presented to
mniversitp of ZToronto Xtbrars

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1bume iJBlafee, JEsq.

from tbe boohs of

Ube late Ibonourable B&warD Blafte
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(1876=1900)

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BIOLOGY

A II rights reserved

Denies Scientific *Pr inters
Edited by J.^ynolds Green&fD., F.R.S.

BIOLOGY

BY

R. J. HARVEY GIBSON, M.A.

Professor of Botany in the University of Liverpool

WITH

NUMEROUS

ILLUSTRATIONS

London: /. «5I/.

& JO Bedford Street, W.C

PREFACE

Ix the following pages an attempt is made to out-
line some of the more important principles of the
science of Biology, and to illustrate in simple terms
the inter-relationship of structure and function in
living organisms. Since Biology is founded on Physics
and Chemistry, an elementary knowledge of these
sciences is presupposed, such as may be gained from
a study of the volumes of this series dealing with them.
Similarly, the present volume is introductory to the
Primers of Zoology, Botany, and Physiology, where
the fundamental principles here dealt with are treated
in greater detail in special relationship to plants and
animals respectively.

It is quite possible that I may have omitted much
that another author would have inserted, and enlarged
on points which some might consider as out of place
in a booklet of this size. Be that as it may, I can only
hope that what I have written may prove of interest
and service to those whose studies have lain in
other departments of knowledge than Biology.

If errors of fact or exposition are not numerous
(it is perhaps too much to hope that they are non-

vi PREFACE

existent), it is due to the kindness of my friends and
colleagues, Professors C. S. Sherrington, F.R.S.,
W. A. Herdman, F.R.S., and Dr. J. Reynolds Green,
F.R.S., in reading and criticising the MS. or proofs
of this work, and to them I tender my heartiest
thanks.

R. J. HARVEY GIBSON.

LIVERPOOL, 1908.

CHAPTER I. INTRODUCTORY

Plants and animals, 1 ; intermediate organisms, 3 ; vitality,
3 ; definition of Biology, 4 ; morphology and physiology, 4 ;
limits of the present volume, 5.

CHAPTER II. THE FUNCTIONS OF THE ORGANISM
Nutrition, 6 ; sensitivity, 7 ; reproduction, 8. Illustrative
examples, 8 ; Vaucheria, 9 ; frog, 13. Summary, 16.

CHAPTER III. DIFFERENTIATION OF STRUCTURE AND DIVISION

OF LABOUR

Cells, 18 ; unicellular organisms, 18 ; Amoeba, 19 ; leucocyte,
19 ; Pleurococcus, 19. Nutrition and locomotion, 20. Multi-
cellular organisms, 20 ; Hydra, 21 ; Enteromorpha, 23. Differen-
tiated multicellular organisms, 23 ; tissues, 23. Division of
labour and specialisation of structure, 24 ; comparison of a
differentiated multicellular organism and a human society, 24 ;
progressive differentiation in individuals during development
from the germ, 25.

CHAPTER IV. FOOD AS A SOURCE OF ENERGY
Chemical analysis of an organism, 27 ; source of the chemical
elements, 27 ; nature of food, 27 ; average daily diet of a human
individual, 28 ; work and energy, 28 ; conservation of energy,
29 ; measurement of energy, 29. Food as a store of potential
energy, 31 ; oxidation and release of energy, 31 ; deficiency of
oxygen in organic compounds, 32 ; respiration, 33 ; other
methods of releasing energy, 34.

CHAPTER V. THE TRANSFORMATION OF FOOD
Digestion, 35 ; in animals, 35 ; enzymes, 36 ; alimentary tract
and associated glands, 37 ; in plants, 39 ; carnivorous plants, 40.

CHAPTER VI. THE MANUFACTURE OF ORGANIC FOOD
The chlorophyll apparatus, 42 ; absorption from the soil, 44 ;
root- hairs, 44 ; law of osmosis, 45 ; transpiration, 46 ; stomata,
46 ; nature of solar energy, 47 ; absorption spectrum of chloro-
phyll, 47 ; photosynthesis, 48 ; chemosynthesis, 48. Summary,
48. Parasites, 49 ; saprophytes and saprozoa, 49 ; symbionts,
49 ; carnivorous plants, 50. Circulation of materials, 50.

vii

viii SYNOPTICAL INDEX

CHAPTER VII. THE LIBERATION OP ENERGY AND EXCRETION

OF WASTE

Composition of air, 52 ; relation of organisms to oxygen, 52 ;
respiration in animals, 53 ; respiration in plants, 54 ; demonstra-
tion of respiration, 56 ; excretion, 57. Circulation of energy, 58.

CHAPTER VIII. SENSITIVITY IN PLANTS AND ANIMALS
Stimuli, 60 ; intensity of stimulus, 60 ; latent period, 61 ;
protoplasm the sensitive substance, 61 ; sensitivity in plants, 61 ;
nature of the reaction, 62 ; sense organs in animals, 64 ; sense
organs in plants, 65 ; importance of sense organs to the animal, 66 ;
transmission of stimuli, 68. Orientation of fixed organisms, 69 ;
gravity, 69; light, 71 ; water, 72; chemical stimuli, 72 ; oxygen,
73. Sensitivity in animals, 74 ; muscle-nerve preparation, 75 ;
tetanus, 76 ; central nervous system, 76 ; afferent and efferent
nerves, 77 ; reflex action, 77.

CHAPTER IX. MOTION AND LOCOMOTION. THE SKELETON
Motion and locomotion, 78 ; movements of protoplasm, 79 ;
organs of locomotion, 80. Skeleton, 80; functions, 80; material-!.
82 ; chemical composition, 82 ; structure, 83 ; relative strength
of materials, 84 ; arrangement of skeletal materials, 84 ; principle
of the girder, 85 ; principle of the hollow column, 86 ; principle
of the arch, 88 ; principle of the crane, 90 ; venation of leaves, 91.

CHAPTER X. THE ADAPTATION OF ORGANISMS TO THEIR

ENVIRONMENT -

Nature of the environment, 92 ; mechanical influences, 93 ;
chemical influences, 93 ; physical influences, 95 ; vital influences,
95. Adaptation of plants to habitat, 96 ; adaptation of organs
to special functions, 97 ; experimental work in adaptation, 98 ;
mimetic resemblance, 100.

CHAPTER XL REPRODUCTION

Antagonism of individual and tribal life, 101 ; asexual reproduc-
tion, 101 ; sexual reproduction, 102 ; cell division, 102 ; relation
of mass to surface, 103; ovum and sperm, 104. Duration of
organisms, 105 ; protection and nourishment of embryos, 106 ;
seeds and seed dispersal, 109 ; illustrative examples, 109.

CHAPTER XII. THE STRUGGLE FOR EXISTENCE AND NATURAL

SELECTION

Types of organisms, 112; powers of increase of organisms, 113;
destruction of life, 114; struggle for existence, 115; variation and
heredity, 116; natural selection, 1J7; mutations, 119. Con-
clusion, 119,

CHAPTER I

INTRODUCTORY

No one, even though ignorant of Biology, can fail
to recognise that living things may be arranged
under one or other of the two categories, known
familiarly as plants and animals ; but although it is Plants
comparatively easy to appreciate the differences
between an oak tree and a horse, and to recognise
that these differences are sufficiently great to justify

FIG. 1. A coral (A) and a coralline seaweed (B)
(J natural size.)

us in keeping such organisms far apart in any scheme
of classification we may adopt, it is by no means so
easy to say offhand what characters they possess in
common. We recognise that they are both alive,
but, without some expert knowledge, we are likely
to find ourselves nonplussed if asked to say why we
regard them both as alive.

\Vhen we come to consider organisms obviously
l A

2 A PRIMER OF BIOLOGY

of lower grade than horses or oak trees, such, for
example, as corals and seaweeds, we may well find
it difficult, without special training in zoology and
botany, to decide to which of the two great classes
above mentioned these organisms respectively belong.
Even so great a naturalist as Linnaeus included
representatives of both in the same category. A
seaweed like that shown in Fig. 1 looks, at first sight,
not at all unlike the coral by its side. Both are, when
alive, reddish-white in colour ; both live in the sea ;

FIG. 2. A, Flustra, a Polyzoon ; B, Padina,
an Alga. (^ natural size.)

both are attached to rocks ; both are stony in texture
and, in the main, composed of calcium carbonate ;
yet the living substance of the one is vegetable, of
the other, animal. Albums of pressed seaweeds
frequently contain specimens of marine animals,
which in many cases bear a strong superficial resem-
blance to genuine seaweeds.

In studying the very lowest types of life we en-
counter even greater difficulties, and the trained
zoologist or botanist may well hesitate before pro-
nouncing a definite opinion on their essential nature.
The remarkable Slime^ Fungi, for example, which

B

FIG. 3. A, Myxomycete (a Slime Fungus) ;
B, Amoeba (an animal), x 350.

INTRODUCTORY 3

find their home in tan-pits and on decaying timber,
&c., are, under some conditions and at certain stages
in their life-history, quite similar in appearance to
some of the simplest animals, so much so, indeed, that
many zoologists claim them as members of the animal
kingdom (Fig. 3). It has even been suggested that
all such lowly
organised
forms of life,
about whose
relationship
to undoubted
plants and
animals dif-
ference of
opinion exists,
should be grouped into a sort of " no man's land,"
a territory inhabited by living things which do not,
so to speak, exhibit any pronounced affinities to
'either of the adjoining nations. Without entering
into a discussion of such dubious cases we may at
present simply recognise the existence of the two
great groups known as plants and animals respec-
tively, each containing organisms of lower and of
higher grade, and of gradually increasing complexity
of structure, the two series diverging from a neutral
region inhabited by forms that fail to show the
distinguishing characters either of the one group or
of the other.

Without at present making any attempt to
enumerate the marks which distinguish plants,
animals and neutrals, let us endeavour to find some
character which all of them agree in possessing, vitality.
Obviously, they are all alive ; they all possess
vitality. But what is meant by vitality ? The

4 A PRIMER OF BIOLOGY

term is much more readily understood than defined
in so many words, and hence it is fortunate that,
for our present purpose, each one may work on his
own individual conception of the meaning implied
by the word and leave verbal interpretations alone ;
a study of the characteristics of living things will
furnish us with a basis on which to formulate defini-
tions subsequently. Meanwhile, let us begin by
taking a summary view of the organisms themselves.
Before commencing any detailed study of the
subject we must recognise that, if our aim be to
Biology, understand the fundamental principles of Biology
— that is to say, of the science which deals with
living things — we must study organisms alive, not
merely investigate the structure of their corpses ;
we must watch the machine at work, not simply pull
it to pieces when it is at a standstill. A knowledge,
however detailed, of the size, shape, minute structure,
chemical composition, mode of manufacture and so
on, of the several parts of a marine engine or of a
chronometer, would give to one entirely ignorant of
the uses of these instruments but a poor idea of the
purpose for which they had been constructed, or of
the part which each component unit played in the
complex whole ; the study of the form and structure
of the machine must be accompanied by a study of
its mode of action and of the manner in which its
different parts co-operate in the performance of its
functions. If this be true of apparatus relatively
so gross, how much truer must it be of the infinitely
more complex and delicate mechanisms we term plants
and animals. To understand what an animal or a
Morpho- plant really is and how it lives, we have to study
phfr3sio-d both its structure and the functions carried out by
logy. its several parts, in other words, both its morphology

INTRODUCTORY 5

and its physiology : for the study of the structure
of an organ will often suggest the function it fulfils,
while the study of function not infrequently aids us
in the interpretation of structure.

There are, however, several other lines of inquiry
in reference to organisms that may be followed
out with interest and profit. There are, for example,
problems connected with the relation of the organism
to its environment (Ecology), its occurrence on and Ecology,
migrations over the earth's surface (Distribution),
and its genealogical relationship to other organisms pistribu-
obviously allied to it (Taxonomy), whether these be [j^ l^nA
now living or represented by more or less perfectly space,
preserved remains imbedded as fossils in the earth's onomy.
crust. It would be impossible, without greatly
increasing the size of the present volume, even to
indicate the nature of these problems, let alone discuss
them ; nor is it necessary or expedient to do so,
seeing that succeeding primers of this series will deal
with certain of these questions in greater detail.
At present we may confine our attention to the task
of endeavouring to obtain some elementary concep-
tions of the principles of the science of life, and
more especially those on which morphology and
physiology are founded, in other words, to gain some
idea of the structure of the living machine and of
the way in which it works.

CHAPTER II

THE FUNCTIONS OF THE ORGANISM

ADOPTING, as our point of departure, the con-
ception of an organism as a machine adapted to
the performance of certain duties or functions, let
us, first of all, inquire what, speaking generally, these
functions are ? What are the essential kinds of work
that all organisms carry out ? A little reflection will
lead us to the conclusion that every living organism
Functions exhibits three fundamental capacities, viz., (1) The
organism caPacity f°r feeding itself ; (2) the capacity for
responding to stimuli — to impulses from within or
from without ; (3) the capacity for multiplying
itself ; in other words, the three fundamental physio-
logical characteristics of the organism are nutrition,
sensitivity, and reproduction. Further, just as a
butcher's knife, a cavalry sabre, an anatomist's
scalpel, a surgeon's lancet, are all of them knives,
and all alike fulfil the general purpose of making an
incision, although each is constructed in the way
best suited to produce the special kind of incision
intended, so organisms show the greatest variety in
constructive detail, in all cases adapted to carry out
these functions in very varied ways. Let us, quite
briefly, summarise what we understand these three
general functions to consist in.

Every organism must obviously be able to absorb

from without certain materials which, however

Nutrition, complex they may be in chemical composition,

are still not themselves alive. These materials,

THE FUNCTIONS OF THE ORGANISM 7

after undergoing appropriate and usually very
complex changes within the organism, having for
their purpose the alteration of these substances into
" food," are built up into the mechanism and become
more or less permanent parts of it, or are employed
in other ways, which at present need be referred to
only in very general terms. If properly nourished
the organism is able to perform work, not merely
visible work, but also internal work not necessarily
apparent to the senses ; it repairs waste in its various
parts, in many cases adds new parts or increases
the size and complexity of parts already in existence
— a series of phenomena commonly united under the
term growth — and also lays aside surplus material over
immediate needs in appropriate forms in some part
of its body for use on a subsequent occasion.

During the entire course of its life-history the
organism is exposed to an ever changing " environ-
ment," a term used to indicate everything, living or
non-living, palpable or impalpable, outside it. The
stimuli exerted by the environment may be in some
cases injurious, but in other cases they are distinctly
advantageous ; these stimuli are constantly varying sensi-
in character, in time of application, and in intensity, tivity.
Manifestly, it must be of the utmost importance to
the organism to be capable not only of appreciating
these impulses, but also of responding to them in
such a way as to protect itself from such as are hurtful,
and to take every advantage of such as are favourable
to its well-being. The most superficial observation,
in fact, teaches us that the organism is sensitive to
stimuli and capable of responding to them by move-
ment, structural adaptation and so on. It must be
noted, however, that the possession of sensitivity
does not necessarily involve that power of apprecia-

8 A PRIMER OF BIOLOGY

tion of a stimulus which we are accustomed to term
" sensation," at least we have no means of determin-
ing whether sensitivity in plants and in lower animals
is accompanied by consciousness or not.

Again, the organism, plant or animal, is, as obser-
vation tells us, one of many of the same kind, type
or species. In some cases at the completion of, but
in most cases, at some stage in its life-history, the
organism makes provision for the continuance of the
race by separating off a part of its body capable of
giving rise, under suitable conditions, to a new
organism of the same type. In some cases, the co-
operation of two individuals is necessary for the
formation of this unit, in other cases it may be pro-
duced by one parent only. The " germ," as we may
for the present term- this isolated part, is, in short,
either simple in the sense of being a single part,
segmented off from one individual, or compound,
i.e., the product of the fusion of two parts separated
from two individuals or from two different parts of the
same individual. In the latter case one of these parts is
termed the male element, or sperm, the other the
female element, or ovum. As every one knows, organ-
isms produced by either of these methods show all the
chief characters of their parent or parents, while at
the same time exhibiting many, and often consider-
able, variations from the parental type. The off-
spring inherit the fundamental characters of the parent
or parents, but show individual peculiarities or
variations of their own.

Let us now quite briefly consider a couple of
illustrations, one taken from the plant, and one from
the animal world.

Growing on the surface of the soil of old flower-
pots or in damp situations near farmhouses may

THE FUNCTIONS OF THE ORGANISM 9

frequently be found a dark green filamentous plant
known as Vaucheria (Fig. 4), so named after the Swiss Vaucheria.
theologian and professor, Dr. Jean Vaucher. Structu-
rally it consists of a sparingly branched thread,
anchored to the substratum by short, branched,
colourless filaments. Microscopic examination shows
that each filament consists of a colourless wall lined
by a viscid substance in which are imbedded very
numerous ovoid
green particles and
a number of small
rounded granules.
The central space
(or spaces) in each
thread is occupied

by a colourless fluid. Jt ^ 4 Young plant

The wall is in close of Vaucheria ( x 10).

contact with the

damp soil, and its external layer is soft and muci-
laginous, owing to absorption of the water with which
it is in touch.

Manifestly such substances as occur in the soil,
and are themselves soluble in water, might readily
be absorbed by the wall, and so be transferred to
the interior. Thus not only might the inorganic
salts, of which the soil is in the main composed, find
entrance, but gases also, which are soluble in water,
might be absorbed by the filament. Let us assume
for the moment that these bodies are absorbed through
the wall ; they would then at once reach the viscid
substance lining the inner side of the wall, and might
possibly be absorbed by it in its turn and passed on
to the solution filling the central cavity or cavities.
The viscid lining of the limiting wall is known as
protoplasm and the central cavity as the vacuole,

10 A PRIMER OF BIOLOGY

which latter is filled with sap. A chemical analysis
of the sap shows it to be composed mainly of water,
but to contain also variable quantities of salts and
gases in solution, such as might have reached it from
without, and also of certain other substances not
derived, at least directly, from the exterior, which,
since they are always found in association with living
or dead organisms, have been termed organic com-
pounds. Since the plant
grows and, as we shall see
presently, multiplies, it is
obvious that the organic
material must gradually
increase in amount, and the
only sources of supply of
__ the raw materials required

FIG. 5. Vaucheria. (A) For- for the manufacture of such
mation of a zoospore at organic substances • must
the apex of a vegetative necessarily be the soil, the
branch (x 25). (B) A fully water and the air. These
developed zoospore (x 50). materials must further

(AfterOltmanns.)

undergo certain changes in

the plant to make them of such a nature that they
may be assimilated by the protoplasm — the only
living part of the mechanism. Into these changes
we need not at present inquire, though, later on,
we shall find that the green particles, which are known
as chloroplasts, play a very important part in the
process. At present it will be sufficient for us to
recognise the fact that Vaucheria must absorb certain
materials from without and transform them into
pabulum, which it uses for the increase of its body
and for other purposes. It is to this series of
phenomena that we give the name nutrition.

