Adaptation for the Species
The strife, as we have described it thus far, is a purely selfish struggle. Every point gained is a point favorable to the welfare of the individual animal. But nature is uncommonly careless of the individual unless the advantage gained is also of use to the species as a whole. Very often the life of an animal ceases when provision has been made for its young. The male garden spider may have a long and dangerous courtship, in which the uncertain temper of his ladylove may lead her to bite off four or five of his eight legs. But her ingratitude is not yet complete. He may have barely accomplished his desperate purpose of fertilizing her eggs at all hazards, when she ends the process by eating him. The male bumblebee fertilizes the female in the late summer and then dies. She does not lay her eggs before the next season. So it happens that no bumblebee ever sees its own father, and no father bumblebee ever sees his own children. In the honey bee the male, which has been fortunate enough to fertilize the queen, pays for his honor by deathwithin the hour. Superfluous bachelors, among the honey bees, when the bridal season has passed, are driven from the hive to die of starvation.
An animal need not always be successful himself, but it is more essential that he hand down his successful traits to those who come after him. It is more important for the future generation that an animal should have had it in him to do great things, though he himself really have never done them, than that he should have learned to do great things on a meager original endowment. Not what an animal accomplishes is important to his children, but what he has it in him to accomplish. Accordingly Nature is full of devices by which those who have proved their original endowment by winning out in the struggle shall hand on this endowment to a subsequent generation. In other words, Nature is anxious that they may successfully mate. Here we are again on distinctly debatable ground. Darwin himself believed thoroughly in what he called sexual selection. It is the essence of this idea that the males and females have grown unlike, more technically have developed secondary sexual characters, through the choice of the mating pair. It would usually be the more serious loss if accident should come to the female, for she may carry fertilized eggs for some time. Hence, if both sexes may not become attractive, it is usually the male that develops fine colors, ornamental appendages or a captivating voice.
An interesting reversal of this process has taken place in civilized man. His more savage ancestor adorned himself more lavishly than he permitted his mate to do. With the advance of civilization man has undertaken to defend his own mate most valorously. The result is it is safe for her to be beautiful. Under these circumstances, however, it is more necessary to her welfare that her consort be vigorous rather than that he be handsome. Hence in the human species beauty has become the prerogative of the woman, and this is increasingly the case the higher the civilization. Whether woman suffrage and self-support will reverse this process remains to be seen. There are indications that point that way.
There are many biologists who are at present expressing serious doubt as to the validity of sexual selection. As in the previous cases of protective coloration, I believe it will be wise for us to retain, even though with an interrogation point behind it, the idea of sexual selection until such time as those who object to it have furnished us with another theory which will more nearly account for the observed facts. While entirely conscious of the possibility that there is a weak spot in the theory, we will still tentatively hold to sexual selection. The fact that beauty inwomen is so intensely attractive to man, and that vigor and manliness in man are so attractive to women, leads us to infer that among the lower animals, although of course in a vastly less degree, vigor and beauty are also attractive. The weakest point of the position lies in the fact that it probably presupposes a higher degree of capacity for appreciation on the part of lower animals than they possess. Those who deny the truth of the theory laugh at the idea that a butterfly can see clearly enough and care enough for what it sees to notice whether its mate has wings of one type or of another. The size, number and position of the spots on the wings of many butterflies are so nearly constant that they cannot of themselves have been entirely determined by the choice of the insect. Yet this may not preclude the possibility of the fact that, while the spots were produced through some other agency, certain types of them were selected by sexual preference.
If attractive coloration is effective anywhere in the animal world, it will possibly be found among the insects, but it is especially likely to be found among the birds. Very many field workers in these groups feel quite sure of the value of attractiveness. When butterflies chase each other up and down, circling and doubling, following each other for long distances, it would certainly seem as if they were pleased witheach other's appearance. Some naturalists, especially those who have worked chiefly in the laboratory, insist that it is the odor, not the color of these insects, which is attractive, and some experiments which have been made would seem to point in this direction. But the creatures experimented upon most carefully were night-flying moths, and it is quite possible that the sense of sight in the night-flying moths has lost its vigor.
The great difficulty in understanding sexual attraction in insects, as based upon beauty, lies in the undoubtedly lower development of their nervous activity; in other words, in the apparent absence of anything worth calling mind. I think no one imagines that a butterfly, looking upon two other butterflies who are competing for her affections, deliberates between them and determines to admit to the circle of her friendship the more brilliantly colored male. Moths are so irresistibly attracted to a light as to fly into it without apparent power to withstand its influence. They repeat the flight again and again until they are destroyed. If they react so vigorously to the stimulus of the light, it seems not impossible that they may also act vigorously to the stimulus of color pattern, and that the male most beautifully colored, according to the nervous ideal of the female, should win her unconscious regard. At least it is certainthat, in very many of the butterflies and moths, the attractive coloration is chiefly displayed when they are moving actively about; and when they alight and their enemies can the more easily capture them, they conceal their brilliant colorings. Most butterflies are very brilliant on the upper surface of the wings and very much duller on the under surface. Hence in flight they show their colorings exquisitely, but when they alight, and are thus more likely to be captured, they fold the brilliant surfaces together in an upright position. In this way not only is the dull side of the wings placed outward, but the wings themselves are placed edgewise to the sky, and it is from this direction that their enemies, the birds, are most likely to see them. Once upon the wing these creatures display their beauty with much greater safety because they can escape the birds very readily by use of their exceedingly jerky flight. The butterfly's motion is as irregular as any we have except the bat's. This eccentricity is one great element in their safety, and makes it less dangerous for them to display their attractive colorations.
One very large group of the night-flying moths have been named the "underwings," because of the fact that their hind wings are very much more brilliant than the front, and in lighting they fold the dull pair back over the bright, completely concealing them.These creatures are in the habit of resting in the daytime against walls, or stones, or the bark of trees. The similarity in color between their front wings, which alone show while sitting, and the background on which they rest, is most remarkable. One may pass them again and again, although they are of considerable size, and not notice them at all. Once let them display their hind wings and the brilliancy of their color always attracts immediate attention.
It is among birds, however, that brilliant coloration serves its most effective purpose. The birds are alert, exceedingly quick of sight, and are much more discriminating than insects in almost every respect. It is not so impossible that these creatures might even voluntarily prefer a distinctly more brilliant mate, though the voluntary character of the process is not essential to its success. Men certainly are constantly attracted to women for whose charm it would puzzle them to account. If this is true with regard to men, it is certainly probable that birds would be largely influenced by phases of attractiveness, of which they were observant, but unconscious.
