be for the land
to be uplifted relative to the ocean
bottoms. During the actual yielding
the oceanic segment may settle
downward with respect to the con-
tinent somewhat as a sinking wedge.
Such action would tend to squeeze
the continent with horizontal forces as well as to produce a rela-
tive elevation. While the idea of balancing great areas by a sink-
ing of the heavier and rising of the lighter areas is highly
speculative, such processes do explain in part the formation of
certain deep ocean beds and of some mountains that resulted
from faulting. Likewise, they would account for some broad
shifting of land levels where mountain building has not occurred.
In terms of this theory, we can get some explanation of the
changes taking place around the Great Lakes. During the ice
ages large glaciers covered Canada and the northern part of the
United States. The enormous weight of this ice caused the land
beneath it to sink. The land to the south was elevated to balance
this pressure. The glaciers have long since melted and the water
flowed back to the sea. The higher lands to the south were
unbalanced by the sunken lands to the north with their glacial
weight removed. So, the process has reversed itself. The lands
of southern Canada are being pushed up again while those to
the south are sinking. It is a sort of giant geologic seesaw.

A greater sinking of the denser
ocean beds near continental margins
than that of the land areas would
produce a relative uplift of the
shore; also the sinking wedge would
exert horizontal, squeezing forces
on the continents.

SOLID SURFACES 55

Glaciers constitute one of the agencies that wear away the land.

Wasting of the Land

Another type of change of the earth's surface which is con-
tinually going on is erosion and the distribution of the eroded
materials to other areas. Erosion is the mighty chisel of nature
that is always operating to sculpture the land and to reduce it to
broken fragments. As soon as mountains are elevated they begin
to be worn down again. Forces are at work which can destroy
the hardest hills and reduce them to the level of the sea. Chief
among the agencies that wear away the land are winds, freezing
water, chemical action, glaciers, ocean waves, and running
water. The eroded materials are then transported and deposited,
usually to lower elevations, by such agencies as winds, running
water, waves, and glaciers.

Winds are an active force in weathering the surface of the
earth. Dust particles carried by winds are blown against exposed
bedrock and pebbles and boulders on the ground. Additional
particles are worn from the rock by an abrasive action. This
action is most pronounced in arid and desert regions, where
large quantities of dust and sand may be picked up by the
moving atmosphere. The effects of wind-abrasive action are
pronounced in the Mojave Desert, where much of the sand and
silt has been cut away by the wind. Many of the deep canyons
and large caves in the Great Basin of Arizona and Utah have
been partly formed by such sand-bearing winds.

Winds are active, also, in transporting eroded materials and
depositing them in other localities. In some of the dust storms
which occurred during recent summers, finely divided particles

56 THIS LIVING WORLD

of soil from the prairie states west of the Mississippi River were
carried as far as the eastern seacoast and out into the Atlantic
Ocean. This was small in amount, however, as compared to the
great volumes of dust deposited in areas closer to the dust-
storm centers. In many instances houses and barns were partially
or completely covered with the deposited dust. In desert
countries and along the shores of some lakes great quantities of
sand are carried by the winds and deposited as sand dunes.
These dunes usually are continually shifting. The sand dunes
along the south end of Lake Michigan, for example, cover many
square miles of territory in northern Indiana.

It has been the experience of many people to have water pipes
or automobile radiators cracked by the force of freezing water.
The fact that water expands when it freezes is generally known.
This principle operates to crack rocks in favored climates as
well as to burst water pipes. Water seeps into their pores and
crevices, and upon repeated freezing and thawing exerts forces
that are capable of disrupting the rocks. This process is usually
confined to relatively thin-surfaced layers of the rocks and
tends to break off small pebbles or fine grains. Eventually large
amounts of the hardest as well as softer rocks may be reduced to
minute particles in this manner. This is particularly noticeable
on steep slopes and in high mountains, where the loosened
material is removed by gravity and new surfaces are exposed so
that the process may continue.

The mechanical forces of running water, ocean waves, and
glaciers are also effective in wearing away the land. Moving
water and ice exert powerful forces against land areas, which
break away fine particles from the rocks or loosen the particles
of rock debris and soil. When fine sediments are carried by the
water, the erosion is accelerated by the scouring action of the
suspended particles. The incessant beating of waves along a
seacoast results in wearing away the shore. The water, which
usually has in it an abundant supply of sand and gravel, sweeps
back and forth along the coast and wears it down to a depth
somewhat below the general water level. As these forces cut
farther and farther into the land that has an original high eleva-
tion, the shore eventually becomes terminated as a sea cliff.
The waves act to cut a notch at the water level, and thus under-

SOLID SURFACES 57

mine the land above. This causes the overlying rocks to break
off and form relatively steep or ragged cliffs. Out beyond the sea
cliff the surface that has been cut away usually forms a gentle
slope seaward beneath the water, producing what is called a
wave-cut terrace.

However, a far greater erosion than that affected by all the
waves of all the oceans is produced by running water. The aver-
age annual rainfall over the land part of the earth is about
thirty-six inches. Even allowing for evaporation and seepage into
the ground, a considerable portion of this total rainfall flows off
the slopes, forming streams that eventually reach the seas.
This running water wears away an enormous amount of rock
debris and soil. It cuts the hardest rocks, particularly after
being collected into streams and rivers. Much of the country in
the Great Basin of Arizona and Utah has been eroded into
thousands of canyons primarily by running water, the most
spectacular of all being the Grand Canyon. The same type of
erosion is repeated throughout the world, although in most
places it may be less conspicuous.

The smaller streams and runoff water continually carry away
some sediments wherever rain falls. Soil surfaces are also loosened
and carried away. In many areas where farming has been
carelessly practiced or the climate is particularly arid, runoff
water reduces the land to a series of outwash gullies and wasted
fields. It has been estimated that the United States as a whole
is being lowered by the action of running water at the rate of one
foot in about seventy-five hundred years. At this rate it would be
entirely reduced to sea level in fifteen million years were there
no other forces operating in the earth's surface.

Simultaneously with the mechanical weathering of rocks by
the agents mentioned above, chemical agents act upon them
and reduce them to different materials. In some cases these new
products are soluble in the water passing through the rocks,
and they are thereby dissolved out and carried away. One of the
most active agents in chemical weathering is carbon dioxide.
When this gas is dissolved in water it forms a weak acid, known
as carbonic acid. Carbonic acid reacts readily with the mineral
salts of calcium and magnesium to form the soluble carbonates
of these elements. Carbonates thus formed as well as native

THIS LIVING WORLD

Royal Gorse near Canyon City, Colorado, has been eroded to a depth of 1,060 feet
by the Arkansas River. A transcontinental railroad passes through the canyon, and the
highest suspension bridge in the world crosses it (Photograph by Ewing Galloway.)

SOLID SURFACES 59

calcium carbonate, or limestone, are slowly dissolved by the
water percolating through rocks that contain these compounds.
This process accounts for the caves in limestone regions. The
Mammoth Cave in Kentucky, for example, is one of the well-
known caves which was formed in this manner. Some of its
caverns are several miles long, and one contains an underground
stream large enough for canoeing and boating.

Water produces chemical weathering of rocks in a manner
other than that of dissolving carbonates and similar soluble
constituents. That is, the water molecule becomes an essential
part of the molecule of the rock substance. The chemical com-
position of the rocks does not change; however, their molecules
increase in size by the addition of the water molecule. This
process is known as hydration. It produces a swelling of the
new molecules and thereby an expansion of the parts of the
rocks so affected. As hydration takes place irregularly in rocks
because of unequal evaporation, the resultant expansion will also
be irregular. Even the hardest rocks give way under the forces
of unequal expansion, and there will be a spalling off of slabs
and flakes until the rock material is reduced to fine particles.

Thus the various agents of weathering, acting through long
periods of time, bring about rock decay. Winds and running
water transport this debris from the high places and deliver it to
the sea. The "eternal hills" are not everlasting, but are eventu-
ally worn down. However, during the same time other great
forces are operating to elevate other land areas and to push up
large and small mountains. Changes are taking place everywhere,
in most cases slowly. The processes go on and on.

Soil Surfaces

The various changes in the earth's crust have produced a
thin layer of mantle rock, from which soils are developed. Soil,
water, and air constitute the earth resources necessary to plant
growth. It should be kept clearly in mind that all animals are
dependent either directly or indirectly upon plants for food.
The development of soils is a slow, gradual process. It results
from mineral substances and organic compounds, along with
microscopic living organisms, being brought together in proper
condition to support plant life.

60 THIS LIVING WORLD

The minerals are supplied by the soil mantle, while the
organic matter is derived from plant and animal remains. Or-
ganic remains when reacted upon by certain microorganisms
living in the soil are broken down into complex compounds that
go into solution, producing a substance known as humus. The
humus furnishes some of the food for plants, produces organic
acids also required by plants, and, in addition, gives the soil a
high capacity for dissolving water. Much of the water retained
in soils is held in these microscopic humus particles. They also
produce tiny air sacs and provide the soil with sufficient air to
support the microorganisms and to aid in plant growth.

Any prolonged condition that tends to upset this balance of
mineral substances, humus, and water in soil composition is
destined to produce an unfertile area for plant growth and
therefore a poor habitation for animal life.

There is a great variety of specialized soil types, the dis-
cussion of which is beyond the scope of this book. However,
there are four general types that cover great areas of the earth's
surface, and are worthy of a brief consideration here. One of
these is the acid soils of little or no humus found in the tropical
forests. Another is the low-humus soilo found in evergreen
forests in temperate or frigid climates. A third is the high-
humus soils of areas in the temperate zones, where there is
moderate rainfall, and the fourth is the gray desert soils.

In the great forest lands of the humid tropics, the soils
generally take on a decided character. Although they support
dense forests, the refuse vegetation collects mainly on the surface.
This material oxidizes rapidly under the influence of high tem-
perature and abundant microorganisms, which are stimulated
by the hot climate. As a result, little or no humus is formed. If
no humus is present, the water from the heavy rainfall, by
percolating through the soil, is exceedingly effective in dissolving
out certain critical mineral elements of soil fertility. The absence
of humus and a scarcity of these critical minerals make these
soils poor for ordinary field crops and many kinds of natural
vegetation.

In climates that have cold winters and only moderate sum-
mers, other forest areas are found. However, these forests usually
consist of evergreen trees such as pines, spruce, and the like.

SOLID SURFACES 61

These colder climates favor the accumulation of a thick layer
of raw humus material that has only partially decomposed be-
cause the lower temperatures retard bacterial action in reducing
organic material into mature humus. This spongy material
retains much water and becomes definitely acidic as a result of
the slow humus decay. These acid waters, by percolating into
the soil, render it acidic and many desirable soil microorganisms
are destroyed. Further, the acid waters dissolve out much of
the iron minerals and give the soil a whitish appearance. Such
soils are usually unfavorable to agriculture and the growth of
smaller vegetation.

In other large areas in the temperate climates somewhat
reduced rainfall has prevented the growth of dense forests. These
are what might be called the grass lands, as represented, for
example, by much of the central part of the United States.
Scattered trees and a dense and luxurious growth of grasses
predominate. A wealth of humus is formed, and mineral and
moisture content are usually sufficient to render the soil fertile
for agriculture and a great variety of food-bearing plants.

The other great division of soils we should take note of here
are the gray deserts. These are arid lands in relatively hot
climates. Only widely spaced desert grasses and scrubs grow
there. These soils are, therefore, light in organic matter. As a
result, such soils usually are scarce in nitrogen compounds. Lime
and other alkali minerals accumulate near the surface, giving the
soils a decidedly alkaline character. The desert sands contain
considerable amounts of undecomposed rock materials, as a rule.
All these conditions tend to prevent any luxuriant or sustaining
plant growth. Wide areas are subject to rapid erosion and they
support only a reduced amount of life of any kind.

Landscapes and Life

The different types of soils and the plants which they produce
have a decided effect upon the animal life which inhabits an
area. This is particularly noticeable when the areas are relatively
large and when no artificial conditions, such as irrigation, exist.
These interrelationships constitute what is usually referred to
as the biotic conditions existing within a given community. This
interdependence of plants and animals, and their absolute de-

62 THIS LIVING WORLD

pendence for existence upon soils and climatic elements, is one
of the fundamental conditions of life on earth.

These biotic relationships may be illustrated in a general way
by considering one small area of country that has been thor-
oughly studied. This area has within narrow geographical
boundaries a variety of soils and a considerable range of climatic
conditions. The area in question is Zion Canyon in southwestern
Utah. For much of this information we are indebted to the work
of Dr. Angus M. Woodbury of the University of Utah.

Zion Canyon is a picturesque gorge about twenty miles
long, a half mile or so wide, and about 3,000 feet deep. It has
been carved out of the Colorado Plateau by the Virgin River.
This river begins in the mountains just to the north of the canyon
at an altitude of about 10,000 feet and drops down rapidly
through the canyon onto the desert plain at the south end of the
canyon, where the altitude is approximately 3,500 feet. Within
this short course of the river there are forest-covered mountains,
where the climate is relatively humid and cool, the arid top of
the plateau, the nearly perpendicular bare rock walls of the
canyon, the cool humid floor of the canyon bottom, and the hot
desert at the canyon's mouth. Let us consider these briefly.

On the desert plain there is only scattered vegetation of scrub
plants such as mesquite, yucca, and cactus. Animals there are
limited to the small and hardy types that can endure the rigors
of desert climate and food supply. They are the smaller insects,
a few reptiles, a scattering of birds, and such small mammals as
the desert wood rat and ground squirrel. The insects feed upon
roots of the desert plants and microorganisms. The reptiles
subsist upon the insects and probably eggs of the birds. The
birds live upon the insects and upon the seeds and fruits that
are available. The ground squirrel feeds upon such vegetation
as it can find, while the rats probably consume a mixed diet of
any organic material that can be secured.

Farther up the canyon and along the rock walls are to be
found the beginning of soil development and its accompanying
life forms. This is on the bare rocks and fallen boulders. Where
there is a trickling of water over such rocks, algae and mosses
begin to grow. Soon they cover the rocks, and, when sufficient
humus has been formed by their decay, ferns get a start. These

SOLID SURFACES 63

are followed by seed-bearing plants, such as orchid and colum-
bine. Thus a plant community becomes established. Animal life
keeps pace with this development, often assisting the plants to
get a foothold. Spiders, including the black widow, are found
here, as well as other smaller insects. Certain insect-eating lizards
soon move in, to be followed by the canyon wren, hummingbird,
and a few other species. Bats inhabit the rock areas, and usually
at least one species of the field mouse will soon make its
appearance.

In the canyon bottom are to be found extensive biotic commu-
nities. Here the abundant moisture from the river and numerous
springs has produced luxuriant plant growth. This soon makes
a rich soil. The climate is generally mild, the area being pro-
tected from much of the intense sun's rays by the steep canyon
walls. In addition to a great variety of flowering and seeding
plants, small trees of oak, willow, and maple are to be found.
Such communities support many species of insects, a variety
of mollusks, fish in the river and ponds, several varieties of
snakes, numerous birds, and many kinds of mammals. Many of
the mammals are vegetarians, and some are carnivores. Rock
squirrels, chipmunks, porcupines, cottontails, and mule deer find
food in the roots, grasses, seeds, and berries. Such carnivorous
mammals as the ring-tail cats, skunk, and badger live regularly
in the canyon and depend upon the above-noted animals for
food. The mountain lion inhabits the cliffs or the mountain
slopes high above, but once or twice each week may descend
to the canyon bottom to kill a deer or some domestic animal for
food.

Above the canyon walls is the table land of the plateau. It
is subjected to a relatively dry and hot climate. The most
prominent vegetation features are the juniper and piiion pine
trees. Such smaller scrubs as sage, manzanita, and silverberry
are to be found along with scattered grasses and other seeding
plants. The largest native vegetarian mammals are mountain
sheep and the mule deer. They, along with smaller mammals,
furnish food for the mountain lion and bobcat which make their
homes in the canyon cliffs or near-by mountains.

Thus, it is seen, physiographic and climatic factors largely
determine the type? of plants which can develop within an area.

64 THIS LIVING WORLD

The dominant plants along with these same factors in turn con-
trol the animal life that can use these areas as natural habitats.

REFERENCES FOR MORE EXTENDED READING

SCHUCHERT, CHARLES, and CLARA M. LEVENE: <%The Earth and Its
Rhythms," D. Appleton-Century Company, Inc., New York, 1927.

This is a popularly written story of the rocky framework of the earth. Chapters IV
to XI inclusive give an account of factors producing change in the earth's surface,
such as running water, chemical action, glaciers, and winds. Chapters XVI to XIX
inclusive are a discussion of the action of earthquakes and volcanoes and the processes
of mountain making.

CRONEIS, CARY, and WILLIAM C. KRUMBEIN: "Down to Earth/' University
of Chicago Press, Chicago, 1936, Chaps. V to XXII inclusive.

The book is written in an informal and readable style and profusely illustrated
with drawings and many photographs. The chapters referred to include an account
of the changing features of the earth's surface, the nature of its rocky crust, and the
action of volcanoes.

HEIM, ARNOLD, and AUGUST GANSSER: "The Throne of the Gods," The
Macmillan Company, New York, 1939.

This is a thoroughly interesting arid authoritative account of an expedition con-
ducted by two geologists for the Swiss Scientific Society for the purpose of studying
the formations of the Central Himalaya Mountains. The book is illustrated with over
two hundred very remarkable photographs; also there is a relief map of The Himalaya.

LOBECK, A. K.: "Geomorphology, An Introduction to the Study of Land-
scapes," McGraw-Hill Book Company, Inc., New York, 1939, Chaps. II,
III.

These two chapters include an exhaustive discussion of rock structures and processes
of weathering. They are illustrated with analytical drawings and a number of re-
markable photographs. The illustrations are so excellent that the reader's interest
will not be confined to the two chapters referred to here, once the book has been
examined.

LONGWELL, C. R., A. KNOPF, and R. F. FLINT: "A Textbook of Geology,"
John Wiley & Sons, Inc., New York, 1939, Chaps. V, VIII, IX, XI, XII,
XIII, XIV, XVI, XVIII.

This is a well- written and extensively illustrated elementary text in geology. The
chapters referred to provide interesting and instructive material on the earth's surface
formations and the various natural processes which affect changes in them.

FINCH, V. C., and G. T. TREWARTHA: "Elements of Geography," McGraw-
Hill Book Company, Inc., New York, 1936, Chaps. XIV to XXI inclusive.

These chapters are a discussion of the processes of 'erosion and transportation of
earth materials and the formation of soil surfaces. The topics are fully treated in a
readable style and are well illustrated.

SOLID SURFACES 65

WOODBURY, ANGUS M.: "Biotic Relationships of Zion Canyon, Utah," Con-
tribution from the Museum of Vertebrate /oology, Ecological Monographs 3 :
1933, University of California, Berkeley, Calif.

A carefully document ated and well-illustrated account of the biotic relationships in
Zion Canyon with specific reference to succession of life in relation to soil and climatic
environment.

The National Geographic Magazine, published by National Geographic Society,
Washington, I). C.

A monthly magazine devoted to the increase and diffusion of geographic knowledge.
It contains articles, all of which are profusely illustrated, on a great variety of
explorations and studies of unique places and peoples.

Journal of Geology, published by University of Chicago Press, Chicago.

This journal of eight issues per year contains illustrated research articles on geo-
logical formations and other articles and news of interest to the geologist and inquiring
laymen.

3: LIFE'S DOMAIN

In Turbulent Oceans of Water and Air

A 5 THE earth pursues its long journey around the sun, it goes
wrapped in a flimsy garment of a gaseous atmosphere.
Transparent and mostly invisible this garment offers a protective
covering against extremes of heat and cold. Endowed with oxy-
gen, it provides the breath of life. Beneath this outer coat
three-fourths of the earth's surface is covered with a flexible
film of water. This film projects itself into the air above and
then down onto the land, providing there a considerable amount
of its liquid covering. Air and water are integral parts of the
earth's surface.

These conditions not only affect the physical characteristics
of the outer portions of the earth, but they also make it possible
for life to exist here. Life is dependent upon water for drinking

LIFE'S DOMAIN 67

and upon oxygen for breathing. All life must have some flexible
lubricant in which to move. Water serves as this medium. Even
for the higher forms of land life, including man, the same is
true, the water being carried around as a part of the body.
Oxygen is a substance necessary for most living forms on the
earth. Most plants and animals must have it for breathing.
Most of this oxygen is in the air. This air may be either in the
gaseous envelope above the earth or dissolved in water of the
oceans, lakes, and soil. Without air and water, the earth would
be dead and barren.

Furthermore, these oceans of water and air are constantly in
turbulent motion. Such motions result from the unequal heating
of the earth's crust and from the rotation of the earth on its
axis. They are greatly modified by the presence of oceans, plains,
and mountains. They are all mixed up by forces that are con-
tinuously at work and over which man has little control. Such
complex movements of air and water produce for us that much-
discussed phenomenon, the weather.

The average succession of all weather conditions over a
period of years constitutes climate. Climate, then, gives a more
general and complete picture of atmospheric and temperature
conditions for a location or country. It includes not only the
average of weather but also the extreme variations of weather
elements. Climatic conditions are more decisive factors than
weather in influencing the course of life as well as the physical
conditions of the earth. It is a well-known fact that the climate
of different sections of the earth has modified living conditions
and life during the present and past historical times. It is equally
well known that great changes in climatic condition during the
remote geologic ages have had a profound effect upon the
development of life on the earth.

Let us, therefore, give some brief consideration to these con-
ditions of water and air and their effects in producing weather
and climates.

Earth's Waters

The water constitutes a sort of envelope surrounding the
earth's solid crust. In the beginning of the earth's long history,
the atmosphere probably contained in the form of vapor all the

68 THIS LIVING WORLD

" . . . about seventy per cent of the earth's surface is covered by oceanic waters."

water now in lakes, oceans, and rivers. As water vapor began to
condense when the earth was cooling and forming its present
shape, the liquid collected, mostly in the low places of the
earth's surface. This formed the oceans. The first oceans were no
doubt mostly fresh water. We know with certainty that the salt
now in the sea has been washed into it from the dry land.

Slightly more than half of the water of the earth is now in
the oceans. When it is realized that about seventy per cent of the
earth's surface is covered by oceanic waters, this is not un-
expected. The average depth of the oceans has been estimated
to be about 13,000 feet, 'while some of the deepest points of the
ocean bed go down to 30,000 to 35,000 feet. The deepest point
yet recorded is off the east coast of the Philippines and is 35,433
feet below sea level. It has been possible to calculate the volume
of water in the ocean, and it is approximately 300,000,000 cubic
miles. This is far greater than the volume making up all the
continents and islands.

Water in lakes, inland seas, and rivers makes up a volume
somewhat less than half of this amount. Also, the amount of
ground water in the crust of the earth is enormous. This water is

LIFE'S DOMAIN 69

held below the surface of the ground in the soil and rocks in
depths ranging from a few inches to several thousand feet. It
furnishes the water for many growing plants, springs, wells,
mines, geysers, and an immeasurable volume which man cannot
tap. There is no way of measuring the total amount of such
underground water. However, it has been estimated from the
best data available to be about one-third the amount of the
oceanic waters.

These three sources — oceans, inland surface water, and
underground water — constitute somewhat less than the grand
total of the earth's waters. To them must be added the ice
sheets that cover much of the Arctic and Antarctic zones as
well as the present existing glaciers. This is more than most
people realize. Should all this ice melt, the ocean level would be
raised 150 feet, and New York City would be submerged beneath
the sea, as would most of the Atlantic Coast line and a part of
the Mississippi Valley.

Some water vapor is always present in the atmosphere. This
water vapor goes through an endless circulation of evaporation
from land and sea water, and precipitation again as rain and
snow. The annual precipitation of water from the atmosphere,
if averaged for all the earth, would be about thirty-six inches for
its entire surface. Of course, it varies enormously in different
areas. The extremes range from 0.02 inch per year for a spot in
Chile to 451 inches in Kanai, Hawaii. These figures are averages
based upon many years of observation. However, the great areas
of extreme dryness are- in the Sahara Desert and a great stretch
of Asia from the Caspian Sea to China. Large areas of excessive
rainfall are to be found in the Amazon Valley and in India,
considerable portions of which exceed 100 inches per year.

Ocean waters have one thing in common that is quite in
contrast to the air or land; this is that their temperature remains
much more constant than does the temperature of the air or the
dry land. Water absorbs relatively a large amount of heat. This
heat is radiated slowly. Therefore, large bodies of water contain
exceedingly great amounts of heat energy. As the ocean warms,
some water is evaporated into the air. The heat required to
evaporate a given quantity of water is over five times the
amount required to increase the temperature of the same

70 THIS LIVING WORLD

volume from freezing to boiling. Some of this heat for evapor-
ation comes from the water itself, thus tending to cool it. These
two transfers of heat operate to keep the oceans relatively con-
stant in temperature; also they make water a great storehouse
of heat energy from the sun.

There is, however, some variation of ocean temperatures.
The temperature of the surface of the ocean near the equator is
about 80 to 90°F. It gradually gets cooler as the polar regions are
approached. The Arctic and Antarctic oceans vary in temper-
ature from 28 to 50°F. The variation of daily temperature
changes is about 1°F., while the yearly variations at any one
point may be as high as 40 to 50°F. in oceanic waters.

The temperatures mentioned above are for the surface
waters of the oceans. At depths of about 1,500 to 2,000 feet the
ocean waters remain constant. At 1,200 feet the temperature is
50°F. throughout all the oceans, at 6,000 feet it is 36.5°, and at
9,000 feet it is 35.3°F. This is brought about because water is a
poor conductor of heat. At those depths little heat from the sun
penetrates, and what heat is there is retained by poor conduc-
tion and the lack of movements of the water. Ocean water
freezes at lower temperatures than fresh water because of the
salt dissolved in it. Even at the low temperature of the Arctic
Ocean the water beneath the ice sheet does not freeze, as normal
sea water freezes at a temperature slightly below 28°F.

Moving Oceans

There are many large movements of surface water in the
ocean, setting up ocean currents and drifts. One of these cur-
rents, for example, is the Gulf Stream. It flows northeast along
the east coast of the United States, then east across the Atlantic
to the west coast of Europe. The movements of ocean waters are
brought about by a number of different factors. Some of these
are inequalities of water level, unequal temperatures, variations
in barometric pressure, tides and constantly blowing winds.
Winds blowing constantly in one general direction drive the
water along with them. The actual path of the water will be
determined somewhat by the shape of the shore line. Thus, the
northeasterly trade winds and the shape of the eastern shore of
North America produce the Gulf Stream.

LIFE'S DOMAIN

Important oceanic currents in the North Atlantic.

Let us see how this takes place. Just north of the equator the
trade winds blow continuously from the northeast. This tends to
push the ocean water along in this direction. Thus water of the
Atlantic is piled up along the northeast coast of South and
Central America, where it is warmed. Of course, it cannot remain
piled up in any one place. It begins to flow north along the coast
line. The shape of the east coast of North America is such as to
give the water also an easterly direction as it flows north. This
warm water follows, therefore, the well-known path of the Gulf
Stream out into the northeast Atlantic and across to Europe.

Added to the Gulf Stream there is a general movement of the
ocean waters of the north Atlantic toward the northeast; this is
referred to as the North Atlantic Drift. Such movements of
ocean waters have a very definite and important effect on the
climate of many lands. For example, Scotland and Norway are
as far north as the Hudson Bay and frozen Labrador in North
America; yet they have a relatively mild climate as a result,
primarily, of warm, moist, winds blowing from the warm sea

THIS LIVING WORLD

High tide at a Hudson River wharf.

that comes to those shores. The prevailing westerly winds of the
North Temperate zone force other waters of the north Atlantic to
follow this same northeasterly direction. Therefore, warm cur-
rents continuously bathe the coasts of Europe. Here they are
deflected southwest and the circuit begins again.

This drift is slow. It is about four months before the warm
waters off South America reach the Straits of Florida. Some eight
to ten months later this same water, still retaining some of its
warmth, arrives in the vicinity of the European coast and
influences European weather. Thus, unusually strong winds, in
mid-Atlantic blowing away from England to the southwest might
produce unusually warm weather in England a year or so later.
Should such strong winds persist for several months, a period of
extreme coldness would follow this warm period in England as
colder waters from the north would be drawn down and less time
allowed for the warming of the Gulf Stream. However, no such
cycle has ever been observed. It is quite likely that the change of
temperature of the Atlantic waters would serve to check the
strong winds producing the cycle.

A striking example of a change of ocean currents producing a
change of weather of near-by lands is reported for the coast of
Peru from January to April, 1925. Ordinarily a current of cold
water from the south flows north along these shores. The cool
air blowing off the ocean current lessens the heat there, but
the air is warmed so much in passing over the land that its
relative humidity is reduced and no precipitation can take place.
Rainfall rarely occurs, and the country is mostly a desert land.
However, during the early months of 1925 the ocean current
changed for some unknown reason. A current of warm water

LIFE'S DOMAIN

Low tide at same wharf.

from the north set in and continued for about four months.
During this time warm, moist air from over the ocean current
blew inland, was cooled somewhat, and its relative humidity
thereby raised. Remarkable changes in the weather occurred.
Great floods spread destruction over the land. However, there
came a quick and luxuriant growth of grass, giving the half-
starved animals such a feast as they had never known before.
By the end of April the ocean current had returned to normal,
the rains ceased, and the country reverted again to its arid
type.

While we have dwelt upon the movements of ocean waters
of the Atlantic, it must be remembered that similar currents
exist in other great sea areas. The ocean current flowing along
the eastern coast of Asia, which makes a mild climate for Japan,
is produced in much the same way as the Gulf Stream.

One thing about oceanic water movements that frequently
puzzles people is the tides and their causes. The raising and
lowering of the water two times in about one day is a condition
common to all seashores. From a bird's-eye viewpoint these
are two great waves separated by about twelve hours that circle
the globe in approximately one day of twenty-four hours. Tides
are caused by the earth's daily rotation under the gravitational
attraction of moon and sun. The moon pulls up water on the side
of the earth toward it. That much is generally understood by
most people. However, a bulge or ridge of water also rises on
the side opposite from the moon. This is not so generally
understood.

The explanation of these conditions is that on the side near-
est the moon, the moon attracts the water more than it does the

74 THIS LIVING WORLD

solid ball of earth, thus tending to separate the water and
the earth. On the farthest side of the earth the moon shifts the
earth ball more than it does the water, causing the water to be
left a few feet behind. Again, there is a tendency for a separation
of earth and water. There are, therefore, two separations of
water and earth, or two high tides, that flow around the earth
as it rotates on its axis each day.

Tides make themselves felt in many rivers which empty into
oceans. They extend up the Hudson River to Albany, up the
Delaware River nearly to Trenton, and up the James River to
Richmond. In many harbors, especially where the water is
shallow, the rise and fall of tides is enough to have an important
effect on navigation. Vessels arriving at such harbors at low
tide are often compelled to wait for high tide before entering.
The current of tides through narrow openings to harbors some-
times is so strong as to interfere with navigation. The current
through Hell Gate near New York City is an illustration of this
point.

Atmosphere

There is an old saying to the effect that "seeing is believing."
Even though we have come to rely upon sight to such an extent
that we consider it the most important of all the senses, we do not
depend upon it exclusively. While we cannot see the atmosphere
under ordinary conditions, no one doubts its existence. Shake-
speare in his "Richard III" refers to it as "the empty, vast,
and wandering air." The atmosphere is one of the great and
important divisions of the earth's surface. The present atmos-
phere is composed of the gases which have not yet condensed as
the earth has cooled. If the earth cooled to a much lower temper-
ature than it now is, these gases would condense to a liquid.
In that instance the oceans might be filled with liquid air.
Water would then be in a solid glass-like rock form, or ice. It
is not unlikely that such conditions do exist on some of the
outer planets, which are much colder than the earth.

The air is a mixture of its several gases, rather than being a
gaseous compound. The largest per cent of the atmosphere is
nitrogen gas, the next largest per cent is oxygen, there being
about seventy-eight and twenty-one per cent of each, respec-

LIFE'S DOMAIN 75

lively. There is present about three hundredths of one per cent of
carbon dioxide, nearly one per cent of argon, and traces of other
rare elements. These are the constant elements of the air, and in
any discussion regarding the composition of the air they are
considered as composing it entirely. However, in addition to
these elements, the space occupied by the air contains other
materials that are somewhat variable, mainly dust particles and
a considerable amount of water vapor. These are finely divided
particles or molecules which are in the space of the atmosphere
around the molecules of oxygen, nitrogen, and other constant
gases. The beautiful color effects which are often observed in
the sky at sunset and sunrise are produced by the water vapor
and transparent dust particles refracting sunlight into its dif-
ferent colors.

The atmosphere at present is at least three hundred miles
thick. There is some evidence to indicate that traces of it extend
as high as 600 miles above sea level. However, it decreases rap-
idly in density as the height increases. At least half the weight
of the air is below the elevation of three and one-half miles. At
five miles above sea level the air is too thin to support human
life. Aviators and balloonists who ascend to these heights must
take an oxygen supply with them. Even larger mammals that
inhabit mountain slopes above three miles elevation must de-
scend below that level in order to breed and raise their young,
so thin is the atmosphere of the higher elevation.

We may think then of a sort of ground floor of the air. It is
the layer around the earth up to about seven miles, the height
varying considerably for different localities. Although not very
high, relatively speaking, this layer nevertheless contains most
of the oxygen. Here are the whirling storms, the cold currents,
the hot ones. Here gentle breezes may blow or hurricanes and
tornadoes devastate the earth. Here clouds form, producing
rain, thunder, and lightning. This ground layer is in a state of
turbulence. However, here it is that all higher forms of land life
exist.

Above this floor is a second layer, called the stratosphere. It
is a region or layer of the atmosphere with its lower surface
about six to eleven miles above the ground, the exact height
depending primarily upon the latitude. It is highest above the

76 THIS LIVING WORLD

equator and gradually sinks toward the earth in the direction
of the north and south poles. From this lower surface upward
to heights of twenty to thirty miles, the temperature remains
practically the same. Without any change in temperature there
is obviously no convection current in this layer, and it is in a
stable equilibrium. Here, then, is a region of the atmosphere
that is in marked contrast to the ground layer below. No violent
winds blow, and there are no storms. At most only general drifts
proceed in a given direction.

Temperatures in the stratosphere are very low compared to
those prevailing at the earth's surface. Above the equator the
temperature in the stratosphere layer is about 110° below zero
Fahrenheit. It gets somewhat warmer as the poles are ap-
proached, the temperature above the North Pole being about
46° below zero Fahrenheit. As would be expected at such high
altitudes, the atmospheric pressure is low. Recordings that have
been made by sending balloons into the stratosphere show that
the barometric pressure in the stratosphere is about one-thou-
sandth that at sea level. Should this region ever become the
highway for airplane travel, it will be necessary to carry oxygen
for crew and passengers and probably to get a new method for
propelling the plane.

Above the quiet, cool region of the stratosphere and extend-
ing up to about one hundred miles is a third layer of the atmos-
phere that has some marked differences from the stratosphere.
For one thing, it probably is much warmer than the stratosphere
or even warmer than the earth's surface. But this warmth would
seem unreal to us, as the air is exceedingly thin. For all practical
purposes it is as rarefied as a vacuum. However, some air does
exist there. It is in this region that the incoming meteors first
encounter enough resistance to heat them to incandescence,
producing for us the streamers of "shooting stars" across the
sky as the meteor is burned to dust. It includes what is called
the Heaviside layer of ionization, which reflects certain radio
waves back to the earth. However, it should be remembered
that we have little accurate information about conditions so
high up.

Even above one hundred miles traces of atmosphere exist.
These upper reaches contain sufficient ionized or electrified

LIFE'S DOMAIN

140

miles

Appleton

layer

Ionized
particles

— V

70 miles
Heaviside
Kennel ly
layer

Strato-
sphere
layer

\ i*~^^ v^'aL /

v \ r ^vv/i /

\ / rt * * ^"f

1 .-ss. ------ - - 50 miles

^ * ' *' = ~^r- *" ~^_ Night
W..*». _4-^ luminous

~~ 23 miles record
C_J -*- Sounding
1 ^ y balloon

5]/2 miles _
Mt.Everest

- 10 miles
Piccard

Layers of the atmosphere. (After Dorothy Fislc in "Exploring the Upper Atmosphere.")

particles of the air to provide for us those beautiful northern
electrical displays, the aurora borealis. Here, also, are other
ionized layers of the atmosphere, known as the Appleton Layer,
that reflect certain radio waves back to the earth and permit
our long-distance radio transmission around the earth. Such
traces of ionization must extend up to approximately six hun-
dred miles, based upon the data of these reflections. Beyond
this the atmosphere shades off into nothingness, and the story
from there out becomes astronomy.

Heat Blanket

The atmosphere acts to trap the heat from the sun and to
keep the earth warm. If there were no air, there would be
extreme daily variations in the temperature of the earth's sur-
face. The temperature at noonday would probably approach
that of boiling water; at night it would probably be far below
freezing. The dry land heats rather rapidly, particularly near

78 THIS LIVING WORLD

the surfaces, and loses its heat quickly. The full intensity of the
sun's rays would make the earth very hot during the day, if
there were no atmosphere. After sunset this heat would soon be
radiated and the temperature would drop to a very low point,
probably as cold as dry ice. Under such conditions life here
would be strenuous, if not altogether impossible.

The atmosphere acts as a blanket to regulate the heat of the
earth because of the way it transmits heat waves. During the
day some of the heat from the sun is absorbed by the atmosphere,
and thus the earth escapes extreme tempera tures. However, most
of the heat waves go through to warm the land and water. At
night this heat is radiated in the form of longer heat or infrared
rays. These longer heat waves do not pass through the air, being
absorbed principally by the carbon dioxide and water vapor pres-
ent. The air next to the ground thus is kept warm during the
night and keeps the land from cooling off rapidly. This warm
blanket of air acts exactly the same as the glass roof of a green-
house. It lets some of the sun's heat in but prevents longer heat
waves from being reradiated from the earth.

Movements of the Atmosphere

Winds are winds. Gentle breezes and violent tornadoes are
different intensities of the movement of the atmosphere. We
complain when no breezes blow and we suffer from the destruc-
tion wrought by tornadoes, yet breezes and tornadoes result
from the same general forces. So long as the physical conditions
of the earth remain what they are, each must occur to torment
men's souls and to manifest the changeableness of weather. The
catalogue of things that keep the winds blowing begins with
the sun's heat and includes the temperature and pressure of the
atmosphere, the irregular distribution of land and water over
the earth, the rotation of the earth on its axis, the presence of
mountains and valleys on the continents. It ends with winds,
just where we started at the beginning of the paragraph.

Great circulations of the atmosphere over the earth are set
up by unequal heating of the earth's surface at different places;
that is, the force that started winds blowing in the beginning and
has served to keep them blowing ever since is the sun's heat.
Anyone who has ever watched the flames leap up from a burning

LIFE'S DOMAIN

yt>-;' ! <y " WV-'

P- i'K -" '..' " ' t'+'iff'?'

*."'.1^ ' !',/'lh;;; rrVh!^i

^f/'Xv;;''''tV "''; 4^S

lornadoes are revolving movements of the air of small diameter and destructive vio-
lence. A funnel-shaped cloud extending downward from a heavy cloud mass may have
revolving winds of 300 miles per hour and an updraft of 200 miles per hour at the center.
(Science Service photograph.)

building, has seen on a small scale a condition similar to the
great circulations of the atmosphere. The heated gases of the
flames go up; they do not spread out over the ground. These
gases are lighter than the surrounding air, and they are thus
pushed upward by heavier air rushing in to equalize their pres-
sure and, incidentally, to fan this fire to greater burning.

When great bodies of air are heated they expand and become
lighter than cooler air. This difference in weight, while generally
imperceptible, is really enormous. Suppose we consider two

THIS LIVING WORLD

Flames leaping upward in lumber yard fire at New Orleans. (Photograph by Ewing

Galloway.)

cubic miles of air adjacent to each other. Each of them is a
layer of air one mile thick over one square mile of surface. It is
not so large a volume after all. If the two cubes differ in tem-
perature by 1°F., the colder cube weighs about 10,000 tons more
than the warmer. Should the difference in temperature be 20°F.,
their difference in weight is approximately 200,000 tons. With
such enormous forces unbalanced something must take place.
The cooler, heavier air flows in and pushes up the warmer,
lighter air until their weights are balanced. Should the incoming
air become likewise heated, it too is subject to the same upward
push by other layers of cooler air. If the heating continues, the
process goes on ad infinitum.

Strangely enough this does take place on certain areas of the
globe. Where would we expect to find such a place? It is, of
course, over the equator. In the equatorial belt the earth is
warmest, in fact continually warmest, and the air is heated most.
Here it rises, probably up to near the stratosphere, where it cools

LIFE'S DOMAIN 81

and overflows toward the poles. Therefore, the chief movements
of the air on each side of the equator are slow vertical drifts.
Since the vertical drifts are generally imperceptible, the area
constitutes a belt of calms, known as the "doldrums." The dol-
drums are characterized by only light winds and heavy rainfall.

The doldrums, then, are areas of heated air which have ex-
panded and become lighter. The cooler, heavier air from each
side the equatorial belt flows in to balance this reduced pressure.
This flow is near the surface and from the region of the temperate
zones to the north and to the south. North of the equatorial belt
the flow is toward the south. South of this belt it is toward the
north. These rather gentle and constant winds are called "trade
winds." The name has nothing to do with commerce, but is
derived from an older English meaning of the word signifying
a straight path.

Now, if the earth were all sea or flat lands and not spinning
on its axis, the trade winds would blow directly north and south,
and such breezes would probably be the only pronounced ones
we would know. Weather would be simple, but probably un-
comfortable and uninteresting. However, the earth is not all
seas or flat plains and it does rotate. Therefore even the large
general movements are complicated somewhat and smaller ones
much more, as we shall note presently.

^Ls the earth rotates on its axis, it carries the air along with
it. Every school child who has played "crack the whip" knows
that the speed in a circle is much greater near the circumference
or at "the end of the line" than it is at the center or the "pivot"
of the school game. The air, then, that is circulating around the
earth in the temperate zones is traveling with much less speed
than it is at the surface of the equator, since the circumference
of the earth is greater at the equator than nearer the poles. The
surface speed of the rotating earth at the equator is about one
thousand miles per hour while at Mexico City, for example, it is
about nine hundred miles per hour. At the north pole this speed
would be, of course, exactly zero or imperceptible.

The air moving into the region of the equator from the lati-
tude of Mexico City is traveling into faster territory. It has no
innate ability to increase its speed. In fact, like all material
bodies, it possesses inertia and tends to remain at its present

82 THIS LIVING WORLD

Great prevailing air movements of the earth's surface.

condition of motion. Consequently, it lags behind the faster
moving surface of sea and land of the equatorial regions. The
eastward motion of the earth is too great for it, and this lag
causes the air to drift to the west as it flows south in the Northern
Hemisphere and north in the Southern Hemisphere. Therefore,
the trade winds blow toward the southwest and the northwest
in the two hemispheres, rather than directly north and south.

Let us not forget the air that has been displaced above the
equator. It cannot remain perpetually aloft. These upper winds
spread out to the north and south above the trade winds. When
they have cooled arid contracted sufficiently, they sink back to
the surface. Just north of the north trade winds and south of
the south trade winds this sinking will be greatest, producing
two other belts of little horizontal movement, giving us two
other regions of calms, known as the "horse latitudes."

Some of the air from aloft spills over the horse-latitude calms
to the north and the south before reaching the surface. It should
be kept in mind that this atmosphere still retains its surface
speed of rotation approximating that at the equator. However,

LIFE'S DOMAIN 83

now it is in higher latitudes, where surface speeds are much
slower. When this air approaches land and sea level it is literally
running ahead of the surface beneath it. As it spreads out to the
north in the North Temperate zone, it also flows to the east,
since it is outdistancing the surface speed of the rotating earth
there. Such winds, then, blow rather constantly toward the
northeast. In the Southern Hemisphere the same condition holds,
and winds blow toward the southeast. These are called the
"prevailing westerlies."

Such are the great prevailing winds of the earth. These are,
however, in no sense the total atmospheric movements, particu-
larly in the North Temperate zone, where continents and
mountains and irregular seas and ocean currents operate to
upset this well-ordered scheme. The prevailing westerlies are
greatly altered by the unequal heating and cooling of land and
water. This air, being swept over high mountains into low-lying
plains, is churned in all sorts of fashions. Some of it comes in
contact with cold air from the polar regions, which also adds
to the turbulence.

Under the influence of the sun's heat the land is warmed
quickly during the day. It cools off some at night. However, the
water of the ocean heats much less quickly and cools off much
more slowly, as we have seen. This unequal heating of air above
land and sea causes unequal expansion of the air. It thus brings
about areas of high pressure adjacent to areas of low pressure.
These are in truth as well as name "centers of action." During
the winter months, for example, low temperatures and high
pressure tend to prevail over the land, while over the ocean
higher temperatures and low pressures prevail. This is particu-
larly true of the North Pacific along the coast of northern United
States and Canada. Unequal conditions are so marked there that
great seasonal circulations of air are set into motion.

Local movements of air thus get started. These movements
travel east under the general influence of the prevailing wester-
lies. The air may be deflected sharply upward over mountains,
where it cools quickly and precipitation begins. Should the
winds come in contact with a cold layer from the north, other
disturbances are produced. All is turmoil. Weather begins.

84 THIS LIVING WORLD

Weather

Until a few years ago people took the weather pretty much
for granted and did not worry a great deal about solving its com-
plexities. As Charles Dudley Warner (not Mark Twain as usually
stated) once said, "Everybody's talking about the weather, but
nobody's doing much about it." However, today it may be said
that we are doing a great deal about understanding the sources of
weather, making short-time, accurate predictions, and regulating
our activities accordingly, with great advantages to many people.

And what is weather? A rather comprehensive and complex
set of phenomena are all put together under this one word. It
includes temperature changes, rainfall, snow, sleet, humidity,
atmospheric pressure, wind direction and velocity, clouds, and
electrical disturbances. These things are all so variable that even
the Weather Bureau, with its technical staff and far-flung set of
observation posts, cannot fathom all their secrets. "As fickle as
the weather" is an axiom with significant meaning. However, the
weather experts have solved many of the problems relating to
atmospheric movements and the physical changes that accom-
pany them. We can decipher some of the physical factors in-
volved in producing weather changes and thus get some insight
into weather phenomena and the reasons for such complexity
and fickleness.

For example, rainfall is so commonplace that we never think
about it unless it upsets some of our best laid plans. However, a
complex series of events must take place before water is lifted
from the earth's surface and returns as rain.

Water vapor is removed by evaporation from the oceans and
inland surfaces containing water. The sun's heat is the energy
that converts water into a gas and brings about its escape into
the air. Heat energy when absorbed by the water sets the mole-
cules of the liquid into a more rapid state of vibration, and, hav-
ing acquired this additional energy of motion, they break away
from the surface. This process goes on continuously, and great
quantities of water vapor are raised to considerable heights above
sea level. It has been calculated that the weight of water vapor
necessary to produce rainfall of one inch over the entire state of
Georgia, for example, is about four billion tons. The tremendous

LIFE'S DOMAIN 85

energies of solar heat provide the only forces on earth capable of
doing such great amounts of work.

The source of most all water vapor is the ocean. After
being removed from the oceans by the expenditure of solar en-
ergy, water vapor is carried over the continents by winds and
diffusion methods. Moist land surfaces, vegetation, and inland
bodies of water contain much water and provide a significant
amount of evaporation. Plants give off more water vapor than
does dry ground, but not so much as a freely exposed water
surface. f

Water vapor gives us humidity, a condition that is the
despair of summer residents of New York City and many other
low-altitude sections of the country. The water vapor is always
present in variable amounts in the space not occupied by the
constant elements of the air. That is, the molecules of the con-
stant elements are not in actual and continued contact with
each other, and as a result there is considerable space between
them. It is this space that is occupied by the molecules of water
vapor. It should be kept in mind that any reference to water
vapor in the air means that the water vapor occupies the space
not occupied by the other molecules; it does not mean that the
water vapor molecules are actually a part of the molecules of
the constant elements of oxygen, nitrogen, carbon dioxide, argon,
etcv or that they are an integral part of the regular atmospheric
composition.

When people generally speak of or complain of humidity,
they are usually referring to conditions brought about by rela-
tive humidity. By relative humidity is meant the amount of
water vapor in the air at a given time compared to the amount
this same space could hold without condensation at the same
temperature; that is, the capacity of the atmospheric space for
water vapor depends very largely upon its temperature. As an
illustration, this space at 90°F. has a capacity for moisture
about fifteen times greater than it does at 20°F. The hot, dry
winds of the Mojave Desert may actually contain more water
vapor than the drizzly December atmosphere of a New England
coast, but the latter place always has a higher relative humidity.
The maximum water vapor capacity of air at 90°F. is about fif-
teen grains per cubic foot while at 20°F. it is about one grain per

86 THIS LIVING WORLD

cubic foot. Should the air over the Mojave contain one grain of
moisture per cubic foot, the relative humidity would be one-
fifteenth or about seven per cent. However, with the same
amount of moisture in air at 20°F. over the New England coast,
the relative humidity would be one hundred per cent and the air
would be saturated at that temperature.

The water vapor that is present in the air must condense and
form into raindrops before it will fall back to the earth as rain.
Condensation will occur only when saturation (or a relative
humidjty of 100 per cent) is reached at a given temperature.
It is obvious, therefore, that condensation depends upon the
amount of water vapor present and the temperature of the vapor
which is, of course, the same as the surrounding air. With a given
amount of moisture present in the air condensation occurs when
the temperature drops to the point just below that at which this
amount produces saturation. Thus, cooling of the air is the main
factor in producing condensation.

Most cases of cooling great masses of air are brought about by
expansion of the air in rising currents. When air rises, it expands
because there is less weight upon it than at the lower altitudes.
Rising currents may be produced by convection when air in one
area is heated more than that in an adjacent area. Air may be
forced upward also by the movements of winds over a mountain
range. No matter what the reason for the upward rush of air and
its consequent expansion, cooling inevitably results and con-
densation is likely to occur.

Should condensation take place, clouds are formed and float
above the surface. However, before the moisture can condense, it
must have something on which to condense. In the air this
medium is either ionized air particles or fine dust particles,
usually the latter. These solid particles cool quicker and to a
lower temperature than the water vapor, and thereby form small
nuclei around which water vapor condenses. Someone has said
that the heart of every raindrop is a dust particle. A continued
rising of these fine droplets produces more condensation, thus
adding to their size. Eventually they become large enough to be
visible, and clouds are formed.

In fair summer weather it is not uncommon for local areas to
become heated and give rise to ascending air currents' that pro-

LIFE'S DOMAIN

Lumulo-nimbus clouds result from updrafts of heated air. (Photograph by Gale Pickwell.)

duce clouds with flat bases and beautiful, towering, cauliflower
top§. These are called cumulus clouds. They are usually well
isolated in a blue sky, attesting that the rising air currents are
very much localized. Another type of cloud that produces
beautiful effects in the sky are similar fair-weather ones of
localized heating, known as cirrus clouds. They occur at alti-
tudes of from five to ten miles, where the temperatures are low
enough to freeze the condensed water. Thus they are composed
of minute ice crystals and they assume various forms, some-
times appearing like white curls, ringlets, or wisps of hair.
Clouds which are formed by a large-scale cooling of the air and
which usually extend from horizon to horizon constitute some
form of stratus clouds. Often they form a gray ceiling to the sky,
or they may be thick and dark masses from which rain or snow
falls.

Clouds will produce rain when the air continues to ascend
well above the condensation level, where further condensation

THIS LIVING WORLD

When the expansion of the upper atmosphere is such as suddenly to lower the tem-
perature below the freezing point of water there will be a simultaneous condensation and
freezing of the water vapor, producing snow. (Photograph by Ewing Galloway.)

takes place around the minute drops of water. When these drops
grow large enough to overcome the upward movement of the
ascending air, the force of gravity pulls them downward and
they eventually reach the earth. And so the rain falls.

Precipitation may occur in other forms than raindrops. The
moisture in the clouds may freeze as it condenses. If the expan-
sion in the upper atmosphere should suddenly produce temper-
atures below the freezing temperature of water, this will always
happen. Such simultaneous condensation and freezing produces

LIFE'S DOMAIN

Houses of Parliament and Westminster Abbey in a fog over London. (Life Magazine

photograph.)

snow. The delicate crystals of ice thus produced are of infinite
design and unsurpassed artistic beauty. No more remarkable
sight is in store for anyone than that of some flakes of new-
fallen snow under a microscope.

On the other hand should the raindrop freeze after condensa-
tion, it might fall to the earth as sleet. Such freezing may be
brought about even in summer if the raindrops in the clouds are
violently blown upward by rapidly rising air. Thus these rain-
drops cool more and may freeze. However, when this happens in
the summer, they will eventually fall back to the cloud, where
more condensation of water occurs around the frozen center.

90 THIS LIVING WORLD

Then without being able to fall to the ground, they are hurled
aloft again by the rising air, and the outer layer likewise is frozen.
Should this up-and-down condensation and freezing repeat itself
enough times the frozen sphere may build itself up to an inch
or even more in diameter. Eventually, of course, it becomes so
heavy that it breaks through the cloud and falls to the earth as
hail. Hailstones in August as large as hen eggs are not unheard of.

Condensation may occur near the surface of the earth rather
than at higher altitudes. In such an event fog is produced. Ideal
conditions for this type of cooling are a clear sky, no wind, and
a relatively long night. Radiation of heat from the earth cools
the surface rapidly, and the thin layer of air next to it has its
temperature reduced below the point necessary to produce a^
saturation of the moisture present. These conditions are also
aided by the presence of large quantities of dust particles com-
posed partly of organic materials, a condition that is common
around many large cities. These particles absorb water vapor
easily, and thus produce persistent fogs. These fogs are more
common in areas near seacoasts, where the moisture content
of the air is likely to be high. This accounts in part for the intense
fogs of London.

Thus expansion, cooling, freezing, and condensation of mois-
ture on microscopic solid nuclei furnish us with some of our
weather phenomena.

Cyclones

The large-scale condensation and precipitation that is com-
mon to much of the country in the North Temperate latitudes
takes place around storm centers. These are areas where great
volumes of air are lifted and consequently cooled. The uprush of
the air in such magnitude occurs in well-established regions of
low barometric pressure, where the air has been warmed some-
what and thereby becomes lighter because of expansion. These
areas are Hnown as "lows," and they constitute the centers of
great revolving cyclones that move across the United States and
many other countries of the north latitudes every few days from
west to east. They are so large in area and mild in wind velocity
that they rarely are considered as "cyclones" in the popular
connotation of that word. They produce much of the weather

LIFE'S DOMAIN

Simplified weather map of September 1 1, 1939, for the United States, showing lows and
highs, and their general path eastward across the continent.

changes common to the northern part of this country. However,
these cyclones are definitely not to be confused with tornadoes or
hurricanes.

These revolving storms range in diameter from about 100 to
2,000 miles, the average diameter in the TJnited States being
1,000 miles or more. Quite in common with the general tendency
of the air to whirl because of the effect of the earth's rotation,
the cyclones revolve in a counterclockwise direction. They usu-
ally form over the North Pacific and more majestically eastward
across the country. The transcontinental journey across the
United States requires from three to five days.

The path they follow is generally southeast to about the
center of the United States, then a turn to the northeast along
the south part of the St. Lawrence drainage and out into the
Atlantic. To be sure, they often take different routes but almost
always in an easterly direction. Frequently the cyclones dissolve
after getting far out to the sea. However, many of them cross the
Atlantic to Europe. Most of these, together with those which
begin in the North Atlantic, move northeastward across the
British Isles into Northern Europe and Russia. A few of them

92 THIS LIVING WORLD

have been followed entirely around the globe. They reflect the
workings of the earth's gigantic weather machine. A glance at the
weather map of the United States for any day will usually show
one or more lows or centers of such cyclones.

The cause of these cyclones is not definitely known. However,
the best explanation is that they are caused by a polar front. It
is well known that great masses of cold air accumulate in the
polar regions, and likewise masses of warm air accumulate in the
warmer zones. In areas of the prevailing westerlies, these masses
of warm and cold air are driven together. There is a well-marked
and distinct surface of separation between the two. This surface
is the polar front. Since the earth's atmosphere affecting weather
is several miles thick, this front is not a line but a surface, ranging
from an inclined plane to an almost vertical wall.

Across this front there is a marked change in the temperature
and humidity of the air. The front is a sort of battleground on
which there is a ceaseless pull and tug between these two moun-
tainous masses of air. The irregularities of air movement along
the polar-front set up eddies and swirls which initiate the de-
pressions in barometric pressure that form the lows of the
cyclones. When this occurs, weather gets started. There are
storms and cold spells, rains and droughts, winds and dust
storms.

The winds of a cyclone blow in certain definite directions
around a low. This is easily understood, if it is remembered that
the cyclones revolve in a counterclockwise direction around the
center, looking down on it ftf>m above. Therefore, east of the
center of such a storm the winds blow north. West of the center,
they blow south. This means that as a whirl approaches New
York City, for example, from the west or northwest, warm moist
air from the Southern Atlantic Ocean is drawn northward or
northwestward toward the city. Accordingly, as such a whirl
approaches, the weather is likely to be warm. Some condensa-
tion is likely to occur because of the slight cooling of this air,
producing cloudy skies.

As the center of the whirl passes, which it usually does at
some distance to the north of the city, the warm air from the
south, which has been drawn northward on the east side of the
whirl, is forced upward by cold, heavier air from the north,

LIFE'S DOMAIN 93

which is being carried southward on the west side of the whirl.
This updraft of the warm air brings about its expansion and
cooling. Condensation and precipitation of the water vapor is
likely to occur, producing rain or snow. The rains or snows,
therefore, usually come to New York City when the whirl is
north or northwest of the city, so that the city is in the southern
half of the whirl.

As soon as the center has passed entirely, so that New York
is in the western half of the whirl, cold northern air, with strong
winds, blows in from the northwest. Cold air drawn into a
warmer climate has its humidity reduced, and clear weather
results. That is why such cold, clear days usually follow promptly
rains or snows. The usual succession as a whirl approaches and
passes is (1) warm, moist weather; (2) cloudy, warm weather;
(3) rain or snow; (4) relatively sudden change to clear, colder
weather with strong northwest winds. This succession follows
about every four or five days.

About ninety per cent of all weather experienced in New
York City and in many other parts of the United States can be
interpreted on the basis of the passage of cyclonic whirls, as
explained above. The chief exceptions, which cannot be so inter-
preted, are continued cold spells in winter, caused by persistent
areas of high air pressure in Western Canada; and persistent hot
spells in summer, caused similarly by persistent areas of high
pressure over the South Atlantic Ocean.

Earth's Climates

A summation of the factors and conditions producing weather
gives a general picture of climate, since climate is the average
succession of weather conditions for a considerable period of
time. Climatic conditions, therefore, usually refer to large areas
or zones of the earth's surface; they involve long stretches of
time. Climate does have and has had throughout the past a mate-
rial effect upon life on the earth. Climates are referred to as warm
or cold; hot or dry. Temperature and precipitation are without
doubt important elements. In addition, such things as relative
humidity and winds are hardly less effective in determining
climate, and in affecting life conditions.

94 THIS LIVING WORLD

One classification of climates relates to climatic zones. Those
commonly recognized are the torrid or Tropical zone, which is
centered about the equator; the Temperate zones, which occupy
areas both north and south of the Tropical zone; and the frigid
or polar zones, which surround the poles. The boundaries of these
are designated in various ways. One that serves as well as any
is the system which uses lines of average temperatures as bound-
aries. Usually the temperature lines of 68°F. (one on each side
of the equator) are taken as the limits of the Tropical zone,
while the temperature lines of 50°F. for the warmest month are
respectively the northern boundary of the North Temperate
zone and the southern boundary of the South Temperate zone.
The polar zones extend from these two boundaries to the North
and South poles. The temperature lines around the earth are
somewhat irregular, since the presence of oceans and continents
has considerable effect upon average and seasonal temperatures.
The general result is that on the western boundaries of conti-
nents in the Northern Hemisphere the warmer temperatures
will extend farther to the north, while on the eastern boundaries
the opposite is true.

The leading characteristic of the tropical climate is a rela-
tively high temperature; uniformity of winds and high humidity
are also typical. So long as the winds blow over low lands they
are usually dry. Many lands in the path of such winds are desert
areas, the most notable ones being the Sahara and Australian
deserts. However, when such winds blow over mountains or
plateaus the moisture in them is precipitated by an uprising
of the air, producing heavy rainfall. The abundant rain on the
tableland of Brazil, on the east slope of the Andes Mountains,
and on the higher parts of the Hawaiian Islands is produced in
this manner.

In addition, the Tropical zone includes some areas where
strong monsoon winds blow. These are winds which are produced
by a greater heating of the land than of the bordering oceans and
which blow in an opposite direction to the prevailing trade winds.
Monsoons are the most active agent in bringing heavy rainfall to
India. In the Tropical zone near the equator the convection
currents of the doldrums give almost daily rains, such as are
common to the Amazon Valley and the central part of Africa.

LIFE'S DOMAIN

Rhythm in sand dunes in the sun-scorched desert basin of Death Valley, California, 210
feet below sea level. (Photograph by Ewing Galloway.)

Thus high temperatures and abundant rainfall or high tem-
peratures and extreme dryness predominate in tropical climates.
In some sections, then, there will be abundant vegetation and
dense forests, in other sections there will be the most pronounced
deserts on earth. The forest areas are conducive to the extensive
existence of varied animal life, particularly some larger forms of
mammals, reptiles, and amphibians. It is there that we find the
natural homes of many specialized types, as for example, the
great apes and monkeys of the primate group.

The range of temperatures is much greater in the North
Temperate zone than in the Tropical zone. The result is that the
summers may be relatively hot and the winters cold. Rainfall is
partly influenced by the prevailing westerlies. These winds
blowing from the oceans over the continents will have only a
moderate precipitation of rainfall. This does not hold true in
winter, when the land is colder than the oceans, nor on the west
side of mountain ranges bordering the west side of continents.

In North America the Sierra and Rocky Mountains produce
an upward movement of the prevailing westerlies on the western
slopes. Thus an expansion of the air with resultant cooling pro-

96 THIS LIVING WORLD

duces abundant rainfall on the slopes. To the east of the moun-
tains the drier air will be warmed again by descending to lower
altitudes, resulting in little rainfall. There we find the semidesert
areas of Utah and Arizona, and the arid regions of the Western
Great Plains. In Europe there are no great mountain ranges on
the western coasts. Consequently there are no great areas of
excessive rainfall or extreme dryness. The prevailing westerlies
produce a somewhat warmer climate in Europe than in inland
North America of the same latitudes, as has been previously
noted.

Life conditions in the temperate zones are more varied and
often rather extreme. An enormous variety of living creatures
finds its natural habitat there. These are mainly mammals, birds,
and insects that can live under changing climates. Creatures that
can move about easily, and often migrate long distances, flourish
best; at least this is true for the larger animals. Here, too, are
found both plants and animals that can subsist upon small
amounts of moisture during the dry seasons and those that can
withstand the rigors of relatively low temperatures.

In the polar zones the temperatures are uniformly colder
than in lower latitudes. Much of the surface is covered with snow
or ice-cold water. Precipitation is usually not heavy, and much
of it falls as snow. Only in certain areas, where local conditions
produce warmer temperatures and greater rainfall, are there
exceptions to this general condition. Life in these zones must be
adapted to this colder, drier climate. The important mammals
are the furbearing mammals. Reptiles are scarce, and native
birds have made special adaptations to these conditions.

Climatic conditions in the different zones do not change
rapidly. Only such minor variations as are produced by periods
of sun spots are clearly established. Within a generation of
human life or within a century, actual records show little climatic
fluctuation. However, over longer intervals of time, such changes
may be quite large. From about A.D. 600 to about 1100 the
climate of Arizona was apparently different from that at present.
There is much evidence to show that there was more abundant
rainfall and a lower average temperature. It was during those
centuries that the extensive Pueblo Indian civilization de-
veloped there. Furthermore, there was a more luxuriant vegeta-

LIFE'S DOMAIN 97

tion and a more extensive animal life than at present. This is also
the case in Southwestern Asia and Northern Africa. Within
early historical times these areas supported great civilizations
and a fairly abundant plant and animal existence. Now they are
arid countries, supporting only a meager distribution of life.

Climates in Geologic Times

Farther back in the earth's history there were great climatic
changes. These resulted from widespread changes in the earth's
surface, such as the elevation of mountain ranges or the covering
of large continental areas by inland seas; and probably from
some long-time fluctuations in the amount of heat received from
the sun.

There is conclusive evidence that the earth is now emerging
from the glacial climate of the Pleistocene epoch. About twenty
thousand years ago these glaciers receded from most of North
America, Europe, and Asia. Within the period beginning at
about this time and extending back about one million years, at
least four great glaciers crept down over a good part of the North
Temperate zone. At the maximum these ice sheets of several
hundred feet thickness covered most of North America as far
south as the Ohio River, Europe nearly to the Mediterranean
Sea, and Western Asia into the southern part of India.

The development of such great ice fields and the reduction of
temperature produced great areas in which little or no life could
exist. The forms which previously inhabited these lands and
seas had to migrate south or perish. Even in the southern cli-
mates the pace of life was quickened, and there was keen com-
petition for space and food. Only those forms best suited to the
more rigorous conditions survived. Thus reindeer, moose, wal-
ruses, and woolly mammoth were found as far south as South
America and Africa. During the interglacial times the climate
became much milder, and tropical conditions existed as far north
as Canada and into Northern Europe. At such times lions, saber-
toothed tigers, horses, elephants, and sea cows were distributed
over practically all the United States, for example.

During still earlier geologic times there were long periods of
uniformly mild climate. This was the condition that existed
during most of the Mesozoic era. Great inland seas covered

98 THIS LIVING WORLD

much of Central North America. Such mild climates and large
areas of swampy land favored the development of the large
reptiles. In the latter part of the Mesozoic era the great dinosaurs
inhabited most of the United States. Following this, the Rocky
Mountains began to be elevated, and the inland seas subsided.
These changed conditions brought about cooler and drier cli-
mates. It is unlikely that the dinosaurs could withstand such
changed conditions. At least they have all perished from the
earth, and in their place other animals have come into existence
and flourished.

There have been many other periods in the earth's history
when mountains were upheaved or oceans were pushed in over
large areas, each period bringing about marked changes in humid-
ity and temperatures. Such times were especially dangerous to
plants and animals that were highly adapted to exist under the
old conditions. Great groups of flora and fauna would be blotted
out. Some small and probably insignificant types would be best
suited to the new conditions; they, in turn, would develop into
the predominating forms.

For example, in Central Africa today, the fauna consists
primarily of animals that were common over the earth in the
tropical climates of the Pliocene epoch, which just preceded the
Pleistocene. These are the elephants, giraffes, lions, monkeys, and
the like. Should this country become cold or dry, or should a
glacier move down over it, these creatures could not possibly
continue to exist, even if man did not continue to destroy them
in wholesale fashion.

Thus, the climates of the ages have exerted a profound effect
upon life on the earth.

REFERENCES FOR MORE EXTENDED READING

FREE, E. E., and TRAVIS HOKE: "Weather," Robert M. McBride & Company,
New York, 1928.

The authors have discussed and illustrated in this book many of the practical,
dramatic, and spectacular facts about the weather. It includes lively written
answers to a great many questions which people generally yearn to know regarding
this much-discussed subject.

PICKWELL, GALE: "Weather," McGraw-Hill Book Company, Inc., New York,
1938.

LIFE'S DOMAIN 99

A popularized elementary discussion of weather phenomena, the chief merit of
which is a large number of remarkable photographs that are unusually well
reproduced.

SHAW, Sin NAPIER: "The Drama of Weather," Cambridge University Press,
London, rev. ed., 1939.

This is a somewhat dramatized account of weather changes which is aided con-
siderably by a number of well-chosen photographs and neatly made charts.

TANNEHILL, I. R. : "Hurricanes," Princeton University Press, Princeton, 1938.

This book, written in popular language, is an account of the nature and history of
tropical cyclones of the West Indies and the southern coasts of the United States.
Vivid descriptions are given of the great hurricanes of these regions. The book in-
cludes a large number of maps and some remarkable photographs of hurricane
destruction.

BLAIR, T. A.: "Weather Elements," Prentice-Hall, Inc., New York, 1937.

This book presents systematically and in some detail the science of meteorology
and the physical processes underlying observed weather phenomena. Written by the
senior meteorologist of the 11. S. Weather Bureau, it contains much information that
has been secured in studying the weather and used in making weather predictions.

FINCH, V. C., and G. T. TREWARTHA: "Elements of Geography," McGraw-
Hill Book Company, Inc., New York, 1938, Chaps. VIII, IX, X, XI, XII.

The chapters referred to include a readable and explicit discussion of the earth's
climates, their classification, characteristics, and weather changes.

KENDREW, W. G.: "The Climates of the Continents," the University Press,
Oxford, England, 1937.

This English text is an extended account of the climate conditions of the different
continents of the earth. It includes a wealth of material regarding weather, rainfall,
temperature changes, winds, and other meteorological conditions. Many maps and
tables of data of interest to aviation and to students of climatic phenomena are
recorded.

4: LIVING CHEMICALS

The Nature and Physical Basis of Life

IN THE mythology of the ancient Norsemen the most glorious
of all cities was Asgard, the shining home of the gods. Only
one person within this city was given the secret of life. This was
Iduna, the lovely goddess of youth. When she smiled, it was al-
ways spring. When the triumph of the evil giants of Jotenheim
saddened her, icy winter set in. Iduna had a magic casque filled
with wonderful apples which the Fates had allowed her to pluck
from the Tree of Life. Whoever partook of these apples gained
immortal youth and loveliness.

Once upon a time Loki, the god of mischief, conspired to
steal Iduna and her life-giving apples away from Asgard. For
refusing to give any of her fruit to the giant, Iduna was impris-
oned in a cave. The gods and people of Asgard grew old and

100

LIVING CHEMICALS 101

gray and tired. At last, learning of Loki's perfidy, they forced
him to rescue Iduna and return her to them. Eating again of
her apples, they at once grew young and strong. Then the god-
dess leaned over the wall of Asgard and smiled with pity on the
cold white earth below. Soon the grass sprang up, the trees
turned green, the birds came back, and men, too, gave thanks
for her return.

The story of Iduna and her magic casque of apples, indeed
the whole fabric of Norse mythology, like that of the early
Egyptians, Greeks, or Romans, represents an attempt to ex-
plain the mysteries of the animate world. Seeing the raging
elements, the procession of the seasons, the awesome panorama
of nature with its cycle of life and death, the ancient peoples
tried to explain these phenomena in terms which were under-
standable to them. The riddle of existence has fascinated and
challenged mankind since the very dawn of thought. Still no
satisfactory answer to it has been found.

It is true that everyone of us can tell a living plant or animal
from a stone. Yet no one can say exactly what makes the one
alive, the other not. No simple solution to such a stupendous
problem as the exact nature of life should be expected. But,
while no one has succeeded wholly in formulating a definition of
life, biologists in the last 250 years have reached some definite
conclusions regarding the characteristics which set apart the
living from the inanimate world.

Characteristics of Living Things

When an animal or a plant is broken down into the chemical
elements which make up its substance, it is found that these are
the same elements that go to make up the composition of inani-
mate bodies. It is possible to go a step further without arriving
at a fundamental distinction between the living and nonliving.
A great many chemical compounds which occur in living matter
are also found in nonliving things. Many other compounds, al-
though not normally occurring except in the bodies of plants
and animals, can be made artificially in the chemical laboratory.
By mixing these compounds in a definite manner, or by further
combining some of them, it is possible to produce substances

102 THIS LIVING WORLD

that have some of the manifestations of living things. That
which we have produced, however, is but an imitation of life.

What is the difference between our mixtures and true living
matter, aside from an obvious one in their degree of complexity ?
The difference seems to lie in the organization of the latter, in
the way in which the various constituents of living substance
are combined and possibly also in the way in which they are
arranged in space. In recent years biochemists have come to
recognize that the intramolecular arrangements which a sub-
stance possesses in the stable form with which we are familiar
in the laboratory do not necessarily hold for the structure of the
same compound as it actually occurs in a living animal or plant.
The first characteristic, then, in which living things differ from
nonliving ones is in the organization of their substance.

We have spoken of living matter as though its composition
were fixed and unchanging. As a matter of fact, nothing could
be farther from the truth. The stuff which constitutes the
physical basis of life is continually changing. Like the flame of a
candle which retains its general form although its burning mole-
cules decompose and are replaced by others, living matter, while
maintaining its essential characteristics, is never the same from
one instant to the next. In every living organism chemical reac-
tions are continuously taking place. Some of them lead to the
breakdown of materials to provide energy for carrying on the
vital activities of the body; others result in the formation of
new body substance, either for growth or for repair.

The sum total of all these chemical reactions is termed
metabolism. The ability to carry them on constitutes a second
fundamental characteristic which serves to distinguish living
things from inanimate ones. Metabolism is the very essence of
life. It implies an orderly set of reactions and interactions which
at death become uncontrolled, some of them slowing down or
ceasing while others speed up to the point of destruction.

Growth, or increase in size, is one of the most obvious fea-
tures of living things. We can all remember a time when we
were much smaller, some perhaps more vividly than others.
Some of us have seen a rather indefinitely shaped bundle of
fluff develop into a fine big police dog. Others have watched a
spindly seedling grow to be a tall shade tree. But increase in

LIVING CHEMICALS 103

size is not alone a property of living plants and animals. A tiny
crystal of copper sulphate dropped into a saturated solution of
the same substance will give rise to a great, branching " copper
sulphate tree" in the course of a relatively few hours. In this
case, the new structure is always built up by the addition of
molecules of copper sulphate withdrawn from solution.

In a living organism, however, growth takes place from
within. In the process many different types of raw materials are
converted into the specific stuff which makes up the body of the
individual. This growth of a plant or an animal is orderly. As it
grows, moreover, each living thing preserves not only its own
individuality but also the characteristic shape and structure of
others of its kind. It takes materials of various chemical com-
positions and converts them into the substances of its own body.
This ability to form new specific substances from nonspecific
materials distinguishes the growth of living organisms from
that of nonliving systems.

Perhaps one of the most remarkable features of living things
is the power of adaptation. In general, adaptation is the modifica-
tion of a part or the whole of an animal or plant to meet the
needs of special living conditions or to perform a special function.
The wing of a bird is a modification, which is an adaptation for
flying, of the forelimb of other four-legged types of animals. A
different type of modification for the same purpose is seen in
the wing of a bat. The brain of man is an example of another
adaptation. Remarkably free from other structural modifica-
tions, except those necessitated by the assumption of an erect
posture, man has developed in his brain the most remarkable
adaptation known. The power of reason confers on human
beings the greatest survival value among living things, for the
ability to think gives mankind a greater capacity to adjust
himself to changing conditions than any other form of life
possesses. Nonliving systems do not exhibit any characteristics
comparable to the power of adaptation.

Another characteristic of living things is illustrated by an
experience which probably every person has had at some time
during his life. If someone aims a blow at our eyes we instinc-
tively blink and duck our heads. Our subsequent actions depend
upon many complex factors, among them, perhaps, the size of

104

THIS LIVING WORLD

3 4

Series of photographs illustrating responses of amoeba to various environmental
changes. (1) In the absence of stimuli, for example, in darkness and cold, the amoeba
rounds up into a ball. (2) In rapid movement a single large pseudopod is formed and the
amoeba flows into it. The large dark nucleus is clearly visible, also the smaller, clear con-
tractile vacuole. (3) Among slime-covered algae the amoeba throws out numerous short,
blunt pseudopodia to support it in its slippery surroundings. (4) In clear, moving water,
amoeba forms many long thin pseudopodia which help to buoy it up. (Photomicrograph
by Roy Allen.)

our opponent or our own aggressiveness. In any case, our be-
havior exemplifies the power of living creatures to respond to
stimuli. This power is termed irritability. It is manifested by
even the simplest kinds of plants and animals, but not by
inanimate bodies.

Any change in the environment of an organism may serve
as a stimulus for some kind of response. Thus, a sudden drop

LIVING CHEMICALS 105

in temperature soon brings on shivering if we are not properly
clothed to withstand it. After strenuous exercise we perspire.
Under different types of conditions the same stimulus is capable
of eliciting quite different responses; that is, the nature of the
response is subject to intrinsic control. Our reaction to the
odor of cooking is apt to be different when we are hungry from
when we are not. It is in this respect that the response of a
living organism to a stimulus differs from the response of an
inanimate object to the application of a force. A billiard ball
has no choice of motion when hit with a cue. It always moves in
a direction and with a speed which are determined by the direc-
tion and angle of incidence of the cuehead and the force with
which it is applied.

We have reserved until the last the discussion of what is
perhaps the most striking characteristic which serves to dis-
tinguish living things from nonliving ones. The ability to re-
produce their kind is exhibited by all forms of life but in general
is not manifested by inanimate objects. The so-called filtrable
viruses and the bacteriophage are borderline cases which possess
the power of reproducing themselves, although not yet properly
referable to the class of living matter. The essential feature of
the reproductive process is the faithfulness with which every
structure of the parent is duplicated in the offspring down to the
most minute details. Horses can give rise to nothing except
other horses, and an acorn will produce nothing but an oak.

The Stream of Life

The title of the current discussion embodies one of the most
fundamental concepts of modern biological thought, namely,
that life is continuous and has been so since its origin. The con-
cept is summed up in the celebrated aphorism "Omne vivum ex
vivo." In its simplest terms it states that living things are always
descended from other living things through the exercise of the
fundamental process of reproduction.

This concept has not always enjoyed the scientific reputation
or widespread acceptance which it does at present. No less em-
inent an authority than Aristotle, writing in the fourth century
B.C., described the spontaneous origin of insects from the dew
which falls on the leaves on warm spring and summer evenings.

106

THIS LIVING WORLD

" . . . maggots developed only in the jar which was left uncovered. ..."

This idea that some forms of life are constantly generated in
some such mysterious fashion proved scientifically acceptable
until the latter half of the eighteenth century. It persists even
today in the folklore of uneducated people, who believe that
horsehairs falling in a barrel of rain water are transformed into
worms or that maggots arise directly from decaying meat.

That the latter is not true was shown by Francesco Redi, an
Italian physician, in 1768. Redi, in one of his experiments, al-
lowed meat to decay in a series of jars placed near an open
window. One of the jars was left uncovered, a second was cov-
ered with gauze, while a third was sealed with parchment. He
found that maggots developed only in the jar which was left
uncovered, although flies laid their eggs on the gauze covering
the second jar. He concluded quite correctly, therefore, that the
maggots came from eggs laid by the flies on the meat to which
they had free access. Since no eggs were laid on the parchment,
which is impervious even to the odor of meat, he also concluded
that flies when about to lay their eggs are attracted by the odor
of decaying meat.

The theory of spontaneous generation received its deathblow
about the middle of the nineteenth century, when Louis Pasteur
demonstrated that even bacteria do not arise de novo. Pasteur
showed that beef broth can be indefinitely prevented from un-
dergoing putrefaction by the simple expedient of boiling it in a

LIVING CHEMICALS 107

vessel which can be sealed off from direct contact with the air.
Under these conditions no bacteria appear in the broth, although
the latter still is capable of supporting bacterial growth on ex-
posure to air which contains bacteria or their spores. If this had
been Pasteur's sole contribution to scientific knowledge, he
would still rank today as one of the greatest benefactors of
mankind, for this one discovery has made possible the cultiva-
tion of single species of bacteria in pure strains and so opened the
door to recognition of the causes, prevention, and cure of infec-
tious diseases.

The Physical Basis of Life

In considering the remarkable manifestations of living organ-
isms which distinguish them from inanimate matter, we are led
to wonder of what sort of stuff living things are made. What
enables living plants and animals to carry on the activities which
are so essential to their existence? The answer to this question
has already been mentioned in part. Living things differ from
nonliving in their organization; that is, in the manner in which
their chemical constituents are combined and possibly also in
the way in which these constituents are arranged in space. Only
the physical substances which show this complex type of or-
ganization possess the peculiar properties of living things.

The essential part of every living organism, from the smallest
bacterium to the largest whale, is a unique substance, called
"protoplasm." It occurs nowhere in the inanimate world, a fact
first recognized during the eighteenth century by the German
botanist, Hugo von Mohl. He not only recognized the signifi-
cance of this substance arid gave it its name but also began a
systematic study of its complexities that has continued until the
present. Somewhat later, Thomas Henry Huxley, the great
English naturalist, defined protoplasm as the "physical basis of
life." In its chemical composition and physical properties it sur-
passes in complexity any other substance or mixture of sub-
stances known to man.

The Chemical Composition of Protoplasm

Of the ninety-two known chemical elements thirteen are
almost invariably present in protoplasm. They may be divided
roughly into four groups in the order of their abundance. The

108 THIS LIVING WORLD

first group, comprising about ninety-nine per cent of the living
substance, consists of carbon, hydrogen, oxygen, and nitrogen,
occurring in the order named. In the second group are potas-
sium, phosphorus, sulphur, and chlorine, making up nine-tenths
of the remaining one per cent. The third group comprises sodium,
calcium, and magnesium. Iron, iodine, and fluorine make up the
last group. Occasionally also there are traces of a few other
elements such as copper, vanadium, and silicon. With the excep-
tion of chlorine, the elements comprising the first two groups are
essential to all organisms. Curiously, sodium and chlorine ap-
parently are not essential for plants, while calcium is not essen-
tial to certain lower animals. The proportions of these different
chemical elements in the human body are aptly set forth in a
short article from Reflector entitled, "What Are Little Girls
Made Of?"

Chlorine, enough to sanitate five swimming pools; oxygen, enough to fill
1,400 cubic feet; thirty teaspoons of salt, enough to season twenty-five chickens;
ten gallons of water; five pounds of lime, enough to whitewash a chicken coop;
thirty-one pounds of carbon; glycerin, enough for the bursting charge of a
heavy navy shell; enough gluten to make five pounds of glue; magnesium,
enough for ten flashlight photos; fat, enough for ten bars of soap; enough iron
to make a sixpenny nail; sulphur enough to rid a dog of fleas; and only one-
fourth of a pound of sugar.

The chemical elements which we have listed as components
of protoplasm do not exist here in elementary form. They occur
partly in combination with one another as chemical compounds
and partly as electrically charged particles, called " ions," formed
by the action of water in dissolving them. Thus, most of the
molecules of salt (sodium chloride, NaCl) appear as positive
sodium ions, Na~*~, and negative chlorine ions, Cl~. Many of the
processes peculiar to living matter are possible only because of
the existence in it of ions.

Water is the most abundant single chemical compound occur-
ring in protoplasm, comprising from seventy to ninety per cent
of its substance by weight. Water is not itself ionized to any great
extent although, as we have seen, it is responsible for the ioniza-
tion of compounds dissolved in it. It is not only the most abun-
dant constituent of living matter; it is essential to life. In the

LIVING CHEMICALS 109

absence of water we cannot even imagine any kind of life re-
motely resembling what we see on earth.

The remaining compounds of protoplasm are the organic
compounds, so called because they occur naturally only in living
organisms, although many of them have been synthesized
artificially in the chemical laboratory. These substances all
contain the element carbon and usually also hydrogen and
oxygen. On the basis of their structure and elementary composi-
tion they are divisible into four classes: carbohydrates, proteins,
fats, and lipoids.

The Carbohydrates

These are the simplest organic constituents of protoplasm,
being composed of carbon, hydrogen, and oxygen alone, the
latter two in the proportion to form water. An example is cane
sugar or table sugar, Ci2H22Oii, the molecules of which are
formed by the union of two molecules of a simpler sugar, glucose,
C6Hi2O6 with the elimination of a molecule of water. Glucose is
the simple sugar formed by green plants from carbon dioxide and
water, using the energy of sunlight. The structure of the glucose
molecule is represented by the chemical formula presented
below.

i O 1

H OH H OH

.!•• i I

H-C-C-C-C-C-C-H
OH H H OH H OH

The arrangement of the hydroxyl (OH) groups and hydrogen (H)
atoms on opposite sides of the carbon atoms (C) in glucose con-
fers upon its molecules in solution the peculiar property of
rotating the plane of polarized light rays. This is so characteristic
of carbohydrates as they occur in living matter that it has been
regarded as a key to the understanding of life processes. Glucose
rotates the plane of polarized light to the right. Other naturally
occurring sugars rotate polarized light rays to the left.

Carbohydrates are particularly important as foods, since
they constitute the most readily available energy source and
since they are stored in the body to a considerable extent for

110 THIS LIVING WORLD

this purpose. The process by which the energy of the carbo-
hydrate molecule is released is a straightforward combustion,
similar to the burning of wood. In fact, wood, or its principal
constituent, cellulose, is a carbohydrate. Another typical exam-
ple of this group is starch, (C6HioO&)n, the chief storage substance
of plants. The starch molecule is formed by the combination of
an indefinite number (n) of simple-sugar molecules with the
elimination of a molecule of water between each uniting pair.
The starches are very complex substances, sometimes composed
of as many as 200 simple-sugar molecules.

The Fats and Lipoids

The fats and lipoids are oily and waxy substances which are
insoluble in water and occur in protoplasm either as granules or
globules, depending upon whether they are solid or liquid at the
temperature of the organism. Like the carbohydrates, they are
made up of carbon, hydrogen, and oxygen, but the lipoids may
contain sulphur, phosphorus, and nitrogen in addition. Unlike
the carbohydrates, the proportion of oxygen in both fats and
lipoids is less than that required to form water. As a result of
this fact, the combustion of fat molecules yields a greater quan-
tity of heat and chemical energy than does that of carbohydrates.
The fats are therefore important as energy sources as well as
being one of the chief storage bodies of animals.

The true fats are formed by the union of molecules of simpler
substances, called fatty acids, with glycerin (C3H8O3). One of
the simplest fatty acids is acetic acid (C2H4O2), found in vinegar.
These two substances have these structural formulas:

H OH

H-C-OH HO-C-C-H

H-C-OH H

1 acetic acid

H-C-OH

glycerin

One of the simplest possible true fats would be glycerin triace-
tate, which has the structure:

LIVING CHEMICALS 111

H OH

i ii i

H- C- O- C- C- H

H
O H

H- C- O- C- C- H

O H

H i

H- C- O- C- C- H

i i

H H

The molecule of glycerin triacetate is formed by the union of
three molecules of acetic acid with a single molecule of glycerin,
as indicated above, with the elimination of three molecules of
water. It will be noted that the union takes place by the sharing
of a common valence bond between the acid

HO- C-

carbon atom of each acetic-acid molecule and one of the three
oxygen atoms in the glycerin molecule. As a matter of fact, the
simplest naturally occurring fats are usually of mixed composi-
tion with respect to their fatty acid particles and contain fatty
acids of much higher molecular weight and complexity than
acetic acid. The structural formula of glycerin triacetate is given
to illustrate the way in^hich fats are formed.

The lipoids are of the utmost importance as constituents of
semipermeable membranes in living organisms. They are also
apparently concerned in many types of activities which take
place in living protoplasm. Evidence of this is seen in the close
correlation between the distribution of lipoids in the various
organs of the body and the degree of activity displayed by the
latter. Thus, the brain, which has the greatest variety and
extent of function, has the highest lipoid content. The liver ex-
hibits the next highest percentage of lipoid, then the pancreas,
kidney, and lung in order.

