It is believed by many geologists that this great luxuriant forest points to a time when the climate was far warmer than it is to-day, when the air was moist and heavily laden with carbon dioxide, and when a great ma.s.s of clouds practically enveloped the earth. In this way only do most geologists account for the enormous wealth of vegetation in the Carboniferous period and for the abundance of plants up to the Arctic Ocean, of the kinds that now grow chiefly in the tropics. But of recent years a few geologists point to the fact that the peat bogs of to-day, which seem to be the beginnings of future coal deposits, are found almost entirely in cold countries. Hence it is a serious matter to attempt to describe the climate of any part of the Palaeozoic era. Certainly of the climate earlier than the Carboniferous it is very risky to say anything definite.
The forests of the coal period seem actually to have cleared the air; at least now we begin to find creatures related to our salamanders and frogs moving about among the stumps of the marshes. These amphibians are evidently the descendants of some of the fishes of the Devonian times. Among these fishes were some which bear a great resemblance to a few found in South America, in Africa and Australia to-day, and which we know as lungfish. Anyone who has cleaned our fresh water fishes in preparation for the table will remember that inside of them there is a long slender bladder filled with air. This bladder a.s.sists in making the fish light, hence making it easier for it to support itself in the water. In certain swampy regions these lungfish swim freely in the water of the marshes. When the dry season comes, however, the water evaporates, draining the marshes completely. This would prove the death of most fishes. The lungfish have a curious habit which keeps them over the dry season. They cover themselves with a coat of mud, inside of which there is a lining of slime produced from their bodies. In such coc.o.o.n-like cases they survive the drought.
The means by which they breathe during this dry season is interesting. The swim-bladder which we have just described in other fishes is, with this lungfish, peculiarly spongy in its walls, presenting a large surface full of blood vessels which absorb the air on the inside of the bladder. This air the fish changes with moderate frequency, the result being that the swim-bladder serves him exactly as the lung serves a higher animal. To this fact he owes his name of lungfish.
We sometimes gain much light concerning the past history of any particular form of animal by studying the development of that animal in the egg, or, in the case of the mammals, before birth. It is an interesting fact that when the lung begins to form in the embryo it starts as a simple sac which is an offspring from the gullet, and occupies the position of the swim-bladder of the fish. This sac later divides into two, and develops into the lungs of the animal. This a.s.sures the zoologist that the origin of the lungs in the higher animals is found in the swim-bladder of the so-called lungfish. In this Silurian time certain of these lungfish were perhaps trapped in the basin in the marsh by the uplifting of the border. The waters becoming progressively shallower and more crowded, these fishes took to the land, their fins developing into awkward limbs which slowly became more perfect.
To state the fact in this simple fashion is to make it seem far less probable than is really the case. The simple forms of the life of lowly creatures, as well as the simple character of the legs and feet in the salamander cla.s.s, make the explanation not so unlikely as would at first sight appear. Suffice it to say that the scientist now believes that out of the lungfish of the Devonian came the amphibians of the Carboniferous period.
At the end of the coal period came the greatest change the face of the globe had seen for many millions of years. Slowly the continent rose on both sides of the old interior sea. A great plateau formed in the region of the Alleghenies and another in the western district, though this latter uplift was to be completely washed away, and later to rise again into the Rocky Mountains and the Sierras. With the uplift at the edges of the continent came a steady rise of the internal marshes, until what had previously been swamp land became progressively first dry land and, in the western part, even desert, in that respect being somewhat like what it is now.
The amphibians of to-day (animals like the salamander and frog) all lay their eggs in the water and their young have a tadpole stage. This doubtless was true of the amphibians of the coal period. With the beginning of the Mesozoic, or "middle life" period, a change and a progression comes over the animal world. The tadpole life of the frog is a rather lengthened one, while the toad has learned to crowd its tadpole life within a few weeks. It would seem as if, in the earlier times of the Mesozoic, this same change of habit had been going on.
With the drying up of the swamp, some of the amphibians crowded their tadpole stage further and further back, until it was completely accomplished before their young left the egg. An examination of the development of the reptile in the egg will show a stage very similar to the fish and to the amphibians, but this is all experienced before the reptile emerges from the egg. The reptilian egg, unlike that of the frog, is covered with a sh.e.l.l, packed away under the surface of the ground, and left to its own fate. If, as most geologists believe, the climate of the Mesozoic was distinctly warm, this habit of the parent of forsaking the egg was not a serious matter. However the creatures arose, it is certain that in this Mesozoic age reptiles roamed the forests, swam the seas, and even flew in the air. Probably at no other time in the earth"s history has any one cla.s.s of animals so completely dominated the situation as did the reptiles of this age.
