TRANSCRIBER'S NOTE
In this etext the oe ligature is represented as [oe]. In the table of
Bat arm measurements, the graphic symbols for male and female are
represented as [Male] and [Fem.]. Proofreaders found a number of
typographical errors which have been corrected, none of which affected
the sense of the text. Inconsistent hyphenation has been retained.
[Illustration: Kentish Plover with Eggs and Young. From the Exhibit in the
British Natural History Museum.]
ANIMAL LIFE AND INTELLIGENCE.
BY
C. LLOYD MORGAN, F.G.S.,
PROFESSOR IN AND DEAN OF UNIVERSITY COLLEGE, BRISTOL;
LECTURER AT THE BRISTOL MEDICAL SCHOOL;
PRESIDENT OF THE BRISTOL NATURALISTS' SOCIETY, ETC.
AUTHOR OF
"ANIMAL BIOLOGY," "THE SPRINGS OF CONDUCT," ETC.
BOSTON, U. S. A.
GINN & COMPANY, PUBLISHERS.
1891.
TO
MY FATHER.
PREFACE.
There are many books in our language which deal with Animal Intelligence
in an anecdotal and conventionally popular manner. There are a few,
notably those by Mr. Romanes and Mr. Mivart, which bring adequate
knowledge and training to bear on a subject of unusual difficulty. In
the following pages I have endeavoured to contribute something
(imperfect, as I know full well, but the result of several years' study
and thought) to our deeper knowledge of those mental processes which we
may fairly infer from the activities of dumb animals.
The consideration of Animal Intelligence, from the scientific and
philosophical standpoint, has been my primary aim. But so inextricably
intertwined is the subject of Intelligence with the subject of Life, the
subject of organic evolution with the subject of mental evolution, so
closely are questions of Heredity and Natural Selection interwoven with
questions of Habit and Instinct, that I have devoted the first part of
this volume to a consideration of Organic Evolution. The great
importance and value of Professor Weismann's recent contributions to
biological science, and their direct bearing on questions of Instinct,
rendered such treatment of my subject, not only advisable, but
necessary. Moreover, it seemed to me, and to those whom I consulted in
the matter, that a general work on Animal Life and Intelligence, if
adequately knit into a connected whole, and based on sound principles of
science and of philosophy, would not be unwelcomed by biological
students, and by that large and increasing class of readers who, though
not professed students, follow with eager interest the development of
the doctrine of Evolution.
Incidentally, but only incidentally, matters concerning man, as compared
with the dumb animals, have been introduced. It is contended that in man
alone, and in no dumb animal, is the rational faculty, as defined in
these pages, developed; and it is contended that among human-folk that
process of natural selection, which is so potent a factor in the lower
reaches of organic life, sinks into comparative insignificance. Man is a
creature of ideas and ideals. For him the moral factor becomes one of
the very highest importance. He conceives an ideal self which he strives
to realize; he conceives an ideal humanity towards which he would raise
his fellow-man. He becomes a conscious participator in the evolution of
man, in the progress of humanity.
But while we must not be blind to the effects of new and higher factors
of progress thus introduced as we rise in the scale of phenomena, we
must at the same time remember that biological laws still hold true,
though moral considerations and the law of duty may profoundly modify
them. The eagle soars aloft apparently in defiance of gravitation; but
the law of gravitation still holds good; and no treatment of the
mechanism of flight which neglected it would be satisfactory. Moral
restraint, a higher standard of comfort, and a perception of the folly
and misery of early and improvident marriage may tend to check the rate
of growth of population: but the "law of increase" still holds good, as
a law of the factors of phenomena; and Malthus did good service to the
cause of science when he insisted on its importance. We may guide or
lighten the incidence of natural selection through competition; we may
in our pity provide an asylum for the unfortunates who are suffering
elimination; but we cannot alter a law which, as that of one of the
factors of organic phenomena, still obtains, notwithstanding the
introduction of other factors.
However profoundly the laws of phenomena may be modified by such
introduction of new and higher factors, the older and lower factors are
still at work beneath the surface. And he who would adequately grasp the
social problems of our time should bring to them a mind prepared by a
study of the laws of organic life: for human beings, rational and moral
though they may be, are still organisms; and man can in no wise alter or
annul those deep-lying facts which nature has throughout the ages been
weaving into the tissue of life.
Some parts of this work are necessarily more technical, and therefore
more abstruse, than others. This is especially the case with Chapters
III., V., and VI.; while, for those unacquainted with philosophical
thought, perhaps the last chapter may present difficulties of a
different order. With these exceptions, the book will not be beyond the
ready comprehension of the general reader of average intelligence.
I have to thank many kind friends for incidental help. Thanks are also
due to Professor Flower, who courteously gave permission that some of
the exhibits in our great national collection in Cromwell Road might be
photographed and reproduced; and to Messrs. Longmans for the use of two
or three illustrations from my text-book of "Animal Biology."
C. LLOYD MORGAN.
UNIVERSITY COLLEGE, BRISTOL,
October, 1890.
CONTENTS.
CHAPTER I.
THE NATURE OF ANIMAL LIFE.