The Proteins

The presence of protein has been stated to be "the most
characteristic chemical property of a living system." Certainly,

112 THIS LIVING WORLD

next to water, proteins are the most abundant components of
protoplasm, where they occur either as microscopic granules or
in colloidal solution and make up about fifteen per cent by
weight of its bulk. They are the most complex organic sub-
stances known. In addition to the carbon, hydrogen, and oxygen
of carbohydrates, all proteins contain nitrogen and sulphur,
many contain phosphorus, and a few contain iron or other
elements. Certain of the properties peculiar to living matter are
attributable to the relatively tremendous size of protein mole-
cules. The simplest known proteins have a relative molecular
weight of about 17,000 as compared to hydrogen, with an atomic
weight of about 1.0, or water, with a molecular weight of ap-
proximately 18.0. These complex molecules are built up of many
simpler molecules of amino acids- simple substances containing
carbon, hydrogen, oxygen, nitrogen, and occasionally also
sulphur. There are twenty-one amino acids that are important
as components of protoplasm, of which the simplest is glycine
(C2H5O2N), having the structural formula:

H O

i H

N- CH2- C- OH
H

Two molecules of glycine may be combined by splitting
out a molecule of water between them in the following manner:

H
N-
H

0
M

CH2- C-

H20 *

T H 0

0-H N-CH2-C-

i
H

When this is done the result shown below is obtained, the two
molecules being linked together at the point indicated by the
broken-line box.

H !0 H; O

N- CH2- JC- N|- CH2- C- OH
i ' ' -'

The resulting compound is an example of a simple dipeptide, the
first step in building up a complex protein molecule. The portion
of the structural formula enclosed in the broken-line box repre-

LIVING CHEMICALS 113

sents the so-called "peptide linkage." This type of linkage makes
possible the formation of exceedingly complex molecules from
relatively simple substances.

^ A single protein molecule may contain as many as fifty
amino-acid groups and over 50,000 individual atoms. Even as
the number of combinations of the twenty-six letters of the
alphabet to form words seems inexhaustible, so the number of
different kinds of proteins made possible through different com-
binations and arrangements of the twenty -one common amino
acids is infinite. It is this diversity of chemical composition of
the protein molecule which makes possible organic diversity
itself, that is, the great variety of the forms of life. It is well
known that the protoplasms of no two kinds of living things are
exactly alike with respect to their protein constituents, a condi-
tion which has been found to be one of the most important bases
of the body's defense against disease. There seems to be at least
one specific protein corresponding to each kind of living creature.

The Enzymes

A special class of proteins of great importance to living things
are certain enzymes, some of which have recently been isolated
in pure crystalline form. One of the most striking characteristics
of living systems from the point of view of the chemist is the
great variety of reactions which take place in them at ordinary
temperatures and pressures and which cannot be duplicated in
the laboratory under these conditions. This remarkable chemical
activity of protoplasm is made possible by the enzymes occurring
in it and produced by it, some of which are now known to be
complex protein substances. Enzymes are organic catalysts. It
will be recalled that catalysts are substances which possess the
peculiar ability to alter the speed of a chemical reaction without
themselves being used up in the process and without changing
the point at which the reaction stops.

An example is amylase, the enzyme of the human pancreatic
juice which greatly accelerates the conversion of starch to the
simple sugar known as maltose :

amylase „

(CeHioOsJn » •=

starch ^ malto

114 THIS LIVING WORLD

In the equation, n signifies an unknown number of starch units
(C6HioO5), and w/2 represents one half of this number. A very
small quantity of pancreatic amylase is capable of bringing
about the rapid conversion of many times its own weight of
starch to maltose. Moreover, the same small quantity of amylase
may be used over and over again in changing successive portions
of starch to sugar without losing its power.

Enzymes differ from inorganic catalysts in several important
respects. One of these is that enzymes are generally specific in
their action; they catalyze one and only one definite kind of
reaction. Thus, pancreatic amylase accelerates the change from
starch to sugar and only that change. This specificity is so rigid
that enzymes are identified and named according to the material
upon which they act. Moreover, there are a large number of dif-
ferent enzymes, each capable of catalyzing only one type of
chemical reaction, even in a single kind of living organism. An-
other feature in which enzymes differ from inorganic catalysts
is that the enzymes are destroyed by heating above relatively
moderate temperatures (140 to 176°F.). Inorganic catalysts,
such as finely divided platinum, are not affected in this manner
by heating.

The Borderland to Living Things

In the preceding paragraphs the chemical composition of
protoplasm has been discussed in a manner to bring out the
peculiar tendency of living matter to build up ever more com-
plex substances. Reference has been made to the fact that proto-
plasm itself is the most complex of all substances, combining in
its organization molecules of carbohydrates, fats, lipoids, and
proteins. The enzymes have been described as a special group of
proteins tremendously important in the economy of life. Bridging
the gap which formerly was thought to exist between living and
nonliving things is another group of protein substances. These
are the filtrable viruses, so called because they will pass through
the pores of the finest porcelain filters and because they are asso-
ciated with diseases such as yellow fever, smallpox, infantile
paralysis, influenza, and the mosaic diseases of plants.

The viruses are so small that they are beneath the limits of
visibility of the microscope. It is their small size that accounts

LIVING CHEMICALS

115

The picture at the left is of an epithelial cell from a person with a sub-acute throat
infection. The dark spots are accumulated virus bodies. The picture at the right shows
a tissue culture from a patient with measles. In this disease the virus bodies often collect in
a crescent formation within the cells as shown by the dark area. (Photomicrographs by
Dr. J. Broadhurst, Columbia University.)

for their ability to pass through filters which will hold back
bacteria. The virus of smallpox is made up of particles estimated
to have a diameter of 0.00017 millimeter, whereas microscopi-
cally visible typhoid bacteria are approximately 0.002 milli-
meter across. This is about the limit of the resolving power of
the microscope, and typhoid bacilli are among the smallest bac-
teria known. Yet, Vaccinia virus of smallpox is nearly a thou-
sand times smaller.

Because of the extremely small size% of their particles, ob-
viously not much can be determined about the structure of the
filtrable viruses at present. They are chiefly known by their
physical and chemical properties, which place them among the
largest known protein molecules. They exhibit many of the
properties of enzymes, but unlike these they possess the ability
to reproduce themselves, although the process by which they
do so is believed to be more closely analogous to the growth of
crystals than to the multiplication of living organisms. The so-
called "mosaic disease," important in the curing of Havana to-
bacco leaf for cigars, is caused by a filtrable virus. Recently, Dr.
W. M. Stanley of the Rockefeller Institute for Medical Research
in Princeton, New Jersey, has succeeded in reducing to a dry
crystalline form several grams of pure tobacco mosaic-disease
virus, a treatment no living substance has ever been able to
survive. The crystals can be weighed, analyzed, and redissolved

116 THIS LIVING WORLD

in the same way as other crystals, such as those of sugar and
salt. Nevertheless, on being injected into the leaves of a living
tobacco plant, they will resume their activities and reproduce
themselves with astonishing rapidity. They are in a sense
chemicals at the threshold of life, standing on the border line
between the living and nonliving. No one has yet succeeded,
however, in cultivating a filtrable virus in the absence of living
matter.

There is another group of substances, intermediate between
living and nonliving things, which closely resembles the filtrable
viruses in certain respects although differing markedly from them
in others. These are the bacteriophages, so named from the fact
that they possess the ability to destroy and feed upon bacteria.
Like the filtrable viruses, the bacteriophages are submicroscopic
in size and are capable of self-propagation. They likewise re-
semble enzymes in their physical and chemical properties and
are placed among the larger protein molecules. On the other
hand, unlike the filtrable viruses, bacteriophages have not been
obtained in pure crystalline form. They are so small that they
are known only from their action upon the bacteria on which
they feed. They cannot be grown in the absence of bacteria but
appear to be able to survive for considerable periods of time in
mediums from which bacteria have disappeared.

The Physical Properties of Protoplasm

By examining a bit of protoplasm under a high-power micro-
scope and by manipulating very fine glass needles in it, some-
thing niay be learned of what this living substance is like. Under
high magnification it is a grayish transparent material not un-
like the uncooked white of an egg in appearance, except that it
frequently is full of tiny bubbles or granules which give it a
foamy structure. When these particles are carefully watched,
they are seen to be constantly moving back and forth, around
and around, without evident purpose or direction. This vibration
is called Brownian movement after its discoverer, Robert
Browne. Brownian movement is not peculiar to protoplasm but
occurs wherever fine particles are suspended in a liquid. This
movement is due to the bombardment of the particles by the
molecules of the liquid. It is evidence of the fact that molecules

LIVING CHEMICALS 117

An ovum cell of a starfish shows the granular nature of protoplasm. Within the cell the
nucleus is clearly visible, and within it is the smaller nucleolus. (Photomicrograph by G. C.
Grand, New York University.)

possess mass and motion sufficient to impart a visible force to the
suspended particles. It is also evidence of the fluid consistency of
protoplasm. Further evidence of this latter fact is obtained by
carefully pushing a fine glass needle about in a bit of protoplasm.
Most of it is quite liquid, although more viscous than water;
however, certain portions are more solid, like a soft jelly. On
withdrawing the needle the stuff is found to stick to it, stretch-
ing out, and, on finally breaking loose, snapping back. Thus,
protoplasm resembles unhardened glue in that it is sticky, elas-
tic, and has a rather high degree of tensile strength.

If the bit of protoplasm is surrounded by a watery medium,
it will be found to behave like a drop of oil in that it does not
mix with the water but preserves a sharp boundary between
itself and its environment. When this boundary is investigated
with the glass needle it is found to be a true membrane which
may be punctured, stretched, and otherwise distorted, showing
that it has thickness, elasticity, tensile strength, and a semisolid
consistency. Thus, protoplasm always surrounds its tiny units
with a true containing membrane.

The Colloidal State

As a result of observations and experiments similar to those
just described, scientists have concluded that protoplasm is an

118 THIS LIVING WORLD

extremely complex colloidal emulsion. What is a colloidal emul-
sion? The word colloid is derived from the Greek, meaning
"glue-like." It was first used over ninety years ago by the Eng-
lish chemist Thomas Graham to refer to peculiar and at that
time little understood substances such as egg white. A colloid is
now known to be a mixture of very fine particles of one sub-
stance suspended against the force of gravity in another sub-
stance, neither one of them dissolving in the other. In such a
system the suspended particles are referred to as the dispersed
phase, while the substance in which they are suspended is called
the continuous phase.

In an emulsion, both the dispersed phase and the continuous
phase are liquids. The mayonnaise used as salad dressing is an
example of a simple emulsion in which the dispersed phase is
fine droplets of a solution of vinegar in water, while the con-
tinuous medium is oil. In other types of colloids the suspended
particles may be solid and the continuous medium gaseous, as
is the case of smoke in air. India ink is a colloidal mixture of
solid carbon particles in water. These are examples of the sim-
plest kind of colloidal system, in which there are but two phases.

In more complicated systems there may be more than one
kind of particle suspended in the continuous phase, and the dis-
persed particles may themselves be colloids. Such a colloidal
system is said to have many phases or to be poly phasic, and
protoplasm is the most complex polyphasic colloid known.

An extremely important property of colloids is their ability
to undergo reversal of phase; that is, the suspended particles
may coalesce to form a continuous phase, in which the former
continuous phase becomes broken up and in turn suspended. An
excellent example is the change which takes place when cream is
churned to make butter. Cream is an emulsion of oil and fat par-
ticles in a watery solution. During the churning process the sus-
pended fatty and oily particles coalesce into the continuous
phase, while the water solution breaks up into fine droplets
which become suspended. In this way butter is produced. Since
fats and oils are much thicker and more viscous than a watery
solution, and since the continuous phase is the more important
in determining the properties of a colloid, butter is more like a
solid or c/el, while cream is more like a liquid or sol.

LIVING CHEMICALS

119

Milk is an emulsion of fat and oil droplets in a watery solution. In the left photograph
the fat and oil droplets are shown as bright spheres, while the watery solution forms the
gray background. In making butter, when the milk is churned and the phases are reversed,
the watery phase breaks up into droplets, which appear in the right photograph as light
spheres scattered in a darker background, representing the fatty material which has
coalesced to form the dispersed phase. (Photomicrograph by Roy Allen.)

Many biologically important colloids can change rather
freely back and forth between a sol and a gel state. Indeed, a
very large number of the peculiar properties of protoplasm may
be traced to its colloidal organization and to this ability to
change from the gel to the sol state and back again.

The Energy of Life

In order to understand the phenomenon of life it is necessary
to think in terms of energy as well as matter. The business of
living does not depend upon some mysterious force ; in the strict-
est physical sense it involves work and energy, the capacity for
doing work. In the first section of this chapter metabolism was
pointed out as one of the distinctive characteristics of living
organisms in comparison with inanimate objects. The chemical
reactions referred to collectively as metabolism constitute a
unique system of energy changes associated with life.

Physical and chemical processes involving inanimate mat-
ter tend to take place in such a way that energy is dissipated
throughout space. Thus, water always tends to run downhill,
and moving bodies tend to come to rest or to lose their kinetic
energy by encountering other bodies. A hot body tends to lose

120

THIS LIVING WORLD

"Physical and chemical processes involving inanimate matter tend to take place in
:h a way that energy is dissipated. . . . water always tends to run downhill." (Photo-
graph by Ewing Galloway.)

heat by radiation or conduction, and ordinary chemical reac-
tions yield energy to their surroundings. Living organisms, how-
ever, possess the singular ability of storing energy in particular
masses of matter to a greater or lesser extent.

The principal differences among living things relate to the
degree in which they have developed this ability to store energy.
With certain exceptions, the plant kingdom comprises a group
of organisms which are able to store large quantities of energy
in chemical form, making use of common inorganic substances
from the soil and from the air as raw materials. The animal
kingdom, on the other hand, is made up of organisms which are
directly or indirectly dependent upon plants for their energy
sources.

The great majority of plants utilize inorganic substances as
energy sources, either directly or in the synthesis of other energy
sources, without the aid of other living things. Among these, the
forms with which we are most familiar are the green plants.

LIVING CHEMICALS 121

Such plants are green because of the presence in their proto-
plasfri of a mixture of substances known as "chlorophylls."
These are among the most important substances in nature.
The chlorophylls comprise a system having the not altogether
unique property of converting light energy into chemical energy.
The chemical energy available in this process is utilized by green
plants in the manufacture of carbohydrates, in which form the
energy is stored. The first step in this process is the formation
of a simple six-carbon sugar from water and carbon dioxide.
In chemical language the process is represented by the following
equation:

6CO2 + 6H2O + light energy « C6H12O6 + 6O2

A synthetic reaction is one in which two or more simpler
substances are combined to form a more complex one. The term
"photosynthesis" is applied to the process in green plants be-
cause the energy utilized is received as light. The carbon dioxide
is taken from the air, while the water is taken up from the soil
through the roots of the plant. This photosynthetic process is
the very foundation of life. It is the principal process whereby
organic substances are built up from inorganic compounds.
Without it, life as we know it on the surface of the earth would
be impossible.

Among the plant-like types of organisms, certain bacteria
are able to obtain the energy needed for carrying on their vital
activities from the oxidation of simple inorganic compounds
found in the air and in the soil. Among these are the nitrogen-
fixing bacteria, which utilize the energy released in the conver-
sion of ammonia and the ammonium of ammonia compounds
to nitrites and in the conversion of nitrites to nitrates. Another
example are the sulphur bacteria, which obtain energy from the
oxidation of hydrogen sulphide to sulphur.

The process of oxidation mentioned in the preceding para-
graph is as fundamental in the economy of nature as is photo-
synthesis. In order that the energy stored in photosynthesis
may be useful to living things, providing them with the energy
necessary to life, it must be released. The process of oxidation
provides the mechanism for releasing stored chemical energy
and transferring it. Oxidation is defined as the loss of electrons.

122 THIS LIVING WORLD

It is always accompanied by reduction, which involves the gain
of electrons. Oxidation really involves the transfer of electrical
energy, in the form of negative electric charges, from one atom,
which is thus oxidized, to another, which is thus reduced. It is
not a case of creating energy, since energy can neither be created
nor destroyed under the conditions existing on the earth.

The processes of oxidation and reduction may be illustrated
with a simple example such as the conversion of ammonia to
nitrous acid. In chemical language the reaction is written as
follows :

2NH3 -f 3O2 = 2HNO2 + 2H2O

For the oxidation of nitrogen:

N — = N++f + 6 electrons

For the reduction of oxygen:

3O -f 6 electrons = 3O —

The nitrogen atom and three hydrogen atoms are held to-
gether in the compound ammonia by the transfer of one electron
from each hydrogen atom to the nitrogen atom. Therefore, the
nitrogen atom has three extra negative charges, and may be writ-
ten N . When ammonia is oxidized to nitrous acid (HNO2),
the nitrogen not only gives up its three extra electrons, but also
releases three additional ones. Having lost these three additional
electrons, the nitrogen atom becomes charged positively, and
may be written N+~H~. Oxygen is the element which gains the
electrons released by the nitrogen. It so happens that the oxy-
gen atom can take on only two additional electrons. When it
gains two electrons, it may be written O . In order to take up
all six of the electrons released by the nitrogen atom, three
oxygen atoms must be used.

The reactions which are chiefly utilized by living organisms
as energy sources ultimately involve the oxidation of carbon and
hydrogen and the reduction of molecular oxygen. As a result,
the principal waste products of metabolism are carbon dioxide
(CO2) and water (H^O). The utilization of molecular oxygen
and the production of water and carbon dioxide by living plants
and animals is called "respiration." When the material which is

LIVING CHEMICALS

Light energy

Leaf system

123

Conducting system
for water and food

Water and
inorganic salts

Green plants use the energy of sunlight in manufacturing carbohydrates from atmos-
pheric carbon dioxide taken in through the leaves, and soil water taken up through the
roots.

oxidized is a simple sugar, the process is just the reverse of photo-
synthesis, as indicated below:

C6H12O6 + 6O2 = 6CO2 + 6H2O + energy

Photosynthesis and respiration are carried on simultaneously
by green plants in the light. The rate of photosynthesis consider-
ably exceeds that of respiration. As a result, in the daytime a
plant will use up more carbon dioxide from the air than it puts
back in, and will return more oxygen than it removes. At night,
or in darkness, photosynthesis ceases, but respiration con-
tinues. Consequently, at night the metabolism of a plant is
like that of an animal. Oxygen is used up from the atmosphere
and carbon dioxide is given off to it. Animals do not possess
chlorophyll and consequently are unable to carry on photosyn-
thesis. Respiration of both plants and animals is closely analo-
gous to the burning of fuels such as wood, coal, or oil. This was
first demonstrated by a French chemist, Lavoisier, in 1774.
Lavoisier's discovery, more than any other, has been responsible
for the development of the modern interpretation of vital phe-
nomena in purely physical and chemical terms.

124 THIS LIVING WORLD

Respiration and photosynthesis are examples of two differ-
ent and opposing kinds of metabolic activity. Photosynthesis is
a constructive process which results in the storage of energy in
forms which are useful to living organisms. The sum total of
metabolic activities by which living organisms store energy is
called "anabolism." The other phase of metabolism, termed
"catabolism," is necessary for the release of the stored energy
and is typified by respiration. Life depends upon a nice balance
between these opposing tendencies, for catabolism is a destruc-
tive process which leads to the wasting away and death of the
individual if not properly compensated by anabolic activities.
This is what happens when a person starves. When food is not
taken in, the body draws upon its own substance to provide the
energy needed for carrying on those fundamental processes which
are essential to the maintenance of life. The starving individual
not only grows thinner, but actually shrinks, since water plays
an important part in many of the chemical reactions upon which
the body depends for its energy. Catabolism predominates also
during the old age of living organisms. During youth, on the
other hand, a preponderance of anabolism finds its expression in
growth and high vitality.

The Interdependency of Living Things

With the exception of the independent types of plants which
we have discussed, all living organisms are dependent in some
degree upon other living organisms. Even some plants depend
upon others for their energy sources. Thus, the yeasts require
sugar, which, as we have seen, is manufactured by green plants.
The putrefactive and fermentative bacteria utilize dead organic
material of both animal and vegetable origin. These are exam-
ples of plants which live on materials produced by other living
things. Among the most interesting of all plants are the car-
nivorous ones, such as the pitcher plant and Venus's-flytrap,
which feed upon insects.

We have already indicated the dependency of animals upon
plants. This is rather obvious in the case of a large group of
animals, called "herbivores/' which feed exclusively upon vege-
table matter. It is not so apparent for the "carnivores," or
flesh-eating animals, yet it is only necessary in the case of these

LIVING CHEMICALS 125

meat-eaters to trace the food chain back a little further and
ultimately some inconspicuous plant feeder will be found. The
voracious shark feeds upon other fishes which in turn feed upon
smaller ones, until finally we come down to the forms which eat
plants, sometimes microscopic plant life. The great agricultural
industry attests man's dependency upon other living things.
The linking of animal and vegetable life is closer here, more-
over, than in the case of the carnivores, for man is an "omni-
vore" — his diet includes both plants and other animals, chiefly
strict herbivores.

Perhaps no better way could be found to illustrate this inter-
dependency of living things than to trace the paths of an indi-
vidual carbon atom and an individual nitrogen atom in the
energy cycle at the earth's surface. Let us begin with an atom of
carbon forming part of a molecule of atmospheric carbon diox-
ide. Eventually our carbon atom will encounter a green plant
and will enter into the process of photosynthesis, becoming part
of a molecule of simple sugar. The sugar molecule may be
broken down to provide energy for other processes in the plant,
or it may be used in the synthesis of starch or new protoplasm.
In the former instance our carbon atom is returned to the at-
mosphere as carbon dioxide. Although the pathway may not be
so direct in the case of sugar molecules used in the synthesis of
starch and protoplasm, we shall see that the net result is the
same. When the plant dies, decay usually sets in through the
activities of putrefying bacteria, with the result that the carbon
atom may become incorporated in the protoplasm of the bac-
teria. Ultimately, however, it is returned to the atmosphere in
•the same form as it was removed, namely, as gaseous carbon
dioxide.

If, on the other hand, the plant is eaten by an animal, the
carbon atom may participate in many interesting and varied
reactions. As part of a simple sugar molecule, it may be oxidized
to carbon dioxide at once to provide energy utilized by the ani-
mal in finding and devouring other plants; or, as part of a com-
plex glycogen molecule, it may be stored in the liver of the
animal. It may be utilized in the formation of new protoplasm
in the body of the animal. If the animal is eaten by still another
animal, the cycle may be repeated with some variations. Even-

126

THIS LIVING WORLD

rC02 in atmospher

Bacteria and other
organisms of decay

Respiration

Photosynthesis

Respiration

Green plants
build up

fats and proteins

Assimilation

Animals use
carbohydrates,
fats and proteins

Death

Dead organisms'1

Living organisms are characterized chemically by the predominance of carbon com-
pounds in their makeup. Perhaps no better way could be found to illustrate the inter de-
pendency of living things than to trace the path of a carbon atom in the energy cycle at the
earth's surface.

tually, however, the carbon atom again reaches the atmosphere
as carbon dioxide, as is shown in the accompanying diagram.
Here the travels of the carbon atom are represented as occurring
in a closed cycle, although, as we have seen, the pathway is
quite likely to be much more devious and complicated.

Nearly four-fifths of the total volume of the earth's atmos-
phere is made up of nitrogen in elementary form. A part of this is
rendered available to living organisms through the activities of
the so-called "nitrogen-fixing'' bacteria, which are able to use
atmospheric nitrogen in the formation of complex organic com-
pounds. These organic compounds are decomposed by other
bacteria into ammonia and ammonium compounds, in which.
form the nitrogen is oxidized by nitrifying bacteria to form ni-
trates and nitrites. These are deposited in the soil. Green plants
in turn utilize the nitrates of the soil in synthesizing their proto-
plasm. Through the processes of decay the nitrogen of the
plant tissues is returned to the soil, and the cycle is repeated
endlessly. The soil nitrates and nitrites may also be broken down

Denitrifying
bacteria

LIVING CHEMICALS

Nitrogen in atmosphere

Symbiotic
nitrogen- fixing
bacteria

127

Leguminous Free- living,
plants nitrogervfixmg

bacteria

Animals and
non- green
plants

| Death

Death /

^ I /

^Organisms of decay ^
yield ammonia
in soil

The wanderings of an individual nitrogen atom, like those of the carbon atom, may be
represented by a closed cycle.

by denitrifying bacteria, their nitrogen being returned to the
atmosphere.

Among the nitrogen-fixing bacteria is an interesting group
that lives in small nodules or tubercles on the roots of certain
plants, chiefly the legumes, which include beans, peas, and
clover. These nitrogen-fixing bacteria render invaluable service
to their plant hosts in making available to them the nitrogen
which they require. In return they receive other materials from
the plant which they are unable to synthesize themselves. The
relationship is an example of a mutual benefit association between
living organisms, called "symbiosis." Soil which has been
depleted of its nitrogen through cultivation of other crops may
be replenished by growing leguminous plants upon it and plow-
ing them under, as has long been practiced in scientific farming.

If plants are eaten by animals, the nitrogen may be excreted
as ammonia, urea, or uric acid, or it may enter into the formation
of animal protoplasm. Eventually, however, it reaches the soil
or the atmosphere once more, either through excretion or on
death and decay. The wanderings of an individual nitrogen atom,

128 THIS LIVING WORLD

like those of the carbon atom, may be represented by a closed
cycle, as shown in the accompanying diagram, but with the same
reservations which were made in the case of the carbon atom.

REFERENCES FOR MORE EXTENDED READING

MASON, FRANCES: "The Great Design," The Macrnillan Company, New
York, 1934.

This book consists of a number of articles by distinguished English scientists. The
primary objectives seem to be to summarize the scientific facts in a number of fields
and in each case to indicate to what extent there is some great design of nature under-
lying the universe. The sections "The Earth as the Home of Man," "The Oneness and
Uniqueness of Life/* and "The Chemical Romance of the Green Leaf" should prove
interesting and not too difficult reading in connection with the present assignment.

KERMACK, W. O., and P. EGGLETON: "The Stuff We're Made Of," Longmans,
Green & Co., New York, 1938.

This book is admirably designed and clearly developed to interpret biochemistry to
the public. The authors explain how the complex molecules of living things are built
up from the chemical elements. Enzymes, hormones, and vitamins are entertainingly
discussed, and substances which fall in the borderland between the living and non-
living are adequately treated.

PLUNKETT, CHARLES R.: " Outlines of Modern Biology," Henry Holt & Com-
pany, New York, 1931, Part I: Protoplasm, Chaps. I, III, IV, VI.

An excellent general account on the elementary level, one of the best texts in the
field.

HOLMAN, RICHARD M., and WILFRED W. ROBBINS: "Elements of Botany,"
2d ed., John Wiley & Sons, Inc., New York, 1936, Chaps. II, III, IX.

A standard introductory account of the principal phenomena of plant life.

BAYLISS, W. M. : " An Introduction to General Physiology," 1st ed., Longmans,
Green, & Co., New York, 1919, Chap. VII.

This is practically a classic in the treatment of the physical basis of life.

BEUTNER, R. : "Life's Beginning on the Earth," Williams & Wilkins Company,
Baltimore, 1938.

The author presents herein his theory that life originated on the earth after a
gradual development of preparation processes necessary to life. He tells in plain
words how science today understands single life phenomena and the working mechan-
isms of life and presents scientific evidence which indicates that life originated
gradually through chemical and physical processes. *

CHILE, GEORGE: "The Phenomena of Life," W. W. Norton & Company, Inc.,
New York, 1936.

LIVING CHEMICALS 129

A physician and scientist sets forth in this book his interpretation of life phenomena;
this being mainly that the phenomena of life are due to radiant and electrical energy.
The discerning reader will find many points for thought in this extensive discussion.

CALKINS, GAKY N.: "The Smallest Living Things," The University Society,
New York, 1935.

A brief survey of microscopic forms of life written in essentially nontechnical
language. This little book contains only 116 pages, and it would pay everyone
interested in microbiology to read it at least once.

SEIFRIZ, WILLIAM: "Protoplasm," 1st ed., McGraw-Hill Book Company,
Inc., New York, 1936.

A standard reference book for those interested in a detailed study of protoplasm,

The Scientific Monthly, published by The Science Press, Lancaster, Pa., for
the American Association for the Advancement of Science.

This is an illustrated monthly magazine containing a variety of articles of general
interest to scientists and inquiring laymen.

Journal of General Physiology, published by Rockefeller Institute for Medical
Research.

A bimonthly technical journal that is devoted to research articles relating to the
explanation of life phenomena on the basis of physical and chemical constitution of
living matter.

5: THE PATTERNS OF LIFE

Organization and Development of Living Things

IN THE "Rubaiyat" of Omar Khayyam the great Persian
scholar of the twelfth century compares the creator of the
heavens and earth to a potter who molded the hollow shape of a
man and then breathed into it in order to give it life. The poet
must have marveled at the skill of the fingers of his imaginary
potter, which fashioned the human form from a shapeless mass of
wet clay by bending and folding it, stretching it here and pinch-
ing it there. Even more marvelous to us today is the intricate
process of embryological development by which a human being
actually arises from a fertilized egg cell. In a flat, three-layered
mass of cells produced by successive divisions of the egg, the
precise sequence of bending, folding, stretching, and pinching
takes place as growth proceeds which a potter might follow in
molding a clay vessel or the model of a man.

But we are getting ahead of our story. In the preceding chap-
ter we learned something of the nature of living matter. Here we

130

THE PATTERNS OF LIFE 131

shall see how protoplasm is organized into units called cells, how
these units divide, and how they become highly differentiated
and associated to form the bodies of complex animals and plants.
In the growth and development of every creature on earth, the
process begins with a single cell. By cell division, cell differentia-
tion, and cell specialization a new individual is formed. This
differentiation and this specialization of cells produces the pat-
terns of organs and body systems. A given specific set of patterns
brings about the production of one specific kind of adult creature,
such, for example, as a starfish. A different set of patterns pro-
duces a different type animal, say an elephant. Still other pat-
terns would produce, other life forms, such as mice, cats, fish,
roses, or cedars.

Let us trace here the history of the individual from the time
of conception through the various stages of growth. We shall see
that the individual's development is a brief and sketchy history
of the evolution of his ancestors. This is not surprising when we
realize that the problems of the race and those of the individual
are really identical. Some two thousand million years were re-
quired for the development of present life forms from the first
protoplasm, and into this struggle have gone the efforts of count-
less individuals to survive and to adapt themselves to the chang-
ing conditions of the earth's surface.

Cellular Organization

Viewed from a distance, a modern skyscraper appears to be
carved from a single large mass of stone. Closer inspection
reveals, however, that it is made up of many kinds of smaller
units, including stone blocks, brick, steel girders, rivets and nails,
plate glass, sections of pipe, and various other materials. Just so,
minute examination of a large animal or plant shows that its
body is not a single large mass of protoplasm. One of the out-
standing characteristics of living matter, and one which sets it
apart from nonliving colloids, is that it is organized into tiny,
individual, and self-perpetuating units called cells. A typical cell
is a bit of protoplasm surrounded by a thin partition wall, the
cell membrane, and differentiated into nucleus and cytoplasm.
The nucleus is a centrally located, characteristic organ provided

132 THIS LIVING WORLD

with a membrane of its own, while the cytoplasm is the rest of the
cell contents.

Upon this type of organization depends the orderly and con-
trolled sequence of activities which take place in protoplasm and
which are essential to life. Indeed, the cell is the unit of life. An
entire group of animals, the protozoa, comprising about 15,000
kinds of organisms, and a similar group of plants are unicellular;
that is, their bodies consist of a single cell. The bodies of multi-
cellular plants and animals are made up of from a few hundreds
to many billions of cells forming a coordinated whole. The single-
celled plants and animals are capable of an independent existence.
Within each cell there are all the conditions necessary to life. On
the other hand, the cells of the larger and more complex plants
and animals have usually become so specialized and dependent
that they cannot continue to live when separated from the parent
community or body.

It is only where some special environment is provided that
the cells comprising the body of a multicellular organism are
capable of a limited degree of independent existence. If a small
fragment from such an animal or plant is removed and placed in
a suitable nourishing fluid under proper conditions, although the
crushed or injured cells soon disintegrate, the intact ones will go
on living, growing, and multiplying as long as they receive proper
care. One of the most famous experiments in modern biology is
that carried on by Dr. Alexis Carrel of the Rockefeller Institute
for Medical Research. In 1921 he took a living fragment of a
chicken's heart and placed it in a medium that provided food,
air, and the proper environment for living conditions. During
the years that have followed, this tissue has lived and grown.
It probably will continue to live indefinitely so long as properly
cared for. Without such care, however, the isolated cells of com-
plex animal bodies soon die. Death is the penalty which they
must pay for the specialization they have undergone in order to
perform particular functions in the economy of the great com-
munity of which they once formed a part. In a somewhat similar
manner, a civilized man if left to Kis own devices in a primitive
world would most probably be unable to survive.

The word "cell" was first applied in a biological sense by an
English amateur microscopist, Robert Hooke, who used it to de-

THE PATTERNS OF LIFE 133

scribe the structure of cork, which is composed of dead cell walls
of the bark of the cork oak. The great French biologist Felix
Dujardin first drew attention to the contents of the cell in 1835,
but it was Hugo von Mohl who first recognized the importance of
protoplasm. The cell principle, which states that living organisms
exist only through the reciprocal action of the cells of which they
are composed, was enunciated by two Germans, Jakob Schleiden
and Theodor Schwann, one hundred years ago in 1839. Today
this concept has been established by a great body of experimental
knowledge. In addition, it has been extended to include many
cell products, substances formed by the action of living cells, as,
for example, the cellulose walls of plant cells, bone, lymph, blood,
and elastic fibers. Thus, it is now well known that protoplasm,
which is the very physical basis of life, is universally organized
into these small but complete units, the cells. It is only because
of the delicate adjustments made possible by this cellular organ-
ization that life continues.

Typical Cells

What, then, are the special features and details of living cells
that make them capable of maintaining this unique place in
nature? We have seen that a typical cell is composed of nucleus,
cytoplasm, and cell membrane. Of these, the nucleus is in many
ways the most important. That it is an organ essential to the life
of the cell is readily demonstrated by microdissection experi-
ments. If a cell is cut into two parts in such a manner that one
of these contains all the nucleus, this fragment will form a
protecting membrane at the cut surface and continue to live and
multiply in a perfectly normal fashion. The other portion, having
no nucleus, never multiplies and usually disintegrates and dies
almost immediately. The experiment proves not only that the
nucleus is an indispensable organ of the cell, but also that it
plays a role in the ordinary cell activities. If this were not so, the
fragment with no nucleus might be expected to survive for a
longer period.

Chemically, the nucleus is composed largely of a peculiar kind
of protein substance, called "nucleoprotein," a material found
nowhere else in the cell in such great quantities. At times this

134 THIS LIVING WORLD

extraordinary substance behaves as though composed of many
separate and distinct bodies. These bodies are the units that have
come to be called "genes." The genes, therefore, are exceedingly
small units within the nucleus. Just how large they are is not
definitely known, but they are probably of the same order of
magnitude as protein molecules. Even though extremely small,
these units are exceedingly important, as they constitute the
physical basis of heredity. Each and every kind of living thing
has a specific and different aggregate of genes, numbering up to
many thousands. The genes determine the exact body patterns
and characteristics to be transmitted to the offspring. They
might be considered as a sort of blueprint in the cell nucleus
which causes the offspring to be the same kind of creature as the
parent.

Genes are believed to be similar to enzymes, since they con-
trol various chemical reactions which take place in the living
cell without themselves being altered or destroyed in the process.
A specific gene tends to regulate a particular kind of reaction in a
particular way. It is in this manner that the genes exert their
controlling influence in heredity. The genes in the nuclei of
human body cells, for example, direct the activities of the cells
in such a way that they have the composition and perform the
functions which characterize human organisms and not some
other type of animal life. Even those qualities which serve to dis-
tinguish human individuals from each other are ultimately
traceable to the action of genes.

The genes have the property of becoming organized in chains
or strings, called "chromosomes." Chromosomes are typically
rod-like in shape and are clearly visible in the nuclei of most cells
under a high-power microscope. They undergo certain definite
changes during cell division. The number of chromosomes in the
nuclei of the cells of any given kind of organism is specific and
constant; this number, however, is much smaller than the
number of genes. The cells of the human body ordinarily contain
forty-eight chromosomes in their nuclei. Those which make up
the body of a fruit fly contain eight chromosomes, while the
nuclei in the cells of a frog's body have twenty -six. Other
creatures have from two to several hundred chromosomes, but
this number is in no way related to the size of the animal or plant.

Vacuole

THE PATTERNS OF LIFE

Cen^ome

135

Nucleus

Cell
membrane

A typical animal cell is a bit
of protoplasm surrounded by a
thin membrane and differentiated
into nucleus and cytoplasm.

Nucleus Cell wall

The ceil membrane of a plant
cell is surrounded by an outer wall
of cellulose.

Although the nucleus is perhaps the most important organ of
the cell, it is very nearly rivaled by the cell membrane. The
very organization of protoplasm into cells depends upon the
presence of such a structure, as may be demonstrated very readily
by disrupting it. If the injury to the membrane is sufficiently ex-
tensive that the cell itself is unable to repair it, the entire cell
contents will flow out and almost immediately disintegrate. The
cell membrane is essential, therefore, for preservation of the cellu-
lar organization, which in turn is necessary for the very existence
of protoplasm. Careful experiments have demonstrated that the
cell membrane serves to preserve and protect the cell contents
not only by preventing mixture with the external medium but
also by regulating the influx and outgo of materials into and out
from the cell. For the cell membrane is one of the finest examples
of what is known as a semipermeable membrane. Such a mem-
brane permits the passage of certain materials through it while
preventing the passage of others.

Chemically, the cell membrane appears to be a film or layer of
protein and lipoid substances just one molecule thick. The thin
cell membrane often builds up a coating around itself to give it
protection or rigidity. Most plant structures are rather rigid.
This condition is brought about by the fact that the cell mem-

136 THIS LIVING WORLD

brane of each plant cell is surrounded by a second, rather thick,
outer wall of cellulose. Cellulose is a complex carbohydrate
substance manufactured by the plant cell. The production of
cellulose cell walls is an ingenious adaptation on the part of plants
which enables them to grow to great sizes and to endure under a
great variety of conditions. On the other hand, the membrane of
animal cells is frequently uncovered, giving the cell greater
pliability and motility. Most muscle cells, nerve cells, and other
body cells have naked cell membranes. However, the membranes
of animal cells may be reinforced by hard materials secreted by
the cells themselves or deposited from the outside. Such mate-
rials may form bone structures, cartilage, horn, and similar
substances.

The cytoplasm of cells frequently contains numerous granules
and vacuoles, each surrounded by its own membrane, presumably
of a semipermeable character. Examination of the contents of
these vacuoles in certain large plant cells has shown that they
are capable of storing materials against a relatively tremendous
osmotic pressure. The conspicuous vacuoles in the cells of Valonia,
a marine plant, contain potassium at a concentration many times
that in which this element occurs in the surrounding sea water.
In order to store materials in this manner, it is necessary to do
work against the osmotic pressure built up within the vacuoles
and against the hydrostatic pressure of the sea water.

Osmotic pressure is the pressure exerted by the molecules of
a substance in solution against any barrier to their free diffusion.
When two solutions of different concentration are separated by a
membrane which is impermeable to the dissolved substance, there
is a difference in osmotic pressure on the two sides of the mem-
brane. In order to equalize the pressure there is a tendency for
the liquid in which the material is dissolved to pass through the
impermeable membrane from the more dilute to the more con-
centrated solution. The extra liquid thus makes the concentrated
solution more dilute and tends to balance the pressure on the
two sides of the membrane. The passage of the liquid through
the membrane is known as osmosis. The construction of osmotic
systems — that is, systems surrounded by semipermeable mem-
branes— provides one of the mechanisms by which cells are
enabled to store energy and to do work. It is a straightforward

THE PATTERNS OF LIFE 137

The paratyphoid uacmus, illustrating a rainer complex type wun nageua. vrnou?micro»

graph by Roy Allen.)

physical process which provides living cells with some of the
energy of life.

The Smallest Living Things

The smallest things generally conceded to be alive are the
bacteria. Although millions of these tiny colorless plants can
occur in a single drop of water and although their very existence
was unknown until after the perfection of the microscope, they
are relatively large compared with the largest known molecules.
Some of the very largest of them can just be discerned with the
naked eye, but the great majority fall within the size range of
objects which must be examined microscopically. Because they
are so small, not much is yet known about the internal organiza-
tion of bacteria. They cannot be described as true cells since they
do not possess a nucleus as a distinct organ, although the presence
of scattered granular masses of a nucleoprotein substance has
been demonstrated by delicate microchemical tests. In common
with true cells, bacteria possess a semipermeable limiting mem-
brane and their protoplasm contains vacuoles and granules
surrounded by similar structures. Some of the more elaborate
forms resemble fhe simplest algae, or lowest plants.

Because of their relative simplicity of organization, it is
generally recognized today that bacteria can be more efficiently

138 THIS LIVING WORLD

Spirillum volutans, a free-living type found in stagnant water. Scattered nuciear granules are
clearly visible and the flagella are well shown. (Photomicrograph by Roy Allen.)

characterized and distinguished by chemical means and by cul-
turing them than by simple microscopic observations. Indeed,
bacteria are little more than packages of enzymes with hulls or
sheaths of lipoid, protein, and carbohydrate materials. They may
be compared to chemical factories equipped to carry on a limited
number of operations, for one of the outstanding characteristics
of bacteria from the chemical viewpoint is their specificity of
action.

We all know that one kind of bacteria produces tuberculosis,
another typhoid fever, another syphilis, and so on down through
a long list of diseases to which mankind is heir. Actually, there
are at least three kinds of tubercle bacilli which look exactly
alike. One of these will grow only in cows, another only in birds,
and the third only in human beings. Again, certain bacterial
organisms will ferment lactose, or milk sugar, to butyric acid,
one of the simpler fatty acids found in butter. Other bacterial
organisms will convert lactose to lactic acid, which is a constitu-
ent of sour milk. This specificity of growth and effect is accounted
for by the specificity of the enzymes found in each type of
bacterium. The lactic-acid-forming bacteria do not possess the
enzymes for converting lactose to butyric *acid, while the
butyric-acid-forming types do not have the enzymes needed to
form lactic acid. Truly, bacteria act like keys in locks one set of

THE PATTERNS OF LIFE

139

A very unusual photograph of Neisseria gonorrhea, the causitive agent of gonorrhea,
taken with practically monochromatic light (the sodium line). A large nuclear granule
appears in the center of the cells, some of which appear to be in stages of division.
(Photomicrograph by Roy Allen.)

internal properties fitting only a certain set of external environ-
mental conditions. ,

Cell Differentiation

The large number of different kinds of bacteria gives us some
idea of the enormous range of differentiation and specialization
manifested by living cells. They differ not only in structure,
shape, and function, but also in size. A one-inch square drawn on
the back of a man's hand would circumscribe about six million
cells. A cubic millimeter of human blood contains about five
million of these protoplasmic units, and the total number making
up the body of one person is an astronomical figure. However,
some particular kinds of cells may be rather large. The single-
celled animal, Paramecium, is large enough to be seen without
the aid of a microscope, and the marine protozoan, Porospora,
may reach a length of over half an inch. Unspecialized types of
cells tend to assume a spherical shape when not restricted by
conditions of their environment. However, nearly every other
conceivable shape may occur, from the long thread-like cells of
the human spinal cord to the snowflake design of the pigment
cells on the scales of fishes. Torpedo shapes and flattened pancake
types are common. Plant cells are often compared to a shoe box.

140 THIS LIVING WORLD

Paramecium. A large nucleus can be seen in about the center of the cell, and near it, a
small nucleus. The surface of the cell bears numerous cilia. (Photomicrograph by Roy
Allen.)

Many different kinds of cells that are specialized for perform-
ing different functions occur in the bodies of multicellular plants
and animals. Here there is a close analogy to the structure of a
mechanical device with its many different parts, each assembled
from different types of materials and differently shaped units.
The tissues of living things are made up of groups of cells special-
ized in structure for performing similar functions. For example,
the exposed surfaces of plants are covered by epidermal tissue,
comprising a layer of flattened protecting cells. The supporting
tissue of plants is made up of thick-walled cells, which give ri-
gidity to the plant structure. Finally, water and food materials are
transported to the various parts of a plant by conducting tissue
made up of tube-like cells.

In animals, both the internal and external surfaces of the
body are covered by epithelial tissue composed of thin, flattened
cells. The supporting tissues of the animal body, including bone
and cartilage, are largely made up of products secreted by cells.
There are elastic and collagenous fibers that form a framework
upon which or within which the softer tissues are suspended.
Muscle tissue is composed of greatly elongated fibrous cells
specialized for contracting, and nervous tissue is made up of
rounded or flattened nerve cells and their filamentous processes
specialized for transmitting impulses.

THE PATTERNS OF LIFE

141

The various tissues of animal and plant bodies are often com-
bined in the structure of organs for performing definite functions.

B?n°S©

8°

Some kinds of animal tissue cells: A, glandular epithelium/ B, mammalian blood cells/ C
nerve cell/ D, smooth muscle cells/ E, pavement epithelium/ F, bone.

The leaf is an organ of a plant, while the liver, heart, and lungs
are examples of organs found among animals. Organs are in turn
often grouped into systems concerned with the same functions.
The foliage of a green plant as a whole forms such a system, as do
the roots. Likewise the brain, spinal cord, and nerves form a com-
plex system of communication within the bodies of higher
animals.

Growth and Division of Cells

One of the distinguishing characteristics of living things is
growth, or increase in size. The conscious desire of every boy
and girl to grow into adulthood is but one manifestation of an
unconscious condition that pervades all animate nature. All
creatures with multicellular bodies grow by an increase in the
number of cells making up theii* physical structures rather than
by an increase in the size of the cells. Cells themselves grow by
adding to their protoplasm and to their storage materials. This

142 THIS LIVING WORLD

process does not continue indefinitely, however. If it did, there
soon would not be room on the earth's surface for more than a
single cell. The rate of growth of protoplasm is so enormous that
even one cell would increase sufficiently within a few days to
cover the entire earth were it not checked somewhere along the
line.

The limits of growth for a given type of cell are governed in
part by intrinsic factors, such as the influence of the genes con-
tained in the nucleus; and in part by extrinsic factors, such as
the pressure exerted by adjacent cells, and particularly by the
factors controlling the exchange of respiratory gases. Oxygen is
taken up and carbon dioxide is given off at the surface of the cell,
and the rate of exchange of these gases is controlled by their
diffusion rates and by the amount of cell surface available. In
order to insure an adequate supply of oxygen at all points in the
cell and proper elimination of carbon dioxide from all points, the
ratio of cell surface to cell volume must be kept within certain
well-defined limits. As a cell grows, its volume increases as the
cube of its radius, while its surface increases only as the square. It
is not difficult to see that the surface-volume relationship soon
becomes unbalanced in the growth process. This would be detri-
mental or even destructive to the cell if it were not corrected.

This difficulty is corrected by one of the most unusual proc-
esses which living organisms manifest. When a cell reaches a
certain size, it usually divides into two parts, called daughter
cells. In this complex and quite remarkable process a favorable
ratio of cell surface to cell volume is restored. After a period of
growth, each of the daughter cells in turn may divide, and the
process continues indefinitely. In this manner, a single cell can
give rise to many thousands in a relatively short period of time.

The rate of increase of a population of cells is a geometrical
progression. The number of individual cells produced by equal
division (barring accidental death) after a given number of
generations, designated by the letter n, is equal to 2 raised to the
/I — I power (2n~1) multiplied by the initial number of cells. For
example, suppose there are 20 bacteria in a bottle of milk. Sup-
pose, also, they are permitted to remain there long enough for
15 generations to occur by cell division. How many bacteria
are now adding life to the bottle of milk? Two raised to the

THE PATTERNS OF LIFE 143

Paramecium, dividing. The division of the large nucleus and of the cytoplasm is well
shown. Numerous food vacuoles are visible in the cytoplasm. (Photomicrograph by Roy
Allen.)

fourteenth power is 16,384. This figure multiplied by the 20
starting bacteria produces a total of 327,680 cells! Should
the growth be allowed to continue for 15 more generations, the
number would be increased to over 21 billions. The growth of a
multicellular animal or plant is the result of cell division and
growth. Likewise as a multicellular organism grows larger, its
component cells increase in number in geometrical progression
until adulthood is reached or some other limiting factor comes
into operation.

When it is realized that so far as is known every living cell
in the universe arose by division of a preexisting cell, it is appar-
ent that the process of cell division is of fundamental importance
in the scheme of life. If we could know all about it, we should go
far toward understanding the riddle of existence. Incidentally,
we would have discovered the cause of cancer, which always
involves abnormal division of cells.

The way in which one cell becomes two is important for
several reasons. It is the way in which the torch of life has been
handed down through the ages and is continuing to be handed
down at present. Moreover, it provides the mechanism by which
multicellular forms of life are believed to have arisen from single-
celled organisms, through the formation of colonies and applica-
tion of the important economic principle of division of labor.

144 THIS LIVING WORLD

Finally, it provides the physical basis for the phenomena of
inheritance, by which the integrity of each kind of living organ-
ism is both preserved in future generations and permitted to
vary in such a way as to account for the great diversity of organic
forms.

The ordinary method by which cells divide is called indirect
division or "mitosis." It is not surprising that this is a rather
complicated process when it is considered that in it the complex
structure of the parent cell is preserved in each of the resulting
products. Of singular importance is what takes place inside the
nucleus when a cell divides by mitosis. A series of complicated
and invariable steps is followed. In the nucleus of a cell in the
resting state, the material of which the chromosomes are com-
posed appears to be scattered irregularly upon a framework of
material of a different sort. When the cell is about to divide, this
material, called "chromatin," becomes aggregated to form the
chromosomes. The appearance of the chromosomes marks the
first step in the mitotic process. This is followed by contraction
and thickening of the chromosomes and by the disappearance of
the nuclear membrane.

In the next step the centrosome divides, one half migrating
around the nucleus and taking a position at the opposite side
from its sister half. It should be noted here that the centrosome is
a tiny granule resting on or near the nucleus. It is surrounded by
a larger zone of clear protoplasm from which numerous fibrils, or
rays, extend out in all directions, forming an aster or star.
Centrosomes generally are present in the cells of higher animals
and many lower plants, although they are absent in the cells of
higher plants. When they do occur, they play a prominent role in
the division of the cell. After the two centrosome halves have
taken their positions on opposite sides of the nucleus, a cigar-
shaped structure is formed between them, called the "division
spindle." It is made up of fibrils derived from the astral rays and
partly formed anew. In cells which lack a centrosome, the spindle
represents an entirely new structure formed at every division.
Certain of the spindle fibers begin to invade the nucleus and
appear to become attached to the various chromosomes at specific
points.

THE PATTERNS OF LIFE

145

In the meantime, another step in the division of the cell has
occurred. Each chromosome has divided longitudinally, or dupli-

E F

When a cell divides by mitosis, a series of complicated steps is invariably Followed:
A, the centrosome divides and one half moves to the opposite side of the nucleus/ B/
the nuclear membrane begins to break down, and the cytoplasm begins to coagulate around
the centrosomes/ C, each chromosome divides longitudinally, the nuclear membrane has
disappeared, and the chromosomes have become arranged about the equator of the
division figure/ D, the division figure elongates with the cell and the sister chromosomes
separate, moving toward opposite poles/ E, the chromosome groups reach the opposite
poles, the elongated cell body begins to constrict in the equator, and the division figure
begins to disintegrate/ F, two daughter cells nearly completely separated.

cated itself. The genes or gene strings which make up the chro-
mosomes have been doubled. During the next step a remarkable
phenomenon takes place. The corresponding halves of each
original chromosome begin to move in opposite directions, as if
drawn by the fibrils of the centrosome. One half goes toward one
pole of the division spindle, the other half toward the opposite

146

THIS LIVING WORLD

5 6

A photographic record of cell division. The first cleavage division of the fertilized «99
of Ascaris, a worm having only two meiotic chromosomes. (1) When a cell is about to
divide the nuclear chromatin becomes aggregated to form the chromosomes. (2) The
nuclear membrane disappears, the centrosome divides and one product migrates to the
opposite pole of the cell. The division figure appears, made up of the two asters and the
spindle fibers. (3) The chromosomes line up at the equator of the spindle. (4) The chromo-
somes divide longitudinally and the corresponding half-chromosomes begin to move in
opposite directions. (5) As each group of daughter chromosomes approaches its centro-
somc, the spindle fibers disappear. The dividing cell elongates in a plane parallel to the
long axis of the division figure. (6) The cell constricts in a plane passing through the equa-
tor of the division figure. The asters begin to disappear. (Photomicrographs by Roy Allen.)

THE PATTERNS OF LIFE 147

pole. During this process, the traction fibrils attached to the
chromosomes shorten and eventually disappear.

As soon as each group of chromosomes reaches its centrosome,
the spindle fibers appear to break at the mid-point and the cell
begins to constrict in a plane passing through the equator of the
spindle. A membrane soon forms in this plane, and the cell is
divided into two daughter cells, each one of them by this process
containing the equivalent of half of the original. The chromosome
groups in each daughter cell presently become reorganized, form-
ing a nucleus for the cell and secreting a nuclear membrane.

The significance of this complicated process of mitotic cell
division lies in the careful manner in which the hereditary factors
are distributed among the products, so that each daughter cell
receives an exact duplicate of the set of genes contained in the
other and both of them have an exact duplicate of those in the
original parent cell.

Production of a New Individual

The process, just described, by which one cell becomes two,
together with cell differentiation, forms the basis of biological
development. The production of a new individual occurs by divi-
sion of the cells constituting a preexisting individual or a
part of it, and, in the strictest sense, this is the only method of
reproduction.

In order to see completely the moving picture of early em-
bryonic development, it is necessary to view it in the sequence
of the acts as they occur. This sequence of development may
be described in eight more or less arbitrary steps. These are arti-
ficial in that they are not sharply marked off from one another,
but they are useful in untangling and understanding the patterns
of life in the growing embryo. In another sense, however, each of
these steps constitutes a distinct act of form building. Each one
leads to the production of visible organs and special structures
in the embryo, which to early outward appearance is not differ-
entiated into special organs. In our own lives they cover the
first twenty-one days of existence, up to the time we are about
an eighth of an inch long.

The first stage in early embryonic development is cleavage, or
cell division, leading to the production of many cells from a single

148 THIS LIVING WORLD

fertilized egg. The second stage is blastulation, in which the
cells become arranged in a hollow sphere. The third is gastrula-
tion, or the infolding of the hollow sphere of cells to form a
multiple-layered embryo. In the fourth stage, evocation, the
primary axes of the body are set up. The fifth stage is induction,
or organization of the embryonic cells into organs and organ
systems. The sixth is differentiation, by which the cells become
specialized for future jobs. The seventh is the stage of interac-
tion, in which the various systems begin to exert their effects
upon each other. The eighth stage is that of coordination and
growth, when the parts of the body begin to grow and to become
adjusted to one another in function. All these steps are now
objects of careful experimental research, most of them being well
understood. Embryology has become one of the exact sciences.

Normal Development

In any great theatrical production, the scenes follow one
another in orderly sequence. First, the background and the plot
are laid. The acts of the drama then unfold until the climax of the
story is reached. Finally, an ending results from the natural order
of events which have preceded. A developing embryo likewise
presents a moving picture which should be viewed in the
sequence of the scenes presented. The growth of a new individual
from a fertilized egg is in reality a continuous and moving process
which begins with the laying down of certain fundamental
structures and continues with the development of all the complex
patterns which make up the adult body.

In order roughly to chart the course of normal development,
let us follow the embryological history of a simple animal such as
the sand dollar. Some of the critical scenes in the story are shown
in the drawings which accompany this text. The first scene
is cleavage. When an egg has been fertilized by a sperm, a set of
age-old forces is unlocked anew; they proceed in orderly, neat,
and perfect fashion. About an hour after penetration by the
sperm, the egg divides by mitosis into two smaller cells, that is,
the egg "cleaves." The two cleavage products soon divide to give
four, and the process is repeated until several hundred small cells
are formed. As shown in the drawings, the individual cells be-

THE PATTERNS OF LIFE

149

come smaller as they increase in number during cleavage. Growth
is another and different matter, which comes later.

The embryo sand dollar develops from a fertilized egg by an orderly series of events
involving the growth, division, rearrangement, and differentiation of cells: A, immature
unfertilized egg; B, polar spindle formed; C, two-cell stage, after first cleavage division;
D, four-cell stage following second cleavage division; E, eight-cell stage following third
cleavage division; F, optical section of thirly-two cell stage; G, optical section of early
blastula; H, optical section of ciliated blastula; I, optical section of blastula, flattened at
antapical pole; J, optical section of early gastrula showing inpushing of endoderm (b,
blastopore); K, optical section of later gastrula showing beginning of mesoderm (m);
L, optical section of free-swimming embryo with mouth (o), anus (a), and complete
primitive gut (g). (Drawn from sketches of living embryo sand dollar by W. R. Duryee.)

The first act in laying the groundwork for the critical scenes
which are to follow is blastulation. In this process the cells
arrange themselves to form a hollow sphere, called the "blastula."
Each cell moves into its little niche with all the precision of a
dancer performing an intricate routine in a musical revue. The
wall of the hollow sphere, as shown in the accompanying draw-
ings, is a single layer of cells in thickness. This layer is called the

150 THIS LIVING WORLD

"blastoderm," or "germinal skin." It will be noted that the cells
comprising it bear hair-like or whip-like structures called cilia.

In the next scene, known as "gastrulation," a unique move-
ment begins to take place. Within about twenty -four hours after
the blastula is formed, one side begins to sink inward in much
the same way as one side of a soft rubber ball sinks in when one
pushes his finger into it. The infolding continues until the embryo
sand dollar acquires an inner sac-like structure, termed the
"primitive gut." This sac-like structure is its first elementary
organ, becoming later the digestive tract. It is easy to see that a
two-layered condition has now been reached. The outer blasto-
derm remains as before, but a new layer has been formed by the
infolding. This layer forms the gastric cavity or stomach. It is
made up of cells which have passed over the rim of the opening
made by the infolding. These cells lose their cilia and become
differentiated to form a new body layer, called the inner skin
or "endoderm." The hole leading to the gastric cavity from the
outside remains open, forming what is known as the "blasto-
pore." The blastoderm may now be called the outer skin, or
"ectoderm."

As the infolding of the wall of the blastula continues, the
endoderm gradually extends across the cavity of the blastula,
until it touches the ectoderm at a point opposite the blastopore.
At this point, where the two layers touch, a thin membrane is
formed, consisting of both endoderm and ectoderm tissue. It is
a temporary structure, called the oral plate. Eventually, it
breaks through to the outside, giving rise to an opening which
becomes the mouth of the embryo. It is, perhaps, significant that
the mouth of every known vertebrate, including man, is formed
in this way. Equally important is the fact that the blastopore, the
original opening into the gastric cavity, becomes the anal open-
ing. With the formation of the mouth the primitive alimentary
canal is completed, forming a straight tube passing through the
embryo from the forward to the rear end. The embryo at this
stage is called a gastrula.

During the later stages of gastrulation a third important
body layer begins to form. This is the middle skin, or "meso-
derm." It Appears first as a few cells which bud off between the
outer ectoderm and inner endoderm in the region of the blasto-

THE PATTERNS OF LIFE 151

pore. Gradually they extend across the cavity between these two
layers, forming two layers of mesoderm tissue. In the formation
of the mesoderm the original cavity of the blastula is completely
obliterated. In the embryo sand dollar, the mesoderm forms a
new cavity which becomes the principal body cavity of the adult
animal.

These three basic layers of tissue, the ectoderm, endoderm,
and mesoderm, are known as the primary germ layers. They are
the raw materials out of which all the organs and organ systems
of the body are made. It might be said that they constitute the
fundamental parts of the plot of embryonic growth fpom which
all future acts are developed. For example, the ectoderm later
gives rise to the skin and nervous system. From the endoderm are
derived the alimentary canal, digestive glands, reproductive
cells, and lungs. The mesoderm produces the skeleton, the mus-
cles, and the circulatory and urinogenital systems.

The next scenes in the drama of embryonic development
produce the climax of the story. They present the immediate
circumstances underlying the shaping of bodily patterns. They
will be better understood in relation to the reproduction of a new
human individual if their sequence in the development of a
simple vertebrate type is followed. For this purpose, the embryo
of the salamander or the frog provides the best example, as more
is known concerning the developmental processes in these forms
than in any others.

The first of the crucial scenes to be dealt with here is evoc-
tion, or the determination of the main body axis. In this process
the locations of the head and the tail ends of the embryo are
established. An imaginary line passing through the centers of
these fixed regions of the body constitutes the principal axis
around which and along which the organs and organ systems will
develop. Up to the time when the position of this imaginary line
is determined the cells of any particular layer of the gastrula have
nearly equal potentialities. Under certain experimental condi-
tions, all the cells of a given germ layer are eligible to enter into
the structure of any or all of the organs and organ systems which
normally would be developed from that layer. Like that of
schoolboys who are eligible for any trade or profession of their
choosing, the fate of the cells is not yet fixed.

152

THIS LIVING WORLD

K L

The events of vertebrate embryonic development as illustrated in the development
of the fros: A, fertilized egg; B, embryo after second cleavage division; C, embryo after
third cleavage division/ D, late blastula/ E, embryo at the beginning of gastruiation, the
arrows showing the direction of movement of cells toward the dorsal rim of the blastopore/
F, transverse section of early gastrula showing cavity (h); G, embryo at later stage of
gastrulation showing blastopore (b); H, transverse section of late gastrula showing primitive
Sut (s)/ ectoderm (d), and endoderm (e); I, embryo at early stage of induction of neural
folds (f); J, transverse section of embryo at early stage of induction of neural folds, also
showing the notochord (n), mesoderm (m), and the direction of folding (arrows); K,
embryo at early neural fold stage showing forward-hinder (A — P) and upper-lower (D — V)
body axes; L, longitudinal section of embryo with neural tube completed showing neural
tube (n.t.)/ brain regions (br), notochord, heart (c), and primitive gut with mouth (o), anus
(a), and liver otitpocketing (p).

THE PATTERNS OF LIFE 153

Reference to the drawings illustrating the development of the
frog will aid in understanding both the embryonic processes
which have already been discussed and those which are about to
be described. Comparison with the drawings illustrating the
development of the embryo sand dollar serves to bring out the
fact that the principal differences between these two embryonic
types during cleavage and gastrulation are caused by the pres-
ence of a greater amount of yolk material in the egg of the frog
and by the tendency of this heavier material to accumulate in the
bottom half of the egg. As a result of this unequal distribution of
inert food materials in the protoplasm of the frog's egg, cleavage
is very unequal and leads to the production of a blastula in
which the cells are graded in size and number from top to bot-
tom. They are smallest and most numerous at the uppermost
pole and fewest and largest at the lowermost. The dense ac-
cumulation of coarse yolk granules in the bottom hemisphere
tends both to retard cleavage and to distort gastrulation.

Repeated divisions of the cells in the upper regions produce
crowding and pressure so that the cells on the sides must go
somewhere else. This eventually results in overgrowth of the
lower hemisphere. Moreover, certain cells in the mid-line region
of the hinder ectoderm sink inward at a point which becomes the
dorsal lip of the blastopore. With continued overgrowth, the
rim of the blastopore extends downward on the sides until
eventually a circular opening is formed. Meanwhile, the rapidly
dividing cells of the top and sides are forced toward the rim of
this opening and pass over it into the inside. The cells which pass
over the rim of the blastopore continue to move upward and
forward on the inside, gradually forming the roof of the primitive
gut.

That those cells which move inward over the upper lip of the
blastopore have undergone some reorganization is shown by the
simple fact that they proceed to specialize, forming a peculiar
rod-like axial structure known as the "notochord." This primi-
tive backbone initially develops on the roof of the primitive gut
rtnd is composed of endodermal cells. Eventually, however, it is
inched off from the wall of the gut and forms a core about which
tie vertebral column is laid down. Our own vertebral columns
riginate in this way in the early embryo. Search for the chemical

THIS LIVING WORLD

A photographic record of the early embryonic development of the rabbit. (1) Shortly
after the fertilization the egg divides into two smaller cells, that is, it cleaves. (2) "The
two cleavage products soon divide to give four." (3) By the fourth cleavage, sixteen cells
are formed. (4) The cleavage process continues until many small cells are formed. (5) In
mammalian embryonic development the stage corresponding to the blastula of lower
forms is called a "blastocyst." (a, inner cell mass, from which embryo develops/ b, tropho-
blast, which erodes away the mucous membrane of the uterus/ c, segmentation cavity/
d, egg membrane). (6) The mammalian embryo develops from a restricted portion of the
blastocyst (a) known as the "inner cell mass" (b, trophoblast/ c, follicle membrane).
(General Biological Supply House photographs.)

THE PATTERNS OF LIFE 155

that emanates from the cells of the upper lip of the blastopore to
stimulate those just ahead of them to form a notochord is one of
the most important researches going on in many biochemical
laboratories all over the world. If the chemical nature of the
" organizer " can be found, it will be a scientific event of the first
importance. At present, workers in Cambridge University in
England think it may be a very complex fatty substance, related
in molecular structure to the vitamins and sex hormones.

The upper lip of the blastopore gives rise to the tail bud from
which the tail of the frog tadpole develops. The point of most
forward extension of the notochord marks the division between
the future forebrain and midbrain in the head of the embryo.
Moreover, the primitive gut marks the lower side. Thus, with the
completion of gastrulation, the principal axes of the body have
been established, and, for the first time, the terms forward and
rear, upper and lower, and right and left may be applied.

Building Body Systems

As soon as the principal axis of the body has been established,
another master force comes into action to direct the ensuing
stages of development. This is the process of induction, by which
is meant the organization of the definitive body organs under the
stimulus of interaction between the elementary organs and
tissues. The latter are distinct parts of the early embryo, which
are uniform in themselves. Each of these elementary organs and
tissues is the result of one distinct act of form building. Examples
are the notochord, the primitive gut, and the primary germ
layers. They are typical of elementary organs and tissues with
regard to position, histological properties, form, and, to a certain
extent, size. Their size is partly controlled by the duration of
the form-building processes which bring them into existence. The
duration of each process, in turn, is subject to variation with the
temperature and other physicochemical factors. The form, size,
and histological structure of the elementary organs and tissues
are governed in part, also, by considerations of present or future
function. They at least prepare for this function by a spe-
cific metabolism which begins at a very early stage in their
development.

156 THIS LIVING WORLD

The first, or primary, induction in the embryo is that which
the cells of the notochord exert upon the overlying layer of
ectoderm to form the beginnings of the nervous system. In
short, the brain and spinal cord are "induced" to develop,
presumably by the action of some chemical compound produced
by the notochord cells. This substance apparently diffuses into
the overlying ectoderm, because direct contact between the
notochord and ectoderm is always necessary before the nervous
system can develop.

Our own brains, and likewise those of all other vertebrates,
appear first as a flat layer of ectoderm cells, known as the
"neural plate." This is in the early gastrula stage. Soon, a
change takes place on the outside. First, a groove appears
directly above the notochord and, almost immediately, two folds
or ridges bulge up on the sides of this groove. These are shown in
different views in the drawings. The two ridges or folds join
each other around the front end, forming a sort of hairpin-
shaped loop. A process of rolling up now begins. While the groove
sinks in deeper, the folds slowly come together, arch up, and
meet above the groove. Thus they form a hollow tube, the
neural tube, which soon becomes closed at both ends.

The front part of this tube swells into a sort of bulb, similar
to that on a medicine dropper. The bulb is the rudiment of the
brain. Its walls thicken here and there and by various bulges
and constrictions form the different parts of the brain. Mean-
while, the rest of the neural tube becomes the spinal cord. Since
all nerve fibers grow out from neural tube cells, it can be seen
that our whole nervous system is derived by induced differentia-
tion exclusively from the ectoderm layer.

Formation of the neural folds was called the primary induc-
tion. Now let us turn to the formation of other organs and sys-
tems in the rest of the body. Analysis shows that in nearly every
case as, for example, the eyes, ears, legs, heart, and so on,
differentiation of the organ is similarly caused by inductions
from underlying or adjacent cells. Such inductions are spoken
of as secondary, to distinguish them from the main one of the
nervous system. It thus appears that the pattern of develop-
ment unfolds by a whole series of inductions, each depending on

THE PATTERNS OF LIFE

157

The germinal disc of the rabbit, showing an early stage in the formation of the main
body systems, a, head region/ b, tail region; c, myotome, or muscle mass; d, neural folds;
e, primitive streak, or region of the notochord. (General Biological Supply House
photograph.)

fectly understood, it is becoming clearer that each induction
depends on the chemical action of some sort of "organizer"
near by. In the primary induction, the notochord acts as an
organizer which induces the formation of the neural tube from
the ectoderm. How the organizers first appear is another ques-
tion which is still unsolved. All we know is that they are some-
how precisely controlled by the genes.

Patterning Life

The organizers for the different body systems are not scat-
tered about haphazardly but are progressively localized in
groups of cells arranged in definite patterns, like the designs in

158 THIS LIVING WORLD

rugs. Such localized groups of cells are called "embryonic
fields"; some of these are represented in the accompanying

drawing of the embryo sala-
mand^r. There are fields for the
eye, ear, limb, heart, and many
other organs in the body. The
rolling inside of some cells
during gastrulation and the
rolling up of others into the
neural tube result in the sliding
"Embryonic fields" in the embryo of the layers over one another

salamander: 1, eye field; 2, ear field; 3, an(J distortions of the Original
forelimb field; 4, hindlimb field, 5, heart ^ f , • /s i T i

patterns ot embryonic fields by

stretching. By these so-called

migrations of cells, the embryonic fields are pushed or pulled into
final position. We may now define an embryonic field as a group
of cells which contains a specific chemical organizer but which
has not yet begun to take shape as an organ.

Until recent years there was no way to test or prove the
existence of embryonic fields. Then, some thirty years ago,
Dr. Harrison at Yale University and Dr. Spemann in Freiburg,
Germany, developed techniques for cutting out small squares
or disks of cells and transplanting them elsewhere in the same
embryo, or even to another one, to find out what would happen.
When a definite field, such as the eye field, has been transplanted
to a neutral or less important area, as, for example, the body
wall, only an eye can develop at this spot. It does so in the new
location just as it would in its normal position, the cells under-
going structural changes for their special jobs. By means of
experiments employing these techniques, some of the more
important embryonic fields have been mapped out.

The next act of the moving-picture drama of embryonic
development has to do with the changes by which unspecialized
cells are transformed to become embryonic fields and finally
true organs and systems. This next act is referred to as "dif-
ferentiation," or specialization of cells for definite jobs. The
early ectoderm and endoderm cells merely look like irregular
little bricks in a pavement or tiles in a mosaic. The mesoderm
cells are even less definitely shaped, appearing like small amoebae

THE PATTERNS OF LIFE 159

crawling around. In differentiation the cells change both shape
and function. Such changes have a twofold chemical origin:
first, in an outside stimulus from contact with an organizer;
second, from an internal stimulus depending on the genes com-
prising the chromosomes of each cell nucleus. Both sets of forces
are always necessary, but their relative importance varies.
Let us note briefly how the specialization of cells works out in a
few instances. How does the leg form? How does the heart form?
How does the eye form ? The answers to these questions are now
rather well known.

An understanding of how the leg forms may be gained by
considering the foreleg or arm of the embryo frog. The embry-
onic field starts as a small thickening of mesodermal cells in the
forward parts of the body, as shown in the drawing. This, of
course, causes the overlying ectoderm to bulge out. Soon the
bulge grows out like a finger in a glove. It is now called the limb
bud. This limb bud grows to a length of about a millimeter,
then the tip begins to branch. At first two digits appear. These
are followed immediately by two smaller branches which form
the third and fourth digits of the four-fingered animal. At the
same time, the inside mesodermal cells have largely abandoned
their nomad existence. Some of them settle down in groups and
produce cartilage. These are the arm, wrist, and finger cartilages.
Other mesodermal cells grow more spindle-shaped and link
up in orderly rows. They become muscles. Still others connect
these two sets together, making tendons. These transformations
of wandering mesoderm cells into cartilage cells, muscle cells,
and tendon cells are typical differentiation. Later, bone cells are
differentiated which invade the cartilage, replacing it with bone.

The heart originates in two separate lateral regions of meso-
dermal tissue. These two regions move together and meet in the
midlower line of the embryo, where the wandering cells rearrange
themselves to form tubes. By the process of mitosis more cells
appear, and the tubes grow longer in both forward and back-
ward directions. In the region from which the heart forms, the
tubes unite to produce a single structure. Then the cells in the
walls link together and differentiate into heart muscle. Simul-
taneously, the tubes grow out from the forward part of the heart
to become arteries. Branches from these arteries gradually

160 THIS LIVING WORLD

penetrate into every nook and crevice of the growing embryo
as blood capillaries. The veins form in somewhat similar fashion.

The formation of the vertebrate eye involves differentiation and a secondary induction:
A, brain wall swellins ouf to produce optic vesicles (o)/ B, optic vesicle sinking in and
inducing a thickening and insinking of the adjacent ectoderm/ C, optic cup (c) forming
from optic vesicle, lens vesicle (I) pinching off from ectoderm/ D, lens forming in opening
of the optic cup.

Now a unique thing begins to happen. The differentiated
muscle cells in the walls of the heart start to contract rhythmi-
cally. Alternate relaxations and contractions cause waves of con-
striction to pass forward along the tube. The heart has begun to
beat, even before any blood is present, as may be noticed by
looking at such an embryo under a microscope. Soon, however,
certain mesoderm cells in the lower body region cast off from
their moorings and launch into the fluid serum that bathes
all the interior of the embryo. These round up to become
literally streamlined. We recognize them as differentiated blood
corpuscles.

One further example claims our attention. It is the formation
of the eye. This really involves a double induction and differen-
tiation. We must first go back to the primary step, in which a
neural tube was induced by the notochord. We should recall that
one end of this hollow tube had formed a bulge, the beginning of
the brain. The rudimentary brain further divides into five
smaller bulges. Now, from the second bulge a large swelling
appears on each side. What then happens is shown in cross
section in the accompanying drawing. The brain bulge, or optic
vesicle, pushes out to the ectoderm. Here a secondary induction
takes place. The outer skin is induced to form a sac-like depres-
sion at this point. The optic vesicle itself then cups in. Mean-
while, the new ectodermal sac pinches off, forming a little hollow
sphere that differentiates as the crystalline lens to focus light
rays on the inside of the optic cup. The sensory layer of the

THE PATTERNS OF LIFE

161

inside of this optic cup becomes the retina. Here the cells
differentiate into light receptors called rods and cones.

From this brief account it is
seen that the sensory part of the
eye is formed from an outgrowth
of the brain, while the accessory
structures are produced from an
induced ingrowth of the skin to
meet the brain extension. The
conclusion is obvious, that em-
bryonic differentiation depends
on precise sequences of events,
as the succession of inductions.
Nowhere else is timing more im-
portant or more perfect. The
undeniable fact that such events
are ultimately controlled by the
protein molecules which make up
the genes is still too complicated
to be comprehended entirely.

furth

ter

A later rabbit embryo. At this stage
the main organ systems of the body
have been laid down in outline, a,
brain; b, spinal cord; c, myotome/ or
muscle mass; d, eye rudiment. (General
Biological Supply House photograph.)

Coordination of Activities

It might be thought that the
climax of this moving picture of
embryonic development is reached
when the main organs and systems
of the body have been formed. In
some respects this is true. The
great construction work has been accomplished and the parts
arranged in proper order. However, it must be kept in mind
that the living embryo is a thriving, going concern. The
parts must work together and they must be coordinated and
regulated in their growth. There are two more acts, then, before
the drama is completed.

The first one of these is interaction, or the exchange of effects
of one system on another. Up to this point we have seen that,
once an embryonic field has been organized, it is capable of self-
differentiation; that is to say/ once a forelimb area has been

162 THIS LIVING WORLD

organized or determined, those cells now have no other choice
than to make a foreleg. However, if this were all, an embryo
would develop only to some part- way stage. For complete
development something more is needed. This fresh impetus to
further development comes from the interaction of the parts.

Two examples should make this clear. A rudimentary leg
even with toes, bones, and muscles can form, but alone it cannot
function; in addition, nerve connections from the neural tube
system must arrive, and a blood supply is essential. Although
the leg muscles are formed, they soon degenerate and disappear
unless nerve fibers penetrate into the limb bud. Connections of
the motor nerves to a few muscle cells soon cause limb motions.
These in turn aid the penetration of capillary rootlets which are
destined to bring a blood supply laden with food and oxygen
for the leg cells to grow on. The limb bud, the nervous system,
and the circulatory system must intereact with each other for
normal development to proceed.

A second example of the importance of interaction is some-
what the converse of the one above. For the normal development
of brain and sp'inal cord, it is absolutely necessary for the first
outgrowing nerve fibers to make connections with leg muscles,
or heart, or ear, or tail, as the case may be. Once such connec-
tions have been made, then impulses travel up the fibers into
the cord and back to the brain, where localized cell division is
greatly stimulated. Thus the corresponding brain areas are
forced in their turn to grow larger in order to care for the addi-
tional load of body control. Interaction, therefore, often works
both ways.

We now come to the last important act of embryonic devel-
opment. This is coordination and regulation of growth as the
embryo gets larger and the number of cells increases into astro-
nomical figures. By the seven acts already reviewed, the main
patterns of development are completed. Thus, in a human
embryo, the brain, the skeleton, the alimentary canal, the circula-
tory system, etc., are all established by the end of the twenty-
first day. Such an embryo is approximately four millimeters long.
The remainder of embryological development is mostly a matter
of growth. No new systems are added. Only relatively minor
changes take place, except for increase in size. The relative

THE PATTERNS OF LIFE 163

growth of parts is controlled by the four factors of blood supply,
hormones, mechanical forces, and, especially, factors inherent
in the genes of the chromosomes.

Experiments have shown that decreased blood supply to any
growing organ system limits or stunts its growth, although not
all organs are equally affected. It seems certain that there is
actual competition between all the developing parts of an em-
bryo for the available food in the blood stream. Some hormones,
as, for example, those from the pituitary gland, are able to con-
trol the rate of food uptake of an organ. But just how genes
operating through chemical hormones cause a giraffe's neck
to grow long and a gorilla's to grow short is still a major bio-
logical mystery.

The regulation of proportions in size is likewise but slightly
understood. For instance, when an eye from a giant species of
salamander is grafted into the head of a small species, the eye
causes the host's skull to enlarge appreciably to accomodate it.
Eventually, however, the foreign tissues shrink partially, re-
maining about a third again as large as the normal eye on the
other side. Further experiments in size regulation are needed to
complete our knowledge in this respect.

In Retrospect

If this story of development as presented here seems complex
and difficult, it must be remembered that in the past hundred
years, since Karl von Baer discovered the human egg, embryol-
ogists themselves have made but slow progress in the study of
early growth of a new individual. This is so because of the great
complexities of the subject. To get even a general understanding
of the knowledge man now possesses regarding so complex a
phenomenon is not easy, regardless of how interesting the sub-
ject may be. These few pages, while attempting to give a brief
survey of the processes involved, have scarcely raised the curtain
on the drama of human development.

In this chapter and the preceding one an attempt has been
made to present the essential characteristics of living creatures
as distinguished from inanimate materials and to point out how
cells reproduce their kind by cell division and how a new indi-
vidual develops during embryonic growth. It should be empha-

164 THIS LIVING WORLD

sized that the processes discussed are those that are essential to
life on the earth and also those that have served to make this
life continuous since it first appeared. Let us consider briefly
in the next few chapters the development of life on the earth
throughout the past geologic ages and note to some extent how
it has specialized into different forms so as to produce the great
variety of living creatures now existing or that have existed in
the past,

REFERENCES FOR MORE EXTENDED READING

PLUNKETT, CHARLES R.: " Outlines of Modern Biology," Henry Holt & Com-
pany, Inc., New York, 1931, Part I, Chaps. II, V; Part IV, Chaps. XVIII,
XX, XXIII.

The chapters referred to contain an excellent elementary discussion of the physical
nature of protoplasm, structure and characteristics of living cells, cell division,
reproduction, and heredity in multicellular animals.

WIEMAN, H. L.: "An Introduction to Vertebrate Embryology," McGraw-Hill
Book Company, Inc., New York, 1930, Chap. III.

In this chapter is given an excellent elementary account of the early development
of amphioxus and the frog.

SHARP, LESTER W.: "Introduction to Cytology," 3d ed., McGraw-Hill Book
Company, Inc., New York, 1934, Chaps. I, III, VIII, IX, X.

An excellent reference hook for the superior student who is especially interested in
cells and cell division.

WILSON, EDMUND B.: "The Cell in Development and Heredity," 3d ed., The
Macmillan Company, New York, 1925, Chaps. I, II.

A classical approach to detailed study of cells and their functional activities.

SPEMANN, HANS: "Embryonic Development and Induction," Yale University
Press, 1938.

A treatise on some of the most fundamental developments of modern embryology
by the most eminent living authority in this particular field. Recommended for gifted
students or those especially interested in the experimental approach to study of
embryonic development.

WEISS, PAUL.: "Principles of Development," Henry Holt & Company, Inc.,
New York, 1939.

This book is an advanced discussion of embryonic development. It has been
organized around the problems relating to development, and in connection with each
of these problems there is given the contributions of research which throw light on
their solution. The book represents a thorough analysis of the whole field of embryonic
growth and the summary of a large body of experimental data bearing on this field.

THE PATTERNS OF LIFE 165

Science, published by the Science Press, New York.

A weekly journal containing numerous articles of general scientific interest as well
as special articles in every field.

The Journal of Experimental Zoology, published by the Wistar Institute of
Anatomy and Biology, Philadelphia.

A monthly journal containing technical papers, especially articles dealing with
problems in the field of experimental embryology, heredity, and variation in animal
life.

6: DOWN TO THE SEA

Where Early Life Existed during the First Geologic Ages

IN THE year A.D. 79 the people of the flourishing city of
Pompeii were going about their luxurious and leisurely daily
life. Then in August the quiet and majestic volcano that towered
above the city began a violent quaking. Within a short time a
cloud of lava ash was pouring forth. Soon Pompeii and 2,000 of
its inhabitants were buried from the light of man. The people
and their complex civilization were soon forgotten, since the
memory of man is short. Their story became little more than a
legend as time passed and they lay buried within their lava-
covered city.

The city remained beneath this blanket of death for 1,700
years, and vineyards grew above its ruins. Then a peasant dis-

166

DOWN TO THE SEA 167

covered traces of its walls below his soil. During the eighteenth
and nineteenth centuries the ancient city was dug out. Many
of its houses and treasures have since been restored, and the dead
have been excavated. From these ruins a vivid picture of the life
and buildings of this ancient city is now reconstructed.

The records unearthed at Pompeii tell the story of a small
but prodigal group of the earth's population that inhabited this
favored spot for a few generations of human life. Other excava-
tions over large stretches of the earth have revealed a widespread
and extensive existence of once-living creatures. The span of
time involved in this buried past is not a few generations, as
was the case at Pompeii, but hundreds of millions of years.
The story of this past life is written in the fossils left in the rocks,
and these fossil rocks constitute the only documentary evidence
of life on the earth before man learned to write a few thousand
years ago. Their study "gives an insight into the origin and
development of the different species of life as well as some under-
standing of modern creatures, including man himself.

Slow Unfolding of a Mystery Story

The unraveling of the story of early life on earth has been a
slow and much- varied process. What we now know of this past is
nothing more than the discoveries of our ancestors and the
modern scientists, along with the explanations and deductions
that have been made from such findings. This understanding of
the earth's past is not some supernatural decree or some abstract
dogma. It is one of man's accomplishments, one of the fruits of
his labors. The different views that have been held of these past
conditions have changed many times. Until the rise of the early
Babylonian and Egyptian civilizations, mankind had little
insight into the meanihg of life and the relationships existing
between living creatures. Following these times, for many cen-
turies, only meager and usually fantastical speculations pre-
vailed. As observations of resemblances of living things and
particularly discoveries of imprints in rocks were made, they
were accounted for in some unusual manner or their existence
was entirely ignored.

From the earliest times, those who examined the strata of the
earth's rocks were surprised to find markings and remains of

168 THIS LIVING WORLD

plants and animals in them. Some regarded them as works of an
occult influence in nature to convey some hidden meaning or
lesson. Others believed them to have been produced by the
forces of evil to mislead and terrify mankind. Later is was held
that such figures were formed by vapors generated in the rocks
from fermentation. Even the pottery and urns sometimes found
in deep deposits were explained as having been produced by
the circular movements of these vapors as they escaped from
lower deposits.

Another explanation of these rock figures that was widely
accepted up to about A.D. 1750 was that they "grew" in the
rocks from "seeds" that were lifted from the sea (or sky) and
transplanted to distant lands by a supreme power. Such a
"seminal root" was thought a sufficient cause of these figures.
The final overthrow of this kind of belief was heralded by prob-
ably the greatest hoax in geologic literature. A German scientist
by the name of Johann Beringer taught geology at the University
of Wurzburg about 1730 and collected fossils in the chalk beds
near by. Some of his students prepared a number of artificial
"fossils" of various living and imaginary things, including some
Hebrew characters, and deposited them in the chalk beds. These,
were found of course, by the unsuspecting Beringer, who de-
scribed them extensively and reverently in a number of publica-
tions. The distressing climax was reached when one day he found a
fragment bearing his own name. Having finally discovered his
mistakes, Beringer attempted to recall and suppress his writings
on the subject, but the cruel and silly joke had reached propor-
tions beyond his control. Not only the professor but much of the
whole belief he represented was made ridiculous.

Gradually the true explanation of the origin of fossils became
established; namely, they are the actual remains of once-
living creatures. These creatures had been caught and preserved
in the sediments as the rocks were being formed either beneath
the sea or on land, depending upon whether the fossil was of
water or dry-land organisms. Of course, this implied that much
of the country now dry had in long ages past been beneath the
ocean or inland seas, a well-established fact at present. Such
ideas were first materially advanced by Leonardo da Vinci in
A.D. 1508; however, they were championed by only a few others

DOWN TO THE SEA

169

Fossils arc the remains in rocks of once living creatures. A fossil of an eurypterid,
Eusarcus scorpionis, found near Buffalo, New York. American Museum of Natural
History photograph.)

through the centuries until about 1800. At about this time such
able scientists as Baron Georges Cuvier and Jean Baptiste de
Lamarck in France and William Smith in England established
geology on a firm research and scientific basis. Since then many
great geologists in both America and Europe have added much
to our knowledge of the geologic past and life during those ages.

170 THIS LIVING WORLD

Geologic Eras^and Periods

Geologic history covers a long period of time. A kaleido-
scopic view of it adds only confusion. Fortunately, it may be
divided into eras and periods for study. In this respect it is com-
parable to the division of human history into eras, such as
ancient history, medieval history, modern history. These eras
in geologic times may be arranged chronologically, as is done in
recording human affairs. As such they help to classify our knowl-
edge and facilitate its study.

