WHY DO ANIMALS BECOME EXTINCT?

It is often asked "why do animals become extinct?" but the question is one to which it is impossible to give a comprehensive and satisfactory reply; this chapter does not pretend to do so, merely to present a few aspects of this complicated, many-sided problem.

In very many cases it may be said that actual extermination has not taken place, but that in the course of evolution one species has passed into another; species may have been lost, but the race, or phylum endures, just as in the growth of a tree, the twigs and branches of the sapling disappear, while the tree, as a whole, grows onward and upward. This is what we see in the horse, which is the living representative of an unbroken line reaching back to the little Eocene Hyracothere. So in a general way it may be said that much of what at the first glance we might term extinction is really the replacement of one set of animals by another better adapted to surrounding conditions.

Again, there are many cases of animals, and particularly of large animals, so peculiar in their make up, so very obviously adapted to their own special surroundings that it requires little imagination to see that it would have been a difficult matter for them to have responded to even a slight change in the world about them. Such great and necessarily sluggish brutes as Brontosaurus and Diplodocus, with their tons of flesh, small heads, and feeble teeth, were obviously reared in easy circumstances, and unfitted to succeed in any strenuous struggle for existence. Stegosaurus, with his bizarre array of plates and spines, and huge-headed Triceratops, had evidently carried specialization to an extreme, while in turn the carnivorous forms must have required an abundant supply of slow and easily captured prey.

Coming down to a more recent epoch, when the big Titanotheres flourished, it is easy to see from a glance at their large, simple teeth that these beasts needed an ample provision of coarse vegetation, and as they seem never to have spread far beyond their birthplace, climatic change, modifying even a comparatively limited area, would suffice to sweep them out of existence. To use the epitaph proposed by Professor Marsh for the tombstone of one of the Dinosaurs, many a beast might say, "I, and my race perished of over specialization." To revert to the horse it will be remembered that this very fate is believed to have overtaken those almost horses the European Hippotheres; they reached a point where no further progress was possible, and fell by the wayside.

There is, however, still another class of cases where species, families, orders, even, seem to have passed out of existence without sufficient cause. Those great marine reptiles, the Ichthyosaurs, of Europe, the Plesiosaurs and Mosasaurs, of our own continent, seem to have been just as well adapted to an aquatic life as the whales, and even better than the seals, and we can see no reason why Columbus should not have found these creatures still disporting themselves in the Gulf of Mexico. The best we can do is to fall back on an unknown "law of progress," and say that the trend of life is toward the replacement of large, lower animals by those smaller and intellectually higher.

But why there should be an allotted course to any group of animals, why some species come to an end when they are seemingly as well fitted to endure as others now living, we do not know, and if we say that a time comes when the germ-plasm is incapable of further subdivision, we merely express our ignorance in an unnecessary number of words. The mammoth and mastodon have already been cited as instances of animals that have unaccountably become extinct, and these examples are chosen from among many on account of their striking nature. The great ground sloths, the Mylodons, Megatheres, and their allies, are another case in point. At one period or another they reached from Oregon to Virginia, Florida, and Patagonia, though it is not claimed that they covered all this area at one time. And, while it may be freely admitted that in some portions of their range they may have been extirpated by a change in food-supply, due in turn to a change in climate, it seems preposterous to claim that there was not at all times, somewhere in this vast expanse of territory, a climate mild enough and a food-supply large enough for the support of even these huge, sluggish creatures. We may evoke the aid of primitive man to account for the disappearance of this race of giants, and we know that the two were coeval in Patagonia, where the sloths seem to have played the rÔle of domesticated animals, but again it seems incredible that early man, with his flint-tipped spears and arrows, should have been able to slay even such slow beasts as these to the very last individual.