PAGE
The characteristics of animals 2
The relation of animals to food-stuffs 15
The relation of animals to the atmosphere 15
The relation of animals to energy 16
CHAPTER II.
THE PROCESS OF LIFE.
Illustration from respiration 21
Illustration from nutrition 25
The utilization of the materials incorporated 27
The analogy of a gas-engine. Explosive metabolism 30
CHAPTER III.
REPRODUCTION AND DEVELOPMENT.
Reproduction in the protozoa 37
Fission in the metazoa 41
The regeneration of lost parts 41
Reproduction by budding 42
Sexual reproduction 42
Illustration of development 51
Parental sacrifice 56
The law of increase 58
CHAPTER IV.
VARIATION AND NATURAL SELECTION.
The law of persistence 61
The occurrence of variations 63
Application of the law of increase 76
Natural selection 77
Elimination and selection 79
Modes of natural elimination illustrated 80
Protective resemblance and mimicry 82
Selection proper illustrated 93
The effects of natural selection 95
Isolation or segregation 99
Its modes, geographical, preferential and physiological 99
Its effects 108
Utility of specific characters 110
Variations in the intensity of the struggle for existence 112
Convergence of characters 117
Modes of adaptation: Progress 119
Evolution and Revolution 120
CHAPTER V.
HEREDITY AND THE ORIGIN OF VARIATIONS.
Heredity in the protozoa 123
Regeneration of lost parts 124
Sexual reproduction and heredity 129
The problem of hen and egg 130
Reproductive continuity 131
Pangenesis 131
Modified pangenesis 134
Continuity of germ-plasm 138
Cellular continuity with differentiation 142
The inheritance or non-inheritance of acquired characters 146
Origin of variations on the latter view 149
Hypothesis of organic combination 150
The extrusion of the second polar cell 153
The protozoan origin of variations 156
How can the body influence the germ? 159
Is there sufficient evidence that it does? 162
Summary and conclusion 175
CHAPTER VI.
ORGANIC EVOLUTION.
The diversity of animal life 177
The evolution theory 181
Natural selection: not to be used as a magic formula 183
Panmixia and disuse 189
Sexual selection or preferential mating 197
Use and disuse 209
The nature of variations 216
The inheritance of variations 223
The origin of variations 231
Summary and conclusion 241
CHAPTER VII.
THE SENSES OF ANIMALS.
The primary object of sensation 243
Organic sensations and the muscular sense 244
Touch 245
The temperature-sense 249
Taste 250
Smell 257
Hearing 261
Sense of rotation or acceleration 269
Sight 273
Restatement of theory of colour-vision 278
Variation in the limits of colour-vision 281
The four types of "visual" organs 293
Problematical senses 294
Permanent possibilities of sensation 298
CHAPTER VIII.
MENTAL PROCESSES IN MAN.
The physiological aspect 302
The psychological aspect 304
Sensations: their localization, etc. 306
Perceptual construction 312
Conceptual analysis 321
Inferences perceptual and conceptual 328
Intelligence and reason 330
CHAPTER IX.
MENTAL PROCESSES IN ANIMALS: THEIR POWERS OF PERCEPTION
AND INTELLIGENCE.
The two factors in phenomena 331
The basis in organic evolution 336
Perceptual construction in mammalia 338
Can animals analyze their constructs? 347
The generic difference between the minds of man and brute 350
Perceptual construction in other vertebrates 350
"Understanding" of words 354
Perceptual construction in the invertebrates 356
"The psychic life of micro-organisms" 360
The inferences of animals 361
Intelligent not rational 365
Use of words defined 372
Language and analysis 374
CHAPTER X.
THE FEELINGS OF ANIMALS: THEIR APPETENCES AND EMOTIONS.
Pleasure and pain: their organic limits 379
Their directive value 380
An emotion exemplified 382
Sensitiveness and sensibility 385
The expression of the emotions 385
The postponement of action 385
The three orders of emotion 390
The capacities of animals for pleasure and pain 391
Sense-feelings 393
Some emotions of animals 395
The necessity for caution in interpretation 399
The sense of beauty 407
Can animals be moral? 413
Conclusion 414
CHAPTER XI.
ANIMAL ACTIVITIES: HABIT AND INSTINCT.
The nature of animal activities 415
The outer and inner aspect 417
The inherited organization 419
Habitual activities 420
Instinctive activities 422
Innate capacity 426
Blind prevision 429
Consciousness and instinct 432
Mr. Romanes's treatment of instinct 434
Lapsed intelligence and modern views on heredity 435
Three factors in the origin of instinctive activities 447
The emotional basis of instinct 449
The influence of intelligence on instinct 452
The characteristics of intelligent activities 456
The place of volition 459
Perceptual and conceptual volition 460
Consciousness and consentience 461
Classification of activities 462
CHAPTER XII.
MENTAL EVOLUTION.
Is mind evolved from matter? 464
Kinesis and metakinesis 467
Monistic assumptions 470
The nature of ejects 476
The universe as eject 478
Metakinetic environment of mind 481
Conceptual ideas not subject to natural selection 483
Elimination through incongruity 486
Interneural evolution 490
Interpretations of nature 492
Can fetishism have had a natural genesis? 493
The origin of interneural variations 496
Are acquired variations inherited? 497
Summary and conclusion 501
LIST OF ILLUSTRATIONS.