Here, then, are some divisions of time that may be new to
you. However, their names and approximate dates are worth
knowing in any consideration of life's ancient past. The Azoic,
Archeozoic, Proterozoic, Paleozoic, Mesozoic, and Cenozoic
eras are the great divisions of geologic time. Let us pause to
study these names. We shall meet them many times as we
proceed. They are chronologically arranged, the first being the
earliest. All the names have the same endings, "zoic." It is
derived from the Greek language and means life. The re-
mainder resolves itself into knowing the meanings and order of
the prefixes. A, the firts one, means "no"; therefore, Azoic is
no life. Archeo is "ancient" or "most ancient." Protero signifies
"former." Paleo refers to "old." Meso means "middle,"
and Ceno, "recent." We have them in order then : no life, ancient
life, former life, old life, middle life, and recent life.

Their dates are given in the accompanying chart. These dates
are based upon ages of certain rocks as determined by nature's
well-regulated time clock, that is, radioactive minerals found in
those rocks. Even though these dates are only approximate, their
order of magnitude is correct. The length of geologic time here
indicated is nearly two billion years. This is a figure that staggers
the imagination, even in these days when many billions of dollars
are the figures that represent the national debt. However, this
long history of the earth is attested to by all contemporary
geologists who have made studies of this age-old question.

The oldest rocks so far examined are some found in Russia.
They have an age of 1,850,000,000 years, and they are younger
rocks than those of the original earth's surface. Radioactive
mineral-bearing rocks belonging to each of the geologic eras have

DOWN TO THE SEA

171

CHART OF AGES

Millions
of
Years

. Eras

Epochs

.Ages of Life

RECENT

CENOZOIC

PLEISTOCENE

PLIOCENE
MIOCENE
OLIGOCENE
EOCENE
PALEOCENE

MAN
MAMMALS

CRETACEOUS

MESOZOIC

JURASSIC

REPTILES

200

TRIASSIC

PERMIAN

AMPHIBIANS

CARBONIFEROUS

COAL

PALEOZOIC

DEVONIAN
SILURIAN

FISH

ORDOVICIAN

CEPHALAPODS

500
IIOO
I600

CAMBRIAN

TRILOBITES

PROTEROZOIC

KEWEENAWAN
HURON I AN

PRIMATIVE
MARINE
LIFE

ARCHEOZOIC

GRENVILLE
DEPOSITS

OLDEST
KNOWN
LIFE

FORMATION

AZOIC

OF
EARTH'S CRUST

2000?

Chart of geologic time, showing divisions into eras and epochs. The ages of these divisions
have been determined by means of radioactive minerals in the rocks.

172 THIS LIVING WORLD

been discovered and dated. Thus, the time order of the different
eras is well established.

The divisions between eras are clear and distinct in most
cases. These divisions were determined by times of great move-
ments of the earth's crust, when mountains were being elevated
and inland seas were being obliterated. They were marked by
definite changes in living forms from one era to another, at least
during the latter eras; that is, the forms that were numerous and
widespread in one era diminish or disappear and gave way to new
forms that multiplied and flourished in the following era.

Let us consider briefly how a geologic era came to an end, and
a new one was ushered in, since the geographic and climatic
changes thus brought about affected life on the earth profoundly.

In the St. Lawrence area of North America there is an exten-
sive series of sedimentary rocks called the Grenville Series. In
some places they constitute masses of limestone that are esti-
mated to be as much as 50,000 feet thick, a thickness of limestone
that is unequaled anywhere else in the world, so far as is known.
Beneath the scattered remains of the Grenville strata is an
extensive complex of gneiss rocks known as the Laurentian
Gneiss which seems to form the basement for the entire region.
The age and structure of the Grenville and Laurentian forma-
tions show that toward the end of the Archeozoic era the Lauren-
tian formations were produced by an intrusion of liquid magma
which cooled beneath the overlying Grenville strata, at that
time evidently forming the bottom of an inland sea. This flow
of magma probably exceeded in magnitude any other flow the
earth has ever experienced. As a result, a great system of moun-
tains, the Laurentian Mountains, was elevated from the sea.
These mountains were rapidly eroded in the succeeding ages
until now they are almost completely obliterated.

This same process of extensive mountain building and subse-
quent rapid erosion is noticeable as having occurred in many
other parts of the earth at approximately the same geologic time.
The end of the Archeozoic era is considered as having been
brought about by an age of widespread mountain building,
usually referred to as the Laurentian revolution, followed by a
time of very active erosion. This period of extensive earth-crust
movements separates the Archeozoic from the Proterozoic era.

DOWN TO THE SEA 173

The end of the Proterozoic era was heralded by another time
of major earth-crust movements. In the United States these are
represented by the elevation of many sections, one of which was a
mountain range, known as the Killarney Mountains, that ex-
tended east and west through the Great Lakes region for perhaps
a thousand miles and with a width of at least a hundred miles.
This time of mountain building was followed by an interval of
perhaps millions of years, when extensive erosion leveled the high
places. The Killarney revolution and the subsequent period
of erosion represent the break between the Proterozoic and
Paleozoic eras.

Toward the end of the Paleozoic the earth witnessed another
long age of extensive mountain building. During this period the
great land area paralleling the present Atlantic Coast was thrust
westward and folded the strata of the Appalachian geosyncline to
the west into a majestic range of mountains, the Appalachians,
which extended from Nova Scotia to Alabama. The geosyncline
as well as much of the area west to the Mississippi River had
been beneath an inland sea at intervals during the Paleozoic, and
the Appalachian elevation raised this country permanently
above the sea. Elevations in other parts of the earth, too detailed
to be included here, occurred during the same period. This exten-
sive mountain building, known as the Appalachian revolu-
tion, did not occur suddenly and probably not violently, except
perhaps in small local areas. Rather it extended over tens of
millions of years. Simultaneous with the mountain building
rapid erosion was taking place. Much of the great height of the
original Appalachian Mountains was worn down. Eventually
the pronounced movements subsided in degree and the Mesozoic
era began.

The closing stages of the Mesozoic produced a series of exten-
sive crustal movements over the earth that were, without doubt,
the most pronounced in western United States. The great geo-
syncline extending from the Gulf of Mexico to Alaska was folded
and upthrusted on a wide scale to produce the Rocky Mountains
by a regional compression from the west. It is also probable that
volcanic activity was common throughout the entire western
part of the United States. This extensive crustal movement is
known as the Laramide revolution, sometimes popularly referred

174 THIS LIVING WORLD

to as the Rocky Mountain revolution. The climax of the Lara-
mide revolution determined the end of the Mesozoic and the
beginning of the Cenozoic eras. These earth disturbances con-
tinued with decreasing vigor, however, long into the Cenozoic.

The Cenozoic was an era of exceptional crustal movements.
The Rocky Mountains were extensively eroded and again
elevated to their present heights. The highest mountains now in
the United States, the Sierra Nevada, were formed. The Colorado
Plateau was slowly elevated, and the Colorado River cut the
Grand Canyon. The Himalaya and Alps were pushed up from
the sea bottoms. The era was brought to a close by the Cascade
revolution, which produced the Coastal Range Mountains along
the Pacific Coast, and the subsequent period of about a million
years, during which time four great glaciers covered much of
North America and Europe. Hence, the Cenozoic era may be
thought of as bounded by two great revolutions, the Laramide,
which started it, and the Cascade, which brought it to a close.

Such broad and pronounced changes in the earth's crust had
an enormous effect upon living creatures. Old habitats were
obliterated or so completely changed that creatures specifically
adapted to them could not endure. They had to give way to less
specialized forms that found the new environments suitable.
Such forms may have been insignificant in both size and number
under the previous conditions, but in the changed environment
these forms increased in number and in complexity of body struc-
ture until they became the most important in the new geologic
era. These emergent forms in turn gave way when the curtain
rang down at the end of the era of conditions to which they had
become adapted. Many specialized types of creatures have arisen
in the past to flourish for long intervals of time and have now
become entirely extinct. In general, these changes occurred with
the change of geologic eras.

Changes within Geologic Eras

In addition to the widespread and pronounced changes of the
earth's surface toward the end of a geologic era, other less exten-
sive changes have occurred within the geologic eras. These less
extensive changes have had decided but usually not extreme
effects upon living creatures. It is possible, therefore, to divide

DOWN TO THE SEA 175

the geologic eras into shorter divisions on the basis of the less
extensive changes. These shorter units are called epochs. For
example, the Paleozoic era is divided into the Cambrian,
Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian,
and Permian epochs. The Mesozoic era is divided into the
Triassic, Jurassic, and Cretaceous epochs. The Cenozoic era
consists of the Paleocene, Eocene, Oligocene, Miocene, Pliocene,
and Pleistocene epochs. During the different epochs one form of
life gradually developed new forms in a rather continuous
process. However, there was no great decay or disappearance of
the earlier existing forms, as in the case of change from one era
to the next.

The above list of geologic epochs constitutes quite an array
of names. There are a few of them that should be learned by the
reader who wishes to obtain even a general picture of developing
life on the earth. Without doubt one of these is the Cambrian. It
is the first epoch of the Paleozoic era, and began about 600 mil-
lion years ago. It is one of the great mileposts in geologic history.
Other epochs worth knowing, along with their place in the order
of this scheme of classification, are the Ordovician, Devonian,
Triassic, Jurassic, Miocene, Pliocene, and Pleistocene.

One illustration will serve to show how these epochs were de-
termined and how they tended to influence life on the earth.
Notice, for example, the Mesozoic era. It consisted of the Triassic,
Jurassic, and Cretaceous epochs. The rocks that immediately
overlie the great coal measures of Great Britain, which were
formed during the latter part of the Paleozoic era, are thick beds
of red sandstones. This same layer of red sandstones can be
traced into the European continent where, in Germany, it be-
comes divided by a layer of limestone. Thus, three layers are
recognized, an upper and lower layer of red sandstone and a mid-
dle layer of limestone. Because of this threefold character, the
name Trias was applied to the series, and the word Triassic
eventually came to designate the epoch when these formations
were laid down.

In Southwestern United States the Triassic rqpks are found
widely distributed. Here the red sandstones predominate. How-
ever, they are often found with thinner layers of shales and
gypsum intermingled, producing now some of our most colorful

176 THIS LIVING WORLD

landscapes, such as the Painted Desert in Arizona. The physical
features of these rocks show that they were laid down, for the
most part, in a warm and arid climate. The sand was deposited
when the land was above water, while the shales and gypsum
were deposited in fluctuating areas of fresh-water swamps or
marine lagoons. In Eastern United States the rocks of the
Triassic epoch are more complex. They show in general, however,
that most of the area was above water and that considerable
sections were lowlands that repeatedly dried between infrequent
rains.

It is reasonable to assume that during the Triassic the climate
was relatively warm and arid or semiarid over great parts of the
land. Much of the United States was above the sea, but some
areas were lowlands or even swamps. Such conditions of mild
climate and extensive land areas facilitated the development of
land animals of the cold-blooded type. This epoch saw the rapid
rise of the reptiles. The previous disappearance of the shallow
seas of the Paleozoic era brought about the decline or oblitera-
tion of some of the older marine forms, and once the reptiles
had become established on land in the Triassic they began to
invade the shallower seas also.

Following these times great inland seas began gradually to
spread over Europe and large areas of Western North America.
This marked the beginning of the Jurassic epoch. For most of
North America it was a time of generkl degredation of land
areas with an evident continuance of mild climates. Inland seas
from Alaska pushed in over what had been arid regions of Utah,
Wyoming, Montana, and the Dakotas, forming what is known
as the Sundance Sea. About the middle of the Jurassic epoch
this sea began to recede and formed great plains or swamps in
this area. It became the scene of luxuriant plant growth and
extensive animal habitation. Sluggish streams must have flowed
across the area, and their deposits buried the remains of the
largest of all American dinosaurs.

These formations as well as many others, in different parts of
the earth, belonging to the Jurassic epoch indicate that the
climate generally was warm and humid. Evidences show that
subtropical climates existed over much of the United States and
over Europe. This condition favored wide multiplication and

DOWN TO THE SEA

177

*'fi*'^^iW7'A
1.^'",|?,,si"f1 f ''•'' a/' '''"'"

i i-iut ir/;"L|ii Ji i rJ"TJ '^^ 't - 'f i

^^Sii'V^^.iv/'V

"Thundersauran", a thirty-ton dinosaur that inhabited the swamp lands of Southwestern
United States during the Jurassic epoch. (Science Service photograph of drawing by
George F. Mason, American Museum of Natural History.)

distribution of cold-blooded animals, particularly the dinosaurs.
They ranged over the plains as far north as Montana, and in
Asia their remains have been found over most of Mongolia.

The end of the Jurassic epoch was marked by local mountain
building in some areas and by a broad increase of inland seas in
Western North America. There was an upflow of basic lava
along the west coast of Canada and also along the eastern part
of California, where the Sierra Mountains now exist. Following
this a great stretch of land, which now constitutes the Rocky

178 THIS LIVING WORLD

Mountains, began to sink, forming there a great geosyncline.
A broad inland sea overflowed the geosyncline. It eventually
extended from the Gulf of Mexico to Alaska and completely
divided the North American continent into two land areas.
The Cretaceous epoch, the last one in the Mesozoic era, had
begun.

This sea evidently overflowed the land very gradually rather
than by any sudden movement of the earth's crust. Then it
began gradually to recede, forming first shallow, connected seas,
then swamp areas, and finally low-lying plains. In many of the
shallower seas great deposits of chalk beds were laid down.
These are often rich in fossils of great marine reptiles, diving
birds, and flying reptiles, which were in abundance at that time.
These beds repeat themselves in many places in the United
States and in Europe and other continents. Dinosaurs continued
to inhabit the lowland areas, as indicated by their fossils. The
climate was mild and humid, perhaps a little less so, however,
than during the preceding Jurassic epoch. Tropical and tem-
perate-zone plants grew as far north as Alaska, and many of the
coal beds of Western North America were formed by luxuriant
plant growth in the lowlands during this epoch.

The Cretaceous epoch came to an end with the great dis-
turbances which elevated the Rocky Mountains from the inland
seas, as well as similar crustal movements in other parts of the
earth. Most of the inland seas of North America were obliterated
and the continent took on much the boundary it has at present.
This was, of course, also the end of the Mesozoic era. The clim-
ates of the earth became much colder, glaciers appeared in
many areas. With such changes in climate and alterations of
land and sea environments, the great dinosaurs were obliterated
and many other forms of life profoundly affected.

Pre-Cambrian Life

The play "Victoria Regina" had a long and successful run in
New York City theaters in recent years. The remarkable thing
about this play was the excellent portrayal by the play's leading
actress of the entire life of England's great queen. Each period in
the queen's long and eventful life, from the time of the slim
girl's coronation to her last days as a plump old empress, was

DOWN TO THE SEA 179

i layer of limestone of the Proterozoic era in the Grand Canyon showing presence of
colonies of fossil algae. (Science Service photograph.)

vividly and artistically revealed by this noted actress. This is
unusual, for in most plays or novels embodying a long stretch
of time, great gaps are only vaguely hinted at or left entirely to
the imagination. This latter procedure is forced upon us in
considering life's early events upon the earth. It is mainly be-
cause there were no great actors to play the leading roles.

Over half of the long span of life on the earth had passed
before the beginning of the Cambrian epoch. The Archeozoic and
Proterozoic eras, not to mention the Eozoic, we see only in dim
outlines. Thus nearly a billion years had elapsed before the
beginning of Cambrian time. This stretch of geologic time is
designated Pre-Cambrian, in somewhat the same manner as the
historian uses the expression "before Christ." The history of
life on the earth since the Cambrian epoch can be pieced together
in such detail as to give a rather continuous picture. Before this
time this is not possible. Therefore, the beginning of the Cam-
brian epOch is an important geologic date.

The search for evidences of life during Pre-Cambrian times
must necessarily be made in the rocks formed in those far-off
ages. Such rocks that have come down to us in unaltered condi-
tion are found at the bottom of the Grand Canyon and in other

180 THIS LIVING WORLD

Cryptoioon fossil formations in limestone; showing one of the earliest forms of life.
(American Museum of Natural History photograph.)

parts of the earth. The Colorado River has cut its way through
the overlying formations and, some 0,000 feet beneath the
surface, has eroded the present gorge into the Pre-Cambrian
sediments. In these sedimentary rocks are found abundant
deposits of algae. These were lowly, microscopic plants that
grew in colonies, over which they deposited calcium carbonate.
They built up, therefore, masses of limestone deposits, some-
what hemispherical in shape, which consisted of one layer upon
another. There are in the seas today certain varieties of algae
which secrete limestone and build up similar "cabbage-head"
masses of this deposit. It is reasonable to assume that the Grand
Canyon deposits were formed by such microscopic plants.
Similar rocks of Pre-Cambrian origin have been found in
Montana, Michigan, and the Hudson Bay regions. These micro-
scopic plants were evidently widespread in the early seas.

Some of the Pre-Cambrian deposits of France have yielded
radiolaria. These are a form of one-celled animal that secrete
delicate mineral layers around their bodies, and there are many
varieties that may be seen in present-day waters: Several
different forms of sponges have been found in the Grand Canyon
and other Pre-Cambrian deposits. These were microscopic
animals that lived in colonies, perhaps somewhat as sponges do
today.

DOWN TO THE SEA 181

Some of the Pre-Cambrian formations contain graphite be-
tween layers of sedimentary rocks. This is a pure form of carbon,
familiar to most people as "lead" in pencils. These streaks of
carbon are now known to have been organic in origin and were
deposited from the bodies of once-living microscopic plants and
animals. The only explanation of this graphite is that such
creatures must have been swarming in the Pre-Cambrian seas.
As their bodies disintegrated the carbon, which is an essential
component of -all living tissue, was deposited in these layers.

Pre-Cambrian rocks of Montana have yielded evidence that
wormlike creatures existed in the seas of those times. This con-
sists of trails and burrows found in the Pre-Cambrian shales and
sandstones. These burrows were most likely made on the sea
bottom by some kind of worms crawling through the mud or
sand much the same as sand worms do today, as may be observed
at the seashore or in a sea-water aquarium. The soft bodies of
these creatures left no fossils, but their burrows in quiet sedi-
ments left holes as the sediments eventually formed rocks. This
lowly creature represents the highest form of Pre-Cambrian life
known at present.

It may be said, then, that there existed a somewhat varied
and extensive sea life near the end of the Proterozoic era. How-
ever, this life was simple in form and consisted of microscopic
plants and animals, and sea worms. This is very much in con-
trast to conditions existing at the very beginning of the Cam-
brian epoch. Then there appeared a great abundance of sea
animals with shells, some of them quite complex in their
physical organization. Why there should be such a scarcity of
more highly developed forms in Pre-Cambrian times and such a
profusion of them in the following epoch has been one of the
great questions of historical geology.

One of the explanations offered is that Pre-Cambrian rocks
have been metamorphosed since they were laid down and all
fossils in them destroyed by the intense heat. However, not all
Pre-Cambrian rocks have been thus heated. Still no shell fossils
are found. Another explanation is that relatively complex forms
of aquatic life had developed during Pre-Cambrian times, but
that none of them had shells and hence left no remains. It is
beyond the scope of this discussion to consider the possible

182 THIS LIVING WORLD

causes for sea invertebrate animals not growing shells at that
time as they did during the Cambrian Period and have at all
times since. However, when all things are considered it seems
likely that all Pre-Cambrian animals were motile creatures and
without any skeletal parts of limestone composition. Few fossils
are likely,' therefore, to have been formed. It seems probable
that our understanding of Pre-Cambrian life forms will always
be rather meager.

Invertebrate Surge

During the Cambrian epoch occurred the first great develop-
ment and diversification of invertebrate life in the sea. These
backboneless creatures bearing shells have left a profusion of
fossils widely scattered in the rocks formed in the Paleozoic era.
Something like 1,200 different species of fossil invertebrates that
existed in North American seas in Cambrian times are now known
to paleontologists. Including fossils from the rest of the Paleo-
zoic era, we increase the number of known fossil species by many
thousands.

The surge of invertebrate creatures that began in this early
geologic age has rapidly increased until the present time. Now
there are some 600,000 recorded living species, a great majority
of which are the insects. In addition to these, there are some
60,000 extinct fossil species now known to man. Of the known
living species about 40,000 now live in the sea, whereas all the
early creatures existed there.

Many of the fundamental kinds of invertebrates found in the
sea today started in the far-off Paleozoic era. It is true that these
Paleozoic creatures were primitive kinds of ancestors to modern
species; however, the similarities of the main groups of modern
invertebrates to these early forms can be noticed. Today the seas
swarm with many thousands of invertebrate animals. Some of
them seem very strange to persons not familiar with sea life.
The remarkable thing is that they not only exist at present, but
that their ancestry can be traced back hundreds of millions of
years.

One group of creatures whose fossils are found in Paleozoic
rocks are the coelenterates. Modern forms of the coelenterates
include the corals, jellyfish, the hydra, and sea anemones. They

DOWN TO THE SEA

183

Imprint of a jellyfish left during the Cambrian epoch in beach sand which later hardened
into a slab of rock, in the Grand Canyon, Arizona. (Science Service photograph.)

are to be found widely scattered in the seas or fresh waters today.
A well-preserved imprint of a jellyfish, one of the most fragile of
animals, has been found in the early Cambrian rocks exposed in
the Grand Canyon. This record thus establishes the long lineage
of the jellyfish, even though it is a record that would hardly be
expected to be left. The group of animals to which the hydra
belong are first represented as fossils from the early Paleozoic.
These are imprints of extinct forms, known as graptolites, on
black shale rocks which have been found in many parts of the
earth, and they must have been world-wide in their distribution.
Sea anemones, having no hard parts in their bodies, have left no
recognizable fossils, but their nearest of kin, the corals, provide
an abundance of records. Fossils of free-swimming corals date
back to the early Paleozoic era. True corals began to build reefs
from their limestone deposits during the Devonian epoch. This
process has continued to the present time.

A distinguishing feature of the coelenterates is that the body
is composed of two layers of tissue, an outer ectoderm and an

184

THIS LIVING WORLD

inner lining, the endoderm. The body is usually in the form of a
simple sac with a single opening at one end, forming the mouth as

Mouth
Body sac
Endoderm
Ectoderm

POLYP MEDUSA

The polyp and medusa have similar body patterns. (Redrawn from Buchsbaum "Animals

Without Backbones.")

well as exit for waste materials. Most of the coelenterates are
armed with stinging cells and oval capsules filled with a poisonous
fluid and containing a long hollow thread, the outer end of which
contains a bristle-like spine. When the animal is disturbed the
poison cell contracts, causing the thread to be shot out with
considerable force so as to pierce whatever conies in contact with
it. The poisonous fluid then flows down the hollow thread into
the body of the captive. This may paralyze or kill some creatures.
The animal feeds on its victims by drawing them down into its
body sac, with tentacles in the case of the sea anemone, or by
wrapping its body sac around the food as in the case of the
jellyfish.

Two main body types are found among these simple and
primitive animals. One is the free-swimming or medusa type
typified by the jellyfish and one stage in the life cycle of many
hydra. The other type has the body sac drawn out to a stem, the
lower end of which is attached to some anchor. This is typical of
the polyps, a common variety being the hydra that may be
easily seen with a magnifying glass in most fresh-water streams
and ponds. The sea anemone, or marine "flower animal," be-
longs to the polyps. These animals have a basal disk or foot, with
which they attach themselves to some rock or solid object.
They have a stout, muscular body and many tenacles surround-
ing the mouth. Different species have a variety of bright
colors, and they constitute an attractive part of the marine
fauna.

DOWN TO THE SEA 185

ConiD jeines are transparent echinoderms that occur in the surface waters of the sea,
mostly near the shore. They are noted for the beauty of their daytime iridescence which is
produced by the tiny rows of combs along the sides of the body refracting light.

The coral polyps are specialized types which have the ability
to secrete lime around and throughout their delicate bodies.
When the animal dies this lime formation remains behind at-
tached to its original base. This ability of these minute animals
to secrete lime has added thousands of square miles to the land
surface of the earth throughout the geologic past. Southern
Florida and many of the low islands of the Pacific, for example,
were originally coral reefs that resulted from such activity.

Another group of animals that originated during the Paleo-
zoic era were the echinoderms. Their descendants are also com-
mon in the seas today, in such forms as starfish, sea urchins, sand
dollars, cucumbers, comb jellies and crinoids. They are animals
which are radially symmetrical. They usually have a set of tubes
which radiate from a large body cavity and which carry water.
This circulating water serves for breathing and to operate the
tube feet that are frequently used for locomotion. Their skin is
strengthened by a deposit of calcium carbonate, usually in the
form of rods and plates.

Since these animals have deposits of limestone in their bodies,
they have left an extensive fossil record. The earliest fossils are
those of ancestral crinoids, which date back to the very begin-

186

THIS LIVING WORLD

ning of the Paleozoic era. Later in the era the true crinoids ap-
peared, and varieties of them have lived in the sea to the present.

Fossils of Paleozoic crinoids
show them to have been plated
and spiny-skinned animals with
many arms radiating from the
larger body cavity somewhat
like the petals of a flower. The
lower part of the body secreted
a limestone stalk that attached
the animal to rocks on the sea
bottom. These feather stars, as
they are called, could produce a
wave-like motion by a slight
bending of the stem, and they
secured their food from the
water that came in contact with
their bodies. They are some-
limes called sea lilies, so much
do they resemble in general out-
line this delicate and glorified
flower.

The starfish is another ani-
mal with a long family history as
well as widespread modern pro-
geny. Early Paleozoic rocks
show fossils of starfish, their
fossils are numerous in later
marine formations, and, as is
generally known, starfish are to
be found in most seas today. The earlier forms are now extinct;
however, these fossils bear such likeness to modern species that
the lines of development are clear. Starfishes usually have five
arms that radiate symmetrically from the body. These arms are
supplied on the underneath side with muscular tubes that end in
suckers. These tubes are filled with water from a water- vascular
system. By a sort of pumping of the water through the system
the animal is able to use these tubular feet for locomotion. The
starfish has the power of regenerating lost parts; that is, it may

Fossil crinoid or sea lily. (American
Museum of Natural History photograph.)

DOWN TO THE SEA 187

The starfish is a radially symmetrical animal, usually having five arms. The underneath side of
the arms are supplied with muscular tubes* that end with suckers.

grow new arms, new tube feet, or even a new stomach, if any or
all of these are lost by its method of feeding or by being chopped
off by man.

The Paleozoic era saw the development of another group of
invertebrates that were destined to become in modern times the
most numerous multicelled animals on earth. These are the
arthropods, which include the modern forms of insects, spiders,
scorpions, centipedes, shrimp, lobsters, and crabs. In the
Cambrian rocks are found fossils of segmented animals known as
eurypterids or "sea scorpions." They apparently lived in the sea
and flourished for several hundred millions of years. At their
zenith they grew to a length of ten feet. However, they declined
toward the middle Paleozoic and became extinct before the end
of the era. Before disappearing it is believed that they gave rise
to the true scorpions and spiders, land animals that have con-
tinued to the present.

Insects appeared on the land during the latter part of the
Paleozoic era. Some of them developed rapidly into remarkable
size. In the coal measures of Belgium has been found a fossil of a
dragon fly with a wing spread of twenty -nine inches. It is the
largest insect known to have lived on the earth. It is fortunate for
man and the other vertebrates that later insects did not develop
into large forms. If so, this might have been an invertebrate

188

THIS LIVING WORLD

The slant crab of Japan, shown above on permanent exhibit at the Buffalo Museum of
Science, reaches eleven feet between its claws. (Science Service photograph.)

world. Cockroaches very much like modern ones were exceed-
ingly numerous during the late Paleozoic. Some of them were
three and four inches long, although most of them were smaller.
During the Mesozoic era insects became widespread on the land.
Since then they have continued to increase in both species and
numbers until today there are over a half million living species
on the earth.

The shrimps, crabs, and lobsters first made their appearance
during the Mesozoic era. Their fossils have been found in rocks
as early as the Jurassic epoch, and these animals are extensive
in the seas and fresh water at present.

Animals Bearing Shells

A group of animals which exist in relatively small numbers
in the sea today but which have ha4 a long and prominent history
during past geologic ages are the brachiopods. They made their
first appearance in the Cambrian seas, and before the epoch was
ended they constituted about one-third of the Cambrian faunas.
They reached their climax of development during the Devonian
epoch and for hundreds of millions of years they were an exten-
sive form of life in the sea. Their fossils are so extensive and their
forms so characteristic of different geologic epochs that they have
long been the favorites of geologists in correlating and determin-
ing the age of the rocks of sedimentary formations.

DOWN TO THE SEA

189

brachiopods made their first appearance in Cambrian seas and still persist in the sea today.
The Terebratella, pictured above, is found off the coast of Japan.

The brachiopod is an animal which is enclosed in a bivalved
shell. The two units of the shell are quite different, however, from
those of the clam or oyster. The upper shell is somewhat smaller
than the lower part, and the two are hinged together at one end
rather than on the side, as is the case with the clamshell. At tjie
rear end of the lower shell there is a sort of upturned opening
through which a fleshy "foot" or stalk projects. The animal
attaches itself to rocks or other objects in the sea by means of
this stalk. The two halves of the shell may be opened quite
widely at the front. Anatomically, the brachiopod consists of a
skin which lines the shell, digestive and circulatory systems, and
two spirally coiled ridges or " arms," in addition to the body stalk
which protrudes through the shell. The spiraled ridges are really
a pair of gills which have on them rows of hair-like tentacles. The
waving of these tentacles sweeps minute organisms toward the
mouth, which is located between the arms. Thus the two "arms "
serve for breathing and entrapping microscopic plants and
organisms for food rather than as an aid in locomotion, as

THIS LIVING WORLD

Shell Muscles for closing shells

Shell

Gills

BRACHIOPOD CLAM

was suspected by the first person to describe and name the
animals.

One remarkable brachiopod is the lingula. In this creature
the shells are held together only by muscles; there is no hinge.
The stalk is usually long and passes out between the shells. Its
fossils are found in rocks as old as the early Paleozoic era. Such
fossils show that the lingula of those days were almost identical
in form to the lingula that still lives in the seas at present. For
600 million years they lived in the sea with their ways and
characteristics pretty much unchanged. They exist at the bottom
of the sea in deep water and live permanently attached to the
sea bottom. The long unchanging persistence of the lingula sug-
gests that living forms may exist without any evolution taking
place and that such forms may long endure after more highly
developed forms have passed away. There are many examples
of this in addition to the lingula. In general, they are forms that
live in an environment that is least subject to change as geologic
time goes on.

One other group of invertebrates that became exceedingly
numerous in the Paleozoic seas were the mollusks. These are
animals with soft muscular bodies, most of which have solid,
limy, external shells. Modern forms include the clams, oysters,
snails, periwinkles, squids, cuttlefish, and octopuses. The oldest
members of the mollusk group are represented by fossils which
appear in the early Cambrian rocks. Such fossils are the shells of
small snails. They had a shell that was spiraled into a small cone.
These animals increased in number and variety as geologic time

DOWN TO THE SEA

Land snails, showing their method of locomotion. (Science Service photograph.)

went on. Not only are they found today in the sea, but other
species are found in fresh water and in damp land areas.

Fossils of other mollusks that are almost as old as the snails
are those of the cephalopods. The cephalopods are a group of
animals that have had a remarkable geologic history and that
justify some acquaintance on the part of anyone interested in
the earth's present varied life forms. The cephalopods not only
were among the first of the mollusks to appear in the Cambrian
seas, but also were for millions of years the most aggressive of
all invertebrate creatures. Today they include -the largest of
invertebrate animals. The most numerous of modern cephalo-
pods are the squids and octopuses, mollusks which no longer
grow shells. However, there are a few species in the sea today
which have shells and are known as nautilids. It was these
shell-bearing types that have made cephalopod history, as well
as having inspired Oliver Wendell Holmes to write one of our
literary classics, "The Chambered Nautilus."

The shells of the cephalopods are cone-shaped and are divided
into chambers. The animal built a new chamber at the end of
the cone as it grew larger. After the group became established
they increased not only in number but also in size. Fossil shells
have been found in middle Paleozoic rocks that are fifteen feet
long, a length never reached again by any shelled invertebrate.
Later the shells began to be coiled, much as is found in modern
nautilids. These coiled cephalopods reached their greatest de-
velopment in the ammonites. These were large animals that

192

THIS LIVING WORLD

Chambered Nautilus, sectioned to show shell structure and animal living in outer chamber.
(American Museum of Natural History photograph.)

developed during the middle Paleozoic and which lived on in
large numbers far into the Mesozoic era. During this time they
must have been the ruling creatures of invertebrate life in the
sea for a hundred million years.

The highest degree of development of the present cephalo-
pods is represented by the squid and octopus. Both of these
animals are characterized by an almost complete absence of the
original shell so typical of other mollusks. Instead they have
developed a strong muscular body entirely devoid of skeletal
parts and usually covered with a tough skin. These animals are
usually vigorous and aggressive. In the matter of size they
include not only the largest mollusks but also the largest living
invertebrate animals. While a great many of the species are
relatively small, a few attain enormous sizes.

One of the largest squids is the giant squid that lives in the
North Atlantic. Sometimes it grows to a total length of over
forty feet. One such specimen was recently stranded on the coast
of Yorkshire, England. The largest octopus is the giant octopus

DOWN TO THE SEA

193

Octopus, showing tubular "suckers" along arms, also muscular coiling of arms. (Photograph
by Karger, Pix Publishing Company.)

of the Pacific. It grows to a total arm span of about thirty feet.
However, the type found in American oceans rarely exceeds a
ten-foot span. These animals are popularly thought to be fero-
cious, and current stories include their attacks on man. Such
attacks are usually limited to their onslaughts on fishing boats
when they have been netted or otherwise disturbed. Their usual
method of resistance to man or large sea animals is flight behind
a murky ink cloud, which they secrete from ink sacs, rather than
a vicious killing attack.

Dominant Groups

In the Paleozoic era there occurs for the first time a phenome-
non that was to continue in some form or other throughout

194 THIS LIVING WORLD

remaining geologic history; that is, a group of animals came into
a position of dominance over the rest of living creatures. This
may have resulted from certain animals being more highly
developed than others or by their existing in much greater
numbers.

The first group of creatures to occupy this position were the
trilobites. They swarmed in the oceans in great numbers during
the Cambrian epoch. They were, as might be said, masters of the
seas, at least of the remainder of life in the sea. They have left
their shells widely scattered over surfaces that were beneath the
sea in Cambrian times. Wherever man has access to such rocks
he finds their fossils. The Cambrian epoch is thus referred to as
the Age of Trilobites.

The trilobites were curious-looking, segmented animals with
their bodies divided into three longitudinal lobes and flattened.
The head was covered by a large shield. There was a set of com-
pound eyes, the lenses of which sometimes reached the great
number of 30,000 in the two great eyes of the head. The rest of
the body had an upper shell, which was segmented, the last few
segments often being cemented together to form a tail shield.

These segmented creatures were the most primitive type of
jointed animals. Their dominance lasted for a period of about one
hundred million years, which is quite an impressive record. They
declined after the close of the Cambrian, and became entirely
extinct before the end of the Paleozoic era. While it is believed
that they gave rise to no other group of animals, they were most
like certain arthropods that appeared much later, such as the
shrimps, crabs, and lobsters. These are animals with segmented
bodies and an outer, segmented skeleton which is shed periodi-
cally as the animal grows larger, characteristics that were com-
mon to the trilobites.

The Ordovician epoch which followed the Cambrian wit-
nessed the development of another group of creatures that
exceeded the trilobites and became the dominant form of
invertebrate life. These were the cephalopods. The earlier types
developed into the ammonites; these grew large coiled shells,
many fossils of which are three feet or more in diameter. The
ammonites must have been ferocious-looking creatures. They
had a mouth surrounded by many tentacles and two large

DOWN TO THE SEA

195

The ammonite lived in the end of its coiled shell. These ancient nautilids probably fed on
the trilobites, as well as other invertebrates of the Paleozoic and Mesozoic seas.

compound eyes. While we would not ordinarily think of them as
building mansions, they did grow compartment shells. The
ammonite lived in the end of its shell, building a larger compart-
ment as its body size increased and leaving the outgrown part
attached behind. It could probably sink or float by forcing water
in or out of these unoccupied compartments on much the same
principle as is now used in our submarines.

The earlier cephalopods and later ammonites probably fed
upon the trilobites, as they were no doubt aggressive, carnivo-
rous animals. This would explain in part why the dominance of
the trilobites vanished. A more highly developed creature had
evolved. The struggle for existence in the ancient seas was
quickened. Those creatures best fitted to survive increased in
numbers and size while the less fortunate ones were reduced to
a position of unimportance or became extinct. Such is the way
of life.

Since the time of the trilobites and ammonites many different
forms of life have developed to dominate the earth for a time.
These have in turn given way to higher forms in later geologic
periods. Today that position is occupied by the highest form of
life ever to develop on the earth. That species is called Homo
sapiens, wise man.

196 THIS LIVING WORLD

Divergence

The variety of living forms was becoming much more ex-
tensive in the seas of the middle and latter epochs of the Paleo-
zoic era. Fossils numbering hundreds of thousand have been
found. These show a great number of different kinds of creatures.
Also, they show that some of them were more alike than others.
There were different kinds of mollusks, different kinds of corals,
different kinds of fish. Thus relationships and kinships may be
traced. Life was diverging into different species, families, and
classes.

When there are a large number of different forms to be con-
sidered, it is necessary to classify them into groups for purposes
of study. In biological classification the creatures that are most
alike in minute details belong to the same species. Thus the
domesticated cat is one species of the group of cats. The species
that are most nearly alike are placed together in larger groups,
called genera; that is, the domesticated cats, the tigers, the
lions, and the leopards belong to the same genus. Similar genera
make up a family, and all living cats, including lions, lynx, etc.,
belong to the cat family. Likewise, families that are most similar
are arranged in classes. Thus the cat family and all other families
of animals that nurse their young from mammary glands belong
to the mammal class. Classes which have relatively close re-
semblances are grouped into phyla. All mammals, birds, and
reptiles belong to the vertebrate phylum. Similar phyla con-
stitute kingdoms; that is, all animal life belongs to the animal
kingdom.

As an example to show how this works out in naming an
animal according to such a classification, consider the domestic
cat. The species is domestica, and the genus is Felis. Therefore,
it is classified Felis domestica. On the other hand, the Rocky
Mountain lion, belonging to the same genus but of different
species, is Felis oregonensis.

This divergence and classification may be represented by a
branching chart which illustrates the relationship of all living
plants and animals. In some cases the relationship is close; in
other cases it is remote, the same as shown by the small branches
or the large limbs on such a diagram.

DOWN TO THE SEA

197

vou

FIRST LIVING CELLS.

A branching chart showing the important divisions and relationships of living creatures.

For example, the first great divergence of life led to the plant
and animal kingdoms. Since that time almost all living creatures
on earth have the characteristics of either plants or animals. In
the animal kingdom a later divergence produced the vertebrates

198 THIS LIVING WORLD

as separated from the invertebrate animals. Within the verte-
brates, divergence and change have produced the reptiles, birds,
and mammals. Likewise, within the mammal class there are
such divergent creatures as mice, pigs, deer, apes, and men.

Early Vertebrate Life

During the Ordovician epoch, or possibly somewhat earlier,
there occurred an event of vast importance to life, at least to
man. This was the development of backboned creatures, or
animals with an internal jointed skeleton. This internal jointed
skeleton distinguishes all the animals called " vertebrates " from
all the others, called <k in vertebrates." It is to the vertebrate
group that man and all the other higher animals belong. Of all
the forms of body structure which have been developed since
life began, the vertebrate is the type which is representative of
the most complex physical and mental organization.

There is still some uncertainty about just how the backbone
was first developed. Fossils of the earliest vertebrates are so
scarce that they fail to give any definite clue as to how they
originated. The best connecting link between the vertebrates
and invertebrates is provided by a few primitive animals living
today, classified as the "chordates." These include the acorn
worm, tunicates, and amphioxus. They possess certain vertebrate
characteristics that are found nowhere else in invertebrate
animals. Particularly is this true of the amphioxus.

The amphioxus is a little animal about three inches long that
lives in shallow marine waters in all parts of the world. The
most distinctive vertebrate characteristic it has is a cartilage-
like rod, called the notochord, which runs the length of the body.
This, of course, is not a backbone, but it is a stout, flexible axis
to which muscles are attached to give support and strength to
the body. The notochord is always present in the embryos of
true vertebrates, including man. In vertebrate embryos a noto-
chord develops first, then is gradually displaced by the jointed
backbone that grows around it. However, in amphioxus and
other lower chordates, development stops with the notochord.
Another distinctive characteristic of the amphioxus is a tubular
nerve cord running the length of the body above the notochord.
This is the type and position of the nerve cord found in the

DOWN TO THE SEA 199

Notochord Nerve cord

Mouth Gill slits

The amphioxus has a notochord running the length of its body and a nerve cord above the

notochord.

embryos of all vertebrates. Furthermore, the amphioxus pos-
sesses gills that are similar to the aquatic vertebrates, and it has
the rudiments of a vertebrate skin.

The amphioxus must not be considered the direct ancestor
to the vertebrates. It is a specialized ancient type that has lived
on to the present. However, in descending the ladder of the
modern vertebrates as far as possible, it is found that the above-
mentioned vertebrate characteristics find their rudimentary
expressions in these animals. They probably are somewhat simi-
lar to the first creatures developing a backbone. Unfortunately,
fossil records available at present do not show the actual first
steps in the development of backboned creatures.

The first vertebrate fossils are found in rocks dating back to
the Ordovician epoch. They were bizarre types which show only
a prophecy of modern fishes. The skin was covered with large
bony scales, and armor plates covered the head. These oldest
known vertebrates were entirely devoid of jaws, the mouth con-
sisting of round openings or a crosswise slit. These "shell-
skinned" creatures lived in the seas for about sixty million years
but finally became extinct about the middle of the Paleozoic era.
However, it is possible that they gave rise to a group of fishes that
have endured to the present. These are the jawless fish, which
resemble in many respects this ancient armored type. Perhaps
the best known of modern jawless fishes is the lamprey eel.

The prophecy of true vertebrates was fulfilled in the Devo-
nian epoch, when the "bony fishes" appeared in great numbers,
establishing the ancestors not only to modern fish but also to all
land vertebrates. These fishes with the fundamental pattern of
vertebrate skeletons quickly divided into the so-called ray-
finned type, which comprise most fishes of today, and into a

200 THIS LIVING WORLD

lobe-finned (or fleshy-finned) type, which gave rise to later land
vertebrates and modern lung fish. The waters of the rivers,
lakes, and seas were swarming with fish of many different kinds
during the latter part of the Devonian epoch, as evidenced by
the profusion of fossils they have left. This is so much the case
that this epoch is referred to as the Age of Fishes.

Vertebrate Migration to the Land

"As helpless as a fish out of water" is a homely saying that
unconciously reveals the difficult but momentous move that
vertebrate life made in first coming out of water onto Land. Back
in Devonian times many kinds of fish developed a sort of primi-
tive lung or air sac in the throat and upper chest region. Thus,
they became the first lung fish. This sac was apparently supplied
with blood vessels, which could absorb some of the oxygen when
the sac was filled with air. The climate of this age must have been
such that there was an alternation of wet and dry seasons, much
as is found in certain tropical regions today. As the pools and
streams became dry or sluggish and refilled again with each
succeeding rain, such air sacs were a necessary adjunct for
survival. By being able to gulp air, these fishes could exist for a
time in stagnant water that was devoid of oxygen.

With the return of more stable climates in the millenniums
following the Devonian epoch this rudimentary lung in the ray-
finned fishes reverted into a sort of vestige which modern fishes
use as a swim bladder. However, the Devonian lobe-finned
fishes seemed to have fared better in the use of air-sac lungs.
Those sacs developed into true lung tissue, which became divided
into two lobes situated toward the upper side of the chest. Thus,
air breathing became more efficient. Also, the lobe fins developed
a bony structure which shows a remarkable similarity to the leg
and arm pattern of land vertebrates. This enabled these crea-
tures to hobble over the mud soil in search of food and more
lasting and better water holes. These old air breathers had about
declined to extinction by the close of the Paleozoic era, but
before disappearing entirely they produced a long line of descend-
ants that have persisted to the present.

Even as archaic as was this first attempt to cross the narrow
dividing line between water and land, it was a red-letter period

DOWN TO THE SEA 201

Diplovertebron was one of the earliest types of amphibians that inhabited the swamps
during the Carboniferous epoch.

for the vertebrates. A new habitat for vertebrate life was dis-
covered, and a new vista for backboned creatures was opened.

A Further Venture — Amphibians

Those Devonian lobe-finned creatures that developed the
best lungs and legs ventured farther onto the land and became
less dependent upon the water for existence. They gave rise to a
group of animals known as the amphibians. The first true four-
legged land vertebrates were the ancient amphibians. Once the
amphibians were established they became common before the
close of the Paleozoic era. However, the primitive amphibians
were very different from modern forms. They were clad in the
armor of their fish ancestors. In many cases the scales of the
amphibians were larger and stronger than those of the fish.
Their brain cases were in many species covered with thick bones
originating in the skin, producing a sort of " armor-headed"
creature. These ancient amphibians ranged in size from about
two inches to six or eight feet long. The majority of them, how-
ever, were small, comparable in size to modern mud puppies
and salamanders.

Perhaps the most famous of the early amphibians are the
armor-headed type, scientifically called "stegocephalian." The
term itself means "mailed head" and refers to the fact that
the upper surface of the actual skull was roofed over by thick
dermal bones. Their fossils are found widely scattered in the coal
measures of the latter Paleozoic era, indicating that they lived ex-
tensively in the swamps where the great ferns that later
produced our coal beds grew. About ninety different species
have been found in North America alone.

202 THIS LIVING WORLD

The early amphibians retained one fundamental characteris-
tic of the fish; that is, the eggs were laid in the water and the
young lived there for a time. Even modern amphibians have
never outgrown this characteristic. The amphibian egg is such
that it cannot hatch on dry land. In this egg, food and air for
the growing embryo are absorbed directly from the water, and
carbon dioxide and waste products are thrown out directly into
the water. Further, the egg has no protective shell around it to
prevent it from drying up or being easily crushed on land. Of
course, the embryo in such an egg cannot grow on dry land.

Although very abundant during the late Paleozoic era, the
amphibians have now shrunken into insignificance, being repre-
sented only by such modern forms as toads, salamanders, newts,
mud puppies, and the like. The life cycle of the toad illustrates
most of the modern amphibian characteristics. Its eggs are laid
and fertilized in the water and develop there without further
care from the parents. This is much like the fishes and very
unlike the higher vertebrates. The eggs hatch in the water into
little animals called "tadpoles" or "polliwogs." They have a
round head and a long tail which they wriggle in swimming, as
do the fishes. The tadpoles have gills that extend from the sides
of the head for water breathing.

In about eight weeks they grow an inside air sack or rudi-
ments of lung similar to the lung fish. Then they undergo a
rather complete metamorphosis. Hind legs are grown first, fol-
lowed by a pair of front legs. The long tail shortens by internal
absorption and finally disappears completely. The gills arc
absorbed or dropped off, and the rudimentary lungs develop
into true lungs. Soon the small toad takes to the land, where it
lives its adult life. Thus, in a few months at most there is a
recapitulation of Paleozoic history that required millions of
selections before the early vertebrates crossed the dividing lino
between water and land.

Hence the amphibians represent the vertebrate animals
that made the first great step in moving out onto the land.
However, as we have seen, their method of reproduction kept
them forever chained to the water. Before the broad reaches of
the land could be populated by the vertebrates a new method of
egg production had to be developed, or a new method of growing

DOWN TO THE SEA

Modern amphibians/ bullfrog and leopard frog. (Photograph by Lynwood M. Chace.)

the young embryo inside the body of the mother had to be
evolved. A land type of egg is now laid by reptiles and birds,
while growing the embryo inside the body of the mother is the
method of mammalian reproduction. The land type of egg
historically preceded mammalian reproduction.

The group of vertebrates that first solved the problem of an
egg that would hatch on the land was the reptiles. They con-
stitute the animals that finally conquered the land for the
vertebrates. After doing so, they were the ruling creatures for
millions of years. Their story constitutes a part of the following
chapter.

REFERENCES FOR MORE EXTENDED READING

CHOWDER, WILLIAM: "Naturalist at the Seashore," D. Appleton-Century
Company, Inc., New York, 1928.

The author has written here a popularized story of the characteristics and habits
of a number of marine invertebrates that are to be found along most seashores of
temperate climates.

SCHUCHERT, CHARLES, and CLARA M. LEVENE: "The Earth and Its
Rhythms/' D. Appleton-Century Company, Inc., New York, 1927, Chaps.

xx-xxv.

204 THIS LIVING WORLD

These chapters are a brief and readable discussion of the geologic ages of the earth
and the important forms of life 'that existed in the sea and on land down to the
beginning of the Mesozoic era.

ROMER, A. S.: "Man and -the Vertebrates," University of Chicago Press,
Chicago, 1933, Chaps. I,' II.

The chapters referred to are a well-organized and nontechnical account of verte-
brate beginnings and their early conquest of the land.

CRONEIS, CARY, and WILLIAM C. KRUMBEIN: "Down to Earth," University
1 of Chicago Press, Chicago, 1936, Chaps. XXXII-XL.

These chapters include an informal account of life on the earth during the early
geologic ages before the appearance of the great dinosaurs, together with a detailed
calendar of geologic time.

BUCHSBATJM, RALPH: "Animals without Backbones," University of Chicago
Press, Chicago, 1939.

This book is a well-written account of the body structure and habits of modern
invertebrates which will prove of interest to the beginning college student of inverte-
brate life and to the inquiring laymen. It is written in nontechnical language and is
profusely illustrated with vivid drawings and photographs.

SCHUCHERT, CHARLES, and DUNBAR, C. O., "Historical Geology," 3d ed.,
John Wiley & Sons, Inc., New York, 1933, Chaps. I, V, VII-XIV.

The third revision of this standard text is a completely rewritten book which is
expressly designed for the beginning student. It contains a wealth of information on
life's ancient past and the rock formations of the geologic ages. It is written in rela-
tively nontechnical language and extensively illustrated with photographs and line
drawings. The chapters referred to treat of.life and formations from the earliest known
times through the Paleozoic era.

Bulletin of New York Zoological Society, published by the New York Zoological
Society, New York.

This is a bimonthly publication which contains articles on natural history and
relating to work carried on at the Zoological Park and the Aquarium. The articles
are usually of popular interest and are extensively illustrated.

The Biological Bulletin, published by the Marine Biological Laboratory,
Wood's Hole, Mass.

A technical journal published monthly. It contains short papers dealing with
problems of general interest to biologists in all fields of science.

7: SIZE AND CUNNING

In the Development of Vertebrate Land Life during the Later
Geologic Ages

THE largest cannon ever built was a superbombard con-
structed by the skilled Hungarian gunmaker, Urban, for the
Turkish Sultan, Mohammed II, in 1453. It was made of hooped
iron and threw a ball weighing 800 pounds. Sixty oxen and two
hundred men were required to move it to the walls of Constanti-
nople, which the Sultan had beseiged. The cannon was fired only
a few times, as it soon blew to pieces, killing all the attendants.
However, the few shots fired so damaged the towers of the Gate
of St. Romanus that the gate fell to pieces. A short time later the
Turks entered and conquered the walled city.

The largest animals ever to inhabit the earth were the
reptiles. These creatures were the first to conquer the land for
the vertebrates. Many different species were evolved after the

205

206 THIS LIVING WORLD

main stem became firmly established. Some of these species
were the mighty dinosaurs. They became in many instances
highly specialized, and one specialization was size. This great
specialization and large size no doubt contributed somewhat
to their undoing, as the dinosaurs have all long since disappeared
from the earth. Their majestic rise and dramatic decline was
not sudden or within a few years, as was the case of the cannon
episode mentioned above, but occupied a period of about a
hundred million years during the Mesozoic era.

The mighty reptiles were followed by the mammals. These
were a type of vertebrate animal more completely developed
physically for rigid land life, and they possessed a larger and
better brain. This larger brain enabled them to adapt themselves
better to their environment and to live under a great diversity of
conditions. They were able to outwit their enemies with a higher
degree of cunning and to survive where other creatures perished.
With the decline of the dinosaurs the mammals multiplied and
developed rapidly. During the Cenozoic era, they became the
dominant form of land life and they have held this position to
the present day, a period of approximately sixty million years of
geologic time.

What Are Reptiles?

There is plenty of confusion when it comes to any accurate
determination of the distinction between "Aryans" and "non-
Aryans" of the human races. However, the distinction between
the reptiles and their nearest of kind, the amphibians on one
side and the mammals on the other, is clear and marked; that is,
reptiles have certain characteristics that serve to set them apart
from other creatures most like them. Their distinctions from the
amphibians are significant, as these characteristics finally enabled
them to become successful vertebrate land animals where the
amphibians failed.

For one thing the reptiles developed a type of egg that could
be laid on land. It is not necessary, therefore, for any part of the
life span to be spent in the water. One other respect in which the
reptiles differed from the amphibians was in the placement of
legs for locomotion. Particularly was this true of the larger
reptiles that came into prominence during the Mesozoic era.

SIZE AND CUNNING 207

The amphibian's legs in most cases were widely spread out on the
sides of the body. While there was a great diversity in the
locomotor adaptation worked out by Mesozoic reptiles as well
as by modern forms, one rather efficient type of leg structure
was more or less typical of the ancient ruling reptiles. The legs
were brought more toward the underneath of the body, and
there was an extensive development of the bones and muscles of
the hind legs. Speedy locomotion was obtained by running not
on all fours but on the hind limbs; that is, most of the great
reptiles were bipeds. Furthermore, the reptiles have scales grow-
ing from the skin and covering the body, lungs for air breathing,
and cold blood.

Growth of a Ruling Class

The ancestry of the reptiles has been traced back to the latter
part of the Paleozoic era. In fact, the family tree of the reptiles
dates back almost as far as that of the amphibians. One of the
most primitive types left its fossils widely scattered in the
deposits of the great coal swamps of the Carboniferous epoch,
where it must have lived beside the early amphibians of those
times. It has been called Seymouria after a town in Texas where
its remains were first found. Its broad mouth and short sprawling
legs are very similar to those of the primitive amphibians, to
which it was no doubt closely related.

From such archaic stem types thousands of different kinds of
reptiles developed during the Mesozoic era. The reptiles that
were the most successful and grew to the largest sizes were the
dinosaurs. The earliest dinosaurs made their appearance in the
early part of the Mesozoic era, and by the time of the Jurassic
epoch they had come into a position of complete dominance of
the remainder of land life. They were apparently widespread
over much of the earth, as evidenced by their fossils and foot-
prints. At the present time their fossils usually constitute the
most impressive display in any museum of natural history. Most
of these great dinosaur fossils have come from a single formation
of deposits that extends from Montana to New Mexico and from
Kansas to Utah. This is the area, it may be recalled, that was
covered by shallow inland seas or great low plains and swamps
during the warm and humid Jurassic and Cretaceous epochs.

208 THIS LIVING WORLD

Seymouria was one of the most primitive of known reptiles. (Drawn from restoration by

L. I. Price.)

The dinosaurs may be divided roughly into two main groups,
based upon the size and shape of the pelvic bone. In the first
group the pelvis is shaped like that found in crocodiles and
lizards today. In the other group the pelvis is bird-like in char-
acter. Reptiles of the first group were almost all carnivorous,
or flesh-eating, with the exception of the giant Brontosaurus
("thunder saurian"); while those of the second group were all
herbivorous, or plant-eating.

One of the first general type of the flesh eaters was the "near
lizard." It was a primitive dinosaur with a light body, long neck,
and long tail. From this type probably developed later all the
great carnivorous dinosaurs, most of which were bipeds with
strong hind limbs.

One of these bipeds, common in Montana, Utah, and Wyo-
ming during the Cretaceous epoch, was the king-tyrant reptile,
called Tyrannosaurus. It had a height of about twenty feet, and
its weight exceeded that of an elephant. It ran on two powerful
hind legs. The front ones were small, usually short, and were
probably used for grasping prey. The tyrant reptile had a large
head on a powerful neck. The jaws reached four feet long and
were equipped with numerous saber-like teeth. No other animal
has ever had a fiercer biting head. The Tyrannosaurus had a
length of about fifty feet. In speed, ferocity, and size it has been
referred to as the "most destructive life engine ever evolved."

Some of the carnivorous reptiles tended to take to a water-
dwelling life, much as do the crocodiles of today. They, of course,
abandoned their bipedal habits and developed limbs suitable for
water locomotion. Such were the ancient crocodiles and plesio-
saurs. They were common inhabitants of shallow seas during the

SIZE AND CUNNING

209

Restoration of plesiosaurus as they probably inhabited the shallow seas of Kansas during
the Cretaceous epoch. In the background is seen an ichthyosaur, a marine reptile, leaping
above the water. (American Museum of Natural History photograph of painting by Kull.)

Jurassic epoch, and their fossils are world wide in their
distribution.

The plesiosaur must have been a weird-looking monster,
growing to a maximum length of forty feet in many instances. It
had an exceedingly long neck and small head. The body proper
was short, broad, and box-shaped and to it were attached four
powerfully developed short limbs. The fingers and toes were
entirely covered with a tough skin, which gave them a somewhat
paddle or rudder-like appearance. The long necks of the plesio-
saurs probably served them well in swiftly darting the head
through the water to catch the fish on which they fed. The plesio-
saurs along with the ichthyosaurs, another water-dwelling reptile,
probably made troublesome times for the fish in these ancient
seas. These reptiles, of course, had lungs instead of gills and
had to keep their heads above the water when breathing.

Another variation of the flesh-eating reptiles were the flying
dragons of the air, known as "pterodactyls." They were biped
reptiles in which the front legs or arms took on a special adapta-
tion; that is, one of the fingers grew exceedingly long and stout.
It was covered with a broad flap of leather-like skin that ex-
tended along the arm and back to the thigh region. This flap of

210 THIS LIVING WORLD

skin constituted the wing. The skin was practically naked and
contained no feathers. Such a wing was probably a fairly efficient
flying mechanism, similar to that possessed by bats of today.

These highly carnivorous flying reptiles were somewhat bird-
like in appearance. However, they are not to be confused with
the birds. Their bodies were covered with reptilian scales, and
there was an absence of feathers. The largest ones had a wing
spread of about twenty-five feet. Many of them had a long beak
armed with sharp teeth. Judging from their skeletons, their
bodies must have been of small and light construction, a sort of
an appendage to support the long beak and the two large wings.
These aerial reptiles must have fed mostly on small fish, which
they probably caught by diving as marine birds do today. The
hind legs and feet were poorly developed, and it is likely that
walking or standing was a slow and cumbersome task for them.
No doubt they spent most of their time soaring over the shallow
seas or near-by lands. Their fossils are numerous, and they
apparently were a common form of reptilian life during the
Jurassic epoch.

The flying reptiles might be called the organic airplanes of
the Mesozoic era. It is said that Langley, from a study of their
fossils as well as modern soaring birds such as the albatross,
obtained ideas for his invention of the first flying machine.

One dinosaur that varied considerably from the flesh-eating
biped type yet retained the same kind of hip bones was the
Brontosaurus. These are the dinosaurs commonly pictured in
modern advertisements. Their teeth show that they changed to
a plant diet, and their bodies became considerably bulkier.
With a lessened need for speed, they slumped back into a four-
foot method of walking. They reached the peak of their develop-
ment during the Jurassic epoch. The greatest of these were the
largest animals ever to live on the earth. They grew to a length
of about 80 feet and weighed something like 40 tons. These huge
creatures had massive hind legs very much like large weight-
bearing columns. The backbone was constructed in a great arch
to support the animal's bulk. The neck was long and terminated
in a small head. The tail usually measured about one-third the
total body length.

SIZE AND CUNNING 211

The head seems absurdly small when compared to the rest of
the body. The jaws were weak and short, and the animals prob-
ably fed on the soft water plants of the swamps in which they
lived. The nostrils were at the top of the skull, indicating that
these giant reptiles spent most of their time in the water. They
could breathe and see with only the top of the head exposed
above the surface, and the water helped support their great
weight. The brain was excessively small in proportion to the
body size. However, there was an enlargement of the spinal
cord between the hips that constituted a sort of hindbrain that
was about twenty times larger than the one in the head. This
hindbrain probably controlled most of the body functions and
the animal's motions. Someone has said that they had more
sense in their hips than in their heads. But without doubt these
ponderous creatures had little intelligence either in the brain
in the head or in the one above the hips.

These giant reptiles attained a world-wide distribution.
They are well known from fossils that have been found in as
widely separated areas as the United States, East Africa, western
Argentina, and Germany.

The Great Vegetarians

The other large group of Mesozoic reptiles were those which
possessed the bird-like pelvis and all of which were plant eaters.
Some of them were bipeds while others walked on all four feet.
One was a bizarre type of heavy-limbed four-footed dinosaur.
These creatures fed on leaves and twigs. They were the plated
and armored types, known as the armored Stegosaurus. They
appear to have lived completely away from the water on the
uplands. They specialized in the production of an elaborate bony
structure as an outgrowth of the skin, which could be used for
protection or as a weapon against their enemies. A double row
of bony plates extended down the back from the head to the tail.
Over the hips these plates were some two feet high, two and a
half feet wide, and about four inches thick at the base. The tail
was covered with sharp spines that grew to a length of about two
feet. These sharp spines must have been their chief weapons of

212

THIS LIVING WORLD

The armored Stegosaurus had a double row of bony plates down the back and sharp

spines on the tail.

offense. With a slashing motion of the tail, they became a sort
of multiheaded spear. The small head attached t6 the ponderous
20-foot body added to its unusual appearance but probably not
much to its intelligence.

There was the weird-looking horned type of plant-eating
dinosaur known as Triceratops. It had three prominent horns
about four feet in length, the cores of bone being covered with a
sheath of horn. From the back of the skull there extended a
broad frill of bone that formed an armored plate over the neck
and shoulders, in some cases exceeding eight feet in length. The
total head was large, being about one-third the entire body,
However, this large size was made up mostly of bony armor
rather than brain cavity. The brain weighed less than twc
pounds out of the total body weight of ten tons.

The horned Triceratops must have been great fighters in
their time, as judged by the dints of deep wounds found on many
of the fossil skeletons. The animals probably charged as does a
rhinoceros, head on, and, with a sweeping uplift of the powerful

SIZE AND CUNNING

213

A horned Triceratops skeleton and miniature reconstructed model on exhibit at the United
States National Museum. (Science Service photograph.)

head, he forced the great horns against or through the impaled
enemy, not unlike the legendary exploits of King Arthur's
armored knights.

Thus the reptiles reached an extensive and varied stage of
development. As we have seen, the climatic and physical con-
ditions of the earth were favorable to reptilian development
during the Mesozoic era. They became exceedingly numerous in
species as well as in numbers. For a period of approximately one
hundred million years they were the mighty rulers of the land
and lords of the air and to some extent masters of life in the
inland seas as well. This era has appropriately been called the
Age of Reptiles. However, when the Rocky Mountain revolution
began at the end of the Mesozoic era their death knell was
sounded. The rising mountains obliterated the inland seas and
the great swamps, ancf there was a marked reduction in tem-
perature that followed the rising mountains. Even local glaciers
began to appear. These changes foretold the end of a great era.
No cold-blooded creatures of the size and habits of the dinosaurs
could withstand cold winters. Their ranks were thinned rapidly,

214 THIS LIVING WORLD

geologically ^peaking. By the end of the Mesozoic era they had
all disappeared from the earth.

Descendants of a Dynasty

Some reptilian groups have had less spectacular careers than
the ruling reptiles. However, they have been successful in surviv-
ing to the present time, and their descendants still hold on in the
struggle for existence. They were the less specialized types that
culminated in modern reptiles. These living forms include gen-
erally the lizards, snakes, turtles, and crocodiles.

The lizards are the most abundant of living reptiles. They
are found in the tropical and warmer temperate regions of all
lands. They have generally retained the primitive sprawling type
of walking. The limbs, however, are usually light, and some
lizards are able to run swiftly. Some species, when rapid motion
is necessary, rise on their hind legs and, balancing the body with
the tail, run with great rapidity. The body is covered with scales.
Most lizards are small ; however, in the East Indies and Australia
there are monitor lizards that grow to a length of about twelve
feet.

The snakes are a development from the original lizard stock.
They probably are the most evolved and progressive of modern
reptiles. Snakes have lost their legs entirely and depend upon a
sinuous twisting of the body for locomotion. The scales are so
placed on the snake as to prevent any backward slip as it twists
its body, but they offer no hindrance to a forward motitm. One
remarkable characteristic of the snakes is the structure of the
skull. It has been greatly modified from that of the ancient
reptiles. The two jaw halves are loosely connected so that they
may be stretched far apart. The snake is, therefore, able to
swallow larger animals. A snake a few feet long can swallow a
rabbit and digest it at its leisure. The python and some other
tropical snakes are able to swallow a good-sized pig or even an
animal as large as a man.

Snakes and lizards have proved themselves very useful to
man. They generally feed upon insects and small mammals, such
as mice and rats, which often prove very destructive to man's
cultivated food plants and stored grain. The natural food and
prey of the lizards and snakes are, then, man's troublesome

SIZE AND CUNNING

;^r ,fl,^ ;, ;, ^ p <\-<^f^

:>>:i;i'i^il1;-.-l^ll1'!'-i.'Vp- -'^i; ri • "•ii ' ^^^^

Gfant tortoise is a land turtle that 3 rows a shell about four feet Ions, and weishs about
six hundred pounds. (Photograph by Ewing Galloway.)

enemies. Most snakes as well as lizards are quite harmless, and
to spare them rather than to destroy them is to add to our own
general welfare.

The turtles are structurally the most remarkable of the living
reptiles. Their most obvious characteristic is the shell. It has
had a profound effect upon the architecture of the body and
served to keep all the turtles pretty much alike. The shell con-
sists of two halves, an upper one and a lower one, firmly united
at the sides but widely separated at the front and behind to
accommodate the head, tail, and legs. The ribs as well as part
of the vertebral column are attached to the upper plate. The
ribs are, therefore, immovable, and the great back muscles
characteristic of other vertebrates have been lost. The non-
flexible ribs have necessitated the development of a different
method of breathing from that of most other vertebrates. This
is accomplished by a system of bones, the hyoid apparatus, which
compresses and dilates the throat and roof of the mouth and thus
forces air into and out of the lungs.

216 THIS LIVING WORLD

Each half of the bony shell is overlaid with horny plates,
which have the same general arrangement as the bones but do
not correspond with them. These plates are not shed periodically,
as are most reptilian scales, but they grow larger as the animal
grows older to correspond to the increase in size. These plates
increase in size by adding a sort of outer ring each year. By
counting the rings on the plates, it is sometimes possible to get
a rough measure of the turtle's age.

The largest of all modern turtles are the luths or leathery
turtles. The oldest ones reach a total length of eight feet and
weigh nearly a ton. Another large species is the giant tortoise,
which was once common around the Galapagos Islands, but now
has been about destroyed by man. Some grow a shell about four
feet long and weigh about six hundred pounds. However, most
turtles seen away from the seashore and larger rivers are small
creatures. They usually feed on animal life such as snails, insects,
or small fish and are quite harmless to man.

Crocodiles, too, have quite an ancient lineage. Their fossil
ancestors can be traced back to the dinosaurs. All of them are
semiaquatic, feeding and passing most of their time in the water.
However, they frequently come ashore to eat, and they are
capable of making long journeys overland in search of water
holes in time of drought. They, of course, lay their eggs on land,
usually in sand or soft clay, to be hatched by the sun's heat.

Adaptation to a water habitat has brought about a peculiar
and special structure of the breathing apparatus. The nostrils
open through a sort of dome on top of the snout. Separate tubes
lead from 'these nostrils along the roof of the mouth down to the
base of the throat. In this way a continuous passage is formed
from the nostrils to the windpipe, an arrangement which enables
the crocodile to drown its prey while still able to breathe itself.
The eyes are on top of the head, and the animal can lie in the
water with only its eyes and nostrils exposed.

Crocodiles have within their ranks the largest living reptiles.
The largest species is the salt- water crocodile. Some of them
probably grow twenty feet long. The average length of most
other species, however, is from four to ten feet long. Most
crocodiles and alligators usually lie quietly in the water, with
only the eyes and nostrils and perhaps a part of the back show-

SIZE AND CUNNING 217

ing, and look like floating logs. They usually feed on fish, but
any unwary bird or beast approaching within range of the
powerful jaws may be seized, dragged under water, and eaten.
They undoubtedly will attack man, but as a rule they recognize
him as an archenemy and in his presence become cautious and
difficult to approach.

Early Birds

The group of animals living on the earth at present in largest
numbers that are most like the great reptiles of the past are the
birds. They have been called by an early writer "glorified
reptiles." Furthermore, it has been often stated that almost
every feature of their body structure, other than feathers and
warm blood, can be matched in some dinosaur type. It is gen-
erally agreed that the birds, while exceedingly unlike the dino-
saurs in appearance, are descended directly from the types of
animals that produced the ruling Mesozoic reptiles. The modi-
fications which they worked out proved effective for existence
through the Cenozoic era and to the present.

The pelvis in the bird skeleton is the kind found in the great
herbivorous dinosaurs. The three toes common to most birds
were also characteristic of many of these dinosaurs; the ankle
bones are similar and are quite different from those of mammals.
The type of egg developed by the dinosaurs has not only been
retained by modern reptiles but by birds as well. The wings of
modern birds possess no claws, but these have been lost by a
fusion of some of the finger bones to form better wing supports.
The breastbone is well developed in the birds in contrast to
smaller ones in the dinosaurs. This seems to be a modification
that was made as flight developed, since the breastbone serves
as an attachment for the strong wing muscles in modern flying
birds.

The earliest fossil bird known is one called by the imposing
name of Archaeopteryx. Incidentally this term is derived from
the Greek language and means "ancient wing/' It was found in
limestone deposits of Germany belonging to the Jurassic epoch.
The features are so well preserved that it seems as if the creature
had come flying out of the Mesozoic past into the present. Had
not the imprints of feathers been so clearly preserved, it might

218 THIS LIVING WORLD

have been easily judged a small dinosaur. It had a long tail,
three toes on each foot, claws on the arms, and teeth in the jaws.
The breastbone was small, indicating weak flying muscles. How-
ever, there were long feathers on the sides of the tail and the
arms and shorter ones over parts of the body. Feathers in modern
birds are known to be a modification of reptilian scales. It is
likely that the feathers developed by this ancient bird provided
it with some measure of flight and retained for the creature some
of its body warmth.

The next glimpse we get of developing bird life is from fossils
found in Kansas belonging to the late Mesozoic. These are for
the most part remains of large reptilian water birds, known as the
"regal western bird." Some of these skeletons measured six feet
in length and about five feet tall. The wings indicate that these
birds had lost the power of flight and apparently spent most of
their time in the water. Also found there are some fossils of
smaller birds, much like those of modern gulls, with powerful
wings. In both types teeth were present in long jaws that took
on the appearance of beaks. The tails had become much shorter
than that of the archaeopteryx, and the feathers grew much as
in modern birds.

By the beginning of the Cenozoic era teeth had been lost,
and a horny beak is common to most well-preserved fossils.
Many of the present families of birds had become established,
and it is not unlikely that bird life generally presented much the
same picture as it does today.

Warm Blood for Cold

Such then is the story, briefly told, of an important group of
vertebrates that once extensively inhabited the earth and their
modern descendants. Now the ruling reptiles have gone, leaving
in their place the mammals as earth's dominant form of land life.
The warm-blooded mammals have a higher and more complex
form of bodily development than the cold-blooded reptiles. They
were able to endure and progress where the great reptiles
perished.

One species of mammals is modern man, whose majestic
development has far exceeded that of any other form of life
ever to exist upon the earth. It is a long story in both time and

SIZE AND CUNNING 219

development from the earliest mammals to man. However, it is
a story that is becoming increasingly better known. Its unfolding
reveals a slow and ever-changing evolution throughout a history
of many millions of years and accounts for many of the forms of
mammal life we know today as well as an exceedingly large
number of extinct ones the remains of which are buried in the
fossils of rocks.

Special Characteristics of Mammals

It is not uncommorf to see a squirrel or cottontail or many
other kinds of wild mammals during even the severest of our
winters. At such times a snake or turtle is rarely encountered.
Instead, they are inactively secluded in some protected spot.
These activities are indicative of one fundamental difference
between mammals and reptiles. Mammals have warm blood;
that is, a high body temperature is maintained regardless of the
temperature of the surrounding air. This is not true of the
reptiles, as they have no mechanisms to regulate the tempera-
ture of their bodies. Accordingly, mammals can remain active
in cold weather and inhabit polar as well as milder climates;
reptiles cannot. A covering of hair on the skin of most mammals
aids in conserving the body heat, while the functioning of sweat
glands serves to cool the body during periods when the surround-
ing air is hot.

The name of the group, mammals, signifies one of their
characteristics; that is, mammals nurse their young. The mam-
mary glands are a well-developed mechanism for supplying
nourishment to the young during postnatal care. Such features
have never existed in the reptilian body, and reptiles rarely mani-
fest any care of their young. Furthermore, in mammals the
fertilized egg and growing embryo are retained and nourished
inside the body of the mother. This constitutes a more complex
body structure and a more efficient form of reproduction than
any other group of animals has ever developed.

The brain of mammals, even the most stupid of them, is
enlarged enormously over that of reptiles. However, the enlarge-
ment has occurred mainly in one particular part of the brain,
the cerebrum. The cerebral hemispheres are the seat of the
higher mental processes while the other parts of the brain serve

220 THIS LIVING WORLD

as the automatic control of bodily functions. Within the cere-
bral hemispheres are the centers of learning that have placed the
mammals as a group far above any other vertebrate stock in
their degree of mental functions.

The method of walking employed by most mammals has
given them ease in movement and made them fleet of foot. The
four legs in most cases have been retained. However, they have
been brought around directly beneath the body, the knees bend
forward, and the elbows swung backward, thus permitting a
more rapid and a more perfect gait.

Geologic Development of Mammals

It is a mistake to imagine that the mammals evolved from the
highly developed dinosaurs. The stem from which mammals
sprang was one of the first to diverge from the primitive reptile
stock. The first mammals appeared almost as early as the first
reptiles.

Fossils of creatures that are mostly reptile but have some
mammalian characteristics are found in rocks dating back to
the very beginning of Mesozoic time. These are the dog-toothed
reptiles, known as "cynodonts." Their fossils have been found
mainly in South Africa; however, a few scattered remains have
been found in Triassic coal beds of North Carolina. Many
authorities are of the opinion that they were either the ancestors
of the mammals or were close to the original mammalian stock.
They possessed the specialized dentition of mammals, having the
teeth divided into incisors, canines, and molars rather than the
unspecialized teeth of reptiles. The skull was intermediate
between that of reptiles and mammals. In the roof of the mouth
a secondary plate had developed, just as mammals have. Other
skeletal features are midway between mammals and reptiles.
It is difficult to classify this creature as either reptile or mammal.
It shows that the original ancestors of these two great groups
of vertebrates were very similar.

Even while the great dinosaurs were dominant there is
evidence that there were in existence some small, more active,
somewhat warm-blooded creatures about the size of rats or cats,
possibly descendants of the earlier cynodonts. Fossils of the

SIZE AND CUNNING

221

Bat

FLYING 'MAMMALS

Prairie
Dog

/Mole
INSECTIVORES

Opossum
MARSUPIALS

Chimpanzee

Simplified chart showing important groups of modern mammals.

Jurassic epoch from Germany, Mongolia, South Africa, and
North America show that these mammals had sharp teeth; how-
ever, they were too small to attack the dinosaurs. Their brains
were larger than the reptiles in proportion to their body size,
They had some features which indicate a tree-dwelling life. They

222 THIS LIVING WORLD

most likely were nocturnal in habit, in order to escape the
carnivorous dinosaurs during the day.

The threat of death from the great carnivorous reptiles lay
constantly over the early small mammals. This probably had
much to do in determining the survival of these creatures, which
had developed a higher degree of cunning, a greater ability to
move more rapidly, and the adaptability to move around in
cold weather, when the reptiles were inactive. As a result, after
the close of the Mesozoic era, when the reptiles declined, the
mammals developed rapidly and during the Cenozoic era they
took the leading place in the drama of evolution.

The fossil records show that as mammals progressed, they
branched out into all sorts of habitats and developed many
specialized features of body structure. As a result there arose
such diversified creatures as the carnivores and the herbivores
and such opposites as mammals with hoofs and mammals with
claws, mammals with swimming limbs and those with flying
limbs, mammals both large and small.

Golden Age of Mammals

The Cenozoic is known as the Age of Mammals. This geologic
age began about sixty million years ago and extended down
through the last great glaciers about twenty thousand years ago.
The mammals of the early Cenozoic were archaic in form and
differed greatly in appearance from modern creatures. However,
with the beginning of the Miocene epoch, itself sometimes called
the Golden Age of Mammals, thousands of forms much like
present-day creatures began to appear. There are present on the
earth now over twenty thousand different species of mammals.
The remains of more than this number of extinct forms have
been discovered in the fossil rocks.

It is obvious that only a thumbnail sketch of the more
important groups can be presented here. For this general con-
sideration let us take brief note of the hoofed mammals, the
carnivores, and the primates. For those interested in more detail
some of the other orders of mammals are represented in the
accompanying chart.

SIZE AND CUNNING 223

Mammals with Hoofs — the Ungulates

Of the hoofed mammals one of the early exceedingly special-
ized forms to develop were the giant creatures known as "titan-
otheres." They first appeared as small animals about the size of
dogs in the early Cenozoic. Before the middle of the era they
had attained the size of elephants and were widely distributed in
North America. They were heavy in body, with four columnar
legs and feet supported on thick pads. In the earlier ones the
heads were long and narrow and devoid of horns. However, as
the animal later developed into greater size, knobs grew over the
eyes. As time went on the knobs shifted forward and grew into
enormous proportions. They were situated on the nose, the head
becoming broad and massive in proportion. The horns must
have been used as powerful weapons. However, the brain was
comparatively small. This indicates that the great beasts were
stupid but large, probably even surpassing the modern rhinoc-
eros in this respect. They rather suddenly disappeared from the
earth — another illustration of the failure of great body size.

In contrast with the titanotheres, the evolution of the horse
has shown a steady and progressive development. Their ancestry
has been traced back to the very beginning of the Cenozoic, and
their descendants are common on the earth today. Their family
tree is one of the best known of all animal creatures, and it con-
stitutes an unbroken lineage of over fifty million years.

The first record in North America is of a small four-toed
"dawn horse," Eohippus, that was a graceful little creature no
larger than a dog. These horses swarmed in the forests and lower
plains. In the Miocene epoch they had increased some in size,
lost one toe, so that they walked on three toes, and were rapidly
becoming roaming grazers on the uplands. Toward the latter
part of the Cenozoic, the horses had progressed to a larger and
swifter animal, running on one toe that had become greatly
developed as the others shrunk to insignificant appendages.
They belonged to the genus Equus, which is the one including
all modern horses.

The fossil remains of horses are found widely scattered over
the continents of the Americas, Europe, Asia, and Africa. They

224

THIS LIVING WORLD

have migrated into all climates and adapted themselves to a
wide variety of environments. In this respect they have had but

two equals, elephants and man.
The brain of the modern horse
is large and convoluted, and its
evolutionary development kept
pace with other body changes.
The intelligence of the modern
horse is notable, as well as is its
emotional disposition. This
emotional instability is prob-
ably an inherited character-
istic growing from a long
*\)l ^SN history of rapid flight as a
™ jj}\ method of protection or escape

if* & if from danger.

The elephants belong to a
group of the ungulates that
have developed a proboscis;
that is, they are mammals with
a trunk. During the middle and
latter Cenozoic they were
among the most widespread of
the mammals. However, they
are distinctly on the wane
today. The first ancestors to a
long and distinguished line
made their appearance in the
early Cenozoic. Their fossils
were left in the delta deposits
of the Nile, an area where the
proboscidians are believed to
have originated. The earliest
ones possessed no tusks. Also,

The development of the horse from the
small, four-toed Eohippus to the modern
one-toed Equus constitutes a well-known
lineage of over fifty million years.

they were no larger than a good-
size dog or pig. From such crea-
tures the true elephant-like or
mastodon stock developed, and they migrated rapidly into
Europe and Asia, At a later time they invaded North America,

SIZE AND CUNNING

225

A remarkable picture of a full-grown African male elephant made in his native haunts in
the Belgian Congo. (Globe photograph taken in Africa by Dorein Leigh.)

probably by way of Siberia and Alaska. They became exceed-
ingly numerous here until within ten to twenty thousand years
of modern times.

Many kinds of elephants or mastodon have lived on the
earth. One was the "woolly mammoth" that lived at or near the
ice fronts during glacial times. Some specimens have recently
been found frozen in the ice sheets of Siberia. No doubt the
unlucky animals fell into great crevices of the ice some twenty to
thirty thousand years ago. They were, of course, quickly frozen,
and in such condition their bodies have been preserved to the
present time. This is a record for long-time refrigeration. These
specimens not only show the skeletal parts, but skin, hair, tissue
structure, and undigested food contents of the stomach as well.

226

THIS LIVING WORLD

Photograph of tllravatha", said to be the largest male elephant in Southern India,
and a twelve year old Indian lad. This picture was made in India during the filming of the
motion picture "Elephant Boy". (Life Magazine photograph.)

SIZE AND CUNNING 227

One such specimen has been carefully removed to the Leningrad
Museum in Russia, where it is now on exhibition.

The "imperial mammoth" roamed the plains of the Western
and Southwestern United States. It grew to large size, measur-
ing some fifteen feet in height. Fossil remains of this great
elephant together with its smaller relatives have been found
scattered from Florida to Alaska, from Connecticut to Cali-
fornia, as well as in South America, Europe, Asia, and Africa.
The proboscidians have, therefore, been world travelers, equaled
only by the horses and exceeded only by man.

Only two species of elephants remain on the earth at present.
These are the Indian elephant and the African elephant. The
Indian elephants usually stand about eight to nine feet tall,
have tusks about three to six feet long, and have ears somewhat
smaller than the African elephant. The African elephant, like
its earliest ancestor, is exclusively an inhabitant of that continent.
It grows to a height of about ten feet, with tusks eight to twelve
feet long, and the males have exceedingly large ears.

The Carnivores

Of the carnivorous or flesh-eating mammals, there is a great
variety of different forms. Their development and modern
progeny can be traced here only in brief outline. They arose from
the small creatures that were in existence during the reign of the
great reptiles and probably fed on insects, grubs, and berries.
As the dinosaurs declined and disappeared, they increased
rapidly, feeding upon plant-eating mammals that developed
along with them. A still later diversification of the archaic
4 'insect eaters" led in one direction toward the hunters and
strictly flesh eaters, and in another direction toward the arboreal
dwellers that fed on a mixed diet. These latter ones gave rise
eventually to the primates, the group to which man belongs.

The various strictly flesh-eating mammals of the early
Cenozoic are usually referred to as "creodonts." They had feet
on which the appendages grew claws rather than hoofs, and the
teeth were more highly specialized. The front teeth, the incisors,
became sharp and even for cutting and tearing, while the
canines, or "dog teeth," were long and pointed, making them
effective in stabbing their prey once it was in the mouth. The

228

THIS LIVING WORLD

Midcis

Simplified chart representing development and relationship of modern carnivores.

earliest such creodont was an animal known as Miacis, which
was a small but likely a progressive cat-like creature. Some of
the later types are comparable in size and general appearance
to modern weasels, some to wolves, and some to tigers or lions.

SIZE AND CUNNING

229

An intimate and artistic photograph of a wild African lion taken unawares at a time of
repose under a shade tree in his native habitat in Tanganyika Territory, Africa. (Photo-
9raph by Dr. W. D. Campbell in 1939 while on the William D. Campbell African
Expedition for the American Museum of Natural History.)

Near the middle of the Cenozoic there appeared fossils of
what are called "dog weasels," which were creatures believed
by many to be the early ancestors to modern dogs, wolves, and
foxes. The body was long, legs were relatively short, and the brain
was rather well developed. These ancient creatures thus had
some characteristics of the modern wolves or dogs as well as of
the modern weasels, two groups that are rather closely related.
They were probably rather close to the original stem of these
two modern types of hunters and killers, if not their actual
ancestors.

Ancestry of the felids or cats goes back to middle Cenozoic.
Such fossil remains show that they had sharp incisors, pointed
canines, and no molar teeth, these being characteristics of
modern forms. The claws were highly developed, and the body
was slim and light, probably agile and suitable for stalking their

S30 THIS LIVING WORLD

The now extinct saber-toothed tigers were once widespread over the earth.

prey and capturing it by a sudden lunge or jump. The modern
descendants of these early cat-like mammals are the lions, leop-
ards, tigers, wildcats, lynx, and household pussies. More dis-
tantly related descendants are such forms as the mongoose,
hyena, and civit cat.

One other strict flesh eater that is worthy of mention is the
saber-toothed tiger. Although now entirely extinct the saber-
toothed tigers once were widespread over the earth; and they
must have stricken fear into all other large mammals of their
time. They grew to the size of the largest tigers. However, they
differed from the other felines in one important respect, that is,
in the killing mechanism of their jaws. The upper canine teeth
were exceedingly long, and they were curved somewhat like
sabers. Furthermore, the lower jaw could be dropped down to
more than a right angle. The large muscles of the neck and head
seem to have fitted these old cats well to use the great saber teeth
for effectively spearing and slicing. They, no doubt, fed chiefly on
the large mastodons of their times.

No discussion of the carnivores would be complete without
some mention of the marine forms, these being the various seals,
walruses, and whales. They differ from all other mammals in that
limbs have been converted into swimming paddles, and they are
distinguished by including in their number the largest animals
on earth at present.

The walrus is a huge animal native to North Atlantic and
Arctic waters. It feeds mostly on mollusks at the bottom of the
sea. Its most pronounced specialized features are the large tusks
and the absence of outer ears. The herd instinct is strongly
developed, and it often shows remarkable cooperation in defense

SIZE AND CUNNING 231

when attacked. However, the walrus has been so persistently
slaughtered by man that it is now relatively scarce and is to be
found only in the Arctic Ocean.

A great many species of seals inhabit the oceans. One variety,
the northern fur seal, is a most prized animal to man because
of its valuable fur. It was formerly common in the Pacific.
Man's extensive hunting of these animals has reduced their
number, until now their breeding places are strictly protected
in order to prevent complete extinction. They come ashore for
breeding purposes. The males are the first to occupy a coast line
or island and contest among themselves for a space to accommo-
date the females. These arrive a few weeks later, and each male
secures as many as he can entice to his chosen area. The young
pups are born soon afterward, and the pairing season for the
following year's families begins. The seals inhabit these chosen
areas for two to three months, during which time the young have
learned to swim. Then about November they take to the sea
and follow the fish southward for the winter. The following May
or June they return to northern islands again.

The whales are other mammals that live in the sea; in fact,
they have become so modified to sea life that people generally
confuse them with the fishes. All external traces of hind limbs
have disappeared, and the flipper tail is provided with a hori-
zontal fin. There is also a fin on the back. Whales live exclusively
in the water and come to the surface only for breathing or feed-
ing. One, known as the blue whale, has been found to grow tc
about one hundred feet long and to weigh over a hundred tons
The whales most hunted by man are the great sperm- and whale-
bone-bearing animals. These have been so extensively destroy ec
that their number is rapidly decreasing. They are only one of i
large number of great and interesting animals that man hai
completely or materially eliminated from the earth.