Certain it is that in many birds the males are far more beautiful than the females. Perhaps the commonest illustration, and, at the same time, one of the best is found in the so-called red-wing or swamp blackbird. The male of this creature is a brilliantblack, excepting that upon the angle of the wing, spoken of roughly as his shoulder, though in reality it is equivalent to our wrist, there appears a splendid orange patch with a border of lemon yellow. When he folds his wing he pushes this colored angle of the wing so deftly under the feathers of his shoulder as almost to conceal it. When in flight the bird is exceedingly conspicuous, showing, with every bend and twist of his body, his gorgeous epaulets. Meanwhile, the female is likely to pass unnoticed. She is dull in color and streaked like the grass among which she lives. During the mating season the male hovers about her, swaying from side to side in such a way as certainly to make it appear as if he realized his good points and was bringing them to bear as effectively as he knew how. After his mate has nested and is rearing her young, it would appear that the male uses his brilliancy to lure the observing enemy away from the nest containing his wife and children.
Another illustration of the remarkable superiority of the male over the female, in many parts of the bird world, is seen in the case of the common barnyard fowl. The rooster is so much more gorgeous than the hen that anyone reasonably acquainted with these birds cannot have failed to notice the fact. In some of our modern varieties we have by breeding colored them nearly alike. The original chicken iscolored much like the common Leghorns. Shades of red and yellow decorate his neck and back, while the flight feathers of his wings and of his tail and the sickle feathers which ornament the rear of his back and hang over his tail are lustrous dark green. The hen meanwhile is very much less brilliant in her contrasts. I shall speak more fully of this in discussing polygamy.
The attraction of beauty is not the only lure by which a creature may win its mate. Sound may captivate as effectively as beauty. This is true of insects as well as of birds. Certain insects at least advise their mates of their presence by means of a sound which they emit. This is particularly noticeable among the group of straight-winged insects to which the grasshopper, katydid and cricket belong. The grasshopper has a ridge on the angle of his wing and a roughness on the side of his leg. When these two are rubbed together the result is sometimes a fiddling, sometimes a snapping or cracking sound, differing in different grasshoppers. I doubt not these sounds are pleasing to the female of the species, for they are always made by the male. The katydid, instead of fiddling in this way, has a sort of drum on the angle of his one wing, which he can rub over a tooth in the corresponding angle of his other wing, thus producing the familiar "katydid" sound. I have neversucceeded in making a dead grasshopper fiddle, but I have long known how to make a dead katydid say "ka." Quite recently I have added to my accomplishment in this respect and can make it say "katy." The "did" part of the song still lies beyond my power. The crickets produce their sharp notes in much the same fashion as the katydids.
One observer of the chirping of the cricket says that the pitch of the song varies with the temperature. He has even worked out a formula by which one can tell the pitch of the chirp, if he knows the temperature, or, knowing the temperature, can determine the pitch. Of course this is too mechanical; yet it indicates that there must be considerable relation between the two; the warmer the cricket the happier he is.
It is the males among insects that chirp their love songs. The females never answer them. There is a peculiar notion that the female katydid, when thus accused of some offense, replies "katy didn't." The truth of the matter is that no female katydid ever replied to the accusations of her lover, if accusation it be. She is absolutely dumb, not having the drum upon her wings with which to reply. She is provided with ears wherewith to hear, and, strange to say, she keeps them on her elbow, as does also the cricket,while the grasshopper has his ears upon the side of his body.
Everyone who lives in the country, or goes into the country in the summertime, is sure to know the humming of the so-called locust. It is an unfortunate fact that the word locust may have several meanings. It is properly applied to one group of the grasshoppers. The creature most commonly called a locust is a cicada, or harvest fly. When the weather gets quite warm the cicada starts his love song. He has two long flaps to his vest, and under each flap he has a vibrating drum head. This is set shivering by a muscle on its under side. The female cicada again is silent.
It is among birds that the love song reaches its finest development. It may consist simply of a little chirp as in the chippy. It may consist of two notes of a different pitch repeated steadily, as in the tufted titmouse. It may attain considerable variation, as in the robin. But in the choir of our best singers, like the catbird, thrasher, and mocking bird, there is unending variation of notes. It seems almost impossible to doubt the charming quality of this voice upon the mate. It certainly is chiefly confined to the mating season, and is indulged in almost entirely by the males. This does not mean that a male does not sing excepting when he wishes to charm his mate. Butthe time when he is in his most exquisite feather and most charming mood is the time when he sings most sweetly, and this is the time when he is taking to himself a mate. The love joy may so overcrowd his life that he sings much and often, but the increase in its amount and character during the mating season seems to proclaim its purpose beyond a doubt.
In addition to the allurements above described there are certain peculiar behaviors of the animal during the mating season which are intensely interesting. Sometimes they consist simply of a wild delirium of joy, which overpowers the animal completely and makes him do wonderful things. Birds will fly with impetuous leaps in the air, mount higher and higher, singing wildly, only to turn suddenly at the top of the flight and drop promptly to the ground. I have seen such ecstatic flights in the oven bird and in our rollicking gold finch. I have seen a catbird on his way to a tree turn three somersaults, much like those performed by a tumbler pigeon, after which he alighted upon the bough. None of these acts seemed deliberately performed in front of the females, but I have seen three or four killdeer parading in most stately and precise manner, spreading their wings and fluffing their feathers, performing a sublimated cup-and-cake walk amid a circle of attracted females.
Even our little English sparrow, as I have previously mentioned, fluffs himself up and spreads his wings and prances around in front of his presumably adoring ladylove. But the weirdest performance of this sort I have ever seen is that shown by the male ostrich. When he becomes excited, swaying his body from side to side, he sinks slowly upon his knees, until his body touches the ground, his wings spread on either side and the feathers fluffed up so as to show every exquisite plume in all its splendid beauty. The long neck is laid back until the head, which is doubled sharply forward, is pressed almost against the back, and in this strange position he sways from side to side, apparently utterly oblivious, for a time, of everything. After about a minute of this performance, he seems slowly to come to himself and rise again to his feet. Now he is particularly likely to make vicious attack upon anything within reach.