FIG. PAGE
Kentish Plover with Eggs and Young: Frontispiece
1. Spiracles and Air-tubes of Cockroach 3
2. Gills of Mussel 4
3. A Cell greatly magnified 11
4. Am[oe]ba 12
5. Egg-cell and Sperm-cell 13
6. Diagram of Circulation 23
7. Protozoa 38
8. Hydra Virides 43
9. Aurelia: Life-cycle 45
10. Liver-Fluke--Embryonic Stages 47
11. Diagram of Development 51
12. Wing of Bat (Pipistrelle) 64
13. Variations of the Noctule 67
14. Variations of the Long-eared Bat 68
15. Variations of the Pipistrelle 69
16. Variations of the Whiskered Bat 70
17. Variations adjusted to the Standard of the Noctule 73
18. Caterpillar of a Moth on an Oak Spray 85
19. Locust resembling a Leaf 86
20. Mimicry of Bees by Flies 91
21. Egg and Hen 141
22. Stag-Beetles 180
23. Tactile Corpuscules 247
24. Touch-hair of Insect 248
25. Taste-buds of Rabbit 250
26. Antennule of Crayfish 259
27. Diagram of Ear 263
28. Tail of Mysis 266
29. Leg of Grasshopper 266
30. Diagram of Semicircular Canals 270
31. The Human Eye 274
32. Retina of the Eye 274
33. Variation in the Limits of Colour-vision 281
34. Pineal Eye 288
35. Skull of Melanerpeton 288
36. Eyes and Eyelets of Bee 289
37. Eye of Fly 290
38. Diagram of Mosaic Vision 291
39. Direction-retina 295
40. Antennary Structures of Hymenoptera 297
ANIMAL LIFE AND INTELLIGENCE.
CHAPTER I.
THE NATURE OF ANIMAL LIFE.
I once asked a class of school-boys to write down for me in a few words
what they considered the chief characteristics of animals. Here are some
of the answers--
1. Animals move about, eat, and grow.
2. Animals eat, grow, breathe, feel (at least, most of them do), and
sleep.
3. Take a cat, for example. It begins as a kitten; it eats, drinks,
plays about, and grows up into a cat, which does much the same,
only it is more lazy, and stops growing. At last it grows old and
dies. But it may have kittens first.
4. An animal has a head and tail, four legs, and a body. It is a
living creature, and not a vegetable.
5. Animals are living creatures, made of flesh and blood.
Combining these statements, we have the following characteristics of
animals:--
1. Each has a proper and definite form, at present described as "a
head and tail, four legs, and a body."
2. They breathe.
3. They eat and drink.
4. They grow.
5. They also "grow up." The kitten grows up into a cat, which is
somewhat different from the kitten.
6. They move about and sleep.
7. They feel--"at least some of them do."
8. They are made of "flesh and blood."
9. They grow old and die.
10. They reproduce their kind. The cat may have kittens.
11. They are living organisms, but "not vegetables."
Now, let us look carefully at these characteristics, all of which were
contained in the five answers, and were probably familiar in some such
form as this to all the boys, and see if we cannot make them more
general and more accurate.
1. _An animal has a definite form._ My school-boy friend described it as
a head and tail, four legs, and a body. But it is clear that this
description applies only to a very limited number of animals. It will
not apply to the butterfly, with its great wings and six legs; nor to
the lobster, with its eight legs and large pincer-claws; to the limbless
snake and worm, the finned fish, the thousand-legs, the oyster or the
snail, the star-fish or the sea-anemone. The animals to which my young
friend's description applies form, indeed, but a numerically
insignificant proportion of the multitudes which throng the waters and
the air, and not by any means a large proportion of those that walk upon
the surface of the earth. The description applies only to the backboned
vertebrates, and not to nearly all of them.
It is impossible to summarize in a sentence the form-characteristics of
animals. The diversities of form are endless. Perhaps the distinguishing
feature is the prevalence of curved and rounded contours, which are in
striking contrast to the definite crystalline forms of the inorganic
kingdom, characterized as these are by plane surfaces and solid angles.
We may say, however, that all but the very lowliest animals have each
and all a proper and characteristic form of their own, which they have
inherited from their immediate ancestors, and which they hand on to
their descendants. But this form does not remain constant throughout
life. Sometimes the change is slight; in many cases, however, the form
alters very markedly during the successive stages of the life of the
individual, as is seen in the frog, which begins life as a tadpole, and
perhaps even more conspicuously in the butterfly, which passes through a
caterpillar and a chrysalis stage. Still, these changes are always the
same for the same kind of animal. So that we may say, each animal has a
definite form and shape or series of shapes.
2. _Animals breathe._ The essential thing here is that oxygen is taken
in by the organism, and carbonic acid gas is produced by the organism.
No animal can carry on its life-processes unless certain chemical
changes take place in the substance of which it is composed. And for
these chemical changes oxygen is essential. The products of these
changes, the most familiar of which are carbonic acid gas and urea, must
be got rid of by the process of excretion. Respiration and excretion are
therefore essential and characteristic life-processes of all animals.