Arboreal Mammals — the Primates

The other great divergence of the earliest "insect eaters " le<
toward specialization in another direction. Such specializatioi
was in tree-dwelling habits and a mixed diet of fruits, berries
and small animals. These arboreal insectivores gave rise to th
primates, the order of mammals including the lemurs, monkeys

232

THIS LIVING WORLD

Modern Apes

Dryopithecus

Propliopithecus

Old World Monkeys

New World Monkeys

Notharctus

The primates are the order of mammals thai include the lemurs, tarsiers, monkeys, great
apes and man, as well as a number of extinct forms.

SIZE AND CUNNING 233

great apes, and man himself. The fossil record of the primates is
much less complete than that of the other great mammal groups.
Therefore, the pedigrees of man and his nearest of kin are much
less documented than those of horses, elephants, and cats. Their
habitats probably account for such scarcity of fossil remains.
Being tree dwellers, their remains were left in the forests, and
fossil remains of vertebrates are normally not generally formed
in forested areas. Particularly is this true in tropical climates,
where the early primates most certainly must have lived.

The earliest tree-living insect eaters were probably not unlike
certain tree shrews now existing. These animals are usually
smaller than squirrels and have long bushy tails. They are found
in large numbers in Borneo, South China, and the Malay Penin-
sula. Fossil remains of animals similar to these and belonging to
the early Cenozoic have been unearthed. These creatures are
called Northarctus. The head was about two inches long and in
the fine characteristics of its bones it was partly similar to tree
shrews and partly similar to the Old World monkeys. No doubt,
then, it was very close to the stem leading to modern primates.

The most primitive of the primates are the lemurs, found
today mainly on the tropical island, Madagascar. The lemur is a
small animal with busy hair; it is nocturnal in habits and lives
in trees. Fossils of lemurs not unlike the modern ones of Mada-
gascar have been found in Europe and North America in rocks
dating back to the early Cenozoic. This primitive stock has been
able to endure in Madagascar to the present, apparently because
the island has long been separated from the mainland of Africa.
In consequence, few flesh eaters have been able to enter this
region and dispute the territory with the lemurs. However, no
fossil remains in their former continental homes have been found
that are later than the early Cenozoic. It is likely that these early
primates were all killed in such localities or developed into more
advanced forms.

Next up the ladder of modern primates is a curious little
animal found in the East Indies, known as Tarsius. It has a
few specialized features such as a long rat-like tail, long hind legs,
and exceptionally large eyes turned completely to the front of the
face. However, the rest of the body structure is remarkably inter-
mediate between lemurs and monkeys. The brain is large,

THIS LIVING WORLD

Capuchin howler monkeys photographed in their native wild haunts of Barro Colorado
Island, Panama Canal Zone/ expressing contempt for some captive monkeys nearby.
(Photograph by Dr. Frank M. Chapman, American Museum of Natural History.)

the head round, and the face an archaic prototype of that of the
monkeys. While the modern Tarsius is a specialized type, the
early fossil Tarsius remains which date back to early Cenozoic
appear to be close to the stem leading to monkeys and apes.

Above the Tarsius in primate development are the monkeys.
Modern forms are found in both the Old World and the New
World. They, too, have large eyes that look to the front, as is
true of the Tarsius. However, in the case of the Tarsius the
creature has two fields of vision, one in each eye, as the nerves

SIZE AND CUNNING 235

leading from each eye do not cross or mix in the brain. Therefore,
it cannot blend the picture into a single image, but rather must
see two images of the same scene. The monkeys have a crossing
of the optic nerves before entering the brain, which permits a
blending of the images. This gives them true stereoscopic vision,
such as man possesses.

Another line of development led to the higher primates, in-
including the anthropoid apes and man. This divergence appar-
ently took place before the Miocene epoch, for in the same bed
in Egy|)t that was referred to above have been found the remains
of a small ancestral ape which is known by the imposing name
of Propliopithecus. The jaw is about two and a half inches long
and has the beginnings of characteristics found in modern apes.

Fossils of apes are exceedingly scarce; however, those that
have been found seem to show that these anthropoids were the
last to separate from the primate stem that eventually led to
man. In the Miocene and more recent rocks of India have been
found fragmentary remains of large ape-like primates, known
as Dryopithecus. These bones possess features found today in
the chimpanzee and gorilla. Another highly significant fossil is
one that was found in South Africa; it has been assigned the
long name of Australopithecus. Considerable difficulty has been
encountered and weighty controversies have ensued in the classi-
fication of this fossil. It has some characteristics that are typical
of modern apes and some that are distinctly human. This un-
certainty of classification is indication of the relatively recent
divergence of the ape and human stems from some common
ancestry.

Anthropoids of Our Times

There are at present on the earth four species of the anthro-
poid apes. These are the gibbon, orangutan, chimpanzee, and
gorilla. In size they range from the light-bodied gibbon, about
three feet high, to the heavy -bodied gorilla, which stands about
five and one-half feet high. They have broad chests in contrast
with the narrow chests of monkeys and all other mammals
except man. The hands are quite similar to man's, except that
the fingers are long in comparison to the thumb. In many other
respects, the skeletons are close to the human type. The main

236

THIS LIVING WORLD

Chimpanzee mother teaching baby to walk. Upper picture, mother calls baby to come

. ,

to her arms. Middle picture, teaching baby to step. Lower picture, proud moment when

.
baby walks alone in fifth month. (Science Service photograph.)

SIZE AND CUNNING 237

differences are in the shape of the skull, the short legs, and the
long grasping type of large toe.

The gibbons, found chiefly in the Malay region, are the most
primitive of the anthropoid apes. They have extremely long arms
and usually walk erect, with the hands reaching the ground, and
live chiefly in wooded slopes of hills and valleys. Being arboreal
in habit, they move through the trees with great agility. They
swing themselves from the limbs of trees with their hands and
arms, being able to clear spaces of fifteen to thirty feet with the
greatest ease and finest precision. They are, therefore, the great
acrobats of the anthropoids. The gibbon is the most specialized
ape in the length of its arms and other adaptations to arboreal
life. Its brain cavity is small as compared to other apes. How-
ever, the top of the skull is smooth, and the forehead lacks the
prominent ridges above the eyes. Its profile, therefore, closely
resembles that of man.

The orang is confined mostly to the swampy forests of
Sumatra and Borneo. This red-haired creature grows to about
four feet in height, and its body is bulky. It has long arms,
which reach to its ankles. It is a good arboreal type and remains
in trees most of the day, being able to swing from one treetop
to another. It builds a sort of platform or nest in a convenient
crotch in the tree, on which it reposes much of the time. Usually
these nests are about twenty-five feet above the ground. The
orang is a highly intelligent ape, but sluggish in its disposition
and habits. It runs laboriously on all fours. It never stands
erect. It is a highly specialized type and represents one distinct
branch of ape development.

The chimpanzee is a native of Equatorial Africa. This ape
is essentially a tree dweller, but is much more at home on the
ground than the gibbons or orangs. It grows to about five feet in
height and is not so bulky as the orang. It sometimes stands or
walks on the hind limbs, but it runs on all fours. It responds
readily to human association, and it is the interesting little ape
that is seen so frequently in the captivity of man. The chim-
panzee has a comparatively well-developed brain and seems to
adapt itself easily to its environment. Much experimentation by
psychologists has recently been done in testing the intelligence
of these animals. They show the rudiments of human intelli-

238

THIS LIVING WORLD

"Bamboo", adult male gorilla raised at the Philadelphia Zoo. (Science Service photo-
graph.)

gence; for example, they have a good memory, and they show
some reasoning powers of the human type.

The gorilla is found only in Central Africa. It is by far the
more impressive of the man-like apes, and, it is the most like
man in body structure and in mental development. It has a brain
capacity equal to nearly half that of man. It is not easily cap-
tured, and adults rarely live long in captivity. A few baby
gorillas have been captured. They prove to be both affectionate
and intelligent. The male gorilla grows to about five and one-half

SIZE AND CUNNING 239

feet in height, and it has an arm spread of about eight feet. It
has departed from the slender-bodied, long-limbed type which is
adaptable for arboreal life. It exhibits a transitional stage leading
to ground-dwelling habits, such as those most highly developed
in man. The body is powerful and the legs are strong. The hands
are human in form and shape. Gorillas rarely walk erect, their
bodies being too heavy to be easily supported by the legs.

Quite contrary to general belief, the gorilla is not an aggres-
sive, ferocious animal. According to Carl Akeley, noted African
explorer, the gorilla shows no indication of marked aggressive-
ness, or that he will fight if there is any means of escape. Rather
he is a perfectly amiable, good-natured creature when not har-
assed or attacked by man or other animals. The gorilla has
developed the rudiments of family life. Gorillas are usually seen
in their natural habitats in family groups consisting of the
father, mother, and one child or more. Also, the family may
adopt some particular spot of territory that is considered as
"home."

REFERENCES FOR MORE EXTENDED READING

GREGORY, W. K., and H. C. RAVEN: "In Quest of Gorillas," The Darwin Press,
New Bedford, Mass., 1937.

Here is told a lively and interesting story of a scientific exploration into the moun-
tains and jungles of equatorial Africa. Aside from the personal experiences of the expe-
dition, the authors have included an extensive account of the gorilla's characteristics
and life habits.

AKELEY, DELIA J., "J. T., Jr.": The Macmillan Company, New York, 1937.

The author subtitles this book, "The Biography of an African Monkey." It is an
account of Mrs. Akeley 's exploration in East Africa for the Field Museum of Natural
History of Chicago and the experiences of a small African monkey which accom-
panied the expedition and finally was brought to New York. The records of the daily
life of " J. T., Jr.," prove to be an interesting story and one that has scientific value
in the study of the habits and characteristics of monkeys.

ELY, A., H. E. ANTHONY, and R. M. CARPENTER: "North American Big
Game/* Charles Scribner's Sons, New York, 1938.

This book is a unique combination of a scientific description and distribution of the
large mammals of North America and a fascinating story of their habits and character-
istics as observed by many experts who have hunted them. In addition, it makes a
subtle recommendation for the conservation of these wild animals. Illustrated with
photographs, maps, and drawings.

240 THIS LIVING WORLD

DITMARS, RAYMOND L.: "Reptiles of the World," The Macmilian Company,
New York, 1933.

This book is an excellent general account of the most important present-day rep-
tiles of the world. It is written in popular style but is in general accord with the scien-
tific study of modern reptiles.

CURKAN, C. H., and CARL KAUFFELD: ** Snakes and Their Ways," Harper &
Brothers, New York, 1937.

This publication is a general and thorough treatment of snakes, dealing with their
habits, habitats, appearances, and relationships to each other. It is written in highly
interesting style, yet adheres to the scientific facts regarding the species discussed.

SCHUCHERT, CHARLES, and CLARA M. LEVENE: "The Earth and Its
Rhythms," D. Appleton-Century Company, Inc., New York, 1927, Chaps.
XXVI-XXIX.

These chapters include a discussion of the development and characteristics of
reptiles and mammals during the Mesozoic and Cenozoic eras.

CRONEIS, CAREY, and W. C. KRUMBEIN: "Down to Earth," University of
Chicago Press, Chicago, 1936, Chaps. XLIII, XLIV, XL VIII.

The chapters referred to are a lively discussion of the ruling reptiles and the
warm-blooded mammals of the Mesozoic and Cenozoic eras. The unique drawings
add interest and understanding to the text.

ROMER, A. S.: "Man and the Vertebrates," University of Chicago Press,
Chicago, 1933, Chaps. III-IX.

In the chapters referred to are to be found an excellent nontechnical account of the
origin of reptiles and mammals and the most important modern types of these
animals.

SCHUCHERT, CHARLES, and C. O. DUNBAR: "Historical Geology," 3d ed.,
John Wiley & Sons, Inc., New York, 1933, Chaps. XV-XX.

These chapters include a wealth of well-written material regarding the rock
formations and life of the Mesozoic and Cenozoic eras.

Natural History, published by the American Museum of Natural History,
New York.

Natural History is a monthly magazine published for members of the museum, which
contains articles on a great variety of natural-history subjects. These articles are
interestingly written and usually illustrated with remarkable photographs.

Journal of Mammology, published by the American Society of Mammologists,
Baltimore.

This is a professional journal published quarterly for the members of the society.
It contains nontechnical articles on various types of mammals as well as articles
deeding With original investigations in this field. An extended list of recent literature
and a review of current books relating to mammal life are included in each issue.

8: THE LAST MILLION YEARS

Or Human Development from Early Man to Modern Races

IN 1891 Dr. Eugene Dubois, a medical officer of the Dutch
army, discovered at Trinil in Java the fossil remains of what
is known as the Java ape man. The discoverer at once claimed
that the fossil represented an intermediate state between apes
and men, and it was widely labeled "the missing link." The real
status of this ancient fossil has been the subject of great debate
for five decades. Many experts have strongly argued that the
fossil is human in character, while others formerly maintained
that it was an oversized ape. Likewise, its age has been widely
placed at from a half million to nearly one million years old.
However, in light of more detailed studies of this fossil and the
recent discovery of other human fossils in Java which resemble
it, present-day paleontologists are generally agreed that Java
ape man is quite definitely to be considered human and that he
lived during the early part of the Pleistocene epoch.

341

242 THIS LIVING WORLD

This primitive fossil and the controversies that have cen-
tered around it indicate that human origins are definitely con-
nected with the primate stem. While it is no longer maintained
that Java ape man is the direct ancestor to modern peoples, it is
known to represent one branch of human development. Even in
this case it gives some indication of the steps man has gone
through in his long ascent to the foremost place in the world
of life.

It has been said that man seems so different from all other
animal nature that he stands isolated and alone. However, if the
fabled man from Mars, first chronicled by H. G. Wells thirty
years ago and recently made real in a dramatization, should
actually visit the earth, he probably would not recognize such
isolation in man. No doubt man would be described as a biped
mammal, of specialized development and of large brain capacity.
Upon examination of his life processes and his body structure,
comparing the body organ by organ with that of other animals,
he would be found to be related to other vertebrates and most
like the great apes. The fossil records, even now available, would
show a probable origin from the primate stem and many stages
of development from those primitive types to modern peoples.
Fortunately, it is not necessary to await the arrival of the hypo-
thetical Martians for this enlightenment. Man himself has dis-
covered much information of this type.

Distinsuishins Man from the Apes

When one makes a casual comparison of man with the great
apes, it seems that their differences are so enormous that they
have little likeness in common. The chimpanzee's short legs and
feet adapted to arboreal life, its protruding jaw, and hairy skin
appear to bear little resemblances to man's long legs and feet
specialized for erect walking, his reduced jaws, and the relative
absence of hair on his body. However, a careful analysis of the
physical structure of the bodies of apes and man shows them to
be fundamentally more alike than different.

We would think tllat the apes are more nearly like monkeys
than man as regards their coat of hair. Careful counts of the
number of hairs on unit areas of skin on the back and chest have
shown that the number of hairs on the skin of the apes is much

THE LAST MILLION YEARS 243

less than that on the skin of monkeys. Likewise, the number of
hairs on the skin of man is less than on the skin of apes. How-
ever, the difference between apes and the monkeys is greater
than the difference between man and the apes ; so much so, that
the apes may be considered to have a relatively hairless skin.
The seeming absence of hair on the body of man is more ap-
parent than real. His entire skin, with the exception of the palms
of the hands, soles of the feet, and a few specialized areas, such as
the lips, contain hair follicles. The rudiments of a hair grow in
each follicle, a condition that may be seen under microscopic
examination. In most cases these rudimentary hairs do not
develop on the skin of man to be as noticeable as the hair on the
apes.

The long arms and hands of the apes show marked contrast
to those of man. This is an adaptation to the habit of hanging
from branches and to the use of the hands in walking. The apes
progress over the ground by means of "hand walking."

The feet of the apes have not become adapted to bipedal
walking as have the feet of man. In this respect, however, they
are more like man than they are like the feet of monkeys, which
are strictly arboreal. The feet of the gorilla, in particular, seem
to be about midway between adaptation to arboreal and to
bipedal ground habits. The great toe is opposite the other toes,
a condition that makes the foot a grasping organ. In adult man
the great toe is in line with the others so that the foot rests flat
on the ground. In human infants, on the other hand, the great
toe is somewhat opposable, but rapidly develops into the
human type as walking is learned. The mountain gorilla of the
Eastern Belgian Congo has feet that are most like man of any
of the great apes. In all apes, however, the bones of the hands
and feet match those in man. The main distinctions between
them are slight differences in size and length and in the muscles
which are attached to them.

The bones of the head of man are also distinctive when com-
pared to those of the apes. This distinction is one of size and
shape rather than number or type. The bones of the cranium
have been greatly enlarged to accomodate the expanded brain.
The jaws and teeth are reduced in size. The human canine teeth
are much smaller than those teeth in the apes. Despite the

244

THIS LIVING WORLD

Man

The human brain is more than twice the volume of the gorilla's brain.

conspicuous difference in size between the teeth of apes and of
man, the teeth of both are of the same basic pattern. The large
cranium of man is balanced on top of the vertebral column,
rather than being thrust forward as in the apes.

Biochemically and physiologically the apes and man are very
similar. Only the most accurate and delicate tests show any
difference between the blood of man and of the apes. Apes are
susceptible to practically all human diseases. The normal life
span of the apes is believed to be about that of man. Chimpan-
zees have been captured that are said to be about sixty years old.
The placenta of the developing ape embryo is much more nearly
like that of man than it is like that of the monkeys. Full-time
pregnancy in the chimpanzee is about eight months and in the
gorilla is thought to be slightly longer.

The most outstanding characteristic of man is the human
brain. In this respect he stands greatly superior to the other
anthropoids. This superiority is due to the size and complexity
of organization of the brain rather than to new parts. It is a
difference in degree only. The convolution pattern in the brains
of apes is very similar to that in men. Even the microscopic
structure of nerve cells within the brain stem of the apes is
almost identical with that found in man. In man the cerebrum,
which is the seat of higher mental facilities, is greatly enlarged
over that of the apes. The human brain ranges in volume from
about 1,000 cubic centimeters to about 2,000 cubic centimeters;
however, for most people the volume ranges between 1,200 cubic

THE LAST MILLION YEARS 245

centimeters and 1,500 cubic centimeters. The volume of the
gorilla's brain is about 600 cubic centimeters, while that of the
chimpanzee is somewhat smaller.

The apes are entirely lacking in the capacity for speech,
which is one of the chief criteria of human mentality. Lack of
speech on the part of the apes is not due to the absence of vocal
cords and other apparatus for making sounds. Speech requires
an elaborate association mechanism in the mind to coordinate
sound symbols into intelligent language. In man this involves
several different parts of the brain. No such association mechan-
ism seems to exist in the brains of apes. This deficiency will
probably prevent modern apes from ever attaining any cultural
inheritance.

These bodily differences between man and the apes did not
arise in one swift change. They were slow in their development
and have extended over most of the last great epoch in geologic
history, that is, the Pleistocene epoch.

Period of Great Climatic Changes

So far as we know, practically all human development has
occurred since the beginning of Pleistocene times. It is only dur-
ing this period that any fossils of man are found, and those
belonging to the earlier part of the Pleistocene have exceedingly
primitive characteristics. This indicates that at the beginning
of this time man had just emerged from the primate stem. The
Pleistocene was a period of great climatic changes. It was one
of the critical times in the world's history. It is in order, there-
fore, to review the climatic conditions existing during this
period, since these changes apparently had a great effect upon
mammal life of those times.

The Pleistocene epoch began about a million years ago, and
ended about twenty -five thousand years ago with the receding
of the last great glaciers. It is often referred to as the "Great Ice
Age." In fact, during this time at least four great continental
glaciers covered many millions of square miles of the northern
parts of America and Europe. The ice sheets extended, at the
greatest, as far south as New York City, the Ohio and Missouri
rivers, and the head of Puget Sound in North America. In
Europe they covered most of Great Britain, present Germany,

246 THIS LIVING WORLD

European Russia, and all countries to the north of them. Glaciers
were practically absent from all of Asia, probably because of
lack of precipitation to form snow and ice. It is not known
exactly how thick these continental ice sheets were; however,
they were very great. They must have been at least 4,000 feet
thick, and some authorities more than double this figure.

In America, studies have shown distinctly that there were four
successive periods of glacial maxima. Between each of these
times of the ice sheets there were long periods of time, tens or
even hundreds of thousands of years, in which the glaciers melted
as far to the north as they are today, or perhaps farther. During
these times the climate was temperate or tropical in Northern
United States and Southern Canada. Also, in Europe, there were
four glacial periods separated by periods of mild climate, about
the same as those occurring in North America. The exact time
of the glacial periods has by no means been determined. How-
ever, for Europe, it is probable that the first two, Giinz and
Mindel, occurred relatively close together near the first part of
the Pleistocene, and that the latter two, Riss and Wiirm, were
somewhat associated near the end of this epoch. The first of
these four glaciers began to creep down over the continents
probably about a million years ago. The Wiirm glaciation, the
last, receded about twenty -five thousand years ago.

These glacial periods are indicated on the accompanying
chart. The names given above refer to the glaciers in Europe.
For those who might be interested, it is worth noting that cor-
responding glaciers in America have been named the Wisconsin,
Illinoian, Kansan, and Nebraskan.

The glaciers reduced greatly the habitable land in Europe and
North America. Man as well as the rest of life had to make ad-
justments to these changed conditions. However, the advance of
the glaciers made great areas to the south of them desirable
habitats which are now desert wastes. North Africa and the
Sahara received much greater rainfall than at present and
apparently had mild climates. All of Asia as far north as The
Gobi and North China had abundant rainfall and moderate
climates, so that the whole vast deserts from Southwestern Asia
to Mongolia became habitable areas. It is within some of these
areas that all the great early historic civilizations sprang up

THE LAST MILLION YEARS

247

GEOLOGIC
AGE

Recent

30,000
Years

o
o

r-
tn

j
a

1,000,000

GLAC1AT10NS

EARTH'S
CONDITIONS

Wiirm

Riss

Mindel

Giinz

Years
PLIOCENE

i i
i i

The Pleistocene period was a time of great climatic changes in which four glaciers
extended over much of Europe and North America. These ice ages were separated from
each other by long intervals of time when temperate or tropical conditions prevailed over
the greater part of these continents.

248 THIS LIVING WORLD

and not at the foot of the glaciers. It is likely, therefore, that
Southern or Central Asia was the home of much earlier human
development.

Thus during the Pleistocene epoch, Europe and North
American experienced periods of great glaciers and icy blasts
coming down from the north, interspersed with warmer times
when tropical conditions prevailed far into our present tem-
perate zones. It seems evident that during much of this time
Africa had temperate and semitropic climates. Probably these
conditions existing over such great areas during the Pleistocene
had much to do with the geologically rapid development of
man and the other primates. The Pleistocene epoch is to be
considered the "Age of Man."

Peoples of Bygone Ages

The fossil record of man gives some insight into the develop-
ment and characteristics of peoples in times long since past.
This record is painfully incomplete, and great gaps still exist in
man's ancestral lineage. However, the number of present known
human fossils that predate recorded history is far greater than
most people realize. Over three hundred complete or fragmentary
human fossils of earlier vintage than modern man have been
unearthed from their elusive burials. In addition, thousands of
artifacts, or human tools, have been discovered which add their
bit to the picture of human development. The main outline of
this development we see in clear relief; however, the details are
often obscure.

One of the earliest known of the man-like fossils is the Java
ape man, referred to in the opening paragraphs of this chapter.
The remains in the original discovery consisted of the top of a
skull, a left thighbone and two upper molar teeth. Somewhat
later a third tooth, a premolar, was found in the same deposits,
and it has been added to the original collection. The fossil was
found in a layer of volcanic debris which exists some sixty feet
below the present level of the land in the Solo River bed near
the village of Trinil. River erosion has cut through the overlying
strata and exposed the volcanic eruptions which are believed to
have occurred in the tropical forests of Java during the second
glacial period in Europe. This establishes the fossil as belonging

THE LAST MILLION YEARS 249

to about the middle of the Pleistocene epoch. Thus, Java ape
man lived at a time not later than about a half million years ago.

The name given to this creature
by the discoverer is Pithecanthropus
erectus. This means "erect ape
man." The leg bone is definitely
human and shows that he walked
erect. The bone indicates a height
of about five feet six inches, which
is near the average of modern man.
The skull cap shows from very
careful measurements completed by Java ape man.

Dr. J. H. McGregor of Columbia

University that Pithecanthropus had a brain capacity of
slightly less than 900 cubic centimeters. This puts it near the
minimum range of some modern human skulls. The shape of the
brain as reconstructed from the configuration of the skull cap
resembles more that of man than it does the apes. Motor and
auditory areas were developed to the extent that it may be
inferred that Java man had a rudimentary language.

The skull as a whole is primitive and ape-like in character.
The cranial vault is low and receding. The forehead was narrow,
and there was a very heavy brow ridge over the eyes. These
features must have given an ape-like appearance. However, the
median ridge over the skull is quite reduced, and the area of
attachment of the temporal muscles indicates that the jaws
were much smaller than those of the apes.

The first two teeth found are large and resemble closely those
of an ape. It has recently been substantiated that these two
teeth do not belong to the skull, but rather are those of an
extinct ape living in Java at that time. The third tooth, the
premolar, is decidedly human. If it belongs to the skull, as gen-
erally agreed, it would also indicate that the jaws were more
nearly human than ape in size and shape.

What, then, is the relationship of the Java man to later
human types ? There is no general agreement among the authori-
ties on this subject. It seems unlikely that Pithecanthropus was
the direct ancestor to either modern man or to the Neanderthal
people, a specialized type which lived in Europe in large numbers

250

THIS LIVING WORLD

General location and excavation of some of the Sinanthropus skulls near Peking, China.
Careful records of exact location of all fossils in the formations were made by designating
each point in areas marked off with the white lines. (Photograph by Dr. Franz Weiden-
reich.)

at a much later date and which has now become extinct. How-
ever, its importance is not diminished by this unknown relation-
ship. It is at least one early type of human development.

A series of discoveries of major proportions in the study of
prehistoric man have been made near Peking, China, within the
last few years. They are fossils of a primitive human type which
has been called Sinanthropus pekinensis, meaning " Chinese man
of Peking." The first remains were found in 1926, and they con-
sisted of three human teeth. These teeth were discovered in
filled-in fissures at the base of limestone hills. During Pleistocene
times these fissures were open caves in the limestone which were
used by both man and beasts. In the course of the succeeding
ages the caves became gradually filled with red clay and bones.
These were cemented together by limestone from the caves.
Today these filled-in caves remain as great pillars and columns
of hard sedimentary rock within the hills of purer limestone. A
careful study of the extinct animal bones found in these deposits

THE LAST MILLION YEARS

251

Reconstruction of head and skull of a Sinanthropus woman by Dr. Franz Weidenreich and
Mrs. Lucile Swan. (Photograph by Dr. Franz Weidenreich.)

shows that the deposits were laid down not later than the middle
of the Pleistocene, or approximately one half million years ago.
They may be much older.

In 1927 a systematic evacuation of the sites was organized
under the joint auspices of the Peking Union Medical College
and the Geological Survey of China. Despite the task of remov-
ing sizable portions of the limestone hills and the drawbacks
resulting from a war in the area and its occupation by a foreign
army, the work has continued to the present. Parts of twenty-
six human fossils had been excavated by 1939. Of these, six are
fairly complete skulls. The others are fragments of skulls, jaws,
and teeth.

The shapes of all the skulls are quite similar, although they
vary considerably in size. The vault of the skull was low, the
forehead was narrow and retreating, and the brow ridge over
the eyes was prominent. In these respects Peking man resembled
closely Java man, with whom he is now believed to have been
contemporaneous. In other features Sinanthropus was more like
the later Neanderthals and even modern man. The brain capac-
ity of one of the skulls is about 900 cubic centimeters, which is

252 THIS LIVING WORLD

about the same as Pithecanthropus. The others are larger, with
cranial capacities ranging from 1,050 cubic centimeters to about
1,200 cubic centimeters. These come within the brain sizes of
Neanderthal, as well as exceed the minimum size for modern
man. The cheekbones, where found, are prominent and do not
slope obliquely backward. This is quite unlike Neanderthal and
resembles more the Mongoloids of modern peoples. The teeth
exceed in size somewhat those of Neanderthal and modern man ;
but in shape they are characteristic of Neanderthal, and re-
markably enough, of modern yellow-race peoples.

It is difficult at present to determine Peking man's relation-
ship to later human types. The sizes and shapes of the skulls bear
some close resemblances to Neanderthal man. The dentition as
well as certain features of the jaws indicate that Peking man was
not far removed from the stems which led to Neanderthal or to
modern man. Recent discoveries of human stone tools in
Mongolia that are intermediate in their chipping between those
of Peking man and the later ones of Neanderthal strengthen the
claim that Peking man was the ancestor to the Neanderthals.
On the other hand, it has been clamied by some authorities that
Peking man is the remote ancestor to modern Mongoloids. How-
ever, such a claim is probably immature on the basis of our
understanding of the history of modern races and the immediate
successors to Peking man.

About two thousand pieces of crudely fashioned stone and
bone implements were found in the deposits. Many of these
stones were foreign to the region and must have been carried in
from great distances. There were also found pothole fireplaces,
charcoal, and the charred remains of animals. The charred
remains of animal bones indicate that these people knew how
to use fire and how to cook their foods. In discovering the use of
fire and how to make tools, Peking man had developed the
beginnings of human culture.

Another human fossil of ancient lineage and very unusual
characteristics is the Piltdown man. It is scientifically called
Eoanthropus dawsoni. The first part of this imposing name
means "dawn man" and the second part is designated in honor
of its discoverer, Charles Dawson. It was found near Piltdown,
England, in deposits that have been dated as belonging to the

THE LAST MILLION YEARS 253

middle Pleistocene epoch. Had the skull alone been discovered,
it probably would have been identified as a modern human skull
and forgotten. However, near it was
found a part of a lower jaw that
contained two teeth. This jaw was
remarkably like the jaw of a chim-
panzee, but no fossils of chimpan-
zees were known in England. The
fact that the jaw and skull were
found close together in the same
deposits and have the same degree
of fossilization has led the foremost Piltdown man.

anthropologists of England to assert that they belong to the same
individual. The teeth are primarily ape-like but have crowns that
approach those of man, thus strengthening somewhat the idea
that the skull and jaw belong together.

When the head is reconstructed the cranium is distinctly
human. The forehead is relatively vertical, and there is a com-
plete absence of a brow ridge ^bove the eyes. The head is large
and has the general contour of modern man. The skull bones are
exceedingly thick, however, and the brain capacity is about
1,240 cubic centimeters. The jaw is distinctly ape-like in the
chin region, and the teeth are large. Thus the fossil is a peculiar
blend of anthropoid and human characteristics. Even with this
perplexity, many anthropologists hold that the Piltdown man
was very close to the stem which led to modern man.

The modern human genus, Homo, is first represented by
Heidelberg man, Homo heidelbergensis. It is a fossil type which
belongs near the middle Pleistocene epoch and possibly to the
second interglacial time. This makes the fossil at least 150,000
years old and probably twice that age. This fossil was found near
Heidelberg, Germany, in 1907, in a sand quarry at a depth of
seventy-nine feet below the present surface. It was in a layef of
ancient sand and gravel deposited by a river overflowing into
an old lake bed. Only the lower jaw with the teeth well preserved
was found. It apparently had drifted down with the old river
sands. The rest of the skull and skeleton was evidently washed
elsewhere and probably lost forever. Careful search for many

254 THIS LIVING WORLD

years since the time of this original discovery has failed to locate
any additional fossils of Heidelberg man.

The jawbone is massive and broad; however, it has certain
features in common with the later Neanderthal race. The chin
is distinctly receding. In this respect it resembles the jaw of an
ape, but the teeth are definitely human. The canines do not
project beyond the line of the other teeth, as do those of the apes
as well as of some prehistoric human fossils. It is probable, there-
fore, that Heidelberg was a Neanderthal man in the making, or
very near the direct line of ancestry which led to this race of
ater European inhabitants.

Homo neanderthalensis

Neanderthal man represents an extinct people who would
iierit a description occupying the entire space allocated to this
chapter because of their large numbers, the long period of time
they inhabited Europe and Asia, and the wealth of information
that has been secured regarding them. The discovery which first
led to the recognition of Neanderthal as a definite human type
was made in 1856, when portions of a skeleton were found in a
cave in the Neanderthal valley near Diisseldorf , Germany. Be-
cause these remains showed many distinctive features, they were
made the type specimen of the species, Homo neanderthalensis.
Since then many of their fossils have been found in other parts
of Germany, in Belgium, France, Spain, at Gibraltar, in the
islands of the English Channel, in Italy, and in various parts of
the Balkans. In 1932 a remarkable series of skeletons were
discovered in Palestine, some of which have many Neanderthal
characteristics. Thus the Neanderthal or proto-Neanderthal
peoples are definitely known to have inhabited Southwestern
Asia, as well as Europe.

In addition to these skeletons, many thousands of artifacts
in the form of stone tools and implements of bone that were
fashioned by Neanderthal have been found in more widely
scattered areas of Europe and Asia and Northern Africa. As
recently as 1939 Dr. Alex Hrdlicka of the Smithsonian Institu-
tion discovered unquestionable Neanderthal artifacts in Mon-
golia, and it has been reported that similar artifacts have been
found in Northwestern China.

THE LAST MILLION YEARS 255

Many of the Neanderthal remains have been rather accu-
rately dated. The fossils found in Palestine, usually referred to
as the Mount Carmel skeletons, were deposited during the early
part of the Third Interglacial period, which was the warm
interval between the Hiss and Wlirm glaciers. This would con-
servatively place them as about 75,000 years old. It is believed
that some of these fossils represent people of the Neanderthal
type who migrated into Europe from Asia, where it is not un-
likely the Neanderthals originated. Likewise, some of the
European Neanderthal fossils definitely belong to the Third
Interglacial. Others have been found in deposits as recent as
about 25,000 years ago. Thus, Neanderthal lived in Europe for
a period of approximately 50,000 years, a time that is twice as
long as that from the date of their extinction until the present.
Apparently they were in Asia for a longer period.

There is some variation in the physical features of Neander-
thal man, even as there are variations among modern racial
groups; however, the type can be rather accurately described.
A few details here are sufficient to give a general picture.
Neanderthal man was of short stature, averaging about five
feet to five feet four inches. The limb bones were particularly
robust. The forearm and shin were short in comparison to the
upper bones of the arm and leg. The thighbone was curved for-
ward. The tibia, one of the bones of the lower leg, indicates that
the knee was flexed somewhat in the standing and walking posi-
tions. These conditions of curved thigh and flexed knees warrant
the statement from some anthropologists that Neanderthal
probably walked in a semierect position. However, this is denied
by others who explain these peculiar characteristics as probably
resulting from a squatting habit, rather than portraying a non-
erect walking position. The ribs were heavy, with large muscle
attachments, and indicate a large chest. The vertebral column
was short and massive, but otherwise was probably similar in
shape to modern man.

The most characteristic features of Neanderthal are to be
found in the skull and jaw. The forehead sloped rapidly back-
ward from large ridges above the eyes. The eyes were deep-set
below the overhanging brow. The face was exceedingly long and
narrow as compared to modern man. The nose was of great

256 THIS LIVING WORLD

width, also long and large, not flat. The cheekbones were not at
all prominent. The head was somewhat imperfectly balanced

forward on the neck. The jaw
had no chin prominence, and the
teeth were large in all their
dimensions.

The Neanderthal features
were distinctly human in most
cases, even if in some instances
they are more ape-like than are
those found in modern man. The
brain capacity ranged from about
1,100 cubic centimeters to ap-
Neanderthal man. proximately 1,600 cubic centi-

meters, limits that do not compare unfavorably with modern
peoples. Neanderthal was much more human-like in appearance
than ape-like, even though popular representations of him
have frequently been the opposite. Judged by today's stand-
ards of human features, however, he was probably a stodgy
and unattractive creature. Neanderthal seems to have been a
successor to Heidelberg man and possibly descended from
earlier types in Asia. He is known to have lived in Europe until
the receding of the last glacier, when other peoples began to
migrate there in large numbers. After this he disappeared from
the European scene as a pure type, either by complete extinction
or by inbreeding with other types.

Homo sapiens Appears

As we have just noted, the Neanderthal people lived in
Europe, apparently in large numbers, for thousands of years.
They were the first example of man's occupying a continent in
somewhat the widespread manner in which man occupies the
earth today, at least so far as we know. However, with the reced-
ing of the last glacier another group of people appeared in Europe
and became the dominant type there. Their conquest of the
Neanderthals was not so rapid as was the conquest of the North
American Indians by the white man, but it seemed to have been
about as effective. Not only did this new group introduce into
Europe a new and improved culture, but they were a new race

THE LAST MILLION YEARS 257

of mankind. These people were definitely of the modern type and
belonged to our own species, Homo sapiens. They are generally
referred to as Cro-Magnon man.

Just where these people came from is not known. It has been
held by the eminent authority, Dr. Hrdlicka, that they de-
veloped from Neanderthal man. However, he says there is not
sufficient evidence as yet available to prove this theory. The
fossil remains and cultural material now known make this claim
seem unlikely. There are few, if any, human fossils from Europe
that show any definite intermediate stages between the physical
features of Neanderthal and Cro-Magnon. Further, Cro-Magnon
cultural remains are found in deposits immediately above those
of Neanderthal and sometimes in contemporaneous deposits.
This indicates that the change of culture was sudden, not a
gradual development.

It is likely that Cro-Magnon man originated elsewhere,
probably Asia or Africa, and migrated into Europe as the
climate changed to mildness following the receding of the last
glacier, approximately 25,000 years ago. One substantial bit of
evidence which indicates Cro-Magnon migrated from Asia into
Europe is found in the Mount Carmel skeletons. These skeletons,
it may be recalled, were found in Palestine and predate by
thousands of years Cro-Magnon's appearance in Europe.
Certain features of some of the Mount Carmel skeletons re-
semble greatly Cro-Magnon, although the characteristics were
somewhat more primitive. It may be reasoned from these
skeletons that Cro-Magnon man developed in Southwestern
Asia and later migrated into Europe.

The type specimens from which Cro-Magnpn man was first
described were five skeletons found in the Cro-Magnon cave in
central France in 1868. Since then other skeletons of similar
type have been found in such widely separated areas as Italy,
Spain, present Germany, and many other localities in France.

The men of the Cro-Magnon peoples are usually described
as tall, averaging six feet or over in height, and of large frame.
This is true of the type specimens and many others that have
been found. However, some Cro-Magnon male skeletons are
much shorter. It is likely that there was considerable variation
in height, just as is found in present peoples; but as a race the

258 THIS LIVING WORLD

Cro-Magnon men were evidently a tall people. The women were
considerably shorter than the men, averaging about five feet

five inches.

Cro-Magnon had an extraordi-
narily large cranial capacity. The
forehead was vertical and high-
vaulted, and the brain capacity
averaged about 1,700 cubic centi-
meters. The skull was decidedly
long-headed, but the face was short
and broad with prominent cheek-
bones. This probably gave the face
somewhat of a disharmonic shape.
The nose was narrow. The lower
jaw was strong and the chin prom-
Cro-Magnon man. inent and relatively narrow.
The chief type of change which was ushered in with the Cro-
Magnons was a superior brain power and with it a modern fore-
head and forebrain. This race was evidently an extremely able
people, judged not only from their physical development but
also from their cultural achievements, a subject which will be
considered later. These people existed in Europe for a few
thousand years. As a true type they have now disappeared. It is
unlikely that they became entirely extinct, but rather inter-
mixed with other peoples who came into Europe later, so that
their descendants live today in France, Spain, and other parts
of Western Europe.

Races of Modern Man

There are now alive on the earth four distinct human races,
all belonging to the same species, Homo sapiens. These are the
white, yellow, Negro, and Australian races. Of the first three
there are many subraces. The story of these races is one with
which increasing knowledge brings absorbing interest. This story
involves the place, time, and manner of their origin, how they
spread over the earth, and their characteristics, cultures, and
habits. It gives a better understanding of the relationships of all
peoples and the struggle which man has had in order to reach his
present stage of civilization. Obviously such a broad considera-

THE LAST MILLION YEARS

259

Modern

Australian Negro

Yellow White

./Cro-Magnon.

Million Years

Chart representing prehistoric and modern man.

tion cannot be undertaken here. However, a few points regard-
ing the probable origin of modern races and their physical
characteristics may serve to give a general insight into this
interesting question.

Origin of Modern Races

It is impossible as yet to say where man originated. It may
have taken place in Central or Southwestern Asia. The physical

260 THIS LIVING WORLD

and climatic conditions of the earth there during the late Ceno-
zoic era and particularly the Pleistocene epoch were such as to

have encouraged this development.
The most ancient known human
fossils have been discovered in Asia.
The Java man was living in Java
at a time when this island was most
certainly connected with Southern
Asia. Peking man to the north was
contemporaneous with the Java
man or perhaps somewhat earlier.
However, it is a long step from
these two ancient types to modern

Rhodesian man man, and there is no known link

directly connecting them. The
first known definitely modern man to appear in Europe was Cro-
Magnon. As previously pointed out, his exact origin is unknown.
This means that at present we know very little about the origin
of our own species. Likewise, the steps man went through from
Cro-Magnon to modern races are still in the realm of speculation
and argument. Further studies of the Mount Carmel skeletons
and additional discoveries in Southwestern Asia may shed more
light on the first modern Europeans.

A fossil that it was thought for a time might shed some light
on the origin of the modern species is that of Rhodesian man,
found in Northern Rhodesia in 1921. The skeleton is an odd
mixture of some very primitive characteristics and some which
are like Homo sapiens. The most obvious features seem to
resemble Neanderthal, while some others are like Cro-Magnon.
This was the basis for reasoning that it might represent a step
in the development of the modern species from Neanderthal.
More detailed studies have revealed that the primitive features
are not those of Neanderthal, and it now seems do not show
any relationship between Neanderthal and Cro-Magnon. The
skeleton has not been accurately dated, and there is reason to
believe it is much too recent to be ancestral to Cro-Magnon. It
is now believed by some that it is an abnormal example of Homo
sapiens of relatively recent date.

THE LAST MILLION YEARS 261

Many other human skeletons of considerable antiquity have
been found in Africa. Some of these have been identified as
early examples of present stocks of the Negro race. They seem
to indicate that there were several Negro types before the end of
the Pleistocene epoch, and that the Negro race developed its
distinctive characteristics in Africa. One of these skeletons, in
particular, is the ancestor to the modern Bushmen and yet
resembles Cro-Magnon man to such an extent that it is con-
sidered by some to represent African Cro-Magnons who migrated
from Europe and finally reached Central and Southern Africa.

Australia, too, has yielded some early human fossils. Two of
these closely resembling modern Australians are dated as belong-
ing to the late Pleistocene and give evidence that the ancestors
to those modern people were on that continent during Pleisto-
cene times. Even more important in establishing the ancient
lineage of the Australians is a series of fossils found in Java in
1936, known as the Solo man. They have been dated as belonging
to the Third Interglacial Period. They are remarkably similar
to modern Australians in some respects and help to establish
the idea that the ancestors to the Australians migrated there
from Southern Asia at times during the Pleistocene when
Australia was connected to Asia by a land bridge.

The brief discussion of these few examples has been given to
point out that modern races have probably developed separately
by a slow and gradual process after migrating to different conti-
nents. Since all modern man is of the same species, it is not
unlikely that the different races developed from some common
ancestry. What the relationship of the original Homo sapiens
was to the very early fossil types of man is not known.

Distinguishing Modern Races

Should anyone stand on a busy street corner of almost any
American city and observe the people passing by, he would
recognize that they have certain marked differences. The differ-
ences lead him to know in a general way that different ones
belong to the white, black, or yellow races. However, when more
precise designations are asked for, difficulty is encountered. Not

262 THIS LIVING WORLD

so many can distinguish between people from Mongolia, China,
or Korea; or between people from Arabia and India.

Differentiating between all the modern human races is a
difficult and often complex process. Racial divisions depend
upon fine distinctions, and only a general outline can be at-
tempted here. A complete classification of races is based upon
their physical, functional, chemical, mental, and pathologic
differences. When all these things are taken into account, it is
found that a great many subdivisions of peoples live in different
parts of the earth today.

There is opportunity here for consideration of only some of
the physical differences in the larger divisions or main races of
modern man. Some of the most important of these physical
distinctions are skin color, eye color, hair color, hair texture,
hairiness of the body, size of the bones, body shape, and head
shape or cephalic index. The anthropologists have devised
methods whereby these physical features may be very accurately
measured. With such exact measurements available, they are
able to apply them to living individuals and give us a description
of different peoples.

If people generally were relatively pure racial types, it would
be simpler to analyze and describe them. However, there has
been such intermingling and inbreeding of peoples of all races
that there are few today who show pure racial characteristics.
Most people represent complex admixtures, at least of subracial
strains. It is always true, however, that certain dominant traits
will be so evident that there is little doubt as to any individual's
main racial classification.

The Whites

The most definite physical feature we generally associate with
the white or Caucasoid race is white skin. However, our inter-
pretation of the term "white" is exceedingly broad in many
instances. This liberal interpretation is an indication that there
are also other features which we recognize as just as descriptive
of the white race as is white skin. When individuals or groups
possess these other features, even though their skin is exceedingly
brown or dark, they are included within the white race. Skin
color is the most variable characteristic, as a matter of fact.

THE LAST MILLION YEARS

263

The Nordic type is represented by flying officer G. L. Ingram of the Canadian air forces.
(Life Magazine photograph.)

It ranges from a delicate pink white to an exceedingly dark
brown.

What then are these main features of the white race ? Almost
universally there is a high development of the frontal region of
the skull, or a high forehead. The nose is long, relatively narrow,
and high. The lips are relatively thin. Usually there is an abun-
dance of hair on the face and body of the males.

Within the subraces are more definite and specific resemb-
lances. At least three subraces are recognized in the whites of
today. They are the Nordic, Alpine, and Mediterranean. In
addition, many anthropologists would form another division to
include the Hindu. Still others would increase the number of
subraces so as to have a separate group for the Cermeniaiis and
even other groups. It all depends upon how fine are the distinc-
tions to be made. However, these last-mentioned groups usually
have at least one feature of one of the three main subraces rather
prominently developed. In any simple consideration of the
subject, such as this, they may be considered as some modifica-
tion of the Nordic, Alpine, or Mediterranean.

The Nordic has a blond to light complexion, blue or gray
eyes, and light hair of fine texture. The whiskers grow long, and

264 THIS LIVING WORLD

Edouard H err lot, former President of the French Chamber of Deputies, possesses many
Alpine characteristics. (Life Magazine photograph.)

there may be a considerable hairiness of the body. The head is
long, narrow, and high. Usually the face is also relatively long.
The Nordic has big bones and is of tall stature. The shoulders
are broad and the chest is thin to medium in thickness. These
people dominate in large measure the regions bordering the
North and Baltic seas. They are best typified by the people in
Sweden, Norway, and Denmark, Northern Germany, and parts
of England.

The Alpines are shorter and stockier than the Nordics. They
have a ruddy to reddish complexion, which tans only moderately.
The eyes and hair are, in general, brown. The hair is somewhat
coarser than that of the Nordics and is usually wavy. The beard
is ample and there is much hair on the body. One of the most
distinguishing features is the possession of a high, short, and
broad head. The face, too, is usually broad. This subrace, more
or less mixed, forms the predominating element in the popula-
tion of Central Europe, the Balkans, most of European Russia,
and the steppes of West Turkestan. They are best typified by the
round-headed, stocky-built German and the Polish Slav.

The Mediterranean race is also shorter than the Nordic. The
stature averages about five feet five inches, and the body shape

THE LAST MILLION YEARS 265

Mediterraneans usually have straight black hair, dark eyes, long head and face. (Globe

photograph.)

is slender. The bones are small. The head form is similar to the
Nordics, however. The head and face are long, and the nose is
straight and thin. The skin is olive to very dark brown in shade.
It may become ivory white when not exposed to intense sun-
light, but it tans easily. The hair varies from brown to black and
is usually straight. There is little hairiness of the body. The eyes
are dark, varying from brown to black. The Mediterranean is
the type found generally in Southern Europe, Southwestern
Asia, India, Northern and Eastern Africa, and parts of the
British Isles. These racial peoples have long been prominent
in these native areas. The great ancient civilizations of Sumeria,
Babylonia, Egypt, and Greece were all developed by the
Mediterraneans.

The Negroes

The Negroes -or Negroid race are a group of people who,
likewise, have certain characteristics that set them apart from
other peoples. They have spread widely over Africa and the
islands of the Pacific as far as New Guinea. They are, however,
by no means uniform over this wide area. They range from tall

266

THIS LIVING WORLD

"Ebony Statue" Bell of University of Minnesota football fame in 1938. (Life Masazine

photograph.)

to short in stature, and from yellow to black in skin color. This,
of course, makes for many subracial groups in any detailed study
of them. But, as is true with the whites, all these groups possess
certain features in common.

The eyes are without exception black. The hair on the head
is black, short, and exceedingly curly or kinky. Usually there is
very little hair on the body. The skin is always dark but varies
considerably in intensity of shade from a brownish yellow to
heavy black. The Negroid head tends to be long, narrow, and
relatively low. Usually the lips are* thick and the nose low and
broad.

The native peoples of a large part of Southern Africa are of
relatively short stature and have yellowish brown to olive skin.
The head shape varies from long and narrow to medium broad-
ness. They usually have a small chin and a flat, broad nose; the
cheekbones are relatively wide and prominent. These people are
the Bushmen and Hottentot subraces of the Negroes.

The pygmies constitute another large group of the Negro
race. They are found in Central Africa as well as scattered in the
Malay Peninsula, Andaman Islands, and New Guinea. They are
short in stature, usually slightly less than five feet tall. The head

THE LAST MILLION YEARS 267

shape is short and broad. The body is usually sturdy, with the
trunk long in comparison to the length of the legs. Skin color
ranges from yellowish to dark brown or black.

Within the Sudan region of Africa there is a subrace of the
Negroes that is characterized by tall stature. They average
nearly six feet tall. The head is long and narrow, frequently with
a high forehead. The face is long, and the nose may be relatively
long but is always broad. The skin is very dark.

This very brief mention of some of the different subraces of
the Negroes is probably sufficient to illustrate that there are
well-recognized variations within the Negro race. In some
respects these variations are greater in kind and degree than are
those found in the white race.

Yellow Peoples

The yellow or Mongoloid race constitutes the other large
division of modern peoples. They inhabit the eastern half of
Asia, many of the near-by islands, and the Malay Peninsula.
They once occupied the greater parts of the North and South
American continents.

There is more uniformity in physical features among people
of this race than among either the white or black races. The hair
is black, coarse, and straight, almost without exception. There
is little or no hair on the body proper. The eyes are black or dark
brown. There is usually a fold of the skin of the upper eyelid
over the inner angle of the eye, known as the Mongolian fold.
This gives the eye opening a decided slant toward the nose. In
general the head is short, broad, and high. The face is broad, this
broadness being accentuated by prominent, wide cheekbones.
The greatest variation is found in skin color and body size. The
skin varies from light color with a yellowish undertone to dark
brown and red. Body shape is slightly robust to slim, and some-
what shorter in height than the white race.

The yellow race is probably best exemplified by the Chinese
and the inhabitants of Mongolia. The oceanic Mongoloids, in-
cluding natives of Japan, British Malaya, Dutch East Indies,
and parts of French Indo-China, have certain characteristics in
common and may be grouped as one or more subdivisions of the

268 THIS LIVING WORLD

Madame Rosa Feng of Peking, China, exemplifies many oF the fine physical character-
istics of the yellow race. (Photograph by Ewing Galloway.)

yellow peoples. The American Indian, the Eskimo, and the
Siberians are other subraces of the Mongoloids.

The long existence of the Indian under climatic conditions
of the American continents produced a number of variations in
their racial features, in addition to a deep-tone red skin. These
other features are, however, far from being uniform. The North
American Indians ranged from medium to tall stature. They
were of sturdy build and a vigorous people. The head shape
varied from broad and high to long, narrow, and high; however,
prominent cheekbones were almost universal. Even among the
scattered tribes still living within the United States there are
to be found almost as many subracial variations as are found in
the conglomerate of white races living here today.

Native Australians

The native Australians are sufficiently different from other
subraces of modern man to warrant brief mention here. They
have black skin, but they do not belong to the Negro race. The
Australians have a long and narrow head, but the vault of the
skull is exceedingly low and sloping. There are heavy brow ridges

THE LAST MILLION YEARS 269

across the base of the forehead. The chin is moderately receding,
and the nose is wide. They have a heavy growth of whiskers and
hair over the body. The hair varies from relatively straight to
very curly. While always black, it is not kinky, as is true of the
Negro race. The mouth is relatively large with thick lips. In
stature the Australians range from short to medium.

What remains of this race is found mostly in Northwestern
Australia. They represent a small, retarded group of people who
are rapidly decreasing in number and will probably soon become
extinct.

Race Betterment

The conditions that have served to bring about the formation
and perpetuation of races have also had other effects upon homo-
geneous peoples. They develop somewhat common social and
religious practices, and may even build up strong national ties.
Then when diverse racial groups come into close contact with
each other, each attempts to maintain its own mores and customs
and even to enforce them upon the other. Thus, racial differences
lead to enslavement, discrimination, and wars. It is, however,
not uncommon that under such conditions there is finally con-
siderable mixing of the two races; thereby, new racial strains
come into existence.

It is usually true that each race possesses some quality of
superiority that is valuable in advancing human culture. In
fact, there are no fundamental and important criteria by which
either of the three major races can be judged superior in all
respects to the others. The same thing is true (within limitations)
of the various subraces. It would be to the advantage of mankind
for each of the racial groups to attempt to understand and value
the peculiar characteristics of the others. Probably much that
is valuable to the advancement of human culture could be
promoted rather than destroyed, as has so often been the case
in the interracial conflicts.

The development of racial purity or the retaining of such
purity is primarily a matter of isolation of the group and uniform
habits within. It is not unusual for such attempts to be made.
They are rarely successful. Race mixture goes on. However, the
maintaining of racial purity by isolation has in the past usually

270 THIS LIVING WORLD

resulted in a static condition within the group. The native
Australians and Tasmanians are emphatic examples of such lack
of progress. In contrast to this, it is often true that the greatest
progress made by a group of people has been fostered by racial
admixtures. During the "dark ages" of European history the
Arabs held the torch of learning in Europe and Northern Africa.
For many centuries previously the land of the Arabs had been
the scene of the great migrations, wars, and racial intermixing
of many peoples.

These conditions may prove of great advantage to the United
States. Here there is the greatest mixing of racial groups that
the earth has ever witnessed. Some twelve million Negroes are
now scattered throughout most of the states, being no longer
isolated in one section of the country. There are large elements
of all three subraces of the whites that make up a big percentage
of our population. In addition, considerable traces of Asiatic
yellow peoples are to be found here. In most cases little assimila-
tion and blending of these diverse racial groups has taken place.
Only those elements of different racial groups that migrated here
before the end of the nineteenth century seem to have been
effectively assimilated. Many other racial elements came here
in such great numbers within so short a time around the turn
of the last century that they have tended to retain their foreign
racial, social, and nationalistic characteristics.

However, it is inevitable that assimilation will eventually
occur. Whether the results are beneficial or harmful will most
likely depend upon the type of racial heritage we pass through
within the next century or so. Should the stronger strains of the
different racial groups be perpetuated and cultivated, it is quite
likely that a better racial admixture than now exists will result.
However, should the less desirable and weaker elements be
multiplied and fostered with special care, it is just as likely that
racial deterioration will be the outcome. These conditions
emphasize the need of wise study and the guidance of our
national racial composition and inheritance. They would involve
a close coordination of genetics and social conditions, serious
consideration of any prolonged relief practices, and rigid ad-
herence to a policy of refusing admission to large groups of
foreign elements within the immediate future.

THE LAST MILLION YEARS 271

REFERENCES FOR MORE EXTENDED READING

SCHUCHERT, CHARLES, and CLARA M. LEVENE: "The Earth and Its
Rhythms,'* D. Appleton-Century Company, Inc., New York, 1927, Chaps.
XXV, XXXI.

In these chapters is found a short and easily read account of the climates and
geographical features of the earth during the Pleistocene epoch, together with a dis-
cussion of the physical development of early man and his relationship to the other
primates.

MACCURDY, GEORGE GRANT: '* Human Origins," I). Appleton-Century Com-
pany, Inc., New York, 1984, Vol. I, Chap. VIII.

This chapter includes a comprehensive discussion of fossil man in Europe before
the beginnings of historical times, as well as some account of the distribution and
general relationships of the primates.

MACCURDY, GEORGE GRANT: "Early Man," J. B. Lippincott Company,
Philadelphia, 1937.

This is an edited group of papers that \\ere presented at the International Sym-
posium on Early Man, held in Philadelphia in 1937. It recounts the researches of many
of the world's authorities and students of prehistory, and it is an extensive summary of
the recent discoveries in this field. The topics discussed include work that has been
done in recent years in Java, the Near East, China, India, Africa, Australia, Norway,
and many parts of North America.

BOAZ, FRANZ, and OTHERS: '* General Anthropology," D. C. Heath & Com-
pany, Boston, 1938, Chaps. II, III.

The chapters referred to are a well- written and comprehensive account of early
man and modern races by two of the foremost authorities in these fields.

BEAN, ROBERT BENNETT: "The Races of Man," The University Society,
New York, 1932.

The formation and movements of races of man are concisely discussed in their
relation to geographic and economic factors. There is some brief account given of
prehistoric man and a review of the modern races and their chief physical character-
istics.

COON, C. S.: "The Races of Europe," The Macmillan Company, New York,
1939.

The author traces the racial history of the white race from its Pleistocene begin-
nings to the present. There are also chapters on racial identification and classification
of living white peoples which are extensively illustrated with photographs of peoples
of different racial types.

Asia, published by Editorial Publications, 10 Ferry Street, Concord, N.H.

This monthly journal is concerned primarily with articles that relate to the present
peoples of Asia and their national and political life. In addition, there are usually

272 THIS LIVING WORLD

articles regarding the cultural and racial characteristics of modern as well as early
historic peoples of that continent which may be of interest to the scientifically
minded reader.

American Anthropologist, published by The American Anthropological Associ-
ation, Menasha, Wis.

This is a quarterly magazine which includes a wide range topics regarding modern as
well as ancient peoples and their characteristics, customs, and mores.

9: COMPARATIVE FEATURES

Human Anatomy in Relation to That of Lower Vertebrates

THE long story of life on earth has been briefly traced in the
preceding accounts of the geologic and immediate past. It
has revealed to us an ever-changing picture — one of slow progress
and development from simpler forms to more complex types of
living creatures. Man was one of the latest higher forms to
appear on the earth. Even the early human creatures were quite
different from mp,n today. His struggle upward along the long,
hard road of physical development has just been noted. Man
stands today as the product of the ages. This is no less true of
his cultural heritage than of the structure of his body.

Human anatomy has been studied in minute detail, as a
thorough knowledge of it is indispensable to the medical practi-
tioner. On the other hand, it is of immediate interest to everyone

278

274 THIS LIVING WORLD

because in the structure and well-being of the body reside the
comfort and joys of living. It has been stated poetically that the
body "is the temple of the soul/*

One of the strangest fruits of the extensive study of human
anatomy has been the discovery that many of the problems
which arise in connection with it find their solution in a study
of lower forms of animals. It has been proved that frequently
more may be learned of human development and structure by
the intelligent examination of some lower vertebrate than by the
study of the human body itself. This is due not only to the
greater availability of such animals for dissection and experi-
mentation, but also to the fact that parts of lower animals show
simpler stages through which the human body has passed in
arriving at its present condition.

All the parts of the human body are represented by similar
parts in lower animals. These parts in lower forms are frequently
much less complex in their development than in man. A com-
parative study of these similar parts shows in many cases the
origin and development of human body structures. Often it
accounts for the particular nature and function of some human
organ or system. In getting an understanding of human anatomy,
nothing about the structure and activities of any animal, how-
ever familiar or strange, becomes trivial or insignificant. The
study of even a few points in comparative anatomy will serve
to give some understanding of the development of the human
body and probably a greater appreciation of this remarkable
mechanism.

The Skin and Its Derivatives

In making this brief study it is well to begin with the skin, or
outer covering of the body. This is appropriate, not only because
it is the first part to be encountered in the examination of any
animal, but also because in man and many animals it is a very
versatile organ serving a great variety of purposes. It forms a
pliable covering for the body, protects it from foreign materials,
helps to regulate body temperature, and prevents the excessive
evaporation of body moisture. From the ectoderm of the embryo
are developed the nervous system and sense organs, and from
the skin itself, the special coverings and appendages. The outside

COMPARATIVE FEATURES

275

of the body, including even the exposed surfaces of the eyeballs,
is entirely clothed with the skin or its derivatives. At the

Epidermis

Corium

Fatty tissue

The skin of vertebrates is made up of two principal layers, the epidermis and the
corium or dermis. The epidermis is composed of a layer of closely packed and dead cells
(a), and a layer of growing, dividing cells (b). The corium contains coiled glands (d) and
numerous capillaries (c) and nerve fibers.

nose, mouth, and genital openings it passes over into a related
tissue, the mucous membrane, which lines these passages.

The skin is made up of two general layers : an outer stratifica-
tion known as the " epidermis " and an inner layer called the
"corium" or "dermis." In the young embryo these two parts
are seen to be derived from two separate germ layers. The epi-
dermis arises from the ectoderm, while the corium is formed from
the mesoderm. The epidermis usually consists of several layers
of cells, of which the innermost or germinal layer is constantly
producing new cells, while the outermost layers tend to become
horny. The protoplasm of these horny cells dies, and they are
constantly being worn away by friction and being replaced by
new cells from the germinal layer. From the epidermis are
formed a number of accessory structures, such as epidermal
scales, hair, horn, nails, claws, feathers, and the enamel of teeth.
Also, it may contain sensory cells.

The corium is sometimes called the true skin. It is quite dif-
ferent from the epidermis, having a distinctive composite

276 THIS LIVING WORLD

structure. It is usually thick and is the part of the skin which
forms the leather of commerce. In addition to smooth muscle,'
it contains fibrous and elastic tissues, which give it strength. It
is richly supplied with blood vessels and with numerous types of
sensory cells. These sensory cells are strictly specific in function
and have nerve endings which form a sort of network in various
parts of the corium. Most of the pigment cells which are re-
sponsible for the color of animals are located in the corium.

Bone is commonly developed in the corium, or dermis,
primarily by the formation of scales of the bony type. These
bony formations are often used in skeleton building, this being
true in man as well as in many lower animals. There are many
dermal bones in the skull, and they are now known to be modifi-
cations of scales which have grown together. In many lower
animals these scales remain as such throughout life, but in man
and other higher animals they develop further to form bones
after the embryonic stage is passed. The development of many
parts of the human skull may be traced from such simple
beginnings. The dentine of teeth also is derived from the corium.

Development of the Skin of Vertebrates

It is not known at present exactly how the vertebrate skin
originated. The best indication is secured by studying certain
modern simple forms. In descending as far as possible down the
ladder of animals with a backbone, a little creature known as
"amphioxus" is eventually reached. Amphioxus is the simplest
of the chordates. It possesses a notochord, like the embryos of
all vertebrates, but does not have a vertebral column. The skin
structure of this animal, while characteristic of the vertebrates,
is reduced to its simplest expression. The epidermis consists of a
single layer of cells which in adult life produce a sort of noncellu-
lar layer typical of the single-layered skin of all invertebrates.
This is indicative of the primitiveness of amphioxus and its
nearness to the invertebrate stem. The skin of the amphioxus
also possesses a corium consisting of a thin layer of gelatinous
connective tissue. However, only vertebrate animals possess a
dermal layer of the skin. Thus, amphioxus assumes the dignity
of a vertebrate. It is probable that the many layered vertebrate

COMPARATIVE FEATURES

277

"The scales of fishes usually develop from the corium alone. ..."

skin began in some fashion similar to the condition seen in this
little animal.

The amphibians, such as toads and salamanders, possess a
typical but a simplified vertebrate skin. It has a rather thin
corium of fibrous structure and is scaleless. The epidermis con-
sists of several layers and contains many glands for keeping the
skin moist as an aid to respiration. In those forms which remain
mostly out of water, the epidermis has a dead outer layer which
may be shed all at once, as is also true of the epidermis in reptiles.

In many kinds of vertebrates, scales form a conspicuous
modification of the skin. This is particularly true of fish and
reptiles. The scales of fishes usually develop from the corium
alone, as illustrated in the accompanying drawing. The scale is
a bone-like material which grows from and is nourished by the
corium. It represents the beginning of dermal bone, which is
found extensively in the bodies of many vertebrates. Frequently
the corium as well as the scales in lower vertebrate forms is
pigmented in different colors, which decorate the body with an
endless variety of patterns and shades. The epidermis of fish is
usually a thin, superficial structure, extending over the scales,
which serves to anoint the body with mucus.

278

THIS LIVING WORLD

The scales of the shark are of interest and significance be-
cause they show conclusively the origin of teeth. The shark scale

Enamel membrane

Enamel

Dentine

ilp cavity

Enamel
membrane

Enamel

itine
Pulp cavity

Epidermis
Corium

D E F

The scales of sharks are of special interest and significance because they indicate
clearly the origin of teeth. A, B, C, diagrams showing the formation of a shark scale/
D, E, F, similar diagrams for a tooth.

consists of a flat base, buried deep in the corium, from which a
naked cusp projects to the outside of the skin. The base consists
of dentine, and the cusp is covered with enamel. The scale
originates from two sources, the dentine from the corium and
the enamel from the epidermis. Each scale has a permanent
cavity filled with pulp, by which blood vessels and nerves are
brought to the scale. The shark tooth appears to be a scale drawn
into the mouth. In many sharks there is a perfect intergrading
of the regular scales of the body into the teeth. The origin of
teeth, even in higher vertebrates, where it is not so evident, is
thus made clear.

The development of scales reaches a high degree of perfection
in the reptiles. These animals are also the first vertebrates to
evolve a skin well suited to land life. In contrast to the fishes,
reptiles have a thick layer of dead epidermis which comes in
contact directly with the air. This layer is entirely impervious
to moisture, and there is no loss of water through it. Thus
reptiles can live out of water without any serious evaporation
of the body fluids. Such evaporation would soon prove fatal to
most water vertebrates, even if they had lungs for air breathing.
The scales of reptiles arise from the epidermis alone. They are

COMPARATIVE FEATURES

279

A well-formed set of human teeth. (Life Magazine photograph.)

horny structures formed from dead epidermal cells. Along
with the highly cornual outer layer of the skin, the epidermal
scales are periodically shed and replaced from the germinal
layer. Many of the large reptiles of the past grew, in addition
to the horny scales, enormous bony plates from the corium, some
of them over two feet long. These dermal scales were not shed
but were carried around for life.

The skin of birds is thin, with only a rudimentary epidermis.
As we know, the typical covering of birds is the feather. This is
a kind of modified scale. It grows from the coriurn, and its
development is similar to that of the fish scale. It appears first as
a small papilla, formed from the corium, having a thin epidermal
covering. This papilla sinks into the corium and forms a feather
follicle, from which the growing feather gradually protrudes.
The bill of birds, on the other hand, is not a true scale. Rather it
is a horny sheath produced from the epidermis. True epidermal
scales are found on the legs and feet of birds.

280

THIS LIVING WORLD

Scales form a conspicuous modification of the skin of many vertebrates, as is shown in
this close study of the lizard-like Tuatara. Tuatara inhabits New Zealand and is the oldest
living species of reptile known to man. (Life Magazine photograph.)

The Specialized Skin of Mammals

Mammals have a typical land skin, consisting, of course, of a
corium and an epidermis. There is a layer of dead epidermal cells
on the outside, so that no living cells are exposed to the air. The
greatest specialization of both layers of the skin is reached in
mammals. Such specialization is attained by modifications of
the characteristics of lower vertebrates rather than by the
development of new parts. For example, one of the greatest
specializations of the skin of mammals is the development of
numerous skin glands of various kinds. In other vertebrates,
except the amphibians, the glands are relatively few and un-
specialized. In mammals they are extensive and specialized as
sweat, oil, and mammary glands.

The covering of mammalian skin generally consists of hairs
instead of feathers or scales. The development of the individual

COMPARATIVE FEATURES

281

hair parallels that of the feather and scale. It starts with a
thickening of the epidermis to produce a papilla, which then dips

Hair /haft-

Layer of dead cell/

Layer of
closely
packed
cellr

JEPIDERMIJ

CORIUM

Papilla

The covering of mammalian skin consists of hairs, which develop from epidermal cells
nourished by the corium. The skin of mammals is characterized also by the presence of
sweat and oil glands.

down into the corium. From the resulting pit or follicle a solid
horny shaft is pushed out by rapid growth of the epidermal cells,
which get their nourishment from the corium. Before birth a
mammalian embryo, including the human embryo, develops a
coat of hair which is shed immediately before or after birth.
The hair pattern is quite regular, suggesting that hair began
originally by developing around scales, which have long since
been lost by most mammals. However, there are a few mammals,
such as the armadillo, which still retain some scales surrounded
by hairs. The tails of such mammals as rats, muskrats, and
beavers have scales covering them, interspersed with hairs. The
porcupine has a covering of spines, which are a particular type
of scale, surrounded by hairs. Embryo bears have a complete
covering of spines which are lost after birth.

Particular derivatives of the epidermis of mammals which
have become highly specialized and useful are claws, hoofs, and
nails. Claws are horny caps which fit over the terminal bones of
the feet. They not only serve as a protection in walking but,

282

THIS LIVING WORLD

Claw of Kodiak bear. (Photograph by Ewing Galloway.)

when they become specialized, have a number of useful purposes.
Thus, badgers and moles have broad, strong claws for digging.
The cats have developed sharp, curved claws that are very
useful in holding and killing prey. The hoofs of cattle, horses,
sheep, and goats are a special type of claw, associated with
spongy pads, which aid in running over hard ground or climbing
on rocky surfaces. The nail in man and other primates is a
highly specialized claw which, being flat and merely forming a
plate over the finger tip instead of being a cap, may be used in
handling very small objects. In fact the nails are one of the most
distinctive features of the human hands, permitting man to
perform many delicate movements impossible for other animals.
Among the specialized skin glands of mammals are the sweat
glands. They are the most common and generally distributed of

COMPARATIVE FEATURES

283

.Chest muscles

Gland

-Nipple

Rib

The mammary glands are
tubular skin glands which serve
to distinguish the mammals from
all other vertebrates.

the coiled glands, there being over two and a half million of them
in the skin of man. In some mammals that are abundantly
clothed with hair the sweat glands
become crowded out and highly local-
ized. In cats and dogs, for example,
they are localized on the soles of the
feet and on the muzzle; these are the
only parts of the skin of these animals
which ever feel moist.

The sweat glands are tubular glands
which reside in the corium and have
openings through the epidermis. They
have their early counterpart in the
mucous glands of fishes and amphib-
ians. The character of the secretion of
the sweat glands varies greatly in
different mammals. In man it is a
watery and colorless fluid; in the hip-
popotamus and kangaroo it is red; in
the African antelope it is blue. In each case, however, the
secretion is derived from the blood and contains waste products
from the body cells, along with a considerable amount of water.

Most striking of all skin glands are the mammary glands, by
which the mammals are distinguished from all other creatures
as a separate class of the higher vertebrates. These milk-produc-
ing glands are tubular structures. They develop in connection
with certain areas, known as the milk ridges, on the underside
of the females and are similar in this respect to the ridges which
develop in birds on the underside of the body while they are
incubating eggs.

The fluid from these glands is believed to have poured out
over the surface of the skin of the early mammals, as it still does
in the case of the duck-billed platypus and other monotremes,
which do not have teats. The system is more specialized in the
kangaroo and oppossum, where nipples are present. It reaches
its highest development in the placental mammals, where the
glands are closely associated with the bearing of the young and
their feeding during infancy. The mammary glands dry up when
milk is no longer needed.

284 THIS LIVING WORLD

The development of the mammary apparatus starts in the
embryo, beginning at about the fourth or fifth fetal week in the
human embryo. A milk line occurs down each side of the belly
of the mammalian fetus. It is simply a thickening of the epi-
dermis. It breaks up into small remnants, or beads, which sink
into the corium, forming pits. The mammary glands arise from
the sides and bottom of these pits, or milk pockets. In some
mammals as many as twelve pairs of glands are formed; in
others, some of these fail to develop. In man, ordinarily only one
pair develops, the fourth from the forward end. Occasionally,
however, extra nipples are found in man, the number reaching
as high as three or four pairs. This occurs as often in males as
in females. Also, there are cases of women who have two and
even three pairs of breasts. Such extra nipples and breasts ar-
range themselves along the vanished embryonic milk lines.

Origin and Development of the Skeleton

The problem of support and protection was encountered by
animals at an early date. The first animals were probably small
marine forms which either depended upon the currents in the
sea for transporting them from place to place or moved about
by sluggish efforts of their own. They might have been attached
to objects. In such creatures, buoyed up as they were by the
water, there was little necessity for supporting structures, and
their tissues were soft and unspecialized. Such hard parts as
these animals possessed were in the nature of shells or other
protective coverings. With increase in size and with the develop-
ment of vigorous movement, there was an immediate need for
greater support and for structures which provided leverage for
the body. This need was met independently by the two great
groups of animals. Among the invertebrates, the problem was
solved by the development of a hardened outer covering and
paired jointed appendages. The vertebrates, on the other hand,
developed an internal jointed skeleton in connection with paired
appendages. Three types of skeletal materials were produced
by the vertebrates, namely, notochord, cartilage, and bone.

The notochord seems to have been the first internal skeletal
structure to appear. At least it is present in the simplest types
of chordates, as is well illustrated in amphioxus, where it is the

COMPARATIVE FEATURES 285

The two front teeth in the upper and lower jaws of the beaver are especially adapted to
enable the animal to cut wood with ease and rapidity. (Photograph by Ewing Galloway.)

only stiffening and supporting part of the body. It is a tough,
flexible rod running the length of the body along the back and
below the nerve cord. It gives rigidity to the body and provides
for the attachment of the muscles of the trunk.

Cartilage is a translucent material which is firm and elastic
and capable of rapid growth. It is a derivative of simpler con-
nective tissue. It is a common supporting material in lower
vertebrates. In lampreys and sharks it is the only skeletal ma-
terial other than the notochord. In land forms its importance
has waned, but the ends of the ribs are composed of cartilage
and there are layers of it between the joints of the backbone.

In man and other higher vertebrates bone is the skeletal
material which predominates. It is made up of a great network
of tiny interlacing fibers and irregularly branching cells. Between
the meshes of this network is deposited a matrix of inorganic
salts, chiefly calcium phosphate and calcium carbonate, which
make up about two-thirds of the bone substance. The bones,
however, are not solid structures. Most of the large bones are
essentially hollow. They are solid at the surface, with bony bars
interlaced within for reinforcement of the walls, not unlike the
steel framework of modern skyscrapers. The inner cavities of
the bones are filled with the marrow, composed mainly of living

286 THIS LIVING WORLD

'Shell-skinned" animals.

cells around which blood circulates and in which red blood cell
are manufactured.

Bone is developed in the vertebrates in two separate and
distinct ways. The greater part of the skeleton is formed first
as cartilage and is later transformed into ossified bone material.
Such bones may be thought of as replacing or cartilage bones.
Some bones, on the other hand, chiefly those forming parts of
the skull and pectoral girdle, do not go through a cartilage stage
at all. They are laid down directly as bony plates, or special
scales, in the dermis of the skin. They become closely joined in
adult life, particularly in man and the higher vertebrates, and
their origin can be seen only in the developing embryo. These
bones may be thought of as investing or dermal bones.
Cartilage bones and dermal bones do not differ in structure in
any way. They cannot be distinguished from each other by
microscopic examination; the difference between them lies
entirely in their mode of origin.

Head Structures

In man the skeletal part of the head consists of the skull,
movable lower jaw, the cartilages of the larynx and trachea, and
a few bony structures in the tongue and middle ear. The skull is
an exceedingly complex structure. Obviously, no detailed ac-
count of it can be given here. However, some idea of the reasons

COMPARATIVE FEATURES 287

for its complexity may be gained by a brief review of its com-
parative features in simpler and more primitive animals.

Cartilaginous
gill bars

The head skeleton of the ostracoderms probably consisted of a simple cartilaginous brain-
box and sill bars of the same material.

The earliest vertebrates of which there is any fossil record
were jawless and limbless creatures probably not unlike some
forms living today, such as the lampreys and hagfishes. These
most ancient of all vertebrates were 'covered with thick bony
plates, forming a solid armor over the head region, and a coat of
mail made up of scales over the trunk and tail. With reference
to this armored condition, the primitive vertebrates have been
called ostracoderms, or "'shell-skinned" animals. The skeletal
structures of the head, like those of the modern lamprey and
hagfish, probably consisted of a simple cartilaginous brain box
and gill bars of the same material. A typical example of the
head skeleton in such an animal is shown in the drawing.
There was no skull in the proper sense, nor any jaws, either up-
per or lower. The gill bars gave support to the gills, which were
the respiratory organs of the ostracoderms. Each gill bar was
composed of two parts, upper and lower, as shown in the draw-
ing. Together, the elements of the gill bars formed the gill
arches, seven to nine pairs of them, numbering from the pair
nearest the mouth.

The ostracoderms were small, sluggish creatures, rarely over
a foot in length and often presenting bizarre shapes. They prob-
ably fed upon decaying organic material on the bottom of the
ancient streams in which they lived. Eventually, some of them,
or some creatures closely related to them, migrated into the salt
waters of the ancient seas, abandoning their bottom feeding

288 THIS LIVING WORLD

habits to prey upon other living things. Among them were the
ancestors of the modern lamprey and hagfish, whose jawless

Upper jaw Brain box.

Lower jaw

Jaw prop

That the jaws of vertebrates are derived from gill bars is clearly indicated by the structural
relations of these parts in a primitive shark-like fish.

round mouth forms a sucking disk for attachment to the higher
fishes on which they feed by the rasping action of a horny tongue-
like structure. Among them, also, were certain forms which
developed jaws in order to tear apart the flesh of the creatures
upon which they fed. Correlated with the adoption of predacious
habits was the assumption of a streamlined body shape and the
development of paired appendages to aid in the rapid locomotion
necessary to capture their prey. From such beginnings the ances-
tors of the modern sharks evolved.

The development of jaws was the first important step in the
evolution of the typical vertebrate skull. That the jaws were
derived from gill bars is clearly indicated by the structural rela-
tions of these parts in modern sharks (especially in the ancestors
of the modern sharks), and by their mode of origin from primary
gill arches in the embryonic development of these and all higher
forms. In the change from scavengers to predatory Creatures, the
mouths of the ancestral vertebrates which gave rise to the earliest
shark-like fishes underwent considerable enlargement. It appears
that one or two of the foremost pairs of gill bars interfered with
this process. These became reduced in size and ultimately disap-
peared. They are represented in modern sharks by pairs of
cartilaginous nodules in the angles of the jaws.

One pair of gill bars, however, increased in size and became
associated with tooth-like structures in the skin. The free ends of
the paired cartilages comprising this gill arch apparently rotated
forward. The upper pair became attached to the brain case by

COMPARATIVE FEATURES 289

means of ligaments and formed the upper jaw. The lower pair,
through enlargement and modification of their shape, produced

Jugal Squamosal

Frontal / Parietal

Lachrymal

Skull of primitive bony Ash.

the lower jaw. The point of contact between the upper and lower
paired elements of the gill arches became the hinge joint of the
jaws. The upper paired cartilages of the next gill arch were
utilized in propping the jaw joint against the brain case.

The condition attained through the developments just de-
scribed is illustrated in the drawing of the skull, jaws, and the
skeleton of the gill apparatus of a primitive shark-like fish which
lived in the Devonian seas. The brain case, jaws, and gill arches
were cartilaginous structures, as in modern sharks, but the teeth
were bony modifications of scales with an outer covering of
enamel.

Additional significant developments in the formation of the
skull occurred in the evolution of the higher fishes. The cartilagi-
nous elements of the brain case, jaws, and gill arches were re-
placed by true bone. Moreover, bony plates laid down in the
dermal layer of the skin covering the head region became
associated with the original cartilage bones of the jaws and brain
case. These investing b,ones covered the top and sides of the
head completely, fusing with each other and with the brain case
and upper jaw to form a true skull. They even invaded the mouth
cavity, which is lined with skin, producing a bony palate or roof
of the mouth. The skull thus became a solid structure, pierced
only by openings for blood vessels and nerves and by the orbits
of the eyes and openings of ilqp nostrils.

The cartilage bones of the lower jaw likewise became covered
with dermal bones, which, in fishes, extend over the throat on the

290 THIS LIVING WORLD

underside and over the gill region to the shoulder girdle. The
lines of fusion of the dermal bones are clearly visible in the skulls

Maxillary
Nasal

Jugal

Columella
auris

n ,\ \ 'Quadrate

Uentary \ .. .

Articulare

Skull of a primitive mammal-like reptile.

of primitive bony fishes, as shown in a typical instance in the
drawing, and even to a certain extent in those of modern bony
fishes. Many of these bones, identified by their proper anatomical
names in the accompanying drawings, can be traced down
through the ages in the evolution of the skulls of higher verte-
brates, even to that of man.

When the vertebrates emerged from the sea to live on land,
gill breathing gave way to lung breathing. The retention of gill
bars was therefore unnecessary, and they have been relegated to
a sort of anatomical scrap heap or used in constructing new bones
more suitable to the skulls of land animals. This is true to a cer-
tain extent in the skulls of amphibians, but more so in those of
reptiles and mammals. The dermal bones covering the gill and
throat regions disappeared, becoming restricted to the skull
and jaws and to isolated elements of the shoulder girdle.

In passing from the fish type of skull and jaws to the condi-
tion of these structures in mammals, the most important changes
took place in the articulation of the jaws. In fishes, the jaw joint
is between two cartilage bones, the quadrate and articular, de-
rived from the upper and lower elements of the gill arch which
went into the formation of the jaws. The upper elements gave
rise to a quadrate bone on each side of the skull at the rear end of
the upper jaw. The upper ends of the lower elements gave rise to
the articular bone of each half ofithe lower jaw. The upper ele-
ments of the next gill arch produced the hyomandibular bones,

COMPARATIVE FEATURES 291

which prop the jaw joint against the sides of the skull. In amphibi-
ans, reptiles, and birds the articulation of the jaws is essentially
Maxjlfcry Frontal

•Squamosal
Slirrup

*~. j Dong

The skull of a primitive mammal serves to illustrate the new type of jaw-joint necessitated

in this class of vertebrates.

the same as in fishes, except that the quadrate and articular are
much smaller. In these higher vertebrates, moreover, the jaw
prop is no longer present. Instead, the hyomandibular is very
greatly reduced in size, forming a small bone in the middle ear.

The middle ear is an entirely new structure which appeared
first in the most primitive amphibians. It is derived from a
rudiment of the first gill slit in fishes. It is a chamber formed by
closing off the external opening of the gill slit with a thin mem-
brane, the eardrum. The greatly reduced hyomandibular is
attached to the center of the eardrum and serves to transmit
vibrations of the membrane to the inner ear.

In mammals, there is a brand-new type of jaw articulation,
necessitated by further development of the middle-ear apparatus.
The hyomandibular of amphibians, reptiles, and birds has
undergone further reduction in size and change of shape. It is
now represented by a tiny bone, commonly called the stirrup
because it resembles that object in shape. The broad base of the
stirrup bone fits into an opening in the bony casing of the inner
ear. Associated with the stirrup bone in the middle ear are two
tiny new bones. One of these is roughly anvil-shaped and is there-
fore popularly called the anvil. The other is shaped roughly
like a hammer and takes its name from its resemblance to that
object. These two new auditory structures are derived from the
bones which formed the jaw articulation in lower vertebrates.
The anvil bone is formed from the quadrate and the hammer is
the remnant of the articular. The handle of the hammer is

292 THIS LIVING WORLD

attached to the center of the eardrum. The anvil bone lies be-
tween the free ends of the hammer and the stirrup bone, com-

Jugal
Lachrymal^ J^ S J>^ parietal

NasaU

naA-{ * \

Squamosal

Dentary

Human skull with dermal bones shown in white, cartilage bones in black.

pleting the jointed bridge by means of which sound vibrations
striking the eardrum are transmitted to the inner ear.

The result of the migration of the quadrate and articular
into the middle ear was that a new type of jaw joint had to be
formed in the mammals. As indicated in the series of drawings
representing the structure of the skull in the principal vertebrate
types, there is considerable reduction in the number of dermal
bones in the mammalian skull as compared with that of a fish or
reptile. In the lower jaw the dentary bone is the only remaining
dermal element. It has expanded to cover and replace the entire
original cartilage of the lower jaw. The jaw now articulates with
the squamosal, a dermal bone of the skull.

In man the dermal bones of the upper skull have become
much enlarged in order to make room for man's expanded brain.
At the same time the bones of the upper and lower jaw have
become reduced. These changes in the proportions of the head
bones have greatly changed the contour of the human face from
that of the faces of lower animals. The dermal bones are shown in
black and the cartilage bones are represented in white in the
drawing of the human skull. A few of the more important bones
from a comparative standpoint are labeled to indicate the fusion
that has taken place, as well as the change in size and shape, in
producing the architectural features of the human skull. Only
a few of the original gill arches are present in the human neck,
forming the cartilages of the trachea and larynx, or "Adam's
apple."

COMPARATIVE FEATURES 293

The Vertebral Column

The head bones of vertebrates serve primarily as a protective
covering for the brain and a number of special sense organs and
nerves. The vertebral column is the part of the skeleton which
gives support to the abdomen and some protection to the central
nervous system. As is generally known, the vertebral column is
made up of separate vertebrae, some of them having long rib
extensions. The vertebrae are the end product of a long series
of developments which began with the appearance of the noto-
chord. A notochord is present in the embryos of all vertebrates,
including man, as has previously been noted. It is the only
skeletal structure present in amphioxus and other lower chor-
dates, but in the vertebrates an axial skeleton composed of the
vertebral column, ribs, and sternum is attained.

The lowest forms possessing structures comparable to verte-
brae are the hagfishes and lampreys. In these primitive creatures
there are little paired cartilaginous struts or pegs rising up from
the notochord on each side of the nerVe cord. There are usually
two pairs in relation to each muscle segment along the mid-line of
the body. The notochord is continuous and unconstricted. In the
embryos of all higher vertebrates the vertebrae first appear in
this manner, as paired upper and lower cartilages corresponding
to each muscle segment.

Inserted between each of these primary pairs there may be
additional secondary paired cartilages. In the formation of each
vertebra, the paired upper and lower primary cartilages of one
segment fuse with each other and with the secondary paired
cartilages inserted between them and the primary cartilages of
the next segment behind. The resulting vertebra is therefore
intersegmental, permitting attachment of two segmental muscle
masses to it. The paired upper primary cartilages of the vertebrae
form an arch over the spinal cord, while the paired secondary
cartilages and the central portions of the paired upper and lower
primary cartilages completely enclosed the notochord. In the
tail region the lower pair of primary cartilages form an inverted
arch around the main artery of the body.