Of course, in modern times man has directly exterminated many animals, while by the introduction of dogs, cats, pigs, and goats he has indirectly not only thinned the ranks of animals, but destroyed plant life on an enormous scale. But in the past man's capabilities for harm were infinitely less than now, while of course the greatest changes took place before man even existed, so that, while he is responsible for the great changes that have taken place in the world's flora and fauna during recent times, his influence, as a whole, has been insignificant. Thus, while man exterminated the great northern sea-cow, Rytina, and Pallas's cormorant on the Commander Islands, these animals were already restricted to this circumscribed area[22] by natural causes, so that man but finished what nature had begun. The extermination of the great auk in European waters was somewhat similar. There is, however, this unfortunate difference between extermination wrought by man and that brought about by natural causes: the extermination of species by nature is ordinarily slow, and the place of one is taken by another, while the destruction wrought by man is rapid, and the gaps he creates remain unfilled.

Not so very long ago it was customary to account for changes in the past life of the globe by earthquakes, volcanic outbursts, or cataclysms of such appalling magnitude that the whole face of nature was changed, and entire races of living beings swept out of existence at once. But it is now generally conceded that while catastrophes have occurred, yet, vast as they may have been, their effects were comparatively local, and, while the life of a limited region may have been ruthlessly blotted out, life as a whole was but little affected. The eruption of Krakatoa shook the earth to its centre and was felt for hundreds of miles around, yet, while it caused the death of thousands of living beings, it remains to be shown that it produced any effect on the life of the region taken in its entirety.

Changes in the life of the globe have been in the main slow and gradual, and in response to correspondingly slow changes in the level of portions of the earth's crust, with their far-reaching effects on temperature, climate, and vegetation. Animals that were what is termed plastic kept pace with the altering conditions about them and became modified, too, while those that could not adapt themselves to their surroundings died out.

How slowly changes may take place is shown by the occurrence of a depression in the Isthmus of Panama, in comparatively recent geologic time, permitting free communication between the Atlantic and Pacific, a sort of natural inter-oceanic canal. And yet the alterations wrought by this were, so to speak, superficial, affecting only some species of shore fishes and invertebrates, having no influence on the animals of the deeper waters. Again, on the Pacific coast are now found a number of shells that, as we learn from fossils, were in Pliocene time common on both coasts of the United States, and Mr. Dall interprets this to mean that when this continent was rising, the steeper shore on the Pacific side permitted the shell-fish to move downward and adapt themselves to the ever changing shore, while on the Atlantic side the drying of a wide strip of level sea-bottom in a relatively short time exterminated a large proportion of the less active mollusks. And in this instance "relatively short" means positively long; for, compared to the rise of a continent from the ocean's bed, the flow of a glacier is the rapid rush of a mountain torrent.

Then, too, while a tendency to vary seems to be inherent in animals, some appear to be vastly more susceptible than others to outside influences, to respond much more readily to any change in the world about them. In fact, Professor Cook has recently suggested that the inborn tendency to variation is sufficient in itself to account for evolution, this tendency being either repressed or stimulated as external conditions are stable or variable.

The more uniform the surrounding conditions, and the simpler the animal, the smaller is the liability to change, and some animals that dwell in the depths of the ocean, where light and temperature vary little, if any, remain at a standstill for long periods of time.

The genus Lingula, a small shell, traces its ancestry back nearly to the base of the Ordovician system of rocks, an almost inconceivable lapse of time, while one species of brachiopod shell endures unchanged from the Trenton Limestone to the Lower Carboniferous. In the first case one species has been replaced by another, so that the shell of to-day is not exactly like its very remote ancestor, but that the type of shell should have remained unchanged when so many other animals have arisen, flourished for a time, and perished, means that there was slight tendency to variation, and that the surrounding conditions were uniform. Says Professor Brooks, speaking of Lingula: "The everlasting hills are the type of venerable antiquity; but Lingula has seen the continents grow up, and has maintained its integrity unmoved by the convulsions which have given the crust of the earth its present form."