[Illustration: Fig. 1.--Diagram of spiracles and air-tubes (tracheæ) of
an insect (cockroach).
The skin, etc., of the back has been removed, and the crop (cr.) and
alimentary canal (al.c.) displayed. The air-tubes are represented by
dotted lines. The ten spiracles are numbered to the right of the
figure.]
In us, and in all air-breathing vertebrates, there are special organs
set apart for respiration and excretion of carbonic acid gas. These are
the lungs. A great number of insects also breathe air, but in a
different way. They have no lungs, but they respire by means of a number
of apertures in their sides, and these open into a system of delicate
branching tubes which ramify throughout the body. Many organisms,
however, such as fish and lobsters and molluscs, breathe the air
dissolved in the water in which they live. The special organs developed
for this purpose are the gills. They are freely exposed to the water
from which they abstract the air dissolved therein. When the air
dissolved in the water is used up, they sicken and die. There can be
nothing more cruel than to keep aquatic animals in a tank or aquarium in
which there is no means of supplying fresh oxygen, either by the action
of green vegetation, or by a jet of water carrying down air-bubbles, or
in some other way. And then there are a number of animals which have no
special organs set apart for breathing. In them respiration is carried
on by the general surface of the body. The common earthworm is one of
these; and most microscopic organisms are in the same condition. Still,
even if there be no special organs for breathing, the process of
respiration must be carried on by all animals.
[Illustration: Fig. 2.--Gills of mussel.
o.g., outer gill; i.g., inner gill; mo., mouth; m., muscles for closing
shell; ma., mantle; s., shell; f., foot; h., position of heart; e.s.,
exhalent siphon, whence the water passes out from the gill-chamber;
i.s., inhalent siphon, where the water enters.
The left valve of the shell has been removed, and the mantle cut away
along the dark line.]
3. _They eat and drink._ The living substance of an animal's body is
consumed during the progress of those chemical changes which are
consequent upon respiration; and this substance must, therefore, be made
good by taking in the materials out of which fresh life-stuff can be
formed. This process is called, in popular language, feeding. But the
food taken in is not identical with the life-stuff formed. It has to
undergo a number of chemical changes before it can be built into the
substance of the organism. In us, and in all the higher animals, there
is a complex system of organs set aside for the preparation, digestion,
and absorption of the food. But there are certain lowly organisms which
can take in food at any portion of their surface, and digest it in any
part of their substance. One of these is the am[oe]ba, a minute speck of
jelly-like life-stuff, which lives in water, and tucks in a bit of
food-material just as it comes. And there are certain degenerate
organisms which have taken to a parasitic life, and live within the
bodies of other animals. Many of these can absorb the material prepared
by their host through the general surface of their simple bodies. But
here, again, though there may be no special organs set apart for the
preparation, absorption, and digestion of food, the process of feeding
is essential to the life of all animals. Stop that process for a
sufficient length of time, and they inevitably die.
4. _They grow._ Food, as we have just seen, has to be taken in,
digested, and absorbed, in order that the loss of substance due to the
chemical changes consequent on respiration may be made good. But where
the digestion and absorption are in excess of that requisite for this
purpose, we have the phenomenon of growth.
What are the characteristics of this growth? We cannot, perhaps,
describe it better than by saying (1) that it is organic, that is to
say, a growth of the various organs of the animal in due proportion; (2)
that it takes place, not merely by the addition of new material (for a
crystal grows by the addition of new material, layer upon layer), but by
the incorporation of that new material into the very substance of the
old; and (3) that the material incorporated during growth differs from
the material absorbed from without, which has undergone a preparatory
chemical transformation within the animal during digestion. The growth
of an animal is thus dependent upon the continued absorption of new
material from without, and its transformation into the substance of the
body.
The animal is, in fact, a centre of continual waste and repair, of
nicely balanced constructive and destructive processes. These are the
invariable concomitants of life. Only so long as the constructive
processes outbalance the destructive processes does growth continue.
During the greater part of a healthy man's life, for example, the two
processes, waste and repair, are in equilibrium. In old age, waste
slowly but surely gains the mastery; and at death the balanced process
ceases, decomposition sets in, and the elements of the body are
scattered to the winds or returned to mother earth.
There are generally limits of growth which are not exceeded by any
individuals of each particular kind of animal. But these limits are
somewhat variable among the individuals of each kind. There are big men
and little men, cart-horses and ponies, bloodhounds and lap-dogs. Wild
animals, however, when fully grown, do not vary so much in size. The
period of growth is also variable. Many of the lower backboned animals
probably grow during the whole of life, but those which suckle their
young generally cease growing after a fraction (in us from one-fourth to
one-fifth) of the allotted span of life is past.