The sharks are the lowest forms possessing a true vertebral
column. In these and certain higher fishes the vertebral column

294

THIS LIVING WORLD

Nerve cord
Rib

Neural
"arch

NotocHord

Nerve
cord

The vertebral column is made up of a series of separate bones called vertebrae/ these
being the end product of a succession of developments which began with the appearance of
the notochord. A, vertebra of a fish; B, vertebra of a young eel; C, human vertebra.

is cartilaginous, and the remains of the notochord may be seen in
the center of the vertebrae, as shown in the drawing. In the bony
fishes and in all higher vertebrates, including man, the notochord
becomes completely replaced with bone. In some forms, as
shown in the drawing of the vertebra of a young eel, the arch over
the nerve cord does not quite cover this structure. The drawing
of the human vertebra illustrates the typical structure found in
higher vertebrates, with the complete neural arch forming a
continuous canal in which the nerve cord lies.

In a primitive vertebrate, ribs may be found on every joint
in the backbone. This is true even in some reptiles. However, in
mammals, including ourselves, many ribs have become much
reduced in size and are fused to the separate vertebrae. This
gives the impression that the ribs are not present. However, if a

COMPARATIVE FEATURES 295

careful examination of the vertebrae is made, these rudimentary
fused ribs are clearly visible. In man, for example, there are seven
vertebrae of the neck which have only miniature ribs fused to
them. The following twelve vertebrae have long curving ribs,
the first ten pairs of which extend around to the front of the
body. There they are joined to a cartilaginous and bony struc-
ture, called the breastbone or sternum, forming a sort of basket
for the breathing apparatus. The next five vertebrae have the
ribs greatly reduced and fused to them so as to make them heavier
and stouter. The succeeding five vertebrae are more or less fused
to each other, and have strong projections on each side to form
the sacrum, in the hip region, to which the pelvis is attached.

Legs and Arms

As soon as vertebrates emerged from water to land, legs
became necessary for locomotion. At first these were small,
awkward appendages, as in the case of the amphibians today. In
the higher vertebrates they increase in size until they not only
form a large part of the skeleton of man but also provide him with
ease and delicacy of motion. Therefore, the legs and arms of
man represent one end product of a long series of developments
in vertebrates, many stages of which are quite clear to us now.
Some of these stages are represented in the developing human
embryo. In other cases, similarities between the bones of the
legs and arms of man and those of other vertebrates show the
steps in this long evolutionary process.

The first step in the evolution of appendages in the early
vertebrates probably consisted in the development of a pair of
lateral skin folds extending along the sides of the body from the
head to the tail. These folds were strengthened by supporting
tissue, which grew out of the mesoderm and gave them a fin-like
structure. It is likely that two regionis in each fold, a forward
region and a hinder region, became separated and capable of
independent movement by means of special muscular develop-
ment. These two paired regions became more pronounced by
continued growth, and at the same time the fold between them
decreased in size. A similar condition is observed in some present-
day primitive fishes. The muscular folds were reinforced by
cartilaginous rays or spines, some of which developed into bones.

296

THIS LIVING WORLD

This famous X-ray photograph of a young woman made on a film as large as the human
body shows the entire human skeleton. The bones are more opaque to X rays than arc
the other tissues of the body, and therefore cast a shadow which makes them visible.
(Photograph by Eastman Kodak Company.)

COMPARATIVE FEATURES 297

Forelimbs of three vertebrates: A Australian "walking" fish; B, primitive amphibian/ C,

human.

Other bones of dermal origin entered the upper region of the
forward paired appendages. Thus, the paired fins of fishes repre-
sent the simplest true appendages. No fishes have more than two
sets of paired fins, corresponding in number and position to the
"legs" and "arms" of terrestrial or land-dwelling vertebrates.

The paired appendages of all land vertebrates are built on the
same plan. They consist of the same sequence of bones. These are,
essentially, (1) a trio of bones forming a "girdle," which is
anchored to the backbone; (2) a single shaft-like bone, called
the "femur" in the leg and "humerus" in the arm; and (3) two
long bones side by side, known as the " radius " and "ulna " in the
arms and "tibia" and "fibia" in the legs. In addition, there are a
number of small bones making up the wrist or ankle and, at the
tip of these, five sets of four small bones each arranged end to end
to form the fingers or toes.

The great diversity of the appendages in vertebrates is
brought about primarily by a variation in the shape, size, and
length of these bones and to a certain extent by an increase or
decrease in the number of bones present. For example, consider-
able modification of the bones in the fins of fishes takes place
when the base is constricted to allow more motion. One type of

298

THIS LIVING WORLD

A remarkable study of a pigeon at the beginning of flight made by high-speed photog-
raphy with a motion picture camera employing 2,000 exposures per second. The adapta-
tion of the forward pair of appendages for flight is shown in interesting detail. (Photograph
by Edgerton, Germeshausen and Grier, Massachusetts Institute of Technology.)

"walking" fish has only one long bone of the fin (corresponding
to the femur) attached to the backbone, the girdle being missing.
Two bones constitute the next segment, thus giving the basic
pattern of all land animals. The bone structures of the limbs of
three types of vertebrates are shown in the drawing on the pre-
ceding page. One is the fin of a fish; another, the leg of an
amphibian; and the third is the arm of man.

The development of the hand is again a case of slight varia-
tions in size and length of the bones and in their arrangement
with respect to each other. Illustrations of the bones of the hands
of a few vertebrates are shown in the drawings. The hand,
although structurally most unspecialized, attains a high degree
of coordinated movement in man. It may be said that the human
hand takes its place along with the human brain in placing man
triumphant at the head of the animal kingdom. It is first of all a
grasping organ, capable of holding a tool, a small delicate instru-

COMPARATIVE FEATURES 299

A, hand of primitive four-fooled animal/ B, monkey's hand; C, human hand.

ment, or a weapon. Without its aid the arts and sciences, which
are the flower and expression of human civilization, would not
have been possible.

This brief comparative study of the skin and skeleton could
be continued for every structure of the bodies of animals. Space
and other considerations do not permjt of such extended treat-
meat here, however. The account which has been given may
serve to show in a general way the reasons for the great com-
plexity of the human body, and, in some degree, the manner in
which its various anatomical features have been developed.

REFERENCES FOR MORE EXTENDED READING

GREGORY, WILLIAM K.: "Our Face from Fish to Man," G. P. Putnam's Sons,
New York, 1929.

While not strictly a "popular'* book, this volume is written in such an interesting
style and is so well illustrated that it may be read with understanding by the layman.

HOMER, ALFRED S.: "Man and the Vertebrates," University of Chicago Press,
Chicago, 1933, Chaps. XIII, XV.

The chapters to which the student is referred contain a brief, but comprehensive
account of certain features of the structure of the human body, and the relationship
of the conditions found in man to those of other vertebrate animals.

BROOM, R.: "The Origin of the Human Skeleton," H. F. and G. Witherby
London, 1930.

This small volume presents a very readable account of the history of the vertebrate
skeleton written by one of the foremost authorities on the subject.

KINGSLEY, J. S.: "Outlines of Comparative Anatomy of Vertebrates," 3d
rev. ed., P. Blakiston's Son & Company, Inc., Philadelphia, pp. 29-131.

300 THIS LIVING WORLD

A detailed descriptive account of the anatomy and phytogeny of the vertebrate skin
and skeleton. The book is a standard, almost a classic text.

HOMER, ALFRED S.: "Vertebrate Paleontology," University of Chicago Press,
Chicago, 1933.

Recommended for the student who wishes to gain a better understanding of the
evolution of vertebrate animals.

SNIDER, LUTHER: "Earth History/' D. Appleton-Century Company, Inc.,
New York, 1932.

An excellent reference volume for the student who is especially interested in
paleontology.

The American Naturalist, published by The Science Press, Lancaster, Pa.

The American Naturalist is a bimonthly journal containing a variety of articles
regarding evolutionary changes in living creatures. These articles range from reports
of experimental research to popularized discourses on many fundamental subjects of
biology.

Journal of Morphology, published by the Wistar Institute of Anatomy and
Biology, Philadelphia.

This is a bimonthly magazine which includes articles on original research in animal
physiology and morphology, featuring such fields as cytology, anatomy, and
embryology.

10: THE HUMAN ORGANISM

A Study of Its Digestive and Respiratory Systems

IN THE Museum of Science and Industry in New York City
there is a complete model of a woman, all parts of which are
made of glass. The glass figure forms the main exhibit in the large
rotunda just inside the entrance to the museum, and thousands of
persons view it annually with absorbing interest. This trans-
parent figure has exposed to view the essential structure and
organization of the human body to many people. Were it pos-
sible to produce life and movement in all the intricate parts of
the model, and then photograph it in a colored, sound motion
picture, the story of the human body would be better told than
is possible in these pages.

Such a picture would show that the human body is a complex
structure made up of an almost countless number of tiny cells

301

302 THIS LIVING WORLD

organized into various organs and systems. All of these are
delicately adjusted to each other and function with a degree of
perfection and coordination unrivaled in the most intricate
machine ever designed and built by man. The details of this
picture would present the fruits of man's ceaseless inquiry into
the structure and functioning of his physical being, a search
which has been going on for centuries and which has revealed
many remarkable things, not only about the make-up of the
human body, but also about the processes of life that surge within
it.

From Cells to Systems

It is about a hundred years since the German scientists,
Schleiden and Schwann, first announced the cell doctrine (1839)
that the structural and functional unit of living matter is the cell.
The years that have elapsed have served to strengthen this
viewpoint and to reveal its fundamental character. It is impossi-
ble to overestimate the importance of Schleiden's and Schwann's
contribution to our knowledge of living things. It forms the very
foundation upon which rests the whole structure of modern
biology and medical practice. Indeed, it is quite possible that in
a thorough understanding of the cell is to be found the answer to
the question of what life itself is.

There is a large group of animals whose entire bodies consist
of a single cell. These are the protozoa. Probably the most
familiar protozoa are the amoeba and paramecium. In such
creatures, all the intricate processes necessary to life are carried
on within a single cell. An amoeba is able to digest food and con-
vert it into the living substance of its body quite as well as does
man himself. Digestion is accomplished by the secretion of
substances which react with foods, breaking them down or
changing them into simpler chemical compounds. These are
then absorbed into the cell body ; there they take part in chemical
reactions which result in the synthesis of exceedingly complex
proteins and other substances that make up the living cells or
which yield energy for performing the work of the cell. The tiny
one-celled animal is able to receive stimuli and respond to them
by movement. Stimulation involves chemical and electrical
changes in the body of the cell when energy is received at the

THE HUMAN ORGANISM 303

cell membrane. The amoeba moves by extending a finger-like
projection from its body in a given direction and then causing

A B C

The amoeba moves by extending a protrusion of its protoplasm in a given direction, form-
ins * sort of arm, and then causing the remainder of the protoplasm to flow into it.

the rest of its protoplasm to flow into it. Moreover, the amoeba
is able to reproduce itself. This occurs by division of the cell,
involving intricate processes which are only too little understood
today.

All these activities take place within an animal form so
small that without the aid of a microscope it escapes the eye
entirely. In other similar one-celled organisms the same processes
are carried on with structural modifications in various parts of
the cell. The bodies of all higher forms of life, including man,
however, consist of a large number of cells. With this increase in
the number of cells different parts of the body behave differently.
Some tissues have functions that are quite different from those of
other tissues. One organ, for example, the stomach, is able to
digest food. This it can do much more effectively than can the
amoeba, but to the exclusion of some other life processes. The
stomach is not sensitive to the variety of stimuli to which
the amoeba is able to respond, neither are the stomach cells
capable of the general movements which amoeba can perform.
Another organ, the brain, is able to receive and transmit stimuli.
This, too, it can do much more effectively than the amoeba, but
at the sacrifice of other functions. Nerve cells cannot digest food
nor produce any motion within themselves.

The reason that one organ of the body can perform one func-
tion well while another is efficient at some other function is that
the cells of which each organ is made are different. There are four
main types of cells which make up the bodies of larger animals.
These are epithelial, muscular, nerve, and connective-tissue
cells. Epithelial cells serve the purpose of protection; also, in
some instances they manufacture and secrete various juices.
They cover the surface of the body and line the mouth, lung

304 THIS LIVING WORLD

membranes, stomach, and intestines. Usually they are some-
what flat and oblong in shape, but great variation is manifested.
Muscle cells are long and slender and have the special ability
to shorten, or contract, when stimulated. They are usually
grouped together in bundles and bound with connective tissue
to constitute the muscles of arms, legs, and other parts of the
body.

Nerve cells have a nerve cell body proper, from which extend
two types of fine filaments. One of these filaments may be very
long. It is the special adaptation of having protruding filaments
which enables the nerve cells to serve their function of transmit-
ting impulses from one part of the body to another. Connective-
tissue cells serve to bind together into various units and organs
all other cells and tissues. They also act as filler tissue to close
up spaces in various parts of the body. In addition, bone cells
are modified connective cells.

All the highly specialized cells of the body are forms of or
are derived from these four types of cells. In this high degree of
specialization, the cells of the body are somewhat comparable to
the people living in a big city. Each person in such a community
follows some particular line of activity. One is a baker, another a
lawyer, a bus driver, a clergyman, and so on. Each does his own
task, but to the exclusion of the other's. Together, their activities
make up the life of the city as a whole, just as the activities of all
the specialized cells of the body make up the life of the complex
individual.

In the human body, as well as in the bodies of all other multi-
cellular forms, the specialized cells function together. Usually
they are grouped, forming the tissues of the body. Thus, the cells
whose special duty it is to filter certain waste materials from the
blood make up the functional tissue of the kidneys. The cells
that receive, focus, and respond to light energy constitute the
essential tissues of the eyes. The cells capable of transferring
oxygen to the blood and removing carbon dioxide and water
vapor from it form the active tissue of the lungs. These groups of
specialized cells are more closely knit and function together
more perfectly than does a carpenters* or a barbers' union, to
continue the analogy with the economic organization of human
societies.

THE HUMAN ORGANISM

305

The transparent woman on display at the Museum of Science and Industry is a life-size
anatomical model, showing veins, arteries, nerves, skeleton and every organ of the human
body. A unique lighting system illuminates each organ in turn until the whole body stands
forth in natural color. (Science Service photograph.)

306 THIS LIVING WORLD

Just as the architect, contractor, building supervisors, and
tradesmen must cooperate in the construction of a modern sky-
scraper, so the various tissues of the body must work together,
mutually assisting each other in the welfare of the organism.
The association is closer in the case of the body tissues, which
are grouped together physically to form organs. In many cases
the organs of the body are combined to form systems, compara-
ble to the railroad system or the telephone system of a great
nation. The telephone system may be said to consist of the
executive officers, research scientists, switchboard operators, line-
men, repairmen, bookkeepers, and all the materials and equip-
ment that they use. So, also, in the human body, the brail?,
spinal cord, ganglia, and nerves, together with their connective
tissue, constitute the nervous system. The heart, blood, and
blood vessels form the circulatory system.

It is well known that a painters' strike in an automobile plant
may upset the whole production schedule or that a truck drivers'
strike may discommode an entire city, so specialized and closely
interwoven has our economic life become. To an even greater
extent is the life of the human organism dependent upon the
proper functioning of each of the various organ systems of which
it is made up.

Supplying Foods (or a Great Community

In the performance of its particular services to the body com-
munity, each of the many billions of cells in a complex animal,
such as man, depends on the same vital processes which are
necessary for the continued existence of the body as a whole.
The energy utilized by the cells in carrying on their specific
functions ultimately comes from the oxidation of foodstuffs, with
the production of waste products which would seriously impair
the operation of the cell, or destroy it altogether, if allowed to
accumulate. By far the greater part of these cells are so situated
in the body that they cannot secure food and oxygen directly
nor rid themselves of toxic wastes in some such simple manner as
does the amoeba. Moreover, even if the specialized body cells
could obtain raw food materials directly, they are incapable of
breaking these substances down into forms suitable for their own
use.

THE HUMAN ORGANISM 307

One of the important systems of the body of a complex ani-
mal, then, is the digestive system. It supplies the body with
foodstuffs in a form suitable for use by the specialized cells in
growth and repair and in obtaining the energy for carrying on
these and other more particular functions. It might be likened
to the commissary system of the army or to the agencies which
supply food and fuel to a great city.

The lowest animals possessing a digestive system are the
coelenterates, such as the fresh-water hydra and the sea anemone.
The bodies of these simple creatures are built around a cavity
known as the gastro-vascular cavity, since it combines some of
the functions of the digestive and circulatory systems in higher
animals. Food is taken into this cavity through a single opening
at the top. Certain of the cells lining the cavity secrete chemical
substances which act upon the ingested food masses and help to
break them down into small particles. These particles are en-
gulfed by other cells lining the cavity, which behave very much
like amoebae in this respect. The food particles undergo the final
stages of digestion within the protoplasm of these cells. Digested
foodstuffs then pass from cell to cell in the body wall by diffu-
sion, while undigested masses of food pass out of the cavity
through the same opening by which they entered.

In higher animals, digestion is essentially the same process of
rendering food chemically suitable for absorption by the in-
dividual cells. The apparatus for accomplishing this is much
more extensive, however, because of the magnitude of the task to
be performed. There are no amoeboid cells in the digestive sys-
tem of man for the purpose of engulfing food particles and
digesting them inside the cell. All the digestive process must be
accomplished outside the cells. The human digestive system
consists of the alimentary canal and associated glands and con-
nective tissues. The alimentary canal is a tube, some twenty -five
to thirty feet long, which passes through the body. Its contents
are not inside the body, but only in contact with a part of its
surface. It consists of the mouth, pharynx, esophagus, stomach,
intestines, and rectum. Its work involves receiving such food as is
offered to it, tearing and grinding this into bits, treating it
chemically so that it may be absorbed into the blood stream,
and then getting rid of unused materials.

308

THIS LIVING WORLD

'Sali vary glands
Esophagus

Liver

Gall
bladder

Duodenum

Small
intestine

Appendix

J-arge
intestine

jr "Rectum

The digestive system in man consists of a much-coiled tube about thirty feet long and a
number of accessory glands for supplying digestive enzymes.

The walls of the alimentary canal, except in the region of the
mouth, are composed essentially of three kinds of tissues, ar-
ranged roughly in layers. The innermost of these is a delicate
lining of epithelium, a single layer of epithelial cells usually
referred to as the mucous membrane. Certain of the cells of this
layer secrete a slimy film of mucus, which is chiefly protective in
function. The mucous membrane is the primary functional
tissue of the digestive system. Its cells are specialized for the
purposes of secreting digestive chemicals or absorbing water and
digested food materials. Surrounding this inner lining and com-
posing the main body of the walls are layers of smooth muscle
tissue. These smooth muscle fibers give the walls of the alimen-
tary canal considerable elasticity and, by their contraction and
relaxation, serve to mix the food with the digestive chemicals and

THE HUMAN ORGANISM

309

to push it along through the tract. The muscular and epithelial
layers are bound together by loose connective tissue containing

Ouier layer of

connective

tissue

Muscular
tissue

Mucous
membrane

Part of a cross section of the small intestine showing the three essential layers that
compose most of the digestive tract. (Redrawn from Carlson and Johnson, "The Machinery
of the Body.")

numerous blood vessels and nerve fibers. The stomach and
intestines are suspended from the upper wall of the abdomen, or
main body cavity, by a thin sheet of connective tissue, which
lines the inner surface of the cavity and surrounds the alimen-
tary canal in this region.

The intestines comprise a narrow tube about twenty -two feet
long. In the interests of conservation of space, this tube is coiled
in a complex manner so as to fit into the abdominal cavity. On
first consideration, the system may seem to be needlessly compli-
cated, since a shorter, broader tube would obviously require less
coiling. However, one of the primary functions of the intestines is
to provide for the absorption of digested foodstuffs. This is a
surface phenomenon. Its rate is governed by the amount of sur-
face provided, and a long, narrow tube presents a greater surface
than a shorter, broader one.

The alimentary canal is an extensive chemical plant where
many chemical reactions take place. The substances which
take part in these reactions are the food, water, and certain diges-
tive chemicals manufactured by special cells and organs con-
nected with the digestive tract. Most important of such organs
are the salivary glands, pancreas, and liver. In addition to these
special organs or chemical plants for the manufacture of diges-
tive fluids, there are countless microscopic glands with tiny
ducts opening into the alimentary canal. These are the minute
glands found in the inner lining of the stomach and small intes-

310 THIS LIVING WORLD

tine. They, too, supply special digestive fluids. The digestive
fluids contain organic catalysts, or enzymes, which hasten the
decomposition of different kinds of foods into substances which
can be used by the body cells.

Digestion

As the food enters the mouth it is usually broken up through
mastication, or chewing, and is thoroughly mixed with saliva.
The saliva is secreted by three pairs of salivary glands. These
glands are located in three different regions of the mouth cavity,
as if to insure against any one injury destroying entirely the
salivary function. One pair is located in the corners of the
jaws, just beneath the ears; another is under the jawbone; and
the third pair is located on each side of the floor of the mouth.
Small tubes or ducts lead from them to the mouth cavity, those
from the first pair opening in the cheeks opposite the upper
molar teeth and the others just beneath the tongue.

The slow secretion of the salivary glands most of the time
serves to keep the mouth moist. When food is taken into the
mouth, salivary secretion is greatly increased. The saliva dis-
solves certain constituents of the food and in so doing initiates
the sense of taste, which can be aroused only by substances in
solution. Its chief function is to lubricate the food masses, aiding
in their mastication and in their passage down the esophagus.
The particular material present in the saliva which aids in diges-
tion is an enzyme known as "ptyalin." It brings about the break-
down of cooked starches into sugar substances. The reaction is
rather rapid; however, much of it occurs after the food and
saliva have been carried down to the stomach. The sugars formed
by the decomposition of starches under the influence of ptyalin
are the type called "double sugars/' An example is ordinary
table sugar or cane sugar. Double sugars cannot be used by the
body cells as foods unless they are split into simple sugars.
Remarkably enough, ptyalin does not effect this splitting. The
further breakdown of the double sugars is delated until the food
reaches the stomach or the small intestine, where other enzymes
act upon them.

After mastication and mixing with saliva, the food is forced
into the throat by an upward motion of the tongue. The muscles

THE HUMAN ORGANISM

311

Illustration of how the food is digested, as shown at the New York World's Fair, 1939
(American Museum of Health photograph.)

of the throat contract so as to close the entrance into the larynx,
or passage leading to the lungs, and to open the entrance into the
esophagus, at the same time forcing food into this passage. Thus
we have the process of swallowing. The muscular walls of th<
esophagus contract behind the food in such manner as to produce
ring-like constrictions forming a peristaltic wave that move;
along toward the stomach. In this manner, the food is pushe(
down the esophagus to the point where it enters the stomach
This entrance ordinarily is closed by a thick ring of tightly con
tracted muscle. When food comes in contact with it, the muscu
lar ring relaxes, allowing the food to pass. Immediately thereafte

312 THIS LIVING WORLD

the ring contracts again, closing the opening and preventing the
food from returning up the esophagus and into the mouth.

The stomach is an elongated
Muscular sac located near the center of the
J^n9 abdominal cavity. The upper

and larger portion, called the

Oair-^-F"*^ l^^^^^^k fundus, is a rounded compart-

ment, connected with the esoph-

p I •/ ^^l^^^^^T^^^I agus and extending down the
valve ^^P^^^^^^^ / left side of the abdomen. It

tapers into a narrower portion,
known as the pylorus, which

Peristaltic waves in the stomach mix curves to the right side of the

the food and move it toward the pyloric body and connects with the

valve. (Redrawn from Carlson and John- upper end of the small intestine

son, "The Machinery of the Body.") through the pyloric yalve. The

pyloric valve is, again, a muscular ring which serves to hold
materials in the stomach until they have been thoroughly mixed
with its digestive juices.

The muscular walls of the stomach exhibit two types of
activity. The fundus serves as a storage compartment or reser-
voir. The muscles of its walls undergo powerful and prolonged
contractions, exerting a steady pressure on the food mass and
causing it to be pushed gradually toward the pylorus. The muscu-
lar walls of the latter undergo ring-like constrictions in peristaltic
waves. This action slowly moves the food toward the pyloric
outlet but does not force it through. Owing to pressure effects,
some material escapes back toward the upper end of the pylorus
so that the net result of stomach peristalsis is a sort of churning
of the food, serving to mix it thoroughly with the gastric juice.

The digestive juice of the stomach is a mixture of substances
secreted by tiny glands located in the mucous lining. The chief
components of this mixture are hydrochloric acid and "pepsin,"
an enzyme that causes the hydrolysis of proteins. By hydrolysis
is meant a chemical reaction in which a complex molecule is
broken down into simpler parts by the addition of water. The
large protein molecules of foods are broken down in the stomach
into substances called "peptones." These are materials that are
similar in structure to proteins but of lower molecular weight.

THE HUMAN ORGANISM 313

They are soluble derivatives of proteins. The hydrochloric acid
gives to the stomach contents the acid character so noticeable
when, through an upset condition, they are regurgitated into the
throat and mouth. The hydrochloric acid does not affect digestion
of proteins directly, but it insures the acid environment neces-
sary for efficient action of pepsin. Peptic digestion takes place
only slowly if at all in an alkaline medium. The saliva is slightly
alkaline, so that the stomach contents must be acidified before
peptic digestion can take place to any appreciable extent.

Two other substances are also secreted by the tiny glands of
the stomach lining. One of these is an enzyme known as " rennin."
Its chief function is to hasten the coagulation or curdling of
protein substances in milk. The curdled milk is then subject to
an initial breakdown by pepsin. The other substance is an enzyme
which brings about the hydrolysis of finely divided fatty material
into glycerin and fatty acids. Its action is inhibited by acids and
hence it takes place only in small amounts, chiefly at the begin-
ning of gastric digestion, before the stomach contents have
become too acidic.

While many people consider the stomach as the most impor-
tant organ in the process of digestion, it actually plays little part
in the final preparation of foods for use in the body. The proteins
are only partly broken down, and ordinarily no extensive diges-
tion of fats takes place. Even the ptyalin of the saliva from the
mouth does not ordinarily convert starches and double sugars
into the simple sugars required by the body cells. The primary
function of the stomach is to serve as a storage reservoir in which
the food material is thoroughly mixed and converted to the con-
dition of colloidal suspension, after which it is fed into the small
intestine. This movement of food into the intestine begins ap-
proximately ten minutes after eating and continues slowly for
three or four hours before the stomach is empty.

The mixing of the food in the stomach by the peristaltic
waves flowing along the muscular walls of the pylorus and the
slow passage of the food into the intestine may be strikingly
viewed by means of X rays. If a meal is eaten that contains food
mixed with a little barium sulphate, the salt renders the food in
the digestive tract opaque to X rays; yet it is innocuous to the
person digesting the meal. The barium-impregnated food will

314 THIS LIVING WORLD

cause the X rays to cast a dark shadow wherever it appears.
Thus its movement through the digestive tract may be followed
by making an X-ray motion picture. Dark shadows of the organs
containing this food stand out in clear relief in contrast to the
surrounding tissues, through which the X rays readily pass.
Contractions of the stomach walls or forward peristaltic move-
ments of the intestinal walls may be plainly observed in this
manner.

Within the Intestines

On the basis of its diameter, the intestine is roughly divided
into two regions, the small and large intestine. As the name
implies, the first is the narrower portion. It is also the longer,
consisting of some eighteen to twenty feet of the intestine. It
extends from the pyloric valve of the stomach to the beginning of
the colon. At its upper end it connects with the pyloric valve
through a portion known as the duodenum. A large compound
duct known as the "common bile duct" empties into the duo-
denum. One branch of this duct comes from the pancreas, while
the other comes from the gall bladder. The duct pours into the
intestine the digestive fluids from these organs.

The pancreas is a thin gland, about five inches long, situated
just behind and below the stomach. Its head is encircled by the
duodenum. The gall bladder is a contractile sac located in a
hollow on the underside of the liver. It serves as a storage reser-
voir for the bile that is secreted by the liver. The liver itself is the
largest organ of the human body and the secretion of bile only
one of its several functions. It is located in the upper part of the
abdominal cavity just beneath the diaphragm and slightly above
and to the right of the stomach. Its chief function is to act as a
storage place for sugars, after acting upon them to convert them
into a complex substance, and to discharge them into the blood
stream as needed.

The food is moved along the small intestine by one kind of
contraction of its muscular walls. This consists of peristaltic
waves similar to those of the stomach and esophagus. The waves
occur only slowly and move the food along for but a few inches,
when they seem to die out. Another type of contraction of the
intestine consists of a rhythmic squeezing and relaxing of the

THE HUMAN ORGANISM

315

^Contracted

Rhythmic squeezing* and relaxations
of the small intestinal wall serve to keep
the food in a churning process. (Redrawn
from Carlson and Johnson, "The
Machinery of the Body.")

intestinal wall. This serves to keep the food in a constant sort of

churning process and thereby effects a complete mixing with the

bile and intestinal digestive juices.

It seems that in any particular

section of the intestine these

churning movements will continue

for a time ; then a peristaltic wave

will move the contents along a

short distance to the adjoining

section, where the process is

repeated.

The mucous lining of the intes-
tine is not smooth, as it is in the
mouth and esophagus. Rather it
is deeply folded into ridges, which
run around it. The surface area
of the intestinal lining is thereby
increased so as to permit a greater contact with the food
materials. Even more important in this respect is the fact that
these folds are completely covered with tiny hair-like projections
called " villi." The villi give the intestinal lining a velvety appear-
ance and admirably provide for an increased surface to effect one
of the functions of the small intestine, namely, the absorption of
digested materials.

Digestion is carried on in the intestine through the action
of the pancreatic juice, aided by the bile and certain fluids
secreted by one-celled, tubular glands located in the intestinal
lining. The pancreatic juice contains digestive enzymes, which
are effective, either directly or indirectly, in breaking down each
of the three major substances — proteins, carbohydrates, and
fats. The products of protein digestion in the stomach, the
peptones, are further split up by hydrolysis in the intestine
under the influence of one of the enzymes of the pancreatic
juice, namely, "trypsin." The action of trypsin on the peptones
is usually to produce compounds of several amino acids, called
"polypeptides." These are substances which must be further
digested before they can be used by the body cells. Thus, the
original proteins of foods require still further reaction, and this is
accomplished by digestive enzymes from the small intestine, as

316 THIS LIVING WORLD

will be noted presently. Still another pancreatic enzyme brings
about the hydrolysis of starches and double sugars to simple
sugars. This is merely a continuation of the process begun in the
mouth and stomach through the action of salivary ptyalin and
the hydrochloric acid of the stomach.

Perhaps the most important of the pancreatic enzymes is
"steapsin," which brings about the hydrolysis of fats to yield
fatty acids and glycerol, substances which can be absorbed and
utilized by the body. The pancreas is the only gland which
secretes a fat-splitting enzyme in significant amounts, and the
only appreciable digestion of fat in the body is brought about
through the action of this enzyme. If the pancreas is removed, or
if its functioning is impaired through disease or accident, one of
the most important consequences is that fats pass through
the alimentary canal almost unchanged and, therefore, are lost
to the body. This result can also be brought about by a failure of
bile secretion, since the efficiency of pancreatic enzymes is more
than trebled by the complementary action of the bile. Actually
the bile is a mixture of certain salts called "bile salts," and prod-
ucts of the decomposition of hemoglobin in the blood. The bile
salts aid the action of steapsin by causing the large particles of fat
to be broken down into fine droplets that are suspended in the
intestinal contents in the form of an emulsion. The surface
presented for action of the enzyme is thus greatly increased,
considerably hastening the digestive process. The bile salts also
aid in making the contents of the intestine alkaline, a condition
necessary for the proper activity of the pancreatic enzymes. The
pancreatic juice itself is alkaline, owing to the presence in it of
sodium bicarbonate, which serves to neutralize the acidity of the
stomach contents as they are poured into the intestine.

There remains still another important part of the digestive
process. This is accomplished through the action of enzymes in
the juices secreted by the tiny glands in the mucous lining of the
small intestine. One of these enzymes completes the hydrolysis
of polypeptides to amino acids. These are protein products which
can be used by the body cells. Thus although this enzyme would
be without effect upon whole proteins, it is responsible for the
completion of the digestive process begun on them in the stom-
ach and continued by the trypsin of the pancreatic juice. Other

THE HUMAN ORGANISM 317

enzymes of the intestinal juices bring about the hydrolysis of
double sugars such as milk sugar, cane sugar, and malt sugar. It
will be recalled that these double sugars are the products of
salivary and pancreatic digestion of starch; also they may
constitute a part of the original foods that are eaten. Their con-
version into simple sugars, such as glucose and fruit sugar, is
accomplished to a certain extent by the hydrochloric acid of the
stomach and by the action of a pancreatic enzyme but chiefly
through the action of specific enzymes of the intestinal juice.

Let us summarize the main steps of digestion of the three
major food constituents. We have seen that the first action on
starches is by salivary ptyalin. This begins in the mouth and con-
verts these materials into double sugars. The next action on these
double sugars, as well as on other double sugars found in foods,
is brought about in the small intestine by an enzyme of the pan-
creatic juice. Here some of the sugars are converted into the
simple sugars. Additional specific enzymes in the juices from the
tiny digestive glands of the small intestine hydrolize the remain-
der of the double sugars to simple sugars, such as glucose and fruit
sugar. These are products which can be used by the body cells.

The first action on fatty substances occurs in the stomach,
where small amounts of these materials are converted into
glycerin and fatty acids by the action of one of the gastric
enzymes. However, most of the digestion of fats is accomplished
in the small intestine by the action of the pancreatic enzyme
steapsin with the assistance of certain salts in the bile. The result
of this digestion is fatty acids and glycerol, materials that can be
used by the body.

We have seen that proteins are first hydrolized by pepsin in
the presence of hydrochloric acid to form peptones. This occurs
in the stomach. These are further digested in the small intestine
by pancreatic trypsin to form poly pep tides. The digestion of
proteins is completed by the action of specific enzymes secreted
by minute glands lining the small intestine, which hydrolize the
polypeptides to amino acids.

Absorption

We have seen how in the process of digestion foods are con-
verted into dissolved material chemically suitable for use by the

318 THIS LIVING WORLD

By an effective method of injection, the minute and extensive network of blood
capillaries in the villi lining the small intestine of the cat are clearly shown. (Photomicro-
graph by Roy Allen.)

cells of the body. However, in the strictest sense, these materials
in the alimentary canal are still outside the body proper. They
must be absorbed into the circulating fluids before they become a
part of the internal environment and thus actually available for
the tissue cells. This absorption takes place mostly in the lower
part of the small intestine, although it occurs to a certain extent
throughout the length of the intestinal tract. Very little is ab-
sorbed from the stomach. A notable exception is alcohol, and
this fact accounts for the rapidity with which the effects of
excessive imbibition become noticeable. Practically the only
absorption that occurs in the large intestine, or colon, is of water.

The absorption of foodstuffs from the intestine is a complex
phenomenon involving specific cellular action modified by the
purely physical factors of diffusion and osmosis. The villi of the
intestinal lining are covered with a thin membrane beneath which
is a rich supply of blood capillaries and a network of lymph
vessels. The food materials are passed through the epithelial cells
of the villi into either the blood capillaries or the lymph vessels.

The simple sugars, which are the products of digestion of
carbohydrates, and the amino acids, which result from the action
of the digestive juices on proteins, pass directly into the blood
stream by way of the capillaries of the intestinal villi. They
are carried by these capillaries into the portal vein and through

THE HUMAN ORGANISM

319

Epithelial
cells

Veins

Lymph— *

Lymph

rtery

Veins

Within the villi are found an extensive network of capillaries that connect the arteries and
veins, also an ending of the lymph vessels.

it to the liver. Here the simple sugars are stored in the form of a
complex sugar, which is a product of synthetic activity of the
liver cells. Some of the amino acids pass right through the liver
and finally reach the body cells, where they are converted into
proteins of the living cell protoplasm. Each different type of cell
forms its own specific kinds of proteins. Usually more protein is
eaten than is required to furnish the necessary amino acids for
this purpose. By far the greater part of the amino acids are acted
upon by the liver in such fashion as to remove their nitrogen,
producing fatty acids and ammonia. The fatty acids may be
utilized in the synthesis of fats for use in the body as fuel or
stored energy, or they may be converted into carbohydrate to be
put to the same uses. The ammonia is almost immediately
combined with carbon dioxide in the liver and converted to urea,
which is excreted in the urine.

The other great group of food materials, the fats, are ab-
sorbed in a somewhat different manner. It will be recalled that in
digestion such foods are broken down into fatty acids and glyc-
erol. In the process of absorption the chemical reaction is
reversed. Even in passing through the absorbing cells of the

320 THIS LIVING WORLD

epithelial membrane covering the villi, at least a part of the fatty
acids and glycerol are reconverted into fats again. However, the
fats now formed are of the human type rather than of the differ-
ent types formed by other animals. The composition of fats varies
considerably, even though the substance glycerol is common to
them all, and each species of animal has its own particular kind
of fat.

The tiny droplets of fat formed in the epithelial cells enter
the lymph vessels of the intestinal villi rather than the blood
capillaries. The lymphatics of the villi connect with an extensive
system of vessels, which carry the lymph all over the body. Even-
tually, however, these vessels empty into a large vein in the left
shoulder region. By this indirect route, the fats eventually reach
the blood stream. After reaching the body cells the fats are used
as fuels; their oxidation or burning releases energy and produces
a part of the body heat. The consumption of fatty foods in exces-
sive amounts results in the storage of fat in certain connective
tissues of the body, notably around the heart, kidneys, intestines,
and just beneath the skin.

By the time the food has run the gauntlet of the countless
villi in the long length of the small intestine, most of it has
been absorbed. At least, most of that part which was properly
digested has been absorbed. What remains when the colon is
reached, therefore, is primarily undigested and indigestible sub-
stances and water. Little digestion of food occurs in the colon
except that which is carried on by bacterial action; even less
absorption, except of water, takes place from the colon. The
absorption of water tends to form the feces or give them their
more solid consistency.

The intestine, especially at its lower end, and the colon con-
tain an abundant bacterial flora. The function of these bacteria
is largely unknown, although in animals which feed exclusively
on plants they play an important part in digesting cellulose and
rendering this material useful to their hosts. The bacteria are
acquired shortly after birth and are present throughout life.
They are harmless as long as they remain in the intestine, but
when they invade the blood stream, through a break in the
intestinal wall caused by accident or disease, they produce
serious disorders which often result in death. A similar result

THE HUMAN ORGANISM

321

<3&
0

Connective
tissue

Transverse section of trachea showing important layers.

Epithelium

Submucous
layer

Connective
tissue

1 Cartilage

occurs if they escape into the body cavity, where they produce
inflammation of the peritoneal lining, or peritonitis.

The bacteria are particularly abundant in the vermiform
appendix. This is a small extension of the lower part of the small
intestine, at its junction with the colon. Inflammation of the
appendix produces the condition known as appendicitis, in
which there is infection and more or less destruction of the walls.
Should the inflammation become acute, the walls may swell and
burst unless the organ is removed. Rupture of the appendix
permits the contents of the alimentary canal, including numerous
bacteria, to pour into the peritoneal cavity surrounding the
intestines, resulting in peritonitis and usually death.

Such is the delicate and complicated mechanism which sup-
plies the body with all its food. An understanding and better
appreciation of the structure and function of the digestive system
should influence one to exercise more care in its treatment.

322 THIS LIVING WORLD

The Breath of Life

All the energy that man utilizes in his bodily activities, or
radiates in the form of heat, in the last analysis comes from the
chemical process of oxidation. As ordinarily understood, this
has reference to the combination of oxygen with other atoms or
molecules. In a stricter sense, however, the term "oxidation"
refers to any chemical reaction involving the loss of electrons by
an atom. It is always accompanied, therefore, by reduction, in
which an atom gains electrons. In a biological sense, the popular
interpretation of the oxidative process is permissible only because
the substance ultimately reduced in the living cell is oxygen and
the final products of the reaction are oxides of carbon and hydro-
gen, namely, carbon dioxide and water.

It is evident that a constant supply of oxygen is required for
the continued liberation of energy in the body and that this ele-
ment is one of the raw materials essential to life. It is readily
available in the atmosphere about us, comprising about one-
fifth of it. The problem of obtaining this oxygen is simple enough
in minute, single-celled animals, such as an amoeba, whose
entire body is in direct contact with water containing dissolved
oxygen. In an animal as large, complex, and bulky as man,
however, a complicated organ system is required to extract the
oxygen from the air, and another is needed to provide for its
distribution to the various cells of the body. The latter is one
of the functions of the circulatory system.

The function of obtaining oxygen from the air is assigned to
the respiratory system. Here the oxygen is brought in contact
with a moist membrane of epithelial tissue. It is transferred

1 9. 3

These six pictures of super-fast photosraphy made at the rate of 4000 per second
show the movement of the vocal cords during one cycle of a high-frequency note being

THE HUMAN ORGANISM 323

through the cells of this membrane into the blood stream by way
of numerous tiny capillaries. The moist membrane also provides
for the transfer, from the blood to the air, of the chief products
of biological oxidation, namely carbon dioxide and water.

The respiratory system is composed of the lungs and the air
passages connecting them with the exterior. The latter are rela-
tively large tubes and include the nasal passages, throat, larynx,
trachea, and bronchial tubes. All parts of the respiratory system
are lined with a delicate membrane of epithelial cells. Most of
the lung structure and the air passages have an outer layer of
connective tissue, providing considerable elasticity. In the larger
passages, such as the larynx, trachea, and bronchial tubes, a
third layer grows between these two. It consists of some muscu-
lar tissue and some cartilage or gristle. The cartilage is arranged
in rings which encircle the tubes and give them rigidity. These
rings are most pronounced in the region of the larynx, or " Adam's
apple," where they may easily be felt. They serve to keep the
tubes always open so as to allow free passage of air through them.

The inner layer of epithelial cells secretes a moist substance,
called "mucus/* which tends to lubricate the passages and
moisten the air before it reaches the finer structure of the lungs.
Some of these cells are ciliated; that is, they have tiny hair-like
projections on them. The cilia beat with a continuous forward
wave motion which tends to dislodge dust particles and germs
from the surface and sweep them upward to the throat.

The Breathing Process

The process in which air is taken into the lungs and then
expelled from them is known as breathing. The air we breathe

456
sung by the subject (Photographs by Dr. J. C. Steinberg, Bell Telephone Laboratories.)

324

THIS LIVING WORLD

Nasal. ^
cavities

Larynx

Cartilage
rings

Bronchi -

Pharynx

•Vocal cords
^ — Esophagus
Trachea

Bronchioles

The respiratory system. (Redrawn from Strausbaugh and Weimer, "General Biology.")

is taken into the nose, or should be, where it is freed somewhat of
dust and germs by the fine hairs growing at the nasal openings.
In the nasal passages the air is warmed to body temperature,
saturated with water vapor, and freed of still finer impurities by
the whip-like motion of the cilia of the mucous membrane. After
this "air-conditioning" process, it enters the cavity of the throat,
from which the larynx channel, or windpipe, leads off to the
front, while the esophagus going to the stomach leads off from
the back. The windpipe is normally open and the esophagus
closed. Only during swallowing does the windpipe close and the
esophagus open for the passage of food. If one attempts to
swallow and to breathe at the same time, both passages are open,
and it is likely that the food will enter the windpipe, with a
likely "explosive" result, or strangulation.

From the throat cavity the air passes into the larynx, where
is located a set of muscular bands called the "vocal cords."
They may be tightened or loosened, opened or closed; and by
such movements the air passing through them may be set into
sound vibrations. Just beneath the larynx, the air enters the
trachea. About halfway down the chest the trachea divides into

THE HUMAN ORGANISM 325

two branches, called the bronchial tubes. One of the bronchial
tubes leads to the left lung, the other to the right lung.

Within the lungs there are finer and finer divisions and rami-
fications of the tubes connecting with the bronchi. Soon the air
has reached very tiny passages known as bronchioles. These are
the last of the air passages. Each bronchiole opens into about six
or eight small pockets, called "air sacs." Some three to six
minute, sac-like nodules, the alveoli, are found on each air sac,
so that the final structure is not unlike bunches of grapes in gen-
eral arrangement. The alveoli average about four-thousandths of
an inch in diameter, and their number in the two lungs has been
estimated at seven hundred and fifty millions.

Exchanse of Gases in the Lungs

By the time the alveoli have been reached, the epithelial lin-
ing of the lungs has become exceedingly thin and the connective-
tissue layer has disappeared entirely. Just beneath the layer of
epithelial cells there is a thick network of blood capillaries. The
actual exchange of oxygen, carbon dioxide, and water vapor takes
place across the thin walls of the alveoli and these capillaries.
It might seem that some unique process is necessary for the
oxygen to filter through the alveoli and capillary walls in one
direction and the carbon dioxide and water vapor to pass in the
opposite direction. However, this exchange is accomplished by
the straightforward and well-known physical process of diffusion.

For a simple illustration consider a gas jet. If the jet is turned
on in the kitchen and left unlighted, it will not be long until
the odor of gas may be noticed over the entire house. The gas
molecules move from the place of their higher concentration,
that is, the jet, to areas of lower concentration. If the jet is then
turned off, but no ventilation provided, the gas will eventually
become as concentrated in one part of the house as in any other
part. In other words, the gas molecules continue to diffuse until
their concentration is equalized in all parts of the house. This
diffusion always takes place in the direction of decreasing
concentration.

The air one inhales contains about twenty per cent oxygen
and four one-hundred ths per cent carbon dioxide. These are
the concentrations of respiratory gases reaching the thin walls of

326

THIS LIVING WORLD

The alveoli are surrounded by
capillaries, and an exchange of gases
takes place through the thin walls
separating them.

the alveoli. On the other hand, the blood flowing through the
capillaries of the alveolar walls, having just returned from the

body circulation, is short in oxy-
gen. At the same time it is heavily
laden with carbon dioxide. The
air exhaled from the lungs con-
tains about sixteen per cent of
oxygen and four per cent of car-
bon dioxide. This difference in
Capillaries composition of the expired from
that of the inspired air tells us
there has been an exchange of
gases in the lungs. Since the con-
centration of oxygen within the
alveoli is greater than that in the
blood just across the thin mem-
brane in the capillaries, oxygen diffuses through this membrane
and dissolves in the blood. Similarly, the concentration of car-
bon dioxide in the blood of the capillaries is greater than it is
in the air within the alveoli. Consequently, there is diffusion of
this gas from the capillaries to the air sacs.

The amount of oxygen that may be carried by the blood is
greatly increased by a loose chemical union that takes place
between the oxygen and the pigment of the red blood corpuscles.
As oxygen diffuses through the alveolar and capillary membranes,
it is dissolved in the blood much the same as sugar dissolves in
water. However, the amount of oxygen which will dissolve in the
blood represents only about one per cent of that actually carried
in the blood leaving the lungs. Obviously some other mechanism
than simple solution must be sought to account for the transpor-
tation of oxygen by the blood. Briefly, this mechanism is as
follows. As the oxygen goes into solution in the blood, it is im-
mediately removed by forming an unstable compound with
hemoglobin, the red pigment present in the blood corpuscles.
The capacity of the blood to hold oxygen is thus increased a
hundredfold. The formation of the compound between hemo-
globin and oxygen is accompanied by a change in color of the
pigment from a purplish to a bright red hue. This gives us the
marked distinction between venous and arterial blood.

THE HUMAN ORGANISM 327

The chemical reaction of oxygen and hemoglobin must occur
very rapidly, since the blood in passing through the capillaries
remains in contact with the small alveoli only for a second or two.
Speed in this case is facilitated by exposing large surface areas of
hemoglobin to the oxygen in solution. The red blood corpuscles
are exceedingly small. Furthermore they are disk-shaped, with
somewhat concave sides, and this shape gives them the greatest
amount of surface area possible in proportion to their volume.
Their small size makes it possible for an enormous number to
exist in the blood, so that the total surface area exposed to the
dissolved oxygen is very large. In frogs and other amphibians
that do not require so much oxygen as man, the red blood cells
are much larger, with a corresponding decrease in surface area.
They may easily be observed with a low-power microscope. In
man, however, their exceedingly minute size prevents them from
being seen except with high powers of magnification.

In the tissues of the body, the chemical reaction between
hemoglobin and oxygen is reversed. Here the concentration of
oxygen is much lower in the protoplasm of the tissue cells than in
the blood. The unstable compound of hemoglobin and oxygen
tends to break down, and the oxygen diffuses into the tissues.
This process is considerably hastened by the rapidity with which
the oxygen is used by the tissue cells.

Just as little oxygen is carried by the blood in simple solu-
tion, so also little carbon dioxide is transported in this form. The
dissolved carbon dioxide reacts with the water to form carbonic
acid. This is a moderately weak acid, which reacts with certain
protein salts of the blood to form sodium bicarbonate and potas-
sium acid carbonate. In addition, some carbon dioxide appar-
ently combines directly with hemoglobin, forming a loose union
similar to that of oxygen with hemoglobin. This fraction of the
total carbon dioxide carried in the blood is very small, however,
in comparison with the part transported as bicarbonate.

The formation of bicarbonates and the compound of hemo-
globin and carbon dioxide takes place in the capillaries pervading
the tissues of the body. In the alveolar capillaries of the lungs the
reactions are reversed, since the concentration of carbon dioxide
in the alveolar air spaces is considerably less than in the blood.
Thus, in the lungs carbon dioxide is released from the blood into

328

THIS LIVING WORLD

Metal cast of air spaces and passages of the lungs of a dog. Where the metal filled the
alveoli well/ it formed an almost solid mass/ at other places the branching air tubes can be
seen. The inset shows a cast of clusters of alveoli at the ends of tiny air tubes. (Photograph
by Dr. Victor Johnson, University of Chicago.)

THE HUMAN ORGANISM 329

the air, whereas in the tissues dissolved carbon dioxide and car-
bonic acid diffuse from the tissue cells into the blood.

Quite apart from their significance in the transportation of
carbon dioxide from the tissues to the lungs, the bicarbonates
have an important role in the regulation of the acid-base balance
of the body. The tissue cells are very sensitive to changes in
acidity of the fluids surrounding them. In fact they can live
only within a very narrow range of concentrations slightly on
the alkaline side of neutrality. The accumulation of acids in the
system is prevented by neutralization with the bases of the
blood to form salts and water which are excreted in the urine.
The carbonic acid released in this process of neutralization breaks
down in the lungs into water and carbon dioxide, which are
excreted.

Mechanics of Breathing

The mechanical phases of the breathing process consist of
getting fresh air into the lungs and exhaling the used air. Here
well-known physical principles are beautifully applied by the
body. The lungs lie completely enclosed in two airtight compart-
ments of the chest, one on the right side and one on the left side.
Each lung itself is an airtight sac, opening to the outside only
through the bronchial tubes. The arrangement is one in which a
bag with a single opening to the outside is placed inside a cham-
ber that is entirely closed. The walls of the inner sac or lung are
quite elastic and capable of considerable stretching.

The chamber, or chest cavity, is bounded by the body wall
and ribs on the top and sides and by the diaphragm at the bot-
tom. Its volume may be regulated by two mechanisms. One of
these involves contraction of the muscular diaphragm ; the other,
movements of the ribs. The diaphragm is the muscular mem-
brane separating the chest from the abdomen below. It extends
up into the chest cavity in somewhat the shape of a dome. When
the muscles of the diaphragm contract, the convexity of the
upward arching is greatly reduced, pulling the diaphragm down
so that the size of the chest cavity above is increased. At the
same time, enlargement of the cavity in other diameters is
effected by raising the ribs. Each pair of the ribs, which are
attached to the vertebral column behind, forms a ring extending

330 THIS LIVING WORLD

Mechanics of breathing. During exhalation the ribs are lowered as shown in upper
drawings, and the diaphragm extends up into abdominal cavity. During inhalation the ribs
and breast bone are raised as shown in lower drawings. At the same time the diaphragm is
flattened, giving larger volume to the chest cavity.

around to the front to join the sternum, or "breastbone." These
rings slope downward and to the front. With each inhalation the
front of the ribs is raised by a contraction of certain muscles.
The raising of the oblique rings pulls them outward and increases
the volume of the chest cavity.

.When the chest cavity enlarges, the pressure on the outside of
the lungs is decreased. As a result of this unbalanced pressure,
air is forced in through the nose by the outside air pressure, and
rushes down the air passages into the lungs. This causes the lungs
to expand in size until the pressure is equalized on both sides,
that is, both inside the lungs and in the chest cavity. The elastic-
ity of the lung tissues permits their expansion in much the same
manner as a rubber balloon will stretch because of the elasticity
of the rubber when air is forced into it.

Diminution of the volume of the chest cavity is brought
about by a relaxation of the diaphragm, which permits it to take
its normal shape, and by a relaxation of the rib muscles, which
permits the ribs to resume their natural position. Dropping of the

THE HUMAN ORGANISM 331

ribs and relaxation of the diaphragm take place simultaneously.
When this occurs, the lungs are squeezed into a smaller volume,
thus forcing the air out of them.

How Much Air Do We Breathe?

The lungs contain about three liters (three quarts) of air
under conditions of normal, quiet breathing. Approximately 600
cubic centimeters (about a pint) of air is inhaled and exhaled
with each normal breath while resting. This is called tidal air.
However, not all this air reaches the alveoli to supply oxygen to
the blood and remove carbon dioxide. About 150 cubic centi-
meters remain in the larger air passages. Thus in each normal
breath about 450 cubic centimeters of air are used for rejuvena-
tion of the blood stream. Under conditions of strenuous exercise
or forced breathing about two liters, or 2,000 cubic centimeters,
may be inhaled and exhaled at each breath.

But even with forced breathing all the air cannot be exhaled
from the lungs. The amount left, of which the lungs can never be
deprived, is not less than about one liter. It is generally referred
to as the "residual air" and represents the minimum amount of
air that is present in the lungs of every person from birth until
death. A small part of this residual air cannot be removed even
though the lung be dissected out from the chest and completely
collapsed. Under no conditions can it be squeezed out of the
alveoli. This small amount of so-called "minimal" air is present
in the lungs of infants who have taken even one breath after
birth and, of course, in all persons of greater age. This property
of the lungs is important in certain cases of fatalities in newly
born infants. If the infant is born dead, there will be no minimal
air in the lung. If, however, a single breath has been drawn, a
test will substantiate without any error whatsoever that death
by foul or natural cause occurred after birth.

Thus the body possesses an intricate and nearly perfect sys-
tem for providing the circulating medium with fresh, pure air
and for keeping an ample residual supply always in contact with
the capillaries just beneath the alveolar membranes. Only when
the organs become diseased or damaged by foreign organisms or
misuse do they fail in their functions and cause trouble.

332 THIS LIVING WORLD

REFERENCES FOR MORE EXTENDED READING

BOGERT, L. J.: "Diet and Personality," The Macmillan Company, New York,
1936.

In this interesting and nontechnical little book a well-trained student of nutrition
has presented facts and explanations which may be of help to laymen in adapting and
regulating their diet intelligently to their special physical type and to modern living
conditions. Practical and sound suggestions regarding the relationship of diet to body
size, age, health, infections, indigestion, undernourishment, and overfatigue are some
of the subjects included.

HILL, A. V.: "Living Machinery," Harcourt, Brace & Company, Inc., New
York, 1927, Lecture IV.

This book consists of six Christmas lectures delivered by the author at the Royal
Institute in London in 1926. Lecture IV contains much information on how the lungs
supply the blood with oxygen and how this oxygen is distributed to the body. The
excellent manner in which the author describes how these things are demonstrated and
proven is one of the features of the book.

ROMER, A. S. : "Man and the Vertebrates," University of Chicago Press,
Chicago, 1933, Chap. XIV.

A part of this chapter treats concisely and clearly of the digestive and respiratory
organs and respiratory processes in man.

STILES, P. G. : "Human Physiology," rev. by G. C. Ring, W. B. Saunders
Company, Philadelphia, 1939, Chaps. XIII, XIV, XV, XX, XXI, XXII,
XXV, XXVI.

The authors have written in these chapters an exceedingly clear and complete
elementary discussion of the organs and processes of digestion and respiration. In
Chap. XXII is an explanation of the "transformation of matter" after it has been
absorbed by the body cells. Nutrition and hygiene are discussed in the last chapters
to which reference is made.

BEST, C. H., and N. B. TAYLOR: "The Human Body and Its Functions,"
Henry Holt & Company, Inc., New York, 1932, Sees. IV, V.

These sections are an explanation of the mechanisms and processes of respiration
and digestion. The authors have included a considerable amount of detail of the body
structure and functions. Descriptions and discussions are often by analogy, and the
chapters are clearly illustrated, the illustrations tending to make the book readily
understandable.

CRANDALL, LATHAN A.: "An Introduction to Human Physiology," W. B.
Saunders Company, Philadelphia, Chaps. VIII-XIII.

In these chapters is found a description of the respiratory and digestive organs,
including some detailed account of how respiration and digestion is accomplished.

THE HUMAN ORGANISM 333

EtJLENBURG-WiENER, VON RENEE: "Fearfully and Wonderfully Made," The
MacmiUan Company, New York, 1938, Chaps. Ill, IV, V, VI, VII, XII.

This is a rather comprehensive survey of the digestion of foods written in style
that is quite understandable to the general reader. Chapter XII includes a dis-
cussion of the respiratory system and the processes by which gases are exchanged in
the blood flowing through the lungs.

CARLSON, A. J., and V. JOHNSON: "The Machinery of the Body," University of
Chicago Press, Chicago, 1937, Chaps. VI, VII, VIII.

Chapter VI is a well-written and clearly illustrated discussion of respiration, and
Chapter VII treats similarly of digestion. Chapter VIII deals with the uses of food
elements by the body, metabolism, and the excretion of wastes.

Hitman Biology, published by Johns Hopkins Press, Baltimore.

This quarterly journal contains only articles that are records of research. The
subjects discussed include a wide range of studies in human biology, and many are
subjects that will be of interest to the intelligent layman. An elementary understand-
ing of the statistical methods of presenting data is necessary for a thorough reading
of the text.

The Anatomical Record, published by The Wistar Institute of Anatomy and
Biology, Philadelphia.

The Anatomical Record is a monthly professional journal which publishes original
researches on vertebrate anatomy. The articles are usually accounts of highly special-
ized investigation and cover a wide range of subjects in the field of vertebrate
anatomy.

II: MOVEMENTS OF MATERIALS

A Study of the Human Circulatory System and Excretory Organs

PEOPLE who live in a large city are aware of the necessity for
an adequate system of transportation. This is especially true
when some circumstance produces a partial or complete paralysis
of the regular facilities. People and supplies cannot reach their
destinations. Wastes accumulate, imperiling the health of the
community. Inconveniences or severe hardships result for every-
one. In villages the problem is less acute or does not exist at all.
The grocery, the drugstore, the church, and individual homes are
in close proximity. Intermingling of the people and the exchange
of goods and services are correspondingly easy and simple.

These varying degrees of economic dependency find their
counterpart in organic nature. In the simplest one-celled crea-
tures no circulating or transporting mechanism is required. The
organism is in direct contact with a medium containing both

384

MOVEMENTS OF MATERIALS 335

food and oxygen and obtains these directly by ingestion and
absorption. Higher forms show the beginnings of a system for the
distribution of materials within their bodies. As the latter get
larger and more complicated, this system increases in complexity.
The human body is as intricate and elaborate an organic struc-
ture as any found in nature. It contains the most complex and
delicately balanced of transportation facilities.

Providing a Suitable Cell Environment

In the preceding chapter two great systems were discussed
which supply the body with materials from the outside world
necessary for life. Another system was mentioned which trans-
ports these materials to the individual cells and carries away
their waste products. Most of the cells of the body are far re-
moved from any outside food or oxygen supply. They cannot get
life necessities unless these are brought to them. Likewise, the
waste products formed by the individual cells would soon ac-
cumulate in and around them unless removed. Without such
removal all soon would become poisoned and would die.

It may accurately be said that much of the work of the body
is concerned with maintaining a special environment around the
individual cells in order that they may live and carry out their
specific functions. This special environment is a watery salt solu-
tion derived from the blood. In many respects it is not unlike
the sea water surrounding marine animals. It is known as the
tissue fluid. In many places in the human body, where the cells
are closely packed, this fluid is only a thin film between them;
nevertheless, it is always there.

Dissolved in the tissue fluid are digested foodstuffs and oxy-
gen, which are taken up and used by the cells, and waste prod-
ucts, which are continually being produced within the cells by
their metabolism. The oxygen and food supplies must constantly
be replenished* and the wastes removed in order for the cells to
function normally. This turnover in composition of the tissue
fluid is brought about at an exceedingly rapid rate, and it is the
function of the blood circulating in every part of the body to
maintain a stable condition of this internal environment.

The blood does not actually mingle with the tissue fluid.
Rather it flows in a closed system of vessels which reach all

336

THIS LIVING WORLD

Arteriole

Red
corpurcles

'White
corpuscles
migrating

A network of arterioles and lymph vessels permeate the entire body, maintaining a proper
condition in the tissue Ruid surrounding the body cells.

Body
cells

parts of the body. The finest branches of this system, called
"capillaries," form a network around and between the tissue
cells. The digested foodstuffs carried in the blood stream pass
through the thin walls of the capillaries into the tissue fluid.
This is accomplished by diffusion of the molecules of such food-
stuffs from the center of their higher concentration to one of
lower concentration, since the amount of food materials in a
unit volume of the blood is greater than it is in a corresponding
volume of the tissue fluid just on the other side of the thin
capillary membrane. Oxygen likewise diffuses into the tissue
fluid for the same reason. However, the larger molecules of the
blood, such as those of the blood proteins, cannot readily get
through the meshes of the capillary membranes. The same thing
is true of the red corpuscles. Therefore, they tend to remain in the
blood. A selective diffusion results, in which only the materials
useful to the body cells pass into the tissue fluid to any great
extent.

Certain waste products produced by the tissue cells diffuse in
the opposite direction. The tissue fluids tend to be richer in

MOVEMENTS OF MATERIALS 337

carbon dioxide than the capillary blood. Therefore, this substance
will move from the tissue fluids into the capillaries, to be carried
away and thus prevented from accumulating around the cells.
Also taken away in this manner are various salts and other sub-
stances having relatively small molecules. Other waste products
are removed from the tissue fluids in an entirely different man-
ner. This is especially true of certain materials composed of
relatively large molecules, fragments of dead cells, and bacteria
which generally will not go through the capillary walls. Such
materials pass into a second set of capillary tubes which also
permeate the entire body. These vessels have very thin membra-
nous walls, even thinner than those of the blood capillaries. The
capillary tubes join larger ducts so as to form a continuous
system. This second set of capillaries are the lymph capillaries,
and the larger passages constitute the lymph vessels. The lymph
vessels eventually empty into the blood stream in the shoulder
region. Thus, all the waste products of the cells finally are de-
livered into the circulating blood.

We have seen that very definite and delicately balanced
conditions are maintained around each cell of the entire body.
In order to accomplish this materials must often be transported
great distances within the body at definite, though changing,
rates. The mechanism for facilitating this transportation we call
the circulatory system. It consists of the heart, blood vessels,
and blood, supplemented by the lymphatic system.

Heart Action

It is not necessary to tell anyone that the heart is an impor-
tant part of the circulatory system. Everyone knows this to be
true. It is also generally known that the continued and regular
beating of the heart is necessary to life. When the heart stops
beating a person soon dies. There are few other natural causes of
death that act more suddenly or with greater dispatch than does
heart failure. However, beyond these general concepts, most
people are confused in their knowledge of the precise structure
and action of this important and vital organ. Usually they are
ignorant of a few general precautions that should be observed
in order that the heart may function satisfactorily until mature
old age.

338 THIS LIVING. WORLD

The heart is a thick-walled, muscular organ which acts as a
great pump to force the blood to all parts of the body. The heart

;;/ V -••-", •

x^i^M^ i

• \J^^-'/

The human heart consists of two separate parts, a right heart and a left heart Each of these
parts has two chambers, an auricle and a ventricle, which are connected by valves.

of an adult human really consists of two separate organs, a right
heart and a left heart. As a result of their evolutionary origin
and embryonic development, these two hearts are adjacent to
each other and give the appearance of one organ. The right heart
receives blood from the body circulation and passes it on to the
lungs. The left heart receives the blood coming from the lungs
and pumps it to the rest of the body. While both hearts work in
unison, the blood from one does not mix directly with that of
the other.

Each of the separate hearts consists of two chambers, an
auricle and a ventricle. The auricles receive blood into the hearts;
the ventricles force it out of them. Blood coming into the right
heart from the vessels of the body flows into the upper chamber,
the right auricle. This is a relatively thin-walled cavity which
connects below with the right ventricle. The blood is forced into
the ventricle from the auricle mainly by a feeble contraction of
the latter's walls. The passage which connects the auricle and
ventricle is guarded by a structure consisting of three thin flaps
directed downward, known as the tricuspid valve. No obstruc-
tion is offered by this valve to movement of the blood in the
direction of the ventricle. However, when the blood attempts to
flow back into the auricle, the tricuspid valve closes immediately,

MOVEMENTS OF MATERIALS 339

preventing such movement. The valve is supported from beneath
by strong fibers so that the flaps cannot be turned upward too
far, thus permitting leakage of blood into the auricle.

The right ventricle is a somewhat triangularly shaped cham-
ber with relatively thick, muscular walls. When the walls of the
ventricle contract, the pressure of the blood closes the tricuspid
valve and the blood itself is forced into the vessels leading to the
lungs. In leaving the ventricle, the blood must pass a set of three
pocket-like valves guarding the entrance into the pulmonary
artery. These valves allow; the blood to flow only in the direction
of the lungs. They provide a sort of safety stopgap to prevent
the blood from flowing back into the ventricle and piling up
during the latter's period of relaxation.

The left auricle, which is situated at the top of the heart,
receives blood from the vessels coming from the lungs. It, too,
possesses relatively thin contractile walls. Below, it connects
with the left ventricle through an opening surrounded by two
flaps pointing downward, known as the bicuspid valve. This
valve functions in a manner similar to the tricuspid valve, per-
mitting the blood to flow only one way, that is, from auricle into
ventricle.

The left ventricle is a triangularly shaped chamber, like the
right ventricle but with very much thicker muscular walls. Con-
traction of the left ventricle forces the blood out through the
great aorta to all parts of the body. The greater thickness of its
walls in comparison with those of the right ventricle gives it a
more powerful pumping stroke. This is indeed necessary, since
the left ventricle must pump the blood over a much greater
distance. Just at the beginning of the aorta are a set of three
pocket-like valves which prevent the blood from flowing back
into the ventricle during the period when it is relaxed. These
valves, and the similar ones at the beginning of the pulmonary
artery, serve as exit gates from the heart. They permit outgo
but never entrance of blood to the ventricles. They are the only
valves in the entire arterial system.

The cycle of operations in the heart begins with the contrac-
tion of the walls of the two auricles, the right auricle slightly
preceding the left. Blood is forced thereby into the right and
left ventricles through the tricuspid and bicuspid valvea,

340 THIS LIVING WORLD

respectively. There is a slight pause, then simultaneous contrac-
tion of the ventricles takes place, the tricuspid and bicuspid
valves close, and the blood is forced from the heart. This is
followed by relaxation of the muscular walls of the auricles and
ventricles in the same order. Blood again flows into the auricles
from the veins, and the cycle is repeated.

The total time consumed in a single cycle of contraction and
relaxation of the heart is about 0.8 seconds. This interval is so
brief that the original discoverer of the circulation of the blood,
the English scientist William Harvey, was forced to remark three
hundred years ago that "the motion of the heart is to be com-
prehended only by God." However, since Harvey's day instru-
ments of greater precision have been devised which permit not
only of observing the motions of the heart but of timing them
as well. The time required for the auricular contractions is about
0.05 second; that for the ventricular contractions is 0.30 second;
and the total time for relaxation is about 0.45 second.

The events of heart action produce certain sounds that
accompany the contractions and the closing of valves. The
manner in which the heart is functioning can be judged very
accurately by the exact nature of these sounds. Each normal
beat produces two sounds. The first is rather low-pitched and
prolonged. It is produced from two actions, one the closing of
the tricuspid and bicuspid valves, and the other the contraction
of the thick walls of the ventricles. This sound is definitely
altered when the valves do not close properly. The other sound
of the heartbeat is high-pitched and of short duration. It is
produced by the rapid closing of the valves leading into the great
aorta and the pulmonary artery at the instant when ventricular
contraction is finished. If these arterial valves become damaged
or begin to leak, this second sound is greatly altered or disappears.

In addition to the above normal sounds, heart murmurs are
sometimes heard. They usually result from some irregular flow
of blood through the heart or great aorta. For example, when the
tricuspid or bicuspid valves do not close properly, there is a
backward flow of blood through them during the contraction
of the ventricles. This backward flow will produce a gurgling
sound, or murmur. When there is leakage through the valves
at the entrance to the aorta, blood will flow back into the left

MOVEMENTS OF MATERIALS

341

A striking exhibit at the Golden Gate International Exposition in 1939 to illustrate
the number of times the heart beats within a lifetime of 65 years. (Ciba Pharmaceutical
Products photograph.)

ventricle during its interval of relaxation, again producing a
murmur as well as seriously interfering with the normal heart
action.

Any such irregularities as are revealed by peculiar heart
sounds can easily be detected by a competent physician. The
same is true of certain infections or chronic conditions that may
later produce serious effects upon heart action. A periodic, com-
plete medical examination oftentimes would reveal to the
individual minor disorders that could be effectively treated
before they became serious. Such periodic examinations are
important for discovering the condition not only of the heart
but also of the entire body.

Canals for Circulation

The blood vessels constitute another integral part of the
circulatory system. The vessels carrying blood in a direction

342

THIS LIVING WORLD

Head
and

Right
auricle

Right
ventricle

Left
ventricle

The circulatory orsans are the heart, arteries, capillaries and veins which form a closed
system of tubes that extend throughout the entire body. (Redrawn from Young "The
Human Organism and the World of Life.11)

away from the heart are called "arteries." Those returning the
blood to the heart are called "veins." The two sets connect with
each other, so as to form a closed circuit, through an extensive
network of minute, thin-walled vessels in the tissues, called
"capillaries." This canal system as a whole forms two great
loops linked through the heart in such a way that the blood must
flow through each in making a complete circuit around the body.
One loop consists of the circuit from the left side of the heart
through the body capillaries and back to the right side of the
heart. It is known as the "systemic" circulation. The other loop
consists of the circuit from the right side of the heart through the
lung capillaries and back to the left side of the heart. It is known
as the "pulmonary" circulation.

MOVEMENTS OF MATERIALS 343

On leaving the heart to enter the systemic, or body, circula-
tion, the blood passes through the largest artery of the body.
This is the aorta. It forms a great arch extending upward from
the left ventricle and toward the left side of the chest cavity,
then, turning downward along the back wall near the backbone,
it pierces the diaphragm to enter the abdominal cavity. As it
emerges from the heart, forming the aortic arch, the aorta is
very large, having a diameter of about an inch. It immediately
gives off several large branches. One of these supplies the head,
chest, and right arm; another, the chest and left arm. A much
smaller but extremely important branch supplies the heart
itself. Within the abdomen, several other large branches are
given off which supply the stomach and intestinal tract, the
digestive glands, the spleen, and the urinogenital system.
Finally, the aorta breaks up into branches going to the legs and
to the region of the end of the spine.

With continued branching, the aorta grows progressively
smaller in diameter. The diameter of the branches likewise tends
to get smaller and smaller as they, too, branch. The larger
arteries supplying the limbs, head, trunk, and some body organs
are on the average about one-quarter of an inch in diameter.
They continue to divide and subdivide until, as minute vessels
called arterioles, they permeate all the tissues of the body. The
arterioles themselves undergo further subdivision into many
smaller branches, the capillaries. By the time the capillaries are
reached the diameter of the vessels has decreased to an average
of less than one-thousandth of an inch. Their length is on the
average about one-hundredth of an inch. The capillaries are so
numerous, however, that were their contents all spread out to
form a continuous surface, they would cover an acre of ground.

The walls of the arteries are composed essentially of three
layers. The innermost one is a thin membrane of smooth epi-
thelial cells, called "endothelia," which permit the blood to flow
with minimum friction. Outside this lining is a layer of muscular
tissue which decreases in thickness as the arteries get smaller.
The walls of the arterial vessels, therefore, are capable of con-
traction and relaxation, so that their diameter may be changed.
Outside the muscular walls is a layer of connective tissue, which

344

THIS LIVING WORLD

is both tough and elastic, permitting the arteries to expand when
necessary but resisting rupture even by very high internal

pressures.

The thickness of the arterial
walls decreases as the size of the
vessels diminishes until the cap-
illaries are reached. Here both
the muscular and connective
layers disappear, leaving only a
microscopic layer of epithelial
tissue. This thin membrane per-
mits the exchange of materials
between the blood in the capil-
aries and the tissue fluid outside.
As the blood courses through
T(^ « ^T- he larger arteries the pressure
change with each heartbeat is
quite pronounced. Such pressures
may readily be felt by placing
the finger on any artery near the
skin surface, as, for example,
the one on the thumb side of the
wrist. However, there is a con-

A network of blood capillaries
branching off from an arteriole as photo-
graphed from the mesentery of a living
frog. (Photomicrograph by 6. Zweifach,
New York University.)

tinuous decrease in this pulsating pressure as the distance from
the heart increases. This is due to the friction encountered along
the way and to the resistance offered by the arterial walls to their
wave-like expansion and contraction following each heart beat.
By the time the capillaries are reached the pulsating pressure has
completely disappeared and the blood flows at an even pace.

The network of capillaries penetrates all the tissues of the
body. As it moves along, the blood in the capillaries gives up
food materials and oxygen to the tissue fluid and collects waste
products from it, as has already been noted. The actual sight of
the blood streaming through these fine vessels, or a clear mental
picture of it, makes for a better understanding of the complex
physical structure of the body.

This capillary network and circulation may be observed quite
easily by looking through a microscope at some thin, living,
animal tissue having a rich supply of blood vessels. This is an

MOVEMENTS OF MATERIALS

345

experience that everyone should be fortunate enough to have. A
suitable tissue is the gill structure of the mud puppy, an amphib-
ian which is commonly found in many
streams and ponds in the eastern part
of the United States. The gills are so
thin and transparent that the capil-
laries may easily be observed in them.
The phenomenon may also be seen in
the web of a frog's foot.

The one-way streets of the capil-
laries must have some outlet. They
cannot terminate with dead ends. As a
matter of fact, they unite to form small vessels carrying blood
away from the tissues. At first, these are tiny venules correspond-
ing to the arterioles. The venules drain into veins, which gradu-
ally increase in diameter. The veins are somewhat larger than
the corresponding arteries, and their walls are thinner and less
elastic. Eventually they come together to form the large vessels
emptying into the heart. The large vein from the lower part of
the body is known as the " inferior vena cava," while that from
the arms and head is called the "superior vena cava." They unite
to form one large vessel just before emptying into the right
auricle.

The blood continues to move along in the veins because of
the impetus given it by the beating of the heart. However, this
force is gradually diminished owing to friction and "loss of head "
as the arteries branch. Other agencies aid the heart in moving
the venous blood. Among these are the pocket-shaped valves
with which the veins are richly supplied. These valves are
arranged in such fashion that the blood can flow only toward the
heart in the veins. Thus any piling up of the blood behind a valve
causes it to open, permitting the blood to move on in the direc-
tion of the heart. Piling up ahead of the valve causes it to close,
preventing the blood from returning toward the capillaries. In
addition, the contraction of various skeletal muscles produces
pressure on the veins, helping to move the blood onward. Such
action is particularly valuable in returning the blood from the
legs to the heart against the force of gravity. By the time the
blood reaches the junction of the venae cavae, its pressure has

346 THIS LIVING WORLD

dropped nearly to zero and it moves evenly into the right auricle,
mainly by gravitational pull. Thus one great loop of the circula-
tion has been completed. The blood has delivered its cargo of
oxygen and food materials to the tissue fluids surrounding the
body cells. From this internal environment it has picked up
another load of materials, this time mostly body wastes, and has
returned with them to the right heart.

A part of the wastes picked up in the systemic circulation,
namely, carbon dioxide and some water vapor, must be unloaded
in another part of the circulatory path. Immediately, therefore,
the blood leaves the right heart for the loop through the lungs.
This is the pulmonary circulation. Blood leaving the right ventri-
cle enters the large pulmonary artery. Almost immediately after
leaving the heart this artery divides, one branch going to each
lung. The two arteries so formed continue to diminish in size by
branching, until the capillaries of the lungs are reached. In the
lung capillaries, the blood rids itself of excess carbon dioxide
and some water vapor and takes on a fresh supply of oxygen.
It then flows into the pulmonary venules. The venules unite to
form veins, which continue until finally they produce the large
veins which empty into the left auricle.

The force which moves the blood through the pulmonary
arteries is supplied by the contraction of the right ventricle.
However, by the time the blood has passed through the lung
capillaries its pressure has dropped nearly to zero, and it flows
smoothly back to the heart because of the greater pressure
behind it. This quick drop in pressure within the pulmonary
vessels is effective in speeding up the circulation of the blood
through the lungs.

The average time required for the blood to traverse the
double loop of the circulatory system is relatively short. It
varies from about thirty seconds to about one minute, depending
upon what part of the body it traverses. Suppose we were to
select some small sample of blood and to time it on its journey
around the body. Starting in the right ventricle, by contraction
of the heart the blood sample would be forced to the lungs. About
ten seconds would be consumed in making the circuit to the
lungs and back to the heart again, this time to the left auricle.
If our particular sample were then to journey to the foot, by

MOVEMENTS OF MATERIALS

347

The path of a blood corpuscle in making the circuit of the body, as illustrated at the New
York World's Fair in 1939. (American Museum of Health photograph.)

348 THIS LIVING WORLD

way of the aorta and large arteries of the leg, it would return
through the veins to the right auricle approximately fifty seconds
later. Should it proceed to an organ lying nearer the heart, it
would be back to the right auricle within a shorter interval of
time and ready for its next circuit to the lungs.

It might be thought that in order to complete its circuit
through the body in so short a time the blood would have to rush
through its entire course at a precipitous speed. This is not
actually so. Within the larger arteries and veins, to be sure, it
does move rapidly. Near the heart, in the large aorta, for
example, the blood travels at the rate of nearly three feet per
second when the left ventricle contracts. In the large veins
approaching the right auricle, it flows at the rate of about one
and a half feet per second. As the capillaries are approached the
speed decreases greatly. This results from the fact that, when a
blood vessel divides, the combined area of cross section of its
branches is greater than that of the original vessel. On entering
the branches, therefore, the pressure head on the blood is reduced
and it slows down, just as water in the wide part of a river flows
more slowly than it does within a narrow gorge. Thus, a given
sample of blood will consume about one second in traversing a
capillary so short that it can be seen only with the aid of a
microscope.

Unique Circulating Fluid

The blood is in many respects the most valuable and unique
fluid in the body. It is not only the bearer of food and waste
products; it contains within itself living tissue; that is, it contains
cells. The main difference between the blood and any other tissue
is that the cells of the blood are floating free in a liquid called
"plasma." This is a yellowish fluid, of which approximately
ninety per cent is water. It has in it certain proteins held in
suspension and a number of materials in solution. The substances
in solution are the food materials such as sugars, fats, and amino
acids and various mineral salts such as chlorides, phosphates,
and carbonates; there are also body wastes such as urea, uric
acid, and ammonium. In addition, it contains some oxygen and
carbon dioxide in solution, as well as a material which is able to
produce a substance called "fibrin."

MOVEMENTS OF MATERIALS 349

A remarkable thing about the plasma is that its composition
remains highly constant, despite the fact that materials are
added and removed at many points in the circulation. Often
this exchange is extremely rapid. The constituents of the plasma
react chemically among themselves in such a fashion that any
change in the blood composition calls forth adjustments of these
factors. Moreover, materials are added by certain organs, while
substances are removed by others. The balance is restored and
there is maintained the exact composition necessary for the life
of the body cells.

A particularly important chemical property of the blood
plasma is that it coagulates or clots when it is discharged from
the blood vessels. This is a complex process, as yet not fully
understood. Following a definite series of events, certain protein
materials of the plasma break down to form fibrin. Fibrin is
precipitated in hair-like strands possessing considerable strength.
It forms a fine meshwork of threads in which the red and white
corpuscles become entangled and on which the plasma coagulates.
Thus a sort of dam to the blood stream is formed, the mass
hardens, and a clot is produced.

Clotting occurs when the blood comes in contact with injured
tissues or damaged cells and when it comes in contact with
foreign surfaces which have physical properties different from
those of the smooth lining of the blood vessels. The process is
one which is vital to the life of the individual. When any wound
occurs, it is important that the break in the circulatory system
be stopped immediately by clotting; otherwise excessive loss of
blood would soon occur. It is just as important that clotting
should not occur within the regular blood channels. If such
should happen, circulation would be hindered or stopped, and
the results would be just as disastrous.

The visible breaks in the blood vessels, of course, are those
that occur on the surface of the body. However, breaks can also
take place in the deeper lying vessels. In either event clotting
must stop the loss of blood until the damage is repaired. Internal
breaks are sometimes caused by foreign particles or organisms
which invade the tissues and destroy the capillaries at a given
point. The organisms that produce syphilis probably furnish
the most spectacular example, but they are in no sense the

350 THIS LIVING WORLD

only ones. Where such internal breaks occur, blood clots tempo-
rarily check the germs, or at least prevent death from internal
hemorrhage.

The worst malady associated with defective blood coagula-
tion is the condition known as "hemophilia." In individuals
afflicted with this condition the coagulation mechanism is
deficient to the extent that clotting does not occur. Even the
slightest wound internally or externally may result in a fatal
hemorrhage. The malady has played an important role in his-
tory, for it has affected some of the royal houses of Europe,
notably the late czar of Russia and the recent royal house of
Spain. In 1938 former King Alfonso's son, the Count of Cova-
donga, died in the United States of excessive bleeding from a
minor cut suffered in an automobile accident. The disease is a
hereditary condition which affects only males and is transmitted
directly to the afflicted individual by the maternal parent only.

Formed Elements of the Blood Stream

Although to the superficial observer the blood appears to be
a liquid of uniform composition, when examined through the
microscope it is seen to contain bodies of definite form and size.
They are of three distinct and different kinds, two of which are
readily identified. These are the red corpuscles and the white
cells. The third type of formed element is more difficult to find
and will not be discussed here. The red cells are by far the most
numerous. In fact, an almost incredible number are found in the
blood of a single individual, there being about eighty to a
hundred billion to the cubic inch, or a grand total of some thirty
trillion. They are, of course, correspondingly small in size.
Human red blood corpuscles are biconcave disk-shaped objects
about one three-thousandth of an inch in diameter, and approxi-
mately one-fourth as thick.

The red corpuscles are little more than thin-walled sacs con-
taining a red-colored solution of potassium salts. Perhaps the
most important chemical characteristic of the red cells is the
presence in them of relatively large amounts of hemoglobin.
This is a compound formed by the union of a protein substance
(globin) with a complex iron-containing material (hematin).
Hemoglobin has the peculiar property of combining easily with

MOVEMENTS OF MATERIALS

351

Formed elements of the blood. The red corpuscles differ conspicuously from the larger
white cells, not only in being more numerous but also in that they have no nucleus. The
very small dark bodies are platelets. (Photomicrograph by Roy Allen.)

oxygen to form a loose compound. The compound is likewise
broken down with the liberation of oxygen. This is just what
happens when the corpuscle arrives in a region where the tissues
are short in oxygen. The reaction is reversed in the lungs where
the relative concentration of oxygen is higher in the tissues than
in the red cropuscles, and a recombination with oxygen takes
place. It is by virtue of the presence of the hemoglobin in the red
corpuscles that the blood is able to transport sufficient oxygen
from the lungs to all parts of the body.

The red corpuscles do not contain a nucleus or any evidence
of nuclear substance as such. In this respect they differ funda-
mentally from typical living cells, and this is one reason for
calling them corpuscles rather than cells. Not having a nucleus,
they are incapable of reproducing themselves by cell division.
Nevertheless, as a part of a living system, they must arise from
living cells even though they are themselves of less than cellular
grade. They must, therefore, be produced by other tissues in
different parts of the body. They must likewise be replaced from
the same source eventually, as they do not exist indefinitely.

352 THIS LIVING WORLD

It has been estimated that about one-thirtieth to one-tenth of
the red corpuscles are destroyed daily. However, in a normal
person, the total number in the blood remains remarkably con-
stant from day to day. This means that ten million red cells are
destroyed and replaced each and every second throughout one's
entire life.

The formation of the red corpuscles takes place in the red
marrow found in the bones of the body, chiefly in the ends of the
long bones, for example, the ribs and limb bones. Within this red
tissue are found specialized cells which are in a state of rapid cell
division. These cells bear no resemblance to the mature red blood
corpuscles. They are nucleated giant cells. Some of the daughter
cells formed by division of the red marrow cells begin to develop
hemoglobin within them. As these cells mature, their nuclei
gradually disappear and their hemoglobin content increases.
The mature red blood corpuscles formed in this manner then
pass into the blood stream.

The fact that the number of red corpuscles in the body
remains constant indicates that there must be some stimulus
which acts upon the red marrow, causing it to produce more red
corpuscles when needed. This stimulus is definitely regulated
by the oxygen content of the blood. It seems to be a chemical
substance which emanates from the liver. Thus, when a shortage
of red corpuscles develops, the resulting lowered oxygen tension
of the blood acts upon the liver, causing it to release this sub-
stance, which in turn stimulates the bone marrow. New red
cells are produced and the balance is restored.

When there is a deficiency of this material produced by the
liver, or when the red marrow does not function properly to
produce new red cells, a shortage of red cells develops which
cannot be overcome by the body. This results in a condition
known as pernicious anemia. This is an insidious ailment that
brings about a progressive weakening of the muscular tissues,
deterioration of the cellular structure, and finally death, unless
proper treatment is administered. The treatment consists essen-
tially of administering liver extracts containing the red marrow-
stimulating substance the body itself does not supply.

Such extracts are prepared from the liver of normal animals.
The active principle appears to be produced by the action of the

MOVEMENTS OF MATERIALS 353

normal digestive juice upon some protein component of the diet.
This substance is stored in the liver. When these extracts are
injected into the blood of an individual suffering from pernicious
anemia they speed up the formation of red cells, even though the
red marrow of the bones may be greatly reduced in such persons.
By this treatment the symptoms are usually relieved. However,
in most cases it is necessary to continue administration of the
extracts if the person is to live.

The white cells are larger and fewer in number than the red
corpuscles. They move more slowly through tjie plasma. In con-
trast to the red corpuscles, the white cells are nucleated. They
are capable of moving about independently by pushing out root-
like extensions of their bodies and flowing into them much as an
amoeba does. By this process they are able to slip through the
walls of the capillaries and move about in the tissues. They
do this in large numbers when some foreign organism gets
inside the body or when the skin has been broken by a wound
or laceration.

White cells are constantly being destroyed in the body,
though not so rapidly as the red corpuscles. Their number
fluctuates widely, being greatest when a person has certain in-
fections, for example, appendicitis. While they have nuclei, they
do not reproduce by cell division; rather, they originate within
the red marrow of the bones. Just what is the relationship of
their origin to that of the red cells is not known. In the adult
stages, however, the two are quite different.