It is not only necessary that the animal should be able to attract a mate. There may be more than one claimant for the damsel's affection. In many animals we see provisions whereby the male may effectively deal with his rivals. This is especially likely to be the case if the animal be a polygamist. In every species there are produced about as many males as females. If the polygamous habit leads one male to gather about him a group of females, with whom he mates, it is evident that he is displacing an equal number of rivals, and they are not willingly displaced. Accordingly we find that polygamy is usually accompanied by a belligerent disposition on the part of the males. In our ordinary barnyard fowl this trait is very evident. The rooster not only domineers over the hens, not only struts about among them in stately fashion and gives vent to his feelings by his sonorous voice, he must also drive away from the neighborhood any rivals for the affections of his wives. Hence the rooster attacks upon sight the neighboring rooster, and battles with him to his entire discomfiture and sometimes to the death.
Among the members of the deer family this particular phase of the relation between the sexes has produced in the males, and only very rarely in the females, the magnificent branching horns. These are intended not so much as a protection against the enemy as for an offensive weapon in the battle for the mates.
Beautiful and stately as are these magnificent horns, they last only for a part of the year. We begin to understand their meaning. When the wolf is hungriest, toward the close of the bitter winter, the deer is without horns. When the time for mating comes, the deer within a few weeks grows his horns, which at first are covered with a plushlike coating, known as velvet. After a while this dries and he rubs his hornsagainst the trees until they are clean and smooth. Now he is ready for the battle royal.
In the case of the fur seals polygamy has carried its specialization of the males to a remarkable extent. The bull seals are several times as large as the cows, and are provided with terrific canine teeth. With these they battle with a violence that very often results in the death of one of the combatants. A successful bull seal who has gathered about him a cluster of seal cows is seamed and scarred with the marks of his annual combats.
One more type of adaptation can be profitably considered. Animals have developed many devices which serve for the protection of their young. The wonderful silk spun by the spider was evidently primarily intended to serve as a covering for the eggs. Probably all of our spiders agree in using the silk for this purpose. Many of them employ it for practically no other, though there are half a dozen different uses to which different spiders may put their silk. Under these conditions we have a right to infer that silk was primarily developed as a coating for the eggs. In the case of some of our spiders a little fluffy mass of silk covers the egg, while a firmly woven sheet of silk covers both egg mass and fluff, holding it flat against a wall or the trunk of a tree. In some of the higher spiders, notably our bank spiders, the silken coveringbecomes an effective cocoon, spherical in shape, with a little opening at the top like the neck of a small bottle. The egg cocoon is woven in a mass of tangled silk between the branches of some tough weed which will be sure to outlast the winter. Into the egg cocoon the spider may place one thousand or more eggs. Having thus provided her children with a snug winter home, the spider dies. When spring comes with the warm rays of the sun, the eggs hatch and the cocoon becomes a creeping mass of minute spiders. At the time these spiders appear there is nothing for them to eat. The obvious way out of this difficulty is taken. At once there begins a progressive party. Spider fights with spider, and the prize in each conflict is the body of the victim, which is promptly eaten. The winners in the first round pair off again, and a little later, as hunger drives them, another set of combats comes on, resulting in another halving of the number of spiders in the cocoon. This process continues until not more than one-tenth of the original number of spiders remains. By this time they have gained sufficient strength of leg and jaw, and sufficient dexterity in the use of both, to make it safe for them to venture out and try their fortunes among the accidents of a strenuous world. There can be little doubt after this long process has worked its final results which tenth remains. Chance plays but small part in thisgame. It is the fittest that survive. When this procedure goes on generation after generation, the result must necessarily be that the spiders grow fitter and fitter for their work. This method is hard on the little spider, but it makes good spiders.
Most insects die before their eggs hatch; accordingly they can pay no attention to their own children. Whatever arrangements are provided for the safety and strength of these offspring must be provided before they appear. About the only care the majority of insects take in this direction is to see that the eggs are placed where the young shall find food as soon as they emerge. Insects' eggs are very small, and as a consequence the creatures which emerge from them are likewise exceedingly minute. As a result they cannot be expected to hunt far for their food. Different insects use different devices by which to overcome this difficulty. The katydid, for instance, must die with the approach of fall. Her children will not appear until the following year. Her food consists of leaves, but to lay the eggs in such a situation would be a fatal process, because the leaf will drop off before the eggs hatch. Accordingly, the katydid lays its shield-shaped eggs in a double row near the end of a young twig. Next year when the weather is sufficiently warm to hatch katydids, it is also warm enough to force the buds on the end of the twigs.When the katydids arrive their jaws are young and tender, but so are the leaves upon which they are born. Hence there is little difficulty on the part of the young katydids in finding an abundance of food. By the time the leaves have grown tougher, the katydid's jaws are stronger, and the leaves will still serve as food.
Everyone who is at all familiar with country life and gardening is familiar with what is called the potato or tomato worm. It is a long, green, smooth, caterpillar, as long and as fat as your finger and provided with a horn upon his tail. The gardener may not know that after a while this creature will burrow into the ground, and there change into an oblong brown mass with a sort of a pitcher handle at one side. Next year this pupa will split down the back, and from out of the brown case will come a hawk-moth, which soon will fly with rapidly quivering wings and feast upon the nectar of our moon flowers or on that of the "Jimson" weed. Those who have cleaned these pests from the potato or tomato vines will often have noticed one of them covered with what look almost like grains of rice. This appearance reveals an interesting story. Some time earlier an insect that looked very much like a dainty wasp with a rather long sting in its tail hovered over the caterpillar. This is the ichneumon fly. Eventually lighting upon thecaterpillar's back, it punctured the skin with its sting, and deposited eggs within the caterpillar's body. These eggs soon hatched and the little grubs worked their way through the body of its host. The infested victim feeds upon leaves and fills itself with rich food. These parasites eat the food, and, try as it may, the caterpillar does not succeed in getting fat. After the grubs have gotten their full growth, each of them eats its way through a little hole to the outside of the caterpillar's body. Here it spins around itself a little white case, and looks like a rice grain. As the caterpillar moves about, these seeming rice grains are rubbed off and fall to the ground. Next year there will come up new ichneumon flies to sting fresh caterpillars and repeat the entire process.