5. But animals not only grow--_they also "grow up."_ The kitten grows up
into a cat, which is somewhat different from the kitten. We speak of
this growing up of an animal as its _development_. The proportion of the
various parts and organs progressively alter. The relative lengths of
the arms and legs, and the relative size of the head, are not the same
in the infant as in the man or woman. Or, take a more marked case. In
early spring there is plenty of frog-spawn in the ponds. A number of
blackish specks of the size of mustard seeds are embedded in a
jelly-like mass. They are frogs' eggs. They seem unorganized. But watch
them, and the organization will gradually appear. The egg will be
hatched, and give rise to a little fish-like organism. This will by
degrees grow into a tadpole, with a powerful swimming tail and rounded
head and body, but with no obvious neck between them. Legs will appear.
The tail will shrink in size and be gradually drawn into the body. The
tadpole will have developed into a minute frog.
There are many of the lower animals which go through a not less
wonderful, if not more wonderful, metamorphosis. The butterfly or the
silkworm moth, beginning life as a caterpillar and changing into a
chrysalis, from which the perfect insect emerges, is a familiar
instance. And hosts of the marine invertebrates have larval forms which
have but little resemblance to their adult parents.
Such a series of changes as is undergone by the frog is called
_metamorphosis_, which essentially consists in the temporary development
of certain provisional embryonic organs (such as gills and a powerful
swimming tail) and the appearance of adult organs (such as lungs and
legs) to take their place. In metamorphosis these changes occur during
the free life of the organism. But beneath the eggshell of birds and
within the womb of mammals scarcely less wonderful changes are slowly
but surely effected, though they are hidden from our view. There is no
metamorphosis during the free life of the organism, but there is a
prenatal _transformation_. The little embryo of a bird or mammal has no
gills like the tadpole (though it has for a while gill-slits, pointing
unmistakably to its fishy ancestry), but it has a temporary provisional
breathing organ, called the allantois, pending the full development and
functional use of its lungs.
All the higher animals, in fact--the dog, the chick, the serpent, the
frog, the fish, the lobster, the butterfly, the worm, the star-fish, the
mollusc, it matters not which we select--take their origin from an
apparently unorganized egg. They all, therefore, pass during their
growth from a comparatively simple condition to a comparatively complex
condition by a process of change which is called development. But there
are certain lowly forms, consisting throughout life of little more than
specks of jelly-like life-stuff, in which such development, if it occurs
at all, is not conspicuous.
6. _They move about and sleep._ This is true of our familiar domestic
pets. The dog and the cat, after periods of restless activity, curl
themselves up and sleep. The canary that has all day been hopping about
its cage, or perhaps been allowed the freedom of the dining-room, tucks
its head under its wing and goes to sleep. The cattle in the meadows,
the sheep in the pastures, the horses in the stables, the birds in the
groves, all show alternating periods of activity and repose. But is this
true of all animals? Do all animals "move about and sleep"? The
sedentary oyster does not move about from place to place; the barnacle
and the coral polyp are fixed for the greater part of life; and whether
these animals sleep or not it is very difficult to say. We must make our
statement more comprehensive and more accurate.
If we throw it into the following form, it will be more satisfactory:
Animals exhibit certain activities; and periods of activity alternate
with periods of repose.
I shall have more to say hereafter concerning the activities of animals.
Here I shall only say a few words concerning the alternating periods of
repose. No organism can continue in ceaseless activity unbroken by any
intervening periods of rest. Nor can the organs within an organism,
however continuous their activity may appear, work on indefinitely and
unrestfully. The heart is apparently restless in its activity. But in
every five minutes of the continued action of the great force-pump
(ventricle) of the heart, two only are occupied in the efforts of
contraction and work, while three are devoted to relaxation and repose.
What we call sleep may be regarded as the repose of the higher
brain-centres after the activity of the day's work--a repose in which
the voluntary muscles share.
The necessity for rest and repose will be readily understood. We have
seen that the organism is a centre of waste and repair, of nicely
balanced destructive and reconstructive processes. Now, activity is
accompanied by waste and destruction. But it is clear that these
processes, by which the substance of the body and its organs is used up,
cannot go on for an indefinite period. There must intervene periods of
reconstruction and recuperation. Hence the necessity of rest and repose
alternating with the periods of more or less prolonged activity.
7. _They feel--"at least some of them do."_ The qualification was a wise
one, for in truth, as we shall hereafter see, we know very little about
the feelings of the lower organisms. The one animal of whose feelings I
know anything definite and at first hand, is myself. Of course, I
believe in the feelings of others; but when we come to very lowly
organisms, we really do not know whether they have feelings or not, or,
if they do, to what extent they feel.
Shall we leave this altogether out of account? Or can we throw it into
some form which is more general and less hypothetical? This, at any
rate, we know--that all animals, even the lowest, are sensitive to
touches, sights, or sounds. It is a matter of common observation that
their activities are generally set agoing under the influence of such
suggestions from without. Perhaps it will be objected that there is no
difference between feeling and being sensitive. But I am using the word
"sensitive" in a general sense--in that sense in which the photographer
uses it when he speaks of a sensitive plate, or the chemist when he
speaks of a sensitive test. When I say that animals are sensitive, I
mean that they answer to touches, or sounds, or other impressions (what
are called stimuli) coming from without. They may feel or not; many of
them undoubtedly do. But that is another aspect of the sensitiveness.
Using the term, then, with this meaning, we may say, without
qualification, that all animals are more or less sensitive to external
influences.