Even though fewer in number, the white cells are no less
important than the red corpuscles. Their chief function so far
as the body is concerned is to destroy bacteria. They can sur-
round a foreign particle or organism and digest it much as an
amoeba engulfs and digests its food. The white cells are found
in largest numbers where bacteria have the best chance to enter
the body, and they tend to congregate at points of bacterial
infection. In such places the white cells slip through the walls
of the capillaries, surround the bacteria, and proceed to digest
them along with fragments of tissue cells killed by the action
of the bacteria or by toxic substances produced by the bacteria.
Thus, the fate of millions of bacteria which invade our bodies is
to be eaten by the white cells.

354 THIS LIVING WORLD

A Helping Hand

An auxiliary to the circulatory system is the lymph system.
This seems to be a secondary system for moving materials,
principally coarser wastes, from the vicinity of the body cells
and thus providing double assurance that unnecessary sub-
stances will not accumulate in places where they are unwanted.
It is a sort of one-way drainage system from the tissue fluids
surrounding the individual cells to the blood stream. The lymph
system consists of a network of lymph capillaries which lie
adjacent to the cells and which come together to form the lymph
ducts. These ducts eventually combine to form two large
vessels, one on either side of the neck. They empty into veins in
the shoulder region not far from where these veins join the left
auricle of the heart.

The lymph capillaries permeating the body tissues collect
the lymph largely through a simple process of diffusion. The
lymph is a watery fluid very similar in composition to the tissue
fluid from which it is derived. In addition it contains numerous
particles which are too large to diffuse through the walls of the
blood capillaries. Included among such particles are bacteria
and fragments of destroyed cells, materials that would soon
clog the body if they were not carried away. Inside most of the
lymph ducts are numerous valves so arranged as to prevent the
lymph from backing up toward the capillaries. The lymph is,
therefore, gradually moved along toward the veins by muscular
movements of the body and eventually is returned to the blood
stream near the heart.

At frequent intervals along the lymph ducts are found struc-
tures known as lymph nodes. These are essentially made up of a
connective-tissue framework enclosing large numbers of white
blood cells and amoeboid cells, which are capable of breaking
down the larger particles contained in the lymph and of destroy-
ing bacteria. As the lymph trickles through the nodes, the large
particles are filtered out so that only the proper materials return
to the blood stream. The nodes in the lungs of city dwellers, for
example, often become black with the soot and dirt filtered out
of the lymph during a lifetime of breathing filthy air.

MOVEMENTS OF MATERIALS 355

tissue from a person who had lived most of his life in a large city where the air is
often laden with smoke. The black material in the picture is carbon granules adhering to
lung tissue cells. The light circles are cross sections of small bloodvessels. (Photomicro-
graph by G. C. Grand, New York University.)

Another function of the lymphatic system is to transport the
fats that are absorbed from the small intestine through the villi.
The fats are carried in the lymphatic system until the large
veins near the heart are reached. Here they are deposited in the
blood stream. In this manner, they are diverted from going
directly to the liver, as do the other foods absorbed from the
intestine. It is not clear just why the fats should be absorbed
and transported in a different manner from other food materials.
The explanation may lie in the fact that in the cells of the in-
testinal wall the fats are present in the form of an emulsion of
fine droplets too large to diffuse through the walls of the blood
capillaries but capable of penetrating the lymphatic walls.

Excretion and Elimination of Wastes

It is common knowledge that industrial operations nearly
always involve the production of wastes. This is especially true
in those industries which have a chemical basis. While research
has pointed out uses for many of these by-products, improve-
ments in production methods have not yet succeeded in eliminat-

356 THIS LIVING WORLD

ing them. It is to be inferred that a group of processes so manifold
and complex as those which go to make up the metabolism of
the human body would likewise yield a variety of waste sub-
stances. This is indeed the case. Unfortunately, provision has
not been made for finding uses to which these by-products may
be put in the body. Some of them are distinctly harmful to the
living cells and if allowed to accumulate would ultimately cause
death. Others are detrimental when present in greater than
certain prescribed quantities. One of the important tasks of the
body, therefore, is to rid itself of these toxic materials.

Two types of physiological processes play a part in accom-
plishing this end. One of them is termed elimination; the other,
excretion. The main avenues of elimination are the passages
leading to the exterior of the body from certain organs and the
pores of the skin. Elimination is largely a mechanical process of
forcing waste materials out of the body confines. Here it should
be noted that materials in the lungs, rectum, and bladder are
actually already outside of the body proper, since they no longer
form a part of the internal environment of its cells. The me-
chanical changes involved in exhaling air from the lungs have
previously been described. The emptying of the bladder is
brought about by reflex contraction of its muscular walls.

The elimination of materials from the colon through the
rectum rids the alimentary canal of undigested substances taken
in with the food and not absorbed into the blood stream. In
addition, the colon serves as an exit organ for certain metabolic
wastes of the body. Excess amounts of calcium salts and salts
of the heavier metals in the blood are excreted into the colon,
from which they are eliminated from the body. Also, certain
bile salts, particularly bile pigment resulting from destruction
in the liver of the red blood cells, pass into the colon and are
thus eliminated. The brownish color of the feces or colon excre-
ment is produced by these and products of red-cell destruction.
Large numbers of living and dead bacteria are also eliminated
from the colon.

The dangers of colon poisoning have frequently received
much attention, particularly in advertising cathartic and "help
nature" remedies. Much of this represents an exaggeration of
the facts. A knowledge of the general composition of the feces,

MOVEMENTS OF MATERIALS 357

however, emphasizes the necessity of regular colon elimination.
This is effected in most people naturally by including in the diet
a variety of foods, some roughage (though not in excess), and
considerable amounts of liquids. Such natural regularity of
elimination is greatly to be desired above that produced by the
use of artificial remedies.

Excretion is a process of removing the waste products of
metabolism from the body fluids and passing them into tempo-
rary depositories, such as the bladder and the lower end of the
alimentary canal. The principal excretions of the body are
perspiration and urine. The first is excreted by the sweat glands
of the skin; the latter by the kidneys.

Perspiration is a very dilute solution, chiefly of salt, or sodium
chloride. It has only about one-eighth the concentration of
solids found in the urine. On a comfortable day, or in a properly
heated and ventilated room, about a pint of perspiration is
excreted in a day. On a very hot day, or in a poorly ventilated
and overheated room, this figure may rise to two or three quarts.
Contrary to popular belief, the perspiration is of little impor-
tance in ridding the body of wastes. Even with the greatest
activity of the sweat glands, the amount of urea eliminated per
day in this way is less than one-tenth of the normal daily output
in the urine. The chief function of the perspiration is to aid in
regulation of the body temperature. Heat is dissipated in the
evaporation of water from the skin, particularly in the evapora-
tion of the perspiration. When functioning of the kidneys is
impaired through disease, there is a compensatory increase in
concentration of the perspiration, especially as regards the
urea content. Unfortunately, this compensating adjustment is
never sufficient to alter significantly the consequences of serious
kidney failure.

The chief metabolic wastes of the body are carbon dioxide,
water, and certain products of protein decomposition. We have
already noted that carbon dioxide is removed from the blood by
processes taking place in the lungs. Water is both a food requisite
and a waste. It is also important as a carrier of other wastes in
solution, as in the urine and perspiration. With the exception
of the small quantity incorporated into the tissues in their
growth, all the water which is taken into the body is passed out

358

THIS LIVING WORLD

again. In addition to the water lost in the perspiration and
through evaporation from the general surface of the skin, a very

i jiia considerable quantity,

of cortex X^j^^SSS^^^ "" amounting to as much

as a pint a day, is passed
out by way of the lungs in
the form of water vapor.
Nevertheless, the urine
Ureter constitutes the major por-
tion of the water loss of
"Branch the body, about a quart

artenf ' an<^ a ^a^ being excreted
daily by normal adults.
It carries with it, in
Calyx ^SBMHIIIPB^^ solution, the by-products

Longitudinal section of the kidney showin3 main of protein metabolism.
divisions. (Redrawn from Starling, "Human Physi- Chief among these are

olosy.11) urea, uric acid, sodium

chloride, and the potassium salts of sulphuric and phosphoric
acids.

The Work of the Kidneys

The urine is formed through the work of the kidneys, which
are the principal excretory organs of the body. In the removal of
waste materials from the blood they are second in importance
only to the lungs. They act partly as a great filtering system,
separating from the blood many of the wastes that have been
formed in other parts of the body and emptied into it. Even
more than this, the kidneys regulate the composition of the blood
and help to maintain the proper environment within the body
necessary to life.

The kidneys are paired organs located at the back of the
abdominal cavity just beneath the diaphragm and to the right
and left of the spine. Each is a bean-shaped body, weighing a
little less than a pound in most people. For their size these small
organs do an enormous amount of work. The activity of any
tissue in the body may be measured in terms of the amount of
oxygen it consumes. It has been found that the kidneys use about
nine per cent of all the oxygen consumed by the body although

MOVEMENTS OF MATERIALS

359

they represent less than one per cent of its total weight. Corre-
spondingly, the blood supply of the kidneys is exceptionally large

Bowm&n's capsule

Urinary
tubule

Knot of arteries inside Bowman's capsule/
and network of capillaries surrounding urinary
tubule furnish the mechanism for filterins the
blood of waste materials.

in proportion to their size.

When cut in two, each
kidney is seen to be a compact
mass roughly divided into two
regions, an outer, narrow zone
called the "cortex," and a cen-
tral portion called "medulla."
At the base of the central
region, and continuous with its
inner surface, is a hollow por-
tion, known as the "calyx,"
which is continuous with the
ureter leading from the kidney.

Microscopic examination
of the organ reveals that it is
made up of a very large number
of minute tubules which open
into the calyx. The tubules are the functional units of the kidney.
They are straight in some places, very much coiled in others.
They extend outward through the medulla toward the cortical
zone, branching and tapering in diameter as they go. Each tubule
ends in the cortex in a blind sac-like structure, known as "Bow-
man's capsule." This is a double-walled cup, shaped as though
formed from a hollow sphere by pushing one half down inside
the other. Each of the two walls thus formed consists of a thin
membrane only one cell thick. The space between the walls is
continuous with the tubule leading from the capsule. Within the
hollow of the capsule and lying next to the inner membrane is
a small knot of blood capillaries. Some four and a half million
of these cups and capillary knots are present in each kidney.
It is across the thin membranes of the capillary knots and the
walls of Bowman's capsules that filtration of impurities from
the blood takes place.

Blood enters each kidney from the dorsal aorta by a short
but large artery; it leaves by a correspondingly large vein which
empties into the inferior vena cava. Blood from the arteries
which supply the kidneys flows into the capillary knots within

360 THIS LIVING WORLD

the Bowman's capsules under considerable pressure. This pres-
sure is greater than in the capillaries in other parts of the body.
It is as though it were maintained here actually to force materials
through the capillary walls. Much water and substances dis-
solved in the water do filter through the capillaries and pass into
the capsules. Chemical analysis of the material in the capsules
shows that it has a very different composition from the urine
finally eliminated from the kidney. It contains a great deal more
water and considerably greater amounts of salt and sugar, in
addition to urea, uric acid, and the various salts derived from
the decomposition of proteins in metabolism. In fact, it has
pretty much the composition of the blood plasma. A notable
difference consists in the absence of the plasena proteins and the
blood corpuscles, which do not normally pass through the capil-
lary walls to any significant extent.

The step which immediately follows this first act of filtration
is the one which determines the final composition of the urine.
As the fluid from Bowman's capsules flows down the kidney
tubules, reabsorption of certain substances takes place. This
reabsorption occurs from the upper ends of the tubules, which
are surrounded by networks draining into the kidney veins. It
is quite obvious that one of the materials reabsorbed is water,
since the urine is more concentrated than either the blood plasma
or the fluid from Bowman's capsules. The movement of the
water from the urine to the blood takes place in spite of the fact
that there is less of it in the urine. Consequently, work must be
done by the cells responsible for this movement, and, indeed,
the structure of the tubular epithelial cells resembles that of
secreting cells, which are characterized by their ability to do
such work.

But this is not the only work done by the cells of the kidney
tubules. All of the sugar and much of the salt is removed from
the fluid and returned to the blood. On the other hand, urea is
not returned, and its concentration in the urine is about seventy
times greater than that in the blood. Likewise foreign materials,
such as caffeine and alcohol, are never returned to the blood.

Should there be an excess of any of its normal constituents in
the blood, the reabsorption process permits only the proper
amounts of these materials to go back. For example, people

MOVEMENTS OF MATERIALS

361

^-Urethra

Relationship of the urinary organs.

suffering from diabetes have an excess of sugar in the blood. The
cells of the tubules in such cases permit only a part of the sugar
to return to the blood, the remainder being retained in the urine.
Likewise, if there is an excess of any mineral or organic salts in
the blood stream, some of these are retained in the urine, and
only the proper amounts are allowed to return to the capillaries
around the tubules. In this manner, the exact composition of the
blood stream is regulated by the kidney. This also makes it
possible, through analysis of the chemical composition of
the urine, to diagnose accurately a person's general physical
condition.

It is fortunate that man is equipped with two kidneys. They
serve an exceedingly important role in the body processes. If
there were only one and it became diseased or inoperative, death
from the accumulated poisons would soon result. As it is, only a
fraction of the total tubules are active at any one time. There is
here a considerable margin of safety, and destruction of con-
siderable kidney tissue by disease does not seriously impair the
work of the kidneys. One kidney may even be removed, and the
other will carry on the work of purifying the blood.

362 THIS LIVING WORLD

Thus the urine as finally excreted by the kidneys contains
some water, urea (which is produced partly by cellular metab-
olism and partly by the breakdown of amino acids in the liver),
some salts, excess sugars, and useless materials which get
absorbed into the blood stream. This material flows down the
small tubules and eventually empties into the calyx. From there
it drains into a tube known as the ureter, which leads below to
the bladder. There is one ureter from each kidney to the bladder
through which urine continually drains. The bladder is a muscu-
lar bag which serves as a sort of storage tank and from which
the waste liquid is eliminated through an exit tube known as
the urethra.

REFERENCES FOR MORE EXTENDED READING

HARVEY, WILLIAM: "The Motion of the Heart and Blood in Animals," trans-
lated from the Latin by Robert Willis, E. P. Dutton & Company, Inc., New
York, 1906; Everyman's Library series, London, 1628.

This historical document of the man who first discovered and described the circu-
lation of the blood is still of interest. It is a sincere and cirgumentative account by a
great physician of the discoveries he had made. Shows style of arguments and
language used 300 years ago when attempting to introduce a new discovery.

BEST, C. H., and N. B. TAYLOK: "The Human Body and Its Functions/
Henry Holt & Company, Inc., New York, 1932, Sees. II, III.

This reference is for those who wish a somewhat detailed and nontechnical expla-
nation of the blood and its circulation.

CRANDALL, LATHAN A. : "Introduction to Human Physiology," W. B. Saunders
Company, Philadelphia, 1938, Chaps. I-V, VII, XV.

An accurate and relatively complete discussion of the blood, heart, blood vessels,
and the processes of circulation is included in these chapters and a short explanation
of the function and action of the kidneys.

EuLENBURG-WiENER, RfiNEE: "Fearfully and Wonderfully Made/' The
Macmillan Company, New York, 1938, Chaps. VIII-XI, XXI.

A concise and well-written account of the blood, heart, blood vessels, lymphs, and
kidneys and their relationship to the complete functioning of the human body.

CARLSON, A. J., and V. JOHNSON: "The Machinery of the Body/' University
of Chicago Press, Chicago, 1937, Chaps. III-V.

The authors have presented in these chapters a detailed account of the blood and
its characteristics, the structure and functioning of the heart, and blood flow and the
meaning of blood pressure. The work is illuminated by a number of well-made
illustrations.

MOVEMENTS OF MATERIALS 363

STARLING, E. N.: "Principles of Human Physiology," 6th ed., rev. by C. L.
Evans, Lea & Febiger, Philadelphia, 1933.

This is a well-established advanced textbook and reference in human physiology.
Chapters XXXI to XXXVI relate to the physiology of the blood, blood vessels, and
heart and to the regulation of circulation.

Life and Health, published by Review and Herald Publishing Association,
Washington, D. C.

This is a journal that is issued monthly and contains articles relating to many
phases of body structure and health. The articles are usually substantial although
popularly written, usually by practicing physicians.

The American Journal of Physiology, published by the American Physiological
Society, Baltimore.

Three or four volumes are published each year which contain technical papers on
significant new research in the field of vertebrate physiology.

12: LIFE CONTINUES

The Process of Reproduction

PERHAPS the most remarkable property of living things is
their ability to give rise to other living things. It is just as
truly remarkable that in so doing all the different living forms
always reproduce their own kind. It is a law of life that the
general characteristics of the parents are retained and trans-
mitted by the offspring, thus tending to preserve resemblances.
On the other hand, heredity is not perfect. There is always some
variation. With the exception of identical twins, triplets, etc.,
which occur only rarely, no two individuals are exactly alike, no
matter how closely related. There is a balance in heredity be-
tween conservative and liberal tendencies. The general charac-
teristics of the type are rigidly preserved, but their detailed
expression in the individual is permitted to vary widely.

Because of the ability of living things to reproduce, life is
continuous and in a sense immortal. This is as characteristic of

364

LIFE CONTINUES 365

the lower animals as of man. Science has revealed a great deal
in recent years about the inner mechanism and processes of
reproduction. There is only one way by which reproduction can
take place, namely, cell division. In man and all other higher
animals, special cells are concerned. They are known as the
reproductive cells. They are of two distinct types, sperm cells
and egg cells, produced by male and female, respectively. Under
normal conditions, there must be a union of these two kinds of
cells, one of each type, before a new individual may be produced
by cell division.

From a biological standpoint, the entire process of human
reproduction is concerned with bringing together these two
types of germ cells and with nourishing and protecting the
product of their union so that growth and development may
continue until such maturity is reached that the individual may
care for himself. These are the essential facts of sexual reproduc-
tion. The continued existence of the human species, which is
probably the first postulate of any philosophy of life, is depend-
ent upon sexual expression. Through it the race survives. It is
upon the basic biological aspects of the reproductive phase of
life that the searchlight of science has been able to shed some
light for us.

Production and Dissemination of Human Male Germ Cells

The body of every normal male person is provided with
certain tissues and organs which serve to produce the male germ
cells and to facilitate their union with the female germ cells.
Likewise, every normal female person possesses certain tissues
and organs concerned with production of the female germ cells
and ensuring their union with those of the male, as well as special
organs and tissues which receive the product of this union and
provide for its early development. The germ cells are specialized
and unique in many respects directly related to their function
in reproduction. To distinguish them from the ordinary tissue
cells of the body, they have been given a special name, the
"gametes." The sperm cells are the male gametes, while the
egg cells are the female gametes.

The mature sperm cells are minute structures about 0.00002
inch in diameter. It seems remarkable that a function so special-

366

THIS LIVING WORLD

Interstitial
cell

Sperm cell

'Mother

sperm

cell

Tubule
membrane

A transverse section of a seminiferous tubule shows sperm being formed from the
sperm cells of the gcrminac epithelium by cell division and differentiation. (Redrawn from
Gray, "Anatomy.")

ized and important as reproduction could be assigned to cells
so small. The sperm cells are so small, in fact, that a drop of fresh
seminal fluid, seen under a high-power microscope, swarms with
them. It is estimated that about 200 million sperms are contained
in the seminal fluid discharged at a single ejaculation. Each one
consists of a head, which is little more than the cell nucleus in a
very condensed state, and a whiplash tail. The tail is a fine fila-
ment of cytoplasm about ten times as long as the head and much
narrower. Its lashing movement propels the sperm through the
seminal fluid at the rate of about one-tenth of an inch per
minute.

The sperm cells are produced in the testes. These are paired
organs which serve a double purpose; not only do they produce
the sperm cells, but they also secrete a male hormone. In many
animals the testes are located within the abdominal cavity in
about the same position as the primary female sex organs, or
ovaries. In man and certain other animals, however, shortly
after birth they descend into a fold of skin which forms a sac sus-
pended from the pubic region. This sac is called the " scrotum. "

Within each testis there are some eight hundred to a thousand
small tubes, known as the seminiferous tubules, in which the
sperm cells are produced. The walls of these tubules are made
up of a tissue, known as "germinal epithelium," which is formed
from the primordial germ cells set aside early in embryonic
development. At puberty this tissue begins to produce mature

LIFE CONTINUES

367

Bladder

sperm cells by cell division and differentiation. Within the adult
lifetime of a man some 340 billion sperms are thus produced.
These germ cellos, like all
other cells of the body,
are descended directly
from the original fertilized

egg cell. The route of male ) ^ — ^ fW\ Seminal A

'vesicle. '

Prostate
gland

Vas deferens

Teslis

/"~* . \^& Jit /
Glans penis

Diagrammatic representation of male reproductive
organs.

inheritance, therefore, is
from the fertilized egg
through the seminiferous
tissue to the sperm cells.
No other tissues of the
body can produce sperms.
The mature sperms
developed within the sem-
iniferous tubules pass into
the "epididymis," which
is a long, fine tube that
is coiled against the testis.
This tube connects with
a duct known as the
"vas deferens." There is a pair of these ducts, one from
each testis. They extend upward into the lower abdominal
cavity, then, bending downward, eventually connect with
the urethra. Near the base of each vas deferens, where it
joins the urethra, is a sac-like structure, called the "seminal
vesicle,'5 which is variously described as a storage depot for
sperms and as a gland contributing to the fluid in which they are
suspended. It may serve either or both functions. It is well
established that the prostate gland, which surrounds the urethra
at the point of union with the vasa deferentia, secretes one of
the constituents of the seminal fluid. Also contributing to this
fluid are a pair of glands located on either side of the penis near
the base, known as "Cowper's glands." All these structures, with
the exception of the seminiferous tubules in which the germ cells
are actually produced, are secondary reproductive organs. Their
functions have to do directly or indirectly with facilitating
movement of the sperm cells and providing for their transfer to
the place where fertilization occurs.

368 THIS LIVING WORLD

Production of Human Female Germ Cells

The female germ cell, or ovum, is a rather generalized type
of cell, roughly spherical in shape and relatively large in compari-
son with the sperm. Its diameter is about fifty times that of the
sperm head, and its total volume is over a hundred thousand
times greater. Its size is such that it is just visible to the unaided
eye. It consists of a nucleus surrounded by a layer of granular
cytoplasm, just as does any other typical cell. However, the
cytoplasm of the egg cell is full of minute bits of yolk, or food
substance for the growth of the early embryo. The ovum is
enclosed within a delicate cell membrane, around which a thin
layer of jelly-like substance develops when a sperm enters the
cell. This substance effectively prevents the entrance of more
than one sperm into the ovum. Unlike the sperm, the ovum is
inactive; it is unable to move except as it is carried along in
currents set up by surrounding structures.

The human egg cells are produced in paired organs, the
ovaries. They correspond to the testes of the male and likewise
have a double function. In addition to producing the ova, they
also secrete the female sex hormones. Each ovary is a somewhat
flattened organ, a little smaller than the testis, being less than
two inches long, one inch wide, and about one-fourth of an inch
thick at maturity. The ovaries are situated at the back and in
the lower part of the abdomen near the pelvic walls, a position
where the testes of the male develop before they descend to the
scrotum shortly after birth.

The ovary is in some respects rather simple in structure. It
consists principally of cells that are to become eggs or ova at
maturity and fibrous material and other cells that serve for sup-
port and nourishment of these. The cells that are to produce the
ripe ova come directly from tissue originating from the primor-
dial germ cells which differentiate at an early stage in embryonic
development. Soon after birth this tissue has produced about
400,000 of these immature eggs. By the time puberty is reached
this number has been reduced to about 17,000 immature cells.
Of these, about 200 in each ovary may reach maturity during
the sexually active period of a woman's life. No others are ever
formed. The path of heredity in the female is seen to pass

LIFE CONTINUES

369

Follicle

approaching

maturity

Simplified representation of development of ovum within the ovary. (Redrawn from
Patten, "Embryology of The Pis.*1)

directly from the original fertilized ovum, by way of the develop-
ing cells of the ovary, to the mature ovum again.

The immature egg cells are not in tubules, as are the sperm
of the male. Rather each ovum is surrounded by a number of
nourishing and supporting cells, which form a hollow structure
filled with fluid, called the "Graafian follicle." As the ovum and
follicle mature, they move toward the surface of the ovary.
There is no tubule opening to the surface of the ovary to provide
an outlet for the ripe ovum. Instead, the follicle ruptures, break-
ing through the surface of the ovary and releasing the ripe ovum
into the body cavity. Release of a mature ovum occurs about
once in twenty -eight days, beginning at puberty and continuing
until a woman has reached the age of about fifty, after which
no more ova ripen and are released.

The ruptured follicle soon forms an actively secreting
endocrine body known as the "corpus luteum." The chemical

370

THIS LIVING WORLD

Ovary

substance secreted, called "progestin," is carried by the blood
stream to the uterine tissues and brings about a preparation of

the uterus for the reception
of the fertilized egg. Should
the discharged ovum not
become fertilized, the corpus
luteum soon degenerates into
scar tissue. However, should
the ovum become fertilized
and attach itself to the uter-
ine wall, the corpus luteum
continues to secrete pro-
gestin during the period of
pregnancy, this hormone
being necessary for the
growth of the embryo.

Diagrammatic representation of female repro- The ovaries constitute

organs.

Vagina

primary female organs of
reproduction. The other parts of the female reproductive tract
might be thought of as secondary structures. They receive the
sperm of the male, provide for conducting the ovum to the place
where fertilization occurs, furnish protection and nourishment to
the growing fetus if fertilization should occur, and provide for
birth of the embryo at the proper time.

The ripe egg, after its escape from the ovary, is drawn into
the horn-shaped opening of a duct known as the "Fallopian
tube." There are two of these tubes, one from each ovary, which
lead into the uterus, or womb. This is a thick-walled, muscular,
pear-shaped sac about three inches long. If fertilization of the
ovum takes place, it usually occurs as passage is made through
the Fallopian tube. The ovum is moved through the tube by
means of a feeble current which slowly carries it along until it
enters the uterus. If it remains unfertilized, the ovum continues
to be washed down the uterus. Within a few days it is discharged
into the vagina through the muscular neck of the uterus, known
as the "cervix/' The vagina is a muscular canal extending ob-
liquely downward and forward and opening to the outside of the
body. Once the ovum is in the vagina, it soon dies and fertiliza-
tion can no longer occur.

LIFE CONTINUES 371

The maturing and discharge of the ova are governed by well-
known biological laws. In typical instances only one egg cell
matures at a time. The ma- ~ ,
turation process is completed Folar bodies
by about the eighth day after
menstruation ceases, at
which time the ovum breaks
loose from the ovary and
enters the Fallopian tube.
Here it remains from three
to six days, during which nucieus ^*- -^ "membrane

time it is Carried along to the Human reproduction is started when the

uterus. The mature ovum $Perm cc!l of the male combines with the

•J.L* j-i, j. ovum cell of the female.

may remain within the uter-
us for three days. It is only during the interval when the egg
is in either the Fallopian tube or the uterus that fertilization
may occur and usually only during the former. Unless
fertilized, the ovum perishes at the end of its normal life, a
period which, with slight variations, terminates about seven
days after its discharge from the ovary. At about this time, some
fifteen days after menstruation ceases, the ovum reaches the
vagina. About thirteen days later menstruation begins again,
and the cycle is repeated.

The process of fertilization is accomplished by the union of
a sperm cell with the mature ovum. In order for this to occur
sperms must be deposited within the vagina. From there they
pass into the uterus, partly as a result of contractions of the
vaginal wall and partly by a swimming motion of their own.
This swimming motion is brought about by the lashing of the
tail of the sperms acting against the liquid environment in which
they are found. As the movement of fluids down the Fallopian
tubes, uterus, and vagina is feeble, the sperms swim upward and
enter the Fallopian tubes. This sperm migration usually requires
a period of from one to six hours. However, the small sperm cells
seem to possess a tenacious hold on life, and may remain alive
within the female reproductive tract for about two days. If an
ovum happens to be passing down the Fallopian tube or in the
uterus at the time the sperms reach such a point, fertilization
is likely to occur.

372

THIS LIVING WORLD

The tiny dark area labeled A is an eleven-day-old human embryo. It constitutes a
remarkable discovery in that it is the youngest ever seen. The outer ring, indicated by the
arrows B, is the shell of the ovum eroding and consuming the maternal tissues of the uterine
wall. (Science Service photograph by Dr. A. T. Hertig of the Carnegie Institution of
Washington.)

Development of the Human Embryo

If fertilization takes place, the fertilized egg begins to divide
immediately, forming a cluster of cells. It seems that the very
first task this cluster of cells sets for itself is to form tissues and
structures to provide protection and facilitate growth of the
embryo. This begins even before any of the cells start to differ-
entiate into the original structures of a new human being. A
cavity soon appears in the cluster from which is set off at one
point an inner cell mass, and the remaining tissue forms a surface
layer of cells that serves as a sort of feeding layer for the embryo
soon to be developed from the inner cell mass. This hollow clus-
ter then attaches itself to some spot in the uterus and literally
bores its way into the uterine wall. This is accomplished by the
cells of the feeding layer digesting or "eating" the maternal
tissues. Once embedded, this layer grows at a rapid rate, sending

LIFE CONTINUES

373

Relative sizes of human embryo at twenty-eight weeks, sixteen weeks, twelve weeks, and
eight weeks. About one-half actual size. (Photographs by Sam Kaufman.)

out villus-like processes into the uterine wall and forming a
sac, known as the "chorion," that surrounds the entire
cluster.

During the first two weeks the developing embryo seems to
be fed by the digested material from the uterus. Then the
thickened part of the chorion and the invaded tissue of
the uterine wall cooperate to form a feeding organ, called the
"placenta." It develops into a disk-shaped mass of tissue which
is richly supplied with capillaries from the mother's circulatory
system. The placenta is connected with the growing embryo
through a narrow stalk of tissue which becomes the umbilical
cord. This cord contains two arteries and a vein which join the
blood vessels of the embryo with capillaries in the placenta
adjacent to the mother's capillaries.

374 THIS LIVING WORLD

The placenta increases in size to keep pace with the growing
embryo. Food materials and oxygen from the maternal blood
are absorbed into the capillaries of the villi and are then trans-
ported to the embryo through the umbilical vein. Carbon dioxide
and other metabolic wastes are brought to the placenta by the
umbilical arteries, where they are transferred to the maternal
circulation and excreted through the lungs and kidneys of the
mother. It should be pointed out here that there is no direct
physical connection between the circulatory system of the
mother and that of the embryo, a belief that many people seem
to hold.

Meanwhile, the inner mass of cells has been developing in a
unique fashion. At first they produce two hollow sacs which are
attached in only one region. Remarkably enough it is only the
small group of cells at the line of contact of the two sacs that
later grows into a human being. The upper cavity develops
rapidly to form what is known as the "amnion," a sac that soon
surrounds the entire embryo and lies just inside the chorion.
This sac, incidentally, is useful only to the growing embryo and
is shed at birth. Its function is to protect the embryo. To accom-
plish this it becomes filled with a watery solution which provides
a fluid environment, thus preventing injury to the delicate
embryo by taking up any undue shocks that the mother's body
may receive. The lower compartment immediately forms an
empty vesicle, called the "yolk sac," which becomes attached to
the belly side of the growing embryo. However, the yolk sac in
human development is no more than a vestige of a useful organ
in lower animals, because it contains no yolk. During the second
month of growth it becomes disconnected from the embryo and
adheres only to the placenta, with which it is expelled at birth.

Thus during the very early stages of growth, provision is
made for supplies and protection of the truly embryonic part
of the developing tissues. Then the double-layered structure
between the amniotic sac and the yolk sac begins definitely to
build a human embryo. By the end of the fourth week it is
about one-fourth of an inch long. The head region is marked, and
limb buds have started. It is somewhat fish-like in appearance,
having the beginnings of gill slits, a segmented back, and a tail.
In fact, about eighty-five per cent of the development of all

LIFE CONTINUES

375

A <% ^^ B

The human embryo is about one-half inch long at the end of five weeks' growth. At
that time many of the organs of the body have begun to develop. Diagram A, represents
outward appearance with umbilical cord and placenta indicated. The hand has differen-
tiated more than the foot. Four external clefts are present as numbered. Diagram B is a
longitudinal section showing major body parts: 1, brain and nerve cord/ 2, notochord/
3, alimentary canal; 4, heart/ 5, liver/ 6, kidney/ 7, gonad/ 8, yolk sac/ 9, villi on placenta
to absorb nourishment from maternal uterine wall.

primary body structures has now been completed. At five weeks
the embryo is almost one-half of an inch long. The head is now
clearly differentiated, and the arms and legs are distinguishable
with fingers on the hands. The feet, however, are slower in devel-
oping. This same gradient of development is also seen in the em-
bryos of the lower vertebrates. For example, in amphibian
embryos the fingers of the hands always develop earlier than
the toes of the feet. Gill slits and a tail are still present after five
weeks of development in the human embryo.

At the end of the eighth week the embryo is a little over one
inch long. The face has begun to take shape, the eyes and ears
have appeared, and the beginnings of all other organs of the
body have been made. During the next seven months the embryo
continues to grow and differentiate and gradually takes on the
proportions seen at birth. Anatomically speaking, our lives are
ninety-five per cent over at the time of birth.

Birth takes place at approximately forty weeks or 280 days
after conception. However, exact knowledge in this respect is
not available, and the time probably varies from about 270 to

376

Body of
uterus

THIS LIVING WORLD

Umbilical cord

Amnion

Placenta

Cervix

Human fetus within the uterus. (After Logan Clendening.)

290 days. At about this time, the uterine walls begin to contract
and force the amniotic sac containing its fluid and the fetus, or
young child, down toward the cervix of the uterus. These con-
tractions usually last from twelve to sixteen hours and cause
some of the chief pain at childbirth. Meanwhile the bones of the
pelvis separate along the mid-line of the body, and the muscles
of the cervix gradually relax so as to permit the fetus to go
through. Finally the amniotic sac breaks, and its fluid is dis-
charged from the body through the vagina. Then the fetus passes
through the cervix and vagina and is born, usually about an
hour later. Approximately one-half to one hour later, the placenta
and its attached membranes, constituting what is called the
"afterbirth," pass out of the uterus and vagina. During the
following six or eight weeks the reproductive organs return to
their normal size and consistence.

Childbirth is sometimes of serious consequence to the mother.
This is more often the result of infection than any fundamental
difficulty with the birth process. It is highly desirable that the
expectant mother be in a hospital during the period of childbirth
and be attended by a well-trained obstetrician. Such facilities

LIFE CONTINUES 377

and medical aid can usually prevent any severe consequences
resulting from the delivery. However, in many families in vari-
ous parts of the country, the only assistance at childbirth is that
provided by none too well-trained attendants and this under
none too sanitary conditions. Mortality cases <of the mothers
under such conditions are usually large in number. It should be
understood generally that such high mortality rates can be
materially reduced by taking proper medical care of the mother
during this critical period.

It is not uncommon for birth to occur before the normal term
of development is completed. In such cases, the premature baby
may be sufficiently well developed to live, if given the proper
delicate care, and may grow into a normal and healthy child. Not
infrequently, also, the fetus may cease developing after a few
weeks and perish. In such cases, a normal abortion results, and
the contents of the uterus are expelled without injury to the
mother. If this takes place when the embryo is between five and
seven months old, it is referred to as a miscarriage. The cause
of such an event is usually some definite physical disturbance,
such as inflammation of the genital organs, displacement of the
uterus, infectious disease of the mother, or glandular upset. It
is very unlikely, however, that mental agitation alone can cause
a normal abortion or miscarriage, as is generally believed. The
administration of drugs will usually not produce one. In-
duced abortion, or the deliberate surgical removal of the
embryo, is technically perfected and may be employed in cases
where the woman is physically unable to sustain pregnancy or
childbirth or was a victim of rape. The operation is illegal, of
course, in the United States and most other civilized countries
if performed for other reasons.

Germ Cells

The mature germ cells, or sperm and ova, are unique cells in
three remarkable respects. In the first place, they contain nuclei
yet they cannot divide to form daughter cells as do most other
nucleated cells, except under the special condition that the nuclei
of a sperm and an ovum have been united in the process of
fertilization. In the next place, after such union has occurred,
the fertilized ovum does not give rise to similar germ cells by

378 THIS LIVING WORLD

Simplified representation of developing gametes, ovum on the left and sperm on the right-
cell division, but possesses the potentialities for producing a
complete adult human being. Finally, these cells constitute the
only direct physical link between parent and offspring and as
such provide the physical basis for heredity and variation.

The nucleus of any typical human body cell contains forty-
eight chromosomes. This is true of all so-called "somatic cells,"
such as skin cells, muscle cells, nerve cells, and so on. However,
the nuclei of the mature germ cells, both male and female, have
only one-half this number, or twenty -four. This reduction in the
number of chromosomes is brought about by the manner in
which the germ cells are formed. These cells arise from products
of the division of the primordial germ cells of the testes and
ovaries by two successive cell divisions of a peculiar character,
known as the "maturation divisions." In the first maturation
division to produce sperms, the forty -eight chromosomes of the
parent cell move together to form twenty-four chromosome
pairs. There is a rather close adherence (or synapsis) of the
members of each chromosome pair. The cell then divides by the
breaking apart of the pairs, one chromosome of each pair going
to the nucleus of each of the two daughter cells. Consequently
the nuclei have only twenty-four chromosomes. There is, how-
ever, much recent evidence to indicate that, before the separation
of the pairs, each chromosome splits longitudinally to form two

LIFE CONTINUES 379

sister halves. In most instances, the sister halves seem to remain
closely bound together in this cell division process. In the event
of such splitting the daughter cells will still contain twenty-four
chromosomes, each consisting of two sister halves that have
already formed. In the next maturation division each of the
twenty-four chromosomes splits longitudinally, and one of the
halves of each goes into the nuclei of the two resulting daughter
cells; or the sister halves merely separate when the splitting has
occurred earlier. The four daughter cells thus produced from
the original germ cell contain twenty-four chromosomes and
develop into mature sperms.

The maturation of ova takes place in essentially the same
manner, except that in the first division the two daughter cells
are very unequal in size. The larger one retains all the cytoplasm.
The smaller cell is referred to as a polar body. The second
division usually involves only the larger of the two cells thus
formed. Like the first maturation division, one large and one
small cell are formed. The larger cell leads to the production of
the mature ovum, while the small one constitutes another polar
body. The polar bodies remain attached to the mature ovum for
a time but soon disintegrate. Thus only one functional ovum is
formed as a result of the maturation divisions.

Unless fertilization occurs, the mature germ cells of both
sexes soon perish and either degenerate or are passed out of the
body. However, should a sperm come in contact with the ovum,
its head (i.e., nucleus) enters the ovum. The sperm nucleus grows
rapidly by absorbing fluid from the substance of the ovum. It
migrates toward the ovum nucleus and soon fuses with it. At
this moment the forty-eight chromosomes are restored, and it is
the real moment of fertilization. Cell division can now take place.
In the first division of the fertilized egg nucleus, the maternal and
paternal chromosomes become Jndistinguishably intermingled
and conditions comparable to those in the normal body cell
nucleus are reestablished.

Brief reflection upon this process shows the significance of
the maturation divisions and of the eventual union of the sperm
and ovum. Normal cell division can now begin, and the human
embryo develops. Yet the constancy of forty-eight chromosomes
is maintained, one-half of them being obtained from the mother

380 THIS LIVING WORLD

and one-half from the father. If no such reduction division
occurred, the nuclei which unite in fertilization would each
contain forty-eight chromosomes and the fertilized nucleus
formed by their union would contain ninety -six chromosomes.
Since the chromosomes are discrete bodies the fertilized nucleus
would have to increase in size each time that fertilization took
place in order to accommodate them. If the process of doubling
the chromosome number were continued indefinitely, a sphere
the size of the earth would eventually be too small for the nuclear
material of a single germ cell.

Sex Determination

Of the forty-eight chromosomes which make up the normal
number in the nucleus of a human body cell, two are structurally
different from the rest. These two chromosomes are known to
influence the sex of the individual and are therefore called the
"sex chromosomes." In the nuclei of the cells of a female indi-
vidual, the two sex chromosomes are identical in appearance.
They have been called the X chromosomes. The chromosome
complement of the nucleus of a female somatic cell may be
written 46 + X + X, where the forty-six represents the normal
chromosomes and the X + X are the two additional sex chromo-
somes. In the nuclei of the cells of a male, however, the sex
chromosomes are quite different from each other. One of them
is indistinguishable in size and shape from the sex chromosomes
in a female. It is, therefore, an X chromosome. The other is very
much smaller and different in structure. It is called the Y
chromosome. The formula for the nucleus of a male somatic cell
is 46 + X + Y, where, as before, the forty-six represents
the normal chromosomes and X + Y the two additional sex
chromosomes.

Keeping this in mind, let us see what happens during the
course of the maturation divisions in the production of mature
germ cells. The sex chromosomes are distributed to two different
cells during the reduction division, one going to each cell. The
nucleus of a mature ovum always contains twenty -three normal
chromosomes and a single X chromosome, represented by
23 + X. However, two kinds of sperms are produced as a result
of the reduction division in maturation. The nucleus of one of

LIFE CONTINUES

381

When an ovum is fertilized with a sperm
containing the X chromosome/ a female results.
An ovum fertilized with a sperm bearing the Y
chromosome produces a male.

necessary to pro-

them will receive the X chromosome, while the other will receive
the Y chromosome. Accordingly, the chromosome composition
of one will be 23 + X,
while that of the other will
be 23 + Y. Should an ovum
be fertilized by a sperm
containing the X chromo-
some, the resultant nucleus
will have 46 + X + X
chromosomes. This is the
condition necessary to pro-
duce a female, and the child
will be a girl. Should an
ovum be fertilized by a
sperm containing the Y
chromosome, the resultant
nucleus will have 46 + X
4- Y chromosomes. This is the condition
duce a male, and the child will be a boy.

Since the two kinds of sperms are initially produced in equal
numbers, the chances are even that in a large number of in-
stances fertilization will be accomplished as often by a sperm
bearing an X chromosome as by one bearing a Y chromosome. In
the general population, therefore, as many girls as boys are born.
There is little doubt that, normally, the modeling of the embryo
to either the male or the female pattern is primarily dependent
upon the chromosome composition given to the nucleus at the
time of fertilization. It is also well known, however, that in rare
cases the primary influence of the sex chromosomes may be over-
ridden by effects of other hereditary factors possibly located in
the other chromosomes, causing the offspring to be an intersexed
individual. Just what these other factors are is not well under-
stood at present.

The query as to whether man can control the sex of his off-
spring is still to be answered in the negative. It should be remem-
bered that the fundamental factor that appears to determine
the sex of the offspring is the composition of the nucleus estab-
lished at the time of fertilization, particularly as regards the sex
chromosomes. This is purely a chance factor. Another factor

382 THIS LIVING WORLD

believed by some to modify the sex of a child is the rate of
metabolism of the embryonic cells at the time the sexual organs
are being differentiated. It is known that in general males have
a higher rate of metabolism than do females. The presence of a
Y chromosome in the nuclei of the body cells may predispose
to a high metabolic rate and the production of maleness. Corre-
spondingly, the presence of two X chromosomes would give the
body cells a lower rate of metabolism, inducing femaleness. If
the influence of the sex chromosomes could be overridden, per-
haps by chemical means, the sex of the embryo might be altered
or controlled. On the basis of such reasoning, a number of doctors
and people generally interested in sex determination have tested
the effect of regulating the diet of mothers during the early
weeks of pregnancy so as to produce a high or low rate of
metabolism in the embryonic cells. If the reasoning upon which
this treatment is based be correct, it should be possible in this
manner to produce boys or girls at will. The results to date
have not been consistent enough to warrant forming definite
conclusions.

Another widely held idea is that the sex of a child is influenced
by the acidity or alkalinity of the female reproductive tract at
the time of conception. It is claimed that conditions of less
acidity tend to favor the production of daughters. If this is true,
it probably means that the sperms containing X chromosomes
are a little more active in such a medium than those containing
Y chromosomes, and, hence, are more likely to effect fertiliza-
tion. In order to control the sex of offspring, then, it would only
be necessary to determine the time in the menstrual cycle when
the female reproductive tract was most or least acid and to
make use of such information in selecting a time for conception.
Artificial control of the conditions in the Fallopian tubes, uterus,
and vagina at the time of fertilization might be achieved by the
use of acid or alkaline douches.

Some slight statistical support for this idea has been obtained
in breeding experiments with smaller mammals. Where hundreds
of mice have been bred after an alkaline douche, a greater number
of females than males have been observed among the offspring.
When an acid douche has been used, on the other hand, a slight
majority of the offspring have been males. However, for indi-

LIFE CONTINUES 383

Located within the chromosomes are genes or genes groups, which are the bearers
of heredity. Microphotograph of chromosomes in the salivary gland of the fruit fly,
Drosophila, showing many genes loci. (Taken by Roy Allen.)

vidual litters no exact prediction could be made. When man
himself has tried the plan, the few individual cases have not
worked out with a high degree of certainty. Results seem to revert
to the well-known law of chance variation in sex determination.

Inheriting Definite Characteristics

There is a definite and positive tendency for the offspring of
all living things to resemble their parents rather closely. This is
particularly true of fundamental traits, so that each species of
creature reproduces its own kind. It is also true of very specific
and individual traits. For example, resemblances in such things
as the shape of the nose, size of the wrists, or color of the hair
tend to be transmitted in human family groups. The expression
of these hereditary characteristics is controlled by the nature of
small bodies called "genes" which make up the chromosomes
within the cell nuclei. The forty-eight chromosomes in the
nucleus of the fertilized ovum contain large numbers of genes,
each of which influences the development of one or more tissues,
organs, or systems of the individual. Thus, there are genes which

384 THIS LIVING WORLD

call for the development of tall or short stature, for one or
another eye color, for mental ability or lack of it, and so on for
every attribute of the adult.

The results of carefully controlled breeding experiments with
lower animals and with plants have shown that the expression
of every hereditary trait is influenced by at least a pair of genes.
One of these is located in a chromosome contributed to the
fertilized nucleus by the ovum, the other in a chromosome con-
tributed by the sperm. Therefore, the expression of every heredi-
tary characteristic may be affected as much by maternal factors
as by paternal ones. This is true of all traits, with the exception
of certain ones which are governed by genes located in the sex
chromosomes and which are therefore said to be sex-linked.

Evidence from the same sort of experiments indicates that
the genes are arranged in a linear fashion within the chromo-
somes. Accordingly, the chromosomes may be regarded as
strings of genes arranged in a manner not unlike beads in a
necklace. The normal make-up of forty-eight chromosomes in
the nucleus of every human body cell actually comprises twenty-
four pairs of gene strings, although this becomes apparent only
at the time of the maturation divisions in the formation of
mature germ cells. The chromosomes of any given pair are said
to be homologous, since each comprises a string of genes affecting
the expression of the same hereditary traits. One member of
such a homologous pair comes from the father, the other from
the mother.

An enormous amount of experimental data has also been
gathered which tends to show not only that the expression of a
given hereditary trait is determined by one or more pairs of
genes located in definite pairs of chromosomes, but also that the
genes occupy exact positions in the chromosomes. For example,
the chromosomes of the fast-breeding fruit fly, Drosophila, have
been mapped with a degree of precision that is unbelievable to
persons not acquainted with the extensive work of modern
genetics. It is possible to locate the exact position in a chromo-
some of the gene that causes the eye of a fruit fly to be white
rather than red. At another known location in the chromosome
or in another chromosome will be a gene which produces black
body color instead of gray, and so on. Several hundred heredi-

LIFE CONTINUES 385

tary factors have been located in the chromosomes in this
manner.

During the maturation divisions in the formation of mature
ova and sperms from primordial germ cells, a remarkable separa-
tion of the homologous chromosomes is effected. In the early
stages of the first maturation divisions the homologous chromo-
somes come to be arranged in pairs, as we have previously
noted. The pairing takes place in such fashion, moreover, that
genes influencing the expression of the same trait are directly
opposite each other. In many instances immediately following
this pairing there is a splitting of each of the homologous chromo-
somes into sister halves. Often there is a cross-over of correspond-
ing parts of sister halves between homologous chromosomes.
Then the cell divides by a separation of the homologous chromo-
somes, including what cross-overs have occurred. In the next
division of the maturation process there is a separation of the
sister halves to produce the nuclei of the mature germ cells. As a
result of these two divisions, a mature germ cell contains only
one of the two genes which influence the expression of a given
trait in the offspring.

In the union of the sperm nucleus and egg nucleus, when
fertilization occurs, the condition found in the nuclei of somatic
cells is restored in the fertilized nucleus. It then contains the full
complement of forty-eight chromosomes, consisting of twenty-
four homologous pairs. Both maternal and paternal genes in-
fluencing the expression of the same traits are again present.

It is a curious fact that not all the genes which influence the
expression of a given trait have the same ability to do so. Some
genes appear to exert a more powerful influence than others.
They are said to be dominant, or they are called dominant genes.
A single such factor will determine the expression of a trait just
as surely as will two. For example, brown eye color in man can be
regarded, for practical purposes, as an expression of the influence
of a dominant gene upon this trait. If a person receives the factor
for brown eye color from either parent, that individual will have
brown eyes, even though one parent may not have brown eyes.
The alternative to brown eye color in man is generally some
shade of blue. The gene which produces blue eye color does not
exert so powerful an influence on development as does the factor

386 THIS LIVING WORLD

for brown. In order for the color of the eyes to be blue, it is
necessary that both genes affecting the expression of this trait
be of the type which produces blue color. Such a factor is said to
be recessive, or to be a recessive gene.

The influence of a recessive gene is masked when a dominant
gene which affects the same trait is present. Thus, in order for
an individual to have blue eyes, he must receive a recessive
factor for the expression of this trait from both parents. What
has been stated provides an explanation for the occurrence of
blue-eyed children among the offspring of parents both of whom
are brown-eyed. Obviously, each parent must have the genes
from his ancestors for both brown and blue eyes. Since the gene
for brown eye color is dominant over that for blue, the parents
will both have brown eyes. However, when sperms are formed in
the seminiferous tubules of the male parent, half of them will
receive the factor for blue eye color and half will receive that for
brown. Similarly, when an egg is released from the ovary of the
female parent, the probability that it will contain a factor for
blue eye color is just as great as the probability that it will con-
tain one for brown eye color. When fertilization occurs there is
just as great a chance that the uniting germ cells will be alike
as that they will be different with respect to the nature of the
genes affecting eye color; that is, it is just as probable that a
sperm bearing the factor for blue eyes will unite with an egg con-
taining a similar factor, as that it will unite with an egg bearing
the gene for brown eye color. The same reasoning may be ap-
plied in figuring the probability that a sperm bearing the gene
for brown eye color will unite with an egg having a similar gene
for this trait or will unite with an egg having the gene for blue
eye color. In either case, brown eyes will result. Thus, the chance
that the offspring will have blue eyes is one in four, because
there are three combinations out of a possible four which will re-
sult in the production of brown-eyed offspring, owing to the
factor of dominance.

The complicated machinery of sexual reproduction provides
for the occurrence of variation in individuals. It has been cal-
culated that the possible number of chromosome combinations
when the human ovum is fertilized is over sixteen billions. The
chance that any particular combination will be reproduced in

LIFE CONTINUES 387

the fertilized ovum and thereby produce a given set of charac-
teristics in the offspring is, accordingly, about one in sixteen
billion. Furthermore, the exact gene composition of a given
chromosome may be altered by the interchange of chromosome
parts between homologous pairs of chromosomes during the
early stages of reduction division in the production of mature
germ cells. This interchange of parts is brought about by a cross-
over of corresponding parts between homologous chromosomes.
This increases the possible gene combinations at the time of
fertilization to a figure approaching infinity. Thereby the chances
become almost limitless that there will be some slight difference
in the exact gene composition of any particular fertilized ovum
from that of another fertilized ovum and that the individual
resulting will have some traits that are characteristic of him
alone. These remarkable facts make it possible to understand
why each individual tends to be somewhat different from every
other individual.

Heredity in Man

The discovery that has done more than anything else to re-
duce the manifold phenomena of heredity to law and order was
made by an Austrian monk, Gregor Mendel, about the middle
of the last century. In the gardens of his monastery he experi-
mented for many years in crossing different varities of peas. He
kept an accurate record of his results and published them in
1866. This paper ranks as one of the finest achievements in ex-
perimental research. However, his contemporaries did not appre-
ciate its value. It was only after the turn of the present century,
when more knowledge of chromosomes and genes had been dis-
covered, that its real worth was realized.

Mendel's techniques have been applied since then to the cross-
ing of different varieties of mice, rats, flies, dogs, horses, cattle,
corn, tobacco, squash, and a great many other animals and
plants. The results have served to strengthen the fundamental
laws formulated by Mendel. It is impossible, of course, to apply
Mendel^s experimental methods to the study of human heredity.
However, there is no reason to believe that man should be the
one exception to these biological laws. In fact, where large
numbers of accurate records of human matings and offspring

388 THIS LIVING WORLD

have been kept, the same principles of heredity are found to
hold true.

These laws may be briefly explained and illustrated with a
few well-established examples of the inheritance of specific
human traits. One of these which has been extensively observed
is feeble-mindedness. Of course, we are concerned here only with
hereditary feeble-mindedness, not the many instances which
have resulted from some injury before or after birth or from in-
fectious disease; such feeble-mindedness is not inherited nor
transmitted to the succeeding generations. It is also to be kept
in mind that the quality of one's environment may have much to
do with his mental development. The truth of the whole question
seems to be that heredity fixes the limits of individual possibili-
ties while the environment determines to what extent these pos-
sibilities are realized.

Let us suppose a family to be started through a marriage be-
tween a man of well-established normal traits and a feeble-
minded woman. Let us designate the genes for normalcy by the
letter N and those for feeble-mindedness by/. The man would be
represented, therefore, by AW to designate the sets of genes
from his two parents. The woman would be represented by ff,
indicating she had received such genes from both parents. The
children of this marriage appeared to be normal, since the normal
genes are dominant and genes for feeble-mindedness are reces-
sive. However, each child contained in his germ plasm the mix-
ture of genes, Nf9 and such a combination is known as a hybrid.

Suppose that one of these children later married a normal-
appearing woman whose mother had been normal and whose
father had been feeble-minded. Contrary to general appearances
she, too, was a feeble-minded hybrid. Let us further suppose that
they had four children; now comes the unpleasant surprise. One
child of this third generation was feeble-minded. The mixing of
the sets of genes to account for this is shown in the accompanying
chart, which represents the statistical average of inheritance of
characteristics when gene patterns are mixed in the offspring.

Since the man of the first marriage was normal, all his genes
would call for normal-mindedness, as represented by the N in
the small circles. All the genes of the feeble-minded woman of
this marriage call for feeble-mindedness. as renresented bv the f

LIFE CONTINUES

389

Statistical average of inheritance of characteristics when gene patterns are mixed in the

offspring.

in the small circles. The three children of this marriage get a
mixture of N and /genes, as shown by the crossover lines. They
are hybrid types, since each contains the feeble-minded genes as
recessives. The marriage of one of the second generation to a
feeble-minded hybrid permits of other combinations of these
matching genes. Each of these individuals produces germ cells
having either N or /genes, as shown by the small circles. There-
fore their children would surely be made up by combinations of
these genes. Of the four children of the third generation born to
this union, one would be normal, with the AW grouping of genes;
two would be hybrids, with Nf genes; and one would be feeble-
minded, with the ff grouping of genes.

Dr. H. H. Goddard in one of his studies, entitled "Feeble-
mindedness," reports 42 matings in which the persons marrying
had the gene combinations of Nf + ff. There were 144
children whose mentality was known. Of these, 73 were normal
and 71 were feeble-minded. This is very close to the expectation
as would be predicted by the Mendelian laws of heredity. How-
ever, the details of representing the various gene combinations
are too complex to be given here.

390 THIS LIVING WORLD

Perhaps the most interesting case of the inheritance of mental
traits known in America is that of the Kallikak families, re-
ported by Dr. Goddard. A Revolutionary War soldier named
Martin Kallikak, of good ancestry, met a feeble-minded girl in
a tavern. As a result of this rendezvous, the girl had an illegiti-
mate son whom she called Martin Kallikak, Jr. Young Martin
married a feeble-minded girl and raised a family of ten children.
The progeny of this union by 1910 had reached 480 known in-
dividuals. Of these, only 46 are known to have been normal,
while 143 were definitely feeble-minded.

However, there is another side to the story of the Kallikak
family. After the episode with the feeble-minded girl, the soldier
Martin finished the war and later married a Quaker woman of
good ancestry. Seven children were born to this union, all of
whom married into good families. Their direct descendants have
reached the number of 496. They have included doctors, judges,
educators, lawyers, and prominent citizens of many kinds. There
are no cases on record of feeble-minded offspring among them.

A remarkable case of hereditary deformity is reported from
Brazil. A man having a hereditary absence of hands and feet
married a normal woman. This deformity seemed to be a domi-
nant trait. Twelve children were born into the family, of whom
six were likewise deformed. Two of the deformed died in infancy,
but four of them lived to adulthood. No record is available of
their matings or offspring.

In the case of stature inheritance, an interesting study is on
record of a family of four generations of tall South African na-
tives. A man six feet eight inches in height married a woman six
feet four inches tall. Six children resulted, of whom one brother
and sister later intermarried. Nine children were born to this
union. Two pairs of these intermarried, and they had nine chil-
dren who grew to adulthood. Of the twenty-four individuals
resulting from these unions who were carefully measured, eleven
were six feet or over, ten others were more than five feet nine
inches tall, while three (all of the fourth generation) were five
feet eight inches in height.

The examples that have been cited have been explained with
considerable overemphasis on the simplicity of heredity. It is
well known that the inheritance of any human trait as complex

LIFE CONTINUES

391

The inheritance of white forelock in man for four generations is shown in this series
of pictures. The first individual of this family who had the white forelock was Nils Rosland
of Osteroy, Norway. Three of his children had this trait, two of whom are shown in the
upper and lower left photographs. Seventeen grandchildren, two of whom are shown in
the center photographs, inherited the trait, while twenty-four great-grandchildren, two
shown in the upper and lower right pictures, inherited the characteristic. (Courtesy of
Journal of Heredity.)

as mentality, or stature, or various other traits such as skin color,
eye characteristics, or resistance to infectious diseases is the re-
sult of many different gene patterns. These produce many blend-
ings and graduations of types between the extremes. However,
these blendings are well understood and may be accurately ac-
counted for when all the gene combinations of the immediate and
distant relatives are known. They show conclusively that the
gene patterns persist either as dominants or recessives through
the succeeding generations and that the physical and mental
make-up of an individual is determined by the particular heredi-

392 THIS LIVING WORLD

tary traits that were matched in the fertilized ovum cell from
which he sprang.

REFERENCES FOR MORE EXTENDED READING

GILBERT, MARGARET SHEA: "Biography of the Unborn," The Williams and
Wilkins Company, Baltimore, 1938.

This little book is the publication of a prize-winning essay on the general subject of
human reproduction. It is a well-written story of human reproduction from the time
of fertilization to birth. Numerous illustrations are used to illuminate the discussion.
An interesting and vivid account is completely told in language that is devoid of the
extensive use of technical terms.

TIETZ, E. B., and C. K. WEICHERT: "The Art and Science of Marriage,"
McGraw-Hill Book Company, Inc., New York, 1938.

This is a volume in the Whittlesey House Health Series, published under the
editorship of Dr. Morris Fishbein, editor of the Journal of the American Medical
Association. The book presents an analysis of the problems of marriage from both
a mental and a biological point of view.

DAVENPORT, CHARLES B.: "How We Came by Our Bodies," Henry Holt &
Company, Inc., N. Y., 1936.

First section of this well-written book is devoted to tracing development from a
single cell to the complicated organism of the adult human being. The second part is
a study of the mechanism by which development takes place, such as the structure of
cells, genes, and heredity. The third part explains how physical changes in the body
are passed on to succeeding generations through the genes. It is written in a popular
style, yet adheres to scientific accuracy.

PARSHLEY, H. M.: "The Science of Human Reproduction," W. W. Norton &
Company, Inc., New York, 1933.

The author has prepared here a frank and comprehensive discussion of the anatomy
and physiology of human reproduction. The text is organized and written in a manner
to provide a biological basis for a scientific attitude toward sex and its problems.

WIEMAN, H. L.: "An Introduction to Vertebrate Embryology," McGraw-Hill
Book Company, Inc., New York, 1930, Chaps. X, XI.

These chapters constitute a discussion of embryonic development in man and an
explanation of the growth of different organs and structures of the embryo at various
ages, each section being well illustrated. This is an excellent reference for the
student who wishes advanced knowledge of this subject.

HOLMES, S. J.: "Human Genetics and Its Social Import," McGraw-Hill Book
Company, Inc., New York, 1936.

Chapters IV- VI deal with chromosomes and genes as the physical basis of heredity.
Chapter IX is a general discussion of heredity in man. The remainder of the book is
an extended and not too difficult discussion for those who are interested hi social
aspects of hereditary factors in man.

LIFE CONTINUES 393

SCHEINFELD, AMRAN: "You and Heredity," Frederick A. Stokes Company,
New York, 1939.

This is a book that was specifically written for the layman to show the applications
of scientific findings to human heredity. Many aspects of heredity, such as eye color,
ancestry and offspring, the Dionne quintuplets, are presented in a popular fashion
while adhering to the best scientific information available on the subject of how and
what we inherit.

STURTEVANT, A. H., and G. W. BEADLE: "An Introduction to Genetics,"
W. B. Saunders Company, Philadelphia, 1939.

This text in genetics is based primarily on studies that have been made on the fruit
fly and maize. It has an extended amount of material on chromosomes and genes and
is adaptable only to the reader who is a thorough student of the subject of genetics.

ALTENBURO, EDGAR: "How We Inherit," Henry Holt & Company, Inc., New
York, 1938, Chaps. IV, V.

A concise, yet specific, discussion of the genes as the hereditary basis of inheritance
and their influence in sex determination. The material is sufficiently nontechnical to
be understood by the intelligent reader.

Nature Magazine, published by the American Nature Association, Washington,
D. C.

This monthly magazine is devoted to stimulating public interest in nature and the
out-of-doors. The articles are written in popular fashion, and some are relatively well
illustrated. The subjects treated are usually those plants and animals with which the
inquiring laymen has some little acquaintance.

The Journal of Heredity, published by American Genetic Association,
Baltimore.

This is a monthly magazine that is devoted to promoting a knowledge of the laws
of heredity and their application to the improvement of plants, animals, and human
racial stocks. The articles are extensively illustrated and may be read with under-
standing by the intelligent layman.

13: SENSATIONS

ByWhichWe Receive Communications from the OutsideWorld

IT IS said that in most state and federal penitentiaries there is
an efficient " underground " system of communication. It is
nonmechanical, invisible to the uninstructed observer, and
unsuper vised, but it works. The prison authorities very defi-
nitely control the information from outside sources which comes
to the inmates through the regular channels of communication.
Usually this information is carefully censored. In addition to
such regular channels of intelligence, however, the prisoners
learn about what is going on in the outside world by devious
means, which they alone know.

Many intelligent individuals are only dimly aware of the
fact that a large part of what they believe to be true about

SENSATIONS 395

the world is determined not by impressions gained through the
physical senses, but by integrating and coordinating mechanisms
which function below the level of consciousness. This subcon-
scious intelligence service may be likened to the so-called
"grapevine" system by which prison inmates receive informa-
tion denied them through official channels. It is true, neverthe-
less, that all our direct knowledge concerning the external
physical world comes to us through our organs of sense. Besides
being limited in number, these sense organs are susceptible only
to certain special kinds of stimulation. They may be compared
to the censored official channels of prison communication in that
the sensory impressions which they transmit to the brain are
modified as much by factors inherent in their own structure as
by the physical character of the stimuli which excite them.

The essential part of any organ of special sense is a group
of cells or tissues which have developed to an extraordinary
degree the fundamental protoplasmic attribute of irritability. In
contrast to the primitive protoplasm of the simplest living
organisms, which is sensitive to all sorts of stimuli, the special-
ized sense organs of higher animals usually respond only to a
very limited range of specific stimuli. The eyes respond to
radiant energy between certain limits of wave length; the ears,
to sound waves, also within a restricted range of wave lengths;
the sense organs of the skin respond to mechanical and physical
stimuli of certain intensities; and the organs of taste and smell, to
chemicals dissolved in certain concentrations in the saliva of the
mouth and in the mucous membranes of the nose, respectively.

In addition to the sensitive tissues, or receptors, the well-
developed sense organs of higher organisms usually contain
auxiliary tissues which are not particularly irritable but are
designed to bring about proper contact between stimulus and
sensory nerve. The fundamental characteristics of the special
sense organs, namely, their limited responsiveness to a particular
kind of stimulus and their composite structure, are clearly
illustrated in the most highly developed of them all — the eyes.

Vision

Undoubtedly most of our knowledge about the world in
which we live comes to us through our eyes. We have only to

396 THIS LIVING WORLD

This low-magnification microphotograph of an actual section of the eye of a young
mouse shows all the parts of the eye, closed eyelid (left), cornea, crystalline lens, retina,
and optic nerve. (Science Service photograph.)

close them momentarily to appreciate the wealth of beauty and
variety which our eyes bring to us and to realize how helpless
we would be without them. The eyes are special sense organs
designed to receive radiant energy and to convert it into the
energy of nerve impulses. The optic nerves, which convey these
impulses to the brain, contain over one-half of all the sensory
nerve fibers in the body. The sensation created by the impact of
radiant energy upon the eyes is what we know as light.

Although our behavior is more definitely influenced by the
information we secure from light than by that from any other
stimulus, the eyes, after all, constitute only a small part of the
entire body. Even within the eye, the sensory receptors which
give us visual imagery are confined within a small area. Most of
the eye as an organ is composed of supporting tissue and struc-
tures to collect the light energy, control the intensity entering
the eye, and to bring it to focus on the cells containing the optic
nerve endings. The latter are the sensitive elements and are in
reality an outgrowth of the brain. These receptive structures

SENSATIONS

Pupil

397

Aqueous
humor

Outer
coat

Vitreous humor

(Vascular
wall

Retina

die nerve

If the eyeball is sliced horizontally through the center, each hemisphere will be seen
to be made of three distinct layers of material. (After Starling, "Human Physiology.11)

are buried within the protective body of the eye and are sur-
rounded by special tissues which originate from the skin.

The eyeball might be referred to as the camera box of the
eye. It is lodged in a bony orbit of the skull, which forms a
protection for it from mechanical injuries. It is a hollow, some-
what plastic sphere filled with a thick, transparent fluid, called
the "vitreous humor." This fluid helps to maintain the shape of
the eyeball. Should the eyeball be sliced through the center so as
to form two hemispheres, it would be seen to consist of three
distinct layers of material.

The outer layer of the eye is a tough membranous coat. About
five-sixths of this layer constitutes the opaque "white of the
eye," which is mostly out of sight in the orbit. The remaining
one-sixth forms a transparent circular window, the "cornea,"
which covers the front face of the eyeball. The second or central
layer of the eyeball is what is known as the "vascular wall." It
is a thin membrane characterized by an abundance of blood and
lymph vessels. The front part of this layer is the iris, in the center
of which is a round opening, the pupil, through which the light
enters the inner chamber. The iris is supplied with radial muscles
which, by their contraction, enlarge the size of the pupil. Another

398

THIS LIVING WORLD

Opiic
nerve
fibers

Connector
neuron

group of muscle fibers, arranged circularly about the pupillary
margin, lessens the size of the pupil by contraction. Pigments of
Liaht ravs various kinds are pres-

* ent in the iris, giving the

distinctive color to the
eye.

Between the iris and
the cornea there is a
small chamber filled
with watery fluid, the
"aqueous humor,"
which bathes the sur-
rounding tissues. Just
behind the iris is the
lens, consisting of dense,
transparent tissue that
serves to focus on the
retina the rays of light
that come from different
objects being viewed.
The inner wall of the
eyeball is the "retinal
layer." It extends
around approximately
two-thirds of the eye-
ball, forming a sort of
cup with the opening
toward the front. This
wall is itself composite

LD,*«

Layer of
rods and
cones

r^rffiSx

Pigmented layer

Nerve cells and layers in the retina. The back
of the retina consists of two layers: an outer layer
of pigmented cells and, in front of this, the layer
comprising trte light receptors themselves — the rod-
and cone-cells — facing to the back, away from the
light

in structure. It may be roughly divided into two parts. The
outermost portion at the back of the eye consists of two
layers, an outer layer of pigmented cells and in front of this the
layer comprising the light receptors themselves. These are the
rods and cones. The rod- and cone-cells contain the specific
sensory endings of the fibers of the optic nerve. A single optic
nerve fiber may supply several rod or cone cells, especially
toward the outer margin of the retina.

Strangely enough the sensory ends of the rods and cones do
not face to the front of the eye but to the rear away from the

SENSATIONS 399

light; that is, the fibers of the optic nerve are linked to the ends
toward the front of the eye. These nerve fibers, together with
certain ganglion cells, form a layer facing the front of the eye.
The nerve fibers pass through this layer until they reach the
region of the optic nerve near the center of the retina. There
they bend backward and pass through an opening in the retina
to connect with the optic nerve. This point has no rods or cones
and, of course, does not respond to light energy falling on it.
When the rays from some object are focused on this point, the
object is not seen. It is, therefore, called the " blind spot."

If this picture of the retina is clear, it is seen that the sensory
part of the eye is turned wrong side out, so that the rods and
cones face away from the source of light rather than toward it.
The light rays, after being focused by the lens, must first pass
through the layer of nerve cells and fibers of the retina, then the
meshwork of rod and cone cells, before they fall on the sensitive
ends of the latter. The rods and cones are distributed differently
in the retina. In the region of the fovea, a point directly behind
the pupil, it is estimated that there are 150,000 cones per square
millimeter. This number decreases rapidly as the distance from
the fovea increases. Thus, at a point 0.016 millimeter from the
fovea the number per square millimeter is about 145,000; it is
about 132,000 at twice that distance. On the other hand, there
are practically no rods in the fovea, but the number per square
millimeter increases rapidly toward the margin of the retina,
then falls off again. There are approximately eighteen or twenty
times as many rods as cones. At the retinal margin there are
practically no cones at all, but only rods.

The rods convey sensations of light and darkness, but they
do not play any part at all in color perception. They are the
receptors primarily concerned with perception in very dim light.
Sharp images are obtained only when light rays are focused on
the region around the fovea, where the cones are most numerous.
The outer zones of the retina, containing rods almost exclusively,
produce only indistinct images. That the cones are more highly
differentiated and specialized than the rods is shown by the fact
that in all those nice discriminations of form and color which
make the human eye such an efficient sense organ, it is the fovea,
made up almost entirely of cones, that is principally concerned.

400 THIS LIVING WORLD

1 2

Section of the fro 3'$ retina, fixed before and after exposure to light. (1) In darkness,
the pigment granules are collected around the base of the receptor cells at x. (2) On
exposure to light, the pigment granules migrate out toward the light source, forming a
protective layer about the receptor cells which prevents the escape of light from one cell
to another. (Photomicrographs by Roy Allen.)

Color can be discerned in objects only when they are almost
directly in front of the eye, so that the rays of light from them
fall on these cones.

It may be wondered how light effects its stimulation of the
optic nerve endings. The rods and cones have been found to be
rather complex devices for bringing about this stimulation by
means of photochemical changes. Within the rods there is a
substance called "visual purple" because of its color. When
light falls on it, a partial bleaching takes place in which it
changes to a yellowish color. This photochemical reaction starts
a series of chemical changes which set up nerve impulses in the
optic nerve endings. The gradual improvement of vision after
one has been in a dark room for a while seems to depend upon
the behavior of visual purple. After one remains in the dark for
a time, more visual purple is formed, and the rods become more
sensitive.

Since the rods are thickest at a point a little off the center
of the retina, a dimly lighted object slightly off to the side may
be seen, whereas it becomes much dimmer or entirely invisible
if looked at directly. This, no doubt, has had much to do with
people seeing " ghosts " at night. A dim object at the side may be

SENSATIONS 401

slightly visible, but ' 'mysteriously5' disappear when looked at
directly.

Just how the cones permit us to distinguish form and color
is not definitely known. They do not contain visual purple.
Recently Dr. George Wald of Harvard University has isolated
from the retinas of chicks, which contain cones almost exclu-
sively, a substance which he calls "visual pink." The material
had to be extracted in total darkness. On exposure to red light,
a rapid bleaching took place. This could be demonstrated by
spectroscopic comparison of solutions of the substance before
and after exposure to the red light. That visual purple and
visual pink are different is shown by the fact that red light is
almost without effect upon visual purple, producing only a very
slow bleaching.

The evidence seems to indicate that there are three kinds of
cones, distributed about equally in the region of the fovea. When
one of these different kinds of cones is stimulated to a greater
degree than the other two a peculiar and characteristic color
sensation is produced; red, green, or violet, as the case may be.
When light of only the longer wave lengths enters the pupils of
the eyes, the color sensation of red is evoked. Similarly, the
medium wave lengths of light produce the sensation of green
color, while the shorter wave lengths cause the color sensation
of blue or violet. It is known that certain intensities of selected
wave lengths of light in the red, green, and violet parts of the
visible spectrum produce the sensation of white, apparently by
stimulating the three types of cones simultaneously to the same
relative extent. Other colors are believed to be perceived through
stimulation of one or more of the different types of cones to
unequal degrees.

One of the most remarkable features of our vision is the
ability to perceive distance or depth in objects. This is what is
known as "stereoscopic vision," and it is possessed only by man,
the great apes, and monkeys. It is made possible primarily by
the fact that the eyes are so situated in the skull that they may
look directly to the front and secondarily by a very ingenious
crossing of the optic nerves before they enter the brain.

In all vertebrates except the primates, all the fibers of the
optic nerve from each of the two eyes cross one another and go

402

THIS LIVING WORLD

The cornea of the normal eye is a thin transparent tissue covering the front of the eye*
ball. It is spherical in shape, as shown in the picture at the top. When a radial pattern is
held in front of an eye with a perfect cornea, the pattern is reflected without distortion
and may be so photographed, as shown in the lower picture. (Photographs by A. Mar-
faing, New York.)

SENSATIONS

403

to the opposite sides of the brain. Thus, the sensations from the
right eye are received on the left side of the brain and those from
the .left eye on the right side
of the brain. In man, apes, and
monkeys, however, at the cross-
ing point of the optic nerves,
half of the fibers from each side
turn at an angle and go to the
corresponding side of the brain,
while half of them cross and go
to the opposite side of the brain,
that is, nerve fibers from each
eye enter the optic centers of
both sides of the brain.

As a consequence of this
fact, the image produced by the
stimulation of one eye is super-
imposed upon that produced by
stimulation of the other. There-
by, we get the perception of
distances and depth in objects.
The principle is similar to that
employed in the old-fashioned
stereoscope, which gives the
impression of depth and dis-
tances by superimposing the
projections of two flat pictures of the same object which have
been taken from different angles corresponding to those of the
two eyes.

Correlated with the distribution of the fibers of the optic
nerves to facilitate binocular vision, the motor control of the
eye muscles is such that the movements of the two eyes are in
unison. It might be thought that each eye, by virtue of possessing
an independent set of motor muscles, is capable of movement
without regard to the other eye. Actually, however, an inde-
pendent movement of each eye is not possible at all. Quite
remarkably, newborn babies do possess the ability to move one
eye independently of the other. However, within a few months
after birth, babies develop the ability* to move the two eyes

Occipital lobes

Stereoscopic vision is made possible
partly by an ingenious crossing of the
Optic nerves before they enter the brain.
(Redrawn from Carlson and Johnson, "The
Machinery of the Body.1')

404 THIS LIVING WORLD

in unison. It seems that from earliest infancy our efforts are
concentrated toward achieving single and distinct vision with
the two eyes, and one phase of this is that they be moved
simultaneously. In some abnormal cases a muscle of one eye
pulls more strongly than the corresponding muscle of the other
eye so that the lines of sight of the two eyes are not correctly
directed. This makes it impossible for the person to focus both
eyes simultaneously on one object. Such a person is said to be
cross-eyed. Under such conditions two images fall on the two
retinas of the eyes, and the person would have double vision
except that his brain soon learns to ignore one of the images.

The perfection of an optical mechanism for color perception
and binocular vision was a circumstance of tremendous conse-
quence in the early development of man and his culture. There
can be no doubt of the necessity of such equipment as a condi-
tion for the development of skilled manipulative operations of
the hands and fingers, for such movements are exhibited only
by those forms which possess stereoscopic vision, namely, mon-
keys, the great apes, and man. With increasing assumption of
an upright posture in walking, man's earliest forebears found
their hands free to grasp objects. This freeing of the hands
aided early man not only in locomotion but also in many other
ways. Skilled manipulations developed from crude grasping
movements with the appearance of close coordination between
hand and eye.

Moreover, these factors in man's advance were reciprocal in
their action. Widening the scope of uses of the hands tended
ever more to force the exclusive adoption of bipedal locomotion
and an erect body carriage. These habit changes, in turn, tended
to broaden the field of man's vision and further to release the
hands for new uses. Furthermore, skilled operations of the hands
are associated with increased mental activity. Those individuals
possessing the physical and mental attributes to excel along the
new lines of activity tended to prevail over their less fortunate
fellows. They tended to survive in the struggle for existence and,
perhaps, to pass on to their offspring the very traits which
conditioned their survival. In some such manner were developed
the physical characteristics which make for skill and the mental
organization which has been responsible for the development of

SENSATIONS 405

man's culture from the primitive beginnings represented in what
we know of the old stone age to modern society with all its
complexity.

Hearing

Second only to visual impressions as a source of information
about events in the world around us are sounds. These are sensa-
tions produced by waves or vibrations transmitted by the air.
The organ by means of which sound waves are translated into
nervous impulses is, of course, the ear. In the brain the impulses
originating in the sensory fibers of the ear are interpreted as the
sensation we call hearing.

The ear is usually described as consisting of three parts, the
outer, middle and inner ear. It is the cochlea of the inner ear,
however, which contains the sensory receptors that are stimu-
lated by the sound vibrations. The other parts are conducting
mechanisms which serve to convey the waves to the cochlea.

The outer ear consists simply of a cartilaginous funnel-like
organ for collecting the sound vibrations and an auditory canal
to lead it from the outside to the eardrum at the inner end of the
canal. The eardrum is a thin membrane of muscle and connec-
tive-tissue fibers which separates the outer from the middle ear.
The latter is an air-filled cavity from which an open canal, the
Eustachian tube, leads into the throat. This canal is sometimes
the cause of considerable trouble, as it forms a passage through
which disease-producing organisms lodged in the mouth or
throat can rather easily reach the middle ear, often leading
to serious infections of this region which may back up into the
cavities of the spongy mastoid bone.

The origin of the middle ear from the spiracle of fishes in
the evolution of the mammalian skull has already been men-
tioned elsewhere. The Eustachian tube represents the portion
of this first gill slit which connected with the throat. The middle
ear chamber is bridged by three small bones, the hammer, anvil,
and stirrup, whose origin and functions were described in a
previous chapter. It is necessary to repeat here only that the
mechanical vibrations of the eardrum, produced by the impact
of the sound waves upon it, are transmitted through these three
bones to the inner ear. In this transmission the vibrations are

406

External
ear

THIS LIVING WORLD

Semicircular
canal

8th nerve

External

ear

opening

^^gy^^fmauL 7w^^»ss5flr

cousiic

Ear drunTV \V ..._, \/,-... / ^%W^ fTnwch of

8th nerve

Eustachian
tube

In a transverse section, the ear is seen to be a complex organ for collecting sound
vibrations, amplifying and transmitting them, and converting their energy into the energy
of nerve impulses.

amplified. The middle-ear bones act like a bent lever which
theoretically has a mechanical advantage of 3 to 1. Friction
between the bones, however, and the air pressure in the middle-
ear cavity, tend to reduce this value by about one-half, so that
the effective leverage is about 3 to 2.

In a previous chapter it was stated that the base of the stirrup
bone forms an oval plate which closes an opening in the bony
casing of the inner ear. This is the so-called "oval window." It
is the upper one of two openings by which the bony labyrinth of
the inner ear communicates with the middle-ear chamber. The
other opening, the "round window," is closed by a tough mem-
brane. The area of the oval window is about one-twentieth that
of the eardrum. As a result, the pressure exerted by the eardrum
is increased about twenty times at the oval window, making the
final pressure of the stirrup moving in the oval window approxi-
mately thirty times that of the eardrum.

The bony labyrinth of the inner ear contains a spirally coiled
structure known as the "cochlea." The coiling of this structure
resembles that of a snail shell, from which it takes its name. It
consists of a tube which gets progressively smaller and comes to

SENSATIONS

407

Organ of

corti ^Cochlear
canal

Auditory

'nerve

fibers

In a cross section of the cochlea, the organ of Corti may be seen to consist of ciliated
cells resting on the basilar membrane. The cilia of these cells are in contact with an over-
hanging membrane. The ciliated cells are anatomically in connection with fibers of the
auditory nerve. (After Carlson and Johnson, "The Machinery of the Body.")

a point at the apex. The tube is partitioned off into three parallel
canals, which traverse its entire length and likewise taper to-
ward the apex. The canals are filled with a fluid which vibrates
in response to the vibrations transmitted to the stirrup from the
eardrum.

The middle canal (or cochlear canal) contains the true organ
of hearing — the so-called "organ of Corti." The middle canal
is separated from the upper and lower canals by membranous
partitions. The lower one of these is the basilar membrane, on
which the organ of Corti rests. The details of the structure
of the cochlea are shown in the accompanying drawing. The
organ of Corti is composed of "hair cells." These are in reality
ciliated cells which are the end organs of the auditory nerves.
There are some 15,000 to 50,000 of these ciliated cells in the
human ear. They extend along the entire length of the coch-
lea from its base to its apex. The cells vary in size, the largest
being.located near the tip. The cilia of the hair cells come in con-
tact with an overhanging structure, or "roof" membrane.

The mechanism that has just been described provides the
means whereby the vibrations of the eardrum are amplified and
transmitted to the fluid of the cochlear canal. How are these
vibrations converted into nerve impulses in the fibers of the
auditory nerve? It seems clear that the hair cells of the organ of

408 THIS LIVING WORLD

Corti are directly involved, since they are anatomically in con-
nection with the nerve fibers themselves. Moreover, they number
roughly twice the number of pitches, or sound frequencies, to
which the ear is attuned. How are they stimulated?

The basilar membrane, upon which the organ of Corti rests,
is composed essentially of transverse connective-tissue fibers at-
tached firmly to the walls of the cochlea and stretched taut,
somewhat like the strings of a piano. The fibers differ in length at
different levels of the cochlea, gradually becoming longer toward
the apex of the spiral. The analogy is very close between the
structure of this membrane and a musical instrument with
strings of graded length and, correspondingly, of graded intrinsic
vibration frequencies. It is thought, therefore, that the fibers of
the basilar membrane, like the strings of the musical instrument,
are set into vibration by sounds of the specific pitch correspond-
ing to their own intrinsic frequencies. The hair cells resting on
the fibers are thus set in motion and their cilia, which are in con-
tact with the "roof" membrane, are stimulated so as to initiate
impulses in the auditory nerve fibers. Sounds of different pitch,
that is, of different frequency, are believed to stimulate hair
cells in different regions of the cochlea. The mode of stimulation
is analogous to that involved in the sense of touch.

This view has been confirmed to the extent that it has been
shown clinically that tone deafness is associated with injury or
destruction of the hair cells in a given restricted portion of the
cochlea. Under these circumstances, the individual is unable to
perceive certain tones. By sounding intensely loud high-pitched
sounds into the ear of an experimental animal, moreover, it can
be demonstrated that deafness for tones of high frequency can be
produced, owing to the resulting injury to the organs of Corti
near the base of the cochlea, where the fibers of the basilar mem-
brane are shortest. Injuries at the apex, where the fibers are
longest, similarly produced by loud low-pitched tones, cause
deafness to low tones. Dr. Harvey Fletcher of the Bell Telephone
Laboratories in New York has been able to plot the different
parts of the cochlea which respond to the various sound fre-
quencies within the range of normal hearing. He has shown that
auditory patterns of vibrating cells along the cochlea are built
up when we hear a sound of a given set of frequencies. As this

SENSATIONS 409

The auditory pattern in the cochlea produced by the 518-cycle fundamental note of a
bugle playing "taps." The spiral represents the distance along the cochlea of the typical
human ear. It is divided into 1 00 equal parts for purposes of identifying locations of nerve
endings which respond to definite frequencies. The bulges on the diagram represent the
positions of nerves which give the maximum response to the pure tones in this complex
note of the bugle. (Drawing reproduced from "Auditory Patterns" by Dr. Harvey Fletcher,
Bell Telephone Laboratories.)

sound changes into a different set of tones, the patterns likewise
change, stimulating different nerve endings so that we perceive
the changing notes.

Besides the cochlea, the labyrinth of the inner ear comprises
other structures not concerned with the phenomena of hearing.
Here, of course, reference is made to the organs having to do with
the sense of balance, or equilibrium. These organs consist of the
three semicircular canals and two tiny sac-like chambers with
which they are associated, the "sacculus" and "utriculus." The
semicircular canals arise from the walls of the utriculus. Each
lies in a plane at right angles to the planes of each of the other
two. The inner walls of the sacculus and utriculus are lined with
ciliated or "hair" cells, from which arise the nonacoustic fibers
of the auditory nerve leading to the brain. The cavities of the
canals and sac-like structures are filled with a watery fluid, and,
in addition, the utriculus and sacculus contain tiny stone-like
bodies. These "ear stones" are secretions of calcium carbonate
attached to the ends of the filaments of the hair cells. When the
head is rotated or inclined it seems that the earstones in the sac-
culus or utriculus are displaced. The resulting slightly unequal
pressure on the filaments of the hair cells stimulates the nerve

410 THIS LIVING WORLD

endings in them and the sensation of rotation or overbalance
is experienced.

The semicircular canals are equipped with receptor organs,
similar to the hair cells of the sacculus and utriculus, located
in little swellings at their upper ends. Experimental evidence
indicates that these receptors are stimulated by movements of
the fluid in the canals. These movements are due to inertia,
causing a lag in the movement of the fluid when the head or body
is moved or rotated in any plane. Stimulation of the end organs,
or receptors, not only makes us aware of the movement or rota-
tion but also causes us to adjust ourselves to the change.

Chemical Senses

Chemical reactions brought about by actual contact of sub-
stances with certain nerve endings provide us with two senses,
smell and taste. It might be said that gases are the substances
smelled and liquids are the materials tasted. A solid substance
must first be reduced to a liquid form or put into solution before
it can be tasted; and, it seems that even gases must be dissolved
in a liquid before they can be smelled. Both of the special senses of
smell and taste are located near the entrances to the respiratory
and digestive tracts, as if to act as sentinels and to pass upon the
character of the materials taken into the body. Smell has the wider
range of the two senses, since the odorous gases may travel con-
siderable distances from their source and then affect the sensory
nerve endings.

