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Presented by

Marion Irwin Osterhout
February 1966

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THE COLVER LECTURES
IN BROWN UNIVERSITY
1922

THE NATURE OF LIFE

BY

W. J. V. OSTERHOUT

COLVER LECTURES

HUMAN LIFE AS THE BIOLOGIST SEES IT
By Vernon Keh^logg

THE RISE OF UNIVERSITIES
By Charles H. Haskins

THE NATURE OF LTrE

By W. J. V. OSTERHOUT

Published by
HENRY HOLT AND COMPANY

BROWN UNIVERSITY, THE COLVER LECTURES, 1922

THE NATURE OF LIFE

BY
W. J. V. OSTERHOUT

PROFESSOR OF BOTANY, HARVARD UNIVERSITY

NEW YORK
HENRY HOLT AND COMPANY

COPTRIGHT, 1924
BT

BROWN UNIVERSITY

FEINTED IN THE
UNITED STATES OF AMEEICA

The Colver lectureship is provided by a fund
of $10,000 presented to Brown University by
Mr. and Mrs. Jesse L. Rosenberger of Chicago in
memory of Mrs. Rosenberger's father, Charles
K. Colver of the class of 1842. The following sen-
tences from the letter accompanying the gift ex-
plain the purposes of the foundation : —

"It is desired that, so far as possible, for these
lectures only subjects of particular importance
and lecturers eminent in scholarship or of other
marked qualifications shall be chosen. It is
desired that the lectures shall be distinctive and
valuable contributions to human knowledge,
known for their quality rather than their number.
Income, or portions of income, not used for lec-
tures may be used for the publication of any of
the lectures deemed desirable to be so published."

Charles Kendrick Colver (1821-1896) was a
graduate of Brown University of the class of 1842.
The necrologist of the University wrote of him:
"He was distinguished for his broad and accurate
scholarship, his unswerving personal integrity,
championship of truth, and obedience to God in
his daily life. He was severely simple and un-
worldly in character."

The lectures now published in this series are: —

1916

The American Conception of Liberty and Govern-
ment, by Frank Johnson Goodnow, LL.D.,
President of Johns Hopkins University.

1917

Medical Research and Human Welfare, by W. W.
Keen, M.D., LL.D. (Brown), Emeritus Pro-
fessor of Surgery, Jefferson Medical College,
Philadelphia.

1918

The Responsible State; A Reexamination of Funda-
mental Political Doctrines in the Light of
World War and the Menace of Anarchism,
by Franklin Henry Giddings, LL.D.,
Professor of Sociology and the History of
Civilization in Columbia University; some-
time Professor of Political Science in Bryn
Mawr College.

1919

Democracy: Discipline: Peace, by William Ros-
coE Thayer.

1920
Plymouth and the Pilgrims, by Arthur Lord.

1921

Human Life as the Biologist Sees It, by Vernon
Kellogg, Sc.D., LL.D., Secretary, National
Research Council; sometime Professor in
Stanford University.

1922

The Nature of Life, by W. J. V. Osterhout,
Professor of Botany, Harvard University.

1923

The Rise of Universities, by Charles H. Haskins,
Ph.D., LL.D., Litt.D., Gurney Professor of
History and Political Science, in Harvard
University.

VI

CONTENTS

PAGE

I. The Origin of Life .... 3

II. Growth 8

III. Reproduction 22

IV. Motion 29

V. Irritability 34

VI. Metabolism 45

VII. Selective Permeability ... 74

VIII. Electrical Forces 84

IX. The Control of Life .... 88

Index Ill

vu

THE NATURE OF LIFE

THE NATURE OP LIFE

THE ORIGIN OF LIFE

At the present time astronomers present
a picture of the evolution of the universe
which holds the imagination captive.
Some of them believe that all kinds
of matter have been evolved from one
original substance, hydrogen, and that
out of the material thus created solar
systems were built up. They are able to
give us a fairly satisfactory description
of the processes which formed bodies
like our earth. Their account is supple-
mented by the geologist, who pictures the
progressive changes on the surface of the
earth whereby it became fitted to support
life. The fascination of these researches
is heightened when we consider that they
lead directly to a question of universal
interest which lies in the province of the

3

4 THE NATURE OF LIFE

biologist, How did life make its appear-
ance on our planet?

To this question an answer was given
long ago by Lucretius and others, who
said that life arose out of lifeless mate-
rials. This is known as the doctrine of
spontaneous generation.

The adherents of this doctrine be-
lieved that life could arise from non-
living materials whenever the conditions
were favorable. For a long time this
belief found favor with many thinkers.
But the experiments of Pasteur and Tyn-
dall showed that if all the living organ-
isms in a nutrient solution were killed,
and if it were kept free from contamina-
tion by germs from without, no life sub-
sequently appeared.

In spite of this evidence the doctrine
of spontaneous generation was revived
from time to time. One of the ablest
botanists of the past generation pre-
dicted that we should one day discover
living forms too small to be seen by our
microscopes; these, he said, represent

THE ORIGIN OF LIFE 5

the earlier steps in the evolution of liv-
ing forms from lifeless matter. This
prediction has been verified in so far as
we now know a considerable number of
such forms (filterable viruses) some of
which cause important diseases. They
can not be detected by the ordinary mi-
croscope; they pass through filters which
retain all the ordinary bacteria. But we
do not think of them as lending support
to the doctrine of spontaneous genera-
tion, since there is no proof that they
can arise from lifeless material.

How then did life originate.^ Are we
not forced to assume that somewhere,
at some time, spontaneous generation
must have taken place .'^ Although no
such process appears to occur at present
we may nevertheless suppose that in
earlier geological epochs and under more
favorable conditions it might have hap-
pened. And if, as Arrhenius supposes,
life can originate on any appropriate
heavenly body and spread thence to
other bodies we have an immense extent

6 THE NATURE OF LIFE

of time and space in which to find con-
ditions favorable to the origin of Hfe. It
may be that such conditions have never
existed on our planet and perhaps have
occurred but rarely in the history of the
universe. It is not impossible, however,
that we may learn of their occurrence,
in the past or the present, since the spec-
troscope gives us accurate information
about the composition of heavenly bod-
ies and, in the case of distant stars, tells
us what they were like thousands of
years ago. If we do not observe on
the earth the conditions necessary for
the origin of life we may perhaps hope
to find them in some of these heavenly
bodies which might differ su£Bciently
from our planet to provide the necessary
combination of factors.

Arrhenius thinks that spores of bac-
teria might be carried to the upper limits
of our atmosphere and thence be expelled
into interstellar space, poetically called the
"ether sea." There the spores might be
driven away from the sun by the action

THE ORIGIN OF LIFE 7

of light, which might exert on such small
bodies pressure sujBScient to carry them
to the outermost limits of our solar sys-
tem. Thus interstellar space might con-
ceivably be peopled with spores which
could come in contact with any heavenly
body that had reached a stage in its de-
velopment at which life could be sup-
ported.

It has been objected that the spores
might be killed by intense cold, dryness,
lack of air, or the action of light. But
some spores are resistant to these influ-
ences and it is by no means certain that
they could not survive a long time in in-
terstellar space.

The theory of Arrhenius stands out
as a stimulating example of speculative
thought. It is inspiring to picture life,
taking flight from worlds outworn to
fresh fields in younger planets, and per-
sisting as long as the universe can har-
bor it, in cycle on cycle of endless progress.
We may admire this beautiful theory as a
splendid achievement of the creative

8 THE NATURE OF LIFE

imagination but we cannot at present
prove or disprove its correctness. If it
should one day turn out to be true it will
greatly widen the possibility of finding
appropriate conditions for the origin of
life.

n

GROWTH

Leaving this riddle of the origin of
life, let us turn to another question of
equal importance. What new factor
entered into the universe with the first
appearance of life? We may perhaps
put this in a more concrete form by ask-
ing, How may we distinguish the living
from the dead?

It may not seem very difficult to an-
swer this question but the matter is less
simple than might at first appear. As
an illustration let us take some dry
seeds. Their appearance does not tell
us whether they are alive or dead. Most
people if called upon to decide would

GROWTH 9

plant them, and use growth as a test of
Hfe.

If we are to employ growth in this man-
ner it is important to have a clear under-
standing of what it means. Growth is
often thought of as comprising the whole
development of the organism. Ordina-
rily the life cycle of an animal or plant
begins with a single cell, which by
repeated division produces a mass of
cells. The form of the organism then
changes, and its parts become differen-
tiated so as to perform different func-
tions.

The question now arises, What is
essential to the conception of growth?
A simple illustration will make it clear
that growth may go on without cell divi-
sion, change of form or color, differentia-
tion, or assimilation of food. A small,
spherical, green cell, desiccated by the
drying up of a pool in which it has lived,
and blown about by the wind, eventually
falls into water. Such a cell often re-
mains alive and when it again finds it-

10 THE NATURE OF LIFE

self in water begins to grow. No one
will deny that this is genuine growth
but it certainly need not possess all the
features which we have enumerated. In
the jfirst place cell division may be absent
for a long time. Many cells increase
enormously in size and never undergo
division. A nerve cell may grow to be
many hundred times its original length
without dividing; and it will continue to
function for years and finally die with-
out any sign of nuclear or cell division.
We cannot therefore regard cell division
as essential to the conception of growth,
though in most cases it accompanies
growth and is advantageous because it
provides separate compartments in which
the diverse processes of the organism can
go on without mutual interference.

There may be no change of form or
color in the green cell of which we are
speaking, since it may remain green and
spherical while growing. Nor is there
any reason to suppose that in general a
change of form is essential to gro^vth.

GROWTH 11

It commonly occurs but is by no means
indispensable. Nor is it necessary that a
differentiation of the organism into un-
like parts should take place in order that
a process may be called growth. Such
differentiation is not observed during the
growth of the simplest cells, such as bac-
teria, which may have at the beginning
all the parts they possess when growth
is complete.

Of especial interest is the assimilation
of food and the building up of those sub-
stances which are characteristic of each
kind of organism. We know that seeds
can grow for weeks in the dark, absorb-
ing nothing except air and water. Under
these circumstances the food which is
stored in the seed steadily decreases. A
kidney bean grown under such condi-
tions may reach a height of four feet and
gain in weight more than fifty fold. Yet
this great gain in weight is wholly due
to the water it absorbs. Its dry matter
steadily decreases during the whole pe-
riod, undergoing a process of combustion

12 THE NATURE OF LIFE

which results in continually giving off
carbon dioxide to the air. In this way
nearly half the dry material may disap-
pear during growth.

It is true that growth must eventually
cease under these circumstances but the
fact that it can go on for so long although
the plant takes in no food shows that
increase in dry weight is not necessary
for growth.

Since we find that growth may occur
without increase in dry weight, change
of form or color, cell division, or differ-
entiation, we may ask. What is really
essential to growth.'^ The answer seems
to be. An increase in size due to the ab-
sorption of water. Let us now look into
this more closely.

It is a very striking fact that when
dry seeds are planted in moist soil the
dead seeds appear to grow in the same
way as the live ones during the first few
hours. We find, however, that a dead
seed soon stops growing while the living
one continues. This suggests that the

GROWTH 13

water is not absorbed in quite the same
manner in the two cases. Absorption
of water may occur in two ways, which
are known as imbibition and osmosis.
Imbibition is the process which occurs
when a piece of dry wood is placed in
water. The water is taken up into mi-
nute pores, other processes follow, and
the result is a swelling which, though
short-lived, can develop great pressure.
At one time granite blocks were split
open by drilling holes in a straight line
and inserting plugs of dry wood. These
were covered with wet rags, the wood
absorbed water and the granite block
was split. Careful measurements show
that starch may develop a pressure of
thirty thousand pounds per square inch
in taking up water. It is therefore no
wonder that a ship loaded with rice is
quickly burst asunder if water reaches
the cargo.