At certain times and under certain conditions, the

tips of some of the branches become darker in colour
and partitioned off from the remainder of the filament
by transverse walls similar to that which limits the
entire plant. After a time these apices open and
permit of the escape of the contents which, when
free, are seen to have the form of ovoid green masses
furnished with minute motile protoplasmic threads
or cilia, usually distributed in pairs over the entire
surface. These bodies, known as zoospores, move
by the rhythmic wavings of their cilia and are able to
propel themselves through
the water for a short
time. At length they reach
a suitable situation, settle
down, lose their cilia and
become covered by walls
similar to those of their

parents. After a period of FlQ 6 yaucheria. Forma-
restgerminationcommences, tion of sexual reproductive
when from the resting spore
there arises a colourless
and a green filament. The
former branches and takes root in the mud ; the
latter also branches, but less frequently, and gradually
elongates into a plant like that from which the zoo-
spore originally sprang. Here we have a case of
multiplication or reproduction, and since only one
part is concerned, i.e., the zoospore, it is spoken of
as asexual reproduction (compare p. 8).

At other times, however, the " germ " which corre-
sponds to and, at that stage, superficially resembles the
resting zoospore, is produced in an entirely different
manner. From the side of the filament arise, close
together in many forms, two short projections, at
first somewhat like the beginnings of two ordinary

organs : A, male ; B, fe-
male ( x 50). (After Olt-
manns.)

12 A PRIMER OF BIOLOGY

branches. These projections are approximately
alike in their early stages of development and both
are green ; but ere long one of them becomes swollen
and somewhat pear-shaped and isolated from the
main filament by a transverse wall. The other pro-
jection becomes hooklike and a partition appears
just where the bend occurs. The region beyond this
partition loses its green colour and its protoplasmic
contents subdivide into many extremely minute
ovoid particles, each of which, when examined under
a high power of the microscope, is seen to be provided
with two cilia pointing in opposite directions. When
these bodies, known as sperms (or antherozoids), are
ripe, they escape by an opening formed at the apex of
the curved branch. Simultaneously the short adjacent
projection becomes mature by the aggregation of
the green contents into a central mass having a colour-
less apical region, in front of which the wall is much
thinner and finally disappears, allowing part of the
colourless portion of the contents to escape as a
minute drop of mucilage. The sperms are attracted
by this mucilage, and one of them finds its way into
and fuses with the central green mass, which is known
as the ovum. The product of fusion, or oosperm,
as the result of this sexual reproduction may be
termed, then becomes enclosed in a wall and behaves,
after a period of rest, in a manner precisely similar
to the zoospore, i.e., it germinates into a new plant.
The same result is thus arrived at in both asexual and
sexual reproduction but in entirely different ways ;
in the asexual method one part only becomes the
" germ " of the future plant, but in the sexual
method the " germ " is the product of fusion of two
parts, one the relatively large passive ovum, the other
the minute motile sperm.

THE FUNCTIONS OF THE ORGANISM 13

For reasons we shall afterwards appreciate, sensi-
tivity to external stimulus is not so observable in the
plant as in the animal ; still, even in Vaucheria, it
is capable of demonstration. Thus the zoospores
may be made to move in definite directions in response
to the stimulus of light. A bright light causes them
to move away from, a weak light induces them
to move towards, the source of illumination, while
the mucilage which escapes from the short swollen
branch containing the ovum attracts the sperm.
Many experiments have been performed of recent
years on Vaucheria (and on other plants equally low
in rank) which go to prove that these plants are
extremely susceptible to external influences, such
as running versus stagnant water, salt solutions of
varying strengths and chemical character, light and
darkness, &c., to which influences they respond in
various ways.

Let us now consider an illustrative example from
the animal world, e.g., a frog. Obviously such an
animal is of relatively much higher grade zoologi-
cally than Vaucheria is botanically, but it is a familiar Frog,
and easily obtained organism, and illustrates certain
characteristic features of the animal kingdom better
than one of lower rank.

From the nutritive point of view we notice at once
that the frog differs markedly from Vaucheria in that
almost all the materials which it absorbs are organic ;
it makes but little use of the minerals in the soil, and
does not employ the gases of the air at all as food
materials. We shall find later (p. 43) that this is
associated with the absence from its mechanism of
the chloroplasts which we have seen to be one of the
constituents of the filaments of Vaucheria. This
organic " food " material is moreover taken in by

14

A PRIMER OF BIOLOGY

a definite opening, the mouth, and not by the entire
surface of the body as in Vaucheria ; in its passage
through a tract known as the alimentary canal, it
undergoes certain complex changes, due to the ad-
mixture of secretions from glands which line or com-
municate with the
alimentary canal,
changes which are
collectively spoken
of as digestion. The
digested substances
are thereafter ab-
sorbed from the ali-
mentary tract by a
system of vessels,
and are carried by
their means to all
parts of the body,
the motive power
being the rhythmic
contractions of a
muscular organ on
the path of the cir-
culation, viz., the
heart. The mechan-
FIG. 7. Alimentary canal of frog : a, ism of nutrition
gullet ; b, liver ; c, stomach ; d, in- would thus appear to
testine; e, pancreas; /, rectum. be much mQre CQm.

plicated in the frog than in Vaucheria, but a little
reflection will show us that the principle is the
same in both cases, viz., the assimilation of certain
organic substances by living protoplasm after these
have been altered within the organism into suit-
able forms.

Again, at certain seasons the frog multiplies its

THE FUNCTIONS OF THE ORGANISM 15

kind. In the interior of each individual certain
special organs occur, in one individual an organ
producing ova, in another an organ producing sperms.
Sperms and ova are emitted from the male and
female frogs respectively, and after the fusion of
ovum and sperm, the product — or oosperm — passes
through certain intermediate stages, and in time
develops into a new frog. In the life-history of this
animal there is no evi-
dence of an asexual method
of reproduction, such as
has been noted as occur-
ring in Vaucheria.

The feeble, but still
observable, sensitivity to
external influences which
we recognised in Vaucheria
is much more clearly
exhibited by the frog.
Not only is the general
body sensitive to extremes
of heat and cold and other
climatic influences, but
there are also specific

FIG. 8.— D, ovum ; A, B, C,
early stages in embryology;
E, sperm of frog.

eyes for receiving light
sound impulses, special

organs developed for the
express purpose of re-
ceiving special impulses ;
impulses, ears to receive

tactile organs in the skin for the reception of contact
impulses, taste organs in the mouth and adjacent
cavity for the appreciation of food, and so on. More-
over, we find a special system of conducting strands —
nerves — having for their duty the transmission of
impulses to a central organ, the brain, from which
again another set of nerves transmit impulses gene-

16 A PRIMER OF BIOLOGY

rated there to motile organs — muscles — acting at
some times on a jointed skeleton, and thus giving
the entire organism the power of locomotion or
permitting of movements in individual parts of the
body. At other times the central nerve impulses
are transmitted to glands, inducing or regulating
secretion in or from them. Briefly put, the whole
organism is sensitive in every part and capable of re-
sponding to stimuli of many different kinds. Further,
in this case we are entitled to assume that the
frog is able to analyse the impulses by which it
is affected, that it is " conscious " of them ; at all
events we recognise that sensitivity has reached
a higher stage in development than it has in
Vaucheria.

It is quite unnecessary for us at present to go into
further detail by way of illustration ; it will be suffi-
cient if we have learned to recognise in all organisms
the power of self-nourishment, the power of reproduc-
jummary. tion, and the power of receiving and responding to
stimuli and if, further, we have recognised that these
powers are manifested by very different mechanisms
iu the plant and in the animal worlds. We must
also note that the green plant, even one so low in
the scale of life as Vaucheria, must manufacture its
organic food from inorganic constituents derived from
the soil, water and air, before it actually employs
such food as nutriment for its protoplasm, while the
animal absorbs organic materials already manu-
factured ; that plants (although not all plants) have
both a sexual and an asexual method of reproduc-
tion, while animals (with relatively few exceptions)
possess the sexual method only ; and lastly, that
plants are much less obviously sensitive to impulses
from without than are animals. This last charac-

THE FUNCTIONS OF THE ORGANISM 17

teristic we shall find later on is associated with
the feebler powers of movement and locomotion
•possessed by plants, and with their ability to
manufacture their own food materials from inorganic
constituents.

CHAPTER III

DIFFERENTIATION OF STRUCTURE AND DIVISION
OF LABOUR

WE have seen that plants and animals may be
arranged in an ascending series, rising in the case
of plants from such a type as that figured on p. 19
to the large and vastly complicated forest tree, or
in the case of the animal world, from the simple
Amoeba (Fig. 3) up to such an elaborate mechanism
as man himself.

The very lowest types of both series of forms,
simple as they are, are yet living organisms capable
of performing all the functions we have recognised in
Vaucheria and in the frog. Each consists of a minute
mass of protoplasm with a central granule or nucleus
and, in some cases, one or more chloroplasts. Micro-
scopic examination of a higher plant or animal
show's us that each is composed of units of extremely
varied size, shape and structure, yet every unit, at
least when young, consists of protoplasm and a
nucleus. Such a unit we term a cell, and we are thus
able to say that some plants and animals — the lowest
in the scale — are unicellular, while others — the
majority — are multicellular.

Let us glance first at unicellular forms and for that
purpose we may select for study a simple animal,
Amoeba, often met with in fresh water aquaria, and
the plant commonly known as Pleurococcus, which,
together writh forms closely allied to it, gives the
familiar green colour to the bark of trees, rain butts,

18

DIFFERENTIATION OF STRUCTURE 19

damp walls, &c. For comparison we may also study
a single cell from the human body, a white blood
corpuscle or, as it is sometimes termed, a leucocyte.

The animal Amoeba, under a high magnifying Amoeba,
power, appears as a minute mass of protoplasm,
granular in the centre and clearer towards the
margin and provided with a nucleus. It creeps slowly
over the substratum by sending out projections
from its margin, technically called pseudopodia.
Should it meet, in its progress, particles of organic
substances suitable for food, it flows slowly round
them and engulfs them. In its interior all that is

A >W B

FIG. 9. A, Pleurococcus ; B, Chlamydomonas. X 450.

nutritively serviceable is digested and absorbed,
and the remainder is rejected.

A leucocyte from the blood is, to all intents and
purposes, a small Amoeba. Being constantly bathed
in a nutrient fluid, viz., the blood, it is in a particu- Leuco-
larly favourable situation for obtaining fluid nourish- cyte-
ment ; still, it has been found that should extraneous
bodies detrimental to the human organism find their
way into the blood, the leucocyte is capable of
engulfing and destroying them. The parallel with
Amoeba is thus fully maintained.

The third type of unicellular i organism we have
chosen is, however, different in many respects
from those already mentioned. In the first place, pieuro-
Pleurococcus (Fig. 9A) although it also consists coccus.

20 A PRIMER OF BIOLOGY

essentially of protoplasm and a nucleus, is covered by
a cell-wall of relatively firm consistence, through
which no solid body can pass, and. further, it contains
one or more chloroplasts, absent, with very few
exceptions, from the cells of animals. Even where
they do occur there is good reason to believe that
they are in reality very simple plants allied to Pleuro-
coccus which have taken to living in the bodies of
these animals. We at once conclude that, as in
the case of Vaucheria, all the materials used by
Pleurococcus must be presented to it in the liquid
or gaseous state and must be absorbed by and passed
through the cell-wall. Moreover, these materials
are in the form of the relatively simple inorganic
bodies occurring in the situations where Pleurococcus
grows and must be constructed into organic com-
pounds before they can be assimilated by the proto-
plasm. This construction, as in the case of Vaucheria.
is associated with the presence of chloroplasts.

Again, the existence of a more or less rigid
cell-wall renders movement in the case of Pleuro-
coccus impossible, although, in allied forms, the
plant is capable of movement at one stage in its life-
history through the agency of two motile threads
or cilia (Fig. 9B). Still, in general terms, we may
Nutrition note that a dependence on ready-made organic food
motion?0" ig associated with locomotion, while the power of
constructing organic compounds out of inorganic
materials occurring in the soil and air accompanies
an absence of ability to move from place to place. This
important relationship, too often minimised or ignored,
will be discussed more fully later on (compare p. 68).

Turning from unicellular forms, let us consider
the general structure of some organisms slightly
higher in the scale of life, e.g., a simple animal fre-

DIVISION OF LABOUR

quently met with in
fresh water aquaria,
and one of the com-
monest forms of sea-
weed, only too abun-
dant on the bottoms
of ships and known
to sailors as "grass."
The animal goes by
the name of Hydra,
and the plant by that
of Enteromorpha. It
is unnecessary for us
to study the struc-
ture of either of these
organisms in detail it

will be sufficient for -,

... FIG. 10. Hydra : m. mouth ; tent, ten-

our purpose if we re- tacle. ca/? body cavity; end ^ inner

cognise that both are layer of body wall; ect, outer lay-r
of body wall. (x20.)

composed not of one cell but of many.

In the case of Hydra, the body- Hydra,
wall consists of two distinct layers
(Fig. 10). The cells of these two
layers differ from each other in
form and contents, although not very
markedly ; those of the outer layer
are smaller, more regular, and ob-
viously perform different functions
from those of the inner layer. They
are primarily protective, for they
contain stinging appliances, of the
Hydra. effect of w}lich most people who have
Transverse sec- . , , , . i .. i • /> i •

tion of body indulged in sea-bathing or fishing

wall. ( x 100 ) have had experience from contact

22

A PRIMER OF BIOLOGY

with them in allied organisms such as some of
the common jellyfish. The cells of the inner layer,
on the other hand, are devoted to the
absorption and mgestion of the organic
materials which enter the opening at
the top of the tubular body. At certain
times some of the cells of the outer
layer produce bodies which are presently
recognisable as reproductive organs. In
I short, now that the organism has become

* multicellular, some cells
take on one function,
others another, while still
retaining certain general
powers, e.g., that of self-
nourishment. Again, it is
obvious that as the duties
of any particular cell be-
come circumscribed and
specialised, its form and
structure become modified
also, in order that it may
carry out the work allotted
to it in a more efficient
manner. In just so far,
however, does the cell in
question become more de-
' pendent on its neighbours
in other respects. Thus, a

FIG. 12. Enteromorpha: a, cell which devotes itself
portion of plant (natural solely to the formation of re-
size) ; b, body wall in section ; productive elements must
c, in surface view. be fed by other cells which

are specialised for carrying out nutritive duties and
protected by others adapted to perform that function.

DIFFERENTIATION OF STRUCTURE 23

Similarly, an examination of the green seaweed
Enteromorpha shows us that while the body is in the
main composed of a vast number of more or less
similar green cells, some at the base of the tubular Entero-
body are colourless and concerned with the fixing morpha.
of the organism to the substratum, while some of
the general body-cells are capable of taking on
reproductive functions. The contents of these
latter cells divide into minute ovoid bodies, each
provided with four cilia and each capable of pro-
ducing a new Enteromorpha.

Differentiation of structure thus appears in forms
of quite lowr grade, but it is only in plants and ani-
mals of much higher rank that we meet with complete
illustrations of the principle hinted at in Hydra and
in Enteromorpha. In plants like a rose or an elm
and in animals like a frog or a rabbit we have to mffer-
deal with organisms composed of countless myriads ^u!ti-ed
of cells, some more intimately connected with the cellular
duty of nutrition, even, it may be, with that of isms.""
forming one special secretion to be used in the
digestive process, some simply protective, as in the
case of the external cells of the body, some purely
supporting, as those which form the skeleton, some
having for their function the elimination of excreta
or waste materials or the formation of reproductive
cells, and so on, and in every case the form and
structure of the cells in question are specially adapted
for the carrying out of the functions allotted to them.
Moreover, we find that many cells of the same kind
are associated together to form tissues. Thus we
have in the animal, epidermal tissue, muscular or
contractile tissue, glandular tissue, nervous tissue, Tissues,
and so on, and in the plant also glandular tissue,
conductive tissue, epidermal tissue, &c. &c. Lastly,

24 A PRIMER OF BIOLOGY

many different kinds of tissues are bound together
into yet higher units. Thus, such an organ as the
liver consists not of glandular tissue alone, but of
connective, vascular, and nervous tissues as well,
and a leaf, in addition to the nutritive green cells of
which it is mainly composed, possesses also epidermal,
vascular and supporting tissues, arranged so as to
carry out most effectively the purposes for which they
were intended. Hence, as we study successively higher
grades of plants and animals, we come to recognise that
division of labour among cells is accompanied by
specialisation of structure ; that as the community
of cells becomes larger and larger, duties previously
carried out by all cells are now relegated to some
Division of cells only, and that these are, by their form, their
and°UI contents or position, better adapted than others

- for the performance of these duties. We find also
structure, that there is a give-and-take among these cells, so
that the nutritive cells nourish not only themselves,
but all other cells requiring nourishment, while the
protective cells protect not only themselves but also
the nutritive cells which feed them. In a word, we
learn to appreciate one great generalisation in
biology, viz., that progressive specialisation of
structure is accompanied by physiological division
of labour. An instructive comparison may be drawn
between a unicellular and a multicellular plant or
animal on the one hand, and a human individual
and society on the other. Just as the unicellular
organism does everything for itself, so the isolated
human individual — if, let us say, marooned on an
uninhabited island — must be his own butcher,
tailor, shoemaker, grocer, builder and what not.
In a society, on the other hand, certain individuals
assume one duty to the community, others another,

DIVISION OF LABOUR 25

and each by his training, education, even by his
capabilities in hands, feet, eyes, &c., succeeds best
in the trade or profession for which he is best suited.
A false selection is followed sooner or later by failure,
or at least by indifferent success, just as a glandular
cell would form but a feeble protective agent, or a
skeleton cell an ineffective contractile one. Further,
a society is successful, a nation prosperous, only if
there be harmonious co-operation between the units
composing it, when they all, in a word, work for the
common good ; so, too, every cell in the healthy
body must play its appropriate part in the general
economy, and take its due share in the work carried
on by the body as a whole. Should it fail to do so,
disease in the organism results sooner or later. A
strike or a revolution is a disastrous phase in the
history of a society, it is equally disastrous in that
of a cellular community.

As a result of this rapid survey of organic nature
we have seen that we may distinguish successively
higher grades of organisation in both plants and
animals, beginning with unicellular types and pass-
ing through multicellular, almost undifferentiated pro-
forms to such as show complete differentiation of
structure and division of labour. We have also tiation
seen that every plant and every animal starts life as divid
a single cell — be it oosperm or zoospore, as in the
majority of plants, or oosperm only, as in the great
majority of animals. Obviously, the same general
advance from the unicellular to the multicellular and
from that to the completely differentiated condition
must be met with in the life-history of every higher
organism. Look, for example, at the early stages
in the life-history of the frog. The oosperm is a
unicellular organism just like the oosperm of

26 A PRIMER OF BIOLOGY

Vaucheria or the adult Amoeba or (save for the wall
and chlorophyll) Pleurococcus. By division of the
original cell there arises a multicellular body which,
because it is a primary stage on the wray to something
higher, we term an embryo. But such an embryo
as that shown in Fig. 8 B, is composed of cells which
are, speaking generally, similar to each other. Later
on, these cells begin to differentiate, begin, in other
words, to specialise both in structure and in function,
and this differentiation is gradually carried to com-
pletion as the adult stage is approached.

We have thus sketched out in the life-history of
the individual the same general advance that we see
illustrated in successively higher groups of organisms,
so that, in general terms at least, we may say that,
assuming for the moment that organisms are genea-
logically related, the history of the individual is a
very brief epitome of the history of the race.

CHAPTER IV

FOOD AS A SOURCE OF ENERGY

WHEN we carefully analyse a series of organisms
by appropriate chemical methods, we find that
twelve chemical elements are constantly present,
viz., carbon, hydrogen, oyxgen, nitrogen, sulphur, Analysis
phosphorus, potassium, calcium, magnesium, °r°T~

• T j i i • o i .LI ganisms.

iron, sodium and chlorine. Several others may
be present in varying quantities in special cases,
but these twelve elements are always to be dis-
tinguished, and the first four are especially prominent.
These elements must have been introduced from
without, and the building up and subsequent keeping Source of
in repair of the organism must involve their continued elements,
absorption, elaboration and incorporation. The
ultimate sources of all the chemical elements in the
organism are, directly or indirectly, three in number,
viz., soil, water, and air.