Another remarkable provision for the young on the part of insects is seen in the behavior of the big sphex wasp, known as the cicada killer. The cicada, it will be remembered, is what is commonly called a locust. The cicada killer is a magnificent big wasp, whose body is nearly an inch long, banded with black and yellow, while the wings are colored a smoky brown. This muscular wasp digs a long tunnel eight or ten inches deep, which ends in a slightly larger room. Having provided the location, he now sallies forth in search of the cicada. The heavy song of the male probably serves as a guide to the wasp in caseof scarcity of cicadas, but the killer has apparently little difficulty in finding his prey. The wasp pounces upon the insect, and in spite of its strength and the thrashing of its vigorous wings punctures it with his sting again and again. The poison of the sting entering into the nerve centers gradually paralyzes, but usually does not kill, the cicada. Now the killer carries its prey home, pushes it to the bottom of the tunnel and deposits upon it a single egg. The wasp closes up the hole and leaves the place. When the egg hatches and the grub of the wasp emerges, it finds a big cicada just at hand, upon which it feeds. By the time the cicada is completely devoured, the wasp grub has obtained its full growth. After a short period of development a new sphex wasp is ready to work its way out of the tunnel, find a mate, dig a hole, and safely provide for its own children.
Still more remarkable adaptations for the care of the young appear among the birds. Here the eggs are not to be deserted, but are to be cared for until the young appear. These again must have attention until such time as they are quite able to take care of themselves. The birds are warm-blooded animals, and even their young, while they are developing in the egg, are warm-blooded. Consequently the temperature of the egg must be maintained evenly and uniformly, or there will be no development.
The fish may drop its eggs carelessly upon the bottom of the stream. A frog may deposit them in a mass of jelly and leave them forever. A turtle may bury its eggs in a sand bank and abandon them to their fate. The warm blood of the young bird demands more attention than this. Accordingly, the parent bird has learned to make for itself some sort of nest, in which the young may be kept properly warm until they are developed. The ancestral bird, who was to be the progenitor of the entire bird class, must have had some very simple method of providing a place in which its eggs might be hatched. As the descendants of this original bird have passed into new situations, the various lines have taken upon themselves different shapes until we have the multiform birds of to-day. The habits of the birds have also varied. Each has adapted itself to the situation in which it found itself, and no adaptation has been more varied and effective than the adjustment of the nesting site. Nests are found upon the ground, in the bushes, on the lower limbs, in the crotches of the trees, in the trunks of the trees, upon their very summits, and on the tops of inaccessible crags. To every sort of situation some bird has been enabled to adapt itself. This has made it possible for very many more birds to thrive than could have found a place in the world, had they all lived upon the same plan.
In the case of the bank swallow his nest may be a very simple contrivance, consisting only of a tunnel running back into a bank, and widening at the back. Some material that will soften the bed upon which eggs are to be laid must be placed in this cavity. The whole home is a very simple and crude affair. But little better is the arrangement which the woodpecker calls a home. This has been cut into the dry wood of a defective tree. No woodpecker can make his home in absolutely solid sapwood. Hence the first labor of the woodpecker must consist in finding a place in which it can dig. If there is an old stump of a limb sticking up, the problem is readily solved. Such wood has no sap in it, and is brittle enough to be easily dug out. But, if there be no such stub, the woodpecker will find a suitable place in most trees. At some time or other almost every tree loses a big limb. When such accident occurs there will always be in the old trunk a region through which sap once went to this limb. This region, deprived of its function, goes completely dry, like the heartwood of the tree, and it is into such material as this that the woodpecker succeeds in drilling his well-protected home.
As birds rise higher in the scale the nest-building becomes a more complicated affair, and after a while we find a well-woven substantial nest, through which even the air will not chill the eggs enough to preventtheir hatching, while the warmth is supplied by the mother's body. It is often a matter of surprise to many people that a bird should contrive to build a nest so exquisitely circular. The trick, after all, is not quite so difficult as it looks. The robin gathers up a few sticks and places them as the beginning of the platform. More and more are brought and woven into each other, making a framework altogether too big for the nest. Then mud is brought and plastered inside of this. With the plastering of this mud the careful circularity of the work begins. Every time a little material has been added the robin sits down in the nest and revolves her body, in this way shaping the interior much as the potter shapes a pot. In the case of the artisan, it is the pot that revolves. In the case of the robin, the bird itself revolves. The effect is the same in both cases—a circular vessel is produced. A little lining added to the interior of the nest softens it for the reception of the eggs. In this exquisite home the robin lays her eggs, and sits upon them until they are developed enough to hatch, and then feeds the young until they are old enough to feed themselves.
Far more remarkable than any of the devices thus far described are the wonderful developments which have come in the class of animals known as the mammals. Here the most wonderful protection is madefor the care and feeding of the young. But this is to be the subject of a separate chapter.
As long as we thought of each sort of animal as being a separate species shaped in the beginning by the hands of the Creator, each of these devices seemed to us a new manifestation of the Divine Providence, whose fertile planning had conceived so many methods of providing for his children. Unconsciously we thought of God acting as man acted. Each animal seemed a purely separate invention purposely designed for an especial place. Now we understand the plan in creation better, and see that each animal has come from another not quite like itself, some distance back, and this from still another. Our admiration for these devices as they arise through evolution is no less, but takes on another form.
Life in the Past
Anyone who earnestly studies plants and animals as they exist in the world to-day cannot help wondering how the earth began and where it got its life. This is the true end and aim of geological study. The history of man seems to run back into a far distant and gloomy past. Except for the poetical account in Genesis and the traditions of various peoples throughout the world, real history fades away into an earlier time of which there are no written records. When the delvers in the Mesopotamian plain talk to us of kingdoms running back through seven or eight or nine thousand years, we seem to be getting back to the beginnings of things. But seven or eight or nine thousand years are as nothing in comparison with the age of the earth, which runs back into a past so limitless that no man can safely assign any set figure to it. In a recent paper, Dr. Walcott, of the Smithsonian Institution, says that the antiquity of the earth must be measured not in millions, for they are too short, nor hundreds of millions, for this carries us toofar, but must surely be measured in tens of millions of years.
When we attempt to study the past we find its various epochs unequally clear to us. In human history only quite modern times are absolutely clear. The history of the Middle Ages is distinct enough for us to build for ourselves a picture of the time with reasonable hope of gaining a correct view of the state of affairs. Back of this comes the long stretch of the Dark Ages, in which here and there we have bright spots, but it will perhaps long be impossible to portray clearly the life of the people. Getting back to the Romans, things once more become reasonably plain, as is true also in the case of Greek history. Back of this stretches the Egyptian with fair precision, and, older than it, the Babylonian and Chaldean. But these past three have not left nearly so definite an account for us as did the later civilizations of Greece and Rome.
