The childhood of animals

Front matter

THE CHILDHOOD

P. CHALMERS > MITCHELL:

Cornell University

Preface

In December and January of 1911-1912 I delivered the Christmas Course of Lectures, ‘‘ adapted to a Juvenile Auditory,” at the Royal Institution of Great Britain, and took as my subject “ The Childhood of Animals.” The six lectures were not written; they shaped themselves as the course proceeded, partly in relation to the set of lantern-slides, specimens and living animals that I was able to bring, and partly in accordance with the advice of my kind and experienced friend Sir James Dewar. This book is not a printed version of the lectures, although it tells the same story in a different fashion. A lecture must be as direct and as little cumbered with detail as may be; the leaves of a book can be turned backwards and forwards, and its lines skipped or re-read. I have therefore been able to include many details that I had to omit when I was speaking, and to cover my canvas in a different way. In particular, I am no longer trying to address a juvenile auditory ; I have attempted to avoid the use of terms familiar only to students of zoology, and to refrain from anatomical detail, but at the same time to refrain from the irritating habit of assuming that my readers have no knowledge, no dictionaries and no other books.

My object has been to bring together observations old and new that seemed to throw a light on the nature of the period in the life-history of animals between birth and maturity, rather than to write a formal treatise on the subject. I have not found it possible, nor have I tried to keep strictly within the logical confines of the title. Where the subject seemed to lead, there I have followed cheerfully, remembering that I am not preparing readers for an examination where no marks will be assigned to extraneous matter.

It has been pleasant to collect the material, pleasanter when it seemed possible to arrange it so as to display a rational interpretation, perhaps most pleasant when the unruly facts refused to conform with theory. Although it may be true, as Lord Morley once wrote, that the universe will never cease to be “a sovereign wonder of superhuman fixedness of law,” it is at least a mitigating vii

PREFACE

In December and January of 1911-1912 I delivered the Christmas Course of Lectures, ‘adapted to a Juvenile Auditory,” at the Royal Institution of Great Britain, and took as my subject “ The Childhood of Animals.” The six lectures were not written; they shaped themselves as the course proceeded, partly in relation to the set of lantern-slides, specimens and living animals that I was able to bring, and partly in accordance with the advice of my kind and experienced friend Sir James Dewar. This book is not a printed version of the lectures, although it tells the same story in a different fashion. A lecture must be as direct and as little cumbered with detail as may be; the leaves of a book can be turned backwards and forwards, and its lines skipped or re-read. I have therefore been able to include many details that I had to omit when I was speaking, and to cover my canvas in a different way. In particular, I am no longer trying to address a juvenile auditory ; I have attempted to avoid the use of terms familiar only to students of zoology, and to refrain from anatomical detail, but at the same time to refrain from the irritating habit of assuming that my readers have no knowledge, no dictionaries and no other books.

My object has been to bring together observations old and new that seemed to throw a light on the nature of the period in the life-history of animals between birth and maturity, rather than to write a formal treatise on the subject. I have not found it possible, nor have I tried to keep strictly within the logical confines of the title. Where the subject seemed to lead, there I have followed cheerfully, remembering that I am not preparing readers for an examination where no marks will be assigned to extraneous matter.

It has been pleasant to collect the material, pleasanter when it seemed possible to arrange it so as to display a rational interpretation, perhaps most pleasant when the unruly facts refused to conform with theory. Although it may be true, as Lord Morley once wrote, that the universe will never cease to be “a sovereign wonder of superhuman fixedness of law,” it is at least a mitigating vii

Vill PREFACE circumstance that as the laws are superhuman, we need not be. quite certain that we know them. Pleasure in her ways, rather than a cold comprehension of them, is Nature’s surest gift to us, and Iam content if I have provided a setting of theory sufficient to make the facts lustrous.