Many instances of sudden but local extermination might be adduced, but among them that of the tile-fish is perhaps the most striking. This fish, belonging to a tropical family having its headquarters in the Gulf of Mexico, was discovered in 1879 in moderately deep water to the southward of Massachusetts and on the edge of the Gulf Stream, where it was taken in considerable numbers. In the spring of 1882 vessels arriving at New York reported having passed through great numbers of dead and dying fishes, the water being thickly dotted with them for miles. From samples brought in, it was found that the majority of these were tile-fish, while from the reports of various vessels it was shown that the area covered by dead fish amounted to somewhere between 5,000 and 7,500 square miles, and the total number of dead was estimated at not far from a billion. This enormous and widespread destruction is believed to have been caused by an unwonted duration of northerly and easterly winds, which drove the cold arctic current inshore and southwards, chilling the warm belt in which the tile-fish resided and killing all in that locality. It was thought possible that the entire race might have been destroyed, but, while none were taken for many years, in 1899 and in 1900 a number were caught, showing that the species was beginning to reoccupy the waters from which it had been driven years before.

The effect of any great fall in temperature on animals specially adapted to a warm climate is also illustrated by the destruction of the Manatees in the Sebastian River, Florida, by the winter of 1894-95, which came very near exterminating this species. Readers may remember that this was the winter that wrought such havoc with the blue-birds, while in the vicinity of Washington, D. C., the fish-crows died by hundreds, if not by thousands.

Fishes may also be exterminated over large areas by outbursts of poisonous gases from submarine volcanoes, or more rarely by some vast lava flood pouring into the sea and actually cooking all living beings in the vicinity. And in the past these outbreaks took place on a much larger scale than now, and naturally wrought more widespread destruction.

A recent instance of local extermination is the total destruction of a humming-bird, Bellona ornata, peculiar to the island of St. Vincent, by the West Indian hurricane of 1898, but this is naturally extirpation on a very small scale.

Still, the problems of nature are so involved that while local destruction is ordinarily of little importance, or temporary in its effects, it may lead to the annihilation of a species by breaking a race of animals into isolated groups, thereby leading to inbreeding and slow decline. The European bison, now confined to a part of Lithuania and a portion of the Caucasus, seems to be slowly but surely approaching extinction in spite of all efforts to preserve the race, and no reason can be assigned for this save that the small size of the herds has led to inbreeding and general decadence.

In other ways, too, local calamity may be sweeping in its effects, and that is by the destruction of animals that resort to one spot during the breeding season, like the fur-seals and some sea-birds, or pass the winter months in great flocks or herds, as do the ducks and elk. The supposed decimation of the Moas by severe winters has been already discussed, and the extermination of the great auk in European waters was indirectly due to natural causes. These birds bred on the small, almost inaccessible island of Eldey, off the coast of Iceland, and when, through volcanic disturbances, this islet sank into the sea, the few birds were forced to other quarters, and as these were, unfortunately, easily reached, the birds were slain to the last one.

From the great local abundance of their remains, it has been thought that the curious short-legged Pliocene rhinoceros, Aphelops fossiger, was killed off in the West by blizzards when the animals were gathered in their winter quarters, and other long-extinct animals, too, have been found under such conditions as to suggest a similar fate.

Among local catastrophes brought about by unusually prolonged cold may be cited the decimation of the fur-seal herds of the Pribilof Islands in 1834 and 1859, when the breeding seals were prevented from landing by the presence of ice-floes, and perished by thousands. Peculiar interest is attached to this case, because the restriction of the northern fur-seals to a few isolated, long undiscovered islands, is believed to have been brought about by their complete extermination in other localities by prehistoric man. Had these two seasons killed all the seals, it would have been a reversal of the customary extermination by man of a species reduced in numbers by nature.