In osmosis water is absorbed in a dif-
ferent way. This may be illustrated by
the story of the good abbe who hid a

14 THE NATURE OF LIFE

skin of wine in the cistern of the abbey.
When the monk developed an unusual
taste for water he investigated and found
to his horror that the skin had burst.
The wine had taken up water through
the skin because it contained substances
which attract water (the word "attract"
is here used in a somewhat figurative
sense). In the living cell there is a pro-
toplasmic membrane which corresponds
to the skin, and inside this a solution
which attracts water. As water is
taken up the protoplasmic membrane is
stretched, and if there is a cellulose wall
outside the living membrane it shares
the same fate. The living membrane
can be stretched almost indefinitely be-
cause the cell can furnish it with new
material so that it can continue to ex-
pand without rupture. At the same time
the cell can produce substances which
attract water. It is therefore possible
for growth to continue indefinitely.

The growth of the dead seed is due to
imbibition while that of the living seed is

GROWTH 15

due during the first few hours principally
to imbibition, after that principally to
osmosis. We should therefore expect
that the dead seed would soon stop
growing while the living one would
continue.

Osmosis does not ordinarily develop
so much pressure as imbibition but it is
supposed that the pressure it produces
in the living cell may reach three hun-
dred pounds per square inch or even
more: this is as much as is commonly
found in steam boilers. It is sufficient
to drive ferns up through macadamized
roads and concrete sidewalks and to en-
able toadstools to lift heavy flagstones.

Let us now consider whether there is
anything in growth which can be used
as a criterion of life. We have tried first
of all to discover what is essential to
growth. Such things as cell division,
change of form, differentiation, and the
assimilation of food may be taken away,
and yet growth may go on for a long
time. One process cannot be dispensed

16 THE NATURE OF LIFE

with, the absorption of water. This ap-
pears to be the essential thing.

If growth consists of the absorption
of water can this serve as a test to dis-
tinguish the Kving from the dead.f^ As
we have seen, absorption of water takes
place by imbibition or by osmosis. Im-
bibition cannot serve as a mark of dis-
tinction for it goes on in the same way
in dead and in living seeds. If we are
to employ growth as a test of life it can
be only on the ground that osmosis is
in some way peculiarly characteristic of
living cells. Let us see whether this is
the case.

One way of attacking this question is
to attempt to make an artificial cell
which will act like the living. We may
employ for this purpose two solutions,
A and B, such that a drop of A intro-
duced into a vessel containing B will
react with it and form a membrane
which is impervious to both A and jB,
but is permeable to water. We have now
what we may for convenience call an ar-

GROWTH 17

tificial cell. It consists of a membrane
in the form of a rounded sack which com-
pletely incloses a drop of the solution A
and which is surrounded by the solution
B. If now solution A is more concen-
trated than solution B water will be at-
tracted by solution A and will pass into
the artificial cell which in consequence
will expand and stretch the membrane.
Under the proper experimental condi-
tions this may continue for a long time.
We may employ for such experiments
a great variety of materials, as copper
salts in a solution of potassium ferro-
cyanide, metallic salts of various kinds
in a solution of water glass, or tannic
acid in a solution of gelatin. In some
cases the artificial membrane expands
by repeated rupture and repair, in others
it is steadily stretched without rupture,
and at the same time strengthened by
the deposit of new material. The pro-
toplasmic membrane might conceivably
expand in either way. It is not certain
which method is followed.

18 THE NATURE OF LIFE

In both the living and the artilBcial
cell growth is quickened by increase of
temperature. In the living cell there is
an upper limit of temperature beyond
which no growth takes place. This
seems to be due to the proteins of the
living cell. If we could employ such pro-
teins in the membrane of the artificial
cell we might obtain a similar result.

The rate of growth depends, in the
living as in the artificial cell on the sup-
ply of substances within the membrane
which can attract water. In the case of
the living cell these are mostly sugars,
organic acids, salts, and so on, and we
can employ these same substances in the
artificial cell. In the living cell we often
find starch, which takes little part in
attracting water but which may be grad-
ually transformed into sugar which at-
tracts water actively. In the same way
we may place starch in the artificial cell
and have it slowly transformed to sugar
and thereby cause the cell to take up
water.

GROWTH 19

If the artificial cell is placed in a solu-
tion which is more concentrated than
that inside the cell, water is attracted
from the cell to the outside solution and
in consequence the cell shrinks. This
is also true of the living cell. If it is
growing in tap water it can be made to
shrink by putting it into a sugar solution
which withdraws water. If replaced in
water it again expands. Since we regard
this as growth, the shrinkage may be
looked upon as the reversal of growth.
We find that many living cells may be
made to grow and shrink several times
in succession, just as in the case of the
artificial cell.

If the outside solution is concentrated
enough to draw water out of the cell
it may nevertheless prevent water from
going in and so check growth in propor-
tion to its concentration. Consequently
by varying the concentration we may
accurately control the rate of growth.

We might go on to discuss other points
of resemblance between the growth of

20 THE NATURE OF LIFE

the living and the artificial cell but
this hardly seems necessary. If we
accept the definition of growth given
above it is clear that the artificial cell
furnishes an imitation which is suffi-
ciently complete for our purpose. We
must therefore conclude that there is
nothing in the absorption of water by the
living cell, either by imbibition or by
osmosis, which differs essentially from
these processes as found in non-living
systems.

In conclusion we may ask whether life
can go on in the absence of growth. We
know that certain things may be tem-
porarily taken away from living matter
without taking away life itself. Is
growth one of these? Certainly the rest-
ing seed lives for years without any sign
of growth. This is also true of many
animal cells. The suppression of all
signs of growth does not in any way in-
volve the suppression of life. Even when
placed in moist soil with all external con-
ditions favorable some living seeds re-

GROWTH n

main quiescent for months or years be-
fore they start to grow.

Hence it seems possible to have life
without growth and growth without life.

Our analysis of the process of growth
illustrates the method which biological
investigation must very commonly pur-
sue. The biologist wishes to study living
matter in the same manner that the
chemist and physicist study their mate-
rial. His first task is observation, after
that he must analyze in order to discover
what properties are essential and what
are merely accompanying phenomena.
He need not attempt to explain these
phenomena, for, after all, we can never
arrive at ultimate explanations. But he
can attempt to predict and control. The
physicist cannot explain electricity but
he can predict and control electrical phe-
nomena. In the same way the biologist
hopes to be able to predict, and control
life phenomena. One method which he
finds particularly useful is to make arti-
ficial imitations which closely resemble

22 THE NATURE OF LIFE

the phenomena he is studying. If he
succeeds in this he may find the funda-
mental laws of physics and chemistry
on which life phenomena are based.

m

REPRODUCTION

Since we cannot regard growth, as
already defined, as an adequate means of
distinguishing the living from the non-
living we may try to find a better crite-
rion. Let us now consider reproduction
from this point of view.

Reproduction consists essentially in
setting free from the organism a portion
which continues to grow and which ordi-
narily reproduces in its turn.

In some cases the detachment is due
to the action of wind, waves, or other
external forces. Thus there are certain
plants each of which consists of a single
large cell (containing many nuclei) which
may be ruptured by purely mechanical

REPRODUCTION 23

means so that a portion becomes de-
tached. The protoplasmic membrane
at the sm'face of the cell (which has been
mentioned above) is broken by the act
of detachment but it is at once repaired
in both parts of the cell so that each por-
tion is able to continue its growth.

This may also happen in the artificial
cell described above. Here also the mem-
brane is instantly repaired when the cell
is ruptured and both portions continue
to grow. The rupture and repair of the
membrane, therefore, do not serve to
distinguish the living from the non-
living.

The rupture of the living cell, as pre-
viously described, produces two portions
which are similar except in size and each
constitutes a complete individual. An
analogous case (in respect to individual
completeness) is found in the strawberry
plant, which produces runners. If one
of these rimners becomes detached by
mechanical means, or by the decay of
the connection between it and the mother

24 THE NATURE OF LIFE

plant, it constitutes a complete new in-
dividual.

In some cases the detached portion
is incomplete, as when a willow branch
is broken off by the wind, falls into soft
mud and begins to grow. It must pro-
duce roots in order to become a complete
individual. This process of completing
the individual is known as regeneration.
It is of common occurrence, but, as we
have seen, it is absent in the simplest
cases and is therefore not essential to
the idea of reproduction.

The detachment of a portion of the
organism is often brought about by in-
ternal forces as seen in the process of cell
division by which a cell separates itself
spontaneously from the parent organism.

In the simplest cells, where there is no
definite nucleus, the process of cell divi-
sion may consist in a simple constriction
which cuts the cell in two. This process
may be imitated in drops of oil and seems
to be due primarily to the forces which
act at the surface of every liquid. Under

REPRODUCTION 25

the action of these forces a drop of liquid
rounds up and becomes spherical when-
ever it is free to do so. But when these
forces are disturbed by localized chemical
action they may cause a constriction of
the drop which may result in its division
into two parts. The same forces seem
to operate in the case of the cell and in
addition there may be changes in the
consistency of the protoplasm. There
seems, therefore, to be nothing in such
cases which we may not hope to explain
on a physicochemical basis.

For the more complicated cases, where
division of the nucleus occurs, we have
at present no satisfactory explanation.
We need not, however, concern ourselves
with these cases at present, since our
purpose is to reduce each life process to
its simplest form and try to see whether
we can explain it on a physicochemical
basis. If we succeed in this we may be
content to leave the more complicated
cases to future investigation.

In sexual reproduction there is a fusion

26 THE NATURE OF LIFE

of two cells called gametes. In the sim-
plest organisms these may be indistin-
guishable but in the course of evolution
one of the gametes becomes larger and
filled with stored nutriment : this is called
the egg. The other gamete remains small
and is frequently motile.

The fusion of the gametes may be imi-
tated by drops consisting of artificial
mixtures which come together and unite.
Here surface forces are concerned as well
as the consistency of the protoplasm.
The process recalls the ingestion of bac-
teria by white blood cells, an operation
known as phagocytosis, which assists in
protecting the body from invasion by
bacteria. It has been suggested that
the fusion of gametes in the simplest
forms is allied to the process of ingesting
food.

The fusion of the gametes is followed
sooner or later by the fusion of their
nuclei. As far as we can judge this is
subject to much the same forces as the
fusion of the gametes, but we have no

REPRODUCTION 27

direct experimental evidence on this
point.

After the fusion of gametes to form
the fertilized egg, development proceeds
by growth and cell division mitil an
adult organism is produced. One of
the important tasks of biology is to
explain the mechanism of development.
It is possible to control development to
a great extent by physical and chemical
agencies. It would lead us too far to
enumerate the triumphs of this fascinating
field of work. It will suffice to mention
Loeb's discovery of artificial partheno-
genesis by which the egg can be made to
develop without fertilization. Usually
this involves chemical treatment but in
the case of the frog it is sufficient to prick
a hole in the membrane which surrounds
the egg. The egg then develops, with-
out fertilization, into a normal adult.

By another treatment the egg can be
made to begin to develop inside out, with
the internal organs on the outside of the
body.

28 THE NATURE OF LIFE

The experimenter is now able to con-
trol to an astonishing degree the develop-
ment of the organism and the future
offers great possibilities in this iSeld of
work. We should not lose sight, how-
ever, of the fact that this sort of develop-
ment is by no means indispensable to the
conception of reproduction. In the sim-
plest forms, such as the blue-green algae,
there is no development in this sense nor
is there any fusion of gametes or compli-
cated nuclear division. The organism,
which is a single cell without a definite
nucleus, simply divides into two and
each half is at once a new organism like
the original cell. In such a case we need
only to explain the mechanism of cell
division and growth in order to give a
physicochemical explanation of repro-
duction in its simplest form.

We must remember that reproduction
may be entirely absent. Individual cells,
as nerve cells, or entire organisms, such
as the resting seed, may serve as exam-
ples. In these cases there is no reproduc-

MOTION 29

tion, there is no suggestion of nuclear
or cell division, yet life may go on for
many years.

In view of these facts we must look
further for a satisfactory criterion by
which living and non-living may be
sharply distinguished.

IV
MOTION

A striking feature of many living or-
ganisms is their power of motion.