When we try to go back of these we must change our method of study entirely. Writing is absent, and all we know of earlier men must be inferred from a few pictures that were daubed on the rocks or carved in ivory or bone, from tools made of stone or bone, from a few metal or stone ornaments, or from the bones of the men themselves. Even so, the history fades out without telling us its own beginnings. Itis quite as impossible for history to write its origins as it is for man, from his own knowledge, to describe his birth.
What is true of the human story is quite as true of that of the earth. Recent steps are very plain. We may read them with considerable confidence. As we go deeper into the rocks and find older fossils, the evidence becomes less certain. The animals differed enough from those of to-day for us to be less sure what they were like. As we keep on moving backward through time, and downward through the rocks, we find, after a while, strata in which there are evidences of life that existed long ago, but in which these traces are so altered that it is impossible to tell what sort of living things existed; we learn only that they were alive. Going back still further, these fade out. There is no knowing when the earth began; there is no knowing when life began upon the earth. It is not meant that men have not wondered, even reckoned carefully, as to how long ago each of these events occurred. Many speculations have proved entirely useless, a few remain as yet neither confirmed nor disproved, and of such we shall speak.
For the last hundred years the theory of the earth's origin suggested by the Marquis Pierre Simon De La Place, of France, near the end of the eighteenth century, has held almost undisputed sway among menwho were willing to consider the question as open to human solution. This theory is known as La Place's Nebular Hypothesis. When men began to study the heavenly bodies with the newly invented telescope, new ideas naturally sprang up. Among the objects which the glass disclosed were the nebulæ, which are great clouds of fire mist, glowing masses of gas. They are scarcely visible to the naked eye, but are among the most interesting objects in the heavens when seen through a telescope. The other suggestive heavenly body was our sister planet, Saturn. Besides having a full complement of moons, Saturn has around it, as distant as we would expect moons to be, three great rings. These look very much as if one's hat, with an enormously wide brim, should have the connection between the rim and the hat broken out completely, but the rim should still float around the hat without touching it and should steadily revolve as it stood there. The rings of Saturn are not solid like the suggested hat rim. They are evidently made up of a great number of very small particles, each moving around the center of Saturn. But the great cloud of them is spread out flat. At the distance which Saturn is from the earth they look as if they made a solid sheet. Furthermore, they do not form, as it were, one continuous hat rim, but it is as if the rim were broken into three circular sections, each bigger thanthe one inside it and separated from the next by an area nearly as wide as the ring itself.
With such material in the heavens to guide him, La Place suggested that the sun had once been an enormous fire mist scattered over an area billions of miles in diameter. This gaseous material, by the attraction of its particles for each other, began to condense and contract. When the plug is pulled from a washbasin the particles of water, in moving toward the center, in order to get out of the basin, invariably set up a rotary motion. As the particles of this diffused nebula began to gather together they, too, gave to the mass a rotary movement. This grew more and more rapid, with greater contraction, until the particles on the outer edge of the rotating mass had just so much speed that the least bit more would make them tend to fly off as mud would fly from a revolving wheel. When this point was reached there was a balance of forces which made the outermost portion remain as a ring while the rest contracted away from it, leaving it behind.
It was La Place's idea that this process had repeated itself, and ring after ring had been left behind. Finally the sun condensed and grew into a ball, occupying the center of the system. At varying distances from it were to be found either rings or planets which had been formed out of such rings. For LaPlace suggested that in a ring like this the material could not be quite evenly distributed. While every particle in the ring kept revolving around the sun, those in front of the densest part were slowly held back by the attraction of the thicker portion, while those behind it in rotation had their speed hastened until finally all the material in the ring had collected at one spot and a new planet was born. La Place believed that these planets formed their moons in exactly the same way, and that Saturn was simply a planet not all of whose moons had yet been formed. He believed that this happy accident served to tell us how the universe had been created.
Of course, so detailed a theory concerning anything of which we know so little has always had much ridicule thrown upon it, and yet no truly competing theory has been proposed until very recent times.
Within a few years a Planetesimal Theory has been announced, and is gaining considerable prominence, although it is too early yet to say whether it will supersede La Place's idea. In this theory, also, the suggestion comes from the heavenly bodies. With the increasing study of the nebulæ, many forms of these interesting bodies have been discovered. A very common type consists of a great coherent central mass, with two or more arms extending from opposite sides in the form of a spiral. This is as if gaseous revolving nebulæ had come into comparatively close proximity to a passing body. The visitor, by its attraction, drew from the nebula a wisp of gas. The revolving motion of the nebula gave to the attracted arm the spiral form.
These twisted arms are not equally dense throughout, but have thickened knots here and there in their course. The Planetesimal Theory suggests that these thickened knots are embryo planets and the central portion of the nebulæ an embryo sun. After all the material in such a body has condensed either around the knots or about the central mass a new solar system will be complete. As before stated, neither of these theories can be said to be demonstrated. Each of them has points in its favor and each has its difficulties. It is pleasant to know what men have clearly thought concerning such questions, but for a man not a trained geologist neither will carry much conviction. He will still rest with his own early conclusion that whichever shall prove to be true, for him his old formula is still valid, "in the beginning God made the heavens and the earth." He will no longer think of God as having shaped the balls with his own hand and thrown them into space; he will no longer dream that it all occurred within a week not more than six thousand years ago; but still to him will come the reverent conviction that, whatever the plan by whichit was accomplished, it was still God's plan and God carried it out.
Now that we have tried to stretch our imagination back to the origin of our globe, the question not unnaturally comes to our mind, how long ago did all this happen? Is there any possible means of telling when the history of the earth began? All such attempts lead either to indefinite or to uncertain conclusions. Each man who essays the problem approaches it from a different side and ends with a different result. But no matter what the method of approach, all are agreed on at least one point, the enormous length of time, as counted in years, through which the earth has lasted.