I am deeply indebted to Mr. E. Yarrow Jones, who prepared the beautiful designs, painted on Japanese silk, which have been reproduced as the plates in this volume. I was anxious to obtain the co-operation of an artist who would see the animals with his own eyes, adopt his own decorative formula, and not be content with setting down diagrams giving the data of colour and form that we find useful in treatises on systematic zoology. Y confess that my delight was tinged with surprise when I found Mr. Jones’s art revealed individual and specific characters which cannot be described by words and diagrams. I have also to offer my sincere thanks to Mr. R. B. Brook-Greaves for his patience and skill in making the pencil-drawings for the text-figures.

Finally I have to state my indebtedness, in general terms, to the great army of writers on zoological subjects. To have tried to attribute to its proper source each observation that I have used, or each little piece of half-remembered theory, would have overweighted this book with historical pomp, and puffed out its slight figure to unhealthy repletion. Although there are some observations that are new, I claim credit for the mode of presentation rather than, for what is presented.

P. CHALMERS MITCHELL

Lonpon, August 21, 1912

CHAPTER

Contents

PREFACE

. CHILDHOOD AND YOUTH . LARv& AND METAMORPHOSES . THE DuRATION OF YOUTH IN MAMMALS

. THE DuRATION oF YOUTH IN BirDS aNnD LOWER ANIMALS

Chapter I Childhood And Youth

WE look out on the world with human eyes, and see with little wonder whatsoever is like ourselves. We are born, small and helpless, yet visibly stamped with humanity; day by day we change, but move with certainty in one direction. A few years pass, and from childhood we attain youth, a few more and we reach maturity. The changes affect size and structure, character and disposition, but are so orderly and familiar that we accept them without surprise, and demand for them no explanation. Man is only one of many hundreds of thousands of living species, and living beings are only a small part of the world around us. Is the mode by which man attains manhood universal in the living world, and does the living world differ in this respect from things that are not alive ?

The universe throbs with restless change. Oursun with its system of revolving planets is rushing into the recesses of starry space on some errand at which we cannot guess. The little planet that is the home of the only life we know is impermanent in its masses and in its details. The oceans shift on their uneasy beds ; continents and islands rise and fall. Mountains and plains are carved and fretted by air and wind and water, blistered by heat, riven by frost, and smoothed over by vegetation. The chemical elements of which we used to think as eternal counters, passing unchanged through mazes of combination and disintegration, are, some of them at least, in a process of making or unmaking. Everything that we know is gecoming rather than being. None the less there are degrees and differences in change itself. The swift and inevitable routine of life stands in sharp contrast with the vaguer and more capricious rhythms of things that are not alive. All living creatures are born into the world from seeds or eggs or directly from the bodies of their parents, and unless they meet death by the way, meet it at the end, after passing through childhood and youth, maturity and old age. This orderly progress from the beginning to the end is

C.A. A characteristic of all animals, and the parts of it that we call childhood and youth are the most characteristic. Complicated pieces of machinery, like watches or motor-cars, resemble animals in many ways, and like them may be new or old, but are never young. Youth is a property of the living world.