In some cases motion is due to growth.
The opening and closing of flowers is due
to unequal growth of the inner and outer
sides of the petals. When the outside
of the petal grows faster than the inside
the flower closes; when this relation is
reversed it opens. In many cases light
and warmth are the determining factors,
so that some Alpine flowers may open
and close two or three times an hour as
clouds pass over the sun.

30 THE NATURE OF LIFE

Another illustration is afforded by the
well known "Praying Palm" of India,
which stands before a temple and is re-
vered because its head at evening is
bowed as if in prayer, while in the morn-
ing it is raised as if to greet the sun.
Scientific observation has shown that
change of temperature combined with the
action of gravitation causes the upper
side of the stem to grow more rapidly
during the day and the lower side to
grow more rapidly during the night, thus
causing the daily change of position.

More striking is the kind of motion
due to muscular contraction. In biology
the word contraction is used in a special
sense. When we speak of the contraction
of a bar of iron we mean a diminution of
volume but when a muscle contracts
there is no alteration of volume, but only
a change of shape, whereby the muscle
grows shorter and thicker. This is also
true of the contraction of simple masses of
protoplasm, as in the one-celled organisms.
In these forms contraction is a much

MOTION 31

simpler phenomenon than in muscle and
we therefore have a better prospect of
analyzing it and discovering the mider-
lying mechanism.

Motion in these cases and in plant
cells may show great diversity. In plants
protoplasmic streaming may range from
the most irregular sort of motion to a
regular flow which passes up one side of
the cell and down the other in a continual
cycle. In animal cells the type of irreg-
ular motion most studied is that found
in the ameba, and hence called ameboid.
An ameba consists of a single cell which
at first sight looks like a structureless
mass of protoplasm but is in reality not
so simple as it appears. It creeps about
by pushing out irregular projections into
which part of its substance flows, while
other portions of its body are simultane-
ously retracted. Currents of protoplasm
(and corresponding countercurrents) are
seen throughout the cell. The interior
of the mass is liquid while the exterior is
firmer, resembling a jelly. The change

32 THE NATURE OF LIFE

from liquid to jelly or the reverse appar-
ently takes place with great ease and is a
characteristic feature of the creeping
movements.

An eminent biologist of the last gener-
ation said that it would be futile to seek
for any simple explanation of the move-
ments of the ameba. You might as well
try, he said, to explain that the move-
ments of a gunny sack with a man in-
side are due to a very simple mechanism.
How far he was wrong may be judged
from the fact that it is now possible to
imitate the movements of an ameba to a
remarkable extent by means of simple
mixtures of oil and other substances.
Not only does a drop of this kind, which
we may call for convenience an artificial
ameba, creep about, but its interior, un-
der the proper conditions, shows cur-
rents and countercurrents.

We cannot say that we have, as yet, a
complete explanation of ameboid motion
but we have gone far enough to make it
evident that mechanical factors are ade-

MOTION 33

quale to account for the observed phe-
nomena.

A very interesting type of movement
is seen in the contractile vacuole, which
begins as a tiny drop of clear liquid in
the living protoplasm, grows to a def-
inite size and suddenly disappears. The
writer has found that precisely the same
phenomenon can be produced in artificial
cells under proper experimental condi-
tions and this rhythmic process may con-
tinue for days.

The movements of living protoplasm
appear to depend upon the surface forces
which act upon all liquids, as well as on
changes in the consistency of the proto-
plasm. They do not seem to involve
anything sujfficiently different from mo-
tion in non-living systems to be capable
of being considered adequate tests of
life.

The idea of motion due to contraction is
in no way indispensable to the conception
of life. A cell which is capable of con-
traction does not cease to live when it

34 THE NATURE OF LIFE

comes to rest. In many cells no contrac-
tion is ever observed yet there is no ques-
tion that they are alive. Evidently it is
not one of the essential characteristics
of living matter.

IRRITABILITY

Let us now consider irritability as a
criterion of life. Irritability is regarded
as one of the most fundamental charac-
teristics of living matter, since it enables
the organism to adjust itself continually
to its environment.

Irritability is often defined as response
to a stimulus. Let us see what is meant
by this definition.

For the sake of simplicity let us con-
sider a purely physical stimulus, such as
a blow. We might compare the effect
upon a man with that upon such an ob-
ject as a billiard ball.

In the first place the response of the

IRRITABILITY 35

billiard ball is immediate while that of
the man may appear to be delayed. The
delay is, however, in appearance only.
The blow causes immediate changes not
evident to the eye: as the result of these
we may get a striking visible phenomenon
which we call the response. In reality
there has been no delay in the response
if we include under this term the entire
series of reactions which result from the
blow.

In the second place the billiard ball
manifests only as much energy as the
blow itself imparts while the organism
may show in its response very much
more. This is because the organism, like
a powder magazine, has a considerable
amount of stored energy, which can be
released by the application of only a
little. The tiny spark may have a very
small amount of energy as compared to
that which it sets free when it falls into
the magazine. We see therefore that
inequality of stimulus and response is in
no way peculiar to living beings.

36 THE NATURE OF LIFE

After repeated stimulation the organ-
ism shows signs of fatigue which disap-
pear after a sulBBcient period of rest.
Similar phenomena are observed in non-
living systems, as in an electric battery
which runs down somewhat during ac-
tivity but recovers on standing. In gen-
eral such effects are due to temporary
exhaustion of the available supply of en-
ergy or to the production of an inhibiting
substance which gradually disappears
during rest.

It is often said that the response of
the organism differs from that of an in-
animate object, such as a billiard ball,
by being advantageous.

It would be absurd to maintain that
all the acts of the organism are advanta-
geous. Numerous exceptions will be re-
called by all of us. Some actions which
seem advantageous on superficial exam-
ination may assume a different aspect on
more careful study. Putting such cases
aside there remain a large number which
are regarded by common consent as ad-

IRRITABILITY 37

vantageous. Each of these consists of
certam processes which take place as the
result of the stimulus. In what respect
do they dilffer from the reactions found
in non-living matter? With this question
in mind let us consider some typical ex-
amples of irritability.

One of the striking methods of re-
sponse is seen in the tropisms. A tropism
is a bending or moving toward, away
from, or at an angle to the source of the
stimulus. A good example is found in
the germinating seed. Under the influ-
ence of gravitation the stem grows up-
ward, the main root downward, and the
side roots grow out at an angle with the
vertical.

If sprouting seeds are placed on a
wheel which is slowly revolving, the
roots are stimulated equally on all sides
and consequently do not bend, but if the
wheel revolves rapidly, they are stimu-
lated by centrifugal force and grow out-
ward from the center of the wheel. If
they are placed at the center of the

38 THE NATURE OF LIFE

wheel in such a position that the centrif-
ugal force acts on the tip in one direction
and on the rest of the root in the opposite
direction the root bends as if only the tip
were acted on. The stimulus is therefore
perceived at the tip and transmitted to
the region where the bending takes place.
The bending is due merely to the fact that
the side of the root which is stimulated
grows more slowly than the opposite side.

The root bends under the influence of a
variety of stimuli such as light, heat, air,
water and various other chemical sub-
stances, currents of water, contact, etc.
These are all cases of tropism.

The tropisms of plants seem to be sus-
ceptible of a physicochemical explana-
tion. Where, for example, a plant bends
toward the light it is evident that growth
is slower on the illuminated side. As it
is well known that light checks growth
the explanation seems to be fairly clear.
It is, however, complicated by the fact
that in many cases the stimulus is per-
ceived at the tip of the plant and the

IRRITABILITY 39

bending takes place further back. The
stimulus is evidently transmitted to the
spot where bending occurs. But while
this introduces an additional factor it
does not make a physico-chemical expla-
nation impossible.

In some cases no mechanistic explana-
tion of tropism has as yet been reached
but since we can predict and control the
process we can not doubt that it is com-
pletely subject to physicochemical laws.
This may be illustrated by the gravita-
tional response of the root. If we place
the root on a horizontal wheel we can
subject it to a varying amount of centrif-
ugal force by causing the wheel to re-
volve at different rates. The centrifugal
force works horizontally, from the center
of the wheel outwards, while that of grav-
itation works vertically. If the response
of the root is wholly subject to physico-
chemical laws we should be able to deter-
mine its direction exactly by varying the
forces which act upon it. Since we can-
not change the vertical component we

40 THE NATURE OF LIFE

vary the horizontal one. We find that
the direction of growth of the root cor-
responds exactly to the parallelogram
of forces, that is, it obeys the forces act-
ing upon it precisely according to pre-
diction, and in such a manner as to jus-
tify the conclusion that it behaves as a
machine.

Tropism is also found in animals.
Many animals are "the slaves of light":
they can not help turning their heads
toward the light and, if free to do so,
move toward it. In this way we may
explain what at first sight seems to be a
very complicated case of instinct. Cer-
tain caterpillars hatch out in the spring
and climb to the top of the branches of
trees where they wait until the food ap-
pears. This was regarded as a partic-
ularly mysterious case of instinct. It
was later shown by Loeb, however, that
they were forced to climb upward by the
irresistible attraction of light. Accord-
ing to his view the response is a tropism
and is purely mechanical.

IRRITABILITY 41

This illustration serves to show that it
is not always easy to distinguish between
tropism and instinct. In fact many psy-
chologists believe that tropisms are the
basis of instincts and that instincts gov-
ern human conduct to a very great de-
gree. According to this view we have a
gradually ascending scale from tropism
to intelligence with no hard and fast di-
viding line at any point.

Of especial interest are the movements
of simple one-celled animals such as the
ameba. If food is placed near them
they may approach and try to engulf it
by putting out projecting pseudopodia
which surround it. This seems a highly
advantageous action but the writer has
found that it may be imitated by arti-
ficial cells which will approach and en-
gulf a variety of materials in the same
fashion.

If the ameba comes in contact with a
food plant consisting of a filament many
times its own length a very extraordinary
reaction takes place. The ameba en-

42 THE NATURE OF LIFE

gulfs a part of the filament which is then
softened by digestion and drawn into
the body of the ameba. The neighboring
portion then suffers the same fate and
this process is continued until the whole
has been drawn in and neatly coiled up
inside the body of the ameba. After
digestion has taken place the indigestible
skeleton is extruded.

The reaction appears not only advan-
tageous but almost uncanny in its effi-
ciency. It seems to be the last word in
advantageous response. Such reactions
have led certain biologists to attribute
consciousness to these simple animals
and to endeavor to analyze their psychic
processes. In view of this it is very sig-
nificant to find that the whole process
can be perfectly imitated by non-living
materials. A drop of chloroform brought
into contact with a delicate glass filament
covered with sealing wax ingests the fil-
ament just as the ameba ingests the
food-plant. It softens the sealing wax,
bends the filament and draws it in, coil-

IRRITABILITY 43

ing it up neatly inside after the fashion
of the ameba. When the sealing wax is
all digested the glass skeleton is extruded.
The whole process depends upon the
familiar forces which act at the surface
of all liquids.

Such an experiment suffices to show
that we need not despair of finding me-
chanical explanations for reactions which
seem very complex and highly advanta-
geous. In reality we have succeeded in
finding such explanations so often that
many biologists feel that there is no logi-
cal necessity for regarding the simplest
cases of irritability as essentially differ-
ent from certain reactions found in non-
living systems.

Other thinkers have a different atti-
tude. The power of the organism to
make continual adjustment between in-
ternal and external relations, and the co-
operation of its various parts, which we
call organization, seem to them beyond
our present powers of analysis. Not ig-
noramus but ignorahimus expresses their

44 THE NATURE OF LIFE

outlook upon the problems of life. The
practical difference between these two
schools of thought is perhaps not so great
as might at first appear. It is true that
the nature of organization and the fit-
ness of the organism for its various tasks
seem at first sight to present an over-
whelming problem. But this is due in
large part to the notion that all organ-
isms possess a mysterious power which
fits them for successful existence. But as
a matter of fact a goodly proportion of
the organisms which have appeared upon
the earth have had so little of this fitness
that they have perished. Such unfit
organisms continually make their ap-
pearance but we see little or nothing of
them. Among the seeds that we sow are
many unfit but they do not grow and we
overlook their presence. Given a suflB-
cient variety of forms and the power of
natural selection to eliminate all but the
fit it does not seem astonishing that the
small fraction which survives should pre-
sent a uniform picture of successful or-

METABOLISM 45

ganization. In proportion as this point
of view prevails the differences between
the two schools will tend to disappear.
It may be said that the real problem
is the origin of the fit. To explain why,
among all the possible combinations,
some few are able to grow, reproduce,
and adjust themselves so as to assure
continued existence, may well be beyond
our present knowledge. We must per-
force wait for more facts before attempt-
ing such an explanation. But in the
meantime the very search for such facts
implies a hope that the explanation will
ultimately be reached and this hope is
in itself a great incentive to an unremit-
ting search for truth.