One most important fact, too often neglected,
must be especially emphasised at the very outset,
viz., that no protoplasm, whether of plant or of
animal, is able to assimilate such substances, either in
their elemental condition or even when united to form
such compounds as occur in the inorganic world.
Only when they are united into the very complex
groups which constitute what are known as organic Nature of
compounds can the protoplasm actually incorporate "food-"
or " assimilate " them, in other words, make them
part of itself. But these organic substances are
non-existent in Nature save in association with

27

28 A PRIMER OF BIOLOGY

plants and animals, as products of their activity
when alive, or of their decomposition when dead.
We thus appear to have landed ourselves in a " vicious
circle " : protoplasm- — the essential basis of the
living organism— can be supported only by organic
compounds and yet organic compounds are formed
only as a result of the activity of protoplasm. The
problem before us, therefore, is, How are the necessary
organic compounds originally formed ? How are
relatively simple inorganic materials synthesised
into food acceptable to protoplasm ? An attempt
will be made to answer this question in Chapter VI.,
meanwhile, let us endeavour to find how " food,"
properly so-called, is a source of energy or of power
to do work.

A study of dietetics teaches us that an average
man doing average work requires, during the twenty-
four hours, in round numbers, 140 gr., or about 5
oz. of nitrogenous compounds or proteids ; 100 gr.,
or about 3^ oz. of fat, and 420 gr., or about 15 oz. of
such compounds as starch, sugar, &c., which are
known to chemists as carbohydrates, because they
contain (in addition to carbon) hydrogen and oxygen
Daily in the same relative proportions as they occur in
[let' water. Now. it must be at once apparent that,
so long as it is alive, an organism, of whatever rank,
is constantly doing work — whether it be external
and visible or internal and invisible. But to
do work, energy must be expended and this natur-
ally involves a source of energy. How does the
organism obtain the necessary energy, and in what
form ?

Energy, so the physicists inform us, occurs in

Energy, two states or conditions, potential and kinetic. A

weight resting on a shelf possesses potential! energy,

FOOD AS A SOURCE OF ENERGY 29

in other words, though performing no work at the
moment, it is capable of doing so. For example,
if it be attached to a cord and the cord be put in
connection with a clock mechanism, the weight, if
swung free, is capable of making the clock go. Its
potential energy now becomes kinetic or active.
Physicists also tell us that energy may be recognised
in several different forms, such as chemical energy,
thermal energy, photic or light energy, electrical
energy, &c. By chemical energy, for instance, is
meant that form of energy which is exhibited when
constituent units or atoms combine to form a molecule
of a compound. Thus a molecule of water is re-
presented by the formula H.,O, meaning that water
is a compound of the two elements, hydrogen and
oxygen, in the proportion of two atoms of hydrogen
to one of oxygen. When these two gases are brought
together, under certain external conditions, they
combine with each other, and, in the act of uniting,
energy is set free.

Then again, the results of physical research have
led to the conclusion that all these forms of energy are
reducible to one, and that each may be changed,
and, in Nature, is constantly being changed, into conserva-
another. Still, no matter how or to what extent energy,
the change occurs, the sum of the energies in the
Universe (if finite) is a constant quantity. It
cannot be reduced and it cannot be increased, it
can only be altered in form or in state. This is
known as the Law of the Conservation of Energy.

In order to obtain exact data as to the amount
of energy expended we must fix on a standard
by which to measure energy. Manifestly this Measure-
must be relative only and in terms of one or
other of the various forms of energy. But which

30 A PRIMER OF BIOLOGY

one ? We have already said that one type of
energy may be converted into another. Physicists
tell us that thermal energy is the only one into
which all the others are convertible. For this
reason energy is usually measured in terms of heat,
and the unit of measurement is known as a " calorie."
A calorie is the amount of heat required to warm one
kilogram of water from 0°C. to 1°C. and it is possible
to measure the energy of every body possessing it
in terms of this unit. The energy of the living
organism, as well as the energy of the various food
substances absorbed by it, may, therefore, be esti-
mated in calories. ;' The heat value of a substance
is the amount of heat that is produced by its complete
oxidation, and this amount is the same whether the
oxidation be quick or slow, reached by a direct or
by a circuitous path. It is, therefore, possible to
estimate the amount of heat that must be produced
in the body, by estimating the heat-value of the
food daily consumed." (Waller). Thus, if the heat-
value of 1 gr. of proteid be 5 calories, of 1 gr. of fat
9'07 calories, and of 1 gr. of starch 3g9 calories, the
heat-value of the nitrogenous and non-nitrogenous
food iorming the diet of an average man doing
average work (p. 28) for twenty-four hours must be,
approximately, 3,300 calories. The total energy of
the body appears (a) as work, (b) as heat, and it
has been found that these bear to each other a
ratio of about 1 : 4, so that the measurable heat
of the body will amount roughly to about 2,640
calories. These values are only approximate,
since some organic compounds are excreted from
the body in an incompletely oxidised condition
still possessing for that reason a certain heat-
value.

FOOD AS A SOURCE OF ENERGY 31

The organic substances required by living proto-
plasm, whether plant or animal in its nature, for the
performance of its functions, its nourishment and pood-
repair — used, in short, as "food" — are stores of a store of
potential energy. It is of the utmost importance ei epgy>
that this should be clearly understood.

The various chemical elements that are found in
the body may be grouped in series, so far at least
as our present problem is concerned, according to
their affinity for oxygen — their capacity for being
oxidised. Thus, to take a couple of examples,
carbon may be oxidised, i.e., made to unite with
the. oxygen of the atmosphere, when heated to a
temperature of at least 500° C. It may, as every
one knows, be burnt, and the products of com- oxidation
bustion are compounds of carbon with oxygen and
known as carbon monoxide and carbon dioxide, of energy,
represented by the chemical symbols CO and CO2
respectively. In order, however, that carbon may
unite with oxygen, it must be raised, as we have
seen, to a fairly high temperature. But at lower
temperatures other bodies have a greater affinity
for oyxgen than carbon has. For instance, the
metal potassium will unite with oxygen at the tem-
perature of the air, and form the familiar substance
potash. If brought in contact with water, it will
appear to burst into flame. This may be explained
in the following way. Water consists, as we have
seen, of hydrogen and oyxgen in the proportion of
two units of hydrogen to one of oxygen, and these
form an exceedingly stable compound. Potassium,
however, has an even greater affinity for oxygen
than hydrogen has, and it tears the oxygen from
the hydrogen, and that, too, with such energy that
the heat generated is sufficient to set fire to the

32 A PRIMER OF BIOLOGY

inflammable gas, hydrogen, now released from
combination. The compound formed by the union
of the potassium and oxygen, i.e., potash, is repre-
sented by the chemical formula KHO, hydrogen having
been ousted from its union with oyxgen and replaced
by potassium, represented by the symbol K. The
released hydrogen, combining with oxygen present
in the air, forms water once more.

If we submit to chemical analyses the varied
organic compounds used by protoplasm as "food"
we find that they are relatively poor in oxygen,
although most of their constituent elements
are characterised by great affinity for it. The
combinations in which they find themselves, however,
interfere with their satisfying these affinities. They
are, so to speak, clogged and hampered by their
Deficiency neighbours, and cannot readily unite with the ele-
hf organic ment> oxygen> so abundantly present in their vicinity
com- ' in the atmosphere. Let us look at a few examples,
pounds. rp^ re(j colouring matter of the blood, which is
known as haemoglobin, is a most complex body,
and its formula, according to one authority, is
C600H960FeN134S3Om, meaning thereby that there
are in the smallest possible particle or molecule of
the pigment, 600 atoms of carbon, 960 of hydrogen,
one of iron, 154 of nitrogen, 3 of sulphur, and 179 of
oxygen. There is thus not nearly enough oxygen
present to oxidise all these elements ; for to com-
pletely oxidise one atom of carbon two of oxygen
are needed ; one atom of oxygen is required to
oxidise every two of hydrogen, five of oxygen for
every two of nitrogen, three of oxygen for every two
of iron, and three for every one of sulphur. A
little elementary arithmetic shows us that to com-
pletely oxidise all the elements present in one molecule

FOOD AS A SOURCE OF ENERGY 33

of haemoglobin, over 2,000 atoms of oxygen are
wanted, of which only 179 are present in the compound.
Similarly, in the case of grape sugar, represented
by the formula C6H,2OB, twelve additional oxygen
atoms are required for the complete oxidation of the
carbon and hydrogen, and for glycerine, C3H8O3,
seven more oxygen atoms are requisite.

Now let us imagine a crowd of persons unwillingly
associated and restrained from joining hands with
their own particular friends hovering round the
outskirts of the crowd. Let us suppose that this
restraint is suddenly removed, and that permission
be given to friends and relations in the crowd to
fraternise with friends and relations outside. The
crowd will speedily break up into new associations,
smaller groupings, and, if the affinity of individuals
be great, considerable friction and heat may be
generated in the process of regrouping. This analogy
may be crude, but an effort of the imagination will
enable us to conceive of an organic compound as
such a crowd of units, and the oxygen particles in
the atmosphere as their natural affinities outside.
Let us make the conditions favourable for the satis-
faction of these affinities and at once new combinations
are effected, new groupings are established, heat
being generated in the process of rearrangement.
In the act of combining of the atoms of oxygen with
those of carbon, of hydrogen, of sulphur and so on,
energy is liberated — kinetic energy — and the position
of separation of these elements from oxygen is
therefore a position in which energy is potential —
ready to be turned into kinetic energy when the
combination is permitted.

Every organism, while alive, is constantly taking Respira-
in oxygen in the process known as respiration, tion-

34 A PRIMER OF BIOLOGY

and this oxygen is conveyed to the regions of the
body that are doing work, and therefore expending
energy, there to unite with the elements of complex
compounds poor in oxygen and themselves ready to
unite with it, and so liberate energy in the kinetic
form. It will thus be seen that, to the organism
absorbing them, organic compounds form stores of
potential energy, by the liberation of which the
organism is able to do work, to exhibit vital phenomena
— in a word, to live, provided always that oxygen
gas be available for the oxidation of these compounds.
It must not, however, be assumed that these various
organic bodies are in all cases oxidised directly and
undergo "combustion" in the ordinary sense of the
term, like oil in a furnace ; on the contrary, it is
highly probable that it is the extremely complex
protoplasmic molecule or aggregate of molecules that
undergoes decomposition, and that the oxidation and
abstriction of simpler decomposition products is in-
timately associated with the constructive or assimi-
latory phenomena already referred to (p. 27).

There are other ways of releasing the potential
energy of an organic compound, although oxidation
may be considered as the chief method. By dis-
sociation a complex compound breaks up into two
or more smaller and less complex groupings without
the entry of any oxygen. There are also decom-
positions set up by certain secretions manufactured
by the organism itself, but these and other methods
need not be considered in the present relation.

CHAPTER V

THE TRANSFORMATION OF FOOD

THE complex organic compounds found in Nature
are always, as we have seen, the products of animal
or plant activity, for although a few organic com-
pounds have been artificially manufactured, still as
an economic source of " food " these may be con-
sidered, at present, at all events, as insignificant.
Further, these organic substances are not, even then,
in forms capable of being made use of at once by
the protoplasm. They must be readjusted in their
composition and constitution before they can be
actually assimilated. For example, starch must be Digestion,
transformed into sugar, if for no other reason than to
render it soluble, so that it may penetrate the walls
of the cells ; grape-sugar, again, is a food-stuff to
yeast, but cane sugar is useless to it until it has been
altered into grape sugar. Readjustments and
alterations such as these, whether in the plant or
animal — and many of them, as we shall see later on,
are exceedingly complicated in their nature — are
collectively termed digestion. The two essentials
are (1) jthat the food shall have the appropriate
chemical composition, suited, that is to say, to the
wants of the organism, and (2) that it shall be in a
state of solution.

Let us first of all consider digestion in one of the
higher animals.

The "food" in the process of mastication is in
mixed with and affected by a secretion formed by animals-

35

36

A PRIMER OF BIOLOGY

glands which line or open into the mouth cavity,
and. after being swallowed, is mixed with other
secretions derived from glands which line or open by
ducts into the alimentary canal. By these secretions
the food is altered in character, one secretion acting
on one constituent, another secretion acting on
another. In their progress through the canal the
altered food-stuffs, now made assimilable, are slowly
absorbed by vessels which permeate the walls of
the canal, and are by them transferred, directly or

indirectly, to the
B tissues.

In order that
we may obtain
some conception
of the nature and
mode of action
of a digestive
secretion, we may
select that formed
by the glands
which open into

mouth-cavity — the salivary glands. The essen-
constituent of such a digestive secretion is an
organic body knowrn as an enzyme or ferment.
On examining a portion of one of the salivary
glands under the microscope, we find (Fig. 13)
that it is composed of an immense number of
delicate protoplasmic cells, which, before the gland
begins to secrete, are very granular. These cells are
in communication, by means of minute intercellular
channels, with one or more ducts which open into
the mouth-cavity. As secretion proceeds, the
granulation in the cells gradually disappears, and
from the mouths of the ducts there exudes a colourless

FIG. 13. Cells from a salivary gland
A, before ; B, after secretion.

the
tial

THE TRANSFORMATION OF FOOD 37

slightly opalescent fluid, familiar to every one as
saliva. The granular substance in the gland cells
is known as zymogen, or the enzyme-producer, the
enzyme itself is termed ptyalin. If we make a very
dilute starch mucilage by adding a few grains of
starch to a wine-glassful of warm water and pour
into this some saliva, keeping the whole at a tem-
perature about that of the human body (i.e., 100°F.),
a gradual change takes place in the starch as the
result of the action of the ptyalin upon it. If a few
drops of a solution of iodine be added to a sample of
the original starch mucilage, the mucilage takes on
an indigo-blue colour, but no such colouration
results from the addition of iodine to the sample
which has been acted on for a considerable time by
saliva. On the other hand, with the aid of another
re-agent known as Fehling's solution (a mixture of
Rochelle salts, copper sulphate and caustic soda in
certain proportions) we can demonstrate the presence
of a new substance, malt sugar, which, unlike starch,
is soluble in water. The enzyme has effected the
alteration of the starch into malt sugar, and one
remarkable feature in the process is that the enzyme
is not used up nor destroyed during the transformation.
Further, a comparison of the chemical formulae of
starcli and malt sugar shows that the enzyme has
made the starch take up a molecule of water. Thus
2 (C,.H1()0.) — starcli — becomes C^H^O,, — malt sugar
— by the addition of H20 — water. Similar results
may be obtained by preparing an extract of ger-
minating barley, and causing it to act on starch.
The ferment, which acts in the same way as ptyalin,
in this case goes by the name of diastase.

Let us now attempt to gain some general
idea of the nature of the alimentary canal of

38

A PRIMER OF BIOLOGY

the animal and the enzyme-forming glands which
open into it (Fig. 14). Into the mouth-cavity three
chief kinds of organic food materials enter and are
there masticated and mixed to-
gether, viz., proteids, carbohydrates,
and fats (p. 28). There, also, they
are mixed with saliva, and the
starchy constituents are, to a cer- '
tain extent, acted on by the
ptyalin present in that secretion.
The mixed food is then swallowed
and transferred to the stomach, by
whose rhythmic contractions it is
again mixed with secretions derived
from glands in its walls. The chief
constituent of the secretion of the
gastric glands is pepsin, and by it
the proteids are attacked. After a
period of gastric digestion, the food
passes on to the intestine in whose
FIG. 14. Diagram walls another series of glands, intes-
of the alimentary tinal glands, occur, which also add
canal and chief digestive secretions to the mixture,
Mf^mouth^Sa' one' esPecially» changing cane sugar
salivary gland- into glucose and fructose. Further,
G, gullet; St, certain special glands, connected
Stomach; L, liver; wjth the intestine by means of

smar^Sine1; 8PeciaJ *»***> *™ their secretions.

Li, large intestine. One or these glands is the pancreas
which has in its secretion a fer-
ment which attacks any starch left unacted upon
by the ptyalin, another which changes cane sugar
into grape sugar and fruit sugar, another which
attacks proteids and yet another which acts on fats.
Another important gland is the liver, which con-

THE TRANSFORMATION OF FOOD 39

tributes bile — an antiseptic, — and an alkali which
saponifies fatty matters. The food, in its passage
through the alimentary canal, is thus materially
altered, and, in the course of its journey, is gradu-
ally absorbed in its altered form by minute vessels
which permeate the wall of the canal in all directions
and is carried by them directly or indirectly to all
protoplasmic cells of the body requiring nourishment.
The undigested remainder is, in due course, voided
as excreta worthless to the organism.

Our next task must be to study, although more Nutrition
briefly, the problem of nutrition in the higher plant, ^J^ep
Like the animal, the plant is composed of proto- plant,
plasm and the products of its activity, and the food
of the plant protoplasm, as of the animal protoplasm,
must be organic in its nature. Moreover, much of
this food, if stored as a reserve in insoluble forms,
must be altered by enzymes and rendered soluble
and appropriately prepared for assimilation.

At the very outset, however, we meet with a great
difference between the two types of organism. As
we have already hinted, the green plant itself manu-
factures its own organic food from inorganic materials,
while the animal, being unable to do so, has to
depend upon organic material made by the plant or
absorbed by another animal from the plant. In a
word, the plant makes what it wants, the animal
takes what it can get. Manifestly, the digestive
secretions and apparatus of the plant need not be
so complicated or elaborate as those of the animal,
for, being able to make what it requires, it has not
so much altering to do afterwards. Nevertheless,
the first formed compounds are not always in the
most appropriate state for assimilation, and further,
as we shall find later on, the plant has great powers

40

A PRIMER OF BIOLOGY

of storage, and the storage form is naturally in most
cases an insoluble one. The plant on that account
also possesses enzymes, proteid, carbohydrate and
fat-transforming ferments, manufactured in some

cases in special glands, but
more commonly in the
cells which contain the sub-
stances to be transformed.
Even in the animal, diges-
tion may take place within
individual cells, as, for
example, of glycogen — a
starch-like compound — in
the cells of the liver.
Fundamentally, therefore,
the digestion of food in
the plant and the animal
is carried out on the same
principles, and the only
difference really lies in
the mode of production of
the secretions, associated
with the absence of any
specialised digestive tract
in the plant.

It must also be noted
that there are certain
plants (carnivorous plants)
whose leaves are greatly

FIG. 15. Drosera. (Half
natural size. )

modified in form from ordinary terrestrial types

and which possess, at least in some cases, definite

nivorous cavities or pockets into wThich insects are, by a

plants. variety of methods, induced to enter. These

chambers are virtually stomachs and their walls

are more or less lined by glands which secrete enzymes

THE TRANSFORMATION OF FOOD 41

acting on and digesting the bodies of the insects so
caught (Fig. 15). One instance must suffice to
illustrate this type of organism. On boggy hillsides
there is commonly to be seen a plant, known popu-
larly as " Sundew," from the glistening drops ter-
minating the numerous tentacles with which the
circular or ovoid leaves are covered and fringed.
Small insects, attracted by these drops and probably
also by the reddish colour of the leaves, are caught by
the secretion, which is sticky in character. The
contact with the insect induces a movement of the
tentacles towards the point of stimulation, so that
the insect's body becomes bathed in the secretion.
The contact also stimulates the glands at the ends
of the tentacles to secrete a ferment comparable with
pepsin, which attacks the proteids of the insect
body and digests them, the products being afterwards
absorbed by the leaf. The " pepsin " in the digestive
secretion of the Sundew acts only in an acid solution,
as in the case of the pepsin of the gastric juice of
the animal's stomach.