The sense of smell is probably one of the most primitive
senses which animals possess. The olfactory apparatus, even in
man, begins to develop at a very early stage in the growing
embryo. In the human embryo it appears first at about the third
week of development, and the organs are fully formed before
birth. The sense of smell can be aroused by an exceedingly small
amount of gas in the atmosphere, some gases being detected
when the concentration is one part in about eight million. In
lower animals, the sense of smell is much keener than this, and it
is likely that it is their chief source of information about the ex-
ternal world. The sense of smell is now useful to man primarily
in the pleasant or unpleasant sensations which it affords him and
as a danger signal of irritating or poisonous gases.

SENSATIONS

Olfactory nerve cells

411

External

nasal

opening

Internal

nasal

opening

The receptors associated with the sense of smell are located in a patch of epithelial tissue/
less than one square inch in area, in the upper part of the nasal cavities.

In the upper part of the nasal cavities there is a lining of epi-
thelial tissue of somewhat less than one square inch in area. This
tissue contains the receptor nerve structures for odorous sub-
stances. These structures are hair-like cells buried within the epi-
thelial tissue. The cells are the end organs of nerve fibers which
pierce the skull and pass back to the brain as the olfactory nerve.
The olfactory epithelium is bathed in liquid and is somewhat out
of line of the main air passages, so that it does not dry too much
as air is drawn into the lungs. It seems that stimulation of the
receptor cells depends upon the odorous chemicals going into
solution in the liquid surrounding the cells, so that these ma-
terials may produce some chemical reaction with the hair cells.

The olfactory receptors are easily fatigued, and the sensation
of any odqr falls off rapidly in strength. This is particularly
noticeable when one enters a room that has a definite odor.
After being there for a few minutes the odor becomes imper-
ceptible, yet it will be quite noticeable to another person just
entering the same room. However, just how we distinguish the
various odors is not well understood.

The structures for detecting taste are located chiefly on the
upper side of the tongue, although a few are found on the roof of

412

THIS LIVING WORLD

the mouth and in the throat. They are known as "taste buds."
Each of these buds is a small cluster of cells which is embedded

the mucous membrane of

Sensory cells Epidermis

Sensory bristles

i Nerve fibers

A taste bud. (Redrawn from Neale and
Rand.)

in tne mucous
the tongue. Within each cluster
there are a number of slender,
elongated cells which are the
end organs of nerves that con-
vey to the brain the sense of
taste. These cells are stimulated
by substances taken into the
mouth when such substances
are dissolved.

It is believed that man dis-
tinguishes only four fundamen-
tal tastes, namely, sweet, salt,
bitter, and sour. We are able to
distinguish the "taste" of so
many different substances

mainly because of the ability to use other senses in conjunction
with the sense of taste. Certainly the odor of foods is so closely
associated with their taste that we smell them more than we taste
them. The temperatures of foods are detected by thermal nerve
endings which respond to heat and cold. Many such nerves are
located in the mouth. In addition, the roughness or softness of
foods produces certain touch sensations. All these we associate
with taste and by them most foods are identified.

Skin Senses and Pain

It is generally said that man possesses five senses. Oftentimes
a person is referred to as having a "sixth" sense when it is de-
sired to draw attention to some property of perception peculiar
to him. Actually, every person possesses a sixth sense, and many
others in addition. The skin alone contains receptors for at least
five different sensations, such as touch, pressure, pain, heat, and
cold. The nerve endings which respond to these different stimuli
vary in their distribution over the skin. Certain of them are con-
centrated more in one area than another. Others may be absent
entirely from certain areas.

SENSATIONS

413

The organs of taste are small taste buds located chiefly on the upper side of the
tongue, as shown by the dark spots in the picture above, and magnified at the right.
(American Museum of Health photograph.)

The skin senses are probably the most universal of all means
of communication with the outside world. Even though a person
may voluntarily close his senses to light and sound, even taste
and smell, he cannot escape physical contact with his surround-
ings. Touch also serves to confirm the impressions gained from
the other senses. The areas of the skin that are most sensitive to
touch are the underside of the finger tips, the palms, soles of the
feet, lips, and external genitalia. In these regions there are
numerous small capsules of tissue which contain the nerve end-
ings that provide us with the sense of touch. Such nerve endings
are less widely scattered in other parts of the skin, thus making
those areas less sensitive to touch stimuli.

Changes in temperature are detected by special nerves,
which in most cases have free nerve endings in the skin. These
nerves are highly concentrated in the forehead, cheeks, and
palms of the hands. One naturally opens the palms of his hands
before a fire after coming in fr^m the cold. There are two differ-

414 THIS LIVING WORLD

ent types of receptors, one for "hot" and one for "cold," in the
skin. These are usually small areas about the size of a pin point.
They contain nerves which respond to such temperatures and
thereby produce these sensations. Some of these minute areas
are on the back of the hand. Stimulating them, even by pressure
from a sharp pencil, will produce the sensation of cold or warm,
depending upon which area is pressed.

Pain is felt through specialized nerve endings. These are
found scattered over all the skin and in many other parts of the
body. However, these nerve endings are more concentrated in
some areas than others. For example, there are relatively few of
them in the inner wall of the cheek. While we may recognize
pressure, heat, or cold there, we experience little of the sensation
of pain. On the other hand, these nerves are highly concentrated
in most parts of the skin, as well as in the teeth and the upper
skeletal bones of the face. It has been rather carefully estimated
that there are four million of these nerve endings in the skin
alone, this number being more than four times as great as all
other sensory nerves of the skin.

The pain nerves are not very extensive in the interior of the
body. They seem to be located mainly in the throat, intestinal
walls, bladder, and joints. They give us, in general, such pain
sensations as thirst, hunger, aches of the stomach, colon, and
bladder, and joint aches. From other regions little pain is expe-
rienced, except from certain deep-seated muscles. Even these
pains are often very generalized rather than being local to some
definite area. Much of the internal body may be cut in a surgical
operation, or torn, burnt, or pinched in an accident, without
producing pain.

Nerve Action

We have seen how we become acquainted with things and
events in the world about us through the possession of special
sense organs. We have seen, also, that these are groups of cells
specialized in structure for the purpose of receiving energy in
various forms from the outside world and translating this energy
into nerve impulses which are conducted to the brain and there
interpreted in terms of what we know as sensations. It would be
natural at this point to inquire what nerve impulses are. How

SENSATIONS 415

does the energy falling on a sensory receptor reach the brain?
Are there special kinds of impulses which transmit light rays or
sound vibrations as such to the brain, or are nerve impulses a
form of energy, alike in all nerves? We shall see that the lat-
ter alternative comes nearer to expressing the facts of nerve
conduction.

A nerve may be compared with an insulated telephone cable
composed of many wires in that it is a bundle of parallel fibers
encased in a connective-tissue sheath. Here, however, the
analogy ends. Numerous experiments have shown that a nerve
impulse is not simply an electric current such as is concerned in
telephonic communication. Moreover, it has been proved that
conduction of a nerve impulse is not the same thing as that of an
electric current. Each nerve fiber is an elongated outgrowth of
a single living cell, not unlike a very long pseudopodium, or
"false foot" of an amoeba. It contains in its electrolytically
dissociated molecules all that is necessary for conducting an
electric current: in fact, a nerve fiber will conduct electricity.
However, a dead nerve fiber will also Carry an electric current,
but it will not transmit a nerve impulse. Injury to even a small
section of a nerve will effectively prevent the passage across it
of an impulse originating in the uninjured portions. Furthermore,
while transmission of a nerve impulse is very rapid, it does not
even approach the speed of an electric current.

The speed at which a nerve impulse travels may be measured
by a very simple experiment first performed nearly a century
ago by the great German physiologist, Hermann L. F. von
Helmholtz. One of the large muscles of a frog's leg is dissected
out and removed, together with the large nerve which supplies
it. By touching the nerve with an electrode the muscle may be
made to contract. There is a brief interval between the moment
the stimulus is applied to the nerve and the beginning of contrac-
tion by the muscle. The interval is longer, moreover, when the
point at which the nerve is stimulated is farther from the muscle.
Let us suppose that the electrode is applied successively at two
points along the nerve six centimeters apart. The difference of
the two intervals between the time the nerve is stimulated and
the time the muscle begins to contract is 0.0005 second. Since
in this time the nerve impulse travels six centimeters along the

416 THIS LIVING WORLD

nerve, the rate of transmission must be 120 meters or about 400
feet per second.

Unlike the shortening and broadening of a muscle when it
contracts, there are no visible changes in a nerve during passage
of an impulse. Nevertheless, there are changes taking place
which can be detected by indirect methods. There is an increase
in the rate of oxygen consumption and carbon-dioxide produc-
tion, indicating the occurrence of oxidations which presumably
release the energy concerned in conduction of the nerve impulse.
Heat is generated as a by-product of these oxidations. The most
useful index of nerve conduction, however, is the electrical
changes which accompany it. These changes are easily detected
by connecting a suitable device for measuring current at two
points along the nerve. The changes are found to spread from
the point of stimulation to the end of the fiber.

The surface of a nerve fiber at rest is polarized; that is, the
ions in the membrane of a nerve cell and its fiber are so arranged
that the outer surface is positively charged while the inner
surface is negatively charged. This polarization depends in part
at least upon the impermeability of the membrane to the ions
responsible for these electrical charges. The polarization, in
turn, is thought to be concerned in maintaining the membrane
semipermeable. Thus a breakdown of either one of these prop-
erties of the resting nerve would cause the breakdown of the
other also. Excitation of a nerve fiber is believed to be associated
with just such a breakdown of the semipermeability and polari-
zation of its membrane.

The stimulus which initiates a nerve impulse brings about
depolarization of the surface of the nerve fiber at the point of
origin. In this depolarized region the membrane of the nerve
fiber is permeable to the ions in the adjacent as yet unactivated
region. These ions migrate through the permeable gap and
neutralize one another: that is, they combine to produce elec-
trically neutral molecules. Another section of the nerve fiber is
thus depolarized and the permeability of the membrane is
altered, providing for the continuation of these changes in the
succeeding portion. The passage of the impulse along the nerve
fiber is preceded by a wave of electrical negativity resulting in
the depolarization of the surface in the region in front of the

SENSATIONS 417

advancing impulse. The phenomena 'are illustrated diagrammati-
cally in the following drawing.

It is clear that a nerve impulse is a physicochemical disturb-
ance in the nerve fiber. Once started, it is self-propagating, like
the burning of a powder fuse in which the burning portion
ignites that just in front of it. The analogy to a powder fuse may
be extended. Thus, whether set off by the application of heat or
by a hammer blow, the combustion of the fuse is the same as
regards both the rate and the nature of the chemical change.
Moreover, the rate of burning is the same regardless of how
much heat or how hard a blow was initially applied to start it;
the only essential is that enough heat, or a hard enough blow,
be applied to stimulate. Similarly, a nerve impulse is the same
regardless of the nature of the initiating stimulus, the sensation
produced, or the motor response elicited. Unless the stimulus
received is sufficient in intensity, no impulse is set up. The
stimulus of intensity just sufficient to initiate a nerve impulse
is called the "threshold stimulus." The strength of the stimulus
may be increased any amount above the threshold intensity
without affecting the strength of the resulting nerve impulse in
the single fiber, since the energy for conduction of the impulse
comes from the nerve itself, not from the stimulus or activating
agency.

Just as the kind of metal in a wire affects its electrical con-
ductivity, or, to continue the analogy with a powder fuse, as the
dampness or dryness of the powder affects its burning, so the
nature, strength, and rate of transmission of a nerve impulse
depends upon the condition of the nerve itself. This idea is
embodied in the so-called "all-or-none law" of nerve action,
which states that if a nerve fiber responds at all to a stimulus,
it responds maximally for the condition of the fiber at that time.
This may be proved experimentally by inserting delicate metal
electrodes attached to a sensitive electrical meter in a small
nerve at two points between which the fibers have been pulled
apart under the microscope and all cut, except one. If, now,
stimuli of graded intensity are applied on one side of the cut
fibers, it will be found that the deflections of the galvanometer
needle are always of the same magnitude, provided, of course,
that the condition of the nerve does not change.

418

THIS LIVING WORLD

4 44-"

' +J+J+ + + + + + +
"Tv : ~ — /

4 4 + 4 4- 4 4 +4.

The wave of electrical negativity which precedes the impulse
may be measured by this same means, and the resulting data

may be plotted. The curve
will appear as represented
in the drawing. The front
of this curve has a steep
slope, showing that depolar-
ization occurs quickly. The
rest of the curve has a more
gradual slope. It represents
an entirely different process
which occurs more slowly
and which is without paral-
lel in the analogy of nerve
action to the burning of a
powder fuse; that is, the
nerve fiber will restore its
polarity immediately after
it has been depolarized and
the nerve impulse has
passed. The positive and
negative charges are again
established on outer and
inner sides of the fiber sur-
face. The slope of the back
of the curve in the diagram

Current

Time
u

A. The ions in the membrane of a resting
nerve fiber are believed to be arranged so
that the outer surface is positively charged,
while the inner surface is negatively charged.

B. The transmission of a nerve impulse is
accompanied by the ions passing through the
membrane and temporarily neutralizing each
other.

C. The passage of the nerve impulse along
a fiber is preceded by a wave of relative elec-
trical negativity. (After Carlson and Johnson,
"The Machinery of the Body,11)

represents the rate at which this restoration occurs.

When a powder fuse has once burned it cannot repeat the
process. A nerve fiber, however, will conduct impulses initiated
one after the other at intervals of as little as 0.005 to 0.001
second. Shortly after depolarization occurs at any point, the
nerve fiber restores its surface so that again the outside bears a
positive charge and the inside bears a negative charge. This
restoration is accomplished by a reversal of the physicochemical
changes involved in the transmission of an impulse. The actual
changes involved in conduction of an impulse, that is, depolar-
ization and increase in permeability of the surface of the fiber,
require only about 0.0004 second. The remainder of the time

SENSATIONS 419

before another impulse may be transmitted is used by the nerve
in restoring the polarized condition.

REFERENCES FOR MORE EXTENDED READINGS

CARLSON, ANTON J., and VICTOR JOHNSON: "The Machinery of the Body,*'
University of Chicago Press, Chicago, 1937, Chap. XI.

This chapter contains a thorough elementary presentation of the anatomy and
physiology of the sense organs with numerous diagrams and drawings.

MITCHELL, PHILIP H.: "A Textbook of General Physiology," McGraw-Hill
Book Company, Inc., New York, 1932, Chap. VI.

This is a standard textbook of general physiology. The chapter referred to contains
an elementary account of the sense organs for students of college level.

PLUNKET, C. R.: " Elements of Modern Biology," Henry Holt & Company,
Inc., New York, 1937, Part III.

This well-known text for a beginning course in biology contains in Part III a con-
cise and specific treatment of the sensory organs, stimulation and response, coordi-
nation of body functions, and behavior of organisms as regards their habits and
intelligence.

DABHIELL, J. F.: "Fundamentals of General Psychology," Houghton Mifflin
Company, Boston, 1937, Chaps. IX, X.

The chapters referred to are an excellent elementary discussion of the physical
structure of the sensory organs and the physical nature of nerve action. They con-
tain, in addition, a more detailed and advanced account of sensory perception and its
relation to behavior.

HERRICH, C. J.: "Introduction to Neurology," W. B. Saunders Company,
Philadelphia, 1927, Chaps. III-VI.

The author has written a book that has been for many years a standard text for
college courses in neurology. It is one of the best comprehensive elementary texts.
The chapters referred to relate to the structure and functioning of neurons and the
sensory receptors and to the general physiology of the nervous system.

HARTRIDGE, H., in Ernest H. Starling: "Principles of Human Physiology,"
5th ed., Lea & Febiger, Philadelphia, 1936, Chap. VIII.

Here is a standard reference text for gifted or advanced students. It is written with
great clarity but sparsely illustrated.

HECHT, SELIG: "The Nature of the Photochemical Process," Chap. XI in
Carl Murchi^on, "Handbook of Experimental Psychology," rev. ed., Clark
University Press, Worcester, Mass., 1934.

This text is a concise account of the physical and chemical processes underlying
vision, written by one of the foremost investigators of visual phenomena among
modern physiologists. The material presented is of a technical nature but is organized
with the maximum clarity and brevity.

420 THIS LIVING WORLD

American Journal of Ophthalmology, published by George Banta Publishing
Company, Menasha, Wis.

This is a professional journal devoted to research articles and clinical reports related
to structure, functioning, and diseases of the eye and related tissues.

The Journal of Comparative Neurology, published by the Wistar Institute of
Anatomy and Biology, Philadelphia.

This journal is issued bimonthly and is devoted to articles on research in the field
of nerve structure and functioning.

14: CORRELATING MECHANISMS

How the Body Is Integrated into a Smoothly Operating Unit

RW persons living in the United States today realize the
•emendously important role which modern means of com-
munication play in their daily lives. Americans take for granted
their fine roads and automobiles, their railroads, postal system,
and airlines, the telephone, and the radio. It is only when one of
these fails conspicuously in the performance of its expected
duties that we become keenly aware of its importance. Similarly,
most people become conscious of the existence of means of com-
munication among the parts of their own bodies only when some-
thing goes wrong with one of them. The circulatory system,
which has already been discussed, provides an obvious example
of such a bodily channel of communication, roughly analogous
in its functions to the railroads and other carriers of the nation's
heavy goods. Two other important communicating systems of
the body remain to be discussed. These are the nervous system

421

422 THIS LIVING WORLD

and the ductless glands, roughly comparable in their functions
with the telephone and postal systems, respectively.

The Nervous System

From its primary use as a means of communication, the tele-
phone has come to play an extremely important part in the
integration and coordination of business and industry. An official
in the New York office of a California firm can get in touch with
the "home office'* in a few minutes by telephone, where by letter
the transaction would require at least forty-eight hours. This has
had important effects on the decentralization of industry, making
it possible for a manufacturer to locate his plant near the sources
of raw materials while maintaining his executive offices in one of
the big centers of commerce, such as New York, Chicago, or
San Francisco.

The human body presents an organization no less complex
than that of modern industry. The specialized tissues, organs, and
systems of the body are composed of billions of cells which are
themselves units in a very real sense. Even the most specialized
cells and tissues are capable of a limited independent existence.
Coordination of their activities is essential to the welfare of
the body. This coordination is brought about chiefly through the
nervous system. In higher animals, especially vertebrates, the
nervous system comprises a brain, located in the skull; a spinal
cord, enclosed in the vertebral column and directly connected
with the brain; and numerous nerves, extending out from the
brain and spinal cord to all parts of the body.

The primary function of the nervous system, like that of the
telephone, is to transmit messages. The nervous system, how-
ever, particularly the cortex of the brain, is able to arrange
the nerve impulses into definite patterns. Stimulus and response
are thus integrated so that the body functions as a whole. In
addition, man is able to select his responses in such a manner as
to represent intelligent behavior. A brief survey of the physical
structure of the nervous system and of the simplest kinds of
nervous coordination will give some insight into how this is
accomplished.

CORRELATING MECHANISMS 423

Structure of the Nervous System

The structural units of the nervous system are the highly
specialized nerve cells or neurons. In addition to these there are
other types of cells which sup-
port them and provide them with
nourishment. Of interest here is
the specialized type, the nerve
cells proper.

Nerve cells are characterized
by having many branched proc-
esses which extend out from the
mass of protoplasm surrounding
the nucleus. It is these projec-
tions, or fibers, which permit the
nerve cells to perform their
special functions. One of the pro-
jections is usually relatively very
long and slender. This is the
"axon." It is the main trunk
which carries impulses away
from the cell body. The other

processes are usually short and

i « 1 i •, 99 rni Representative nerve cell.

are known as dendrites. Iney

serve to carry impulses toward the cell body. There are a few ex-
ceptions to this general condition of the axon's being longer than
the dendrites. The most important are in the case of the spinal
nerves and in certain nerves that arise from centers lying just out-
side the spinal cord, in which the axons are short and the den-
drites are long. The accompanying drawing is an illustration of
a rather typical nerve cell.

Some nerve cells are very large and complex. An example is
the neurons which conduct impulses to the muscles of the foot,
causing them to contract. They have axons which are over three
feet long in man. Some of the neurons which carry impulses from
the joints of the toes have axons and dendrites which combined
are nearly six feet long. These neurons are, however, the giant
cells of the nervous system. They are single cells and have all the

424

THIS LIVING WORLD

properties of a single cell, such as the ability to regenerate a lost
part of the axon or dendrite, if not too much of it is missing.

This explains why muti-
lated nerve endings in the
skin or muscles will often be
repaired after a minor in-
jury to the tissue.

Nerve cells are of two
types, so far as function is
concerned. One type is the
sensory nerves, which re-
spond to external stimuli.
The other is the motor
nerves, which conduct im-
pulses to a muscle or other
- t . TL j ii effector cell, causing it to

Cross section or a large nerve. The medullary .

sheath of myelinated fibers, which has a whitish respond by Contracting or

appearance in the picture, may be compared to other appropriate action,

the insulation covering a telephone wire, while Stimuli are received by the

the fibers, shown as dark spots, correspond to , , . -

the wires. (Photomicrosraph by Roy Allen.) dendntes of sensory nerve

cells and converted into

nerve impulses, which are transmitted to the brain or spinal cord
through a branched ending of the axon. Similarly, impulses are
received by the dendrites of motor nerve cells and transmitted
to the effector through branched endings of the axon.

The term "nerve" as usually employed refers to a bundle of
fibers or processes from many nerve cells. For example, the
sciatic nerve is the large nerve which supplies nearly the whole
of the skin of the leg and the muscles of the back and thigh and
those of the leg and foot. It is made up of thousands of nerve
fibers going to different parts of the leg. Each fiber passes to some
muscle or section of the skin. It resembles very much a telephone
cable of many wires, each one supplying the telephone of a dif-
ferent subscriber. Thus, a single motor nerve may supply as
many as 150 muscle fibers.

Most nerves are very similar in their make-up or structure.
By special methods of examination it may be shown that each
component fiber or axon is surrounded by its own covering or
sheath. This consists of two layers. The inner layer, next to the

CORRELATING MECHANISMS 425

axon itself, is made up of a white fatty material, which gives a
whitish appearance to the nerve. This inner layer is known as
the "myelin sheath." It is not a continuous cylinder surrounding
the axon over its entire length. At regular intervals it is inter-
rupted, giving a segmented appearance. The outer part of the
nerve sheath is a thin, transparent layer composed of fused
cells.

The reasons for the segmented arrangement of the nerve
sheath are not clear. It is known, however, that myelinated
fibers conduct nerve impulses more rapidly than do nonmyelin-
ated fibers. Thus, as already noted, an ordinary motor impulse
travels along a myelinated fiber at the rate of about 400 feet per
second. The nonmyelinated fibers of the visceral or autonomic
nerves, on the other hand, conduct impulses at about 100 feet
or less per second. It appears that the presence of the myelin
sheath speeds up the transmission of the nerve impulse. The
mechanism may be similar to that which accounts for the more
rapid conduction of an electrical impulse along an iron wire,
when the wire is enclosed in several segments of glass tubing
arranged to simulate the nodes of a myelinated fiber. Instead
of passing along the wire longitudinally, as it does when the sur-
rounding glass tubing is continuous, the current jumps from one
node to the next, greatly accelerating the rate of transmission.

The nerve cells are frequently gathered together in small
groups, those occurring outside of the brain and spinal cord being
known as "ganglia." The cells are not physically connected to
each other, but dendrites of one cell are in close proximity to the
terminal fibers of the axon of another. Some of the ganglion cells
send axons to the spinal cord and brain, while others send their
axons to motor end organs such as the muscles. The largest
ganglion is in the abdomen and is known as the "solar plexus."
These nerve centers control certain definite organs. Especially is
this true of the ganglia which lie outside the spinal cord and
brain, such as the solar plexus. The latter controls the blood sup-
ply to a part of the abdominal cavity.

The spinal cord is enclosed within the vertebral column. It
is made up of combinations of nerve fibers from the brain and
many ganglia. Its main function is to control the trunk and
limbs and to transmit nerve impulses from the body to the brain

426 THIS LIVING WORLD

and vice versa. Normally the spinal cord is under direct control
of the brain, but it may act independently of it. Thus walking

soon becomes a more or less
unconscious effort.

In cross section the spinal
cord is seen to be composed of
a central gray portion sur-
rounded by white matter. The
gray material is roughly ar-
ranged in the shape of a but-
terfly or the letter //. It is
composed of nerve-cell bodies
In cro$$ section the spinal cord is seen to and nonmyelinated fibers,
be composed of a central portion of gray The white matter is made up

matter and an outer portion of white matter. « ,. , „, 1

(Photomicrograph by Roy Allen.) of myelmated fibers whose

fatty sheaths give it its color.

These fibers are of several types. There are ascending and de-
scending fibers, which conduct impulses from all parts of the
cord to the controlling centers of the brain and from the brain
to the spinal cord. In addition there are fibers of intermediate or
connecting neurons which link the brain and spinal cord with
ganglia located outside them.

The brain in vertebrates completely fills the skull. It is
directly connected with the spinal cord at the base of the skull.
In the lower vertebrates its chief function is to control the head,
heart, and lungs in much the same manner as the spinal cord
controls the rest of the body. Its operation is primitive, auto-
matic, and unconscious. In the higher vertebrates, and particu-
larly in the great apes and man, the frontal portions of the brain
are enormously expanded, overshadowing the more primitive
portions in their development. These expanded portions form
the cerebrum or cerebral hemispheres, in which are the centers
of sensory perception and the higher mental processes of thought
and reasoning. Both the cerebrum and the more primitive por-
tions of the brain are made up of nerve cells and nerve fibers.

The brain, spinal cord, and certain ganglia constitute the
central nervous system. In some respects they correspond to the
switchboards in a central telephone exchange, which connect
the wires of an incoming call to those ef the party desired. In

CORRELATING MECHANISMS 427

other respects, the central nervous system may be compared
with the editorial offices of a large newspaper. For example, one
of the functions of the brain and spinal cord is to "edit" reports
about the outside world which it receives via sensory nerves
from the eye, ear, nose, etc. Another function of the central
nervous system is " executive " in character; that is, the brain
and spinal cord "formulate the policy" of the body and give
"orders" which travel over the motor nerves to various kinds of
end organs, there to be translated into some sort of activity.

Reflex Action

The simplest kind of nerve-controlled activity would involve
five components: (1) a sense organ, (2) a sensory nerve, (3) a
ganglion or other nerve center, (4) a motor nerve, and (5) a
muscle or other structure capable of some such response as
movement, cessation of movement, or secretion. The simple
mechanism in such a case might be thought of as a stimulus-
response action, represented by an S-R bond. Actually, no
nerve-controlled behavior is as simple as this, but it illustrates
the principle involved.

What happens, for example, when one's finger is burned by
a gas flame? The pain receptors in the skin are stimulated,
initiating an impulse which travels up one or more fibers of the
sensory nerves from the burnt finger to a nerve center located
in the spinal cord. In the nerve center, functional connection is
established with the appropriate motor neurons so that the
impulse is relayed along the motor nerves of the arm. At the
motor nerve endings the muscles of the arm are stimulated to
contract, and the finger is quickly withdrawn from the flame.
This is a simple S-R bond. It is an example of a reflex. However,
more happens in this case than the simple reflex action of remov-
ing the finger from the flame. The impulse initiated in the pain
receptors of the skin is also transmitted, by way of connecting
neurons, from the primary reflex center to other centers until
finally it reaches a certain part of the middle region of the brain.
Here it is translated into sensation, and the person becomes
conscious both of the pain and of its localization. This sensation
probably is not realized until after the finger has been automati-

428

THIS LIVING WORLD

Brain

Sensory ^
area

Motor neurons

Connector
neurons

A simple reflex arc and an accompanying pathway to the brain.

cally withdrawn from the flame. The action is illustrated in the
accompanying diagram.

The reflex arc is the functional unit of the nervous system.
The essential steps in any reflex action have been presented in
the illustrative example describing what happens when we burn
our fingers in a gas flame. These steps may be summarized
briefly as follows. A receptor organ is stimulated, giving rise to
an impulse, which travels along a sensory nerve fiber to a gan-
glion or other nerve center. Here functional connection is
established with a motor neuron and the impulse is transferred
to a motor nerve fiber. At the motor ending an effector organ is
stimulated, causing a characteristic response such as contraction
or relaxation of a muscle or discharge of a gland.

Reflexes vary in several respects, one of the most obvious
differences among them being one of complexity. A typical
example of a simple reflex is the knee jerk. This is the sudden
straightening of the leg when tapped sharply but lightly just
below the kneecap. The movement is caused by the stretching
of the broad tendon at the knee joint which serves for the attach-
ment of the muscles that extend the lower leg. The tendon and
the extensor muscles contain receptors sensitive to stretch. On

CORRELATING MECHANISMS 429

stimulation of these receptors by stretching, the very muscles
in which they are located are activated, causing the leg to be
straightened with a jerk. A more complicated type of reflex is
called forth when a person turns his ankle while walking. The
injured leg is immediately flexed or drawn up while the opposite
leg is extended or straightened. Simultaneously, the weight of
the body is shifted from the injured to the sound leg. The reac-
tion is automatic in a young person. It is a defensive adaptation
designed to prevent falling. Although turning an ankle is fre-
quently accompanied by pain, the reflex called forth is not
dependent upon the sensation. Thus, a similar response can be
elicited experimentally in lower animals even after the brain
has been removed.

The most complex reflexes involve highly integrated activity
on the part of many muscles. They result in well-coordinated
movements which give the impression of higher nervous control
and purposiveness. It is possible to demonstrate, however, that
such movements are entirely involuntary. If, for example, the
brain of a frog is removed under anesthesia, the animal may be
placed on a table and after a few minutes will be found capable
of maintaining a normal posture. When mildly stimulated by
touching an electrode to the back, the decerebrated animal will
hop away exactly as a normal frog would do under similar
conditions. There can be no question of sensation or volition
in the case of the decerebrated frog, since the centers of sensory
perception and voluntary control have been taken away.

Reflexes may be classified in several ways, depending upon
the point of view of the person doing the classification. An
anatomist classifies them on the basis of what level or levels of
the spinal cord or brain are involved or what pathways in the
central nervous system are followed and to what extent. Physio-
logically, reflexes may be grouped according to the kind and
location of the receptors involved. Thus one group of reflexes is
called forth by stimulation of the special sense organs; another
group is initiated by stimuli arising in the viscera, or internal
organs; and still another has its origin in receptors located in the
muscles, tendons, joints, or parts of the ear having to do with
the positions of the body and its parts. Finally, there is a psycho-
logical basis for classification of reflexes as unlearned, innate,

«0

THIS LIVING WORLD

>r "unconditioned" types; and learned, acquired, or "condi-
tioned" types.

•Axon

^filaments

Association Paths

The brain, spinal cord, and certain
ganglia have been compared with
switchboards in a central telephone
exchange in that they provide for the
transfer of nerve impulses from one
neuron to another. In these nerve
centers the terminal filaments of the
axon of one nerve cell lie closely to
the dendrites of another. There is no
direct physical connection, but nerve
impulses are conducted from the axon
of the one cell to the dendrites of the
other. This kind of nerve bridge is
known as a "synapse," and whenever
neurons are brought into such relation-
ship a synaptic connection is estab-
lished. The manner in which nerve
impulses are conducted across this
bridge is not definitely known. Some
evidence shows the mechanism is
physicochemical in nature; that is, the passage of the nerve im-
pulse across the synapse is accompanied by colloidal phase
reversal in the protoplasm of the nerve cell, not unlike the
hardening or coagulation of egg white by heat. In contrast to
the changes produced in egg white by cooking, the colloidal
changes in the nerve cell are reversible.

Even the simplest reflex action involves at least one synaptic
connection. Complex reflexes may involve very many such con-
nections. It should be noted that synapses are seldom, if ever,
wholly independent of one another. A sensory nerve may have
several end branches in its axon which excite a number of differ-
ent nerves through many synaptic connections. Just which
synapse will be used and which response will be made depends
upon many conditions. In general, the more a synapse is used,
the more readily it is made, and so a given response will

A synapse is a kind of nerve
>ridge. It is a point where the
terminal filaments of the axon
>f one nerve cell lie close to the
iendrites of another.

CORRELATING MECHANISMS

431

Complex reflex action and a precise coordination of eye and muscular movements
result From a high degree of nerve organization in the cerebrum, cerebellum, and spinal
cord. With a swish of his skates and a lunge of his body, Dave Kerr of the Rangers hockey
team grabs the flying puck to prevent a score by the opposing team. (Life Magazine.)

follow a given stimulus. This is the way in which a habit is
established.

Each synaptic connection makes possible some different
response or mental process. In the brain alone there is a possi-
bility of a very large number of these nerve connections and
hence as many different mental activities. It is possible to calcu-
late the total number of different synapses which could occur in
an average human brain. This has been done by Professor C. J.
Herrick of the University of Chicago in his interesting book,
"The Brains of Rats and Man/' The figure necessary to express
this number would require 2,783,000 places. That is, it is 10
raised to the 278,300th power. This means that it is possible for

432 THIS LIVING WORLD

everyone to know that many things or to go through that many
mental processes. Very few people ever develop and use all their
mental capacities.

When synaptic connections have been established, they build
up what are called " association paths " ; that is, certain reactions,
motor or mental, tend to follow given stimuli and these stimuli
may be from a physical sensation or they may be memories,
former experiences, or impressions. Association paths are not due
to simple or single nerve bridges, but usually involve many such
connections, so that our forms of behavior become very complex.

However, most of the things a person does, the emotions he
feels, the attitudes he has, result from the association paths
which are developed in his brain as he acquires his experience
and his education. For example, one's sentiments of or attitudes
toward patriotism result from various association paths which
have been formed in his brain. A person's method of work or
study, whether careless and lax or thorough and accurate, is in
large measure the result of definite types of association paths.
In other words, normal behavior is partly explained on the basis
of the establishment of certain kinds of synaptic connections.

The Brain

In the evolution of higher animals from lower ones there has
been a marked tendency for the nervous centers to migrate to the
head region, which thereby has come to exert ever greater control
over the rest of the body. This tendency has reached its culmina-
tion in the vertebrates, and especially in man, with the con-
centration of central nervous elements to form a brain.

In structure, the brain roughly resembles the spinal cord.
Indeed, it may be regarded essentially as an extension of the
cord into the skull or head region, since in embryonic develop-
ment it arises as an expansion of the head end of the hollow tube
from which the spinal cord differentiates. This expanded portion
of the early embryonic nerve tube may be called the "brain
stem." It soon becomes partially divided off, as shown in the
drawing, so that three regions are recognizable. These are the
forebrain, midbrain, and hindbrain. They persist in the adult
animal, where they can be distinguished on the basis of structural
differences and differences in function, as well as from their mode

CORRELATING MECHANISMS

433

of embryonic origin. The brain stem is the primitive brain of
lower vertebrates as contrasted with parts acquired more re-
cently in the evolution of higher forms, Ce]ebra, hemispheres
especially monkeys, apes, and man. In
these higher vertebrates, control of the
body is taken over largely by the cerebrum
and cerebellum, which arise in the embryo
as paired outpocketings from the upper
right and left sides of the brain stem. The
cerebrum is derived from the forebrain
region, as shown in the drawing, while the
cerebellum develops from the hindbrain.
The outgrowths extend rapidly to the
sides, upward, and backward, until they
cover the primitive brain almost com-
pletely. The greatest development occurs
in the cerebrum, which comes to overlie the
greater part of the cerebellum.

The gray and white matter of the brain
are reversed from the positions they occu-
py in the spinal cord. The gray matter
forms the cortex at the surface of the brain,
whereas in the cord it lies inside the white matter. The latter is
made up primarily of fiber tracts from the spinal cord, which ex-
tend into the brain, and an exceedingly complex network of fiber
tracts, which connect the various brain centers with one another.
The cortex contains numerous "centers," or groups of nerve-cell
bodies, which give it its characteristic gray color. The gray
matter also contains the cell bodies of the motor nerve compo-
nents of twelve head nerves. These nerves supply the special
sense organs and associated structures responsible for their ad-
justment and movements. The sensory components of the cranial
nerves arise from nerve cells located in ganglia lying either just
outside the brain or in the sensory structures themselves.

The brain stem is the seat of numerous reflex centers in
lower vertebrates. The centers which control respiration, heart-
beat, swallowing, vomiting, and various other visceral reflexes
are located in the hindbrain. Here also are found the centers
controlling the postural reflexes having to do with balance and

•Spinal cord

In a horizontal section
of the developing brain,
three regions are recog-
nizable. (After Carlson and
Johnson, "The Machinery
of the Body/')

434 THIS LIVING WORLD

regulation of the body's upright position, with the sensory com-
ponents of the nonacoustic branch of the auditory nerve being
distributed to the postural reflex centers. The midbrain con-
tains the centers of visual and auditory reflexes, that is,
adjustments of the eyes in focusing, regulation of the pupil diame-
ter and regulation of the tension on the eardrum. In lower
vertebrates the reflex centers for general muscular tone and
posture are also located in the midbrain. In monkeys, apes, and
man the centers controlling muscular tone are moved up into
the forebrain.

The chief function of the cerebellum relates to the coordina-
tion of skeletal muscular activities. Its size is roughly propor-
tional to the complexity of movements of the skeletal muscles.
When the surface of the cerebellum is stimulated artificially by
the application of electrodes, muscular responses are called forth
which, as far as observed, are limited to skeletal muscles. The
movements elicited are rather generalized in character. Ap-
parently, there is never any actual sensation associated with
stimulation of the cerebellum, and its destruction by disease or
accident produces no sensory defects. There is a certain degree
of localization of function. Thus stimulation of the mid-region
produces movements of the head, neck and trunk, while stimula-
tion of either hemisphere causes no movements of the limbs on
the corresponding side of the animal. This localization is not
associated, however, with any visible differentiation in internal
structure.

Persons whose cerebellum has been damaged by disease ex-
hibit certain definite defects of muscular activity, but there is no
real paralysis; that is, movements are still possible, but they are
not well coordinated. Movements which are ordinarily performed
smoothly and surely are done hesitantly or jerkily. They are
broken up into their component movements. Movements re-
quiring delicate coordination, such as writing, drawing, or picking
up small objects, cannot be performed at all. Speech may be
impaired, and there frequently is disturbance of balance. Experi-
mental destruction of the cerebellum in lower animals yields
similar evidence of its coordinating and integrating function. If,
for example, the cerebellum of a pigeon is removed, the bird is
unable to walk or fly, although movements of the legs and wings

CORRELATING MECHANISMS

435

These ganglion cells from the cerebrum are
a particularly complex type, having many
branched fibers extending out from the cell
proper. (Photomicrograph by Roy Allen.)

still occur. These movements, however, are quite uncoordinated,
and the bird merely thrashes around, beating its wings aimlessly.

Localization of Function in
the Cerebrum

The part of the brain
which remains to be dis-
cussed is the cerebrum,
concerned with sensory per-
ception and the so-called
"higher mental processes."
The possession and use of
this structure have served,
more than any other factor
or factors, to place man
definitely above the lower
animals. It is chiefly in the
degree of differentiation and
functioning of the cerebral
hemispheres that a sound physical and quantitative basis can be
found for setting man apart from the great apes. In the structure
and function of other bodily systems, man is essentially like the
least specialized of the lower forms. Even in the functioning of
the cerebral cortex the difference between man and his nearest
animal relatives is largely one of degree. The capacity to learn,
that is, to modify behavior according to experience, is found in
all vertebrates and even in a rudimentary way in lower forms.
Thus nearly everyone is familiar with unmistakable instances of
learning in domestic animals such as the horse, the dog, and
the cat.

What is customarily referred to as intelligent behavior is cor-
related in a very general way with certain anatomical features
of the cerebrum. Thus the functional potentialities of the brain
are roughly indicated by its size. The total volume of the normal
human brain averages about 1,500 cubic centimeters, whereas
for the highest apes the corresponding figure is about 600 cubic
centimeters. It is not the absolute size of the brain which is
important, however, nor even the relative size in proportion to
that of the body. Obviously, the ratio of the volume of a rat's

436 THIS LIVING WORLD

brain to his body is greater than the corresponding ratio for a
man. The most significant index of brain size seems to be the
ratio of the weight of the brain to that of the spinal cord. This
ratio is less than 1 in lower vertebrates; from 2 to 4 in lower
mammals; about 15 in apes; and 55 in man!

Another general anatomical index of the complexity of cere-
bral function lies in the complexity of the surface convolutions
of the brain. The ridges and fissures which mark the cerebral
cortex are produced as a result of the more rapid growth of this
region than of the underlying parts, throwing the surface into
folds. In general, this differential growth and consequent folding
has occurred to a lesser extent in the brains of lower vertebrates,
which therefore have a smoother brain surface than does man.
The difference in degree of complexity of the convolutions is
not great enough, however, among different people of human
races to reflect significant differences in intelligence. While on
this basis the brain of an absolute idiot may be recognizable,
nevertheless even a trained specialist cannot distinguish between
the brain of an average individual and that of a genius.

Among the brain convolutions, certain deeper fissures divide
each cerebral hemisphere anatomically into four large areas or
lobes — the frontal, parietal, occipital, and temporal lobes — as
shown in the accompanying drawing. Superficially, these regions
are indistinguishable in structure. Internally, however, there are
definite differences, both in composition and in arrangement,
which are evident on microscopic examination. Even within each
of these four major areas there are local differences, which have
been correlated with differences of function in some instances.
It must not be thought, however, that different kinds of emo-
tions, feelings, and thought processes, such as judgment or
mathematical ability, are located in special brain areas. This be-
lief was once rather widely entertained and is the basis of the
idea of phrenology or "science of the bumps." It is now known
that there is nothing to this. It is not possible, for example, to
tell by feeling the bumps on a person's head whether or not he
will make a great musician or statesman or whether he is a highly
emotional individual.

Modern views of localization of cortical function are based
upon experimental evidence from many different sources. One

CORRELATING MECHANISMS
Sensory area^Sfij femes)

Parietal lobe

Occipital
lobe

437

Motor area

Frontal
lobe

Vision

\
Cerebellum Temporal lobe

From the combined information obtained as a result of clinical studies and several
types of experiments, it has been possible to map out certain areas of the cerebral cortex
and to assign to them definite general functions.

of the most valuable methods of study has been observation of
the effects produced by artificial stimulation of specific regions
in the exposed cortex of anesthetized animals. Thus, when a
particular group of muscles respond by contraction or relaxation
upon stimulation of a certain local area of the cortex, it is reason-
able to suppose that this area controls the movements brought
about by these muscles. Stimulation of certain areas under local
anesthesia has also been practiced with human subjects. Under
these conditions, when the conscious subject responds with the
statement that he sees light or hears sounds, it seems probable
that the sensory areas for visual or auditory perception have
been located. Another fruitful source of information has been
9bservation of the behavior of animals following surgical removal
of a part of the cortex. In this way it was found that complete
muscular paralysis results from the removal of certain areas of
the parietal lobe, which must therefore control the skeletal mus-
cles. A closely related method of study has been the observation
and analysis of human behavior where post-mortem findings
have revealed destruction of local areas of the cortex due to
disease or accident.

From the combined information obtained by these different
methods it has been possible to map out certain areas of the

438 THIS LIVING WORLD

Recording the brain waves of a student at New York University. (Herman Young.)

cerebral cortex and to assign to them definite general functions, as
shown in the foregoing drawing. It will be noticed that the func-
tions whose areas have been established are largely sensory or
simple motor types. The higher mental processes, so far as is now
known, take place over a large part of the entire cortex or surface
of the cerebrum.

Brain Waves

One very unusual condition of the brain has been discovered
in recent years. This is that there are constant low-frequency
electric vibrations in the brain, in addition to the regular nerve
impulses which pass through it. These vibrations are the widely
heralded brain waves, frequently referred to as the "Berger
rhythm." These electric vibrations in the brain of man were dis-
covered by Dr. Hans Berger of Germany in 1929. Since that time
many research workers in various countries, particularly in the
United States and England, have studied these waves inten-
sively. It is now known that if you could view the working of
your own brain, as well as that of any other person, it would be
possible to see these small electric waves emanating from it.

The brain waves are detected and recorded by an extremely
sensitive electric apparatus. Such equipment consists of small

CORRELATING MECHANISMS 439

metal plates that are attached to the outside of the scalp by
means of adhesive tape or glue and are connected by wires to a
powerful electric amplifier. This amplifier strengthens the brain
currents to make them strong enough to operate an electrically
driven recording device, such as a writing pen. The amplifi-
cation required for this purpose is of the order of about ten
thousand billion times. Under such conditions it is desirable to
have the person screened in from all outside electrical disturb-
ances. The recorder can be made to write the brain wave
record on a strip of paper or a photographic film. Such an
apparatus is usually referred to as a brain-wave machine, or
electroencephalograph.

By using such equipment it is possible to show that the brain
waves of different normal persons clearly differ from each other
in character. In general, however, all such records have certain
basic features in common. The most prominent waves have a
frequency of 9 to 12 vibrations per second and are known as the
" alpha " waves. The most extreme variation from these is seen
in certain individuals who have waves of a frequency from 25 to
35 complete waves a second; these are called "beta" waves.
Most normal people come within these two classes.

A large proportion of people who have had their brain waves
tested show the alpha rhythms. In general it seems that it is the
people who are living and working under considerable mental
tension that have the beta waves, while most others have the
alpha waves. However, not enough is yet known about the brain
waves to attempt to use them for any sort of classification of
people according to mental temperaments.

The alpha waves have the unique property of responding to
light and sound stimuli; the beta waves show no such response.
Suppose that a person who has the alpha type is having his
brain waves measured. It is found that the waves are most pro-
nounced when the person sits completely relaxed with his mind
in quiet repose and his eyes closed. The room should be free from
noise and the mind not engaged in any concentrated thinking.
Under such conditions the electric pen writes a wavy -line record
of the vibrations that surge from the brain in wave-like rhythm.
Should the person open his eyes the alpha waves will disappear,
and when the eyes are closed the waves begin again to come

440 THIS LIVING WORLD

/VUto/viAA/\AA/^

Alpha brain-wave record secured under conditions represented in the drawings beneath
the three variations in the wave form.

through in regular fashion. The sudden ringing of a bell or other
pronounced sound causes the waves to die out while the sound
is in progress.

Another unique property of the alpha waves is that they are
affected by concentrated thinking, such as working a mental
arithmetic problem. The waves die out as soon as a person begins
to puzzle through the problem. When the problem is solved and
the mind again relaxes, the original waves return. It is possible
to tell from the brain waves when a person begins to do concen-
trated thinking and when he stops, but it is not possible to tell
what he is thinking. Professor Lee Travis of the University of
Iowa reported in 1938 that he could measure when students were
daydreaming or were paying close attention to the class lecture.
His deductions were, of course, based upon the degree of change
in the alpha rhythm, as it is known that the extent to which
waves are neutralized is in proportion to the degree of concentra-
tion of thinking.

In extensive studies of brain waves made by Doctors Hallo-
well Davis, F. A. Gibbs, and William Lennox of the Harvard
Medical School, it has been shown that people with various mental
diseases have brain-wave patterns which are typical for a given
disease and which are entirely different from the waves of normal
people; that is, for example, when a person has an epileptic
seizure his brain-wave pattern is completely changed. It has
been found that these changes are so pronounced and charac-
teristic that it is possible to predict when a person is going to
have a seizure, in some cases many hours in advance, by studying
his brain waves.

CORRELATING MECHANISMS 441

Brain-wave records are now being used to discover and to
locate brain tumors where other methods have failed. A brain
tumor or other cause of abnormal pressure at some spot on the
brain tends to produce characteristic changes in the brain waves.
One case is reported from the Harvard Medical School of a
patient suffering from dementia praecox who was sent in for
routine measurement of his brain waves. Careful study of these
waves indicated that the patient was suffering from some
physiologically abnormal condition. The electrodes were then
moved from place to place over the head until a spot was reached
where the brain waves showed a definite change. An X-ray
photograph later revealed a tumor, which was removed by an
operation.

The meaning and significance of the brain waves are at
present not understood with any degree of certainty, so recent
and incomplete is this investigation. It is certain that they are
not representative of a person's conscious mental processes, such
as receiving stimuli or thinking. They are not related to degrees
of intelligence of normal people., Dr. George Kreezer of Cornell
University reported in 1939 that he had discovered that highly
intelligent persons have brain-wave patterns that are different
from those of persons of average and low mentality. However,
his findings have not been verified by others. In fact, other
experimenters have failed to find any relationship between brain
waves and intelligence. Dr. R. W. Gerard of the Uniyersity of
Chicago Medical School believes that these waves show the brain
is in continuous action; that is, it is a dynamic organ. He says
that all individual nerve currents which terminate in the brain
act together to produce the waves and that the body is inte-
grated into a single unit by the nervous system, with the brain
dominating all other parts electrically.

Chemical Agents of Body Control

In the opening paragraphs of this chapter the brain, spinal
cord, and nerves were compared with the telephone as a great
system of communication and control. Two other means of
bodily intercourse were mentioned, which operate together to
regulate many activities within the organism. The circulatory
system was compared with the railroads as a means of transpor-

442

THIS LIVING WORLD

General location in the body of the ductless glands as shown at the New York World's
Fair in 1939. (American Museum of Health.)

tation. The other system, which utilizes the great circulatory
network as a carrier, comprises the endocrine organs or ductless
glands. These have been compared with the national postal
system. Together they constitute an exceedingly delicately
balanced and complex regulatory mechanism. Although separate
and definite functions have been assigned to the different glands,
it seems highly improbable that any one of them ever acts alone.
Changes in the functioning of one endocrine organ have been

CORRELATING MECHANISMS 443

found to affect others, and these in turn to affect still others,
so that a disturbance in any one may upset the whole system.

The ductless glands take their name from the fact that they
do not discharge their products directly through tubes or ducts.
Instead, these products are absorbed into the blood circulating
through the capillaries in the glands. The secretions of the
endocrine organs are chemical substances which circulate with
the blood and produce important and profound effects, not only
upon various parts of the body but also upon the growth and
functioning of the entire organism. These chemicals are usually
referred to as hormones. There are probably very many hor-
mones, but the most important are those produced by the
pituitary gland, the thyroid and parathyroid glands, the
thymus, the adrenals, the pancreas, and the sex glands. Of
these, the pancreas and sex glands perform special functions as
ducted glands besides this one of internal secretion. The others,
however, seem to have no other work.

The pituitary gland is located just beneath the brain in the
floor of the skull. It is sometimes referred to as the driver gland,
since it exerts a positive control over many of the organs and
activities of the body. The gland consists of two separate and
distinct parts known as the "anterior lobe" and "posterior
lobe." Each of these secretes one or more chemical substances
or hormones.

One of the hormones of the anterior lobe regulates the growth
of the bones, especially the long bones of the limbs and ribs.
Abnormal activity of the anterior lobe with respect to produc-
tion of this hormone results in excessive growth of the bones.
If this occurs after adulthood has been reached, it is charac-
terized by an overgrowth of the bones of the head, hands, and
feet. If it occurs during childhood, the result is an excessive
growth of the limbs and trunk bones, producing gigantism.
Arrested functioning of the anterior lobe causes a lack of
development of the bones of growing children, resulting in
dwarfism.

Another of the anterior-lobe hormones controls the develop-
ment and normal functioning of the sex glands. Deficiencies of
this hormone are associated with a lack of development of the
reproductive organs and secondary sexual characteristics in

444

THIS LIVING WORLD

young persons, with atrophy or regression of these in adults.

In addition, the anterior lobe of the pituitary secretes hormones

which regulate the growth and
functioning of the thyroid
gland and adrenal bodies and
another, called prolactin,
which controls the develop-
ment of the mammary glands
during the later stages of
pregnancy.

The posterior lobe of the
pituitary operates to control
general bodily welfare. It
secretes a hormone which
causes constriction of the
blood vessels and brings about
a rise in blood pressure. This
substance also regulates the
water balance of the body
through its effect upon the se-
cretion of urine and upon milk
flow of the mammary glands
in women immediately pre-
ceding and following the birth
of a child. The posterior lobe
also produces a hormone
which stimulates smooth mus-
cle to contract.

The thyroid gland is sit-
uated in the front part of
the neck. It consists of two
large lobes, one on either
side of the larynx. The chem-

Robert Wadlow at the age of twenty-one
was 8 feet 8.5 inches tall, and at the time of
his death, when he was twenty-two, he had
grown to a height of 8 feet 10.3 inches, as
a result of abnormal activity of the pituitary

gland. (International News.) ical which it secretes is known

as "thyroxin." This substance

is peculiar among bodily products in that it contains a large
amount of iodine, sixty-five per cent by weight. The chief
function of this chemical seems to be to regulate the basal metab-
olism of the body. In this way it regulates every activity of the

CORRELATING MECHANISMS 445

individual, including mental development and ability. It has
been said that less than one two-thousandth of an ounce of
thyroxin is all that stands between Einstein and imbecility. The
same applies to every normal person. Deficiency of iodine in
the diet disturbs the balance of the thyroid gland. Iodine is
necessary for the production of thyroxin, and a lack of sufficient
iodine in the food of a person usually results in enlargement of
the gland. This condition is known as goiter. Obviously, it can
be corrected by supplying iodine in the food.

Failure of the thyroid gland to develop properly or its
atrophy in later life produces serious consequences. A child
whose thyroid fails to develop and function properly becomes a
"cretin." Such individuals never grow up. Although they may
live to be thirty years old, they present a childish physique.
They are dwarfed, pot-bellied, and ugly, with the mentality of a
child of four or five years. If the condition is discovered soon
enough, and if it is not too severe, it may be corrected by feeding
thyroxin or the fresh thyroid glands of a sheep or a calf. Atrophy
of the thyroid in later life produces mental dullness and obesity
associated with a lowered metabolic rate.

Overfunction of the thyroid leads to an excess of thyroxin in
the blood. This condition is associated with improper regulation
of thyroid activity by the pituitary gland. It results in an in-
creased metabolic rate, leading to nervousness, increased blood
pressure, and enlargement of the thyroid itself so that the gland
presses against the blood vessels and nerves in the neck region.
This causes irregular heart action, a pronounced protrusion of
the eyes, and the various other symptoms of exophthalmic goiter.

The parathyroid glands are four small bodies which are
situated near the thyroid, two on each side. Their function is
entirely different from the thyroid, however. One of the hor-
mones secreted by these glands regulates the development of the
bones. It does this by controlling the calcium content of the
blood and thereby the rate of deposition of calcium carbonate in
the bones. Malfunctioning of the parathyroids results in abnor-
mal bone growth. Removal of the parathyroid glands causes a
condition of excessive excitability, resulting in violent muscular
contractions called "tetany." This can be treated by administer-
ing an extract of the glands.

446 THIS LIVING WORLD

The thymus gland is present in the infant. It reaches its
greatest relative development shortly after birth and gradually
disappears as adulthood is approached, its substance being re-
placed by fatty tissue. It is situated below the thyroid and lies
mostly in the thorax. Its secretion seems to affect metabolism,
especially that of the sex organs. Just how it works is not well
known. Recently it has been determined that repeated injections
of thymus extracts in experimental animals accelerate the growth
and attainment of sexual maturity. When the treatment is con-
tinued for several generations, sexual maturity comes progres-
sively earlier. However, removal of the thymus gland in young
animals seems to have little or no effect upon development. Its
exact function in the body is far from being well understood.

There are two adrenal glands, one located just above each
kidney. Each gland has two parts. The inner part secretes
"adrenalin," which controls the action of the heart under fear,
anger, and similar emotional stresses as well as under normal
conditions. This secretion also circulates through the liver and
stimulates the release of carbohydrates for the muscles. Under
conditions of emotional excitement adrenalin is discharged into
the blood and produces the following results: (1) an increased
heartbeat, (2) greater flow of blood to the brain and muscles,
and (3) an increased discharge of muscle food from the liver.

The outer part of the adrenals, called the "cortex," produces
a different chemical, "cortin." This hormone is indispensable to
life. A gradual failure of the cortex to function produces a fatal
condition known as "Addison's disease." It is characterized by
physical languor, anemia, feeble heart action, a peculiar bronze
discoloration of the skin, and eventually death. Removal of the
cortex in experimental animals is soon followed by death, unless
the cortical hormone is regularly injected. Cortin has a decided
influence on the sex organs and the secondary sexual charac-
teristics. It is not known exactly how this effect is produced.
An overaction of the cortex in childhood leads to a condition
in which there is a precocious sexual development. The male
element is usually emphasized regardless of the sex of the child.
A female child under these conditions develops a masculine
voice, has the male distribution of body hair, and fails to
menstruate. Should such overactivity develop after a woman

CORRELATING MECHANISMS 447

has reached adulthood, somewhat the same conditions are
observed, particularly growth of facial hair, masculine voice,
and radical change of the reproductive organs.

The pancreas, in addition to secreting the powerful digestive
fluid, pancreatic juice, also acts as a ductless gland. It produces
an internal secretion, called "insulin," which is absorbed into
the blood stream. Insulin is necessary in the blood for the nor-
mal metabolism of sugar. If sufficient insulin is not present, the
sugar cannot be used by the body and it accumulates in the
blood and is excreted in the urine. These are the symptoms of
the disease called "diabetes." In severe cases the body rapidly
wastes away and the patient dies in a diabetic coma. Insulin
may be prepared from the fresh pancreas of other animals. This
prepared insulin may be injected into the blood of a diabetic
patient, eliminating the symptoms of the disease. However, in
order to be effective, it is necessary that the insulin be injected
three times daily as long as the patient lives or suffers from the
disease. Insulin does not cure diabetes, it corrects the distressing
symptoms of the sufferer.

The sex glands, or gonads, are the testes of the male and the
ovaries of the female. They produce the germ cells necessary for
reproduction. In addition, they secrete hormones which control
the secondary sexual characteristics. Among these are the deep
voice and facial hair of men; the broad hips and well-developed
breasts of women. Removal or maladjustment of the gonads
always results in definite changes in these secondary sexual
characteristics. In addition to the hormone controlling develop-
ment of the secondary sexual characteristics, the ovaries in the
female secrete another substance which is concerned with the
phenomena of pregnancy. This is the substance called "pro-
gestin," which causes enlargement and vascular congestion of
the uterus in preparation for implantation of the fertilized egg.

Within recent years the sex hormones have been isolated and
chemically analyzed, so that their composition is known. In
addition, these hormones have been made synthetically in the
laboratory. It has been found that they are closely related
chemically to substances capable of producing cancers in experi-
mental animals. Gland chemicals, such as thyroxin, adrenalin,
and sex-gland chemicals or other such substances sometimes are

448 THIS LIVING WORLD

used as medicines. They may be very useful. Apparently, they
are the same in all the higher animals, and animal extracts are
the ones which are used in medicine. However, gland chemicals
are powerful and dangerous. No gland preparation should be
taken except upon the advice and under the care of an expert
physician.

REFERENCES FOR MORE EXTENDED READING

STILES, P. G.: "Human Physiology/' rev. by G. C. Ring, W. B. Saunders
Company, Philadelphia, 1939, Chaps. VII, VIII, IX, XII, XXVIII.

In these chapters are to be found a clear and concise description of the nervous
system and its function of coordinating the body processes, together with a brief
account of the endocrine glands and their secretions.

BEST, C. H., and N. B. TAYLOR: "The Human Body and Its Functions,"
Henry Holt & Company, Inc., New York, 1932, Sees. VII, IX.

There is much in this text for hospital and public health nurses that will be of
interest to the intelligent layman. The sections referred to deal with the structure and
coordinating function of the central nervous system and the effects of the endocrine
secretions on coordination of growth and functioning of the body. They are clearly
and simply illustrated.

CARLSON, ANTON J., and VICTOR JOHNSON: "The Machinery of the Body,'*
University of Chicago Press, Chicago, 1937, Chap. X.

This chapter contains a thorough elementary presentation of the anatomy and
physiology of the nervous system.

MITCHELL, PHILIP R. : "A Textbook of General Physiology/' McGraw-Hill
Book Company, Inc., New York, 1932, Chaps. III-V.

The chapters to which the student is referred contain a detailed account of the
nature of nerve cells, nerves, nervous transmission, reflex action, and the central
nervous system.

ROGERS, CHARLES G.: "Textbook of Comparative Physiology," McGraw-Hill
Book Company, Inc., New York, 1938, Chap. XVII.

An interesting account of the regulatory mechanisms of the body written from the
comparative viewpoint.

STARLING, ERNEST H.: "Principles of Human Physiology," 5th ed., Lea &
Febiger, Philadelphia, 1936, Chaps. VI, VII.

These chapters contain a complete detailed account of the nervous system, except
the sensory receptors.

CAMERON, A. T.t "Recent Advances in Endocrinology," P. Blakiston's Son
& Company, Inc., Philadelphia, 1934.

This is an excellent survey of endocrine glands and their secretions, together with
their effects on body growth and functioning. Some of the clinical aspects of endo-

CORRELATING MECHANISMS 449

crinology are also discussed. The book is a relatively nontechnical treatment of the
knowledge available in this field up to 1983.

References to brain-wave investigations may be found in a number of pro-
fessional journals, among which are the following: Brain, Vols. 57, 58;
Journal of Experimental Psychology, Vol. 19; Journal of General Psychology,
Vol. 14; Science, Vols. 81, 82, 84, 85, 87, 90; The Lancet, August, 1936. A
popularized summary was published in The American Weekly, May 21, 1939.

Endocrinology, published by Association for the Study of Internal Secretions,
Harvard Medical School, Boston.

This is a monthly magazine which contains a review of current endocrine literature
and reports on clinical and experimental endocrine medicine and biology.

15: KEEPING WELL

Through a Knowledge of the Nature and Treatment of Disease

IN THE fifth chapter of the Book of Job it is written that
"Man is born unto trouble." This revelation not only pen-
etrates the extent of human suffering but also implies one of the
fundamental laws of life from which man is not immune. This
is the fact that animal life exists by feeding upon other living
things. When one creature attempts to feed upon another, trouble
or disease usually overtakes the victim selected. If the attempt is
successful, death results for the creature serving as food. Man's
body is continually being invaded by innumerable living organ-
isms which attempt to live there and feed upon his tissue sub-
stance. When this invasion is successful, man experiences trouble.
Some of his body tissue is destroyed, and disease results. Even

450

KEEPING WELL 451

death may occur. However, in most cases the human body is able
to overcome and kill the foreign organisms before they have
caused him noticeable trouble, and thus he remains healthy.

Many other disturbances also bring about human disease.
The complicated physical structure which constitutes the body
requires constant adjustment and repair in order to function
properly. If the cells and organs do not maintain perfect balance
and adapt themselves harmoniously to their internal and ex-
ternal environment the abnormal conditions interfere with the
life activities of specific cells or the entire body. A variation from
this balance and the proper internal environment may result
from long-continued abuse of some particular organs by im-
proper health habits. In such cases, disease of some nature is
likely to overtake the individual. It must be said, however, that
the human body is able to adjust itself to a wide variety of con-
ditions, so that normally we enjoy health rather than suffer from
disease.

Causes of Disease

Occasionally we do get sick, and it is necessary for medical
science to come to our assistance. In such cases the treatment
administered is usually something to help the body cure itself.
In order to understand clearly and to appreciate the provisions
which the body has for warding off and curing disease, it is neces-
sary to know what causes disease, at least the more common
forms of disease. To explain what these causes are is not so simple
as to ask the question. In fact, there are a great many causes of
disease, since disease itself is not a simple thing. Rather it con-
sists of a great variety of complexities which come about when
there is some interference with the normal activity of the body
cells.

These causes may be some change within the internal en-
vironment of the body cells because of the improper functioning
of an organ or improper diet. Some diseases result from the in-
evitable wearing out and destruction of the body cells in old age.
However, the most common and widespread diseases are due to
tiny organisms, called "germs," getting into the body in large
numbers. The chief germs, as far as human disease is concerned,
belong to the group called "bacteria." This is one of the great

452 THIS LIVING WORLD

evolutionary groups of life on the earth which seems to lie about
midway between the plant and the animal kingdoms; however,
bacteria are usually classified as a form of plant microorganism.

Many of the diseases from which man suffers would be un-
known were this form of life not on the earth to keep continually
invading his body. Disease is, after all, an abnormal condition;
that is, it is abnormal in the sense that living organisms are so
constituted that, given proper food and a favorable environment,
they live their span of life without any serious trouble. But
when one organism begins to invest another and becomes a
parasite, then a struggle between the two begins which usually
results in one overcoming the other.

There are several instances in nature where two or more
organisms of different types exist in such close biologic associa-
tion as to be considered living together. However, they have so
adjusted themselves that they work under mutual cooperation
with no harm to each other and in some cases to definite advan-
tages. One type of such association is commensalism, which
means "eating at the same table." For example, the shark sucker
is a small fish which can attach itself to the body of the shark by
means of a sucker at the top of its head. Thus, it gets free trans-
portation and food discarded by the larger animal. In commen-
salism no harm results to either of the individuals as a result of
the association, and usually there may be some small advantage
to one of them. Symbiosis is an internal partnership in which
each assists the other in their mutual existence. One important
example is the bacteria that live inside the root tissues of legu-
minous plants and provide the plants with usable nitrogen and
carbon. In some respects the same is true of certain bacteria
which live inside the human digestive tract. The body supplies
them with food and a suitable environment in which to live, and
they repay by destroying certain strictly disease-producing
bacteria.

However, in most cases when one creature invades the body
of another there is a battle between them which continues until
one or the other is killed. This is true of many kinds of bacteria
which attack the body of man. The result is disease and death
for man, or the death of the bacteria in his body. The disease-
producing bacteria are here, and man has been unable to exter-

KEEPING WELL

453

iiiiirnTnTTHTnnTTTTTTl

Bacteria vary in size and shape.

minate those which are harmful to him. He probably will never
be able to do so. Therefore he should understand how best to
protect his body from them. Modern medicine has gone a long
way in that direction.

Nature of Bacteria

The bacteria have certain very definite characteristics. They
are extremely small, the smallest living creatures on the earth.
They are probably the simplest and most primitive form of life
now in existence. In some respects they are like the individual
cells of the body and in other respects more like one-celled plants.
However, they are much smaller and more competent at many
jobs. They are present everywhere on the earth. They are found
in the tropics and at the poles. They are on the mountain tops
and in the deepest sunless mines. The water people drink and the
food they eat carry them in great numbers. The dust particles
of the air are infested with them, as are the bodies of most all liv-
ing or dead and decaying organisms.

Their sizes and shapes are varied. Some are almost as large
as small one-celled plants. Others are too small to be seen with
the highest power microscope. Some are motile, while others can-
not move. Some are round spheres or are like disks. These are
different varieties of what are called "cocci" bacteria. Some are
like rods. These are the "bacilli" bacteria. Some are bent like a
roll, and still others are like a spiral thread. These are called

454

THIS LIVING WORLD

"spirilla." Many of them have projecting filaments somewhat
like the tail of human sperm cells; still others have many such

filaments. These are "flagel-
la" bacteria. Some of these
types are shown in the ac-
companying drawing. No one
knows exactly how many
kinds there are. There are
probably many millions of
different species of bacteria.
Bacteria may reproduce
in a fashion that is somewhat
different from that of other
cells. In general, the repro-
duction is by the process of

TL ... . t . , ... cell division which is com-

The causitive agent or tetanus or lockjaw,

shown above, is a typical sporeformins m°n to other living Cells,

bacterium. The spores are the round objects at However, when food is scarce

the ends of the bacteria. (Photomicro3raph by Qr other conditions are un.
Roy Allen.) .

favorable, a bacterium passes

into a peculiar condition. This is a condition in which a great
many tiny cell-like objects form inside the body of the bacterium.
They are called spores, or bacterial "seeds." The spores remain
in the body of the bacterium for a time. Then the cell disinteg-
rates and the spores are set free. When conditions are again
favorable, each spore develops into a bacterium of the same kind
as the parent.

These spores can withstand much more severe conditions,
such as extreme heat, extreme cold, and antiseptics, than can the
ordinary bacteria. There are some spores which can stand boiling
water for as long as sixteen hours. These are the Clostridium
botulinum, which may cause poison in canned foods. Spores can
withstand great drying for months or even years. Spores are one
form in which the disease germs float in the air. In this condition
they may exist for many months. Such spores may travel with air
currents over large portions of the earth. There is a record of
bacteria spores having been blown from Australia to New
Zealand, a distance of over 2,000 miles.

KEEPING WELL

455

Different bacteria produce many chemical products.

When food is plentiful and other conditions are favorable, the
bacteria reproduce by a straightforward process of cell division.
This process is very rapid. It has been calculated that if a single
bacterium reproduced by cell division at the rate of one division
per hour under favorable conditions so that all could remain
alive, in three days the resultant mass would equal 7,000 tons.
If they reproduced at their normal rate, which is much faster
than one per hour, and all could remain alive, in three days their
total volume would equal the volume of the earth. One germ
dropped into a glass of milk would accomplish this amazing re-
sult, if it were not stopped somewhere along the line. Of course,
something always does stop bacterial growth. Usually this is an
overcrowding of the bacteria in the medium; for example, in a
glass of milk. Another limit is the lack of food. Like other crea-
tures, bacteria must eat in order to grow and reproduce.

Bacteria are very able chemists when many different species
are considered together. They can decompose by chemical action
almost all kinds of organic tissue or substances. For example,
they can decompose cellulose, which is the woody fiber of plants.
They manufacture substances which sour milk, turn wine into
vinegar, or make flavors in different kinds of cheese and sauer-
kraut. Disease bacteria in the body produce certain chemicals
which poison the body, causing sickness or death. It is these
chemicals which cause the trouble in the case of most diseases.
This is the condition, for example, in diphtheria. In other

456 THIS LIVING WORLD

diseases, however, such as tuberculosis, it seems to be the slow
persistent work of the bacteria themselves which destroy the
tissues and produce death.

Bacteria have a most extraordinary manner of digesting their
food. It is done outside their bodies. All types of bacteria give off
digestive juices or enzymes that digest the food upon which they
live very much as do the human digestive organs. This food may
be dead plant or animal tissue or even living tissue. The process
is the same. For example, the tuberculosis bacilli first digest
certain living parts of the human body, then absorb the digested
material into the bacterial body cell. Practically all bacteria
which invade the body absorb some protein from some of the
living cells. The absorbed protein is either assimilated by the
bacteria or oxidized for the release of energy.

Before closing the discussion of the nature of bacteria it
should be understood that not all bacteria are harmful; in fact,
many kinds of these tiny creatures are our helpers. Some bac-
teria, for example, destroy dead plant and animal matter, pre-
venting it from soon covering the entire earth. This is about the
only method by which organic refuse is decomposed and returned
to the simple inorganic compounds of the earth. If this were not
so, within a few years the earth's surface would be covered with
several feet of such dead material. The thickness of this layer
would increase as time went on, if life continued to exist on the
earth under these conditions, which, of course, it could not pos-
sibly do.

Again, certain bacteria in milk change the milk sugar to
lactic acid. The presence of lactic acid in cream increases the
yield of butter and improves the flavor. Lactic-acid bacteria are
necessary for the production of sour-milk cheeses, such as the
Swiss and Camembert varieties. The special flavors of these
cheeses is partly due to lactic-acid fermentation. Meat is made
tender by the action of bacterial enzymes. When meat is fresh,
that is, soon after the animal is killed, it is tough and more or less
tasteless. The enzymes attack the muscles, loosen the fiber of
the muscles and connective tissue, and produce chemical
compounds which give the meat flavor. So meat is usually kept
for some time to permit this action. However, the process must
not go too far, or products which are objectionable are formed.

KEEPING WELL

457

Plants which grow in the soil remove large quantities of ni-
trates. When these plants are removed by man or other animals,
they take with them the
nitrates in the form of pro-
teins. Soon the nitrates of
the soil would entirely be
depleted and the land be-
come unproductive were
there no way of replacing
it. This is accomplished
mainly by * ' nitrogen-fix-
ing" bacteria, which live
in the roots of clover, al-
falfa, and other similar
plants. These bacteria ab-
sorb nitrogen from the air
and convert it into am-
monia, which is used to
form their own proteins.
Upon the death of these
bacteria, their proteins
are converted into soil ni-
trates by other bacteria,
much the same as is done
with the proteins in the
dead bodies of other or-
ganisms.

"Nitrogen fixing" bacteria residing in the
nodules on the roots of leguminus plants aid in
restoring nitrates to the soil.

Infectious Diseases

Many of the diseases from which man suffers may be trans-
mitted from one individual to another. We refer to them as the
" infectious " diseases. They are produced when bacteria or some
other organisms invade the body and attack some tissues or
organs. Perhaps the most prevalent disease of northern climates
is the common cold. It is produced by a variety of organisms
which invade the soft tissues of the nose, throat, and eyes. One
of these organisms is a coccus type of bacteria; another is with-
out doubt one of the filtrable viruses. Once in these tissues in
sufficient numbers, these bacteria produce chemical products

458 THIS LIVING WORLD

that are poisonous to the body. Some of these circulate in the
blood, producing headaches and other aches of a cold. The efforts
of the body to remove these organisms and their poisons result
in discharges from the membranes, such as water from the eyes
and mucus from the nose and throat.

Pneumonia is a similar infection, really invasion, of the
tissues of the lungs by another species of cocci bacteria. Typhoid
fever is an infection of the lining of the bowel by a bacillus
bacteria known as the Bacillus typhosus. Diphtheria is an infec-
tion of the throat by still another type of bacillus. Spinal menin-
gitis is an infection by bacteria, too small to be visible under a
high-power microscope, of the spinal cord and a part of the brain.
Many other diseases are due to other kinds of bacteria. Different
diseases are caused by different kinds or species of bacteria.
These different species of bacteria respond to different treat-
ment and conditions; therefore each disease is a study within
itself. No patented medicine or general treatment is a*' cure-all"
for many different diseases.

Within recent years other organisms have been discovered
which are known to be the causes of some of the most serious
infectious diseases. These organisms are the filtrable viruses,
so called because they are so small that they will pass through a
porcelain filter. They are too small to be seen, even with a high-
power microscope. There are many kinds of filtrable viruses.
When they get into the body they cause infections or diseases,
each strain producing its own infection. Smallpox is a virus
infection. Measles is produced by another virus. Yellow fever
is another virus disease; so are influenza and infantile paralysis.

Syphilis is a disease produced by the presence in the body
of one-cell organisms called "spirochaeta." They have a sort of
corkscrew appearance when viewed under a high-power micro-
scope. These organisms are considered by some as being bacteria,
by others as being animal protozoans. They seem to be midway
between the two groups in their physical structure and develop-
ment. They will reside and flourish in many tissues of the body.
They usually enter the body through the thin membranes lining
the reproductive organs. However, they may enter through an
abrasion of the skin or protective tissue. They find their way
into the blood stream and then migrate to some favorable

KEEPING WELL 459

tissue, which they infest. Fortunately, however, for man the
spirochaeta can live only in a moist mucous medium. When
they are deposited where drying action occurs they soon die.
Infection from toilets, drinking cups, and crowded cars is not
common.

The disease manifests itself in three rather distinct phases,
unless it is properly treated. The first stage is characterized by
the formation of a small, hard elevation, called a "chancre," at
the point where the infection entered the body. This will persist
for a short time, then disappear. In the course of about two or
three months the second stage begins. The spirochaeta have
invaded other parts of the body. The lymph glands may swell,
or there may be a breaking out of a rash on the skin; also there
may be fever. Either one or all these symptoms may appear.
These symptoms usually disappear within a few weeks or
months, either with or without treatment. Without treatment,
however, the organisms start to migrate to the vital organs of
the body and later will begin to destroy these organs. This is the
third stage of the disease. Usually treatment then is not effec-
tive. They may cause hardening of the arteries and attack the
heart, kidneys, liver, stomach, and finally the brain. The result
is slow, lingering death.

Effective treatment of the disease is possible when it is
begun in the early stages of syphilitic infection. It consists
primarily of injection of arsenic compounds into the blood to
kill the organisms in the tissues. Such treatment must be con-
tinued under expert medical care over a period of years in order to
insure a cure. There is no other way. Since the symptoms of syphilis
are so elusive, it is very difficult for anyone to be sure he does not
have the infection unless he takes the Wassermann test. This is
a chemical test of the blood that is always accurate in determining
the presence or absence of the spirochaeta. In order to eradicate
the disease from the individual as well as the general population,
it is necessary that it be detected in its early stages and treated
in accordance with well-established medical practice.

Some diseases are due to animal parasites or protozoans.
Such, for example, is malaria. In this disease a small animal gets
into the blood, damages or eats the red corpuscles, and sets free
a poison in the blood. These protozoa usually are introduced

460

THIS LIVING WORLD

Blood test for syphillis. The cloudy white solution shows a positive test, while the clear red
(dark in photosraph) is nesative. (U. S. Public Health Service.)

into the blood by the bite of a mosquito carrying the micro-
organism. The discovery of this relationship stands out as one
of the great advancements in medical science. People of many
lands will long honor the memory of Major Ronald Ross for his
work in this respect.

Major Ross was a surgeon stationed with the British army
in India, where some three million natives as well as many
Englishmen were dying each year from malaria. He finally
discovered that, when a mosquito bit a person or a bird suffer-

KEEPING WELL 461

ing from the disease, tiny nodules developed in the mosquito's
digestive tract and that mosquitos biting persons not having

Photomicrograph of male hookworm by Roy Allen, showing body structure and sucker

foot at right.

the disease did not have such lumps. These nodules were dis-
covered to grow rapidly, then burst, and from them a swarm of
microorganisms would stream out to the glands and proboscus
of the mosquito. When such mosquitos were then allowed to
bite a person or a bird, the germs were deposited in the blood
stream, and malaria resulted. From these discoveries it is now
known that the most effective method of preventing the spread
of malaria is to destroy the breeding places of the mosquitoes.
Such practice has already made the disease relatively uncommon
in the United States.

African sleeping sickness is another similar disease. Here
tiny worm-like protozoa get into the blood and destroy the red
corpuscles, thus cutting off the oxygen supply of the body. The
brain and nervous system are the first to be affected by this
shortage of oxygen, hence the patient becomes more or less
unconscious long before death. The other organs needing a large
supply of oxygen are the kidneys. They begin to suffer and are
unable to perform their function of ridding the body of wastes
and regulating the composition of the blood. Death inevitably
results unless the organisms are removed from the blood before
too great damage is done.

One of the infectious diseases that has afflicted great popula-
tions in certain sections of the United States as well as many
foreign countries is the malady caused by hookworms in the
intestine. The hookworm is scientifically called Necator ameri-
canus, which means "the American killer." It is usually about
one-half an inch long. When they infect man, they penetrate

462 THIS LIVING WORLD

the body tissues until they reach the intestine. There the worms
grasp bits of the intestinal lining and suck in blood and tissue
fluids. They produce a sort of starving effect upon the individual.
Symptoms of the disease are anemia, laziness, and a general
lack of physical and mental vigor. Infection in children leads to
retardation of physical and mental development. The disease
is seldom fatal but it may spread through large populations,
causing them to live the dullest and most unproductive lives.
Hookworms are spread from one individual to another by
feces contamination. The eggs produced by the worms in the
intestine are excreted from the body in the feces. In certain great
areas of countries, particularly in China, India, and parts of
Southern United States, it had been the practice up to the
immediate past to leave human feces on the open ground. Hook-
worm eggs existing in such feces hatch into tiny larvae. These
larvae in crawling around on the ground come in contact with
the soles of bare-foot persons. They penetrate the skin and
eventually make their way to the digestive tract, where they
grow into adult hookworms and the cycle starts again. To
eradicate the disease it is necessary to teach the people to use
exclusively properly constructed toilet facilities. Some of the
earth's great unsung heroes have been the doctors, nurses, and
social workers who have influenced whole populations to stamp
out the disease by such practices.

Defenses against Bacterial Diseases

The body has four more or less distinct defenses against dis-
ease bacteria and a few disease protozoans. One of these is a sort
of protective wall, to be compared in some ways to the great
walls the ancient Chinese built around their cities and provinces
to protect them from foes who were less adept than their modern
adversaries. This protective covering consists of the skin and the
mucous linings of the lungs and digestive tract. The skin is a
tough, dry substance that is impervious to the tiny organisms
that continually swarm around us. Even the multitude of such
creatures that regularly get into the lungs, eyes, and stomach
have great difficulty in penetrating the membranes lining these
organs. These membranes secrete a mucus which entangles the
bacteria. Thus they are finally thrown off the body. In addition,

KEEPING WELL

463

One of defenses the body has against bacterial diseases is the skin, a sort of protective
wall that might be compared in some respects to the great walls the ancient Chinese built
around their cities and provinces.

the digestive tract secretes chemicals which are destructive to
the millions of bacteria that are taken in with the food we eat.

The next, and first internal, line of bodily defense consists
of the white corpuscles of the blood. They might be compared to
the policemen of a city, protecting its citizens against its danger-
ous characters. When bacteria in large numbers enter the body
at any point the white corpuscles gather there in large numbers.
They are probably summoned by the chemical poison in the
blood produced by the first invading bacteria. They endeavor to
remove the bacteria by eating them. In this process they usually
surround the bacteria with their body cells and digest them.
This is very much the same process as the amoeba follows in its
normal feeding.

This feeding process may kill the white corpuscles but the
"arrest" works and the body is rid for the time being of bacteria.
To be thus engulfed and eaten by the white corpuscles probably
is the lot of most of the billions of bacteria that we eat or drink
or inhale or get into cuts every hour of the day. Only when for
some reason the white corpuscles fail to do their job successfully
do we realize that a germ has invaded us at all.

In some cases, the white corpuscles can restrict an infection,
if not repel it. An abscess, for example, is usually an invasion
by bacteria of the staphylococcus type, which the white cor-
puscles have walled off with their own bodies. In this case the

464

THIS LIVING WORLD

This photomicrograph shows a white
blood cell which has engulfed a large
number of gonorrhea bacteria. The
bacteria are the smaller black objects.
(Photograph by Roy Allen.)

trouble is only local. The yellowish discharge from such an
abscess consists of a considerable amount of body tissue fluids

that have been carried to the
site by the blood, and the bodies
of millions of white corpuscles
which have given their lives in
the struggle. A boil is similar, ex-
cept usually the infection is by
a different staphylococcus bac-
teria.

The body's second internal
defense against bacteria is fever.
All body cells possess in some
degree the same power as the
white corpuscles to destroy
bacteria. These self-protective
powers, like the ability of a citi-
zen to fight a burglar if no police-
man is at hand or if he fails, are greatly increased by heat. The
human body cannot be heated to a high temperature with
safety, but it is all right to heat it to 102 or 103°F. The normal
temperature is 98.6°F. Accordingly, one of nature's defenses is
to heat the body by fever, because the body cells become more
active at higher temperatures. In this manner the germ-repelling
powers of the white corpuscles and other cells become greater.
The fever is believed to be caused by effects of the bacterial
chemicals working on the glands. Fever, when not too high, is
a desirable thing, therefore, when one is sick.

The body has still another, often more efficient, defense
against bacteria and some viruses. This is called immunity.
There are some diseases which one has only once. An example is
smallpox. In fact, one seldom has smallpox at all after once
having had a mild form of the disease produced by vaccination.
This vaccination is the process of putting into the blood stream
a small quantity of the smallpox virus after they have been
weakened or killed. The presence of the weakened germs causes
the body cells to manfacture a chemical substance that is de-
structive to smallpox virus should they later enter the body in
large numbers. Such a chemical is known as an antitoxin. Other

KEEPING WELL

465

disease viruses or bacteria stimulate the body cells to manu-
facture different antitoxins that are antagonistic to these specific
germs. Such a chemical de-
stroys the bacteria present.
Furthermore, the antitoxin
may remain permanently in
the blood stream and de-
stroy any future similar
organisms which enter the
body. This is a sort of pro-
tectivs chemical warfare
conducted by the body cells
against bacteria and similar
infectious agents.

This immunity is built

up temporarily when the T, , , , * ....

, , , | 'iv 'h* rod-shaped objects are anthrax bacilli

body has any bacterial dlS- ln ti$suc. Thls Organi$m also illustrates one fea-

ease. With many diseases ture of certain bacteria, namely, the ability to

it does not last. This is form a protective capsule oMatty material sur-

... , . ,. ,., rounding the cell. (Photomicrograph by Roy

illustrated in diseases like a >\llen.)
cold. The first day one has a

cold it spreads, attacks new tissues each hour, saturates the body
with its bacterial poisons. After a few days it stops spreading. One
begins to feel better. The body's cold antitoxins are getting to
work. This takes a little time, but soon one is cured of the dis-
ease. This immunity to colds soon disappears. After a few days
the bacteria and virus-repelling antitoxins have faded out of the
blood, and one can catch cold again. However, some diseases
produce an immunity which lasts the remainder of one's life.
Measles and diphtheria produce antitoxins which remain in the
blood stream more or less permanently. The organisms that
produce these diseases are usually not able, therefore, to infest
the body in sufficient numbers to cause the diseases a second time.
It is possible to bring about artificial immunity by injecting
into the body various serums or antitoxins. In treating diph-
theria, caused by one of the most virulent and deadly germs, an
antitoxin is injected into the blood stream of the person having
the disease. This antitoxin serum is the same chemical which the
body itself would produce to kill the bacteria, if it were given

466 THIS LIVING WORLD

time. However, the injection works much quicker. Without it the
person is usually killed by the bacteria before his body can pro-
duce sufficient antitoxin to destroy the bacteria. This serum is
secured by causing some animal, usually a horse, which does not
die from the disease, to have diphtheria. The horse's body manu-
factures the same antitoxin as the human body. The horse's
blood is drawn off and the chemical is extracted from it.

This is known as the antitoxin treatment. In dangerous cases
of diphtheria a toxin-antitoxin treatment is used. In this treat-
ment the antitoxin is injected as before. In addition some toxin,
that is, poison produced by the diphtheria bacteria but not the
live bacteria themselves, is injected at the same time. This poison
causes the body to produce its own antitoxin more rapidly, and
at the same time the toxin does relatively little harm to the
individual.

In some cases injections are used to stimulate the manufac-
ture of the antigerm chemicals in the human body before the
bacteria enter and thus prevent one from having the disease.
Such is the inoculation against typhoid. This serum consists of
dead typhoid bacteria, Bacillus typhosus. Their presence in the
body does little or no harm, but it does cause the body to manu-
facture typhoid antitoxin, which will kill any live typhoid
bacilli which enter. Typhoid antitoxin thus produced remains in
the blood stream from one to three years.

The question is often asked, why do some people "catch"
disease, like colds, more easily than others? There are at least
two reasons for this. One is lowered defense abilities of the body,
resulting from illness which has weakened the white corpuscles,
the body cells, or the fever mechanism; or the body may be in a
generally poor physical condition. The second reason is the pres-
ence of a very large or virulent group of the bacteria or viruses
that infect a person. From the individual standpoint, therefore,
the chief factor in avoiding bacterial disease is high resistance.
This high resistance is largely determined by one's practice in
proper eating, health habits, and care of the body.

Functional Diseases

The infectious diseases do not include all the ailments with
which mankind suffers. There is another type of disorder which

KEEPING WELL

467

is usually referred to as the "functional" diseases. In such
diseases no known foreign organisms play any significant part.
Some of these are diseases
common to old age. Some
others result from improper
diet, they being generally re-
ferred to as the " deficiency "
diseases.