8. _They are made of "flesh and blood."_ Here we have allusion to the
materials of which the animal body is composed. It is obviously a loose
and unsatisfactory statement as it stands. An American is said to have
described the difference between vertebrates and insects by saying that
the former are composed of flesh and bone, and the latter of skin and
squash. But even if we amend the statement that animals are made of
"flesh and blood" by the addition of the words, "or of skin and squash,"
we shall hardly have a sufficiently satisfactory statement of the
composition of the animal body.
The essential constituent of animal (as indeed also of vegetable)
tissues is protoplasm. This is a nearly colourless, jelly-like
substance, composed of carbon, hydrogen, nitrogen, and oxygen, with some
sulphur and phosphorus, and often, if not always, some iron; and it is
permeated by water. Protoplasm, together with certain substances, such
as bony and horny matter, which it has the power of producing,
constitutes the entire structure of simple organisms, and is built up
into the organs of the bodies of higher animals. Moreover, in these
organs it is not arranged as a continuous mass of substance, but is
distributed in minute separate fragments, or corpuscles, only visible
under the microscope, called cells. These cells are of very various
shapes--spherical, discoidal, polyhedral, columnar, cubical, flattened,
spindle-shaped, elongated, and stellate.
A great deal of attention has been devoted of late years to the minute
structure of cells, and the great improvements in microscopical powers
and appliances have enabled investigators to ascertain a number of
exceedingly interesting and important facts. The external surface of a
cell is sometimes, but not always in the case of animals, bounded by a
film or membrane. Within this membrane the substance of the cell is made
up of a network of very delicate fibres (the _plasmogen_), enclosing a
more fluid material (the _plasm_); and this network seems to be the
essential living substance. In the midst of the cell is a small round or
oval body, called the nucleus, which is surrounded by a very delicate
membrane. In this nucleus there is also a network of delicate plasmogen
fibres, enclosing a more fluid plasm material. At certain times the
network takes the form of a coiled filament or set of filaments, and
these arrange themselves in the form of rosettes and stars. In the
meshwork of the net or in the coils of the filament there may be one or
more small bodies (nucleoli), which probably have some special
significance in the life of the cell. These cells multiply or give birth
to new cells by dividing into two, and this process is often accompanied
by special changes in the nucleus (which also divides) and by the
arrangement of its network or filaments into the rosettes and stars
before alluded to.
Instead, therefore, of the somewhat vague statement that animals are
made of flesh and blood, we may now say that the living substance of
which animals are composed is a complex material called protoplasm; that
organisms are formed either of single cells or of a number of related
cells, together with certain life-products of these cells; and that each
cell, small as it is, has a definite and wonderful minute structure
revealed by the microscope.
[Illustration: Fig. 3.--A cell, greatly magnified.
c.m., cell-membrane; c.p., cell-protoplasm; n.m., nuclear membrane;
n.p., nuclear protoplasm; n.f., coiled nuclear filament.]
9. _Animals grow old and die._ This is a familiar observation. Apart
from the fact that they are often killed by accident, by the teeth or
claws of an enemy, or by disease, animals, like human beings, in course
of time become less active and less vigorous; the vital forces gradually
fail, and eventually the flame of life, which has for some time been
burning dimmer and dimmer, flickers out and dies. But is this true of
all animals? Can we say that death--as distinct from being killed--is
the natural heritage of every creature that lives?
One of the simplest living creatures is the am[oe]ba. It consists of a
speck of nucleated protoplasm, no larger than a small pin's head. Simple
as it is, all the essential life-processes are duly performed. It is a
centre of waste and repair; it is sensitive and responsive to a
stimulus; respiration and nutrition are effected in a simple and
primitive fashion. It is, moreover, reproductive. First the nucleus and
then the protoplasm of the cell divide, and in place of one am[oe]ba
there are two. And these two are, so far as we can tell, exactly alike.
There is no saying which is mother and which is daughter; and, so far as
we can see at present, there is no reason why either should die. It is
conceivable that am[oe]bæ never die, though they may be killed in
immense numbers. Hence it has been plausibly maintained that the
primitive living cell is by nature deathless; that death is not the
heritage of all living things; that death is indeed an acquisition,
painful indeed to the individual, but, since it leaves the stage free
for the younger and more vigorous individuals, conducive to the general
good.
[Illustration: Fig. 4.--Am[oe]ba.
1. An am[oe]ba, showing the inner and outer substance (endosarc and
ectosarc); a pseudopodium, p.s.; the nucleus, n.; and the contractile
vesicle, c.v. 2. An am[oe]ba dividing into two. 3. The division just
effected.]
In face of this opinion, therefore, we cannot say that all animals grow
old and die; but we may still say that all animals, with the possible
exception of some of the lowest and simplest, exhibit, after a longer or
a shorter time, a waning of the vital energies which sooner or later
ends in death.
10. _Animals reproduce their kind._ We have just seen the nature of
reproduction in the simple unicellular am[oe]ba. The reproduction of the
constituent cells in the complex multicellular organism, during its
natural growth or to make good the inevitable loss consequent on the
wear and tear of life, is of the same character.