CHAPTER VI

THE MANUFACTURE OF ORGANIC FOOD

ON p. 27 we saw that it was only the green plant that
was able to manufacture organic compounds from
inorganic materials, for long erroneously spoken of
as the " food " of plants. The " food " of the plant,
just as much as that of the animal, must be organic
in its nature, and since, in the liberation of energy,
these compounds are constantly being reduced once
more to simpler inorganic compounds by the process
of oxidation, it follows that a mechanism must be
forthcoming for the remanufacture of the complex
compounds so destroyed, else the whole living machi-
nery of the globe would come to a standstill. Further,
not only must there be a constructing apparatus, but
energy must be supplied to it else the mechanism
would be unable to work. It must now be our task
to inquire into the nature of this apparatus and the
source of the energy — in other words, to study the
chlorophyll machinery, the chloroplasts, referred to
on p. 10, and the nature of sunlight.

Our conception of the sequence of events that
take place in the process of nutrition in plants and
animals will become much clearer if we endeavour
to realise that the chlorophyll apparatus is not neces-
sarily a part of the plant only ; there are many plants
which are destitute of chlorophyll, and not a few
animals which possess it. Indeed, a near ally of the
species of Hydra which we studied from another
aspect in a previous chapter, is, owing to the presence

42

MANUFACTURE OF ORGANIC FOOD 43

of chloroplasts in its cells, quite as green as any plant,
and behaves, from the nutritive point of view, exactly
like a green plant. Some forms allied, though dis-
tantly, to Amoeba, to which we have referred above,
also possess chlorophyll as do also some of the lower
worms. It does not affect the physiology of the
process whether these green par-
ticles are, as some biologists have
attempted to show, plants living
in intimate association with the
animals in question, or, actual
constituents of the animal itself,
i.e., not introduced from without.
Let us examine a chloroplast
from the cell of a leaf. What is
it made of, and how does it
operate ? In the first place, we
may note that although some
plants have chloroplasts in the
forms of bands, stars, &c., in the
vast majority of cases the chloro-
plasts are minute ovoid bodies, .

i ' FIG. 16.
occurring singly or in large

numbers in the cells which con-
tain them (Fig. 16). Each con-
sists of a basis of protoplasm permeated by an
oily matter in which the chlorophyll, or pigment
proper, is dissolved. The chloroplast is, further, in
intimate relation with the protoplasm of the cell.
Chemically, the pigment which can be extracted
from the plastid by means of alcohol, ether and a
variety of similar substances, is of extremely complex
composition ; indeed, it is probable that it is a mixture
of several compounds. This apparatus does not
perform its function of manufacturing organic sub-

Plant cells
with chloroplasts.
( x 300.)

44

A PRIMER OF BIOLOGY

stances save when exposed to sunlight, and we shall
see later what particular rays of light are most useful
to it. The cells containing chloroplasts are (with
certain exceptions) found only in such parts of the
plant as are exposed to sunlight, not only because it
would serve no useful purpose to develop them under-
ground or in deep-seated tissues, but also because,
apparently in most cases, sunlight is itself essential
to the formation of the pigment. In darkness the
plastid is of a pale yellow colour, familiar to every
one in the leaves of celery, or of grass which has been
overlaid by a plank of wood or other opaque object.

The raw materials of the food of plants, as we
have already seen, are derived from three source?,
soil, water, and air. Let us consider, in the first
place, absorption from the soil. The soil consists
of a mixture of various minerals in the form of
granules of varied size and shape, the interstices
between which are filled with air and water. The
minerals are more or less soluble in the water which
circulates through these capillary channels, and
round each soil particle there is an extremely thin
film of water spoken of as hygroscopic water. From
the surfaces of the finest roots, for a short distance
behind the apices, arises a dense felt work of fine
hairs — the root-hairs.

These root-hairs are really elongations of the sur-
face cells of the root and find their way into the
minute crevices between the soil particles, and come
into intimate union with them (Fig. 17). The walls
of the root-hairs, where they come in contact with
the hygroscopic water, become swollen and mucilagi-
nous, and any mineral matter dissolved in the water
may pass through the wall of the root-hair and proto-
plasm lining it. This entrance takes place primarily

MANUFACTURE OF ORGANIC FOOD 45

in accordance with the physical law of osmosis. The
soil water contains about TV per cent, or less of mineral
matter in solution, whilst the fluid in the vacuole
of the root-hair may contain as much as 2 or 3 per
cent, of solid in solution. Physicists tell us that if
an organic membrane — and the cell-wall is such a
membrane — separates two fluids of different density,
both of which are capable of passing through it, the
less dense solution will
pass through with
greater rapidity into
the more dense, than
the more dense into
the less dense, and
hence the very dilute
solution in the soil
forces its way into
the interior of the
root-hairs. This entry
of a more dilute solu-
tion renders the cell
contents less dense than
those of the cell next
further inwards, and consequently a further flow from
the outer to the inner cell occurs. This, however,
will result in an increase in the density of the outer
cell, permitting of the entry of more of the dilute solu-
tion from without. In this way a constant stream
is set up from the soil outside to the interior of the
root, where the solution enters the vascular system
and is conducted upwards to the stem and distributed
to all parts of the leaf. The solution that reaches the
leaf is much too dilute for the chloroplastids to
operate upon, so that arrangements must be made
for its concentration by evaporation of the excess

FIG. 17. Root-hairs, with soil par-
ticles attached. ( x 250. )

46

water. This escape of water in the form of water
vapour is known as transpiration. Let us see how
this is effected. On the underside (as a rule) of the
leaf occur innumerable minute apertures — known
as stomata (Fig. 18). These are in communication
with a complicated system of spaces between the
inner cells of the leaf, through
which latter also run the vascular
cords. The excess water evaporates
into the intercellular spaces whence
it escapes to the exterior through
the stomata.

Not only does water vapour
escape by the stomata but air
enters by them also, and one of
the constituent gases in the air is
carbon dioxide. In this way water,
mineral matters and carbon di-
oxide gas are brought into the
immediate neighbourhood of the
chloroplasts.

These raw materials are, how-
ever, already fully oxidised and,
as we have seen above, are, in
that condition, useless as sources of
energy. In the form of carbon
dioxide the affinity of the carbon for oxygen has
been completely satisfied, as also that of hydrogen
for oxygen when in the form of water, and the same
is true of most of the other salts absorbed and carried
upwards by the water. The potential energy of
position of separation has already been liberated by
the union of oxygen with these other elements, so
that to make the elements again valuable as stores
of potential energy the combined oxygen must be

FIG. 18. Stomata.
(X 250.)

MANUFACTURE OF ORGANIC FOOD 47

got rid of and the position of separation of elements
re-established by the formation of complex com-
pounds deficient in it. How is this to be effected ?
The process requires an apparatus, and, further,
the expenditure of a large amount of energy. The
apparatus we have already seen is the chloroplast ;
the energy is derived from the sun.

A beam of sunlight may be analysed by means
of a glass prism into rays of different colour. The
rays as observed in Nature give us the colours of the
rainbow — namely, red, orange, yellow, green, blue,

Aa? C 10 3)i«

FIG. 19. A, absorption spectrum of chlorophyll ;
B, solar spectrum.

indigo, violet (Fig. 19). Let us now examine a solar
spectrum by means of a spectroscope — and let us solar
also make an alcoholic solution of chlorophyll and spectrum,
introduce, in a flat-sided glass vessel, a thin film of
the solution between the source of light and the
prism ; we shall find that the previously continuous
coloured band is now interrupted by a dark band in
the red region, and also by paler bands in the yellow
and green, while the violet, indigo, and part of the
blue regions are almost completely wiped out.
We speak of this incomplete band of colour as the
absorption spectrum of chlorophyll, for we may Absorp-
assume that the chlorophyll absorbs some of the ^p°enctpum
sun's rays and allows others to pass through. What

48

happens to those rays which are absorbed ? We are
still very much in the dark on this subject, but in all
probability the rays absorbed are transformed into
some other form of energy and used by the proto-
plasm in the construction of organic substance.
What we do know for certain is that, given the condi-
tions above described, oxygen gas is evolved from
the green leaf and almost immediately thereafter
carbohydrates appear in the cell.

We have now to ask what becomes of the energy

of the solar rays which are absorbed ? Undoubtedly,

a large amount of it is used up in the decomposition

of the highly oxidised mineral compounds, water and

Photosyn- carbon dioxide, and in getting rid of the excess water

thesis. absorbed by the roots, but part becomes stored as

potential energy in the carbohydrates which have been

manufactured. This constructive process is spoken

of as photosynthesis.

While the detailed stages of the photosynthetic
process are as yet very imperfectly known to us, we
are even more in the dark as to the nature of the
further constructive efforts of the protoplasm by
which higher compounds, such as proteids, are manu-
Summary. factured. We know, however, that for these higher
constructive efforts no sunlight is necessary, and in
all probability the energy required is obtained by
the oxidation of primary organic compounds, and
possibly of protoplasm itself (chemosyn thesis), but
into these recondite physiological problems it would
be out of place to enter in such a preliminary sketch
as the present. Thus far we have learned that the
process of nutrition is fundamentally the same in
plant and animal, but that the green plant adds a
new department of work, entirely absent from the
normal animal economy, namely, the manufacture

of the organic matter necessary for nutrition. We
have further learned that this organic matter has to
undergo certain transformations, summed up under
the word digestion, before it can be incorporated
into the protoplasm of either kind of organism. At
the same time it is worthy of note that the diges-
tive apparatus is less complex in the plant than
in the animal, because the green plant is able
to construct the primary organic compounds best
suited to its wants, whilst the animal, being unable
to do so, must accept those already formed by
other organisms, and these compounds are not
always those most appropriate to its immediate
necessities.

A few words of explanation must be added as to
the modes of nutrition of non-green plants. Some of
them are parasites living at the expense of living
plants or animals ; such plants are virtually thieves, Parasites
since they appropriate the compounds manufactured
by others for their own use. Parasites also occur
in the animal world. Other organisms, again, be-
longing either to the vegetable or animal world, are
either saprophytes, or saprozoa, that is to say,
plants or animals which live on non-living organic Sapro-
substances, compounds which have been manu- ^5tes
factured by living organisms, or which result from saprozoa.
the decomposition of their dead bodies.

There are, however, other types of nutrition in the
plant world worthy of mention ; it will suffice to specify
two of these. Other plants, moreover, are symbionts, Symbionts.
that is to say, organisms which live with others but
not precisely at their expense, since, although they
depend upon their partners for certain products,
they give to their partners certain other products
which they themselves have manufactured. There

50

A PRIMER OF BIOLOGY

is thus a mutual give-and-take between them, the
one helping the other.

Finally, yet another type of nutrition is illustrated
by the so-called carnivorous plant which, though
green and rooted in the soil and thus in reality in-
dependent of organic nutriment, supplements its
supplies personally manufactured by absorbing
proteids and other organic compounds from insects
and other small animals caught by one or other of
the various mechanisms with which these carnivorous
plants are provided. Many of them possess special
digestive glands, and the enzymes produced by them
show striking resemblances to those secreted by
animals (p. 40).

Let us now attempt a summary in diagrammatic
form of the whole problem of nutrition (Fig. 20).

H,0

FIG. 20.

cot

Circulation of materials.

From this diagram it will be seen that kinetic solar

energy acting on green cells in the presence of carbon

Circuia- dioxide, water, and simple inorganic salts brings about

materials, the formation of organic compounds which normally

would go to the nutrition of the organism possessing

MANUFACTURE OF ORGANIC FOOD 51

such green cells. These compounds, however, may
be appropriated by an animal or by a plant or animal
parasite. In all these cases — the green plant, the
normal animal and the parasitic plant or animal —
the organic compounds formed by their decomposition
when dead, form the food of saprophyta or saprozoa.
The final products of decomposition of all dead
organisms, as well as those resulting from the general
liberation of energy in all living organisms are, as
we shall see in the next chapter, carbon dioxide,
water, and inorganic salts, which replace the original
simple compounds absorbed by the green plant from
the soil and air. There is thus a constant circulation
of material from the inorganic, through the organic,
back once more to the inorganic world. We shall
see presently that there is also a circulation of
energy.

CHAPTER VII

THE LIBERATION OF ENERGY AND THE EXCRETION
OF WASTE

THE maintenance of life involves a continual ex-
penditure of energy. This energy is derived directly
or indirectly from the potential energy stored in
organic compounds manufactured during photo-
synthesis and the further constructive activities of
protoplasm. The potential energy becomes kinetic
in the satisfying of the oxygen affinities of the ele-
ments of the organic compounds, as we have already
seen in Chapter IV. The supply of oxygen is derived
from the air and the process of intaking of oxygen,
decomposition of organic compounds and excretion
of the simpler and more or less fully oxygenated
compounds are known as respiration and excretion.
Let us look at these processes rather more in detail.

The primary composition of the atmosphere must

be our first concern. It consists essentially of three

gases, nitrogen, oxygen and carbon dioxide. In

Composi- an average sample of air these gases occur in the

air?° following (approximate) percentages, viz., nitrogen,

79*02 per cent. ; oxygen, 20' 95 per cent. ; carbon

dioxide, 0*037 per cent.

Detailed research has shown that all varieties of
protoplasm ultimately die in the absence of free oxy-
gen, and animal protoplasm is more sensitive in this
respect than plant protoplasm. There are, however,
some organisms of lower rank which can, for a con-
siderable time or during their entire life, exist in the

52

THE LIBERATION OF ENERGY

53

absence of free oxygen, being able to obtain any
necessary supplies of that gas from compounds
containing it. Most plant embryos, also, are able for
a time to take the oxygen they need from like sources,
but, generally speaking, we may say that free oxygen
is essential to life. The chief compounds ultimately
resulting from the oxidation of organic compounds
are carbon di-
oxide and water.

So long ago
as 1757 Black
showed that the
final product
both of combus-
tion and of re- I
spiration was
carbon dioxide,
but it was not
until Priestley,
twenty years
later, discovered
oxygen, that it
became possible
to compare the
two processes
in detail, as was
Saussure.

As every one knows, in all the higher animals,
oxygen, along with nitrogen and carbon dioxide,
enters the lungs, gills or other respiratory organs,
either as a free gas or in solution in water. In mam- Respira-
mals, the respiratory organ, the lung (Fig. 21), consists animals,
of an immense number of minute cavities in whose
walls lies a network of extremely delicate blood-
vessels known as capillaries. The oxygen becomes

FIG. 21. A.r spaces in .ung. The darker
lines are capillaries. ( x 350.)

done by Lavoisier and De

54 A PRIMER OF BIOLOGY

associated with the haemoglobin or red-colouring
matter of the blood and forms a feeble compound
with it, known as oxyhaemoglobin. This loosely
combined oxygen is carried by the blood to such a
seat of activity as, let us say, a muscle. There oxida-
tion of muscle substance, or of organic compounds
present in the muscle, takes place, thus liberating
energy which enables the muscle to do work, i.e., to
contract. Amongst the substances formed as a
result of this oxidation is the gas, carbon dioxide,
•which is transferred from the contractile cells to the
blood, and thence by its means back to the lungs
from which it is expired. The difference between
the air inhaled and that exhaled shows approximately
how much tissue destruction has taken place. Thus
inhaled air consists approximately of 79 per cent, of
nitrogen, 21 per cent, of oxygen, and '04 per cent,
of carbon dioxide, but exhaled air consists of only
16 per cent, of oxygen, and about 4 per cent, of carbon
dioxide, the percentage of nitrogen — a neutral gas —
remaining approximately constant.

It may readily be shown that in plants the process
of respiration is, in principle, fundamentally the
same — the method of entry and exit of the gases,
however, differs, for in them there is no special respi-
ratory apparatus beyond the intercellular spaces in
the tissues themselves. Air enters by the stomata
in green parts or by special pores, left in older parts
which have become covered with cork, known as
lenticels, and diffuses to all parts which may require it,
there to break down organic substances and so release
energy required for carrying on vital processes. So,
too, the carbon dioxide formed diffuses outwards and
finds its way to the exterior by the same channels.
During the day, while the green cells are exposed to

THE LIBERATION OF ENERGY 55

sunlight, carbon dioxide united with water, i.e.,
carbonic acid, is, as we have already seen, decomposed,
and photosynthesis of primary organic compounds
takes place. It will be at once manifest that this
nutritive process must mask the respiratory process,
if the amount of carbon dioxide required for photo-
synthesis be greater than that formed in respiration,
and that the formation and excretion of carbon
dioxide will not be apparent ; the carbon dioxide
will be decomposed and rebuilt by the green cells as
soon as it appears in their vicinity. For that reason
the green plant appears not to be respiring during
the day ; on the contrary, it appears to be giving
off oxygen by day, and carbon dioxide by night.
But this is easily explicable if we remember that
during the night no photosynthesis is going on
although respiration is, whilst during the day,
although both photosynthesis and respiration are
taking place, the carbon dioxide required for photo-
synthetic purposes so much exceeds in quantity the
carbon dioxide produced by the tissues that none of
the latter is able to escape, whilst, from it, as well
as from the surplus carbon dioxide taken in, oxygen
is released and exhaled as a by-product in photo-
synthesis. Hence the statement often made that
one of the essential differences between a plant and
an animal is that " whilst the animal takes in oxygen
and gives off carbon dioxide, the plant takes in
carbon-dioxide and gives off oxygen." Both state-
ments, as a matter of fact, are perfectly correct,
but the actual comparison is entirely misleading,
since the taking in of carbon dioxide and giving off
of oxygen is a nutritive or constructive process,
whereas the converse process is respiratory or de-
structive. That respiration takes place in green plants

56

A PRIMER OF BIOLOGY

Experi-
mental
demon-
stration
of respira-
tion.

by day as well as by night, is easily proved by the
following experiment (Fig. 22).

Place a vigorously growing green plant beneath a
suitable bell-jar (A) which communicates by means
of two bent glass tubes with gas-wash bottles,
and cover the bell- jar with black cloth or black
paper to shut off sunlight from the plant. The
two gas-wash bottles B and C, and D and E are half-
filled with lime-water. The outer end of the bent
tube B' communicates with the air directly, but since

"«-N + C<\.

rh f

FIG. 22. Apparatus for demonstrating respiration in
plants during the day.

the end of the tube inside the wash- bottle passes
below the level of the lime-water, all the gas which
enters the apparatus must pass through the lime-
water in B. Now when carbon dioxide and lime-
water meet, carbonate of lime (or calcium carbonate)
is formed. This latter substance is insoluble in
water and appears as a white precipitate in the lime-
water. The second bottle (C) is interpolated between
B and the bell- jar to catch any carbon dioxide that
may have passed over unaffected by the lime in
bottle B. On the other side of the bell-jar, the two
bottles D and E also contain lime-water. If now
any carbon dioxide be formed by the plant it will
give rise to a white precipitate in bottle D, while

THE LIBERATION OF ENERGY 57

bottle E is a trap for any carbon dioxide produced
by the plant which has managed to pass bottle D.
Bottle E is in connection with a pump or aspirator,
by which air can be sucked through the whole appara-
tus. (Care must be taken that all joints are made
absolutely air-tight and that the bell-jar is vaselined
to a glass plate.) If the pump be started it will be
found that bottle B becomes milky owing to the
presence of carbon dioxide in normal air, but so long
as C remains clear, we may be sure that the plant in
A is receiving nitrogen and oxygen only. Since
oxygen is being supplied, respiration is possible.
Very soon bottle D, next the glass bell- jar on the
other side, also becomes milky from the formation
of calcium carbonate — the carbon dioxide being
produced by respiration in the living plant. The same
experiment may be performed with a frog or other
small animal, which will live under the bell- jar, A, with-
out suffering any injury or inconvenience, beyond
imprisonment. Under these circumstances the
bottle D will be found to become milky much more
rapidly than when a plant is placed beneath the
bell-jar, for respiration in an animal is, under ordi-
nary conditions, much more vigorous than in the
plant. It is, of course, immaterial in this case whether
the bell- jar be darkened or not, since the animal has
no photosynthetic power.