Some of the diseases
which produce the greatest
mortality are those that de-
velop as one approaches old
age. These represent the slow
disintegration of the body

tissues. Such disintegration

i fv . i i_ Remarkable photograph of a cancer cell

may be affected by some pre- Mdr|nlcln9 ,, The fluld i$ tokcn |n by the ruff|e.

vious bacterial impairment, like edge and appears as clear globules in the
hereditary traits, Or personal cel1- (Microphotograph by Warren H. Lewis,

i i •, -r» it £ 4.U Carnegie Institution of Washington.)

habits. Regardless of these, * * '

the body tissues do wear out, and death eventually overtakes one.
As the average life of a people is increased by the elimination of
premature "accidental" deaths from infection or improper care
in childhood, the deaths from diseases of old age increase. Such
conditions now definitely exist in the United States. The most
common of the diseases of later life are cancer, heart diseases, and
kidney diseases. The percentage of deaths from these diseases
has increased greatly during the last generation.

Cancer is a condition in which a group of cells begins to grow
and multiply at an enormous rate. Just why and how they begin
is not well known. They invade and destroy the surrounding
tissue, producing in its place a tumorous growth. Some of the
cancerous cells may break off and enter the blood system or
lymphatic system, where they spread to other parts of the body.
Should they lodge in some particular area, the growth would be
repeated. After a time any cancerous growth interferes with the
effected parts of the body to the extent that these parts cannot
function, or the cancer saps the vitality of the entire body. In
such cases death inevitably comes to relieve the patient from his
suffering.

468 THIS LIVING WORLD

The only cure for cancer known at present is to remove the
cancerous tissue from the body by surgical methods or to de-
stroy it by X-ray and radium treatment. These methods will
usually cure a local cancer. If such treatment is applied before
the cancerous tissue has invaded the circulating systems and
other parts of the body, the cure is effective and permanent.
However, if the tumorous cells have migrated to other parts of
the body, another cancer will soon develop. In such cases, cure
is usually impossible. A part of the fight against cancer, then, is
to discover it and treat it while it is still localized. At least, that
is the situation at present. When cancer is discovered in its
initial stages, it can be cured. Any person of adult age or over
should report immediately any symptoms he may have of can-
cer, such as persistent sores or lumps, or disorders in the digestive
tract.

Disorders of the heart become more pronounced as age in-
creases. One of the most common disturbances results from an
inadequate supply of blood to the heart-muscle capillaries. The
general hardening of the body arteries, which is a natural condi-
tion of advancing age, may reduce the blood supply to the heart
muscles to the extent that these muscles cease to function, or
there may be an occlusion of the arteries supplying the heart
muscles. Should this produce a clotting or rupture of the vessel,
the blood supply is stopped, and death will result in a few min-
utes. Damages to the heart muscles may be produced as an after-
effect of certain infections. Scarlet fever or diphtheria bacteria
produce toxins which leave the heart muscles scarred. In later
life this damage may seriously interfere with proper heart action.

Defective valves of the heart constitute an important group
of heart disorders and sometimes they result in fatalities. The
valves may have been rendered defective by syphilis spirochaeta
having invaded the muscular tissue. Leaky valves may also be
the aftereffects of rheumatic fever. This disease does not affect
the heart at the time it is noticeable in the joints. However, it is
thought that some of the bacteria lodge in the heart, where they
slowly multiply and years later produce the heart disease. In
many other cases, defective valves appear to develop from
natural deterioration.

KEEPING WELL

469

cancer pain with "frozen sleep." The patient is packed in ice and the body
temperature is lowered to about 82 degrees. After a period of "hibernation" due to the
reduced temperature, the patient awakes with the pain relieved, and with no memory of
the "sleep.1* (International News.)

470 THIS LIVING WORLD

Kidney diseases can be brought about by many contributing
factors which accumulate with advancing age. When the kidneys
become so damaged that wastes are improperly removed from
the blood stream, weakness and uremic poison results, and this
usually ends with death. Infectious agents are often responsible
for kidney damage. Bacteria from a sore throat sometimes invade
the kidneys and produce a gradually increasing destruction of th(
glands. The hardening of the arteries will reduce the blood supplj
to the kidneys^ and produce disintegration of the cells from lacl
of oxygen and nourishment. High blood pressure also is likely t(
produce chronic kidney disorders.

As age increases there is a tendency for some of the solid ma
terials which the kidneys filter from the blood to be deposited ir
these organs, forming stones. These obstruct the passage o
urine and produce kidney degeneration after long standing. Ai
effective method of treatment for this disorder is to remove th<
stones and diseased parts. As has previously been mentioned
there is a sufficiently large number of the Bowman's capsules am
capillary knots to permit the removal of a part of them withou
greatly endangering the work of the kidneys.

Vitamins and the Deficiency Diseases

Vitamin deficiency diseases have been known since the Mid
die Ages. However, a knowledge of their real relationship t<
minute quantities of certain specific materials in foods has com
as a result of scientific discoveries made within the last twenty
five years or so. Ancient sailors on long sea voyages were we]
acquainted with the painful and bleeding disease of scurvy
They knew, too, that the malady was cured when they reaches
land and could get a diet containing fresh fruits and vegetables
What the real relationship was between the disease and diet
however, they did not know.

The disease called " beriberi " has long been prevalent in th
Orient, where the diet was largely confined to polished rice. I]
certain years during the past century about one-third of the mei
in the Japanese navy would fall victims of the disease. The dig
covery that it was caused by a deficient diet was made by
Dutch physician stationed with the Dutch army in Java near
hospital for the disease about 1900. Since then its prevention an«

KEEPING WELL

471

Parasitic infection is in no sense limited to man. Shown above are two turtles that have been
attacked by the parasitic leech Placobdella.

cure by including certain food materials in the diet has become
known throughout the world.

The vitamins, then, are substances that exist in small quan-
tities in different foods. The discovery of most of them has been
associated with some particular disease. However, we now think
of them as being requirements for normal health rather than as
connected with some malady. The lessons they have taught have
been largely that a diet of a variety of natural foods is essential
to physical well-being. It is desirable that a variety of foods be
carefully selected so as to provide a proper balance of vitamins as
well as other food elements. This variety is much to be preferred
to some restricted diet of highly prepared foods, however cheap
or palatable they may be.

There has been such an extensive discussion of vitamins in
the popular press and in advertisements within recent years that
almost every one has become "vitamin-conscious." Many firms
have attempted to increase the sale of their food products by
claiming that they are particularly rich in some one or more of
the vitamins or that they contain a vitamin in concentrated

472 THIS LIVING WORLD

form. Such statements are usually based upon scientific evidence.
However, one very important condition regarding the vitamin
needs of the body is overlooked, and, because of this, erroneous
and even dangerous impressions are sometimes created. This
condition is that the action of vitamins in the body is often an
interrelated and dependent process. They are related in that
certain vitamins seem to depend upon others for their desired
effects; they are often also, dependent upon other food elements
of carbohydrates, fats, proteins, and minerals for proper reaction
in the body. The ingestion of vitamins in concentrated and
specialized forms, therefore, is likely to be of little value unless
administered under the care of a competent physician.

The term "vitamin" arose from a name suggested in 1912 by
Dr. Casimir Funk, who believed that these food elements be-
longed to a group of nitrogen-containing chemicals known as the
"amines." Since they are considered essential to life, the prefix
"vit" was added, producing the word "vitamine." However, it
was later discovered that all these substances are not amines,
and the word was changed to vitamin. The vitamins are a group
of organic chemicals which act as catalysts in the body, produc-
ing certain chemical and physiological reactions which cannot
occur without them.

These substances are not all chemically related to each other,
but they may be classified as derivatives of (1) the amine sub-
stances, (2) fatty alcohols, (3) sugars, or (4) carotene sub-
stances. There are now a great many of the vitamins known to be
essential to general health and well-being, all of which are
identified by letters. The chief ones are vitamins A, B complex,
C, D complex, and E. In addition, others have been discovered
to be required in small amounts, these being known as F, H, K,
and P.

Perhaps it is in order to mention that vitamins are not a
panacea for all the ills of man, but their importance in helping
man attain a higher level of buoyant health should be empha-
sized. Although each vitamin has a specific part to play, it is now
well known that any practice of taking vitamins in individual
"doses" is of little value. Some knowledge of their general dis-
tribution in various foodstuffs and their effects upon the body
may be of value to everyone.

KEEPING WELL 473

Vitamin A is necessary for growth of the young; it increases
resistance of the body to infections of the respiratory and
urinary tracts and maintains appetite and normal digestion. A
complete lack of the vitamin causes a dangerous eye disease, in
which there is almost complete destruction of the eye tissue by
replacement with an abnormal cellular growth. During the
World War it was discovered that a large number of Danish
children who were on restricted diets not containing butter and
whole milk developed the disease. When fresh butter was added
to the diet, the disease disappeared. Milder deficiencies of the
vitamin may produce physical weakness, formation of kidney
and gallstones, night blindness, and sterility. This vitamin seems
to be closely related to a chemical called carotene, which makes
the yellow color in carrots. The vitamin is present in a variety
of green vegetables, apricots, prunes, butter, milk, cheese, beef
liver, and kidneys.

Vitamin B complex includes a number of vitamins, all of
which were once thought to belong to vitamin B. In addition,
what was formerly designated as vitamin G is now known to be
a part of this complex. The entire group now includes vitamins
BI, B2, B3, B4, B6, B6.

BI is the vitamin that is necessary to produce a normal
condition and normal functioning of nerve tissue. It also pro-
motes growth, aids digestion and absorption of foods, and
increases resistance to infection. A complete lack of it causes
the dreaded disease beriberi. This disease is characterized by
atrophic paralysis of the legs and arms, dropsy, and heart
trouble, accompanied by a degeneration of the nervous system.
The disease is largely prevented now in some of the countries
where it was once prevalent by a substitution of unpolished rice
and unbleached flour as the staple diet. This is particularly true
in the Philippines, where many beriberi hospitals have now been
closed, largely because of the lack of patients. Vitamin BI is
present in such foods as nuts, whole grains, peas, grapes, spinach,
liver, and eggs.

B2 is itself a mixture, one factor of which promotes growth.
The other factor prevents the disease of pellagra and is some-
times designated B6. Pellagra is characterized by eruptions on
the hands, arms, or neck and other symptoms. The disease may

474

THIS LIVING WORLD

result eventually in insanity or death. A mild deficiency of
vitamin B2 produces low vitality and slow growth of the young

and interferes with muscular
action. Both of these vita-
mins are present in a
number of vegetables, par-
ticularly those of the green-
leaf type. They are also
present in liver, kidney,
veal, and eggs.

The other B vitamins
have been found to produce
certain effects in experi-
mental animals, and indica-
tions are that they represent
different chemical sub-
stances. One of those seems
to be effective in regulating
the formation of red blood
cells and the prevention of
anemia. However, at present
knowledge of these other B
vitamins is in a nebulous state, and actually we possess little
definite information about them.

Vitamin C is a substance that favors good bone and tooth
formation and endurance. It maintains the health of blood
vessels, improves appetite, and stimulates growth; it is also,
involved in the mechanism for the prevention of bacterial infec-
tion. A shortage of this vitamin causes general weakness, head-
aches, slow healing of wounds, and joint pains. A complete lack
produces scurvy, characterized by multiple hemorrhages, ane-
mia, progressive body weakness, mental decay, and delirium.
The vitamin is found in oranges, lemons, peppers, cabbage,
spinach, and tomatoes.

Vitamin D is sometimes referred to as the "sunshine vita-
min," since it is produced in certain fatty substances of animal
tissue by the effects of ultraviolet radiation. However, it should
be kept in mind that ultraviolet radiation is itself not a vitamin,
a mistaken idea that many people seem to have. The composi-

Lack of vitamin D produces rickets in
children, characterized by poor development
and malformation of the bones. (Courtesy of
Standard Brands, Inc.)

KEEPING WELL 475

tion of vitamin D is unknown, but it seems to be related to the
fatty alcohols. It is believed that it contains five or six factors
rather than being a single substance. It regulates the metabo-
lism of calcium and phosphorus and is essential to bone growth
and tooth formation. It promotes normal glandular function
and maintains the proper calcium level in the blood. It is practi-
cally absent from most vegetables, and its most potent source
is fish-liver oils. However, the body itself will manufacture a
sufficient amount of it when there is a reasonable amount of
exposure to sunlight.

There is still much to be discovered about vitamin E,
especially regarding its effect on the human body. It has been
found to regulate sexual maturity and functioning in experi-
mental animals. There is some clinical evidence to show that a
complete lack of it produces sterility or sexual impotence in
both men and women, but there is at present far from universal
agreement on such effects. The natural sources of the vitamin
are lettuce, spinach, whole wheat, peanuts, milk, eggs, and lean
meats.

Certain other vitamins have been discovered to have an
effect upon experimental animals. These have been labeled F,
H, K, and P. It is likely that in the future they may be found to
be related to certain human physiological processes and may
prove to be the causes of a few elusive human disorders that are
at present not well understood by medical science.

The very extensive work that has been done in the study of
vitamins during the last quarter century has given us wide and
valuable knowledge of their specific effects upon normal health
and body functioning. The vitamins are so interrelated that a
normal diet should contain all of them in approximately the
proper amounts. These conditions are usually satisfied when one
eats the natural foods which the body has become adapted to
through long ages of its development.

The problem of vitamin supply for most people, then, is
mainly one of eating a variety of foods. Such foods should include
some fresh fruits and vegetables, some dairy products, some
bread and potatoes, and as wide a choice of different kinds of
meats as is available. This variety is not necessary in each meal,
of course, but should be included in one's regular eating habits.

476

THIS LIVING WORLD

When such habits are practiced one is practically sure to obtain
all the vitamins necessary, not only for the prevention of the

deficiency diseases but
also for normal health.

Maintaining Health

The best way to pre-
vent having disease is to
keep the body healthy.
This is not so foolish a
truism as it may seem on
first reading. The human
body, while an ex-
tremely delicate and
complicated mechanism,
is so constituted as to be
able to ward off disease
when it is kept in a per-
fectly normal condition.
Some of the ways the
body has for fighting
bacterial infection have
just been explained. It
has been seen that many
other ailments to which mankind is subject are also prevented
when the body is functioning properly. Health, in its broader
sense, means more than freedom from disease. There is a differ-
ence between buoyant health and merely not being sick. The
superior vitality of some people need not be accepted by others
as a measure of luck which can only be inherited. Intelligent care
of the body will provide for each individual a large measure
of this vitality.

The relation of nutrition to health constitutes, in some
respects, the most remarkable discovery of modern times. It is
of concern now not only that food be free from poisons but also
that it be considered from the point of providing the body with
all the elements necessary for growth, energy, and protective
purposes. It is now known that for an adult engaged in various
kinds of activities definite amounts of energy expressed in

Intelligent care of the body will provide for
most individuals a large measure of superior vitality.
(Courtesy of Selby Shoe Company.)

KEEPING WELL 477

calories are needed. All foods which man eats have been analyzed
in terms of their energy values as well as their vitamin and
mineral contents, so that it is possible, if one desires, to regulate
one's diet accurately.

Directly related to health is physical exercise. This involves
more than mere gymnastics; however, it does not mean a strenu-
ous athletic program. Physical education for health includes
some outdoor activities in the form of games or hobbies which
provide some measure of mental relaxation as well as physical
exercise. Walking, swimming, dancing, or tennis constitute
activities which even busy people in a crowded city may partic-
ipate in and enjoy to some extent. For the average adult such
forms of exercise are to be recommended more than strenuous
competitive athletic games.

A periodic physical examination by a physician is important.
Any deficiency or maladjustment in the body may then be
detected before it becomes serious, and a remedy applied. Yearly
physical examinations of children are now provided in most
primary and secondary schools. To a certain extent this practice
is followed in colleges. Some life insurance companies provide
their policyholders with a free medical examination every two
or three years. However, whether one secures such an examina-
tion through the school health department, insurance companies,
or from his personal physician, it is a valuable protective phys-
ical analysis which should be obtained at least once yearly by
every person.

REFERENCES FOR MORE EXTENDED READING

BIGGER, JOSEPH W.: "Man against Microbe," The Macmillan Company,

New York, 1939.

•

In addition to a short history of microbiology, with emphasis on its relationship
to mankind, the author discusses the nature of disease germs, their characteristics,
how studied, and how acquired and resisted by the human body.

RIESMAN, DAVID: "Medicine in Modern Society," Princeton University Press,
Princeton, N. J., 1939.

The author has written here a short history of medicine up to the present which
includes a summary of discoveries about most modern diseases. However, the book
is more than a history. It gives a thought-provoking analysis of the effectiveness and
proper position of medicine in our modern social order.

478 THIS LIVING WORLD

ZINSSER, HANS: "Rats, Lice and History," Little, Brown & Company, Boston,
1935.

This book is primarily an account of typus fever, its causes, manner of transmission,
and the plagues it has produced. The style of writing utilizes a wide command of
language and is thereby interesting, although somewhat laborious for the untrained
to follow.

FULOP-MILLER, RENE: "Triumph over Pain," Bobbs-Merrill Company,
Indianapolis, 1938.

This book contains an interestingly written history of almost all attempts to elimi-
nate pain connected with medical treatment and operations.

WALL, F. P., and L. D. ZEIDBEHG: "Health Guides and Guards," Prentice
Hall, Inc., New York, 1936.

Here is a concise and practical book of hygiene. The structure and functions of
different parts of the body are briefly treated, and specific information of their proper
care is clearly set forth.

DIEHL, HAROLD S.: "Healthful Living," McGraw-Hill Book Company, Inc.
New York, 1940.

This is primarily a textbook for college courses in hygiene. It contains a wealth of
information of value to everyone interested in developing and preserving the body to
the best possible advantage.

FROBISHER, MARTIN: "Fundamentals of Bacteriology," W. B. Saunders
Company, Philadelphia, 1937.

The author has written here a text for students of bacteriology which gives a grasp
of the essential facts concerning both pathogenic and nonpathogenic bacteria. While
the book includes a considerable amount of technical detail, it contains many accounts
of interest to the intelligent layman.

STITT, E. R.: "Practical Bacteriology, Blood Work and Animal Parasitology,"
P. Blakiston's Son & Company, Philadelphia, 1927.

This is primarily a laboratory manual for advanced students of bacteriology and
interns. It is a good reference for less specialized persons who may wish to secure some
particular information in this field.

Hygeia, published by the American Medical Association, Chicago.

This monthly magazine contains popularized articles relating to health conditions
and practices, general body welfare, and frequently an article discussing body
structure.

Journal of the American Medical Association, published by the American
Medical Association, Chicago.

The Journal is a professional magazine that is published weekly. It is devoted to
research and clinical reports on a wide variety of subjects within the field of medicine,
some of which are of interest to the intelligent layman.

16: THE LONG ROAD

In the Development of Human Culture

THE subject of this chapter is the title of a delightful little
book written by Dr. Fay-Cooper Cole of the University of
Chicago and published for the Century of Progress Exposition
in 1933. It is a brief account written in simple but descriptive
language of man's hard struggle from savagery to civilization.
In it is told the story of the beginnings and development of
many of our cultural heritages, as it has been told more exten-
sively in many other publications. Such publications give some
descriptions of the cultural remains which prehistoric mankind
left to form a part of the debris of the earth's crust long ages
before he learned to write. They also represent our modern
attempts to reconstruct the picture of man's early past as accu-
rately as possible on the information we now possess.

479

480 THIS LIVING WORLD

As might be expected, the earth itself is the best witness of
how man lived in the early days of his development. Primitive
man at the very outset developed kinds of tools from the objects
of nature which he found. Later more elaborate and effective
instruments were designed. Pottery and baskets were added, as
well as clothing. As he moved from place to place in search of
food, the articles were often left behind. A surprise attack or
sudden death brought about the leaving of still others. Many
times our remote ancestors built houses, temples, burials, even
cities. Then as time went on, these treasures and structures
became covered over by soil and rock and by dust borne by wind
and water or by volcanic eruptions and thus became a part of
the earth's crust.

Many of these remains have been preserved to the present
time and are now being excavated. They constitute the only
records of primitive man's activities. Their careful handling and
unbiased interpretation can give us the only accurate under-
standing we will ever have of prehistoric human activities. While
the discovery of these records is in no sense complete at present,
sufficient discoveries have been made to give some insight into
the origins and development of human culture.

Chronology of Cultural Development

Man has been on the earth for quite a long time. Our pres-
ent information intimates it has been about a million years.
Throughout most of this great age he has been fashioning tools,
or making pottery, or domesticating plants and animals, or
building cities. The way and extent to which he did these things
has been one of progress. This progress divides itself naturally
into periods in which the developments continued along certain
lines. Gradually new developments would arise. Man's ways of
doing things would slowly change. The old order eventually
passed, and a new period would be ushered in. There are, then,
great periods or ages in human cultural development. The
periods are named and their boundaries determined chiefly by
the materials which were used in the making of tools and instru-
ments. These periods are based primarily on the cultural de-
velopment of man in Europe. They do not follow the divisions
of geologic history, except insofar as geologic factors affected

THE LONG ROAD 481

Four distinct cultural periods are clearly recognized. These
are the Paleolithic, or Old Stone Age; the Neolithic, or New
Stone Age; the Bronze Age; and the Iron Age. Each of these
ages is further subdivided into epochs. These epochs are deter-
mined in most part by the types of implements made, regarding
both form and methods of manufacture. The Paleolithic age
consists of the Pre-Chellean, Chellean, Acheulean, Mousterian,
Aurignacian, and Magdalenian epochs. The Neolithic age in-
cludes the Campignian, Asturian, and Azilian epochs. In the
Bronze Age four numbered epochs are generally recognized;
while in the Iron Age the Hallstatt and LaTene epochs are well
defined.

The Paleolithic is practically coextensive with the Pleistocene
geologic epoch. It began with the making of the earliest crude
stones at least a half million years ago, and continued until the
beginning of the New Stone Age. This age probably began in
Asia about twenty thousand years ago. The Bronze Age began
with the use of copper in Chaldea some seven thousand years
ago, while the Iron Age began with the first use of that metal
in the Tigris and Euphrates valleys about 1200 B.C. These cultural
ages, together with their approximate dates and relation to the
Ice Ages of the Pleistocene, should be familiar to everyone who
desires to get even a general understanding of man's early
cultural development. In addition, the epochs of the latter part
of the Old Stone Age should be remembered in their proper
chronological order and some idea of their significant develop-
ments kept in mind.

In making an extensive study of cultural development, it
is more convenient to consider in detail each one of these periods
and man's accomplishments during those times. However, for a
brief account it is more significant to trace the growth of some
of our most important heritages perpendicularly through these
periods and note the important steps in their development. This
method will be followed here.

Tools through the Ages

Perhaps the first development man made that started him on
the long road to modern culture was the fashioning and use of
tools. He learned early to make crude instruments, which served
as aids in securing food or in warding off his enemies. Few other

482

THIS LIVING WORLD

animals have ever used tools of any sort to supplement their
physical body in maintaining an existence or in protecting

themselves. Certainly none
have ever fashioned such tools
or modified their environment
through this development.
However, some of the very
oldest rocks that contain
human fossils also contain
stones that had been chipped
and carried to those areas by
man. The deposits near
Peking, China, from which the
oldest known human remains
were excavated, yielded ap-
proximately two thousand
pieces of crudely shaped tools
One of the earliest known human tools made of stone or bone. Some

from Black, Fossil Man in China. )

that had been shaped into

rough picks or axes by striking off flakes with other stones. Most
of them were short, thick, and of irregular form, the edges being
poorly shaped. However, they served a purpose that was better
than no weapons at all. These implements are at least nearly a
half million years old and are the earliest known human tools.
At a much later age, man in Europe was using flint and bone
tools, often leaving them buried in the earth's debris as he
perished. Now some of them have been excavated and give us a
clear picture of the development of tools throughout the ages be-
fore written, historical civilization began. Near the middle of the
Paleolithic age the coup de poing, or hand ax, came into general
use. These hand axes had one end somewhat rounded, with a
rather well-shaped point at the other end. Some of them showed
a high degree of skill in chipping. Probably the best workman-
ship in making chipped hand axes was first reached by the
Neanderthal people during the middle part of the Paleolithic
age, as represented by the Mousterian cultural epoch in France.
It may be thought that these stone axes were crude and ineffec-
tive tools. However, the opposite is more nearly true. In a recent

THE LONG ROAD 483

experiment in Denmark a carpenter who had never seen a stone
ax was given one of these prehistoric implements with which to
work. In ten hours working time he cut down twenty-six pine
trees. In eighty-one days he had hewn them into boards and
timbers and built a house, using no tools except the prehistoric
stone ones supplied him.

In addition to the hand ax, other kinds of useful implements
are found. There are stone scrapers, arrow points, and lance
heads. Neanderthal man showed far more skill than peoples be-
fore him in making such tools. He would mine the flint in crude
pieces, then knock large, thin flakes from the pieces. These thin
flakes, some as long as seven inches, were then chipped into the
desired shape for scrapers, points, or lance heads.

With the beginning of the Aurignacian epoch a decided ad-
vance is noted in the making of tools. By this time Neanderthal
man was beginning to disappear, and a new race of men appeared
in Europe from Asia. These were the Cro-Magnons. The kit of
tools of man up to this date had been extremely simple, consist-
ing almost entirely of hand axes, scrapers, lance heads, and
points. The big-brained Cro-Magnons brought with them many
new advances in the art of toolmaking. These were improve-
ments they had perfected, at least to a considerable extent,
before coming into Europe. They must have considered Neander-
thal tools primitive and crude.

The Cro-Magnons not only made implements from flint, but
they also used bone, horn, and ivory very extensively. These new
materials permitted the manufacture of finer and more delicate
instruments, as well as a much wider variety of tools. From the
antlers of reindeer they made javelin points of varying size.
These were often ornamented along the sides with engravings
and carvings. Harpoons ranging from two to fifteen inches long
were fashioned out of reindeer horn. The harpoons had well-
defined rows of barbs cut on one side or on both sides with a
dagger-like point at the forward end. They were evidently widely
used for spearing large fish, which were numerous in the streams
at that time. Cylindrical chisels of reindeer horn were common.
They too, were often richly decorated with engraving.

Bone needles must have been extensively and almost univer-
sally used by the Cro-Magnons, as many of them have been

484

THIS LIVING WORLD

found in most of their deposits. Long, slender bones would be
brought to a fine point by being polished with stone. In some
cases there would be a hole or eye at the other
end, made by a sharp flint drill. These needles,
along with bone awls, indicate a refinement in
the making and finishing of clothing. Appar-
ently animal skins remained the only clothing
material, but these skins must have been cut
and sewn into a variety of garments.

Flint was used for making chisels, drills,
knives, and crucibles in addition to the more
common uses of axes, points, and scrapers. The
stone crucibles were adapted to the grinding of
mineral pigments which the Cro-Magnons are
known to have used extensively in color deco-
ration and painting. The following epoch, Solu-
trean, witnessed some of the finest examples of
flint chipping that have come down from an-
cient times. These were the laurel-leaf blades,
which were deftly chipped to a remarkable de-
gree of thinness. Many of them attained an al-
most perfect degree of symmetry. Some of these
spearheads were shaped with a lateral base
notch, probably for fitting into a shaft to make
a sword or a javelin. Others were chipped on one
edge only, with a notch or handle on the other,
forming an instrument similar to a knife or
s|nf-") razor.

With these finer and wider varieties of tools, these people
were able to secure their food more easily and to make more use
of their environment. In some parts of Europe the big game of
the Cro-Magnon hunters was the mammoth which was common
there as the glaciers receded. They must have been extremely
adept at spearing these large elephants. One camp site recently
excavated in Moravia has yielded more'than two thousand mam-
moth vertebrae. At another site a tomb containing twenty
human bodies was found. It was constructed entirely of the
shoulder blades of mammoth. Another extensive deposit of cul-
tural remains of this epoch is one near the village of Solutre in
Southern France. In some of the levels of these remains are found

Laurel-leaf blade.
(Redrawn from Mac-
Curdy, "Human Ori-

THE LONG ROAD 485

great fireplaces with flint utensils and the remains of what ap-
parently were abundant feasts. The animal bones include the
horse, wolf, fox, hyena, bear, badger, wild cattle, reindeer, and
mammoth. The improved tools of the late Paleolithic apparently
served man well for collecting a wide variety of game for food.

Such, in general, was the development and use of tools during
the Paleolithic or Old Stone Age, a period lasting for over a half
million years down to the decline of Cro-Magnon man. As we
have seen, most of the advances were made during the last few
thousand years of this period. The stone tools of this age were all
made by chipping the flint until the desired shape was secured.
Apparently during all these centuries man in Europe conceived of
no other or better way of shaping his stone tools than to chip
them.

However, at about the time when the Cro-Magnon peoples
were at their zenith there was being developed in Northern
Africa and Southwestern Asia a new technique for shaping stone
tools. Later this technique was introduced into Europe by the
slow migration of peoples into that country. This technique was
polishing the stone. The general shape of the ax was first made by
chipping. Then the edge was ground or polished by rubbing it
against other stones. This gave a sharper cutting edge. Later the
whole ax was polished, making a still better instrument. Like-
wise other stone implements, daggers, knives, and spear points,
were polished. This finer grade of workmanship of stone tools by
peoples who followed Cro-Magnon man into Europe established
a new period in cultural development. It is called the Neolithic
or New Stone Age.

The introduction of the new technique of polishing stone into
Europe was by no means sudden or universal. For many cen-
turies it was only sparingly used. During this transition period
there was a decided decline in the art and extent of fashioning
implements of all sorts. This period is sometimes looked upon as
the "dark ages" in prehistoric times. These conditions were
effected not only by a transition in techniques of making tools
but by many other changing factors. The climate of Europe was
shifting to a milder one. The animals of postglacial times were
disappearing; dense forests and grasslands came into existence.
New modes of life were in order. It took the peoples of Europe,
particularly those migrating into this area, considerable time to

486

THIS LIVING WORLD

examples or airrerent stages in maKing poiisneo scone axes during tne new atone /\ge.
(American Museum of Natural History.)

establish a new order of living. Making tools was only one phase
of it.

However, as these new cultures developed, finely made
polished stone tools became more universally used. Flint was
mined and exchanged on a commercial basis for the first time in
man's history. Some of the finest and most extensive deposits of
polished axes, knives, and spear points have been found in village
sites along lake shores or streams far from the flint beds. Flint
mined in Southern France has been traced by its peculiar color
to Belgium, Switzerland, and Italy as an article of exchange in
Neolithic times.

Another revolutionary advance in the making of tools and
other implements was made by man before the time of recorded
history. This was the discovery and use of copper and its alloy,
bronze. Copper was first used in Egypt and Southwestern Asia
about 5000 B.C. Soon after this the art of extracting copper from
its ores was discovered and methods of casting it by melting it
and pouring it into molds. With this early discovery our modern
industrial civilization really began, even though it was many
centuries before it acquired much momentum. Copper began to
be used for making weapons, tools, razors, and decorations and
in constructing buildings and vehicles of transportation.

Ceramic and Textile Arts

During the Neolithic age there was developed another phase
of human culture that was of vast importance to mankind.

THE LONG ROAD 487

This was the making of pots and jars from clay. During all the
previous thousands of years man had been on earth he had not
discovered how to make pottery. Even
Cro-Magnon man, regardless of his
many other accomplishments, seemed
never to have developed the idea. How-
ever, Cro-Magnon was essentially a
hunter and cave dweller. The climatic
and geographic conditions of Europe
at the times when he lived there were
such as to favor this type of existence. Early Neolithic pottery.
The forests and caves were not con- (From a photograph by J.
ducive to developing the ceramic arts. fchrajnl1 °' c«rl* ba"ded pottcry

.~ ~ -T found in Bohemia.)

But as Cro-Magnon man was su-
perseded in Europe by other peoples, coining by different routes
from the east, pottery was introduced. These new races were the
people who had discovered how to make pottery, probably in the
warmer, drier climates of Southwestern Asia and Northern
Africa. They brought this knowledge and industry with them into
Europe. With pottery at his disposal, man was able to store his
food effectively. This made him less dependent upon following
and hunting wild game. It allowed him to establish definite
homes rather than to continue being a wanderer. Where his food
was stored, there he would continue to live. Man for the first time
in his long history became a community dweller.

Some of the most extensive of the early Neolithic communi-
ties were the pile villages along the lakes of Switzerland and
Germany. These consisted of homes built over the water's edge
on wooden piles that had been driven into the lake bed for a
foundation. At that time the lake levels in those countries were
somewhat lower than they are at present. Accordingly, many of
the pile villages were long ago covered with rising waters, and
some have been preserved in fair condition to the present time.
The first of these was discovered in 1845, when an extremely dry
season in Europe caused the lakes of Switzerland to be lowered
below the level of the ancient dwellings. Since then a great many
others have been discovered.

The lake villagers must have lived a peaceful and prosperous
life, as judged from the remains they left behind. Their homes

488

THIS LIVING WORLD

were comfortable wood shelters. They had wooden furniture and
wooden tools as well as many implements made of stone, horn,

and bone. They made pottery
in various shapes and sizes,
bowls, jars, and dishes. Many
large, baked clay kettles that
were adapted to cooking foods
or storing large quantities of
grain have been found in some
of the village remains. Al-
though the earlier types were
crudely made without the pot-
ter's wheel and unevenly
burned without a baking oven,
these pottery vessels must
have made the household life
much easier and more stable.
They served well to transport
and store food and water.

As Neolithic times contin-
ued, the art of making pottery
progressed rapidly. This invention was admirably adapted to
serve both a utilitarian and an artistic purpose. Before the
beginnings of written civilization Neolithic man had become
adept at making fine pottery and decorating it in a great variety
of ways.

Another development of Neolithic times was the weaving of
textiles and the making of cloth garments. It is not known
whether the invention of weaving preceded or followed the first
making of pottery. They probably both came into practice some-
what simultaneously. The earliest known specimens of textile
fabrics are those that have been found in the remains of the lake
dwellings of Switzerland. These were the homes of relatively
early Neolithic peoples. These remains are sufficiently extensive
to give a good picture of the development of the textile art at
that time.

The materials used included both flax and wool. The art
included not only the spinning of thread and yarn, but also
weaving, knitting, embroidering, and basket making. Samples
of all these have been found. Some of the deposits also included

Late Neolithic pottery was often finely
made and decorated in a variety of ways.
(Redrawn from MacCurdy, "Human Ori-
gins.")

THE LONG ROAD 489

raw flax fiber, coarse linen thread, and thick ropes. Neolithic
spindle wheels and loom weights of stone and clay were found.
Remarkably enough, these loom weights were very similar to
those used on early Greek looms several thousand years later.
It is believed by some, therefore, that the lake dwellers* looms
must have been very much like those of the Greeks since the
weights of both were so similar; and, as preserved specimens of
the Greek looms have been found, we can judge what the Neo-
lithic looms must have looked like.

Agriculture and Domestication of Animals

At the very outset of his existence, man was dependent upon
wild life for his food. This consisted of the flesh of such animals
as he could capture, supplemented by berries, nuts, seeds, and
fruits when they were in season. This of necessity made him a
wanderer. He had to follow the game he cherished. While this
probably served to take his footsteps over much of the earth, it
kept him in a dependent and barbaric stage. The vicissitudes of
the hunt kept him a nomad, subject to the uncertainties which it
involved. Even should fortune favor him and an abundance of
food come into his possession, early man had no way to store or
preserve it.

It was only during Neolithic times that mankind learned to
make pots for carrying and storing food. This tended to keep
him nearer his stored supplies. He began to settle down. He did
not entirely forsake the hunt, but he began to domesticate
animals and to cultivate plants.

The first animal to be domesticated was the dog, it being
tamed from the wolf that was common in all countries. Remains
of domesticated dogs have been found in practically all Neo-
lithic deposits, and today the dog remains the most universally
domesticated qf animals. Paleolithic man at much earlier times
represented the ox and horse in many of his paintings and works
of sculpture; however, there is no direct evidence to indicate that
he ever domesticated these animals or any others. It is, there-
fore, generally agreed that Neolithic man was the first to tame
and use animals.

The hog was another animal that was domesticated in early
Neolithic times. It must have been kept close around the village
homes as a source of food. The horse had become a domestic

490 THIS LIVING WORLD

animal by the time the Bronze Age was ushered in, as it was
extensively used then as a beast of burden. It is likely that it was
also domesticated to some extent in Neolithic times, although
there is no direct evidence of this. Before the Neolithic age, the
horse was one of the staples of diet. However, there is no evidence
that Neolithic man used it for food, probably because he had
begun to use it as a work animal. The other animals that man
had tamed and taken into his fold during the Neolithic age
included cattle, sheep, and goats. Altogether, about 170 differ-
ent species of animals had been tamed and were being used by
man when recorded history began.

The cultivation of plants was also introduced into Europe
during Neolithic times. The knowledge of certain grains as
desirable elements of food and an understanding of how to
cultivate them were evidently brought in from Southwestern
Asia with the Neolithic migrations. One variety of barley found
in the lake-dwelling ruins of Switzerland, Germany, and Den-
mark was the same as was cultivated by the ajicient Greeks and
Romans. One kind of Neolithic wheat was identical with an
Egyptian wheat. Other fragments of plants and seeds found
widely scattered in Neolithic deposits are millet, flax, peas,
apples, pears, oats, grapes, strawberries. Thus, these peoples
were using and cultivating quite a wide variety of field products.

Very little is known of the earliest Neolithic agriculture.
However, these people most likely cultivated the fields near the
villages, using crude wooden tools for working the soil.

The factors of domestication of plants and animals along
with the development of pottery and the invention of weaving
had a very profound effect on man's cultural development. They
were all first developed by Neolithic man and seemed to have
been introduced into Europe from Asia or Northern Africa.
Wherever they began to be used, communities began to grow,
and eventually great cities developed which showed many
remarkable prehistoric cultures.

Development of Art and Writing

The artistic nature of man is made evident to us long before
the time of recorded history. Works of art have been found
dating back as far as the Aurignacian epoch of the Old Stone Age.

THE LONG ROAD

491

$';'^ '-v" V' ' ^;'l^J^'T^ll|l'l;^^:!^;Lii/ '^ll^f^'1l^!fl
* "" ' '" '

Examples of Old Stone Age engraving on bone and stone. (American Museum of

Natural History.)

Since that time many new phases and perfections of artistic
accomplishments have been developed. Man's production of
works of art has not been in a steady, ever- widening stream.
There were periods when art flourished and periods of lesser
developments, just as there have been in historical times. How-
ever, artistic abilities have never been lost.

Some of the first works of art consisted of engravings of
human and animal forms on bone and ivory. The harpoons of
reindeer horn were highly decorated. The ivory javelin thrower,
often made in the form of a lioness, shows delicacy and beauty
in its carvings. The bison form was frequently carved, as well as

492 THIS LIVING WORLD

the forms of birds. The bone dagger, carved as a model of a
reindeer, represents a high degree of perfection in sculpture.
Many of these have been found in the caves of France. The
human form was sometimes pictured in well-made sculpture in
stone. Most of these are the works of Cro-Magnon man during
the latter part of the Old Stone Age.

' A remarkable example of Paleolithic sculpture was dis-
covered in 1912 in a cave in the Pyrenees Mountains. The cave
had been completely closed off long ago by a formation of
stalactites across the entrance. No human being had been in its
recesses for thousands of years. Yet there on the floor were the
figures of two bisons, modeled in clay in as fine a fashion as any
modern sculpture. The footprints of the artists remain on the
hardened floor. Another bison figure was being modeled, and
marks of the artist's fingers are to be seen on the scattered clay
parts. It seems in every way as if the artists had just stepped out
for a few minutes; yet thousands of years have elapsed since
they discontinued their work.

However, it is in the paintings in caves that Paleolithic art
reaches its golden stage of development. These paintings, again,
are mostly the works of Cro-Magnon man. They are found in
the caves of France and Spain. The paintings of one generation
are frequently overlaid with those of later generations. As time
went on the workmanship was greatly improved. The first were
reliefs, outlined with charcoal and done as monochromes. Later
were added polychromes in red, brown, and many shades of
yellow. Many animals were depicted. Some of the finest of these
paintings are found in the caverns of Marsoulas and Font-de-
Gaume in France and of Altamira in Spain.

In the drawings of these caves as well as many others there
is always manifested a seriousness of purpose and almost a
reverence in the workmanship. There is an absence of trivial
work and meaningless drawings. Each drawing seems to have
been executed with the greatest of care. Probably only the Cro-
Magnon artists were allowed to enter the darker and more
remote caverns where the greatest paintings are found. These
artists were no doubt looked upon as a class that was especially
gifted by nature, and they probably were accorded considerable
privileges and distinctions. It would appear that the love of art

THE LONG ROAD 493

Cro-Magnon artists painting the woolly mammoth. (Photosraph by Ewing Galloway of a
painting by Charles R. Knight in the American Museum of Natural History.)

for art's sake was the controlling factor in inspiring and execut-
ing these paintings and drawings, a love akin to that which
inspired the e^rly Greeks in their great works.

With the disappearance of the Cro-Magnons, painting suf-
fered a decline. It was not to reach such development again
until long after recorded civilization began. Likewise there was
a decline in engraving and sculpture. It was as if an age of
culture had ended. It was a period of adjustments to new condi-
tions. But the peoples of the later Neolithic age developed
another means of artistic expression which reached great heights
in many instances. This was in the modeling of fine pottery and
the decoration of this pottery in relief and with paintings. The
Neolithic pottery was decorated with lines and bands to form a
variety of patterns, some of which were of exquisite design.

Neolithic sculptured figures of the human female are found
in the valleys of the Marne and Seine. These, however, hardly
compare in workmanship with similar Paleolithic sculpture of
Cro-Magnon man. Neolithic man also decorated many of his
burial places. On one of the large granite rocks forming the cover
of a burial place in Locmariaquer, France, and on some of the
supporting stones are found remarkable sculptured figures. In
fact, many thousands of specimens of Neolithic engraving and
sculpture have been found. Such artwork was widely practiced,
even though little of it represents outstanding quality.

494

THIS LIVING WORLD

Illustrations of Azilian painted pebbles.
(Redrawn from Osborn, "Men of the Old
Stone Age.")

The Neolithic and Bronze ages are sometimes referred to as
dark ages in the realm of art. However, there were definite

contributions made during
the last part of the Bronze
Age in sculpture and archi-
tecture. A fine example of
sepulture ornamentation is
found in a tomb at Kivik in
Switzerland. Many of the
bronze vases, shields, and
chariots of this age are elab-
orate in design and exten-
sively decorated with
delicately shaped figures. At
the beginning of the Iron Age
recorded history had begun,
and any account of develop-
ments during this time does not come within the scope of this
discussion.

Perhaps by far the most important thing that was going on
in all these centuries was the development of writing. It was a
new tool of the human mind which gave an enormous enlarge-
ment to its range of action and a new means of continuity. The
beginning of a system of writing was probably made toward the
end of the Paleolithic period. During the Magdalenian epoch
small stones were engraved with circles, many of which were
dotted. Animal bones of this age were engraved with many signs
which were alphabetical in form. Painted pebbles belonging to
the early Neolithic period are numerous. These apparently repre-
sented a system of writing. In fact, it is believed by some that a
number of these symbols form the basis of letters in the later
writing of the Phoenicians, Greeks, and Romans. However, it is
exceedingly difficult to prove any such relationship.

The development of writing beyond mere pictographs must
necessarily have followed the development of a spoken language.
The origin of language is shrouded in the uncertainties of the
past. Just how it began will probably never be known. It may
have originated through imitation of animal sounds. It may have
been a slow evolution from sounds uttered by man in emotional

THE LONG ROAD 495

states of joy, sorrow, anger, and pain. Or it may have grown up
from vocal expressions emitted by the group as they worked
together at something in unison. But whatever its origins were,
it involved the representation of objects or ideas with sounds.
It seems that in developing the art of writing the most difficult
thing was to represent sounds and the ideas they conveyed with
some sort of symbols.

For thousands of years writing consisted of a series of symbols
which pictured objects without a highly definite indication of
their relationships. The early Egyptian writing was largely pic-
tographic. Then the form of the picture was abbreviated. A part
of an animal, for example, represented the whole animal. Still
later the abbreviation continued until finally a mere symbol
stood for a particular object. The latest prehistoric attempts
were to represent syllables and letters with symbols. Egyptian
writing became a beautiful but complicated and awkward com-
bination of pictures, abbreviated pictures, and symbols.

As early as 6,000 years ago the Sumerians had perfected a
rather complete system of writing. This system predated the
Semitic language of the ancient Assyrians. The Sumerians lived
in the Tigris and Euphrates valleys and developed a flourishing
culture before the time of the Assyrian and Egyptian civiliza-
tions. The Sumerians used first the pictorial script, which they
later developed into cuneiform signs. These signs were used not
only to express the idea represented by the pictures, but also to
express the sound of the word in other relations which did not
involve the original pictures at all. Thus it became a phonetic
system of language. The Phoenicians later improved upon this
cuneiform system of writing, which in time came to be the basis
for modern systems of written expression.

From all of this, one thing is evident; that is, modern alpha-
bets were slow developments and they began in their fundamen-
tals with prehistoric peoples.

From Caves to Cathedrals

During the long stretch of time when Neanderthal and Cro-
Magnon peoples were flourishing in Europe, the most popular
place of abode was in the caves or beneath well-sheltered rock
cliffs. These were the natural sites that would provide some pro-

496 THIS LIVING WORLD

Restoration of lake dwellings by M. Gbtzinger. (American Museum of Natural History.)

tection from the weather elements. It is natural that man would
seek them as dwelling* places, since, because of his nature, such
protection is of grave concern to him. However, it seems that a
long period in human history had elapsed before man even used
the caves, as no evidences have been found of their consistent
use before the time of Neanderthal man. But once the caves
were "discovered" and cave dwelling became the vogue, they
served man in Europe as a home for over fifty thousand years.
If there were any artificially made dwellings dotting the land-
scape during all these centuries, they must have been exceedingly
temporary and crude. Not the faintest trace of any of them re-
mains. It was only during Neolithic times that man began to
construct homes, burial tombs and markers, and places for
ceremonies and worship.

The first evidences of human building operations are the re-
mains of crude dwelling places dating back to the early Neo-
lithic age. They were simple huts over a shallow pit in the
ground. They were constructed by digging a round pit a few feet
deep, walling it with poles or stones up to a few feet above the
ground, and covering the structure with branches and a coating
of clay. Frequently a number of such houses have been found
close together, these earliest villages being along waterfronts in
Central Europe. These earlier houses were followed by the pile
villages which were built entirely above ground, either over the
water's edge along some lake or river or over land with a small
brook beneath.

THE LONG ROAD 497

These pile villages became quite numerous about six to ten
thousand years ago and must have sheltered a considerable por-
tion of the late Neolithic population. One of the most elaborate
of the villages and one of the first to be discovered was one on an
old lake shore of Switzerland. Since then their remains have been
found in Germany, France, Italy, and Austria as well as in
England, Ireland, and Scotland. Most of the villages were small,
probably being limited to one or a few family groups; some how-
ever, were of considerable size. One of the largest was on Lake
Constaaice in what is now Southern Germany. It covered an
area of about 230,000 square feet. The number of piles driven
into the ground is estimated to have been about 60,000. In these
pile villages the floors were supported on beams fastened to the
upright piles, and the walls were made of upright slabs or tim-
bers. The roof was supported on poles and probably consisted
of branches or grasses.

Simultaneous with the building of pile villages, Neolithic man
constructed large stone edifices in other parts of the country.
Such buildings were usually on the more fertile plains away from
the streams and lakes of the mountain valleys. Remains of such
structures have been found in France and England. They prob-
ably were used for ceremonial and religious purposes and were
evidently the gathering places of large groups of people from all
the near-by communities. One of the largest was Avebury in
England. It was a complex of stone circles rather than a closed
house, built upon a circular embankment of earth which was
one-fourth of a mile in diameter. From this central theme two
avenues each lined with stones extended about a mile to the
south and southeast to enclose a flat-topped and nearly round
hill.

Of similar design and of a somewhat later period is Stone-
henge in England. It, too, consists of a series of circles marked by
stone walls or large stone trilithons. The outer circle originally
consisted of thirty large upright stones connected on top by a
continuous band of stones. This circle was ninety-eight feet in
diameter and inside it were smaller circles and semicircles of
stones. At the center was a large stone which has been called the
" Alter Stone." Stonehenge was constructed at least four thou-
sand years ago, and parts of it still remain.

498 THIS LIVING WORLD

Dr. James Breasted in "The Conquest of Civilization" con-
cisely points out the significance of these edifices as follows

Furthermore, the stone structures furnish us very interesting glimpses
of the life of the Neolithic towns. Some of them suggest to us whole communi-
ties coming out from the towns on feast days and marching to such places as
the huge stone circle at Stonehenge. It has been thought that here they held
contests and athletic games in honor of the dead chief buried within the stone
circles. Festival processions may have once marched down the long avenues,
marked out by mighty stones. Today, silent and forsaken, they stretch foi
miles across the fields of modern farmers, to remind us of forgotten human
joys, of ancient customs, and of beliefs long revered by the vanished peoples
of Stone Age Europe.1

During this period tombs for burial became plentiful. These
are well represented by the dolmens of France. They were con-
structed of stone slabs. The walls were upright stones, and the
roofs were large stone slabs. Some of the dolmens are as much as
350 feet long and thirty feet high. Many of the covering stones
were very large, some of them being over sixty feet long, twenty
feet wide, and eight feet thick, weighing many tons. To place
these stones on their pillars several feet above the ground was an
engineering feat of no mean order.

A great engineering people lived on the island of Crete in the
Mediterranean Sea as early as six or seven thousand years ago.
They built many intricate structures. Their palaces were rugged
structures of stone and were decorated with frescoes. They knew
the principles of drainage, and many of their buildings had sew-
age disposal systems quite modern in their arrangement. These
people were not some new and superior race, but they developed
from earlier Neolithic stoneworkers, as is evidenced by some of
their remains that have been excavated. One of the cities sur-
rounded a large hill about ninety feet high which was long
thought to be a natural elevation. However, recent excavations
have shown that one city was built on top of the ruins of another
and that earlier cities continued down to the base of the hill.
At the lowest levels have been found the ruins of an early Neo-
lithic village, with the crude tools and culture of their times.

The arch and vault were probably first invented by pre-
historic Assyrians, the Sumerians. The arch was in use in the

1 BREASTED, JAMES H., "The Conquest of Civilization," Harper & Brothers, New
York, 1938, p. 40.

THE LONG ROAD 499

Tigris and Euphrates valleys at least 5,000 years before the time
of Nebuchadnezzar. Since there was a lack of stone and timber
in this country, building materials were chiefly sun-dried clay
bricks. This necessitated the use of an arch for building any
large structures. Some of these early temples were remarkable
for their great height, built chiefly on the pyramid plan, their
large external mass, and the brilliant coloring of the receding
stories.

This brief discussion has mentioned only a few of the cultural
developments of man. Perhaps sufficient has been said to show
that the fundamental beginnings of most of our modern culture
can b6 traced back into the dimly lighted past. These beginnings
came about through a slow process of evolution and they were
worked out under the hard and unrelenting conditions of nature.
The road to civilization has not been easy.

Early Man in America

The cultural development of man in Europe is of particular
significance to people in America because these cultural heritages
form the basis of modern life here. Our present American civiliza-
tion began at the point of development which European culture
had reached at the time of settlement of this country. However,
a culture had been developed by a people which inhabited the
whole of this continent long before the time of Columbus. That
culture was soon destroyed by the white man. It is of particular
interest to us now in throwing some light on the development of
early man in America and in giving some appreciation of the
achievements of a race that was quickly vanquished by our
immediate forefathers.

Just when man first appeared in North America is a question
of great uncertainty at present. It is a question about which
there is as much controversy among anthropologists as there has
ever been among physicists about the nature of cosmic rays.
Many facts indicate that man has been here since Pleistocene
times, that is, for twenty-five thousand years or more. They
are discoveries of human skeletons and artifacts in deposits that
seem to be that old. However, it has not been possible to date
these deposits with a high degree of certainty, and exactly how
old they are is still somewhat undetermined. Furthermore, many

500

THIS LIVING WORLD

Mammoth pit near Clovis, N.M., restored in exact position as found. Bones are of an
extinct mammoth which was common in America during the Pleistocene epoch. Some of
the small objects are human artifacts found with the bones. (Reuben Goldberg.)

of the discoveries have some particular feature about them that
could mark them as being rather recent. Therefore, a great many
students of the question hold that man first came to America not
more than about five thousand years ago.

It is generally agreed that these first people migrated to
America by way of the Bering Straits and Alaska from the
frozen and barren stretches of Siberia. From Alaska, man
wandered over the whole of North America and far into South
America. This is a wide stretch of land, and migrations were slow.
But some five thousand or more years is sufficient time for
primitive people to cover such magnificent distances. These early
Americans were surely a nomad people, only the last two or three
thousand years showing any evidences of community or seden-
tary life. The earliest definitely dated dwelling in the United
States is an Indian house in Arizona which is dated A.D. 660.
However, man had been on this continent long before that time.
He had even developed flourishing cities before the beginning of
the Christian era.

THE LONG ROAD 501

The best indication of man's real antiquity in America is the
remains of human origins which are found in burials associated
with the bones of long-extinct animals. In an old lake bed near
Clovis, New Mexico, have been found stone spear points,
knives, and scrapers in the same strata with bones of mammoth,
extinct horses, and camels. These animals have not been in
North America since shortly after the receding of the last ice
sheet, about twenty -five thousand years ago. It is difficult to ac-
count for this association of human relics except by concluding
that man was living in America at that time. There were buried
in the same strata the bones of mammoth and about twenty
flint knives which showed signs of having been heated by fire.
Only man makes hearth fires and cooks his food. There is no
other known explanation of such charred remains. All this is ex-
cellent indirect evidence that man was in America during
Pleistocene times.

Perhaps the most spectacular evidences of man in America
during the Pleistocene are some of the human skeletons that
have been discovered. One of these is the so-called "Minnesota
man." In Minnesota a female human skeleton has been found
beneath a layer of glacial-lake clays that was the bottom of a
lake which was in existence there during glacial times. The lake
has long since been filled in and is now a part of the rolling
country. The skeleton of this young lady was lying on its side
with the head and shoulders somewhat lower than the rest of the
body, the angle corresponding to the inclination of the stratum
in which it was found. This is quite an unnatural position for a
primitive burial. It is claimed by some that the girl was drowned
in the lake and her body sank to the bottom, there to be covered
over by clays and sand. As the glacier receded farther, the lake
finally disappeared and the present low hills were formed over
the skeleton.

A number of objects of cultural development were found
with this skeleton. These consisted of a dagger of elk antler and
a number of shells believed to have been worn for decoration.
One was a conch shell, apparently worn as a pendant suspended
at the girdle. A number of smaller shells and bones were found
which might have been worn as a necklace or carried for their
magical value. The dagger had been carefully shaped from the

502

THIS LIVING WORLD

Reconstruction of head of Minne-
sota girl, believed by some to have
lived during late Pleistocene times.

antler, and its position seemed to indicate that it had been
suspended from the neck by a thong. Dr. A. E. Jenks of

the University of Minnesota, who
described this skeleton, is firmly of
the belief that it was deposited some
18,000 to 20,000 years ago and rep-
resents an Indian group of people
living in America at that time.

There is every reason to believe
that the skeleton was laid down at
the time the clays were forming and
was not enterred at a much later
burial. Unfortunately, the discov-
ery was made by a road building
crew, which made no careful note
of whether the layers above the
burial had ever been broken. There
will probably always remain some
doubt as to whether it was a natural or a man-made burial.

Human skeletons seeming to have Pleistocene antiquity
have been found in other parts of the United States. As our
information in this respect increases, it will be possible to deter-
mine more accurately how long man has been in America. A
large number of spear points have been found in the United
States which are definitely unlike any of those belonging to
Indians who have lived here during the last two thousand years.
These have been extensively studied and classified. Some 350 such
specimens are known to archaeologists, and these have been
collected in over thirty diiferent states. If the story these
weapons seem to tell is true, then these ancient hunters were
well scattered throughout the United States long before the
time of any accurately dated ruins or remains.

However, American scientists demand proof beyond reason-
able doubt before accepting the idea that man lived in America
during Pleistocene times. At present many anthropologists insist
that he came to these shores not more than five thousand years
ago. Just when man first appeared in America is a question which
will have to be answered in the future. Many lines of investiga-
tion now in progress, such as excavations of ancient deposits,

THE LONG ROAD

Partially restored famous ruin in the Mayan city of Chichen Itza. The beautiful and
unique round building may have been used as a watch tower, astronomical observatory,
or temple. (Carnegie Institution of Washington.)

studies of plant distribution, and soil analysis may shed much
light on this interesting problem.

Indian Cultures

Regardless of the exact time when man first appeared on these
shores, it is a fact that the Indians had made many cultural
developments long before the coming of Columbus. These
developments were made here almost independently of any early
influence from Europe or Asia. We have learned a great deal in
recent times about colorful Indian civilizations which flourished
as early as two thousand years ago. These were civilizations that
were in progress in Mexico, Southwestern United States, Central
America, and Peru.

504 THIS LIVING WORLD

For example, a burial and temple have been discovered in
Panama which have revealed a brilliant civilization existing
there perhaps before the time of Christ. The burial consisted of a
chieftain's skeleton, along with those of twenty female skeletons,
probably those of his wives. It seemed to have been their un-
happy lot to have to die at the time the king passed on to his
final reward. The burial also contained a glittering display of
gold ornaments and nearly two thousand other objects, arrow-
heads, axes, knives, mirrors, and pottery.

In Yucatan, the Mayan culture had been extensively
developed for many centuries before American historical times.
The Mayan civilization began in A.D. 333 and continued until
the people were finally conquered by the Spaniards in 1541. The
Mayan culture at its height was represented by more than one
hundred cities and towns, the largest having populations of
several thousand people. Some twenty or more of these great
communities have now been excavated and partly restored. The
capital city for several centuries was Chichen Itza (meaning
"Holy Well"), and it had several magnificent temples and
public buildings. It probably was the Mayans' greatest city for a
thousand years.

These people had an extensive agriculture and had domesti-
cated many animals, including bees and fowl. They wove cotton
so fine that the ruthless Spaniards mistook it for silk. They made
large canoes and traded with Cuba. They had hieroglyphic
books, a calendar system, and a considerable amount of astro-
nomical knowledge.

The Aztec culture of Mexico was a little later than the
Mayan. It resembled the Mayan culture but was somewhat
more highly developed. The agriculture of the Aztecs was more
extensive; their cities were fortified and skillfully constructed.
They made tools of brass as well as of stone. They had books of
paper and a system of schools. Excavations in the heart of
Mexico City have revealed the ruins of the Old Aztec capital
which stood on the site of the modern city. At Monte Alban in
Mexico a whole city has been excavated. This revealed temples
built in such a manner that the architects must have been familiar
with many of the facts of astronomy. The city also showed a
court where ball games of a kind were held.

THE LONG ROAD

Cliff Palace, largest of the Pueblo ru:n$ at Mesa Verde National Park. (Science Service.)

506

THIS LIVING WORLD

The Pueblo Indians made fine pottery which
was often decorated in exquisite designs, such
as represented above in a piece dating back to
about 11 00 A.D.

The Aztec and Mayan cultures were about at their height
at the time of the coming of the Spaniards. Not only did these

newcomers to America de-
stroy the culture and cities
of these people, but they
burned their books and
obliterated their written
records, after having killed
their priests, medical men,
and government leaders.
As a result of this destruc-
tion much of the history of
these early people can
never be reconstructed.
In the United States
definitely dated native civ-
ilizations extended from
A.D. 660 to 1565. These are
found in the Southwest. The first peoples there are called Bas-
ket Maker Indians. Their cave dwellings and subterranean
houses are found extensively over several states. These Indians
are believed to have lived there long before the earliest dated
house of A.D. 660. Evidences found at Mesa Verde in Colorado
indicate that some of the caves there were occupied by an agri-
cultural people as early as 1000 B.C.

Following the Basket Maker Indians were the Pueblo
Indians. These were an emergent group of people who built the
pueblos, or houses of stone, mostly in the sheltered caves of the
deep canyons. Pueblos began to be constructed about A.D. 800.
The earliest of the larger pueblos is Pueblo Bonito, the "city
beautiful," begun in 919 and abandoned in 1130. The largest
single ancient cliff dwelling is at Kiet Siel in northwestern
Arizona, consisting of 155 rooms. It was completed in 1294.
The largest and most spectacular group of pueblo ruins in the
United States is at Mesa Verde. These were built in several
caves and contain over 500 rooms. Many others of smaller size
are scattered throughout the Southwest.

These structures are all built of sandstone, often fastened
together with huge wood beams and roofed with reeds from the

THE LONG ROAD

507

1^:1^:^

A prehistoric Indian mound near Knoxville, Tenn., being excavated. (Ewing Galloway.)

near-by canyons. The masonry shows a high degree of engineer-
ing skill. The stones and trees were all transported up precipitous
cliffs, some a hundred feet or more high. The oldest continuously
inhabited site in the United States is the pueblo at Oraibi in the

508 THIS LIVING WORLD

Hopi Indian Reservation in Arizona. This village was begun in
1370 and is still occupied by the Hopi Indians.

Recent excavations in the Tennessee Valley and other parts
of the Southeast have yielded much light on the history and
development of the Early Mound Builders of Central North
America. Some of these mounds have three to ten levels of
Indian occupancy, indicating a long history for the site; that is,
a mound would be built by the Indians and a village con-
structed on it. Then when the village was abandoned another
tribe would build the mound higher and erect another village.
Later this would be abandoned and another tribe would come
in to repeat the process. Some of the mounds covered several
acres of ground.

In one mound found in Alabama the top level of the site
contained Cherokee pipes, pottery, and weaving mixed with
glass beads and scraps of iron. The beads and iron are white
man's articles and probably tell the story of De Soto's coming in
1540. The lower levels contain only Indian-made articles, mark-
ing sharply the pre-Spanish era from the era of exploration and
conquest.

Mound Builders' relics are found all over the lower Missis-
sippi Valley. At their highest point the Mound Builders had
great skill at carving stone, weaving cloth and dyeing patterns
into it, hammering copper into ornamental objects, and grow-
ing farm crops. What materials they did not have they obtained
by trade, even from across the Rocky Mountains, a distance of
over a thousand miles. Now it is known that the spectacular
culture of the Mound Builders was not that of a superior,
vanished race, but was that of the Indians who occupied the
territory up to the time of the white man's coming.

So, little by little, the story of the American Indians is being
disclosed. Just when man first arrived on this continent is now
unknown. It certainly was not later than five thousand years
ago. It may have been over twenty-five thousand years ago.
Cultural development had advanced on a wide scale; however,
with only minor exceptions, it was all of the Stone Age type.

REFERENCES FOR MORE EXTENDED READING

COLE, FAY-COOPER: "The Long Road," The Williams & Wilkins Company,
Baltimore, 1933.

THE LONG ROAD 509

A popularly written survey of cultural developments of man in the New Stone Age
and the metal ages down to the beginnings of recorded history. There is also a short
discussion of modern races of man and their relationships to prehistoric peoples.

MACCURDY, GEORGE GRANT: "The Coming of Man," The University Society,
New York, 1935, Chaps. VII-XXVII.

A condensed treatment of the four main periods of prehistoric human culture
as they are manifested in Europe. There is also an account of the development of
some of our cultural heritages through the ages preceding historical times.

OSBORN, HENRY FAIRFIELD: "Men of the Old Stone Age," Charles Scribner's
Sons, New York, 1915.

One of the foremost American paleontologists has produced here a well- written and
extensively illustrated account of Paleolithic peoples in Europe as they were known in
1914. The book treats of the geologic history of man and other forms of life of the
Pleistocene period, as well as the cultural development of man during the Old Stone
Age.

MACCURDY, GEORGE GRANT: "Human Origins/' D. Apple ton- Century Com-
pany, Inc., 1924, Vols. I, II.

This work is a comprehensive treatment of the development of man and the
growth of human culture in Europe during the prehistoric ages that man has been
there. It is extensively illustrated with photographs and drawings, and each chapter
includes a bibliography of original source material.

WISSLER, CLARK: w Indians of the United States," Doubleday, Doran &
Company, Inc., New York, 1940.

The subject of this book is the history and culture of the American Indian during
the last four hundred years. The author describes in a most interesting fashion the
rise of the Indian to the height of his culture and gives an illuminating account of
Indian life today and his contribution to our culture. The great Indian families and
some personalities are described with such detail as to make the Indians a real people
rather than some abstract or foreign group.

COLE, FAY-COOPER, and THORNE DEUEL: "Rediscovering Illinois/' University
of Chicago Press, Chicago, 1937.

This is a comprehensive and technical study of the Mound Builders' culture in
Illinois. A well-documented scientific study that is of primary value to the reader
who is interested in the details of scientific excavations and what they reveal of the
culture of these people.

JENKS, ALBERT ERNEST: " Pleistocene Man in Minnesota," University of
Minnesota Press, Minneapolis, 1936.

A complete discussion of the discovery and physical measurement of a female
human skeleton found in the glacial deposits of Minnesota. The book is profusely
illustrated. Much of the discussion is quite technical; however, introductory and
concluding chapters as well as chapter summaries are easily read.

510 THIS LIVING WORLD

GANN, THOMAS, and J. ERIC THOMPSON: "The History of the Maya," Charles
Scribner's Sons, New York, 1937.

This is a substantial treatment of the origin and history of the Mayan Indians,
together with an account of their cultural and scientific achievements. A number of
illustrations of superior quality appear, and the treatment of the text material is not
highly technical.

WEIDENKEICH, FRANZ: "Six Lectures on Sinanthropus Pekinsis and Belated
Problems," Bulletin of the Geological Society of China, Vol. 19, No. 1, 1939,
published by the Society, Pehpei, Chungking, Szechuan, China.

This recent publication on the China man is an excellent illustrated discussion
concerning the physical characteristics of the Sinanthropus fossils, the relationship of
these early people to each other and to other prehistoric peoples, and their significance
for the problems of human development.

Discovery, published by Cambridge University Press, London, The Macmillan
Company, New York, U. S. agents.

Discovery is a monthly popular journal of knowledge. The articles cover many
fields of science. They are usually well illustrated and written in a popular style.

American Journal of Archaeology, published by the Archaeological Institute of
America, Columbia University, New York.

A quarterly journal which includes an extensive range of articles in the field of
archaeology, all of which are usually extensively illustrated. It contains authentic
and interestingly written series of articles relating to many phases of archaeological
discoveries.

INDEX

Absorption, of foods, 317—321

of oxygen, 326-337
Addison's disease, 446
Adrenal glands, 446
African sleeping sickness, 461
Agriculture, prehistoric, 489-490
Air, 6

amount of, in lungs, 331

composition of, 74-75
Alimentary canal, 308, 309
Alpine race, 264
Alveoli, 325, 326, 328
Amino acids, 112
Ammonites, 195
Amnion, 374
Amoeba, 302, 303
Amphibians, 201-203

early types of, 201

frogs, 203

life cycle of, 202

skin of, 277
Amphioxus, 199

notochord of, 285

skin of, 276
Anabolism, 124
Anatomy, 10

study of, 273-274
Anemia, 352
Animal life, classification of, 17, 196,

domestication of, 489-490
Anthropoid apes, 235-239

as distinguished from man, 242-245
Antitoxins, 465-466
Aorta, 343, 347
Appalachian Mountains, 26

formation of, 50-53
Appendages of vertebrates, 297-299

embryonic growth of, 159

Arch, construction of, 498-499
Archaeopteryx, 217
Aristotle, 6, 7, 17
Arms, bones of, 295-298

embryonic development of, 159
Art, geramic and textile, 486-489

development of, 490-495
painting, 492, 493
pottery, 487
sculpture, 491
Arteries, 342-344
Arthropods, 187-188

crabs, 188

insects, 187

Association paths, 430—432
Atlantic Coastal Plain, 26
Atmosphere, 74—77

movement of, 78-84
Auditory patterns, 409
Australian race, 268
Axon, 423-425
Aztec culture, 504

Bacteria, 15, 137-139, 451

defenses of body against, 462—466
nature of, 453-457
nitrogen fixing, 126-127

Bacteriophages, 116
197 Basalt, 32, 35

Basket Maker Indians, 506

Batholiths, 31

Berger, Hans, 438

Beriberi, 470, 473

Beringer, Johann, 168

Biotic community, 61—63

Birds, 217-218
skin of, 279
wings of, 298
511

THIS LIVING WORLD

Blastopore, 149, 150, 153, 155
Blastulation, 149, 153
Blood, circulation of, 346-348

clotting of, 349

components of, 348, 350-353

regulation of composition of, 358-361
Bone, 286

Bowman's capsule, 359
Brachiopods, 188-190
Brahe, Tycho, 9
Brain, 244, 422, 426

localization of functions of, 433-438

structure of, 432-435
Brain waves, 438-441

characteristics of alpha type of, 439-440

meaning and significance of, 441

method of recording of, 438, 439

types of, 439

Breathing process, 323-331
Brontosaurus, 177, 208, 210-211
Bronze Age, 481

sculpture and architecture of, 494

tools of, 486
Bruno, Giordano, 8

Cambrian epoch, life during, 182, 183, 187,

188, 190, 194
Cancer, 467, 469
Capillaries, 336, 344
Carbohydrates, 109, 110

absorption of, 318

digestion of, 310-317

manufacture of, 121
Carbon dioxide, in blood, 322-326

energy cycle of, 125, 126
Carnivores, 227-231
Catabolism, 124
Cell doctrine, 302
Cells, differentiation of, 139-141

division of, 143-147

growth of, 141-143

nerve, 423, 424

organization of, 131, 133-137

suitable environment of, 335

types of, 303-304
Cenozoic era, 171, 174

dominant forms of life of, 222-235
Centrosome, in cell division* 144
Cephalopods, 191-198

Cephalopods, dominance of, 194, 195

Cerebellum, 434

Cerebrum, 435-437

Chambered nautilus, 191

Chemical senses, 410-412

Childbirth, 375-377

Chimpanzee, 236, 237

"Chinese man," 250

Chordates, 198-199

Chorion, 373

Chromosomes, 134

in cell division, 144, 145-147
and heredity, 383-387

Circulation of blood, 346-348

Classification of animals, 18, 19, 196, 191

Cleavage, 148, 158

Climate, 71, 93-98

effects of, on life, 95-98

in geologic times, 97, 176-178

Clothing, prehistoric, 488

Clostridium botulinum, 454

Clouds, 86-88

Cochlea, 406, 407

Coelenterates, 181-185

Cold, common, 457-458, 466

Cole, Fay-Cooper, 479

Colloidal state, 117-119

Color perception, 401

Columbia plateau, 31-32

Commensalism, 452

Condensation of water vapor, 86-90

Cone-cells of retina, 398, 399, 401

Coordination, in embryonic development,

162-163

Copernicus, Nikolaus, 8
Corium of skin, 275
Cornea, 397, 403
Corpus luteum, 369, 370
Corti, organ of, 407, 408
Crabs, 188
Crater Lake, 35
Creodonts, 227
Cretin, 445
Crinoids, 186
Crocodiles, 216-218
Cro-Magnon man, 257, 258, 483-485,

487, 492

Cryptozoon fossils, 180
Culture, human, development of, 479-510
abodes, 495-499
Agriculture, 489, 490

INDEX

513

Culture, human, in America, 499-503

of art and writing, 490-495

ceramic and textile arts, 486-489

chronology of, 480-481

domestication of animals, 489, 490

Indian cultures, 503-508

tools, 481-486

writing, 494-495
Cyclones, 90-93

cause of, 92
Cynodonts, 220

Darwin, Charles, 20-23
Davis, Hallowell, 440
Dawson, Charles, 252
Deficiency diseases, 470-476
Dendrites, 423-425
Dermis of skin, 275
Deserts, 61, 95

life on, 62
Diabetes, 447
Diastrophism, 45—47
Differentiation, of cells, 139-141

in embryonic development, 158-161
Digestion, 310-317^

in amoeba, 302

of fats, 316, 317

of proteins, 312, 313, 315, 316

of starches, 310, 315-317

of sugars, 315-317
Digestive system, 307, 308
Dinosaurs, 176, 178, 207-213
Diphtheria, 458, 465
Diplovertebron, 201
Diseases, 11, 14, 16

causes of, 451-453

defenses against bacterial, 462-466

deficiency, 470-476

functional, 466-470

infectious, 457-462

prevention of, 476-478
Divergence of life forms, 196-200
Doldrums, 81, 94
Domestication of animals and plants, 489,

490
Dominant life groups, 193-195

mammals, 218

reptiles, 213

Dominant and recessive characters in

heredity, 385-386, 388, 889
Drosophila, in study of heredity, 384
Dubois, Eugene, 241
Ductless glands, 422, 441-448
Dujardin, Felix, 133
Dwelling places, evolution of, 495—499

Ear, origin of bones of, 291, 292

structure of, 405-408
Earth, 6, 8

relief of, 26-27

shifting surface of, 45-47
causes of, 53-55

surface of, 25-65

waters of, 67-69
Echinoderms, 185-187

comb jellies, 185

crinoids, 186

starfish, 187
Ectoderm, 150
Electro-encephalograph, 439
Elephants, 224-227

African, 225

Indian, 226

Mastodon stock of, 224, 225
Elimination of body wastes, 356
Embryonic development, 148, 149

of frog, 151, 163

human, 372-375

of sand dollar, 149-151

sequence of stages in, 147
Embryonic fields, 158
Endocrine glands, 442-448
Endoderm, 150
Energy cycle, carbon atom in, 125-126

nitrogen atom in, 126-127
Enzymes, 113-114

from bacteria, 456

digestive, 310
Epidermis of skin, 275
Erosion, 81, 55-59
Eurypterid, 187
Evocation, 151-155
Excretion of body wastes, 357-362
Eye, embryonic development of, 160-161

structure of, 396-399

514

THIS LIVING WORLD

Fallopian tube, 370, 371
Fats, 110-111

absorption of, 319- 20

digestion of, 316-317
Fertilization of ova, 371, 881
Fever, 464

Fibrin in blood, 348, 349
Fields, embryonic, 158
Filtrable viruses, 114-116
Fish, bony, 199

lobe finned, 200

lung, 200

ray finned, 199
Fletcher, Harvey, 408
Fog, 89, 90
Foods, absorption of, 317-321

and diet, 476

in tissue fluid, 336

vitamins in, 473-475
Fossils, 47, 50

early ideas of, 168

nature and meaning of, 168
Frogs, 202-203
Functional diseases, 466-470
Funk, Casimir, 472

Galen, Claudius, 10-13
Galileo, 7
Gall bladder, 314
Gametes, 365

maturation divisions of, 378-379

product ior of, in female, 369

in male, 366
Ganglia, 425, 426
Gastrulation, 15O-151, 153
Geologic eras, 170-172

boundaries of, 172-174

changes within, 174-178

chart of, 171
Geosyncline, 50-53
Genes, 134

dominant and recessive, 385-386

units of heredity, 383-387
Gerard, R. W., 441
Germ layers, primary, 151
Gibbon, 237
Glaciers, during Pleistocene, 245-248

Glands, endocrine, 442

of skin, 282-284
Goddard, H. H., 389, 390
Gonads, as ductless glands, 447
Gorilla, 238, 239
Graafian follicle, 369
Grand Canyon, 35, 37

erosion of, 57

strata of, 38
Great Lakes, 46-47
Greenville formations, 172
Growth, 102-103

regulation of, in embryo, 162-163
Gulf Stream, 70, 71

Hair in skin, 281
Hands, bones of, 299
Harrison, Ross Granville, 158
Head, bones of, 286-292
Health, maintaining, 476
Hearing, 405-410

auditory patterns in cochlea, 408, 409
Heart, cycle of operation of, 339-340

diseases of, 467, 468

embryonic development of, 159-160

sounds of, 340

structure of, 338-339
Heat blanket, 77, 78
Heidelberg man, 253
Hemoglobin, 326, 327, 350-351
Hemophilia, 350
Henry Mountains, 30, 31
Heredity, in man, 387-392

physical basis of, 383-387
Himalaya Mountains, 26, 47, 174
Hippocrates, 9, 10
Homologous chromosomes, 384, 385
Hooke, Robert, 132
Hookworm, 461-462
Hormones of ductless glands, 443-448
Horse, 223, 224
Horse latitudes, 82
Hrdlicka, Alex, 254, 257
Human body, early study of, 9-13

organization of, 301-306
Humidity, 85, 92
Humus, 60

Huxley, Thomas Henry, 107
Hybrid, 388

INDEX

515

Igneous rocks, £8-35

extrusives, 31-85

formation of, 29

intrusives, 29-81

source of, 35
Immunity, 464-466
Indian, American, racial classification of,

268
Indian cultures, 503-508

Aztec, 504

Basket Maker, 506

Mayan, 504

Mound Builders, 508

Pueblo, 506-508
Induction, embryonic, 155-158
Infectious diseases, 457-462
Inheritance of hereditary traits, 383-387
Insects, 187-188
Interaction in embryonic development,

161-162
Interdependence of living things, 61-63,

124-127

Intestines, 309, 314-320
Iron Age, 481
Irritability, 103-105*

Java man, 241, 248-250

Jaws, articulation of, 290-292

Jellyfish, 183

Jenks, A. E., 502

Jupiter Serapis, temple to, 45-46

Kallikak families, 390
Kidneys, 358

diseases of, 467, 470

structure of, 359

work of, 360-361
Koch, Robert, 15
Krakatoa, 33-34

Laccoliths, 31

Lake dwellings, prehistoric, 487, 496

Lava, 29-36

Legs, bones of, 295-298

Lemurs, 233

Life, continuity of, 105-107, 143, 164

energy of, 119-124

Limbs, embryonic development of, 159
Limestone, 41, 42, 50
Lingula, 190
Linnaeus, Carl, 18-20
Lions, 229
Lipoids, 110-111
Liver, 314

Living things, characteristics of, 102-105
Lung fish, 200
Lungs, 323, 325, 328, 330

amounts of air in, 331
Lymphatic system, 354—355

McGregor, J. H., 249
Maffei, Giovanni, 3
Magma, 28, 33

and metamorphism, 42, 43

source of, 35
Malaria,' 459-461
Mammals, 218-239

carnivorous, 227-231

characteristics of, 218-220

classes of, 221

early development of, 220-222

Golden Age of, 222

primates, 231-239

skin of, 280-284

ungulates, 223-227
Mammary glands, 219, 283-284
Mammoth, 225, 227
Man, "Age of," 248

distinguishing features of, 242-245

early, in America, 499-503

prehistoric, 248-258
Mantle rock, 27-28
Maturation divisions, 378-380
Mauna Loa, 33, 34
Mayan culture, 504
Mechanical balancing in earth crust

movements, 53—54

Mechanics of breathing, 329-331, 380
Medicine, history of, 9-16
Mediterranean race, 265
Medusa, body patterns of, 184
Mendel, Gregor, 887

516

THIS LIVING WORLD

Mesoderm, 150-151
Mesozoic era, 175-178

dominant forms of life during, 207-212
Metabolism, 102, 124

wastes of, 357
Metamorphic rocks, 41-45

formation of, 42-43

kinds of, 43-44
Miacis, 228

Minnesota man, 501-502
Mitosis, 144-146

significance of, 147
Mohl, Hugo von, 107
Molluscs, 190-193

cephalopoda, 191-198
chambered nautilus, 192
octopus, 193
squid, 192

snails, 191
Monkeys, 234
Mound Builders, 508
Mountain building, 47

by faulting, 48-49

by folding, 51-53
Myelin, sheath, 425

Neanderthal man, 254-256

Negroid race, 266

Neolithic cultural period, 481

agriculture during, 489-490

art during, 493

buildings of, 496-497

ceramic and textile arts during, 488-489

domestication of animals during, 489-

490

Nerve action, 414-419
Nerve impulse, nature of, 416-418

speed of, 415
Nerves, 422-424

auditory, 408

motor, 424, 427

olfactory, 411

optic, 397, 401-403

polarization of, 418

sensory, 424, 427
Nervous system, 421, 422

structure of, 423-427
Neuron, 423-424
New Stone Age, 481

Nitrogen-fixing bacteria, 457
Nordic race, 263
North Temperate zone, 83
Notochord, 198, 293, 294

in embryonic development, 153

Ocean currents, 71-73
Oceans, depth of, 68

movement in waters of, 67, 70-74

temperatures of, 69-70
Octopus, 193
Old Stone Age, 481
Olfactory receptors, 411
Optic nerves, 403
Orang, 237

Organ of Corti, 407, 408
Organizers, in embryonic development,

155-157
Organs, of the body, 306

elementary, 150-153, 155
Origin of Species, 2O-23
Ostracoderms, 199, 287
Oxidation, 121-122, 322
Oxygen in blood, 325-327
Ova, maturation of, 378

production of human, 368-370
Ovaries, 369, 370

Pain, sense of, 414

Painting, prehistoric, 492-493

Paleolithic cultural period, 481

art during, 492

tools used during, 482
Paleozoic era, 170, 171, 173

life during, 182-201
Pancreas, 314

as ductless gland, 447
Parathyroid glands, 445
Pasteur, Louis, 13-16
Pellagra, 473-474
Peptide linkage, 112-113
Perception of color, 401
Peristoltic waves, 315
Photosynthesis, 121
Pile villages, 487-488, 496-497
Piltdown man, 253
Pithecanthropus, 249

INDEX

517

Pituitary gland, 443-444
Placenta, 373, 374

Plants, and animals, principal differences
of, 120-123

and soils, 59-63
Plasma, 348, 349
Plato, 5-6

Pleistocene epoch, 171, 175, 245-248
Plesiosaurs, 209
Pneumonia, 458
Polar front, 92
Polyp, body pattern of, 184
Pompeii, 166-167
Pottery, prehistoric, 487, 488
Pre-Cambrian life, 178-182
Precipitation of moisture, 69, 88
Prehistoric man, 248-258

Chinese, 250-252

Cro-Magnon, 256-258

cultural development of, 479

Heidelberg, 253

Java, 241, 248-250

Neanderthal, 254-256

Piltdown, 252-253
Prevailing westerlies, 83
Primates, 231-239
Proteins, 111-116 *

absorption of, 319

digestion of, 312-316

Protoplasm, chemical composition of,
107-114

colloidal state of, 118

physical properties of, 116-119
Pterodactyls, 209-210
Pueblo Indians, 96, 506-508
Pulmonary circulation, 346
Pylorus, 312

Races, modern, 259-269

Australian, 268-269

Negroid, 265-267

origin of, 259-261

physical characteristics of, 262

white, 262-265

yellow, 267-268
Rain, formation of, 86
Red blood corpuscles, 326, 327, 350-352
Redi, Francesco, 106
Reflex action, 427-430

Reflex arc, 428
Relative humidity, 85, 86
Relief divisions of U. S., 26-27
Reproduction, human, 365-377

organs of, 367, 370
Reptiles, 206-217

Age of, 213

distinguishing characteristics of, 206—
207

early development of, 207

modern forms of, 214-217

"ruling" types of, 208-213

skin of, 278

Respiration, 122-123, 322, 323, 324
Retina, functioning of, 399-401

structure of, 398, 399, 400
Rhodesian man, 260
Rickets, 474
Rock, igneous, 28-35

mantle, 27-28

metamorphic, 41-45

sedimentary, 35-45
Rocky Mountains, 53, 95
Rod-cells of retina, 398, 399
Rosland family, and heredity, 391
Ross, Major Ronald, 460
Royal Gorge, 58

Saber-toothed tiger, 230
Scales, in skin, 277-279
Schleiden, Jakob, 133
Schwann, Theodor, 133
Scurvy, 474
Sea anemone, 184
Sea lily, 186
Seals, 230, 231
Sedimentary rocks, 35—41

formation of strata of, 38-39

materials of, 39-41
Semicircular canals, 409-410
Seminiferous tubules, 366
Sense organs, 395-414

ear, 406-408

eye, 397-400

olfactory receptors, 411

pain receptors, 414

taste bud, 412

temperature receptors, 414

touch receptors, 414

518

THIS LIVING WORLD

Sex, determination of, 380-383

Sex chromosomes, 380, 381

Sex hormones, 447

Seymouria, 207, 208

Sharks, 278, 288, 289

Shiprock formation, 30

Sierra Nevada Mountains, 31, 49, 95

Sinanthropus, 250, 251, 252

Skeleton, 284-286

of head structure, 286-292

of legs and arms, 295-299

origin of, 284

of vertebral column, 293-295
Skin, 274

of birds, 279

derivatives of, 281-283

development of, 276-280

layers of, 275

of birds, 279

of mammals, 280, 281
Skin senses, 412-414
Skull structure, 286-292
Smell, sense of, 410, 411
Snakes, 214
Soils, 59-61

effects of, on life, 61-64
Solar plexus, 425
Speech, capacity for, 245
Sperm cells, human production of, 365-
367

maturation divisions of, 378
Spinal cord, 422, 425-426
Spinal meningitis, 458
Spontaneous generation, theory of, 105-

107

Aristotle and, 17
Spores, 454
Squid, 192
S-R bond, 427-430
Stanley, W. M., 115
Starches, 110

absorption of, 318

digestion of, 310-317
Starfish, 186, 187
Stegocephalian, 201
Stegosaurus, 211, 212
Stereoscopic vision, 403
Stomach, 307, 312, 313
Stonehenge, 497, 498
Stratosphere, 75, 76
Sugars, 109

Sugars, absorption of, 318

digestion of, 316, 317
Sweat glands, 282, 283
Symbiosis, 127, 452
Synapse, 430, 431
Syphilis, 458, 459
Systemic circulation, 343-346

Tarsius, 233, 234

Taste, 412

Teapot Dome Rock, 39

Teeth, 278, 279

Temperate zone, 94

Testes, 366, 367

Tetany, 445

Threshold stimulus of nerve impulse, 417

Thymus gland, 446

Thyroid gland, 444, 445

Tides, 73, 74

cause of, 73
Tissue fluid, 335, 336
Tissues, connective, function of, 304

of living things, 140-141
Tools, 481-486

copper and bronze, 486

in deposits near Peking, China, 482

flint and bone, 482-485

hand-ax, 482-483

polished stone, 485-486

used by Cro-Magnons, 483-485

used in hunt, 484-485

used to make clothing, 484
Tornado, 78, 79
Trade winds, 70, 71, 81
Triceratops, 212, 213
Trilobites, 194
Turtles, 215, 216
Typhoid, 458, 466
Tyrannosaurus, 208

Ungulates, 223-227
Urinary organs, 358-362
Urine, 360-362

Vaccination, 15

Vault, construction of, 498

INDEX

519

Veins, 342, 345
Vena cava, 345, 347
Vermiform appendix, 321
Vertebral column, 293-295
Vesalius, Andreas, 18
Villi, 318, 319
Viruses, 114-116
Vision, 395-405

color perception, 401

stereoscopic, 401-403
Visual pink, 401
Visual purple, 400
Vitamins, 470-476
Volcanoes, 33-35
Von Mohl, Hugo, 107

Wald, George, 401
Walrus, 228, 230
Water, 66-67

erosion produced by, effects of, 56-59

sources of, 6&-70

Water vapor, 84-86
Weather, 84-90

ocean currents, effect of, on, 71-74
Weathering of rocks, 56-59

agents of, 55

Weaving, prehistoric, 488, 489
Weidenreich, Franz, 251
Whales, 231
White blood cells, 353

and disease, 463-464
Winds, 78-83

erosion, produced by, 55
Woodbury, Angus M., 62
Writing, development of, 494-495

Yellow race, 268

Zion Canyon, 62

biotic relationships in, 62-64

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "This Living World"

See other formats