In the case of large animals another element probably played a part. The larger the animal, the fewer young, as a rule, does it bring forth at a birth, the longer are the intervals between births, and the slower the growth of the young. The loss of two or three broods of sparrows or two or three litters of rabbits makes comparatively little difference, as the loss is soon supplied, but the death of the young of the larger and higher mammals is a more serious matter. A factor that has probably played an important rÔle in the extinction of animals is the relation that exists between various animals, and the relations that also exist between animals and plants, so that the existence of one is dependent on that of another. Thus no group of living beings, plants or animals, can be affected without in some way affecting others, so that the injury or destruction of some plant may result in serious harm to some animal. Nearly everyone is familiar with the classic example given by Darwin of the effect of cats on the growth of red clover. This plant is fertilized by bumble bees only, and if the field mice, which destroy the nests of the bees, were not kept in check by cats, or other small carnivores, their increase would lessen the numbers of the bees and this in turn would cause a dearth of clover.

The yuccas present a still more wonderful example of the dependence of plants on animals, for their existence hangs on that of a small moth whose peculiar structure and habits bring about the fertilization of the flower. The two probably developed side by side until their present state of inter-dependence was reached, when the extinction of the one would probably bring about that of the other.

It is this inter-dependence of living things that makes the outcome of any direct interference with the natural order of things more or less problematical, and sometimes brings about results quite different from what were expected or intended.

The gamekeepers on the grouse moors of Scotland systematically killed off all birds of prey because they caught some of the grouse, but this is believed to have caused far more harm than good through permitting weak and sickly birds, that would otherwise have fallen a prey to hawks, to live and disseminate the grouse distemper.

The destruction of sheep by coyotes led the State of California to place a bounty on the heads of these animals, with the result that in eighteen months the State was called upon to pay out $187,485. As a result of the war on coyotes the animals on which they fed, notably the rabbits, increased so enormously that in turn a bounty was put on rabbits, the damage these animals caused the fruit-growers being greater than the losses among sheep-owners from the depredations of coyotes. And so, says Dr. Palmer, "In this remarkable case of legislation a large bounty was offered by a county in the interest of fruit-growers to counteract the effects of a State bounty expended mainly for the benefit of sheep-owners!"

Professor Shaler, in noting the sudden disappearance of such trees as the gums, magnolias, and tulip poplars from the Miocene flora of Europe has suggested that this may have been due to the attacks, for a series of years, of some insect enemy like the gipsy moth, and the theory is worth considering, although it must be looked upon as a possibility rather than a probability. Still, anyone familiar with the ravages of the gipsy moth in Massachusetts, where the insect was introduced by accident, can readily imagine what might have been the effect of some sudden increase in the numbers of such a pest on the forests of the past. Trees might resist the attacks of enemies and the destruction of their leaves for two or three years, but would be destroyed by a few additional seasons of defoliation.

Ordinarily the abnormal increase of any insect is promptly followed by an increase in the number of its enemies; the pest is killed off, the destroyers die of starvation and nature's balance is struck. But if by some accident, such as two or three consecutive seasons of wet, drought, or cold, the natural increase of the enemies was checked, the balance of nature would be temporarily destroyed and serious harm done. That such accidents may occur is familiar to us by the damage wrought in Florida and other Southern States by the unwonted severity of the winters of 1893, 1895, and 1899.

If any group of forest trees was destroyed in the manner suggested by Professor Shaler, the effects would be felt by various plants and animals. In the first place, the insects that fed on these trees would be forced to seek another source of food and would be brought into a silent struggle with forms already in possession, while the destruction of one set of plants would be to the advantage of those with which they came into competition and to the disadvantage of vegetation that was protected by the shade. Finally, these changed conditions would react in various ways on the smaller birds and mammals, the general effect being, to use a well-worn simile, like that of casting a stone into a quiet pool and setting in motion ripples that sooner or later reach to every part of the margin.

It is scarcely necessary to warn the reader that for the most part this is purely conjectural, for from the nature of the case it is bound to be so. But it is one of the characteristics of educated man that he wishes to know the why and wherefore of everything, and is in a condition of mental unhappiness until he has at least formulated some theory which seems to harmonize with the visible facts. And from the few glimpses we get of the extinction of animals from natural causes we must formulate a theory to fit the continued extermination that has been taking place ever since living beings came into the world and were pitted against one another and against their surroundings in the silent and ceaseless struggle for existence.

THE END.

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