They were not only abundant; they were frequently enormously large.
Their skeletons are among the most interesting that we find to-day.
Gigantic lizards, seventy feet long and eighteen feet high at the shoulders, dragged their heavy bodies through the marshy edges of the lakes. Out upon the land others, not quite so heavy nor so large, roamed about, some of them feeding upon the soft vegetation, others having teeth fitted to tear down their herbivorous cousins. In some of them the hind legs and tail were very heavy and the front legs so light that it is quite clear they must have hopped around as do the kangaroos to-day. Others of these reptiles went back to the sea, lost the leglike development of their limbs and regained the flipper form, though the bones of the fingers and toes are singularly distinguishable in the paddle.
Strangest of all, a considerable group of these wonderful reptiles lengthened their little fingers, sometimes to three or four feet in length, and had a skin stretched from these fingers over to the body in such a fashion as to give them wings not unlike those of the bat.
In the wing of the bat, however, four of the fingers of the hand run through the membrane and support it. In the pterodactyl, as these flying reptiles are called, the middle finger supports the web, while the remaining fingers can still be used to clasp objects or serve the animal to lift himself, as the bat can do with his thumbs.
Meanwhile an entire change is coming over the plant world. The last third of this age of reptiles is known as the Cretaceous or chalk period. Now, for the first time, the forests begin to take on more of the character of our forests of to-day. Plants like our willow and beech, poplar and sa.s.safras appear in great abundance. Their broad leaves serve better than those of any earlier plants to catch the sunlight. But in addition they offered such effective evaporating surface that they cast off rapidly the moisture obtained from the ground by the plant. Accordingly in the winter season, when the water in the ground is frozen and not available for plant purposes, they were forced to throw away their leaves. It is quite possible that up to and including the time of the Carboniferous, plants were all evergreen. There had been before this little variation in climate over the globe. Life in the Cretaceous begins to take on distinctly its modern form.
Among the reptiles of the forest there appear to have been a few small creatures which to an observer of those times, if there could have been an observer, would have seemed of the utmost insignificance compared with their giant cousins.
These little creatures climbed up into the trees to escape their enemies. There were some in whom the skin, in front of the elbow and behind the wrist, was loose, and stretched across the joint a little like the wing of a bat. This reptile, climbing into the trees to escape its enemies, found that this loose flap of skin served it nicely, and sailed out of the trees in a manner not unlike that of the flying squirrel of to-day. Among these experimenters in aviation, certain forms produced scales which became elongated and finally slit up along the side. These slit scales slowly developed into the feathers of the birds of to-day. Whether the steps by which the change occurred have been correctly stated or not, the result is sure. In the rocks of the chalk period we find the remains of an interesting creature. If nothing but its bones had been found it would have been called a reptile. It had a long tail, it had claws on its front limbs; it had teeth in its mouth; it had a flexible backbone. All of these are reptilian rather than bird characters. Yet on the rocks surrounding these bones are the unmistakable impressions of the feathers of the wings and of the tail. Nothing in the world to-day has feathers excepting the birds, and in this "ancient winged thing," for this is the significance of its name--archaeopteryx--we have perhaps the most remarkable link in the world between two distinct sections of the animal kingdom. Here is a creature half reptile, half bird; perhaps one-third reptile and two-thirds bird. It was about the size of the crow. A little later unmistakable bird skeletons will appear, but still their jaws are provided with long conical teeth.
Still more interesting from our standpoint is another set of primitive animals, utterly insignificant in appearance, but of momentous importance on account of their later history. Among these reptiles were a few small creatures perhaps not much bigger than mice or moles.
Their teeth were a little more complicated and specialized than the teeth of their reptilian cousins. Between their scales were small and spa.r.s.e hairs. Almost nothing but their jaws remain to-day to tell us anything about them. But in this humble little creature of the Mesozoic, utterly insignificant beside the tremendous reptiles of the time, we discern the ancestor of the mammals. These were the progenitors of the horses and cows, of the cats and dogs, of the monkeys and apes, of the men of to-day.