When we come to the higher organisms, reproduction is effected by the
separation of special cells called egg-cells, or ova, from a special
organ called the ovary; and these, in a great number of cases, will not
develop into a new organism unless they be fertilized by the union with
them in each case of another cell--the sperm-cell--produced by a
different individual. The separate parents are called male and female,
and reproduction of this kind is said to be sexual.
[Illustration: Fig. 5.--Egg-cell and sperm-cell.
a, ovum or egg; b, spermatozoon or sperm.]
The wonderful thing about this process is the power of the fertilized
ovum, produced by the union of two minute cells from different parents,
to develop into the likeness of these parents. This likeness, however,
though it extends to minute particulars, is not absolute. The offspring
is not exactly like either parent, nor does it present a precise mean
between the characters of the two parents. There is always some amount
of individual variability, the effects of which, as we shall hereafter
see, are of wide importance. We are wont to say that these phenomena,
the transmission of parental characteristics, together with a margin of
difference, are due to heredity with variation. But this merely names
the facts. How the special reproductive cells have acquired the secret
of developing along special lines, and reproducing, with a margin of
variability, the likeness of the organisms which produced them, is a
matter concerning which we can at present only make more or less
plausible guesses.
Scarcely less wonderful is the power which separated bits of certain
organisms, such as the green freshwater hydra of our ponds, possess of
growing up into the complete organism. Cut a hydra into half a dozen
fragments, and each fragment will become a perfect hydra. Reproduction
of this kind is said to be asexual.
We shall have, in later chapters, to discuss more fully some of the
phenomena of reproduction and heredity. For the present, it is
sufficient to say that animals reproduce their kind by the detachment of
a portion of the substance of their own bodies, which portion, in the
case of the higher animals, undergoes a series of successive
developmental changes constituting its life-history, the special nature
of which is determined by inheritance, and the result of which is a new
organism in all essential respects similar to the parent or parents.
11. _Animals are living organisms, and "not vegetables."_ The first part
of this final statement merely sums up the characteristics of living
animals which have gone before. But the latter part introduces us to the
fact that there are other living organisms than those we call animals,
namely, those which belong to the vegetable kingdom.
It might, at first sight, be thought a very easy matter to distinguish
between animals and plants. There is no chance, for example, of
mistaking to which kingdom an oak tree or a lion, a cabbage or a
butterfly, belongs. But when we come down to the simpler organisms,
those whose bodies are constituted by a single cell, the matter is by no
means so easy. There are, indeed, lowly creatures which are hovering on
the boundary-line between the two kingdoms. We need not discuss the
nature of these boundary forms. It is sufficient to state that
unicellular plants are spoken of as _protophyta_, and unicellular
animals as _protozoa_, the whole group of unicellular organisms being
classed together as _protista_. The animals whose bodies are formed of
many cells in which there is a differentiation of structure and a
specialization of function, are called _metazoa_, and the multicellular
plants _metaphyta_. The relations of these groups may be thus
expressed--
Animals. Plants.
/\ /\
--------- ------------- -------------- ------------
Metazoa. Protozoa. Protophyta. Metaphyta.
----------- -------------
\/
Protista.
There are three matters with regard to the life-process of animals and
plants concerning which a few words must be said. These are (1) their
relation to food-stuffs; (2) their relation to the atmosphere; (3) their
relation to energy, or the power of doing work.
With regard to the first matter, that of food-relation, the essential
fact seems to be the dependence of animals on plants. Plants can
manufacture protoplasm out of its constituents if presented to them in
suitable inorganic form scattered through earth and air and water. Hence
the peculiar features of their form, the branching and spreading nature
of those parts which are exposed to the air, and the far-reaching
ramifications of those parts which are implanted in the earth. Hence,
too, the flattened leaves, with their large available surface. Animals
are unable to manufacture protoplasm in this way. They are, sooner or
later, dependent for food on plant-products. It is true that the
carnivora eat animal food, but the animals they eat are directly or
indirectly consumers of vegetable products. Plants are nature's primary
producers of organic material. Animals utilize these products and carry
them to higher developments.
In relation to the atmosphere, animals require a very much larger
quantity of oxygen than do plants. This, during the respiratory process,
combines with carbon so as to form carbonic acid gas; and the atmosphere
would be gradually drained of its oxygen and flooded with carbonic acid
gas were it not that plants, through their green colouring matter
(chlorophyll), under the influence of light, have the power of
decomposing the carbonic acid gas, seizing on the carbon and building it
into their tissues, and setting free the oxygen. Thus are animals and
green plants complementary elements in the scheme of nature.[A] The
animal eats the carbon elaborated by the plant into organic products
(starch and others), and breathes the oxygen which the plant sets free
after it has abstracted the carbon. In the animal's body the carbon and
oxygen recombine; its varied activities are thus kept going; and the
resultant carbonic acid gas is breathed forth, to be again separated by
green, growing plants into carbonaceous food-stuff and vitalizing
oxygen. It must be remembered, however, that vegetable protoplasm, like
animal protoplasm, respires by the absorption of oxygen and the
formation of carbonic acid gas. But in green plants this process is
outbalanced by the characteristic action of the chlorophyll, by which
carbonic acid gas is decomposed.