In addition to carbon dioxide, water and solid waste
materials of various kinds are produced as the result Excretion
of decomposition processes. Water as a waste
product is got rid of as water vapour along with the
transpiration water (p. 46) by the stomata in the case
of the plant, and by glandular organs, such as sweat
glands and kidneys in the case of animals. The
solid waste substances, some of which are by no

58 A PRIMER OF BIOLOGY

means reduced to their ultimate constituents, are
also got rid of by the kidneys and sweat glands in
animals. Several of these bodies are highly nitroge-
nous, such as urea, uric acid, &c. In plants the solid
waste is stored either in parts of the body which are
periodically thrown off, e.g., leaves, bark, fruits, &c.,
or is permanently retained in tissues, such as old
wood, otherwise of service only for mechanical pur-
poses (p. 88). It is unnecessary for our present
purpose to go further into these subjects since the
task before us is to endeavour to master the principles
of biology, not the details of the different physiological
processes.

In Chapter VI an attempt was made to show
graphically the circulation of matter from the in-
organic world through the organic back once more
to the inorganic. We must now try to express
graphically the circulation of energy.

We have seen already in Chapter VI (p. 48) that
solar energy is in part stored as potential energy
in the complex organic compounds formed by the
green organism during the process of photosynthesis,
and that these compounds are used as " food," that
is to say, as stores of potential energy, by the green
plant itself, or by animals which feed on other animals,
which feed, in turn, on green plants. In a word, we
learned that the ultimate source of all " food " of
non-green organisms is the green plant, and that we
ourselves are dependent for our nutriment in the long
run on the activities of chlorophyll. Further, we
see that the ultimate source of the matter of which
the body of the highest organism is composed is,
ultimately, the soil, the water, and the air ; and that,
in the process of tissue metabolism — as the sum of
all these complex chemical changes is termed — and

THE LIBERATION OF ENERGY

59

in the final decomposition of all dead bodies, the
oxidised products pass back again to the sources from
which they came, in all probability to be rebuilt into
the tissues of another generation of green plants.
The diagram now before us, aims at showing that
a similar generalisation may be arrived at with
reference to the circulation of energy in the Universe.

The solar energy K,NET,C SOLAR ENERGY '
becomes, in part, 4

stored as potential
energy in the organic
compounds manu-
factured by the green
organism. Whether
these are oxidised in
the green plant itself
or in an animal
which has used the
green plant as food,

FIG. 23.

Circulation of energy.

or in a carnivorous animal

which has preyed on a herbivorous one, the result
is the same ; the energy is gradually released.
Before being radiated from the body in its final form
as heat (the one form of energy, it will be remembered,
into which all other forms are ultimately transform-
able), the potential energy of the food makes its appear-
ance as mechanical, chemical or electrical energy. The
energy, even in its final form, is not " lost," however,
but becomes unavailable on its dissipation into space.
There is thus both a balance of matter and a balance
of energy in the Universe, and these two great generali-
sations are among the most important with which
modern science has made us acquainted.

CHAPTER VIII

SENSITIVITY IN PLANTS AND ANIMALS

THE second characteristic or capacity exhibited to
a greater or less degree by all organisms, we have
termed sensitivity, or the power to respond to stimuli
from without or from within (p. 7).

Let us first of all glance, quite briefly, at some of the
chief types of stimuli to which organisms are subject.
These will be found to be (a) mechanical, i.e., those
which might be popularly described as a push or a
pull ; (b) chemical, where the stimulus lies not in the
mass of the material,, but in the chemical properties
it possesses and which are able to induce certain
alterations in the form, position or behaviour of the
organism ; (c) thermal, where the stimulus is of the
nature of a more or less sudden change of tempera-
ture ; (d) photic, the access of light or its with-
drawal ; and (c) electric, the influence of an electric
current or shock.

Not only the nature, but also the intensity of the
stimulus, must be taken into account, for we find
that every vital process varies in its activity within
wide limits, according to the intensity of the stimulus.
Each process goes on best when the stimulus is of a
definite intensity, but there is also a minimum, or
liminal, intensity at which the process commences,
and a maximum intensity at which it ceases.
The best response is represented by the optimum
intensity of the stimulus, but the optimum bv no

00

SENSITIVITY IN PLANTS AND ANIMALS 61

means always lies midway between the minimum
and maximum.

Another important point which has to be noted
is that only very rarely does one stimulus act alone ;
it is generally accompanied and affected by other
stimuli, which may render the organism more or less
sensitive to the special stimulus under consideration,
or may affect the primary stimulus itself, either by
diminishing or increasing its intensity.

The moment of application of a stimulus of any
kind is followed by a latent or quiescent period
during which no visible response can be detected. Latent
Doubtless, however, during this period (which may period,
be of longer or shorter duration) certain molecular
rearrangements and other changes are going on in
the stimulated organ in preparation for the ultimate
visible response.

The sensitive body in plant or animal is in all
cases protoplasm — that mysterious substance whose
analysis has as yet defied the ingenuity of chemists proto-
and biologists. We know only that it is an exceed- Plasm-
ingly complex mixture or aggregate of chemical
compounds, whose relationships to each other in the
living organism are but little known— though these
constituent compounds must be arranged in an infinite
variety of ways, as may be deduced from the varied
behaviour of protoplasm under different conditions,
from an analysis of dead protoplasm from different
situations, from its microscopic appearance at diffe-
rent times, and from the mere fact that in one situation
it constructs a bone, in another a nerve, in another
a green cell of a leaf, in another a hair, and in yet
another an enzyme.

Let us now turn our attention to plant Sensitivity
sensitivity more especially. The gradual origin ofinplants'

62 A PRIMER OF BIOLOGY

the belief that plant protoplasm is sensitive is of
interest. The botanist Jung, in the seventeenth
century, held that a plant was a living, but not a
sentient organism — " Planta est corpus vivens non
sentiens " — while early in the eighteenth century
Linnaeus formulated his famous aphorism —
" Minerals grow, plants grow and live, animals grow,
live and feel." Later still the English botanist
Smith postulated for plants " some degree of sensa-
tion, however low." In our own day biologists in
describing plant activities use terms derived from
animal life, suggested, in the first instance doubtless,
by superficial analogy, but justified by researches
which all tend to show that Smith's view was funda-
mentally correct, and that plants, like animals, are
sensitive to stimuli though perhaps the responses
are not in all cases so rapid or so well marked. The
reason for this sluggishness of response on the part
of the plant will appear later. George Henry Lewes
sums up the modern view when he says " that animal
and plant organisms have with their common structure
common properties, and if we call one of these pro-
perties sensitivity in animals, we must call it thus
also in the plant " (Arthur, " Special Senses of
Plants.").

We have thus found that protoplasm, whether de-
rived from the plant or from the animal, is sensitive
to stimuli. But three things strike us at once when
we begin to study this subject in detail, viz., first,
that two stimuli, qualitatively and quantitatively
alike, may induce very different reactions in proto-
plasm of different kinds ; secondly, that the same
stimulus may induce very different reactions in the
same protoplasm at different stages of its growth
or under diverse general conditions ; thirdly, that

SENSITIVITY IN PLANTS AND ANIMALS 63

the same stimulus applied in different intensities, may
excite very varied responses on the part of the same
protoplasm. A few illustrations will make this
clear.

First, the same stimulus may induce very different
reactions in different varieties of protoplasm. — Select
a young seedling whose shoot and root have attained
a certain development and lay or suspend it, horizon-
tally in a moist chamber, so that root and shoot are
free to move (Fig. 24, a-a').
Both root and shoot are
affected equally by the
stimulus of gravity, yet
after a few hours it will
be found that the root has
begun to bend downwards
towards the earth's centre, f J.'

while the shoot has begun FIG. 24. Geotropic curva-
to bend upwards and away ture in root and shoot
from the earth's centre of mustard. (Natural
(Fig. 24, b-V). The proto-
plasms of the root and of the shoot have thus
responded differently to the same stimulus.

Secondly, the same stimulus may induce different
reactions in the same protoplasm at different stages in
its (jroivth. — It will be remembered that in Chapter I.
we referred to a very lowly organism (Fig. 3), known
as a Myxomycete or Slime Fungus. If exposed to
light in its young state the protoplasmic mass creeps
slowly away from the source of light, attempting to
hide itself, so to speak, in a crevice in, or in the shade
'of, say, a piece of bark. When fully ripe and ready
to produce reproductive organs, however, it seeks
the light, which it previously made every effort to
avoid.

64 A PRIMER OF BIOLOGY

Thirdly, different intensities of the same stimulus
induce different effects in the same 'protoplasm. — If
a number of zoospores of such a seaweed as Entero-
morpha (Fig. 12) be placed in a glass vessel standing
on a window-sill, the zoospores aggregate on the side
of the vessel nearest to the source of light. If a
strong beam of light be now thrown on that surface
of the vessel, the zoospores leave it and aggregate
on the opposite and less brightly illuminated side.
If the vessel be illuminated by ordinary diffuse light,
the zoospores distribute themselves generally in the
medium. The zoospores thus move towards weak
light, away from intense light, but are indifferent to
diffuse light.

Animal protoplasm responds with mucli greater

rapidity and more markedly to stimuli than plant

protoplasm, and it is quite unnecessary for us even

to cite illustrations, for they are amongst those most

familiar to us in Nature. One point, however, we

Sense must emphasise here, and it is that, in addition to

organs in a generai sensitivity to stimuli of various kinds, \\Q

animals. e> J JTJ- • i li,

find special sense organs developed in animals, that

is, tissues which have become differentiated morpho-
logically and physiologically solely for the reception
of special classes of stimuli, e.g., light, contact, vapours,
&c. Thus we have the eye for the appreciation of the
form, size and colour of external objects ; the ear
for the appreciation of sounds ; taste-bodies for
distinguishing the flavours of various foodstuffs ;
tactile bodies in the skin and certain special structures
at the ends of the nerves of external or internal
organs for the appreciation of contact with bodies
likely to produce pain or pleasure ; and the nose,
the duty of whose sensitive inner surface it is to receive
impressions from volatile substances, Why are

SENSITIVITY IN PLANTS AND ANIMALS 65

such sense organs absent as a rule from plants ? We

say " as a rule," for the sense of touch is, at least in

some plants, fairly well developed, but we have no

evidence of the possession by plants of any of the

other special

sense organs

(though attempts

have been made

to show that

some plants at

least do possess

sense organs of

a kind). Let us

try and obtain

an answer to this

question.

Self - preserva-
tion is obviously
of paramount
importance to
every living or-
ganism. It must
obtain food ; it
must avoid in-
jury ; it must
acquire the re-
quisite supplies
of heat, air,
moisture, and so
on, to enable it

to live healthily ; these are the primary necessities
of its existence.

The capacity for responding rapidly to contact Sense
with extraneous bodies is developed in the animal, "
in the first instance for the recognition of injurious

FIG. 25. Cobseci

66 A PRIMER OF BIOLOGY

surroundings and of the presence of food. Where
response to contact is developed in plants it is, with
few exceptions, connected only indirectly with the
acquisition of food. More often it is associated with
the attempt to obtain support, where the plant is
unable by its own unaided efforts to stand erect, and
is developed most prominently in tendrils, such as
one finds in the pea, Cobaea (Fig. 25), passion-flower,
and other climbing plants which possess such organs.
In the case of some carnivorous plants, however,
response to the stimulus of contact is intimately
associated with nutrition as,
e.g., in the carnivorous plant
Dionsea (Fig. 26), the two
halves of whose leaf - blade
close immediately on any
insect that may happen to
touch any one of the six
FIG. 26. Dionsea sensitive hairs which arise

from the upper surface of the

leaf, or in the case of our own Sundew (Fig. 15), where
contact of an insect with the sticky tentacles in the
centre of the leaf brings about a slow infolding of all the
peripheral tentacles over the trapped animal, and also
a rapid secretion of digestive fluid from their glandular
ends. In other plants still, such as the sensitive
plant (Fig. 27), Mimosa, rapid movement, presumably
for protection, takes place in the leaf when touched,
and similar movements, due to other stimuli, are well
known to occur in other forms, such as Oxalis,
Lathyrus, &c.

A little reflection will show us that the animal
on detecting, by contact, an injurious object or an
object fit for food, being motile, can at once move
away from or towards the object as the case may

SENSITIVITY IN PLANTS AND ANIMALS 67

be — while the plant, on the other hand, has no such
power of movement. A sense of contact for this
purpose is thus useless to it, for, being fixed, it could
not benefit by its possession. The ingestion of organic

FIG. 27. Mimosa. A, before, B, after stimulation.
(J Natural size. )

food by the animal necessitates, on its part, power
of movement or locomotion, so that it may seek for
such food (p. 78) ; the plant, on the other hand, does not
require to search for the raw materials, for these are
brought to it by atmospheric currents, or lie round
its roots in the soil. The animal is further liable to
all sorts of injuries during its search for food — the

68 A PRIMER OF BIOLOGY

plant is certainly almost equally liable to injury, but
even though it recognised coming misfortune it could
not escape from it. As a corollary we may note
that the majority of non-motile animals, such as
sea-anemones, corals, zoophytes, barnacles and such
like, are aquatic and have their organic food brought
to them by water currents. Non-green plants, again,
though dependent on organic food materials, make
up for the want of locomotory power by the produc-
tion of enormous numbers of offspring, and distribute
them far and wide on the chance of some few reaching
an appropriate and favourable habitat.

For the same reason the senses of smell, of hearing,
and of sight are well developed in animals, both for
the avoidance of injury and for the procuring of food
in the first instance, whilst such senses would be
useless to plants and are not developed. As for the
sense of taste the raw materials absorbed by plants,
such as carbon dioxide, water, and the salts of the
soil, are absorbed irrespective of whether they are
tasteless or otherwise, while the organic substances
used as food by the animal, have every possible
variety of flavour, and require to be discriminated
by the organism.

The stimulation or excitation of these varied
sensory structures, be they differentiated and
undifferentiated, is often followed by movements
or indications of appreciation or otherwise, in regions,
it may be, far removed from the point of application
Transrois- of the stimulus. Thus, for example, a touch applied
suinufi. ^0 one of the segments of a Mimosa leaf is followed
by movement not only of that segment, but also of
all the segments in the vicinity. It follows from this
that the stimulus must have been transmitted from
the point of application to distant points. How is

SENSITIVITY IN PLANTS AND ANIMALS 69

this accomplished ? In the higher animal, as every
one knows, transmission is effected by specialised pro-
cesses from cells which are elongated very much in
one direction, and known as nerves — but in plants the
transference of the impulse is not so easily explained.

Microscopic research has shown that the cells in
many plants are in communication with each other
through their walls by very fine threads of proto-
plasm, so that there may be direct protoplasmic
communication from one part of the plant to another.
Indeed a recent investigator, Nemec, has gone so far
as to affirm the existence of special tracts in the cells
themselves, along which impulses may be carried.

This general survey of the phenomena of sensitivity,
so far as we have as yet carried it, has thus taught us
one important principle, viz., that, in so far as animals
and plants respond to stimuli from without, develop- Fixed and
ment of sensitivity proceeds along two divergent
lines, the one corresponding to the needs of free
organisms, the other corresponding to the needs of
fixed organisms.

Let us look a little more in detail in the first place
at fixed organisms.

It will be at once obvious that to a fixed organism
orientation is all-important, for the root must pene-
trate the soil, and the shoot must expand in the air.
Now if a seedling be laid on its side, and its shoot and
root in consequence be horizontal, how are these
two parts to ascertain which is the way up and which
the way down ?

In the beginning of the last century Knight dis-
covered that gravity acted as a stimulus to the plant,
and that the root and shoot responded differently to
this stimulus, so that the root, no matter what its ori- Gravity,
ginal position, bent towards the soil and the shoot, no

70 A PRIMER OF BIOLOGY

matter what its original position, bent towards the
sky. If some germinating peas be pinned to the rim
of a vertically revolving wheel, so that their roots and
shoots form all possible angles with the horizon, it
will be found that both roots and shoots grow in the
directions in which they have been originally placed,
because, owing to the slow revolution of the wheel,
the stimulus does not affect the same part continuously
in the same direction. What stimulus is given during
one half revolution woi^ld appear to be neutralised
during the second half ; at all events, even though
the stimulus be appreciated there is no visible response
so long as the wheel is revolving. Gravity has, we
might say, been put on both sides of the equation and
may, as the mathematician puts it, be ignored.

In botanical terminology the normal root is said
to be geotropic, and the normal shoot a- or apo-
geotropic. A very simple and instructive experiment
is to take some moistened mustard seed and throw
them against the inside of a damp empty flower-pot.
The seeds \vill adhere to its surface, and will germinate
in situ. The pot is then turned upside down over
damp blotting-paper or wet sawdust, &c., the pot
being at the same time covered over by a wet cloth.
If the pot be examined after a couple of days, it will
be found that all the young roots have grown down-
wards along the wet wall of the pot, and the shoots
have grown upwards, but without touching the wall.
If the pot be now placed in its normal position, so
that the roots point upwards and the shoot down-
wards, and if the mouth of the pot be covered with
a black cloth and be left for forty-eight hours, the
roots and shoots will then be seen to have bent through
an angle of 180° and regained their originally selected
positions.

SENSITIVITY IN PLANTS AND ANIMALS 71

Let us now study another stimulus, that of light.
Since light is of such transcendent importance to the
plant it is manifestly of the highest advantage that
the shoot should learn to grow towards the source Light,
of light, just as it is equally important that the root
should learn
to grow away
from it. If
some mustard
seed be grown
on damp moss
on a window-
sill we shall find
that the shoots
grow towards
the window
(heliotropism),
while the roots,
if exposed,
grow a way from
it (apheliotrop-
ism). This is
still better seen
if the plants
be grown in
culture solu-
tions. Now if

other mustard seedlings be cultivated in the same
way, but if they be placed during cultivation on a
horizontally revolving disc, it will be seen that the
roots and shoots obey the stimulus of gravity only,
their shoots show no tendency to turn towards the
light, and their roots show no inclination to turn
away from it.

Another illustration of the sensitivity of vegetable

FIG. 28. Heliotropic curvature of
mustard seedlings.

72

A PRIMER OF BIOLOGY

protoplasm to light has been given in the case of the
movements of zoospores (p. 64).