VI
METABOLISM

Let us now consider metabolism as a
test of life. All the chemical activities
of the organism are included in metab-
olism and they are the very essence of

46 THE NATURE OF LIFE

life. The familiar processes of waste and
repair in the body illustrate destructive
and constructive metabolism.

We shall begin with the green plant,
on which all other living things depend
for food. Here are joined together the
simple substances found in air and water
to make the materials which every or-
ganism needs for its existence. This is
the first step in constructive metabolism.
Once these materials (such as sugar,
starch, fats, and proteins) have been
formed by the green plant they can be
used by other organisms to build up
their own characteristic compounds.

We may now turn our attention to the
activities of the green plant. In the first
place it would seem advantageous for
it to spread out as much leaf surface
as possible, so as to get the maximum
amount of air and sunlight. But there
is danger that water may evaporate
faster from the surface of the leaves than
it can be supplied by the root thus caus-
ing wilting or even death. We find that

METABOLISM 47

this is avoided in various ways. Under
arid conditions evaporation from the
surface of the leaf is often checked by a
covering of hairs (giving a gray aspect
to desert plants) or by wax, resin or var-
nish. The leaf surface is often greatly
reduced and the stomata, or openings in
the surface of the leaf by which water
escapes, are fewer in number. In moist
and shady situations the opposite tend-
ency is shown: leaves become larger,
smoother, thinner and have more sto-
mata.

The arrangement of the leaves on the
stem follows a regular order which can
be expressed mathematically. This was
at first interpreted mystically but is now
believed to be due in many cases to mutual
pressure of the young leaves at the tip of
the stem. This illustrates how theories of
development often begin in an atmos-
phere of mysticism but end with a purely
mechanistic explanation.

This arrangement, which often in-
sures a fairly equal exposure of leaves to

48 THE NATURE OF LIFE

the light, is supplemented by a reaction
to light which causes partially shaded
leaves to push out into the sunshine.
This very advantageous reaction seems
to be capable of explanation on a physico-
chemical basis.

A single large tree may expose a leaf
surface of more than an acre but the
surface which absorbs carbon dioxide
from the air is much larger than this.
The carbon dioxide enters through the
stomata and comes in contact with the
green chlorophyll bodies in the cells of
the leaf. Within each of these bodies the
chlorophyll or green coloring matter is
present as very minute particles (almost
ultra-microscopic) which offer an ex-
ceedingly large surface for absorbing car-
bon dioxide.

Under the influence of sunlight the
carbon dioxide combines with water and
oxygen is given off. It is supposed by
many investigators that the first com-
pound formed is a substance familiar to
us as formalin or formaldehyde. If it

METABOLISM 49

accumulated it might poison the leaf
but it changes to sugar and this in turn
may produce starch. It is possible to
observe the chlorophyll bodies under the
microscope and to see the formation of
the starch grains within them.

When starch is burned energy is given
oflF in the form of heat. This energy was
furnished by the sunlight and became
stored up in the starch. The process of
photosynthesis, by which starch is pro-
duced from carbon dioxide and water,
utilizes the energy of sunlight. It is this
which supplies most of the energy needed
by animals and plants and which likewise
runs our steam engines when wood or coal
is burned in them.

The green plant is practically the only
place in nature where such storage of en-
ergy occurs and it therefore occupies a
place of peculiar importance. If we in-
quire how efficient it is we find that it
actually stores less than one per cent of
the energy which it receives from the
sun. This is due in part to the fact that

50 THE NATURE OF LIFE

it is unable to make use of all the dif-
ferent kinds of rays found in sunlight. It
absorbs principally the red and orange
light (as well as ultra-violet). Since it
can use only what it can absorb it is to
be expected that it will be more active
in photosynthesis in red and orange light
than in other colors. This seems in gen-
eral to be the case, as is shown by a very
beautiful experiment in which a spectrum
is thrown upon a green plant placed in
water. Certain motile bacteria which
seek oxygen are placed in the water and
they gather most thickly around that
portion of the plant which lies in the red
and orange light, indicating that oxygen
is given off most freely there. It should
be said, however, that these and other
experiments need repeating under con-
ditions which insure that the intensity
of all the different rays shall be equal:
there is some evidence that in this case
the result will be quite different.

From sugar and starch fats may be
formed, but to make proteins nitrogen

METABOLISM 51

must be added. The plant obtains ni-
trogen from the soil and in some cases
from the air. It has been said that the
air above each acre of soil contains ten
million dollars worth of nitrogen. Many
plants of the pea family have tubercles
on their roots which contain bacteria
capable of taking nitrogen from the air;
many kinds of very simple plants living
independently in the soil can also fix ni-
trogen. The attempt has been made to
inoculate crop plants such as wheat with
organisms capable of fixing atmospheric
nitrogen. It has been stated that the
wheat plant has this power, independ-
ently of the presence of bacteria or other
organisms.

Sulphur and phosphorus enter into
the composition of some proteins and
these substances are often added to the
soil. Lime salts are also very necessary
and in many cases are used as fertilizers
but their fimction is not clear. Lime .
salts have the power of acting as anti-
dotes to the poisonous action of other

52 THE NATURE OF LIFE

salts and may therefore be said to have
a protective action. This applies to
animals as well as to plants. The rela-
tive proportion of lime salts (as com-
pared with other salts) in blood and sea
water is about the same and this is also
true of many solutions used for the nu-
trition of plants. A solution in which the
toxic properties of one salt are inhibited
by other salts is called a balanced salt
solution and such solutions play an im-
portant part in biology.

The role of mineral substances is not
very well understood. The same may
be said of the hormones and vitamins
which are important for plants as well
as for animals.

A hundred years ago it was not be-
lieved that any of the food substances or
compounds related to them could be pro-
duced outside the living cell. But to-day
such a feat is a commonplace of chemis-
try. We are even able to build up such
substances as sugar from carbon dioxide
and water, in imitation of the plant.

METABOLISM 53

This can be done in a variety of ways.
The source of energy is usually an elec-
tric current, radium, or ultra-violet
light. Ordinary light can be used in the
presence of certain metals such as iron
(which also occurs in the plant). By
subjecting a mixture of ammonia, car-
bon dioxide and water, to ultra-violet
light nitrogenous compounds can be
produced. Some of these appear to be
amino-acids from which proteins can be
built up.

It may be added that certain bacteria
are able by oxidizing compounds of iron
or sulphm* to gain suflScient energy to
utilize the carbon dioxide of the air with-
out the aid of sunlight or chlorophyll.
Hence it has been suggested they may
have been the precursors of chlorophyll-
bearing organisms in the process of evo-
lution.

Since many of the most important
steps in constructive metabolism have
already been imitated in the laboratory
it seems probable that others will share

54 THE NATURE OF LIFE

a similar fate. Rapid progress is being
made in this direction and every fresh
discovery strengthens the belief that the
whole process may be explained by the
laws of chemistry.

Let us now consider the processes by
which food is utilized to furnish energy.
This is known as destructive metabolism
and consists largely in the combustion
of food in the organism (respiration).

It may be instructive to compare the
combustion in the living organism with
that in a tallow candle.

The food which enters the body and
is stored, or remains undigested for a
time, may be compared to the inactive
tallow of the candle. If it is to be used
it must first be prepared for absorption.
As a rule solids cannot be taken into
living cells and the food must be liquefied
before it can be absorbed. This is ac-
complished by digestion.

In the candle also the fuel must be
liquefied before it can be absorbed. It
is then transported to the place where

METABOLISM 55

it is to be burned. This takes place in
the candle, as in the simpler organisms,
by such processes as diffusion, capillary
action, assisted by convection currents.

When combustion has ceased we find
certain substances left over which are
given off as gases or as solid particles
which constitute the smoke. These may
be compared with the excreta of the or-
ganism. With a few exceptions, the same
substances can be burned in the candle
as in the organism. If we introduced
into the candle bits of the food sub-
stances which can be burned by the or-
ganism, they would also be burned in the
candle and most of the substances which
the organism is unable to bum could not
be burned in the candle.

The beginning and the end of the proc-
ess of combustion is the same in both
cases, though the intermediate steps dif-
fer. Thus we find that an ounce of
tallow ordinarily consumes the same
amount of oxygen, sets free the same
amount of heat, and produces the same

56 THE NATURE OF LIFE

amount of water and carbon dioxide,
whether fed to a dog, or burned in the
candle.

In the candle flame we may point to
the spot where the combustion begins
and likewise where it ends. The ques-
tion arises. Can we do the same in the
case of the organism.'^ When food is
taken into the organism it is transformed
into living matter, after a time it again
becomes lifeless material. When and
how do these transformations take place?
We might regard the food as lifeless until
it becomes chemically active in the proc-
ess of combustion. We might also say
that when it ceases to be active in this
way it again becomes dead material.

If we use this conception as a working
hypothesis we can at least give to our
ideas a certain degree of precision. It
will be seen that this involves the idea
that life is a process in which combustion
plays a fundamental role. This concep-
tion of life and of living matter leads us
to study it in precisely the same way as

METABOLISM 57

we study the non-living. We endeavor
to discover its underlying mechanism
and to bring it under our control.

When we compare the organism to a
candle flame, we are struck by certain re-
semblances. Both are in dynamic equi-
librium, that is, they retain their form
while continually changing their sub-
stance; both carry on processes of com-
bustion which may start with the same
substances and end with the same prod-
ucts, and set free the same amounts of
energy; both respond to changes in the
environment, such as variations in the
supply of oxygen or in temperature.

We might go on to point out other re-
semblances but it will perhaps be more
instructive to consider the points of dif-
ference. In the first place the combus-
tion of the organism proceeds at a rela-
tively low temperature. This depends
upon the fact that it takes place in wa-
tery solution : such combustion can easily
be imitated in the laboratory. It is
greatly facilitated by certain enzymes.

58 THE NATURE OF LIFE

as is easily shown when we follow the
process of combustion or oxidation by
means of the changes in color which it
sometimes produces. Many plants show
changes during the process of death. We
are familiar with the blackening which
occurs in plants as the result of frost
or of injury due to crushing. These
changes, which are due to oxidation, are
particularly well seen in the common
Indian Pipe, whose cells are well adapted
for observing the whole process under
the microscope. In this case the most
rapid oxidation (and consequently black-
ening) appears to occur in the cell-nu-
cleus. It has been suggested that this
may be connected with the fact that the
nucleus contains more iron than the rest
of the cell.

It is possible to extract from plants
and animals colorless substances which
if allowed to take up oxygen turn yellow,
then red, and finally black. These pig-
ments are responsible for much of the
coloration of animals, in skin, feathers.

METABOLISM 59

and hair, and for many of the autumnal
colors of leaves. In the decay of animals
and plants in the soil a dark coloration
is produced which is due to these pig-
ments.

If an organism is killed by exposure
to boiling water the process of combus-
tion ordinarily stops, but if it is killed by
certain other methods (as by acetone)
the combustion continues after death (in
complete absence of bacterial action).
The explanation lies in the fact that com-
bustion in organisms depends on en-
zymes: these are ordinarily destroyed
by the heat of boiling water but not by
exposure to acetone.

Enzymes play a very highly important
role in the chemical reactions of the or-
ganism. We suppose that the enzyme
does not initiate the reaction but merely
hastens it. It is often said that its
action is analogous to that of oil in
lubricating a machine and it need not,
theoretically at least, be itself used up in
the process. Any substance which acts

60 THE NATURE OF LIFE

in this way is called a catalyzer. There
are catalyzers which hasten oxidation
in non-living systems but the term en-
zyme is reserved for the catalyzers found
in organisms.