Lastly, we have to consider the relations of animals and plants to
energy. Energy is defined as the power of doing work, and it is
classified by physicists under two modes--potential energy, or energy of
position; and kinetic energy, or energy of motion. The muscles of my arm
contain a store of potential energy. Suppose I pull up the weight of an
old-fashioned eight-day clock. Some of the potential energy of my arm is
converted into the potential energy of the weight; that is, the raised
weight is now in a position of advantage, and capable of doing work. It
has energy of position, or potential energy. If the chain breaks, down
falls the weight, and exhibits the energy of motion. But, under ordinary
circumstances, this potential energy is utilized in giving a succession
of little pushes to the pendulum to keep up its swing, and in overcoming
the friction of the works. Again, the energy of an electric current may
be utilized in decomposing water, and tearing asunder the oxygen and
hydrogen of which it is composed. The oxygen and hydrogen now have
potential energy, and, if they be allowed to combine, this will manifest
itself as the light and heat of the explosion. These examples will serve
to illustrate the nature of the changes which energy undergoes. These
are of the nature of transferences of energy from one body to another,
and of transformations from one mode or manifestation to another. The
most important point that has been established during this century with
regard to energy is that, throughout all its transferences and
transformations, it can be neither created nor destroyed. But there is
another point of great importance. Transformations of energy take place
more readily in certain directions than in others. And there is always a
tendency for energy to pass from the higher or more readily
transformable to the lower or less readily transformable forms. When,
for example, energy has passed to the low kinetic form of the uniformly
distributed molecular motion of heat, it is exceedingly difficult, or
practically impossible, to transform it into a higher and more available
form.
Now, both animals and plants are centres of the transformation of
energy; and in them energy, notwithstanding that it is being raised to a
high position of potentiality, is constantly tending to be degraded to
lower forms. Hence the necessity of some source from which fresh stores
of available energy may be constantly supplied. Such a source is solar
radiance. This it is which gives the succession of little pushes which
keeps the pendulum of life a-swinging. And it is the green plants which,
through their chlorophyll, are in the best position to utilize the solar
energy. They utilize it in building up, from the necessary constituents
diffused through the atmosphere and the soil, complex forms of organic
material, of which the first visible product seems to be starch; and
these not only contain large stores of potential energy, but are
capable, when combined with oxygen, of containing yet larger stores. The
animal, taking into its body these complex materials, and elaborating
them together with oxygen into yet more complex and more unstable
compounds, then, during its vital activity, makes organized use of the
transformation of the potential energy thus stored into lower forms of
energy. Thus there go on side by side, in both animals and plants, a
building up or synthesis of complex and unstable chemical compounds,
accompanied by a storage of potential energy, and a breaking down or
analysis of these compounds into lower and simpler forms, accompanied by
a setting free of kinetic energy. But in the plant, synthetic changes
and storage of energy are in excess, while in the animal, analytic
changes and the setting free of kinetic energy are more marked. Hence
the variety and volume of animal activities.
The building up of complex organic substances with abundance of stored
energy may be roughly likened to the building up, by the child with his
wooden bricks, of houses and towers and pyramids. The more complex they
become the more unstable they are, until a touch will shatter the
edifice and liberate the stored-up energy of position acquired by the
bricks. Thus, under the influence of solar energy, do plants build up
their bricks of hydrogen, carbon, and oxygen into complex molecular
edifices. Animals take advantage of the structures so elaborated, modify
them, add to them, and build yet more complex molecular edifices. These,
at the touch of the appropriate stimulus, topple over and break
down--not, indeed, into the elemental bricks, but into simpler molecular
forms, and these again in later stages into yet simpler forms, which are
then got rid of or excreted from the body. Meanwhile the destructive
fall of the molecular edifice is accompanied by the liberation of
energy--as heat, maintaining the warmth of the body; as visible or
hidden movements, in locomotion, for example, and the heart-beat; and
sometimes as electrical energy (in electric fishes); as light (in
phosphorescent animals and the glow-worm), or as sound. It is this
abundant liberation of energy, giving rise to many and complex
activities, which is one of the distinguishing features of animals as
compared with plants.
* * * * *
We have now, I trust, extended somewhat and rendered somewhat more exact
our common and familiar knowledge of the nature of animal life. In the
next chapter we will endeavour to extend it still further by a
consideration of the process of life.
NOTES
[A] An interesting problem concerning the atmosphere is suggested by
certain geological facts. In our buried coal-seams and other
carbonaceous deposits a great quantity of carbon, for the most
part abstracted from the atmosphere, has been stored away. Still
greater quantities of carbon are imprisoned in the substance of
our limestones, which contain, when pure, 44 per cent. of this
element. A large quantity of oxygen has also been taken from the
atmosphere to combine with other elements during their oxidation.
The question is--Was the atmosphere, in the geological past, more
richly laden with carbonic acid gas, of which some has entered
into combination with lime to form limestone, while some has been
decomposed by plants, the carbon being buried as coal, and the
oxygen as products of oxidation? Or, has the atmosphere been
furnished with continuous fresh supplies of carbonic acid gas?