The history of an animal, from its first appearance as a speck of living matter formed from the parental body, to its death, is continuous, and it would be useless to try to define exactly when childhood begins, when it passes into youth, or the point at which the period of youth ends. There is difficulty even in fixing the beginning, for animals of the same kind may be born at different stages of growth, whilst animals of different kinds differ extremely in this respect. A large black newt, brilliantly spotted with yellow, known as the spotted salamander and common in the south of Europe, lays eggs like the spawn of a frog. But unlike the eggs of the frog which show the presence of tadpoles only after some days, those of the salamander appear with fully formed little tadpoles wriggling in them, and hatch almost as soon as they are laid. Sometimes they hatch actually before they are laid, and it is in the tadpole stage that the animals first appear in the world. So also most snakes lay eggs and incubate them for days or weeks, before the young snakes break through the leathery shell. But in some snakes, like the common adder, what corresponds to hatching takes place inside the body of the mother, and instead of eggs being laid, young snakes are born. Most of the warm-blooded, hairy creatures that we know as mammals because they suckle their young, give birth to moving young and do not lay eggs, but two of them, the duck-billed platypus and the spiny echidna of Australia, lay eggs with yolk and hard shells. The platypus incubates the eggs until they hatch; the echidna, after laying an egg, transfers 1t with her mouth to a pouch on the under side of her body, like that of a kangaroo, and in this warm and secure receptacle, safer than any nest, the egg is kept until it hatches. Mammals of the group known as Marsupials, because most of them have a marsupium, or pouch (which is well seen in the kangaroo), at one time laid large eggs and no doubt transferred them with the mouth to the pouch, just as the echidna still does. But now the eggs are retained for a certain time in the body, although the young are still very imperfect when they are born. The new-born young of a kangaroo is less than an inch long, although its mother may be nearly as tallas a man. The figure (Fig. 1) has been drawn from a specimen obtained at the

London Zoological Gardens, and shows the naked little creature, an embryo rather than a young animal, hanging to a nipple inside the hairy pouch of its mother. In higher mammals eggs are not laid, and the young at birth are much more formed than in the case of the kangaroo, but they may be covered with fur, have their eyes open and be able to run in a few minutes, like young hares, or, like young rabbits, may be naked, blind and helpless. Even in one species there may be notable differences ; the kittens in a single litter are seldom alike in size, in the degree of their development and in the date when they begin to see, and although new - born human babies are more closely similar, some may be at least a month older or a month younger than usual, and yet grow up quite normally.

These differences are interesting and important, but I mention them here only to show that there is no exact, fixed point Fic. 1. Part of the inner wall of the pouch of the red in its history when kangaroo, with the young attached to the teat.

ec asekas (Natural size.) a new individual ceases to be an egg or an embryo and may fairly be called a young animal.

In the same way the end of the period of youth is indefinite. Sometimes there is a sharp break. Accaterpillar becomes a chrysalid and from the chrysalid the full-grown moth or butterfly emerges. Sometimes, perhaps more often, the transition is gradual. Even the time when a young animal itself can become a parent does not give adividing line. A few generations ago, girls were thought fitted for marriage when they were fourteen, and not infrequently became mothers whilst they were still children. Amongst animals, parentage is often precocious in individuals or in whole groups. We must be content to take the period of youth in a general way as a subject for description rather than for precise definition.

Young animals can be placed in three groups, notably different in their character. The first group has little claim to existence ; it contains a few animals that have no period of youth. The second group contains very many of the animals with which we are most familiar. The young are sufficiently like their parents to enable us to make a close guess as to what they are going to become. We have no doubt that a human baby is a young human being, that a baby monkey belongs to the group of monkeys, although we may not be quite certain as to the particular species of which it is a member. It is the same with kittens, puppies, calves and lambs ; we place them at once among the mammals, with complete certainty in their own order, and with a probability that depends on our powers of observation and knowledge in their proper family, genus, or species. Young birds may puzzle us a little more, but at the least we are never in doubt that the naked or fluffy creatures are going to be birds. Crocodiles and lizards, snakes and turtles similarly come into the world with their relationships plainly stamped upon them,

In the third group we must place those young animals, of which many insects and marine creatures are familiar types, that are so unlike their parents that their destiny cannot be guessed from inspection. The changes through which many of these creatures pass on their way to adult life are as strange as if a new-born human baby were to have the form of a fish, swimming in a tank, feeding greedily on worms and water-fleas, and then after a few weeks or months were to grow very fat and sleepy, to split along the back, and, discarding its fish-skin, to creep out on land in the form of a hedgehog ; and if the hedgehog were to live for months or years the life of a humble quadruped, growing bigger and fatter until it too reached a limit of growth, broke out of its hedgehog skin and appeared as an adult human being fitted in body and mind to be a bishop or a burglar.