The importance of enzymes is very
great since most life processes are pro-
foundly influenced by them. In view of
this we may ask whether they constitute
a criterion of life. Cells may be killed
in a variety of ways which do not destroy
the enzymes and in that case the en-
zymes may be extracted from the dead
cell. In natural death the enzymes con-
tinue to work after death and the cell
undergoes a process of partial seK-diges-
tion known as autolysis.

In some cases the action of enzymes
may be imitated by simple inorganic
substances, such as colloidal metals.

The special function of oxidizing en-
zymes (and associated substances) is to
activate the inert oxygen of the air and
cause it to combine with various sub-
stances in the organism. Such enzymes

METABOLISM 61

are apparently responsible for the oxida-
tion of alcohol to acetic acid in the pro-
duction of vinegar as well as for changes
in color and flavor in the curing of to-
bacco and tea. They are also active in
producing famous Tyrian purple of the
Romans, obtained from a shellfish.

The production of light by lumi-
nescent animals and plants is due to oxi-
dation carried on by means of oxidizing
enzymes. This may be illustrated by an
interesting experiment in which the juice
of a potato or turnip (which contains
oxidizing enzymes) is mixed with pyro-
gallol (an oxidizable substance) in the
presence of hydrogen peroxide, which
furnishes oxygen. When this mixture is
made in the dark room a distinct lumi-
nosity is observed.

While many of the oxidations of the
organism can be traced to more or less
definite enzymes others have not yet
been explained upon this basis. At pres-
ent we know very little of the various
steps in the process whereby food is

62 THE NATURE OF LIFE

1

I

burned but the fact that it stops when
the organism is killed by heat indicates \
that it is due to enzymes. !

In many animals the tissues are sup-
plied with oxygen by oxygen carriers in I
the blood which take up oxygen in the j
lungs or gills and convey it to the cells. j
In mammals an iron compound, hemo-
globin, performs this office, in the horse-
shoe crab it is a copper compound; in
certain other marine animals compounds
of vanadium or manganese. All of these
are admirably adapted to the role of oxy-
gen carriers and it is remarkable that so
many different metals are employed.

The cells of the body are able to take
their oxygen from these compounds and
hence do not need free oxygen. Many
bacteria, protozoa and other organisms
get along without free oxygen and to
some of them it acts as a poison.

Of late the belief has been gaining
ground that living cells can get the oxy-
gen they need by decomposing water.
This leaves hydrogen, which may unite

METABOLISM 63

with free oxygen if it is present or with
any of the substances in the organism
with which it can combine. If the cell
has been stained with methylene blue
the hydrogen unites with it forming a
colorless compound. This has been pro-
posed as a test of life. It is, however,
not certain that it occurs in all living
cells and it can easily be imitated by non-
living materials.

There are certain organisms which do
not carry the process of respiration to the
point of producing carbon dioxide and
which therefore differ from the majority
of animals and plants.

Plants and animals which require free
oxygen quickly show the effects of a de-
ficiency in the oxygen supply. They be-
have as if anesthetized; but if not kept
too long in this condition they recover
completely when the normal supply of
oxygen is restored. This led Verworn to
think that anesthetics produce their ef-
fects by depriving the cells of their nor-
mal supply of oxygen because they di-

64 THE NATURE OF LIFE

minish the ability of the cell to absorb
oxygen. This would lead to decreased
output of carbon dioxide. But this
seems to be an error as the experiments
carried on in my laboratory have shown
that in many cases the anesthetic instead
of decreasing the output of carbon dioxide
actually increases it.

It has been stated that the combustion
of the organism goes on in aqueous solu-
tion. This might surprise the lay reader
who thinks of his own body as a solid
rather than a liquid. He might suppose
that it was intended to apply to the
blood or other liquids of the body but
not to such an apparently solid structure
as a muscle. It is, however, in the muscle
and other living cells of the body that
most of the combustion occurs and this
is possible because they combine to an
extraordinary degree the properties of
liquids with those of solids. In this re-
spect they resemble a jellyfish which is
nearly all water yet is so firm that you
can not readily squeeze liquid out of it.

METABOLISM 65

In such a structure substances can diffuse
and chemical action can go on as rapidly
as in a liquid. Substances which can
form structures of this sort are called col-
loids (from the Greek word meaning glue).

A familiar example is gelatin which
passes readily from the solid to the liquid
state on heating and becomes solid again
on cooling. The living substance, how-
ever, more closely resembles white of egg
which becomes coagulated at higher tem-
peratures and suffers a permanent change.
Uncoagulated white of egg can be partly
solidified by cooling to a low temperature
and completely solidified by partial loss
of water: it can be liquefied by reversing
the process. Protoplasm seems to pass
very readily from the solid to the liquid
state and vice versa without change of
temperature or loss of water.

In the opinion of some writers proto-
plasm is comparable to an emulsion such
as mayonnaise salad dressing. Such an
emulsion can pass under appropriate con-
ditions from a solid state, resembling

66 THE NATURE OF LIFE

butter, to a liquid state resembling cream
and vice versa.

There is no evident reason why proto-
plasm cannot contain at the same time
jelly-like materials and emulsions. The
question must be left to future investiga-
tion. It is, however, perfectly clear that
the properties of protoplasm are to a
large extent determined by the fact that
it is largely composed of colloids, so that
some biologists speak of organisms as
''colloidal machines." It may be added
that it is possible to construct non-living
colloidal systems which to a great extent
imitate the properties of living matter.
Thus protein solutions are affected by
heat, heavy metals, and various coagu-
lants in much the same way as proto-
plasm.

It is evident, therefore, that combus-
tion, as well as other metabolic processes
may be regarded as taking place in an
aqueous medium.

In conclusion we may emphasize the
fact that destructive metabolism oc-

METABOLISM 67

cupies a peculiar position among life
processes. Growth, reproduction, mo-
tion and irritability, and constructive
metabolism may cease, as in the resting
seed, yet life may go on for many years.
But when destructive metabolism ceases,
life ceases. We can apply this test to the
resting seed: as long as it is alive it pro-
duces carbon dioxide, when this ceases
it is dead.

The cessation of this process is there-
fore a test of life but we can not say that
its continuance is a test for life for we
know that when an organism is killed in
certain ways it can go on producing car-
bon dioxide after death.

These facts would seem to indicate
that life as manifested in the simplest
organisms is a physicochemical process
in which destructive metabolism plays
a fundamental role. This leads us to
attempt to control the process. Some
very remarkable experiments have been
made in this field. It is possible to stop
the development of certain organisms

68 THE NATURE OF LIFE

at a definite stage and to hold them at
this point for a long time; afterward they
can be made to finish their development.
In experiments on rats it is possible to
allow development to proceed on a nor-
mal diet and then by substituting a de-
ficient diet to stop development entirely.
In this case there is no starvation since
the animals do not lose weight. They
remain undeveloped and may be kept
in this condition long after their brothers
and sisters have grown up and had fam-
ilies of their own. If now the normal
diet be substituted for the deficient one
development proceeds normally.

Thus it is also possible to reverse de-
velopment and cause certain animals
(flat worms) to grow smaller, until they
are not as large as when hatched from
the egg. During this process they lose
some of their organs, returning toward
the infantile condition. They can then
be caused to develop normally, after
which development can again be re-
versed, and so on.

METABOLISM 69

We can easily put out the candle and
light it again. Can we also stop life
processes and then cause them to start
up again? When a grain of wheat be-
gins to sprout it shows growth, reproduc-
tion of cells by cell division, motion, and
the ordinary phenomena of irritability.
All these stop if (after 4 or 5 days of
growth) the seed be allowed to dry up
again. We can easily cause them to
start up once more by putting the seed
in water. Can we do the same with re-
spect to destructive metabolism, which
seems to be a process absolutely essential
to liie?

If we stop the destructive metabolism
of the candle by a violent alteration of
its physical structure, as by smashing it
with a hammer, we cannot make it burn
again in the same way as before. Its
fragments may be burned but we shall
not get the same candle flame that we
had before. In the same way such an
organism as a seed may be shattered by
a blow and its fragments may go on burn-

70 THE NATURE OF LIFE

ing for a time but in a somewhat diflfer-
ent way. In neither case can we restore
precisely the same condition with which
we started out.

If we stop the destructive metabolism
of the tallow candle by violent chemical
action, as by heating it in lye, we shall
not be able to relight it any more than
we could restore life to an organism
treated in the same way.

But if we put out the candle by some
means which does not alter its chemical
or physical structure, as by cooling it
down sufficiently, we shall be able by
raising the temperature to make it burn
precisely as before. In the same way we
might expect that if we cooled down a
dry seed to the point where its metabo-
lism stopped, without materially altering
its physical or chemical structure, it
might be able, when brought back to
normal temperature, to resume its ac-
tivities as before.

As a matter of fact dry seeds have been
cooled down to the temperature of liquid

METABOLISM 71

hydrogen, about 250° below zero centi-
grade. At this temperature substances
which under ordinary conditions undergo
violent reactions with each other do not
show any activity when brought into
intimate contact. It is a temperature
at which all chemical reactions practi-
cally cease. If there is any chemical
activity in seeds at this temperature it
is certainly much less than exists in what
we call dead seeds at ordinary temper-
atm'e. Yet when these cooled seeds are
warmed up again and planted in moist
soil they grow and develop normally.

We cannot say what would happen
if these seeds could be cooled down to
- 273° instead of to - 250° C. It does not
seem probable that this relatively small
difference in cooling would prevent them
from recovering. At —273° C (absolute
zero) all chemical processes would come
to a standstill and if we regard life as a
physicochemical process we should say
that it was extinct.

We are led, therefore, to look upon

72 THE NATURE OF LIFE

destructive metabolism as a process upon
which life depends in a peculiarly inti-
mate way. It sets free in available
form the energy necessary to run the liv-
ing machine. In default of this the ma-
chine stops and life ceases. When suffi-
cient food is present the process may
go on for a long time, probably in the
case of the resting seed for a hundred
years, without any constructive metab-
olism.

This leads us to consider the role of
constructive metabolism. It is clear
that photosynthesis is necessary to store
up energy but when this is accomplished
by the plant why should further con-
structive metabolism be needed in the
animal in order that its life processes
may go on? The constructive metab-
olism of the animal does not add energy
but changes some of the food material in
a special and characteristic way. Fats
are usually burned without first being
changed to a different kind of fat: pro-
teins are transformed. It is a familiar

METABOLISM 7S

fact that a dog fed on mutton breaks
down the sheep protein into simpler sub-
stances which are in part built up into
the proteins characteristic of the dog.
Each species of animal or plant appar-
ently has its special kind of protein.
This can be demonstrated by injecting
into an animal the protein of another
species. This is always more or less
toxic.

We are inclined to assume that this
power of transformation is indispensable
to life but it would be interesting to
know what would happen if it were
taken away. Doubtless the organism
would change, but whether all organisms,
including the simplest, would die is an
open question.

We may sum up by saying that metab-
olism, once regarded as too mysterious
to be understood, now appears to be only
a complex chemical process which has
been imitated to a remarkable degree.
If we can continue the rapid progress
of recent years we have reason to hope

74 THE NATURE OF LIFE

that a satisfactory explanation of its
fundamental processes may eventually be
reached.

VII

SELECTIVE PERMEABILITY

Another criterion of life which has
of late attracted especial attention is
the selective permeability of protoplasm.
The life of the cell depends on its power
to retain certain substances which it
manufactures and which are necessary
for its existence. These are prevented
from diffusing out because the outer sur-
face of the cell is impermeable to them.
At the same time this surface allows cer-
tain substances to penetrate into the cell
from the outside, but excludes others.
This is known as selective permeability,
and such surfaces are called semiper-
meable. In some cases, no doubt, a spe-
cial structure exists at the surface which
may be called a semipermeable mem-
brane but the surface itself may behave

SELECTIVE PERMEABILITY 75

as semipermeable in the absence of any
such definite structure.