One great mathematician worked on the basis of the rate of the present cooling of the earth. Counting backward to the time when the earth's surface must have been hotter, according to La Place's idea, he decided that our globe has been cool enough for the existence of life upon it for a period of somewhere in the neighborhood of one hundred million years. Those who try to study the rate at which mud is being deposited in our bays and at the mouth of our rivers, and who hence try to deduce how long it has taken to produce the thickness of all the stratified rock we know, arrive at a figure larger, rather than smaller, than that mentioned above. The same is true of those who try to count the age of the earth by the rate atwhich the present rivers are carrying away their river basins, and hence who calculate how long it has taken the rivers of the globe to wash away all the rocks which it is quite clear have been carried out. Still others have attempted to solve the problem by seeing how much salt the rivers are carrying into the sea, and consequently how long it must have taken the sea to become as salt as it is. A very late attempt has been based on the alteration in the minerals that show radio-activity. Conservative estimates, based on all of these, would give us a figure on which we must not count with any exactness, but which will serve at least to mark the present trend of opinion. We may put this figure at one hundred millions of years.
The followingtablegives us the names of the periods into which the geologist has divided the past history of the earth. The first column gives a simple name, which, in each case, is a translation of the technical name the geologist gives to the era. This technical name is also given in parenthesis. The second column shows the number of years ago at which this period may be placed, while the third column gives a series of names most of which are in use in geology and which are intended to indicate the stage of advancement of the higher animals in that particular period. Some of these names are perhaps giving way to later terms, but all of them will be understood byany geologist. Most of them will serve to keep very clearly before the mind of the ungeological the period which he is studying. Like all such tables, this must be read from the bottom up. This arrangement is used because the oldest rocks in the series are naturally at the bottom and the newest rocks are on the top, though occasionally a region is sufficiently upset partly to reverse the order.
TABLE OF GEOLOGICAL TIMESERASMILLIONS OF YEARS AGO(VERY UNCERTAIN)STAGES OF ANIMAL DEVELOPMENTRecent Life (Cenozoic)0 to 5Age of Man (Quaternary)Age of Mammals (Tertiary)Middle Life (Mesozoic)5 to 10Age of ReptilesAncient Life (Palæozoic)10 to 25Age of Amphibians (Carboniferous)Age of Fishes (Devonian)Age of Invertebrates (Silurian and Cambrian)Dawn Life (Eozoic)25 to 50Earliest Animals and Plants
TABLE OF GEOLOGICAL TIMES
Having seen what the scientist supposes to be the method of formation of the earth itself, it will beinteresting next to consider what the biologist surmises as to the origin of the life upon the earth. Here again two explanations hold. The one, and distinctly the older of the two, says that at some time in the far distant past, under conditions which are rarely if ever duplicated, out of the lifeless material of the globe were produced simple and low forms of life. These could not properly be called either animal or plant, but partook somewhat of the nature of both. Of this there is at present no evidence whatever. The only reason we have for suggesting it is that, if we understand the past conditions on the earth, there was a time when life was impossible. Now we find life. Hence it must have arisen. This of itself, of course, furnishes no proof, but leads us to try to imagine how the transition might have come about. Every scientist who believes in this form of origin holds that if the exact conditions are repeated the result will occur once more. He may believe that no such repetition is possible, but he is confident that, if it could be, life would arise again from lifeless matter.
This process of life arising from matter that is not alive is known as Spontaneous Generation. Two hundred years ago it was supposed to occur frequently. It was common belief that the beautiful pickerel weed which borders our Northern lakes, after freezing, went into a sort of protoplasmic slime out of which pickerel were produced. The eelgrass of the river was supposed to yield eels in a similar fashion. The dead bodies of animals were supposed to turn into maggots. Such crude ideas of spontaneous generation are no longer possible. The whole science of bacteriology absolutely presupposes the impossibility of spontaneous generation in the flasks and test tubes of the laboratory. One or two men of otherwise good standing in science still maintain that they are getting new life in their own test tubes, but they fail utterly to persuade the scientific world. I think it is a fair statement of the position of science to-day to say that there is no evidence whatever of spontaneous generation, excepting the presence of life upon the globe.
Not all has been said, however, on this question. The chemist is learning in the laboratory to produce many substances which, until very recent times, were produced only in the bodies of animals or plants. Dye-stuffs were originally gotten almost entirely from animal or plant material. At present the great majority of them are made in the laboratory, and in not a few cases they not only imitate the color of the older material, but actually have identically the same composition and constitution. The laboratory-made material is exactly like that made by the animals or the plants.
The same is true with regard to a large number ofthe fruit flavors. These are due to the presence of ethereal oils in the plant, and their exact counterparts can now be produced in the laboratory, and can serve every purpose of the fruit flavor itself. Alcohol has been produced artificially, and alcohols, which nature never dreamed of making, so far as we can tell, but which are made on her plan, are manufactured by the chemist. Last of all, sugar has recently been built up by the chemist, though the method at present is so expensive that it cannot possibly compete with the production of the commodity from the cane and the beet. As in the case of alcohol, all the sugars that nature makes can now be made artificially, and others of the same general plan which she seems not to have as yet devised can be produced within the laboratory.
Attempts have been made to manufacture proteids, but these have as yet eluded the efforts of the chemist. He is beginning, however, to come nearer understanding their composition, and when he once clearly comprehends that he may be able to reproduce them.
One of the German chemists is convinced that the nuclein in the nucleus of the cell is not a very complicated compound. Under such conditions it is not a matter of surprise that the physiological chemist should be constantly dreaming that he may at some time produce living matter in the laboratory. To the ordinary mind it scarcely seems possible. We are soentirely sure that life is not amenable to physics or chemistry that we can hardly conceive of the possibility of its originating out of matter in the test tube. If it does so come, and when it does so come, this will not prove that life is a less noble and less wonderful thing than we thought. It will only prove that chemistry and physics are more noble and more wonderful than we dreamed.
There is another way of approaching this life problem, though it seems to be rather a begging of the question than a solution of it. Of recent years it has been discovered that even the very low temperatures obtained by evaporating liquid air, say three hundred degrees below zero, Fahrenheit, do not kill seeds or spores of mold. The space between the planets is undoubtedly extremely cold. We have always supposed it to be entirely too cold for life to exist in it. But we laid little stress on the fact because we had no thought of any possible life existing there. But the discovery that seeds and spores can live uninjured through extreme cold has led to an interesting suggestion. This is that when the earth became adapted to the presence of life it was infected by germs transported on meteors from some other system. According to this theory, organic dust through space is ready to infect any planet which offers the conditions under which life may arise. Of course this theory does notexplain the origin of life. It pushes back that origin a little farther or supposes that life is as old as matter itself. Again we may leave to the scientist the discussion and the elaboration of this or any other theory he may promulgate concerning the origin of life. When he has established clearly the process and can produce life we will accept his explanation; meanwhile, we will always be interested in his attempts to solve the problem, but still our simple formula, "in the beginning God," serves our present needs and will satisfy us better than any as yet unverified hypothesis.