It is not to be supposed that these three different kinds or aspects of youth agree with the divisions in which the animal kingdom is arranged by zoologists. It happens that the creatures without a true period of youth belong to the lowest division of animals, and that the highest animals fall naturally into the second group, but the vast range of living beings between the lowest and the highest divisions show all degrees of close likeness and complete unlikeness to their parents. Nor must it be supposed that the three groups are sharply marked off. The arrangement of facts in groups is more convenient than natural, and we must not forget that many of the divisions of science are concessions to the human mind rather than forms of nature.

Further consideration of the first group need not detain us long. The very small animals known as amcebe (Fig. 2), the largest of which are visible as specks to the naked eye, are mere droplets of granular protoplasm, creeping over the mud in fresh

Fic. 2. Reproduction of Amceba (highly magnified). To the left a full-grown amceba ; to the right successive stages of division.

water or in the sea, or lurking in the bodies of other animals or of plants. The soft, jelly-like material of which they are formed makes it possible for little particles of food to be engulfed wherever these come in contact with the surface of the body. The simple business of their life is to creep in search of food, to digest the food as quickly as possible, and to grow bigger. But although the different kinds of amoeba differ in size, there is a limit beyond which each kind does not grow. When that limit has been reached, or sometimes before it has been reached—for reproduction is a good deal more complicated in its causes than a mere escape from inconvenient size—the amceba becomes oblong in shape and then acquires a kind of waist which becomes more and more slender until only a string of jelly remains. Finally this string divides, and the two halves become rounded again, each forming a complete amceba, exactly like the parent in all respects except size, and these two at once set about the pursuit of food and begin to grow. The two amcebe may be called young animals in the sense that they have just come into existence as new individuals, but nothing in their tissues or characters distinguishes them from their parent. So far as the period of youth has any interest or significance, these animals escape it. Many small creatures belonging to the lowest group of the animal kingdom, the Grade known as Protozoa, reproduce like amceba by a process of simple division, and it is tempting to suppose that this method is older than the more complicated fashions in which most animals multiply. Even amongst Protozoa, however, very many animals begin their individual lives in a form unlike that of their parents, and attain the adult condition only after passing through complicated changes. I am not going to describe any of these here, as they show no characters of youth that are not equally well displayed in animals easier to observe. I wish to recall their existence, however, because it is very frequently the case in the living world that simple structures and events are not primitive, and it may well be that the Protozoa without a true period of youth are not surviving relics of primeval life, but are forms that have become simple and degenerate because of the easy conditions in which they live.

The animals in the second group will engage most of our attention in this book, because they include ourselves and those most nearly akin to us. As their structures, habits, and dispositions are not very remote from our own, they offer problems which it is possible to understand, and perhaps to solve, and they give a hope of interpreting our own history and of predicting, perhaps controlling, our own future. They have this in common, that the young always resemble the parents more or less closely.

Amongst human beings and monkeys, the young are born in so advanced a condition that we think of them as babies and not as embryos. The eyes are open, the voice is lusty, the face, the hands and feet, and the body generally are shapely and well formed. But the senses are deficient, especially in the great apes and man. The hand of a new-born infant will close round and cling to a broomstick or any other object placed in it, almost in the automatic fashion in which the tendril of a creeper will twine round a support which it comes to touch. So also, in the danger of the woods, the new-born gorilla or chimpanzee must cling from the first to the body of its mother, or perish miserably. In a few days the observing and reflecting parts of the brain awaken, automatic action becomes less important, and is replaced by a medley of instinct and intelligence. In the lower monkeys, and especially in lemurs, although the young cling to their mothers, the automatic period is shorter, and the babies, almost from the first, show what looks like conscious, independent movement. Human babies and the babies of apes and monkeys differ from their parents in proportions. The heads are relatively larger, especially in the higher creatures, and the legs and arms are relatively shorter. They all, as a rule, are born with some hair, but this is more scanty and more different in texture and colour than that of the parents in human beings and the great apes, more like that of the parents, in abundance, texture and colour, in the lower monkeys and lemurs. Special growths of hair, like beards and crests, special patches of colour on the face and body, like the brilliant scarlet and blue on the face of the mandrill, are absent. I need not waste time recalling familiar differences like the absence of teeth, and of bony ridges on the head, the softness of the bones, the protruding stomachs and the general plumpness and roundness of the body.

