Time and Change

by John Burroughs

II

That the hazards of the past, to many forms of life, at least, have been real and no myth, is evident from the vast number of forms that have been cut off and become extinct; various causes, now hard to decipher, have worked together to the end, such as changed geographical conditions, changes of climate, affecting the food-supply, extreme specialization, like that of the sabre-toothed tiger whose petrified remains have been found in various parts of this continent, and who apparently was finally handicapped by his huge dental sabre. Probably many more species of animals have become extinct than have survived, but none of these could have been in the line of man's descent, else the human race would not have been here. If the Eocene progenitor of the horse, the little four-toed eohippus, had been cut off, would not the world have been horseless to-day? The horse in America became extinct, from some cause only conjectural, many tens of thousands of years ago. Had the same fate befallen the horse in Europe and Asia, it seems probable that our civilization would have been far less advanced to-day than it is.

The fate of every species of mammal in our time seems to have been in the keeping of a single form in early Tertiary times. The end of the Cretaceous or chalk period saw the extinction of the giant reptiles both of sea and of land, at the same time that it saw the appearance of a great many species of small and inconspicuous mammals, among which doubtless were our own humble forebears. Extreme specialization in any direction may narrow an animal's chances of survival; they have but one chance in the game of life, whereas an animal with a more generalized organization has many chances. Man is one of the most generalized of animals; no special tools, no special weapons—his hand many tools and weapons in one. Hence he is the most adaptable of animals; all climes, all foods, all places are his; he is master of the land, of the sea, of the air.

Animal life is often curiously interdependent. I asked our guide in the Adirondacks if there were any ravens there. "Not nearly as many as there used to be," he said, and his explanation of their disappearance seems thoroughly scientific; it was that the wolves and the panthers kept them in meat, and now that these animals had disappeared, the ravens had little to feed upon. If the moose were compelled to graze from off the ground, like a sheep or a cow, the species would probably soon become extinct. Osborn thinks it probable that the huge beast called titanothere finally became extinct early in Tertiary times owing to the form of its teeth, which were of such a type that they could not change to meet a change in the flora upon which the creature fed. Of course we shall never know what narrow escapes our race had from extinction in the remote past; some forms have ended in a blind alley, like the sea-urchin and the oyster. Arthropoda have continued to evolve and have reached their high-water mark of intelligence in bees and ants. The vertebrates went forward and have culminated in man. Bergson thinks that in the vertebrates intelligence has been developed at the expense of instinct, and that in the invertebrates instinct has been perfected at the expense of intelligence.

Are we not compelled to adopt what is called the monophyletic hypothesis, that is, that our line of descent started from one pair, male and female, somewhere in the vast stretch of geologic or biologic time, and to reason that, had that pair been out of the race, we should not have appeared?

Can we narrow life to a single point, a single cell, in the past? Was there one and only one first bit of protoplasm? If we were to say that life first appeared on the globe in Cambrian times, just what should we mean? That it began as a single point, or as many points? When we say that the primates first appeared in Eocene times, do we mean that one single primate appeared then? If so, what form went immediately before him? This is all a vain speculation.

Does man presuppose all the vertebrate sub-kingdom? Was he safe as long as one vertebrate form remained? Are his forebears many, and not one pair? Can we think of his ancestry under the image of a tree, and of him as one of the many branches? If so, nothing but the destruction of the tree would have imperiled his appearance, or the lopping off of his particular branch. Probably all such images are misleading. We simply cannot figure to ourselves the tangled course of our biological descent. If thwartings and accidents arid delays could have cut man off, how could he have escaped? We cannot think of man as one; we are compelled to think of him as many; and yet in all our experience the many come from the one, or the one pair.

How thick the field of animal life in the past is strewn with extinct forms!—as thick as the sidereal spaces are strewn with the fragments of wrecked worlds! But other worlds and suns are spun out of the wrecked worlds and suns through the process of cosmic evolution. The world-stuff is worked over and over. Extinct animal forms must have given rise to other, allied forms before they perished, and these to still others, and so on down to our time.

The image of a tree is misleading from the fact that all the different branches of the animal kingdom, from the protozoa up to man, have come along with what we call the higher branches, the mammals; the suckers have kept pace with the main stalk, so that we have the image of a sheaf of branches starting from a common origin and all of equal length. Man has brought on his relations along with him.

There is no glamour of romance over that past. It was all hard, prosy, terrible fact. The earth's crust was less stable than now, the upheavals and subsidences and earthquakes more frequent, the warring of the elements more fierce and incessant, deluge and inundation in more rapid succession, and the riot and excesses of animal life far beyond anything we know of. And our line of descent was taking its chances amid it all. The widespread blotting out of life at the end of Palaeozoic time, and again at the end of Mesozoic times, when myriads of forms were cut off, probably from some convulsion of nature or some cosmic catastrophe; and again during the ice age, when the camel, the llama, the horse, the tapir, the mastodon, the elephant, the giant sloth, became extinct in North America—how fared it with our ancestor during these terrible ages? There is no sure trace of him till late Tertiary times, and it is probably not more than two hundred thousand years ago that he assumed the upright attitude and began to use tools. Probably in Europe fifty thousand years ago he was living in caves, clothed in skins, contending with the cave bear and cave lion, using rude stone implements, and hunting the hairy mastodon, etc. In Asia the probabilities are that he was farther on the road toward the dawn of history.