Water is as important a factor in the life of the
green plant as light, and it is therefore obvious that
it is of the utmost value, to the root especially, that
it should be sensitive to its presence ; roots, as a
matter of fact, grow towards water. Hence the
frequency with which drain-pipes are clogged up by
the intruding roots of plants living in the vicinity.
A very interesting and at the same time simple
experiment serves to demon-
strate the predominant effect
of water as a stimulus over
gravity. Remove the bottom
from a cigar-box and replace
it by one made of wide meshed
wire netting, floor the inside
with wet bog moss and plant
in it some peas or other seeds
(Fig. 29). In a few days the
roots, in obedience to the
stimulus of gravity, will have
grown through the wire netting
and into the air below. Finding, however, that the
air is less moist than the moss above them, they
change their direction of growth and bend back again
into the box, thus showing that the hydrotropic
stimulus is more vigorous and effective than the
geotropic stimulus.

Let us briefly consider, in conclusion, the influence
of some chemical stimuli on plants. One of the com-
monest weeds in our rivers and canals is an American
aquatic plant, known as Elodea. If some of the young
leaves of this plant be placed under the microscope
it will be seen that the chloroplasts and other contents

FIG. 29. Hydrotropism.

SENSITIVITY IN PLANTS AND ANIMALS 73

of the cells are in a continual state of movement. It
is, of course, the protoplasm which moves, and in
doing so carries with it the chloroplasts, and by
watching these the rate of motion of the protoplasm
may be, at all events approximately, measured. If
such a leaf be exposed to ether vapour, the streaming
gradually comes to a standstill, to be resumed when
the ether vapour is removed, unless the exposure to
the vapour has been too prolonged, under which
circumstances the protoplasm is paralysed.

Again, if we examine some of the extremely minute
organisms known
as Bacteria, we
find that, in the
motile state, they
are sensitive to
the presence of
oxygen gas, being
attracted to it
wherever it is pro-
duced. We have
already seen that
a green cell in
sunlight and in the presence of carbon dioxide manu-
factures organic substances and evolves oxygen gas
during the process. Let us place such a green cell,
say, of a unicellular plant, in the centre of a cover-
glass preparation in water (Fig. 30). Obviously,
if exposed to light under the microscope, oxygen will
be given off from it and will accumulate in the water
in the immediate vicinity of the cell. If we introduce
some motile Bacteria below the cover-glass they will
aggregate round the green cell. If the preparation
be darkened for a time and then examined, we shall
find that most of the Bacteria have now betaken

FIG. 30. Bacteria and green cell. A, ex-
posed to light ; B, in darkness. (After
Engelmann. )

74

A PRIMER OF BIOLOGY

themselves to the margin of the cover-glass, since,
near the edges, oxygen will have been absorbed by
the water from the air. Engelmann has made use
of this fact in a very ingenious manner to prove that
the rays absorbed by chlorophyll are those chiefly
concerned in the processes which result in photo-
synthesis with its accompanying evolution of oxygen.
1'or if a filament of an alga be placed on the field of
the microscope and illuminated from below by the
solar^speetrum, obviouslyrsome cells will be affected

FIG. 31. Muscle- nerve preparation and recording drum.
(After Waller.)

by red, some by orange, some by blue rays, and so
on. Since the rays which are absorbed by chloro-
phyll, viz., the red and the violet, are believed to be
those chiefly concerned in photosynthesis, the Bacteria
will congregate near these regions, for there oxygen
will be given off during photosynthesis.

The general conclusion we arrive at, then, is that
plants as well as animals are sensitive to external
stimuli, and that the protoplasm alone is the sensitive
substance.

Sensitivity Perhaps we may most easily gain some ele-
animais. mentary acquaintance with the general mechanism

SENSITIVITY IN PLANTS AND ANIMALS 75

of sensitivity in animals by studying what is termed
in physiology a muscle-nerve preparation (Fig. 31).
It is well known that the tissues of the lower animals
retain their vitality for some considerable time after
death, and thus permit of the performance on them
of certain simple physiological experiments which
cannot conveniently be carried out on the living
animal. One of the muscles of the hind leg of a frog
is dissected off a recently killed animal and the nerve
supplying it is also carefully exposed. If one end
of the sinew attached to a muscle be now fixed in
a rigid clamp, and the other free end be attached to
a weight, we are able, by applying a stimulus to the
nerve, to cause contraction in
the muscle, thereby raising the
weight. Moreover, if we attach
to the weight a pointer, placed
in such a way as to write on
smoked paper covering a re-
volving drum, we are able to Fl°: 32- Tra<>ing of a
i , . c j f ,-, simple muscular con-

obtam a record of the amount traction,
of the muscular contraction

in relation to the nature or intensity of the stimulus
applied to the nerve. Let us suppose the nerve to
be stimulated by an electric shock, we obtain a pro-
nounced contraction on the part of the muscle, but
the beginning of the contraction and the moment of
application of the stimulus are not synchronous ;
a longer or shorter period elapses between the applica-
tion of the stimulus and the response (Fig. 32). This
period is known as the latent period, and during it.
various chemical and molecular rearrangements are
no doubt taking place, both in the nerve, in carrying
the message along, and in the muscle fibres, pre-
liminary to their contraction. If the stimulus be

76

repeated at very short intervals the muscle becomes
at length rigid in the contracted condition ; it is
said to be in a state of tetanus (Fig. 33). Gradually,
however, the muscle becomes less and less contracted
as fatigue sets in, until, finally, it is unable to raise
the weight at all, and in this condition it remains for
some time. If the stimulus be reapplied after a
short period of rest, the muscle is again able to raise
the weight, but not so far as it did at first.

The nature of the stimulus applied may be of the
most varied character ; it may be a chemical reagent,
an electric shock, or merely a tap from a pencil.

It is not difficult to see
"~"*\ that the chief character-
V_ istic of the animal as con-

r-r iiniiiiinimiiiuiir trasted with the plant in

****** relation to sensitivity is

FIG. 33. Tracing of imperfect wnat may be termed the
tetanus in muscle. centralisation of adminis-

tration. The plant has, as

we have seen, diffused sensitiveness to certain stimuli,
but in the animal, not only is the perception of
many of these stimuli localised, but one or more
centres are developed to which these stimuli are
transmitted ; there they are analysed before a
reaction takes place, which reaction is caused in
turn by a stimulus generated in the centre and
transmitted to the region of response. In the
simplest condition the same cell that receives the
stimulus also brings about the response, but in most
multicellular organisms the element that receives
the stimulus and ths element that reacts are distinct,
but put in communication with each other by means
of a central element, so that the motive impulse
to contract or secrete as it may be, is transmitted

SENSITIVITY IN PLANTS AND ANIMALS 77

from the central element by an efferent nerve to the
contractile or secretory cell (Fig. 34A).

In the higher types of animal life a fourth
element is added, so that there are two central
elements, one to receive the impulse from the sensory Nerves,
organ, and another connected with the former to
transmit, by means of the HERVE

efferent nerve, the impulse
to the muscle, gland, or
other body affected. When
the response takes place
without any consciousness
being aroused, it is termed
a reflex action. Usually,
however, the two nerve -
centres are connected with
a nerve cell complex form-
ing a central nervous
system, by whose means FIG. 34. Scheme of nervous

a definite and determinate
co-ordination of the various
parts of the organism is
insured.. It is the develop-
ment and elaboration of this central nervous system
that furnishes the key-note to the history of the
evolution of the animal line of life.

system. A, simple reflex
action; B, 1, 2, 3, 4, reflex
action; 1, 2, 5, 6, 3, 4, con-
scious nervous response.

CHAPTER IX

MOTION AND LOCOMOTION. THE SKELETON

EVERY organism is capable of exhibiting motion in
some part, even though such movement may be
visible only with the aid of a microscope. Most
animals, are capable of locomotion, or movement
from place to place, while but few plants have
that power. The power of movement possessed
by the animal and the fixed condition of the
plant forms a popular distinction between the
two branches of the organic world, but although
for the most part ignored as a characteristic
difference by science, it is still worthy of close
attention, since it is bound up with other
differences which are fundamental.

We have said that most animals are capable of
locomotion, and that this is necessary to their
existence, since, without that power, it would be im-
possible for them to obtain organic food, which is
only local in distribution. Fixed animals, such as
zoophytes, sea- anemones, barnacles, &c., live in a
medium, the sea, wherein organic food is distributed
more uniformly and where currents of water bring
the organic food to them, just as atmospheric currents
bring the necessary carbon-dioxide to the plant.
On the other hand, the vast majority of plants are
fixed organisms, but the raw materials which they
require for the purpose of constructing organic
compounds are to be found everywhere ; there is
no need to move about in search of them. Loco-

78

MOTION AND LOCOMOTION 79

motion among the higher plants when it does occur
is purely physical, and dependent on the absorption
and evaporation of water and the consequent bending
and unbending, extension or shrinkage of parts of
the organism, and not, as in the animal, on the
movements of special contractile tissues. Further,
locomotion in the higher plant is not associated
with the problem of self-nutrition, but with the
distribution of offspring. On the other hand,
many of the lower plants have the power of loco-
motion, but these plants are aquatic and their
powers of movement are as much associated with
the problem of dispersal of progeny as with that of
nutrition.

It has already been said that all plants and animals
exhibit powers of motion in some degree. Even in
the higher plants the protoplasm of the cells, at
least in the young state, shows power of movement ;
the leaves of many plants are able to open and close,
according to certain conditions ; fruits and seeds,
growing points, &c., show powers of movement
associated with growth conditions. Illustrations
of the power of movement in individual parts of
the animal body, apart from locomotion of the
whole organism, are too familiar to require citation.

The types of movement that are exhibited by
protoplasm itself are very varied in character. Move-
Apart from the circulation of protoplasm in cells, ™rotoS- °f
already referred to (p. 73) we have the ciliary plasm,
movements of the cells lining various tubes, such as
those of the respiratory organs, the cells covering
the surfaces of gills, &c., and the amoeboid movement
of the leucocytes of the blood, of many gland
cells, of the cells lining the alimentary canal of
many of the lower animals, and, in the plant world,

80 A PRIMER OF BIOLOGY

the ciliary movements of many lower Algee and of
many reproductive cells in higher forms, such as
mosses and ferns, and the amoeboid movements of
the Slime Fungi, of the reproductive cells of many
lower Fungi and of Algae, &c.

Naturally, it is in the animal rather than in the
plant world that we expect and find special organs
for locomotion. These are most varied in character
and comprise such types as the water-tube feet of
Organs of starfish and their allies, the jointed appendages of
motion. insects, crabs, lobsters, spiders, &c., the contractile
massive foot of the molluscs and the wings of all
grades of birds, bats, &c., the fins of fish and the
familiar limbs of mammals. Amongst these we
recognise three types, those adapted to terrestrial,
those adapted to aerial, and those adapted to aquatic
conditions, with, occasionally; as in the birds, ap-
pendages both for locomotion on land or in water
and for locomotion through the air.

The subject of motion and locomotion in organisms
leads us naturally to the question of the skeleton or
Skeleton, hard parts, and to that subject we must devote the
rest of this chapter.

The necessity for locomotion in search of food in
the animal is associated with the condensation of
the skeleton and the jointing of its various parts —
whilst the uniform distribution of the skeleton in the
plant is associated with its fixed habit. We shall
see how this principle is exemplified and established
in the course of our discussion of the skeleton.

Before discussing its composition let us, first of

all, attempt to determine what functions the skeleton

Functions fulfils by considering simple cases from the animal

skeleton, world. Manifestly, it gives protection to soft parts.

For example, the skull and vertebral column protect

MOTION AND LOCOMOTION 81

the central nervous system, while ribs give pro-
tection to the heart, lungs and other important
organs. The exoskeleton of the turtle, armadillo,
&c., protects the entire body, the shell of the snail
and of the limpet and the integument of the beetle
perform similar functions, as do also the scaly hide
of reptiles, the hair of furry animals, &c. Simi-
larly, in plants, cork is protective, and even plants
which do not develop cork develop a thin corky
cuticle which is protective in function, while others
are provided with thorns, hairs, wax, resin, &c.,
which are the analogues of an exoskeleton.

Another function of the skeleton is to give rigidity
to soft parts which require it. Thus the thigh, leg,
arm and forearm bones give rigidity to these
members, while their jointing at the same time
permits freedom of movement ; the skeleton frame-
work of a leaf keeps its green substance expanded,
and so on.

The skeleton of the animal, moreover, performs
a special function in that type of organism, in that
it gives points of attachment for muscles and enables
the individual parts to be moved independently or
collectively, and, at the same time, co-ordinately.
On the other hand, the skeleton of the plant may
perform a function which that of the animal does not
perform, namely, circulation, or the conveyance of
both crude and manufactured food materials from
one part of the organism to another. Manifestly,
it would be excessively inconvenient if the skeleton
of the animal acted also as a circulatory organ, for
circulation would be interrupted or impeded every
time the skeleton was put in motion ; in the plant,
on the other hand, economy of tissue is effected by
combining the function of circulation with that of

F

82

A PRIMER OF BIOLOGY

giving rigidity in an un jointed and, in itself, immobile

framework.

We may now turn our attention to the general

nature of the material of which the skeleton is com-
posed in the two types
of organism. The mate-
rial in the one case is
mainly bone, in the other
mainly woodland we may
consider these two sub-
stances from two points
of view, (a) chemical
composition, and (6)
structure. First, as to
chemical composition.
If a piece of bone and
a piece of wood be
placed in a furnace and
burned so far as they
will burn, we find that,
at the end of the opera-
tion, we still have a
bone, though it has lost
considerably in weight,
whilst the outline of
the piece of wood is
entirely lost. Further,
the ash left over after
burning weighs only a

FIQ. 35. Wood of the Plane small fraction of the
tree in tangential longitudinal original block. Chemical
section. ( x 75.) i • • / i.

analysis, in fact, shows

us that, whilst two-thirds or more of the dry bone is
composed of mineral matter, not more than a twentieth
of the dry weight of the wood is inorganic in charac-

MOTION AND LOCOMOTION

83

ter. Thus, one of the long bones of an ox, after being
thoroughly dried, yields about 60 per cent, of calcium
phosphate and about 10 per cent, of other inorganic
salts, while the remaining 30 per cent, consists of
combustible nitrogenous organic matter. On the
other hand, a piece of perfectly dry fir wood yields
on an average only about 2 per cent, of its weight
of incombustible ash, consisting mainly of salts of
calcium, potassium and sodium, while
all the remainder is composed chiefly
of compounds of carbon, hydrogen,
oxygen and nitrogen.

Again, as to structure, we find that
wood consists of overlapping, spindle-
shaped fibres (Fig. 35), while bone con-
sists of "concentric lamellae surrounding
central spaces containing nerves, blood-
vessels, &c., the lamellae being, so to
speak, nailed together by fibres (Fig.
36).

Taking these two series of facts into
account, let us next inquire whether FlG- 36-
bone and wood form good building tudinal section
. , . ; . ^ of bone. (xoO.)

materials Irom an engineering point ot

view, and for that purpose let us contrast them, with
cast-iron and steel. Obviously, a good all-round
building material should be able to withstand equally
well a crushing force and a tearing force. From the
following table it will be seen at once that a bar of
cast-iron can withstand a crushing force extremely
well, but that it is very liable to snap if subjected
to a bending or tearing one. Steel, on the other
hand, can withstand both tearing and crushing
forces absolutely and relatively better than cast-
iron, hence its constant use as a material for

Structure.

84

A PRIMER OF BIOLOGY

the construction of girders, rails, masts, columns,
&c. The same table shows us that wood and bone
are able to resist tearing and crushing forces about
equally, but that wood is stronger than bone in its
power of resisting a tearing force, while bone is
stronger than wood in its powyer of resisting a crushing
force. How important this point is we shall see
later, when we come to consider the strains to which
these skeletal substances are subjected in the plant
and animal respectively.

Lbs. required to

Lbs. required to

Relative values as

Material.

break a rod
1 sq. mm. in

crush a rod
1 sq. mm. in

building material.

sectional area.

sectional area.

Tearing.

Crushing.

Steel .

225

319

29

41

Cast-iron

28

160

4

22

Bone

26

33

13-5

17

Oakwood

14

10-5

17-5

13

(Note. — The relative values are obtained by
dividing the figures given in the first two columns
by the specific gravity of the materials, viz., steel,
7'2 ; bone, T9 ; oak wood, '9. The values given
are approximate only. The table is adapted from
Macalister, " Encyc. Brit." Art. Anatomy.)

Bearing these fundamental data in mind, the next

point of importance is to determine how we may

best arrange a given amount of material, bone or

wood, as the case may be, so as to combine economy

of material and effectiveness to resist crushing or

Arrange- bending. Obviously the plant has to withstand

sk^ietsfi more tearing than crushing. Thus, pressure of

material, wind tends to bend the stem of a plant, that is, to

MOTION AND LOCOMOTION 85

tear it on the convex side and crush it on the concave
side, and also to tear the roots out of the soil with
a rectilinear strain. The long bones of the leg,
owing to their being jointed together, are not sub-
jected to extreme tearing forces, but have to with-
stand more crushing, since they support the weight
of the body. Broad expansions, such as the leaves
of a plant, have to resist rupture at their edges ;
the body of the animal, on the other hand, being
much more compact than that of the plant, is not
subject to such stresses. A simple engineering
example will make
the arrangement of
skeletal material in
the two cases ob-
vious at once.

Suppose that we
rest a bar of wood
or other elastic ma-
terial on two sup- FIG. 37. Principle of the girder,
ports as represented

in Fig. 37, and place a heavy weight in the centre.
If the weight be sufficiently heavy, the bar will be
bent so that the under side becomes convex and the Principle
upper side concave. Careful measurement reveals
the fact that the underside has increased in length,
while the upper side has decreased ; in other words,
the underside is in a state of tension and the upper
side in a state of compression. Manifestly, the
layers immediately subjacent to these outermost
layers will be slightly less extended and slightly less
compressed, respectively, than they were before the
weight was applied ; a median region of the bar
must, therefore, be neither extended nor compressed.
If neither stretched nor compressed, that region is

86

A PRIMER OF BIOLOGY

obviously doing nothing towards supporting the
weight, and we may therefore cut it away very
.considerably, so long as we leave sufficient to keep
the two outer regions at the same distance apart.
Far less material than what we have available will
be required for that purpose, so that we may hollow
out the sides of the bar and leave a central region or
" web," as it is technically termed, to keep the two
" flanges " apart. The " girder " thus formed will
be almost as strong as the solid bar, whilst its own

weight will have
been greatly de-
creased and
material econo-
mised.

In the illus-
trative case we
have just con-
sidered we have
assumed that the
weight is press-
ing only in one of two directions (for manifestly,
the beam might have to resist an upward as well as
a downward pressure). Let us now, however,
suppose the weight or pressure to affect the beam
Principle laterally as well as vertically. A girder structure
honow must in that case be provided laterally also, and the
column, two webs would cross each other at right angles.
We thus get in cress section such an appearance as
that seen at Fig. 38. Lastly, let us suppose the
pressure to be exerted in any direction ; we must
then provide an infinite number of girders whose
flanges must face every point of the compass. Under
these circumstances, the flanges will obviously keep
each other apart, and we may then get rid of the

FIG. 38. Principle of the hollow column
A, crossed girders ; B, hollow column.

87

webs altogether. We thus reach the principle of
the hollow column, and the hollow column, as every
one knows, is one of the commonest structural devices
adopted in engineering,
in shipbuilding and
architecture. If we
examine the long bones
of the body, we find
that they are all hollow
columns, combining
the maximum of
strength with the
minimum weight of
material (Fig. 39).
The long bones are, it
is true, in the em-
bryonic state, solid,
but are hollowed out
by certain cells which
have this special duty
to perform.