During this chalk period, which forms the last portion of the age of reptiles, life for the first time grew to look much as it does to-day.
Now, apparently, the cold of winter and the heat of summer followed each other in regular succession. There have been colder and warmer periods at various times in the previous history of the earth, but undoubtedly they were more uniformly cold or uniformly warm than now.
Ages were warm, or ages were cold, but now the earth clearly shows the annual alternations of summer and winter, and for the first time clearly shows the bands of climate on the earth which we know as zones.
In the chalk period this new factor of cold works mightily in favor of the mammals. Their reptilian ancestors were cold blooded. When the climate was warm they were active; when the climate was cold they were sluggish. With the continuation of the annual alternations of cold and warm weather that had now set in upon the earth, the little birds and mammals had in their warm blood an advantage which, in the long run, enables them not simply to compete with their reptile forefathers, but to outdistance them absolutely in the race. Here and there, on earth to-day, exist a few big reptiles like the crocodiles and the boa constrictors. But they are few and comparatively insignificant among the mult.i.tudinous population of the globe and are confined to the hotter portions of the earth. For the most part, the reptiles now play an insignificant and un.o.btrusive part. The little molelike creatures, practically unnoticed between their feet in the later Mesozoic, have come to supplant them entirely, and almost to rival them in size.
While the reptiles have grown steadily smaller, the mammals have steadily become larger.
While there is no land mammal to-day as big as the heaviest of the reptiles in the Mesozoic, the whale, which is one of the mammals that has again taken to the ocean, surpa.s.ses in size even those gigantic creatures. There never lived in the world before a creature quite so big as the biggest of our whales. Size, however, is not the most important point in any animal. Speed, sagacity, variability, and power of adaptation, these are the qualities which the world prizes, and these the new mammals possessed.
The next geological era is the Cenozoic, or period of modern life.
This is divided into two quite distinct sections, the Tertiary and the Quaternary. This era began about five million years ago, roughly speaking, and is still going on. The greater half of it is known as the Tertiary. It was during this time that the mammals came to their own. At first these creatures belonged to what the scientist knows as generalized types. They are jacks-of-all-trades. The student of early animal life finds in the little Phenacodus, which was scarcely bigger than a good-sized setter dog, the beginnings from which many forms have subsequently developed. This creature showed points of structure which to-day may be seen in such diversified animals as the dog, the horse, the rabbit, and the monkey. It is not, of course, suggested that Phenacodus was the immediate ancestor of any of these. But there were no animals in those times more like these I have mentioned than was Phenacodus, and from forms like it in main features all of these other animals have since been derived, each species of animal having become adapted to one particular kind of life. The development of diversified situations on the earth, the varieties of climate, the variation between marsh and upland, between valley and plateau, furnish a complexity of environment into each niche of which a new form of animal fitted itself.
With the increased complexity of mammals comes the submergence of the reptiles and amphibians to-day. In all sorts of situations we find mammals. The old-fashioned continent of Australia is separated from everything about it by deep water, impa.s.sable to any animal which lives upon it. In this secluded country evolution is very slow and animals are very antiquated. We still find there mammals with the ancient habit of laying eggs in a hollow in the ground, though after these eggs are hatched the young are nursed on the milk of the mother.
But on the great continental stretches, where compet.i.tion is keen, where the animal must battle for his life against a wide field of other animals, where migration into new situations is possible, the rapidity of the development has been very much greater.
It is in such a situation that man has arisen. In the extreme southeastern portion of Asia, and on the islands lying close to the coast, his highest non-human relatives, members of the ape family, have reached their best development. These, of course, are not man"s ancestors. They are the less progressive members who are left behind entirely in the race. Whether we have to-day any traces of the steps by which man arose from the animal beneath him is vigorously disputed.
Eminent scientists will be found on both sides of this question.
Many scientific writers to-day take it for granted that one form, discovered in Java, while it may not be in the absolutely direct line, must be very close indeed to the line of ascent toward man out of the apelike forms. A scientist by the name of DuBois, working in the banks of a stream in south-central Java, found a thigh bone which seemed to him exceedingly human in its general character and yet not absolutely like the human thigh bone. The oncoming of the rainy season raised the water in the river so that DuBois could not continue his search.