The fact that the surface excludes cer-
tain substances is very important. This
may be illustrated by the case of certain
fungi which grow on fairly concentrated
solutions of copper sulphate. Since cop-
per is poisonous this seems rather re-
markable. Analysis shows no copper in
the cells of the plant and it therefore
seems unable to penetrate.

It is quite possible that hay fever fur-
nishes another illustration. Although the
facts are not by any means perfectly clear
it seems probable that if the proteins
of various kinds of pollen are absorbed
by the body the system becomes sen-
sitized to that kind of protein. The case
is quite the opposite of immunity where
the introduction of a poison renders the
body less sensitive. It is well known that
each species of animal has its own char-
acteristic protein and that all other pro-
teins act as poisons. Ordinarily when
pollen enters the nose or mouth its pro-

76 THE NATURE OF LIFE

tein is unable to enter the body because
the cells of the mucous membrane are
impermeable to it. But if they fail to
exclude it and a little enters the system,
it may become extraordinarily sensitive
to further doses so that whenever the
pollen comes in contact with the mucous
membrane the symptoms of hay fever
ensue. The remedy is to avoid contact
with the pollen and to try to render the
system immune by injecting it in a suit-
able manner with pollen extract. In
case of doubt regarding the kind of pollen
which causes the diflSculty extracts of
various kinds may be applied to a scratch
on the skin. When the extract of the ir-
ritating pollen is applied a red patch
appears on the skin.

Selective permeability makes it pos-
sible for two cells in intimate contact to
exchange certain materials and yet differ
in regard to other constituents and to
carry on entirely different kinds of work.
Thus deeply colored cells are found next
to those which are colorless. If tissue is

SELECTIVE PERMEABILITY 77

crushed, so that the contents of such
cells mingle, new reactions may result.
When a bitter almond is crushed, prussic
acid is at once evolved: in the normal
state the substances which produce this
reaction were kept apart by semiper-
meable membranes. Such membranes
may exist within a cell as well as at its
surface; if these internal membranes are
broken new reactions may start. It is
possible that this is what happens in the
sense of touch. The contact or pressure
may disrupt membranes within the cell
and so cause chemical reactions which
stimulate the nerves. The stimulus of
gravitation which causes the root to
grow downward may possibly come un-
der the same category.

When a root which is growing down-
ward is placed in a horizontal position
there is a mechanical disturbance of the
protoplasm in each cell. A part of the
cell contents tends to sink downward
toward the bottom of the cell and in so
doing may rupture or displace some of

78 THE NATURE OF LIFE

the delicate membranes in the proto-
plasm. If this is the case it is easy to
see that new reactions may be started
which may produce the bending of the
root.

In view of what is said above it is not
surprising that on cutting or crushing a
tissue an increased evolution of carbon
dioxide is observed. Since this does not
occur in tissues killed by heating it has
been called by Tashiro a sign of life. Mr.
Davies, working in the writer's labora-
tory, found that cells killed by acetone
continue to give off carbon dioxide after
death and that when such dead cells are
crushed there is a very marked and im-
mediate increase in the amount of carbon
dioxide given off. Hence it would seem
that this is not properly to be considered
a sign of life.

A similar behavior is to be expected in
any non-living system in which mechani-
cal agitation brings into better contact
substances which produce carbon dioxide.
To what extent this would imitate what

SELECTIVE PERMEABILITY 79

occurs in the organism we can not defi-
nitely say.

As soon as the cell dies, its power of
selective absorption disappears. Sub-
stances can then diffuse in and out of the
protoplasm without hindrance (unless in-
terfered with by the non-living cell wall,
as happens in some cases).

It follows that if we can measure the
permeability of the cell to various sub-
stances, we can tell whether it is alive
or dead. One way of doing this is to put
the cell into a solution of a dye. Cer-
tain dyes will not enter as long as the
cell is alive while others enter and be-
come stored up inside, reaching a higher
concentration than outside. This proc-
ess obeys a definite mathematical law as
has been shown by Miss Irwin. In the
dead cell this storage of the dye does
not occur to so great an extent.

Another way is by sending an elec-
tric current through the cell; in a salt
solution the current is carried by the
charged particles derived from the salt

80 THE NATURE OF LIFE

and if the cell lies in a salt solution the
flow of the current will be in proportion
to the permeability of the cell to these
particles. In the normal condition only
a few particles can penetrate and little
current passes, but as it dies, the salt en-
ters more and more freely and more cur-
rent flows through. By measuring the
amount of current we can follow the prog-
ress of death precisely as we follow the
progress of a chemical reaction in a test
tube. We then find that death is a very-
orderly process, following a definite law
which can be expressed mathematically.
These studies lead us to look upon the
death process as one which is always go-
ing on, even in a normal, actively grow-
ing cell. In other words we regard the
death process as a normal part of the life
process, producing no disturbance un-
less unduly accelerated by an injurious
agent which upsets the normal balance
and so produces death. The activity of
the cell need not cease with death but
may be merely changed in character.

SELECTIVE PERMEABILITY 81

An organism injured by exposure to
a toxic solution may recover if it is re-
turned to the normal environment be-
fore the injury has progressed too far.
But if injury has gone beyond a certain
point, the recovery may not be complete
and the organism may remain in a state
intermediate between normal health and
a moribund condition.

This is of especial interest, since in
physiological literature it seems to be
generally assumed that when recovery
occurs it is always complete, or practi-
cally so, as if it obeyed an "all or none"
law. But it is evident that partial re-
covery may be easily overlooked unless
accurate measurements can be made.
This fact serves to illustrate the im-
portance of quantitative methods in the
study of fundamental problems.

These studies have led to the develop-
ment of equations which enable us to pre-
dict with a satisfactory degree of accuracy
the recovery curves which are observed
under a great variety of conditions.

82 THE NATURE OF LIFE

As the result of these investigations
we are led to look upon recovery in a
somewhat different fashion from that
which is customary. While recovery is
usually regarded as due to the reversal
of the process which produces injury,
the conception of the writer is fundamen-
tally different. It assumes that the re-
actions involved are irreversible (or prac-
tically so) and that injury and recovery
diflfer only in the relative speed at which
certain processes take place.

The experiments of the writer lead to
the view that life depends upon a series
of reactions which normally proceed at
rates bearing a definite relation to each
other. If this is true it is clear that a
disturbance of these rate-relations may
have a profound effect upon the organ-
ism, and may produce such diverse phe-
nomena as stimulation, development, in-
jury, and death. Such a disturbance
might be produced by changes of tem-
perature (if the temperature coefficients
of the reactions differ) or by chemical

SELECTIVE PERMEABILITY 83

agents. The same result might be
brought about by physical means, espe-
cially where structural changes occur
which alter the permeability of the
plasma membrane at the surface of the
protoplasm or of internal structures
(such as the nucleus and plastids) in
such a way as to bring together sub-
stances which do not normally react.

By means of electrical measurements
we can follow changes in the cell with
suflScient accuracy to enable us to give
a mathematical expression to certain
fundamental ideas of biology which have
never been precisely formulated. Among
these are such important conceptions as
vitality, injury, recovery, and death. A
mathematical theory of the mechanism
of injury and recovery has likewise been
developed by the writer.

We find that some aquatic organisms
are killed when placed in a solution of
common salt, but that death can be
hindered by the addition of a lime salt.
We speak of this as antagonism between

84 THE NATURE OF LIFE

the two substances. The rate of death de-
pends on the amount of lime salt added
and the death curve can be predicted
by means of the mathematical formulae
already mentioned. We can also predict
the recovery curves obtained when the
organism is transferred from any of these
solutions to its normal environment.

In conclusion we may say that in many
respects the selective action of the pro-
toplasm can be imitated but in others
it cannot. The subject is peculiarly dif-
ficult because as yet we know very little
about the rates at which various sub-
stances penetrate. But with improved
technique we may hope to get a clear
picture of the whole process and to in-
terpret it in the light of physics and
chemistry.

VIII

ELECTRICAL FORCES

The electrical experiments just de-
scribed lead naturally to a consideration
of certain other electrical tests.

ELECTRICAL FORCES 85

In the first place it is observed that
injury changes the electrical condition
of the cell. If injured and uninjured cells
are connected to a galvanometer by un-
polarizable electrodes a current flows
through the galvanometer from the nor-
mal to the injured cells.

It would seem at first sight that this
might serve to distinguish a dead cell
from a living one. We find, however,
that in many cases the current ("current
of injury") which may be very noticeable
when the cell is first injured gradually
falls off and disappears after the cell is
entirely dead. We also find that in tis-
sues, such as muscle, which can be easily
excited to activity the behavior of active
cells (as compared with one in the resting
state) with respect to the current is very
much like that of an injured cell. The
current therefore seems to be due to a
disturbance in the cell, produced by stim-
ulation or by injury, rather than to
death. In order to use it as a test of life
we must injure or stimulate a part of the

86 THE NATURE OF LIFE

organism and compare its electrical con-
dition with a normal or an unstimulated
portion.

It is well known that activity and in-
jury are commonly accompanied by an
increased production of carbon dioxide
and in view of this it is not surprising
that electrical changes should occur. In
any case we shall probably not go far
wrong in thinking that a study of the
chemical changes will give us the key to
the explanation of the electrical phenom-
ena.

Certain other electrical manifestations
of living matter are apparently of univer-
sal occurrence as, for example, the fact
that if an organism be brought into con-
tact with a salt solution at one point and
with a more concentrated one at another
p)oint the latter is electrically negative
with reference to the other. This, how-
ever, can be easily imitated with non-
living materials.

If an organism is connected to a gal-
vanometer by unpolarizable electrodes

ELECTRICAL FORCES 87

and an induction shock is sent through
it we observe that immediately after-
ward a current (called a "blaze current")
flows through the galvanometer, some-
times in the direction of the induction
shock, sometimes in the opposite direc-
tion. No such current is observed when
a dead organism is used. Waller, who
discovered this phenomenon, calls it a
test of life. He states, however, that as
a rule he was unable to obtain it in ma-
rine algae. This may be due to the fact
that the sea water interferes with the
manifestation of the current by produc-
ing a kind of short circuit, but this seems
improbable because this would abolish
all electrical effects and this is not found
to be the case. Moreover other cells
such as frogs' eggs sometimes fail to
give it. In any case the fact that the
blaze current is not found in all organ-
isms makes it less satisfactory as a test
of life.

88 THE NATURE OF LIFE

IX

THE CONTROL OF LIFE

What shall we conclude from our sur-
vey of the tests of life? Certain of them
can be imitated to a remarkable degree
and seem to be reducible to mechanism.
In other cases we have not yet reached
this stage but we are making continual
progress toward a physicochemical ex-
planation, so that many investigators are
convinced that such an explanation must
eventually be reached.

It is certainly true that all biologists
agree that the organism is in some re-
spects a machine but to what extent it
is a machine, is still a matter of contro-
versy. Some who think that the higher
organisms cannot be reduced to mechan-
ism feel no hesitation in regarding the
simplest forms of life as pure machines.
Such organisms are, however, suflSciently
complex to challenge our best powers of
analysis. While we can imitate more or

THE CONTROL OF LIFE 89

less successfully the various life processes
taken one at a time no one has made an
artificial cell which can imitate all these
life processes simultaneously. It is evi-
dent that the simplest living cell is a ma-
chine of great complexity. This is only
another way of saying that we have a
great many factors, or variables, to deal
with. If we can measure each of these
we may hope to arrive at a clear picture
of the life processes of a cell and to
express them in a definite manner, pref-
erably in mathematical form. If we can
do this we may also hope to predict the
course of life processes as w^e predict the
course of the tides, which also involves a
great many variables. We need not de-
spair of the power of the human mind
to deal with such a large number of
variables providing each of them can be
measured.

Our task then is clearly defined. It
is to measure each of the factors or va-
riables which determine life processes to
the end that we may imitate, predict

90 THE NATURE OF LIFE

and control the activities of the organ-
ism.

When we have done this successfully
we shall doubtless be able to say whether
the simplest cells possess any features
which cannot be explained upon a phys-
icochemical basis.