When we find through scientific investigation how life arises we will simply know how God created it in the beginning.
The next step in the understanding of early life is to study under the microscope the simplest forms which we can find in existence to-day. This, while far easier of execution than the problems which we have thus far considered, is still not without serious difficulties. But every day brings us nearer to the understanding of the structure of living things. Life the scientist cannot see. All he can study is living matter. Whether life can exist separate from living things is a problem outside the range of his, at least present, possibilities. Therefore, concerning it he has no answer whatever to give. But when we come to study living things we find that all life is associatedwith protoplasm. This apparently foamy, jellylike, transparent material is the only living substance in all the world. Animals and plants are larger or smaller collections of the little masses of protoplasm which we know as cells. The lowest animals are each made up of but a single cell. This consists of a small mass of protoplasm surrounded almost always by a thicker skin or covering, known as the cell wall and enclosing a complicated kernel known as the nucleus. The protoplasm seems to be the living substance itself. The cell wall is not a simple dead scum on the outside of the protoplasm, but is itself able to do certain things which can only, so far as we know, be done by living substances. For instance, of two materials dissolved in the water in which the cell floats, the wall may permit one to soak into the animal and keep the other out. The one allowed to enter will usually be found good to be used for food by the cell. The nucleus seems to store within itself the record of its past history and thus enable the cell to do in the future what its ancestors did in the past.
Such simple cells can exhibit in very low form all the activities the higher animals show in much more elaborate development. A one-celled animal can move about, can recognize the proximity of food, can engulf its food and digest it, can build up its own substance out of the digested food, can absorb oxygen,can use this oxygen in the burning of its own substance to produce its own activities, can act in response to sensation gained from outside, can throw off its waste matter produced by its own activities, and can grow. When the proper time comes its nucleus can split in two, the cell itself enclosing the nucleus can separate into two cells, each of which can grow to the size of the parent cell and repeat its life. This is as simple an animal as we have yet discovered. Every kitchen drain swarms with such creatures. On a summer day the stagnant pools are full of them. The simplest microscope will show them clearly. This is life in its lowest terms with which we are acquainted. With such life, it seems to us, the animal and plant world must have started their existence, when first the earth began to teem with living matter.
If, then, we may form any judgment concerning the first living things upon the globe by considering the simplest creatures that live here to-day, certain facts seem clear. In the first place, life began in the water, and for a long time was only to be found in the water. Single cells are so small and dry out so easily that it is necessary to their existence that they should be kept entirely moist by the presence of water all about them. It is true many of them will stand drying, but while they are thus dried they can scarcely be said to be much more than just alive. They are utterly inactive, or, as we say, they are dormant. In such conditions they become covered with a tough skin, almost a shell, and their protoplasm is itself nearly dry. Under these circumstances the life processes hardly continue at all. The protozoa, as these small animals are called, tolerate drought for a time; but they only live, in any sense worth calling living, when water is abundant and is neither very warm nor very cold. It is safe to say that the early life of the world formed in the oceans of the time. So absolutely is the habit fixed upon cells of protoplasm that even to-day the activities of the cells of higher animals depend upon the presence of moisture. The cells of our own bodies are to-day living, as it were, in an ocean. Everyone can remember far enough back to recall some time at which a tear slipped from his own eye onto his own tongue; we know our tears are salt. The tongue has tasted, undoubtedly, the perspiration from the lip on more than one summer day; this perspiration tasted as salt as the tear itself. The lymph that constitutes the "water" of a so-called "water blister" is also salty, and even the little blood one gets into his mouth in trying nature's method of stanching the flow from a cut finger gives the impression that it contains a little salt. Every fluid of the body is salty, and every cell of the body is bathed in salt water. It is too long since the ancestors of our cells swam in the seas of theEozoic time for us to assert with any positiveness that the ancestral habit is responsible for this trait in the descendants. Sure it is that to-day our cells, like their ancestors of old, live in water, and this water is slightly salty—as were probably the Archæan seas.
The geologist tries as best he may to build up the geography of the earth in the past. He endeavors to judge from the rocks as he now finds them, where the seas, the bays, the dry land, and the mountains of earlier geological times lay. The present aspect of the earth is very recent, and earlier ages must have shown an entirely different distribution of land and water. The North American continent was certainly very much smaller than it is now. The first known lands lay close to the Atlantic seaboard and probably extended out into the water some distance beyond the present shoreline. The stretch of continent was narrow, and grew narrower as it went southward. In what is now the Canadian district, a considerable expanse probably existed in very early times. Then a great internal sea, shallower than the Atlantic, stretched its unbroken sheet over almost the entire area now occupied by the United States, while only a comparatively small hump of earth, ending in a narrower strip, lay where the great Western plateau now rears its enormous bulk.
A large portion of the history of the North American continent, with its developing animals and plants, is tied up with the gradual shrinkage of this interior sea. Slowly across the Canadian district, the Eastern and Western lands became connected with each other, while the waters progressively were pushed down the continent, which was steadily growing from the east and from the north, though less slowly from the west, into this internal sea. To-day only the Gulf of Mexico remains as evidence of the broad stretch that once extended through to the Arctic Ocean and west beyond the present position of the Rocky Mountains.
How this great Eastern backbone of the continent was produced, what sort of animals lived while these rocks were being formed, or whether this preceded entirely the existence of life upon the earth, no man to-day may surely say. In the oldest of the rocks there are beds of graphite, from which lead pencils are made. This substance is believed by the geologists to be, like coal, the remains of vegetable life. But these early rocks have been so heated and baked, so twisted and bent, that whatever forms of life they once held are now obliterated, or so altered as to give us no idea of what may have been their character.
So far as anyone can now see, this past history is wiped out forever and it will be impossible for men ever to demonstrate the character of this early life. Speculations, more or less certain, will arise. Theymay, after a while, seem so clear as to receive the acceptance of the scientific mind. Yet the truth remains that the early history of the earth, so far as animals and plants are concerned, is probably lost forever.