Returning a year later, and digging back deeper into this bank, he found a skull cap and two molar teeth which seemed to him to belong to the thigh bone, although they lay several yards farther back, but at the same level in the bank.
When these bones were subsequently presented to a meeting of European scientists by DuBois, he claimed to have found the "missing link" for which there was so eager a demand. Some of the best anatomists of the meeting, notably Virchow, laughed at his claim and said that the skull cap was simply that of a human idiot, and could be duplicated in any large asylum. A committee of twelve naturalists was appointed to report upon DuBois" find. Of this committee three a.s.serted the bones to be those of a low-grade man, three insisted that they belonged to a high ape, of a type somewhat higher than any we know to-day, but still distinctly an ape. Six members of the committee of twelve agreed that the remains were those of a creature higher than an ape and lower than any normal man, and represented, in their opinion, a stage distinctly along the line of development out of the apes and into man.
This so-called "Java find" is known in science by the name of Pithecanthropus, which means the ape-man. Whether we look upon this fossil as a serious find or not, it is very certain that in the caves of Europe belonging to the Quaternary period we find abundant evidences of primitive man. The older these evidences are, the more likely they are to be distinctly below the grade of man of to-day, in the size and shape of the brain case and in the length and ma.s.siveness of the jaw.
There are probably more races than one represented among these skulls.
Some of them are surely well-deserving of the t.i.tle of low brow. Their heavy ridges over the eyes, their small foreheads, their ma.s.sive, heavy-set jaws show a race of men far less endowed mentally and much better endowed in the matter of brute force than the men of to-day.
These skeletons, or parts of skeletons, are turning up every year, and we are just beginning to know much about them. Capable men are studying them with much care. The next fifty years may not improbably make the history of the ascent of man as clear as is now that of the horse, to which we shall refer later.
The whole question of the descent of man from the lower animals, or his ascent from them, as Drummond aptly termed it, is to most people so entirely repugnant as to set them at once, and finally, against all willingness to consider the question of Evolution. This, however, does not solve the problem. Even though truth be horribly unpalatable, it is still to be believed if it is only the truth. There is practically no doubt left among scientific men of the origin of man in lower forms. The evidences grow more and more complete year by year, and from every line of investigation. Whether we study his anatomy, his embryology, his history, his language, or his civilization, all indications point in the same direction. Constant discoveries indicate the fact of an enormously long development from a very humble form. If this proves to be true and remains unpalatable, the fault lies in the palate and not in the truth. Gradually we are coming to understand that there is no reason why this truth should be unpalatable. We consider a rise from humble conditions to be the glory of our heroes; we esteem it an added charm in their strength that they should have developed from untoward surroundings. It is not a disgrace to man to have descended from the apes. It is to the glory of man that he should have ascended from forms not much more promising-looking than the apes of to-day. We must repeat, however, that the apes were the unprogressive members, and hence we must not judge man"s ancestors too harshly. It must have been in them to rise. But the great glory in the thought of the humble ancestry lies in the possibilities of his future. If out of a creature not materially unlike the gibbering ape of to-day there should have come, under the guiding hand of an Almighty G.o.d, creatures with the endowments and capabilities of man of to-day, then this is only an earnest and foretaste of that which may be expected in the future. A time will come when man shall have risen to heights as far above anything he now is as to-day he stands above the ape. Even then there seems no end. With Infinite Power as the agent, and limitless time in which to work, man would be limiting G.o.d to an extent unwarranted by the history of the past to imagine that His process had stopped to-day, and that man, with his many imperfections of body, of mind, and of morals, should be the best that is yet to come. There cling to him still the limitations and dregs of his brute life. Often the brute in him comes to the surface. Little by little he is coming to be dominated by the qualities G.o.d has last given him. Slowly the brute shall sink away, slowly the divine in him shall advance, until such heights are attained as we to-day can scarcely imagine. As we can scarcely conceive the beginnings of this process, so we can with difficulty imagine its end. This only can be seen by the Eternal through whom it shall all come to pa.s.s, and by whom all will in time be accomplished.