In the meantime we are free to form
such working hypotheses as we choose.
It should be said, however, that the
mechanistic point of view has been vastly
more productive in research than the vi-
talistic. If we regard the organism as a
machine we have a powerful incentive
to endeavor to control it as we control
other machines.

The imitation of various life processes
has already been discussed. It has also
been stated that in many cases the behavior
of the organism is predictable, as in the
reaction of the root to gravitation or in
the changes of the electrical resistance
of the organism under various conditions.
If we deal with large numbers of individ-
uals (as the chemist and physicist deal

THE CONTROL OF LIFE 91

with large numbers of molecules) their
average behavior can be predicted in
certain respects with surprising accuracy.
This sort of prediction depends on the
use of statistical methods which are be-
coming very useful in biology.

Let us now turn to another subject
which is destined to play a prominent
role in biological progress, namely, the
control of life-processes.

We have already learned to control
the development and behavior of organ-
isms to such a remarkable degree that it
would be difficult to set any limit to what
we may hope to accomplish.

Every one is aware, for example, of the
way in which the development of the
human body is controlled by the thyroid
gland. If it fails to function properly the
individual may develop into a cretin,
that is, a dwarf with a peculiar cast of
countenance, a dry and parchment-like
skin, defective in mind or even idiotic.
Transplantation of a thyroid gland into
various parts of the body in childhood

92 THE NATURE OF LIFE

makes a great improvement. Feeding
thyroid gland also helps but is less effec-
tive than transplantation.

Lack of thyroid activity lowers the
whole metabolism, excessive activity of
the thyroid has the opposite effect. The
mind instead of being sluggish is over-
active, often excitable and troubled by
anxious fear, the eyes may protrude, the
body tends to be taller than normal. In
short the symptoms are quite the oppo-
site of those found when the thyroid is
subnormal.

To what extent the effect of the thy-
roid depends upon its content of iodine
is not yet wholly clear. We know that
both thyroid tissue and iodine have a re-
markable effect on development in cer-
tain animals. Feeding thyroid to tad-
poles causes them to develop rapidly into
frogs; on the other hand feeding thymus
delays metamorphosis and the tadpoles
become gigantic in size. Feeding iodine in
the form of inorganic salts or free iodine
has less effect than feeding thyroid.

THE CONTROL OF LIFE 93

Another ductless gland which has a
marked influence on development is the
hypophysis, or pituitary gland, situated
in the brain. If its activity is less than
normal in childhood the body becomes
abnormally fat; if its activity is above
normal the body becomes abnormally
tall and slender and under some con-
ditions there is abnormal enlargement of
the hands and feet and alteration of the
face by thickening the skin and connect-
ive tissue. This may be accompanied
by a lessened mental activity. The re-
moval of the hypophysis from a young
animal profoundly affects the develop-
ment. The animal grows but little, and
development comes nearly to a standstill,
but it continues to live in this condition.

The experiments of Steinach on young
rats show the effect of glands in the most
striking manner. He succeeded in trans-
planting testes from the male into the
female and ovaries from the female into
the male. This exchange of organs pro-
duced remarkable results. The males

94 THE NATURE OF LIFE '

which had received ovaries in place of j

testes took on the characters of the fe- !

male, including the more graceful form, !

softer fur, larger nipples and even the ;

habits of the female. The breasts se- \

creted milk and these animals suckled
young.

The females which received testes in
place of ovaries showed less effect but
acquired to a considerable extent the
heavier build, larger head, coarser fur
and the habits of the male.

In all these cases the gland produces
a small amount of substance, called a
hormone, which influences development.
There are doubtless a large number of
such hormones. Their influence on bod-
ily form and on mental states is just be-
ginning to be more fully appreciated.
Doubtless the immediate future will wit-
ness rapid advances in this field of study.

The question arises whether we can
introduce hormones, or artificial substi-
tutes for them, into the body and so in-
fluence development. There seems to be

THE CONTROL OF LIFE 95

no reason to think that this cannot be
done. Herrmann succeeded in extract-
ing a hormone from the corpus luteum of
the female guinea pig which on injection
stimulated the development of the female
characters. The nature of this material
is unknown except that it appears to be
a lipoid (fatlike) substance but it is, of
course, possible that the active substance
is quite different and is merely associated
with the lipoid.

The hormone produced by the supra-
renal capsules, called adrenalin, has not
only been isolated but thoroughly studied.
Its chemical structure is known and it
has been made artificially. It has an
extraordinary effect upon muscle and
hence upon the blood vessels, so that it
can be used in certain surgical operations
(removal of teeth or tonsils) to prevent
excessive bleeding. The injection of
.00000000125 gram of adrenalin into a
frog has a perceptible effect on the rate
of bleeding. Cannon states that in
higher animals it may be poured out into

THE NATURE OF LIFE

the blood stream as the result of anger,
thereby increasing the muscular tonus.
It affects also the nervous system and
other organs.

The fact that the injection or feeding
of hormones has such marked effects
leads us to ask whether other substances
can be used in the same way. As a mat-
ter of fact a great variety of substances
can be used to influence development.
We may mention the fact that when
plant lice are placed upon rose twigs
whose lower ends dip into water the de-
velopment of the insect may be easily
determined by substances added to the
water. Thus a variety of salts can be
added to the water which insure that
the plant lice acquire wings; but if alco-
hol is added to the water they remain
wingless.

Stockard, and later McClendon, found
that by adding alcohol and other sub-
stances to the sea water the developing
fish produced cyclopean eyes, that is, the
two eyes were fused into one.

THE CONTROL OF LIFE 97

In the remarkable experiments of Loeb
it was found that merely treating unfer-
tilized sea urchin eggs for a short time
with butyric acid, followed by somewhat
concentrated sea water, caused them to
develop just as if fertilized by sperm.

Loeb also found that by adding a small
amount of alkali to the sea water he
could cause the sperm of the starfish to
enter the egg of the sea urchin and thus
produce a hybrid which does not occur
in nature.

We may recall some of the eflFects of
special kinds of food. It is well known
that bees create a queen by feeding with
a special substance called "royal jelly."
The other females which do not receive
this food become the sterile "workers."
A very interesting experiment was made
by Marchal upon certain wasps. The
workers or sterile females attend to the
feeding of the larvae. Marchal took away
the larvae but continued to provide the
workers with food. As there were no lar-
vae to feed they ate it themselves and as

98 THE NATURE OF LIFE

a result their ovaries began to develop
and they were able to lay eggs. In this
connection it may be recalled that Evans
has recently announced the discovery of
"vitamin X" which is necessary for fer-
tility. Rats were given a diet (casein,
lard, and cornstarch) on which they
grew and developed in a normal manner
but in some cases they were completely
sterile. The sterility could be cured by
the addition of certain substances to the
diet, such as lettuce, alfalfa, wheat germs,
milk fat, or meat.

Vitamin X is distinct from the other
well known vitamins, such as the fat
soluble A and the water soluble B and C,
which are necessary for growth and
health. Very little is known of their
nature, but another, vitamin D (formerly
called bios), which is needed for the
growth of yeast has recently been ob-
tained in crystalline form and we may
hope to learn its chemical structure.

A great many cases are known where^
as in the case of vitamins, small amounts

THE CONTROL OF LIFE 99

of substances promote growth to an
astonishing degree. Thus peat, inocu-
lated with certain bacteria produces
substances which greatly stimulate the
growth of crops.

Certain marine organisms will not
grow in artificial sea water until a few
drops of natural sea water are added.
The growth of certain fungi is greatly-
increased by minute amounts of certain
metals.

In all these cases the growth-promoting
substance acts at a very low concentra-
tion: often a mere trace suffices. In the
future we shall be compelled to pay more
attention to substances which act at
great dilution.

The form of the plant may be con-
trolled in a great variety of ways. One
of the most potent substances for influ-
encing development is water. This is
illustrated by the behavior of a potato
lying on a dry table-top without food or
water. Under these circumstances a
potato can grow for a long time. It de-

100 THE NATURE OF LIFE

velops into something which superficially
resembles a cactus far more than a po-
tato. The stem is very short and thick,
with nodes close together, the leaves are
reduced to small growths resembling
spines, and the whole aspect is that of a
desert plant. A photograph of such a
plant might easily be mistaken for that
of a cactus, even by botanists.

In some aquatic plants the form of the
leaf is determined by their position with
reference to the water. For example the
Mermaid Weed {Proserpinaca) produces
under water a leaf which is composed of
fine threadlike parts arranged on each
side of a central rib. Above the water
the same plant produces a flat elongated
leaf resembling that of land plants. By
growing the plants in an aquarium and
alternately lowering and raising the water
level the plant can be made to produce
each type alternately.

Light produces remarkable effects upon
plants. Not only does it cause the upper
and lower sides of the leaf to differ

THE CONTROL OF LIFE 101

but it largely determines the position
assumed by the leaf and in many cases
modifies its external form. Thus the
harebell (Campanula) produces leaves
of a very diflFerent shape on a branch
which is covered so as to exclude light.
The water hyacinth (Eichornia) loses in
the shade to a considerable extent the
remarkable swollen leaf stalk filled with
air which assists the plant to float on the
surface of the water.

Light modifies the color and the fra-
grance of flowers, as well as of other parts
of the plant. Reducing the amount of
light by shading plants for a part of each
day causes some species to produce
flowers and fruit very much earlier,
while in other cases the reverse occurs,
as shown by the experiments of Garner
and Allard.

Marked modifications arc produced in
some cases by temperature. Every one
has noticed the manner in which plants
at high altitudes hug the ground, in many
cases forming rosettes which spread out

102 THE NATURE OF LIFE

as close to the surface as possible. In
the lowland the same plants grow up-
right and have a very different aspect.
Experiment shows that by placing the
plants on ice every night in the lowland
they can be made to assume the curious
aspect of the alpine situation.

Very striking effects are observed in
insects. By exposing butterflies in the
chrysalis stage to high or low temper-
atures we are able to change their colors
in a remarkable manner: the same species
treated in this manner may produce va-
rious forms which occur in nature and
were at one time regarded as different
species.

Stockard has found that when devel-
oping trout are cooled at certain stages
of development all degrees of doubling
are observed. In some cases the fish has
two heads, in others two tails, or the body
may be double throughout.

A doubling may also be produced by
simple physical means in the case of cer-
tain eggs. Loeb was able to produce

THE CONTROL OF LIFE 103

Siamese twins in sea urchins by causing
the egg to extrude a part of its contents
and so that there were two portions imi-
ted by a bridge of protoplasm. In many
cases each portion developed, forming
two individuals connected by a ligature.
In the same way triplets could be pro-
duced.

It would seem needless to multiply
instances, for these few examples suffice
to show that the experimenter has power
to control development to a remarkable
degree. We may, however, inquire to
what extent the modifications produced
by the experimenter are hereditary.

It is often stated that simple one-celled
organisms, such as bacteria, can be mod-
ified by treatment so that they and
their descendants are noticeably altered.
Thus it is a common practice to decrease
the virulence of bacteria so that they
may be used for various purposes. We
must bear in mind, however, that the
culture may contain different strains and
that the treatment may do nothing more

104 THE NATURE OF LIFE

than inhibit the multipUcation of a vir-
ulent strain and allow a less virulent one
to develop.

The case is quite different when a cell
takes on new characters as the result of
treatment and its descendants retain
these characters. In this connection we
may mention the remarkable results ob-
tained in the artificial production of can-
cer.

Yamagiwa and Ichikawa produced
cancer by painting the ears of rabbits with
coal tar. They were led to this experi-
ment by the well-known fact that chim-
ney sweeps and tar workers are partic-
ularly subject to cancer. The effect of
the tar is to stimulate normal cells into
an abnormal growth and multiplication.
This continues in the descendants of
these cells since they continue to produce
cancerous tissue and in the case of white
mice Deelman was able to transplant
such cancers to a normal animal where
the cancer continued to grow. This can-
cer was then again transplanted to a nor-

THE CONTROL OF LIFE 105

mal animal with the same result and so
on until the original cancer had been
transplanted eleven times in succession.