In the case of the
plant, as an examina-
tion of erect, and, at
the same time, slender,
stems shows, the sup-
porting or skeletal
tissue is laid down on
the same principle.
For example, the stem
of such a plant as
wheat is a hollow column, and in other plants,
the special skeleton tissue is peripherally placed
in flanges, kept apart and yet held together
by more delicate central tissue. The varieties in

FIG. 39. Longitudinal section of
human thigh bone. (£ Natural
size.)

88

A PRIMER OF BIOLOGY

the mode of deposition of the skeleton or mechanical
tissue in such plants are extremely numerous as
may be seen from the examples illustrated in Fig. 40.

Even in forest trees it
not infrequently hap-
pens that the central
wood decays, and an
old tree may be quite
hollow in the centre
and yet be quite able
to support the super-
incumbent weight of
branches and leaves.

Roots, on the other
hand, have to with-
stand a rectilinear
pull, and are not
subjected to bending
at all, and engineers
tell us that the tissue
required to resist such
a strain should be
centrally placed. This
is precisely the ar-
rangement adopted
in the root (Fig. 41).
Even when there is
a central pith it may
become hardened, or
sclerotic, while the softer tissues are ' peripherally
placed.

We may now turn to the consideration of another
Principle engineering example^the arch or rafter. An ex-
arch6 cellent illustration is obtained from the human
ankle (Fig. 42). In every roof (Fig. 43) where the

June as

Clad, i urn

FIG. 40. Distribution of skeletal
tissue in plant stems. (After Van
Tieghem.)

MOTION AND LOCOMOTION 89

weight to be supported is at all likely to bear too
heavily on the walls, the " struts " which meet at
the apex of the roof, and which would, at their free
ends, tend to force the walls outwards are connected
by a " tie beam." The struts are obviously in a

FIG. 41. — Transverse section of a root, showing aggregation
of vascular and skeletal tissue in the centre. ( x 50. )

state of compression and the tie beam in a state of
tension. When these strains are equal, the rafter
is a rigid system, and will then bear down vertically
on the walls without exerting any tendency to force
the walls outwards. Now the trunk of the body
bears down through the long bones of the legs on
the arch of the ankle. The ankle has, on the one side,

90 A PRIMER OF BIOLOGY

the bones of the foot, on the other, the heel bone
for its two struts, while the tie beam is the muscle

FIG. 42. Human ankle.

FIG. 43. Raf er.

and sinew of the sole. Obviously, the tie beam
must not in this case be permanently rigid, but capable

of being made either
rigid or flexible as
need requires. In
the plant, struts are
frequently adopted
by tall, top-heavy
trees, the earth form-
ing the tie beam.

The principle of
the crane, again, is
very well exempli-
fied in the human
thigh bone (Fig. 39).
When the body
is bent forward,
the weight rests on
the knuckle of the
of theiple thigh bone, revolving in the socket of the hip bone,
crane. and is liable to " sheering." How is this avoided in a

FIG. 44. Crane showing lattice (girder)
shaft and solid head.

MOTION AND LOCOMOTION

91

crane, where also the weight is supported on the end
of a tapering shaft ? A glance at Fig. 44 shows us
that the material of the steel frame is arranged so
as to support any such weight, and counteract the
tendency to sheer off the head of the crane
Similarly, if we examine
the head of the femur,
the bone substance will
be found to be arranged
on an exactly similar
plan, as we see from a
comparison of Figs. 44
and 39.

Lastly, the edges of flat
structures, e.g., leaves of
plants; are often strength-
ened bystrands of skeleton
tissue and these strands
are aided by the veins,
which are usually ar-
ranged, in broad leaves at
all events, in a series of
successively smaller arches
from the midrib outwards
(Fig. 45). Large leaves
which have no such
strengthened margins are
liable to be torn to ribbons by the wind, very much
in the same way as a flag is frayed out unless
protected by a marginal cord.

FIG. 45. Venation at the edge
of a leaf.

CHAPTER X

THE ADAPTATION OF ORGANISMS TO THEIR
ENVIRONMENT

WE may now look briefly at the general relations of
organisms to their environment, how they adapt them-
selves to their surroundings, making the best of such
as are favourable to their healthy existence and the
multiplication of their offspring, and protecting
themselves from such as are injurious to them or
their progeny. Let us, first of all, look at plant
life, and here, at the outset, we meet with differences
once more dependent on the fact that the plant is a
fixed organism while the animal is pre-eminently
a motile one. Manifestly, we may expect the plant
to show more adaptability than the animal, simply
because the animal, in virtue of its locomotory
powers, can remove itself from obnoxious influences,
while the plant cannot escape, and must therefore
adapt itself, temporarily or permanently, to its
surroundings or succumb.

Before going into the consideration of specific
examples, let us briefly summarise the principal
external influences which affect plants.

First, we have what may be termed mechanical

influences of the environment, represented by

amount of space, lateral or vertical pressures

Nature and tensions, and so on; then we have chemical

vfrormfent. influences, such as those of food, air, water, the

nature of the medium in which the organism lives,

&c. ; thirdly, physical influences, which we may

92

THE ADAPTATION OF ORGANISMS 93

summarise under the heads of heat, light, and elec-
tricity ; and, lastly, vital influences, the influences,
that is to say, exerted by neighbours, parasites, and
that most active of all vital agents, man. We may
consider our organism, in short, as a central unit
surrounded by an environment — everything not
the plant — in part aiding, in part retarding the
organism in its healthy development. The case is,
however, not so simple as it looks, for not only may
these various influences be all active at the same
moment, but they act on and modify each other,
and the modified influence may have an entirely
different effect on the organism from that which
it would have had if it had been unaltered by other
conditions.

It will be useful at this point to quote a few examples
from the profusion of literature on the subject of
the influence of the environment on the organism.

It has been shown by several experimenters
that, when bred in confined spaces, the offspring of
certain shrimps, &c., develop into dwarf forms, and
that a definite relation exists between the size of
molluscs and that of the vessel in which they are Meehani-

grown. The influence of changes in the pressure — £?i

T -, ,. i P ., . influences-

lateral and vertical — ot the environment, more

especially on the form of aquatic plants, on the shape
of corals, shells, sponges, trees, &c., has been the
subject of research by many investigators, while
the effect of pressures and tensions on cell form and
planes of division has also occupied the attention
of both botanists and zoologists.

Looking next at chemical influences, tadpoles and
young fish, when well supplied with oxygen, develop chemical
more rapidly than under normal conditions ; drought influences,
induces encystation and latent life in many lower

94

organisms ; dry air induces the formation of a
thick cuticle and much skeletal tissue in many
plants, while excess of moisture is accompanied
by the formation of little cuticle and absence of
strengthening tissue ; the presence or absence of
water has also a marked effect on the mode of develop-
ment of some amphibious organisms (see also p. 98).
The Axolotl of the Mexican lakes, for instance, is
at one stage aquatic and provided with gills, but
develops lungs, like a salamander, when subjected to
dry conditions. The effect of artificially altering
the salinity of water on the movements and forms
of organisms inhabiting it have led to important
conclusions on the origin of fresh-water from salt-
water faunas. Indeed, the fluids of the body also
have been shown to become altered by changed con-
ditions of the medium, affecting, as it would appear,
the character of the blood corpuscles, the amount
of pigment developed, &c., while ciliated cells may
be made to become amoeboid and vice versa by
varied changes in the medium. It has been found
that certain chemicals can induce the unfertilised
eggs of certain animals to segment, but the classic
researches of the Hertwigs and of Loeb on the fer-
tilisation and segmentation of the ovum under
different conditions can only be referred to in this
connection — space forbids their quotation in detail.
Pre-eminently favourable nutritive conditions have
been found to induce ciliated lower forms to become
amoeboid or even to take on cell walls, and it is well
known that asexual reproduction — by purely vegeta-
tive methods — is encouraged by such conditions, while
vigorous pruning of shoot or root tends to the develop-
ment of flowers and fruit. More than one authority
claims to have shown that better nutrition tends to

THE ADAPTATION OF ORGANISMS 95

the excess of female offspring, while relative starvation
tends to the formation of males. Indeed, the great
physiologist, Claude Bernard, went so far as to say
that the whole problem of evolution circled round
the variations in the nutritive factors affecting
plants and animals.

Apart from the changes in the rate of response
of contractile organs immediately observable on
alterations in temperature already referred to, it
has been shown that the rate of multiplication of Physical
certain of the lowest animals is markedly increased ln
by a rise in temperature, whilst cold, in addition to
retarding movement, diminishes the rapidity of deve-
lopment and tends to induce the formation of dwarf
and even larval forms, and to affect the sex of flowers.
Similarly, light influences the formation of pigment
in certain animals, e.g., insects, and affects the
colouration of birds' eggs, while in relation to plants,
we have already quoted numerous instances of the
importance of variations in light in relation to the
distribution of chlorophyll, the anatomy and mor-
phology of leaves, the movements of motile leaves
and of free organisms. Light is also known to
govern the mode of reproduction in certain Algae,
and, in excess, to act injuriously on Bacteria, while
some botanists hold that deficiency in illumination
favours the production of male as opposed to female
cones in certain members of the pine family.

The reaction of organisms of different types
on each other may be demonstrated by endless
examples. To quote only a few cases, we have the vital
alteration in form in both constituents due to the influenees.
constant association of Algae and Fungi in the
composite structures we term lichens, the remarkable
cases of hypertrophy of vegetable tissue in fungal

96

A PRIMER OF BIOLOGY

and insect galls, and the structural changes induced
in some sponges by the constant living with them
of certain polyps. The varied forms of flowers are
now very generally looked upon as direct adaptations
to visits of insects and that the manifold forms of
domestic plants and animals have arisen as a result
of conscious selection and cultivation by man is a
fact too familiar to require proof. Evidence in
abundance is forthcoming in Darwin's classic work
on the subject ("Animals and Plants under Domesti-
cation") and in the extensive literature that has
arisen since its publication.

The conditions of the environment are infinitely
varied in different parts of the world — even, it
may be, in the same district. In no two regions in-
deed are they exactly similar in all respects, and even
in the same spot, the conditions are never the same
for two moments in succession. Under these circum-
stances, it must be obvious that the organism,
whether plant or animal, must be capable of keeping
itself in equilibrium or accord with the ever-changing
conditions. In certain regions some conditions of
the environment are specially emphasised. Thus,
in a desert region the absence of water is the principal
factor to be considered, and unless the plant is adapted
to live in such dry conditions it must obviously
succumb. Again, aquatic plants are adapted to
live either entirely or partially submerged. A
general survey of the plant world enables us to dis-
tinguish certain types of structure specially adapted
to special climatic conditions. Thus we have aquatic
plants, desert plants, arctic and alpine plants,
seacoast plants, swamp plants, &c., as well as plants
adapted to peculiar modes of life, such as parasites,
carnivorous plants, climbers, epiphytes, these latter

THE ADAPTATION OF ORGANISMS 97

being plants which grow on, but not at the expense
of, other plants.

Then, again, we have adaptation of different Adapta-
types of organ to subserve special purposes or organs to
functions. For example, protection from destruc- special

. * £ fiinnt ir\

tion by animals is well exemplified in such
ABC

>

functions.

FIG. 46.. — A, prickles ; B, leaf thorns ; C, branch thorns.

common plants as the rose, the hawthorn, and the
holly. In each case the protection is afforded by
sharp spines, but a little knowledge of morphological
botany teaches us that the spines are of very different
origin in each case (Fig. 46). In the hawthorn they
are modified branches, in the holly they are extensions
of the veins of leaves, in the rose they are merely
hardened and sharpened emergencies from the
surface layers of the stem or leaf-stalk, and have no
connection with the internal vascular system. Once

98

A PRIMER OF BIOLOGY

more, delicate-stemmed plants are able to maintain
themselves in the erect position by holding on to
their stronger neighbours, and so enjoy the maximum
of air and light they would otherwise fail to obtain.
This they do by means of a variety of structures,
all of them performing the same function, but of
the most diverse morphological value. Cobsea,
for example, climbs by means of tendrils which are

the terminal leaflets of
branched leaves (Fig. 25).
The grape also possesses
tendrils which perform
the same function, but,
in this case, the tendrils
are modified flower
branches. The bramble
climbs by means of
prickles, which are, at
the same time, protec-
tive, and the ivy by
throwing out aerial roots
which cling to walls,
trees, &c.

A considerable amount
of experimental work has
been carried out of recent years on several plants
with the object of determining how far they may
be made to adapt themselves to changed surroundings.
Take, for example, the common gorse, familiar to
every one by its bright yellow flowers, spiny green
shoots and absence of genuine photosynthetic leaves.
If a seedling gorse plant be examined, it will be
found that it has no spines, but, on the other hand,
possesses branches with small but quite recognisable
leaves (Fig. 47). On cultivating such a seedling in

FIG. 47. Ulex europseus: A,
grown in moist air ; B, grown
in dry air (1 nat. size).

THE ADAPTATION OF ORGANISMS 99

a moist atmosphere, it develops into an adult without
any such protective arrangements as one sees in

FIG. 48. An amphibious buttercup.

the adult grown under normal conditions. If,
however, the conditions approximate to the normal,

100 A PRIMER OF BIOLOGY

no more leaves are developed, and all further growth
takes the form so familiar to us on our commons
and moors. The white water buttercup, common
in wet ditches, is another illustration in point.
The lower leaves of this plant developed in the
water are much divided into numerous fine linear
segments, whilst those developed in the air are
provided with three to five obovate or rounded
lobes (Fig, 48). Under dry conditions all the leaves
have the lobed form, but if entirely submerged all
are filamentous. It must be understood, of course,
that the one type of leaf cannot, after once being
developed, be transformed into the other ; but leaves
subsequently produced will assume the aerial or
aquatic form according to external conditions.

The wonderful phenomena of mimetic resemblances

seen between animals, between plants, and between

Mimetic plants and animals are well worthy of consideration

bfance". m this connection, but space forbids us even to give

instances, let alone consider any one of these in

detail.

CHAPTER XI

REPRODUCTION

THE life of every organism has two aspects : the
vegetative, or individual, aspect, and the reproduc-
tive, or tribal, aspect. In the one case all the energies
of the organism are devoted to its own individual
nourishment, protection, and so forth ; in the other,
certain organs come into play, previously in abey- Antag-
ance or up to that time non-existent, the activity ""dividual
of which, since they are, as a rule, incapable of nourish- and tribal
ing themselves, immediately brings about a drain
upon the vegetative organs. Further, among the
higher forms, the offspring are, for a time at least,
dependent on the parent for support, and this con-
stitutes a further drain on its resources. Hence we
see that tribal life must be antagonistic to indi-
vidual life. Indeed it may be said, at least in general
terms, that whatever conditions are favourable to
vegetative development are against the interests of
the reproductive processes, while the reproductive
processes must of necessity react adversely on
the vegetative system. Thus gardeners prune fruit
trees when they wish them to bear fruit, or re-
move the flowers when they desire plentiful foliage.
Keeping this fact in mind, let us inquire into the
different modes of increase presented by plants and
animals.

As we have already seen in Chapter II (p. 8), Asexual
at or before the completion of, or at some period ^epro

• i_ !•• T P ,1 . , .r. . tion.

in, the lite cycle or the plant or animal, provision is

101

102 A PRIMER OF BIOLOGY

made for the continuance of the race, and this is
attained in one or both of two ways, i.e., by separation
of a part of the body of the parent capable of giving
rise directly to a new organism of the same type,
in other words, by vegetative and " asexual reproduc-
tion," or by separation of a cell — an ovum or egg-
cell — which is itself, save in exceptional cases, in-
capable of developing into a new organism without
previous fusion with a corresponding cell — a, sperm
or fertilising cell — almost always in animals, and very
generally in plants, derived from another individual.
This latter method is termed " sexual reproduction."
It is customary to speak of the sperm- producing
parent as the male, and the ovum-producing parent
as the female. The ovum, after fusion with the sperm,
becomes the oosperm and develops into the embryo
and, finally, into the adult.

Let us inquire first as to the theoretical origin of
these two kinds of cells.

One of the first things we become acquainted with
when we study the origin of cells is that they are
capable of division. Why does a cell divide ? A
cell is a living unit ; in it, during life, certain con-
structive changes are going on, tending to the accumu-
lation of organic substances and of energy in the
potential form, and, at the same time, certain destruc-
tive changes, tending to the liberation of potential
energy in the kinetic form, the decomposition of
complex compounds, and the formation of simpler
degradation products or excreta. We have seen
already that the surface of the cell is the medium
through which all nutritive substances must enter
the cell, and it is equally obvious that from that
same surface the waste products must be given off.
If, in consequence of adequate nutrition, the cell

REPRODUCTION 103

grows, obviously the surface will increase synchro-
nously with the volume, but not in the same ratio,
for mathematicians tell us that, in a sphere, while the
mass increases as the cube of the radius, the surface
increases only as the square. Under these circum-
stances there will come a time when the mass must
attain a size just such as may be adequately nourished
by the possibilities of the surface as a means of
entrance of food, and adequately purified by the
possibility of getting rid of waste. A further increase
in the volume is obviously impossible, since not only
is there no surface available for the entrance of suffi-
cient food, but the surface is
also inadequate for the ex-
cretion of the waste. The
cell must then either die or
readjust the relation between
surface and volume. If it
divides into equal parts, its
volume is at once halved and FIG. 49.

the surface area of each half is

increased by the whole circular face exposed by the
division (Fig. 49). For instance, in the spherical cells A
and B, let us assume that the radii are two and three
millimetres respectively. The volumes of these
spheres may be calculated from the formula i TT rs,
where r = radius and ?r = a number approximately
estimated at 3i. The volume of A will thus be 33^
cubic millimetres. The surface of a sphere may be
determined from the formula, 4 TT r2, so that the extent
of the surface of cell A amounts to 50S- square milli-
metres. Similarly, the volume of cell B will be 113-1
cubic millimetres, and its area will be 113^ square
millimetres. Assuming for the sake of argument that
an area of 1 square millimetre is sufficient for the

104 A PRIMER OF BIOLOGY

adequate nutrition and purification of 1 cubic milli-
metre of protoplasm, we see at once that the cell
A has more than ample area for its nutritive and
excretory needs, and may go on growing without
detriment, while B has reached the maximum limit
in this respect, and must be insufficiently nourished
and accumulate waste products should it by any
chance increase still further in volume. Let B, how-
ever, divide into hemispheres then each half will have
a volume of 564 cubic millimetres, while the area of
each will be 564 square millimetres + 28f square
millimetres (the area of the circular face exposed),
i e., 84" square millimetres — more than re-establishing
the balance on the side of area.

It is inconceivable, however, that the two new
cells arising in this way should be precisely similar
in all respects. Apart altogether from differences
in the protoplasm, one or other will have an excess
Ovum and of waste products or of reserve products, and thus
sperm. jhere arise differences between the daughter cells
which result from division, both in minute structure
and in activity. It is known that the accumulation
of reserve nutritive bodies is accompanied by a
tendency to sluggishness and non-motility in a cell,
and hence there might arise a more massive and non-
motile ovum, and a smaller and more active sperm.
These cells are the characteristic reproductive cells
of the female and the male respectively, both in the
plant world and in the animal. It is, of course, not
suggested that this is the way in which these reproduc-
tive cells arise in higher individuals, though it is
possible that some such explanation might account
for the original differentiation of cells of different sex.
Our next question must be, at what period in the
life cycle of the organism are such cells formed ?