CHAPTER VII
HOW THE MAMMALS DEVELOPED
When the idea of evolution first began to be much discussed, especially after the publication of the "Origin of Species," there were several points which appeared to be more than commonly difficult of explanation. It did not seem impossible that the various types of domesticated cattle should have descended from a common ancestor. It did not seem difficult of comprehension that the dog might once have been a wolf. Though not quite so credible, it did not seem absurd that the tigers, lions, and leopards should have once all been alike. The resemblance between these are strong enough to make the idea seem conceivable. Though men were willing to concede this much, they insisted that the great branches of the animal kingdom varied so widely from each other as to make it certain that each was a separate creation. It was particularly objected that the mammals differed so entirely from other animals in several important particulars that a special divine act was necessary for their appearance. The mammals have a furry covering entirely different from the clothing of any other animal in the kingdom, and have warm blood, which is found nowhere else except among the birds. But particularly their method of producing their young seemed so entirely different from that of any other group that here a special creation was deemed absolutely necessary.
Other young creatures are produced from eggs laid by the parent and subsequently hatched. The young of the mammals are born alive and comparatively well developed. In addition, their first food, the milk of the mother, is so entirely different from the food of any other creature that this again seemed to involve a separate creation.
Gradually we have come to understand the whole matter of reproduction very much better. Minute and careful dissections of rabbits, of dogs and cats, of animals slaughtered for food, with occasional post-mortem examinations of human beings in various stages of the development of the young, leave us no longer in doubt concerning the main features of the process. The better we come to understand it the more clearly it becomes evident that in the development of the mammals we have no new procedure, but, as in so many other activities, new developments of an old process.
There are two entirely different methods by which new animals and plants may arise. One sees sometimes in the home of a friend a geranium of particular beauty, the like of which he would be glad to possess. The accommodating friend cuts a small piece from the geranium. This is stuck into poor but well-watered ground, develops roots, and eventually grows into a geranium stalk exactly like the one from which it came and of which it is in reality only a detached part.
In similar fashion, if one wants a particular kind of apple, he never trusts to planting an apple seed. Going to the tree of the variety he desires, he takes from it a small twig provided with a bud and inserts this bud into a cleft made in the young branch of another apple tree.
The young bud so inserted starts up into a new branch, resembling almost absolutely, not the tree which feeds it with sap, but the tree from which the bud was originally taken.
When we wish a particular variety of potato we obtain pieces of the potato of the kind we desire. Each of these must contain an eye, which is a bud of the old potato. When the sprout appears the new plant will be practically identical in character with the plant from which the potato was taken. This sort of reproduction, in which a piece of the old parent grows up into the new generation, is called the as.e.xual method. But one parent is concerned in the process, and the offspring are as nearly as may be like the parent from which they arose.
The gardener who wishes to obtain new varieties is not content with this method. If he plant the seed of the potato the outcome will be most uncertain. His seed must be taken, of course, from the fruit of the potato, and most of these plants never fruit. Every grower of large quant.i.ties of potatoes will have noticed occasionally, on the tops of the plant, after the flowers disappear, a globular growth looking not unlike a small tomato, but with a tendency to become purplish green in color. This is the fruit of the potato and in it are the seeds. When these are planted all sorts of potatoes are liable to start up. Most of them will prove worthless. An occasional seed may produce an uncommonly fine plant. This new variety may thereafter be propagated from the tuber, as the potato itself is called, and the new strain will be kept constant in this way. This method of using the seed for reproducing the plant is called the s.e.xual method, because two parents cooperate in the production of the seed. The pollen came from one parent and the ovule, which after fertilization swelled up into the seed, came from another. By this combination of two individuals new varieties become quite possible. Nature seems to be more concerned in improving her strain than in maintaining her older strains. In all of her lowest plants and animals she uses the as.e.xual method of reproduction. As we go higher in the organic world the two-parent method becomes increasingly common. When we reach the higher animals, and most of the higher plants, this plan of double parenthood, the s.e.xual method, alone is used.
In order that we may the more clearly understand how the mammals produce their young and nourish them, we shall begin at the lowest cla.s.s of the backboned animals and note how the process is there accomplished. As we pa.s.s upward through the kingdom the method acquires greater complexity. When we finally reach the mammals, what at first seemed an absolutely new process will prove to be, as is all of nature"s work with which we are thoroughly acquainted, but a modification and an elaboration of some previously existing process.