The most striking feature concerning the earliest layers of rocks in which good fossils are found abundantly is the complexity of the life. With the exception of the backboned animals, every important branch of the animal kingdom is represented, and it is just possible that we have even earlier forms of the vertebrates themselves. This, to the evolutionist, is very disconcerting. To find the great groups all well developed at least twenty-five million years ago and to find only fossils built on the same lines since almost nonplusses him. When the geologist tells him what an enormous length of time preceded the rocks in which he finds these fossils and how absolutely these earlier strata have been altered by the later geological activities he easily understands why it is impossible to find fossils in them. As a consequence, the evolutionist is forced to believe that all the earliest animals have left no clear traces behind them. Life as he first surely knows it is already extremely varied and quite well developed in some of its groups. The early animals were as well adapted to the times in which they lived as are the great majority of the animals of to-day. The reader must not infer this to mean that the animals of those days were like our present animals. They were not. No one traveling in a far country could find there animals as strange to him as would be those of the earlier stratified rocks. In these there were no fishes as we know them to-day, not a single member of the frog and salamander class, not a reptile, not a bird, not a mammal, and probably no air-living insects. It is highly doubtful whether there was any animal living upon the land and breathing the air twenty-five million years ago.
We start our study, then, at the period known as the Palæozoic era, the era of the ancient life of the globe, beginning twenty-five million and ending ten million years ago. The first of the three sections into which this period of life is divided is known as the Silurian age, the age of invertebrates. The word invertebrate is an unscientific but convenient term under which we embrace all the animals below those having backbones. This period is called the age of invertebrates because, although there is an enormous wealth of animal and plant life in the Silurian, there are no backboned animals except the lowest kinds of fishes. It was supposed for a long time that even fishes were absent. Now we know they existed, but they were small and inconspicuous. In this period corals were wonderfully abundant, particularly in the great internal sea which spread over what is now known as the Mississippi Valley. Everywhere over this region must have grown in the shallow water great numbers of creatures called crinoids or stone lilies. They were attached to the bottom by slender stems, sometimes many feet long. These stems are jointed, and when they became fossilized the sections were apt to separate, with the result that over a wide area in the Mississippi Valley it is very common to find these little segments which look not unlike checkers. At the end of the stem was a rounded head, with a mouth at the top, and around the mouth were branched, feathery arms. The creatures must have been exquisitely beautiful, but they have completely disappeared from the face of the earth, with the exception of a very few, found in the obscurity of the almost fathomless depths of the great ocean. Here they remain as peculiar relics, only preserved by the unvarying conditions in the deep sea from the extinction that has met their sisters.
Those who are familiar with our seacoast will know an interesting creature known as the horseshoe crab, or king crab, though in reality it is not a crab at all. It is rather more nearly related to the spiders than the crabs, though no one but a technical zoölogist could possibly associate them together. The ancestors of these king crabs were the finest and best developedanimals in this early Palæozoic time. These creatures had bodies jointed like the tail of a lobster. They were wide and flat, instead of narrow and rounded like a lobster, and each joint of the body was highest in the middle and distinctly lower at the two sides, thus forming three regions along their backs. This structure gives to these creatures the name of trilobites. These animals were the kings of the early ocean. They had an interesting habit of curling up nose to tail before they died, and, as a result, a large proportion of all the trilobite fossils we find are curled in this peculiar manner.
After these forms the most abundant fossils we find in Silurian times were creatures that at first sight looked as if they might be related to the clams. These are known as lampshells, because one shell projects beyond the other and curls up at the tip so as to resemble the clay lamps which are dug out of old Roman towns. The lampshells also have nearly disappeared in modern times. Simple creatures belonging with our present crab and snail had begun to make their appearance, but they were not as abundant as we find them later on.
The third group of the mollusks to which the nautilus and squid of to-day belong is very abundantly represented in the Silurian by fossils with coiled-up shells. As for the plant life of the time, it is exceedingly difficult to say much about it. There must have been nothing but marine plants, and these must have been on the general line of the seaweeds. Little can be definitely said concerning them.
The next period of the Palæozoic is known as the Devonian age, or the age of fishes. Now the backboned animals first make their clear and unmistakable appearance. There are remains in the Silurian which show that there must have been a few fishes at that time. The Devonian is so full of them and they are so well developed and so diversified that this period is definitely known as the "age of fishes." They do not closely resemble the fishes of to-day, but anyone would recognize most of them for what they are. Their bodies were covered, not so much with scales as with heavy plates, often arranged like tiles, those on the forward half of the animal being often larger than those surrounding the rest of the body. The creature was encased, as it were, in armor. These were the rulers of the Devonian seas. The land, as yet, was probably nearly without animal life, the creatures thus far being almost confined to the water. A few insects make their appearance and a few thousand-leggers are running around among the lowly plants; a few spider-like animals have arisen; there are a few snails that have left the water and taken to the land. Altogether only the dawn of a landfauna is to be noticed. In the Devonian the plants are creeping up upon the ground. Ferns are growing about everywhere, though they are not exactly our ferns, but are rather a sort of intermediate form between these and the present seed plants.
Now comes an entire change in the history of the world. By some means a rise in the bottom seems to have cut off a great part of the internal sea from the outer ocean and to have converted it into a widespread shallow bay, much like the sounds which lie back of the islands that line the Atlantic Coast from New Jersey to Florida. Just as this coastal region to-day is covered with salt marshes, so the whole internal sea of the Carboniferous period was converted into a great swamp. Sometimes an oscillation of the crust of the earth brought this marsh above the surface of the sea and a luxuriant growth of plants spread over it. Then a sinking of the bottom allowed the mud and sand to wash down the shores, and spread out over the marsh, and enclose the muck of the marsh under a layer of sand or clay. Another lift of the bottom would start the swamp growing once more, and a series of alternations between marsh land and sound seems to have followed. The plants of this period are not the plants of to-day, though we still have some very degenerate representatives of them. The common horse-tail, with its angular,slender, leaflike branches and its club-shaped spore-bearing body, is a modern degenerate descendant of the treelike calamites of the Carboniferous forest. A creeping evergreen, known by the name of clubmoss, is in like manner the modern degenerate remnant of the scalestem and sealstem, which were the great trees of the forests of the coal period.
All over the surface of the marsh, between these big trees, grew the ferns. While the coal itself was formed generally from the scalestems and sealstems, the most common fossils found in the shales that lie upon the coal beds are the ferns which covered the surface of the marsh.
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 mass 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 describethe climate of any part of the Palæozoic 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 assists 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 cocoon-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 assures the zoölogist 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.