REPRODUCTION

105

Let us consider plants first. Some of them, as every
one knows, last only, it may be, a few hours, a few
days, or a few months. Others again, and these
include all our higher plants, are annual, biennial or
perennial. By annual we mean that the plant starts
life as a seed in the beginning of the year, grows to
maturity and forms flower, fruit and " seed again in
the same year, the parent
dying off in late autumn or
early winter. Biennials, on
the other hand, start life from
the seed, and in their first
year of growth, devote all
their energies to attaining
full vegetative maturity, at
the same time laying aside a
surplus for propagative pur-
poses, to be employed in the
year following, when the flower
and fruit are formed. Lastly,
the perennial starts life in one
year and may grow for several
years before it reaches an age
at which it is able to flower
and fruit. Thereafter it does
so, either every succeeding
year or intermittently. These
three conditions may be expressed diagrammatically
as in Fig. 50.

In the case of the animal the conditions are quite
similar. Some of the lower forms live only for a
brief period, a few hours or days ; but the majority
of animals, including all the higher ones, live for
several years, it may be for a hundred or more,
although in no case as long as some of the highest

Annual
~B • 3/ennial
C Pervimal
at seed; tcuch.
oirclt raorew/z
one seasons
trrourtfi -The, -.-

itmtinaiion. of \ ,•"'
l/iecurre in. A t ^~ "
J3 !ndictif-e.s Ike.
dtaJh oflHt indLivtiu.a.1.

FIG. 50. Duration of
plants.

106

A PRIMER OF BIOLOGY

plants, which, in many cases, measure their duration
by centuries. During these periods, annually or

more frequently in
the year, or at
intervals of two or
more years, repro-
ductive cells are
formed and off-
spring are pro-
duced.

In order that the
offspring may have
a chance in the
struggle for exist-
ence it is manifest
that they must not
only be protected
during the early
stages of their ex-
istence, but also

that some provision must be made for their proper
nourishment during the embryonic
period and until they are capable
Protection of feeding themselves. Both these
necessities are provided for in a
variety of ways.

The lower the rank of the
organism the less provision is
made for it in either respect. In
the very lowest forms, indeed, no FIG. 52. Seed of Cas-
provision at all is made, and the tor-oil = «, testa ; 6,
offspring, newly born, are left to
shift for themselves and take their
chance among the favourable or unfavourable con-
ditions of the environment. But higher up the

FIG. 51. Two stages in the germina-
tion of a bean. In A the radicle has
developed and the plumule is on the
point of escaping from the testa ; in
B the plumule is beginning to unfold.

and
nourish-
ment of
embryos.

food reserve ; c, cot-
yledon ; d, radicle.

REPRODUCTION

107

scale, protection is either afforded by the parent,
or the offspring itself has special protective adapta-
tions. Further, the parent lays aside reserve stores
in association with the embryo to start it in life.

One striking difference makes itself evident in the
early stages of
existence of
higher plants
and of higher
animals respec-
tively. The
embryo animal
is nourished by
its parent and
develops con-
tinuously from
the moment of
fertilisation of
the ovum until
the embryo be-
comes able to
shift for itself,
but in the case
of the higher
plant the con-
ditions are
somewhat dif- FIG. 53. Winged fruits of Maple,

ferent. The

oosperm gradually develops into an embryo up^to
a certain stage and has, at the same time, reserve
food stored in it or round it. Then ensues a
period of rest, and in this condition it, along with
its food supply and protective structures, is known
as a seed. This resting stage, or seed-period, may
last for several months or even years, after which

108 A PRIMER OF BIOLOGY

the latent life of the embryo is awakened and, in the
process of germination, it continues the development
.v so long interrupted. During

this resting period, again, the
embryo is effectively protected.
For example, the seed of the
pea-plant comprises a protective
shell or testa, enclosing a mas-
sive embryo, consisting of an
embryonic shoot or plumule,
an embryonic root or radicle,
and two large swollen "seed-
leaves " or cotyledons, filled
Popla •. with reserve proteids and carbo-
hydrates (Fig. 51). During germination the insoluble
reserves are, by the action of enzymes, transformed
into soluble substances,
and serve to .nourish
the plumule and ra-
dicle until the former
has developed green
leaves above ground,
and the latter has
obtained a firm hold
on the soil and has
developed branch roots
and root-hairs for ab-
sorbing the necessary
salts and water. In
the case of the castor-
oil seed (Fig. 52), the FIG 55 Fruit Of Medicago,
reserves — chiefly pro- closed and open.

teid and oil — are

stored wdthin the testa but outside the embryo,
which latter appears as a minute nodular body

REPRODUCTION 109

with two large but delicate cotyledons, containing
practically no reserves.

We have next to inquire what is the significance
of this difference between the two types of organism ?
The explanation is again to be found in the fact that
the animal is a motile and the plant a fixed organism ; pistritm-
for it is during the hibernating period that the seed
is distributed. Each parent may produce thousands
of seeds, and manifestly it would never do to sow
them in the immediate vicinity of
the parent ; there would be no room
for them to take root, much less find
nourishment. They must be dis-
persed, and it is manifest that there
is more likelihood of their surviving
if they be thoroughly protected
and in a quiescent condition while
dispersal is being effected, than if
they be in an actively germinating FlG 56 Fruit Of
condition. Cherry : a, succu-

The seed being, like its parent, len* layer ; 6> hard-

non-locomotorv, must be aided in ened lay^r c>
j. , -. ?y i j testa; a, embryo,

dispersal, and the agents employed

are, in the main, four, viz., wind, water, animals, and
ejaculatory efforts on the part of the parent plant.

Let us glance at an example of each of these modes of
dispersal. In the case of wind it is obvious that the
seeds must be light and buoyed up by something in the
nature of a parachute. Thus we have the wings on
the fruits of the maple and of the ash (Fig. 53), and niustra-
the hairs on the seeds of cotton and of the willow tions-
(Fig. 54). It comes to the same thing, in the end,
whether single-seeded fruits be dispersed or whether
the fruit wall opens and the individual seeds be
dispersed. Hence the " float " may be developed

110

A PRIMER OF BIOLOGY

either from the wall of a single seeded fruit, as in
Clematis, or from the wall of the seed itself, as in the
willow. Obviously the same
adaptations will be effective in
relation to water dispersal, pro-
vided the protective arrange-
ments are such as to shield the
embryo from injury from water,
be it fresh or salt.

Animals -are by far the most
effective agents in seed dispersal.
Thus seeds and fruits may be
provided with hooks or spines
which stick to the fur or feathers,
1^ as in the case of the burdock,

hedge-burr, medic, &c. (Fig. 55).
\ Succulent fruits, on the other

hand, appeal to the desire for
food on the part of the animal.
In some cases the fruit is re-
moved from the plant and carried
to a distance before being eaten.
The seeds, then rejected, are thus
sown far away, it may be, from
their place of origin. In other
cases the fruit is swallowed
entire and passes through the
intestine, but the embryos are
protected from the action of the
digestive juices of the animal's
FIG. 57. Fruit of Gera- alimentary canal, either by the
nium before and after testa Or by a hardening of the

bursting.

innermost laver of the fruit wall

— as in the cherry (Fig. 56).

In other cases still, the plant itself arranges for the

REPRODUCTION

111

dispersal of its seeds, e.g., by squirting them out, as

in the " squirting cucumber," or by slinging them

to a distance, as in the geranium (Fig. 57). In the

former case the motive power

is the elasticity of the fruit wall

stretched to its utmost limit by

the pressure exerted by the

swollen contents of the ripe

fruit ; in the latter it is due to

the drying and sudden rupture

of part of the fruit-wall.

In some cases it is believed
that insectivorous birds are de-
luded into carrying off fruits or FIG.
seeds on account of their like-
ness to insects, dropping them
at some distance on discovering their mistake.
Examples are seen in the castor-oil seed, the seeds
of Jatropha, and the fruits of Scorpiurus (Fig. 58).

CHAPTER XII

No sketch of the principles of Biology, even though
as brief as the present one, would be complete
without a reference, however short, to the subject
which forms the title of this chapter.

Every day experience teaches us that both in
the plant world and in the animal world there are
many different types of organism, and that these
may be arranged in a gradually ascending series from
the most lowly unicellular types to the highest and
most complicated forms, culminating in a daisy or a
tree, on the one hand, and in man himself on the
other. Under each type there are endless sub-
types, and under these, again, yet other subordinate
types. It becomes at once evident that some expla-
nation of the relationships of these types to each
other must be forthcoming if we are to believe in
life on the globe as an organic unity. Round this
question there has for long raged a vigorous con-
troversy, some authorities holding that each type
represented a distinct act of creation, others holding
that types were not immutable, but that there existed
a family relationship between them, if one had only
the data necessary to construct a genealogical tree.
Without entering in any detail into this controversy
let us attempt to gain some insight into the funda-
mental premises which must underlie any theoretical
explanations that may be advanced.

112

THE STRUGGLE FOR EXISTENCE 113

The first fact to which attention may be drawn is
one not generally appreciated, viz., the enormous
powers of increase possessed by organisms, if con- powers
sidered as living under ideally favourable condi- of in-
tions. A numerical example will bring this fact home of on-
to US. ganisms.

Let us suppose that an organism, say a plant, can
produce fifty seeds in one year. Let us suppose that
all these are sown, and that all grow to adult life,
each in turn producing fifty seeds in the second year ;
suppose that each of these fifty plants again produces
fifty seeds, developing into seedlings in the following
year, and so on. A simple arithmetical calculation
will show us that in the tenth year, if every plant
survived, we should have the prodigious number of
1953 millions of millions of plants then existing
derived from the original one !

To take actual cases, shepherds' purse — one of
our commonest weeds — is calculated to produce not
fifty but 12,000 seeds annually. Burdock is believed
to produce over 40,000, whilst purslane may give rise
to 2,000,000 ! Our estimate of fifty, therefore, is
immensely under the actual facts.

Man himself is calculated to be capable of doubling
his numbers every twenty-five years. Few wild birds
produce less than six young per annum. Let us
suppose that each pair produces young four times
in their lives. Each pair may therefore, if all live,
give rise in fifteen years to many millions of birds
like themselves, including the offspring produced
in turn by these descendants of the original pair.
A carrion fly can produce 20,000 larvse, and each
larva is mature in about five days. In three
months, therefore, if all of them lived and produced
eggs and larvse in turn, the original carrion fly would

H

114 A PRIMER OF BIOLOGY

have given rise to 100 millions of millions of carrion
flies!

These figures are sufficiently startling when they
are put down in black and white, but another
and equally startling fact meets us at once when
we study the subject more closely, namely, that
this prodigious rate of increase is never main-
tained. It is perfectly obvious to every one that
one plant in an incredibly short space of time would
soon cover the globe to the exclusion of everything
else. If every pair of birds produced in a few years
10,000,000 of birds, the sky would be dark with
their wings. There must therefore be an enormous
destruction of individuals, especially in the early
stages of life, by various injurious agencies. Extreme
cold or heat, damp, drought, disease, enemies, all
play a part, and the net result is that, despite these
enormous powers of increase, the number of individuals
of each type, living from year to year, remains fairly
constant.

Nothing, perhaps, brings this destruction of life
more vividly home to us than to consider how many
organisms, in the adult or in the embryonic condi-
tion, are destroyed in order that an average dinner
may be provided for one human being (Arthur : " The
Right to Live," 1897). Suppose that the dinner
consists of tomato soup, fish, roast beef with potatoes
and cauliflower, chicken, a rice pudding, together
with the usual accompaniments of bread, cheese,
and, say, a glass of wine. To produce the plate of
tomato soup at least two tomatoes will be required,
representing at least 200 possible seedlings. Then
there will be one fish, one ox, one chicken, say
three potatoes, representing the possibility of at
least twelve plants, and one cauliflower. The bread

THE STRUGGLE FOR EXISTENCE 115

will represent at least 500 grains of wheat, the rice
pudding at least 1000 grains of rice, not to speak of
a couple of eggs required as an ingredient. In addi-
tion we have, say, 100 seeds of mustard and ten fruits
of pepper. Here, then, to start with, we have 1828
lives sacrificed. But to these we must add millions
of yeast cells, required in the manufacture of the
bread, millions of Bacteria required for the matura-
tion of the cheese and wine, together with thousands
of seeds of the vine, destroyed in the production of
the wine. We need not pursue the illustration further,
for when we begin to consider that not only is man
slaying his thousands of lives at every meal, but that
every herbivorous animal is slaughtering plants all
day long, and every carnivorous one, animals, when-
ever it can get the chance, we need have no diffi-
culty in understanding how it is that, notwithstanding
its enormous powers of increase, no organism ever
succeeds in entirely dominating the earth.

It will thus be seen that relatively only a few of
each generation survive and propagate in turn.
There must be a continuous and intense, though in
most cases unconscious, struggle for existence taking
place among organisms, and this struggle will be struggle
keenest amongst those most closely related, since existence,
obviously these forms will be desirous of the same
location, the same environment, the same articles
of food, and will endeavour to protect themselves
from the same kind of enemy or vicissitude of climate.
Again, the struggle will be keenest among the young,
since every organism is most liable to injury in the
young stages of development, that being the most
critical period in its life-history. We must now ask
ourselves what conditions determine which of the
organisms shall survive and which shall succumb ?

116 A PRIMER OF BIOLOGY

Before we can answer this question we must look
at two series of phenomena of fundamental
importance, viz., those of heredity and those of
variation.

It is a matter of common knowledge that an
organism produces an organism liker to itself
than to any other organism — an oak tree pro-
duces an acorn, which in turn produces an
oak tree ; a lobster produces an egg, which in
turn becomes a lobster. The offspring inherits all
the fundamental characteristics of its parent —
it is hereditarily like its parent. But neverthe-
less no offspring resembles its parent in every
particular ; it occasionally shows features which
recall characters of its grandparents, or even of some
farther back ancestor, and it also presents individual
idiosyncrasies of its own, which, so far as we can see,
are not traceable to any ancestor. We are accus-
tomed to say " as like as two peas " — we might just
as well say " as unlike as two peas," for no two peas
are exactly alike. There are differences in colour,
in weight, in size, in form, in the number and shape
of the cells of which they are composed, in the contents
of these cells, and so on, and the plants arising from
them are also different from each other in every
detail, though we have no hesitation in identifying
both as pea plants. No child is precisely like either
parent — though it may show characters present in
both ; each has an individuality of its own. Some
of these variations may be of such a kind as to lessen
its chances of success in the struggle before it ; some
may be, on the other hand, entirely in its favour. It
must be at once apparent that those individuals
which possess any variation giving them a superiority,
however little, over their fellows will, on the whole,

NATURAL SELECTION 117

be more likely to survive than those which have not

the variation in question. They will in this way

be " naturally selected from among the sum total of

individuals of that generation," much in the same

way as certain plants and animals are, artificially,

i.e., consciously, selected by man, on account of their Natural

possessing some feature of service to him or agreeing selection-

with his taste.

An illustration will make this subject clearer. Let
A be an organism — say a plant — adapted to ordinary
terrestrial conditions ; it will give rise, in any particu-
lar year, to, say, 100 seeds. These seeds will be
scattered far and wide, but some may get eaten,
some may fall on rock, some on water, and none of
these will germinate. Let us suppose that ten get
planted in situations which are, on the whole, suitable
for their germination. Of these, a, let us say, ger-
minates at the bottom of a moist ditch, while 6
germinates on fairly dry arable soil. Both develop
into seedlings and thus start two new centres of
colonisation for A. a gives rise in like manner to
progeny, and let us assume that the variations shown
by a', one of a's progeny, are such as to enable it to
make a home satisfactorily under moister conditions
than A or a, and that b also gives rise to progeny, one of
which, &', is better adapted to live under drier con-
ditions than A or 6. If these conditions are main-
tained for a series of generations, the aquatic cha-
racters of the a! series will become emphasised, just
as the characters of the b' series will gradually become
more and more suited to dry conditions. Manifestly,
in the competition for space, the original a and 6
types are likely to die out and be replaced by the
types a' and b' ; b' and a' have thus been naturally
selected out of a series represented at the extremes

118 A PRIMER OF BIOLOGY

by a on the one hand and b on the other. Either a
or 6 indeed, may again develop characters which in
some respects give it an advantage over the more
constant descendants of A, whose territory it will
therefore invade, and hence instead of having
one type in a particular locality we may get two
types divergent both from each other and from the
original A.

In some such way as this it has appeared to many
biologists to be possible to explain the endless varieties
of related organisms that now cover the surface of the
globe and that peopled it in past ages, whose descen-
dants the former are. To Charles Darwin belongs
the credit of having been the first to clearly expound
the part played by natural selection in the evolution
of new forms in his great classic, " The Origin of
Species " (1859). To other biologists the theory
of natural selection has appeared more or less in-
adequate, even granting the genealogical relationship
of organisms, and many variations and modifications
of Darwin's theory have been promulgated during
the past half century. To one of these only can
reference be made here.

One of the great objections offered to Darwin's
theory has been that the evolution of new forms by
natural selection would involve a quite stupendous
period of time ; and long, undoubtedly, as the earth
has been inhabited by plants and animals, even these
eons of time are considered inadequate for the evolu-
tion, by so slow a method, of the endless types of
organism that are now in existence or have existed
in past ages. Natura non facit saltus — Nature does
not proceed by leaps — has been an axiom with most
biologists since the days of Linnaeus, but during the
last few years, chiefly due to the persevering energy

THE ORIGIN OP SPECIES 119

of Professor De Vries of Amsterdam, we have come
to believe that Nature does make leaps not in-
frequently and, it may be, even generally, if only
there were a sufficiently large army of detectives avail-
able to catch her in the act. De Vries and others
have found that some variations appear suddenly and
spasmodically (mutations), and that these variations
are constant, that is to say, reappear in the offspring
generation after generation. Such variations have been
termed " mutations," and it must beat once manifest
that if mutations be at all frequent in Nature, and have
been even more so in past ages, starting-points for Mutation;
new races of organisms may have arisen and may
now be arising without the need for the long period
of time postulated under the natural selection theory.
Indeed, as a recent critic has put it, natural selection
must have a significance quite different from that
attributed to it by Darwin ; for while, according to
Darwin, the struggle for existence takes place between
individuals, and new species arise by selection of
those possessed of variations most likely to aid them
in the combat, according to De Vries, fully developed
species, produced by a sudden mutation, must come
into conflict with those already in existence. One
tiling, however, is clear, that the last word has not
yet been said on this, the problem par excellence of
Biology.

Science has been defined as the search for unity
amid diversity, and even in the course of our brief
study of the principles of the science of Biology we
have seen this aphorism abundantly exemplified.
Both plants and animals, as we have found, possess
vitality, both are capable of self-nourishment, and
this self-nourishment is effected in both types in
fundamentally the same manner, viz., by the assimila-

120 A PRIMER OF BIOLOGY

tion of organic compounds. Both are sensitive to
stimuli, both multiply their kind. Both have the
power of movement, although not in equal degree,
and the skeletons of both are constructed in accord-
ance with the same laws. Finally, we have seen that
there is good evidence for believing that organisms
are related to each other — in some cases, less, in
other cases, more distantly — but that all of them may
be regarded as terminal twigs of the infinitely branched
trunks of the bifurcate tree of life. It must be left
to other volumes of this series to connect the organic
\vorld with the inorganic, from which in the long run
both obtain their nutriment, and by whose laws they
also are governed.

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