Some time ago I was pa.s.sing the early months of summer by the side of a lake in northern Pennsylvania. Near my tent, on the edge of the water, was a wharf from which it was possible to look down into the shallows about the edge of the lake. In early July the bottom began to take on a strange appearance. Spots as big as a dinner plate became evident because they were cleaned of the finer sand or mud which is common on the bottom. A close examination showed that each of these circular spots was being occupied and cleaned up by a sunfish. The pebbles were lifted into the mouth of the fish and driven out again with force. The water which emerged with the stones seemed to wash away the dirt, while the pebbles themselves became gradually cleaned of the green plant life which ordinarily covers them. After the process was completed each spot was saucer-shaped and free from sc.u.m and mud. Over each of these spots hovered the sunfish which made it, and round and round the fish swam. The circles thus traversed were so near each other that every now and then the occupants of two adjoining nests would meet on the border. The fish which was most nearly on its own ground would at once attack the other and drive him away. In a few days the other partner in each family seemed to appear. Now two fishes swam side by side over each nest, bringing the lower edge of their bodies comparatively close together. In this position they moved around over the pebbly bottom. The female was discharging her mult.i.tudinous and very small eggs, so that they dropped to the bottom of the nest. At the same time the male was expelling what in fish is known as the milt. In this milt are the sperm cells of the male, each consisting of a rounded head and a very slender body. These are attracted by the eggs. Pushing up against them, the head of a sperm cell, consisting almost entirely of the nucleus of the cell and carrying the determinants which were to decide one-half of its future characters, penetrated this egg and fused with its nucleus. This was filled with the determinants of the characters inherited from the mother. Of course many of the eggs, of which probably there are a thousand, must have escaped fertilization. There are doubtless a thousand sperm cells that went to utter waste for one which found an egg to fertilize. These eggs nestled in the crevices between the stones in the warm water of the edge of the lake. Here the sun could easily penetrate to the bottom and hatch them. The little fish, still guarded by one hovering parent, swam around in the water long before the yolk of the egg, containing its large amount of food, had been absorbed into the tissues of the young fish. This fatty store made the abdomen of the fish in which it lay protrude enormously. Gradually the fish grew larger and the yolk grew smaller until all had been consumed. Soon the fish began to forage for himself and no longer to demand or care for the company and protection of its parent. The little sunfish is highly favored among his comrades in having any care whatever by the parent. In the case of most fishes the female, swimming slowly over the bottom, deposits her eggs, which are fertilized by the male, which follows behind her. After the eggs have thus been laid and quickened no other attention is paid to them by either of the parents.
Fish are stupid almost beyond the comprehension of those who are not students of the minds of animals. Frogs and toads are a distinct step in advance, and hence their mental activities play a larger part in the process.
In the love-making of the frogs and toads the song has an important share. In each species the voice is a little different from that of any other. In our familiar garden toad we have an excellent ill.u.s.tration of the method common to the entire group. When spring comes an impulse seems to stir in all the toads of a neighborhood.
Heretofore they have stuck faithfully to dry ground; now they start off for the water. Whether their impulse is simply to move down hill or whether they by some means detect the near presence of water, I cannot say. Certainly a new fountain on a lawn will secure in spring its prompt and full share of the neighborhood"s toads. In any event the toads of a district congregate in great numbers in any pond or along the edge of any moderate stream. Within a short time their flutelike, quivering voice is heard far and wide. That this note has an attractive power over the female there is no doubt. She herself makes no effort to imitate, but the song of her mate is persistent and exceedingly sweet. I have seen a male sit upon a clump of gra.s.s and utter his love call. Before he had been singing for more than half a minute three females hastened toward him from a distance of perhaps twenty feet. Each seemed anxious to reach as promptly as possible the creature whose voice had proved so attractive. When the mating comes, the female discharges a series of small shotlike eggs which are encased in a very tenacious mucous. While they are being deposited the male fertilizes them. No sooner have the eggs, fertilized by the sperm cells, reached the water than the mucous at once begins to swell. The result is that eggs appear encased in two slender strings of jelly, each having a diameter about that of a lead pencil. At intervals of not more than half an inch the shotlike eggs may be seen. The mother toad, in laying these eggs, moves about rather restlessly in the water. By this means she succeeds in wrapping the strings about the gra.s.s and sticks of the pool. This will hold them quite safely even against a considerable current of water, should the stream rise and flood the side pools in which the eggs are laid. With this amount of care, however, the attention of both parents to the young entirely ceases. They are now abandoned to the chances of a fortune to them exceedingly unkind. A toad will lay about five hundred eggs. It is evident that on the average only two of these can attain maturity by the time the parents have died, for the number of toads does not materially alter season by season. The connecting string is made up not of nourishment for the eggs, but of a bitter mucous so unpleasant to the taste that fish are thus deterred from eating the otherwise nourishing material. This secures for the young embryo a chance to mature which in the absence of the jelly it would entirely lack.