I have already said of this group of young animals that although there is a fairly close resemblance with the parents, we cannot always be certain of the particular species to which an infant belongs. The reason of this difficulty lies in the striking circumstance that the young of nearly allied animals are much more alike than are the adults. No one could fail to distinguish a fully grown man, gorilla, orang and chimpanzee, but in many points in which the young of these creatures differ from the adults, they resemble each other more closely. In the slow development of every individual before birth and after birth, the characters of the species are the last to be assumed. We explain this by supposing that the evolution of the individual to a certain extent repeats the evolution of the race. Man, the gorilla, the orang and the chimpanzee had a common ancestor, and the children of these creatures are more like the common ancestor, and so like each other, than are the adults. We have to remember, however, that this explanation is not complete, and we shall find many characters of young animals to which it does not apply. The young animal owes its characters not merely to its ancestry ; as much as the adult, it has to be fitted to the special environment in which it lives. It is not merely a stage in development, but an independent living creature with its own needs and its own aptitudes, presenting characters that are neither a memory nor an anticipation, neither a relic of the past nor a preparation for the future, but suitable for its own purposes. These

Fic. 3. Head of an unborn gorilla. (After J. DENIKER.)

creatures, suckling their mothers, clinging to them and being protected by them, have an environment which is much simpler and more nearly identical than the environment of the adults, and we must expect, quite apart from common inheritance, to find common characters due to common conditions. The figures on the first coloured plate (see Frontispiece) represent young animals two or three years old, and show how much more alike they are when they are still children than when they are grownup. The young gorilla, with its small ears and short upper lip, is not very different in appearance from a black baby ; the very long upper lips of the orang and chimpanzee and the large ears of the latter make them rather less human.

The parental stages of man and the great apes are still more alike than are the young creatures. The text-figures of the young gorilla (Fig. 3), taken from a specimen of an unborn ape obtained by Monsieur J. Deniker, and of a human being of about the same age, after a figure given by Professor Metchnikoff (Fig. 4), show the almost appalling resemblance between man and the ape before birth.

Fic. 4. Head of a human foetus, about Fic. 5. Head of an unborn longfive months old. (Aftey E. METCHNIKOFF.) nosed ape. (After E. SELENKa.)

For comparison, I have given in another figure (Fig. 5) a representation of acorresponding stage in the development of one of the lower monkeys, the long-nosed ape of Borneo, taken from a drawing given by Professor Selenka in his great monograph on the embryology of mammals. The face and features, the domed forehead covering the capacious brain, the practical absence of hair, and every minute detail of the internal and external structure agree with a fidelity that is almost shocking. Professor Metchnikoff was so impressed by such resemblances that he has suggested that the human race may have taken its origin from the precocious birth of an ape. His theory may be regarded rather as a parable than a definite scientific proposition, but it puts in a striking fashion a remarkable character displayed by young animals. When these differ from the adults, it is not merely that they resemble their ancestors, or are specially

Be) CHILDHOOD OF ANIMALS fitted for the purpose of their own stage of life. They sometimes suggest the future possibilities of the race, directions in which the race may move. As the young animals mature they lose promise and flexibility, and settle down to the average characters and average limitations of their kind.