Much more efficient than crude tar is
the distillate which comes off when tar
is heated to over 300° C. An ether ex-
tract is also efficacious and likewise a
watery extract of soot which has been
mixed with lime.

The question arises whether it is pos-
sible to affect the germ cell so that it will
take on new characteristics which will
be shown by its descendants. There is
considerable evidence that this is the case.
When alcohol is given to chickens or
guinea pigs, as in the experiments of
Pearl and Stockard it affects the germ
cells so that the characters of the off-
spring are altered and this persists for
two or more generations. In the case of
guinea pigs the offspring of parents
treated with alcohol are less vigorous
and have a high death rate. Their eyes
show abnormalities and they are in other
ways defective. It would appear that

106 THE NATURE OF LIFE

the weaker germ cells are injured or
killed by the alcohol. The stronger are
uninjured or may possibly be stimulated.
In any case there is a rapid elimination
of the weaker and in the fourth genera-
tion a superior strain results. It should
be understood that the alcohol is applied
to the parents of the first generation only,
the four subsequent generations receiv-
ing no alcohol. In the experiments of
Pearl on chickens this improvement oc-
curred much more rapidly.

A more permanent effect was obtained
by Guyer by introducing materig-l from
the eye of the rabbit into fowls where
it caused the production of a substance
which was injected into the rabbit and
caused the offspring of the latter to
have defective eyes. It is well known
that the introduction of foreign protein
causes the body to produce substances
which tend to destroy or precipitate that
protein and it is possible that a body of
this sort is responsible for the effects ob-
tained. If the fowl produced a sub-

THE CONTROL OF LIFE 107

stance which attacks the protein of the
rabbit's eye it would be natural to ex-
pect that the injection of this substance
into the rabbit would produce defective
eyes. The case is not quite clear, how-
ever, since it appears that many poisons
affect the eye more quickly than other
parts.

Guyer states that the effect of a single
treatment has persisted for nine genera-
tions and that there is no reason to sup-
pose that it will not go on indefinitely
since the imperfections of the eye tend
to become worse in succeeding genera-
tions and also to occur in a proportionally
greater number of young. It seems pos-
sible that the material from the defective
eye of any individual may get into the
circulation of that individual and cause
the production there of substances which
are destructive to the protein of the eye;
if these substances pass into the develop-
ing young they might produce defects
of the eye.

In these cases the poisonous agent

108 THE NATURE OF LIFE

probably acts directly upon the germ
cell. We are particularly interested in
knowing whether it is possible to produce
inherited effects indirectly by inducing
changes in the body cells which then re-
act in such a way as to alter the germ cell.
If this were possible we might expect that
the normal activity of the body cells
might also affect the germ cells and that
in this way the inheritance of so-called
acquired characters could be brought
about.

We are not yet able to answer this
question decisively. Some recent exper-
iments of Griffith, Bentley and Detlefsen
are of interest in this connection. White
rats were kept for several months in
cages which revolved continuously at a
rate of 60, 90 or 120 revolutions per min-
ute. Such rats when returned to normal
conditions showed distiu'bances of the
equilibrating apparatus as evidenced by
modified muscular and ocular movements
and twisting of the head and neck. These
disturbances are inherited and it would

THE CONTROL OF LIFE 109

therefore appear that an efiFect upon the
body cells can be transmitted to the germ
cells. But it is too soon to draw any far-
reaching conclusions from this series of
experiments, especially as disturbances
of equilibrium sometimes appear in rats
which have not been subjected to treat-
ment. We must be the more cautious
in this regard as a great many experi-
menters have vainly sought to prove the
inheritance of acquired characters and
have reached the conclusion that such
inheritance is extremely doubtful.

One of the most important problems
of biology is to discover how changes are
produced in the germ cells. One way of
attacking this is to observe imder what
conditions spontaneous changes occur in
germ cells which produce the so-called
mutations. Such mutations are now ac-
cessible to observation in considerable
numbers. Over three hundred of them
have been observed by Morgan and his
co-workers in the fruit fly {Drosophila),
These embrace the most diverse features.

110 THE NATURE OF LIFE

m

including changes in eye-color, shape of
wings and body, hairiness, and partial or
complete loss of organs.

In order to control these mutations we
must discover the physicochemical fac-
tors on which they depend. If such con-
trol can be established we shall be able to
create new species. The aim of modern
biology is to accomplish this, just as we
control growth, metabolism, irritability,
and other life processes.

Upon man's control of nature rests all
our civilization. The last and most dif-
ficult step in the conquest of nature is
the control of life. This offers so much
promise that no effort is too great to
consecrate to it, and each step forward
is of fundamental importance for human
welfare.

INDEX

INDEX

Absorption, 54

Acetic acid, 61

Acetone, 59

Acquired characters, 108,

109
Adrenalin, 95
Air, 38, 51

Alcohol, 61, 96, 105, 106
Ameba, 31, 32, 41, 42, 43 Capillary action, 55

Blue-green algae, 28
Butterflies, 102
Butyric acid, 97

Campanula, 101
Cancer, 104

Candle, 55, 56, 57, 69, 70
Cannon, 93

Amino-acids, 53
Anesthetics, 63, 64
Antagonism, 52, 83
Arrhenius, 5, 6, 7
Artificial cell, 16, 17, 18,

19, 20, 23, 33
Artificial parthenogenesis,

27

Carbon dioxide, 12, 48, 49,
52, 53, 56, 63, 64, 67, 78,
86

Catalyzer, 60

Caterpillars, 40

Cell division, 9, 10, 12, 15,
24,29

Cellulose wall, 14

Assimilation of food, 9, 11, Centrifugal force, 37

15 Change of form or color, 15

Autolysis, 60 Chemical activities, 45

Autunmal colors, 59 Chemical stimuU, 38

Chloroform, 42
Bacteria, 6, 11, 26, 50, 51, Chlorophyll, 48, 49, 53

53, 62, 99
Balanced solution, 52
Bentley, 108
Bitter almonds, 77
Blaze current, 87
Blood, 52, 62, 64

Coal tar, 104
Colloids, 65
Collodial metals, 60
Coloration, 58, 59
Combustion, 55, 56, 57, 59,
61, 63, 64, 66, 69, 70, 72

113

114 INDEX

Consciousness, 42 Eichornia, 101

Constructive metabolism, Electric current, 53

53, 67, 72 Electrical forces, 84ff.
Contractile vacuoles, 33 Emulsion, 65
Contraction, 30, 33 Energy, 35, 36, 57
Contact, 38 Enzymes, 57, 59, 60, 61,
Control of life, 21, 67flF., 62, 79

SSfiF. Evaporation, 47

Copper, 17, 62 Excreta, 55

Copper sulphate, 75 Eye, 106, 107
Corpus luteum, 95

Cretin, 91 Fatigue, 36

Crushing, 58 Fats, 46, 50, 72

Current of injury, 85 Feathers, 58

Fertilizers, 51

Davies, 78 Filterable viruses, 5

Death 58, 59, 67, 80, 82, 83 Fish, 96

Deelman, 104 Formaldehyde, 48

Detlefsen, 108 Formalin, 48

Destructive metabolism. Food, 56

54, 67, 69, 70, 72 Fowls, 106
Development, 27, 28, 68, Frost, 58

82 Fruit fly, 109

Development, reversal of. Fungi, 75, 99

68

Differentiation, 9, 11, 12, Gamete, 26

15 Garner and Allard, 101

Diffusion, 55 Gelatin, 17, 65

Digestion, 54 Germ cells, 105

Drosophila, 109 Gills, 62

Dyes, 79 Gravitation, 30, 37, 39,

Dynamic equilibrium, 57 77

Griffith, 108

Egg, 26, 27 Growth, 8ff., 29, 67, 99

INDEX

115

Guinea pig, 95, 105
Guyer, 106

Hair, 59
Harebell, 101
Hay fever, 75, 76
Heat, 38, 55, 66
Heavy metals, 66
Hemoglobin, 62
Herrmann, 95
Hormones, 52, 94, 95,

96
Hydrogen, 3, 62, 63, 70
Hydrogen peroxide, 61
Hypophysis, 93

Imbibition, 13, 15, 16,
20

Imitations of life phenom-
ena, 21, 89

Immunity, 75

Indian Pipe, 58

Injury, 81, 82, 83, 86

Injury, current of death,
85

Intelligence, 41

Iodine, 92

Iron, 53, 58

Irritability, 67

Irwin, 79

Jelly fish, 64
Kidney bean, 11

Leaf, 47, 48, 59

Light, 38, 40, 48, 61, 100,

101
Lime, 51, 52, 83, 84, 105
Loeb, 27, 40, 97, 102
Lucretius, 4

Luminescent animals, 61
Lungs, 62

Manganese, 62
Marchal, 97
McClendon, 96
Membrane, 14, 16, 17, 18,

77, 78
Mermaid Weed, 100
Methylene blue, 63
Mice, 104

Mineral substances, 52
Morgan, 109
Motion, 29ff.
Muscle, 31, 64
Mutations, 109, 110

Natural selection, 44
Nerve cell, 10, 28
Nitrogen, 50, 51, 53
Nuclear division, 10, 25,

28, 29
Nucleus, 58, 83

Organic acids, 18
Osmosis, 13, 15, 16, 20
Ovaries, 93, 94
Oxidation, 58, 60, 61

116

INDEX

Oxygen, 50, 55, 57, 60, 61,

62, 63, 64
Oxygen carriers, 62

Pasteur, 4

Pea family, 31

Pearl, 106

Pearl, 105

Peat, 99

Permeability, 74flF.

Phagocytosis, 26

Phosphorus, 51

Photosynthesis, 40, 50, 72

Pigments, 58, 59

Pituitary gland, 93

Plant lice, 96

Plastids, 83

Pollen, 75, 76

Potato, 61, 99

Potassium ferrocvanide,
17

Praying Palm, 30

Prediction of life phenom-
ena, 21

Prediction of life processes,
89

Proserpinaca, 100

Proteins, 18, 46, 50, 51,
53, 66, 72, 73, 75, 107

Protoplasm, 65

Protoplasmic membrane,
14, 17, 23

Protoplasmic streaming,
31

Protozoa, 62
Pseudopodia, 41
Prussic acid, 77
Pyrogallol, 61

Rabbit, 106
Radium, 53
Rats, 68, 93, 98, 108
Recovery, 82, 81, 83
Regeneration, 24
Repair, 46

Reproduction , 22ff., 24, 67
Respiration, 54, 63
Rhythmic process, 33
Root, 37, 38, 39, 40, 51, 77
Rose, 96
Royal jelly, 97

Salts, 18

Sealing wax, 42

Sea urchin, 97, 103

Sea water, 52, 99

Seeds, 11, 20, 28, 37, 67,
69, 70, 71, 72

Selective permeability,
74fiF.

Semi-permeable mem-

brane, 74ff.

Sexual reproduction, 25

Siamese twins, 103

Skin, 58

Soil, 51, 59

Soot, 105

Spontaneous generation, 4

INDEX

117

Spores, 6, 7
Starch, 46, 49, 50
Starvation, 68
Stem, 37
Steinach, 93
Stimulation, 82, 85, 86
Stimulus, 34, 35, 38, trans-
mission of, 39
Stockard, 96, 102, 105
Stomata, 47
Strawberry, 23
Sugars, 18, 46
Sulphur, 51, 53
Sunlight, 49, 50, 53
Surface forces, 26, 33, 43

Tannic acid, 17
Tashiro, 78

Temperature, 18, 30, 57,
65, 70, 71, 82, 101, 102
Testes, 93, 94
Thyroid, 91, 92
Toxicity, 81
Tropisms, 37, 38, 39, 40,41

Turnips, 61
Tyndall, 4
Tyrian purple, 61

Ultra-violet light, 53

Vanadium, 62
Verworn, 63
Vitality, 83
Vitamines, 52, 98

Waller, 87
Wasps, 97
Water, 38, 99; absorption

of, 3, 161
Water glass, 17
Water hyacinth, 101
Wheat, 69

White blood cells, 26
Willow, 24

Yamagiwa, 104
Yeast, 95