Imbedded in this mucous is the embryo itself, surrounded by a small amount of alb.u.men and containing inside of itself a very considerable amount of yolk. This gives to the egg a volume possibly a hundred times that of the egg of the sunfish. Thus, even counting the care the parent sunfish took of its offspring, which care is very uncommon among fishes, the toad stands a distinctly better chance in life. The protection of the bitter mucous and the large amount of yolk permitting considerably larger development before leaving the egg, give to the toad a material advantage. When the toad first emerges from the egg it is amazingly like the fish. It has gills at the side of its neck and swims by the movement of its tail. Later its limbs develop, the hind ones coming first, its tail is absorbed, and it is now a true toad, ready to leave the water.
Altogether a higher state of reproduction is encountered when we reach the reptiles, which are the next higher cla.s.s of backboned animals.
Here very distinct developments of the process are discovered. The turtle, to use the best known ill.u.s.tration, may lay but twenty eggs.
But she will not lay them at random in the water, as do the toads and the fish. Each egg is wonderfully fattened with yolk. This means that it is possible for the creature to develop to a far greater extent before leaving the egg than was possible in the case of the toad.
Accordingly the little turtle, while it begins life not unlike a fish and goes through the gilled and tailed period, during which it is not unlike a tadpole, pa.s.ses beyond this period before leaving the sh.e.l.l and has already acquired its full turtle characters when first it steps upon the scene. So big an egg as this would be highly nutritious and animals would desire it immensely for food. Hence it becomes necessary for the turtle to securely hide her eggs. In order to do this, she scoops out a pit in the sand in which she deposits them and here they develop. If no further provisions were made the eggs of the turtle would dry completely and never hatch. Accordingly it becomes necessary for the turtle to enclose each egg in a tough, leathery membrane, known as the sh.e.l.l. Because the egg is thus encased it is necessary for it to be fertilized before being laid. Accordingly the male must place the sperm cells within the body of the female. These cells swim nearly to the top of the tubes in which they are placed, and there fertilize the descending eggs. Farther down the ca.n.a.l the sh.e.l.l is secreted about the now swollen ma.s.s of yolk and white, completing the egg just before it leaves the parent.
If the evolutionist understands properly the line of descent, the birds and mammals are both the descendants of the reptiles. While there is less exterior resemblance between a chicken and a turtle than between a cat and a turtle, the real relationship in the first case is much closer than in the second. This is perhaps most easily seen in the scaly legs of both bird and reptile. Another remarkable resemblance lies in the fact that in both cases the eggs are large, well stored with nourishment, and protected by a resistant sh.e.l.l.
So few people know the turtle"s egg that it will be better to describe that of the hen, which it largely resembles. Underneath the hard sh.e.l.l is a tough but flexible membrane which lies against the limey coating, except at the blunt end, where a separation between the two gives room for a bubble of air. Inside of this sh.e.l.l and its membrane lies the white of the egg, which is nourishment for the chick, though not nearly so rich as the yolk. This, besides the alb.u.men which it contains, is stored with large quant.i.ties of fat. It will be remembered that upon breaking a hen"s egg and dropping it into a bowl, the yolk holds together because it is enclosed in a delicate sac. As the yolk falls into the bowl there floats to the top of it a lighter yellow spot as big as the end of a lead pencil. This is all of the egg which thus far represents the chick itself. All the rest is nourishment. This disk already consists of three reasonably distinguishable layers of cells, which grow rapidly different from each other. They spread and bend and twist, forming the young chick and a set of organs which serve for its protection and maintenance during its embryonic life. Within a few days these accessory organs will have formed distinctly. Within the upper half of the yolk will be found the small developing chick, which for the first thirty-six hours of its development pa.s.ses through a stage not unlike the fish, or the earlier steps of the turtle. Within a few days it becomes clearly evident that this creature is to be a bird, though it is much longer before it is clearly a chick.