Young Carnivores seldom differ notably from their parents. The cubs of lions, tigers, leopards and jaguars, and the kittens of cats, lynxes and caracals can usually be identified at a glance. They

Fic. 6. Young American timber-wolf, are softer and more rounded, and differ in size and in proportions, and they do not display characters limited to one sex, like the mane of the lion, or special marks like the twisted, hairy tufts on the tips of the ears of caracals and lynxes, and those which are uniformly coloured when they are adult may be spotted when they are young. The puppies or cubs of dogs, dingoes, wolves, jackals and foxes are much more alike than the adults, and point clearly to descent from a common and not very distant ancestor. Young wolves (the ‘drawing in Fig. 6 represents the cub of an American timber-wolf) are quite like the puppies of domestic dogs, except that their ears are erect. The difference is mental rather than physical. When they begin to run about, they betray a shy and furtive disposition, as if they expected no kindness or toleration from man. Young hyenas and civets, bears, raccoons and weasels, seals and sea-lions all closely resemble their parents.

PLATE IT GIRAFFES AND YOUNG

The colorations of the young and the adult are practically identical, but the neck and forequarters of the young are relatively shorter. By an accident of the drawing, the vertical rails in the background make the necks of the adults appear rather shorter than they are, but the proportions are correct by measurement. es

CHILDHOOD AND YOUTH Tr

It would be tedious to go through mammals group by group, making the same general statements about them. Differences of colour and pattern in the coat are often remarkable and will be discussed in a separate chapter (Chapter VI). When the adults have no special weapons or ornaments, they can be distinguished from their young by little that is visible, except size. A young hippopotamus, except for the absence of tusks, a young dromedary or bactrian camel, except that the humps are not so conspicuous, and a kangaroo, as soon as it is able to leave the pouch of its mother, are almost ludicrously exact miniatures of their parents. Baby elephants are more interesting. The smallest that I have seen was a female Indian elephant, presented to the London Zoological Gardens by the Government of the Federated Malay States, and certainly less than a year old and about three feet in height. No one could mistake it for anything but an elephant, but it was thickly covered with long coarse hair, recalling its distant relative, the extinct hairy mammoth. Its ears were much larger in proportion to the size of the head than in the adult Indian elephant, so recalling the African animal, and this resemblance was increased by the smoothly rounded forehead, passing in an even curve from the root of the trunk to the top of the head, and showing no sign of the angular forehead of adult Indian elephants. Its trunk was rather short, the tip being well off the ground when the little animal was standing upright, and was rather an embarrassment to it. It found difficulty in finding its mouth with it, fumbling as a baby does when trying touseaspoon. Nor had it learned to use it in drinking ; it sucked its milk by a rubber tube placed in its mouth, holding its trunk awkwardly out of the way. No doubt if we could see together a young Indian elephant, a young African elephant and a young mammoth, we should find that they were as much alike as are the young of the great apes and man.

A young giraffe (see Plate II) from the first resembles its parents, but neither its neck nor its legs are so long in proportion, and the horns, although erect and tufted with hair like those of the adult, are soft because they have no bony core. In the great assemblage of animals that are armed with horns or antlers the peculiarities of these weapons appear gradually, and the young, at first defenceless, produce little straight spikes like those of their fossil ancestors, and these, as they grow larger, curve or twist or branch until they reach the full splendour of maturity. In antelopes, sheep, goats and cattle, where the horns are “hollow,” that is to say, where they consist of a horny case fitting over a bony core, the first weapons to appear persist throughout life, however they may increase in size and change in shape. In Fig. 7 some of these differences are shown. The takin, a rare and very large goat-like animal from the highlands of Asia, shows little conical horns when it is a few months old. These are placed rather far apart on the forehead, separated by an expanse of hair. As the horns grow they acquire a spiral, goat-like twist and the greatly expanded lower portions meet in the middle line to form a stout rough helmet. In the eland, one of the largest of the African antelopes, the horns first appear as still more slender conical spikes, and as they grow usually become twisted in a straight spiral in the fashion in which a stick of soft candy can be twisted when one end is held firm and the other rotated. Cattle of different kinds also show small spiky horns at first, and these later on acquire the spreading curves of the adult.