I
Bergson, the new French philosopher, thinks we all had a narrow escape, back in geologic time, of having our eggs spoiled before they were hatched, or, rather, rendered incapable of hatching by too thick a shell. This was owing to the voracity of the early organisms. As they became more and more mobile, they began to take on thick armors and breastplates and shells and calcareous skins to protect themselves from one another. This tendency resulted, he thinks, in the arrest of the entire animal world in its evolution toward higher and higher forms. These shells and armors begat a kind of torpor and immobility which has continued down to our day with the echinoderms and mollusks, but the arthropods and vertebrates escaped it by some lucky stroke. Now you and I are here without imprisoning shells on our backs; but how or why did we escape? Bergson does not say. Was it a matter of luck or chance? Was there ever a time when the stream of life tended to harden and become fixed in its own forms like a stream of cooling lava, or has the innate plasticity of life been easily equal to its own ends? True, the clam remains a clam, and the starfish remains a starfish; some other forms have carried the evolutionary impulse forward till it flowered in man. Was this impulse ever really checked or endangered? Was the golden secret ever intrusted to the keeping of any single form? and, had that form been cut off, would the earth have been still without its man? These are puzzling questions.
Thus, when we have come to look upon life and nature in the light of evolution, what vistas are opened to us where before were only blank walls! The geologic ages take on a new interest to us. We know that in some form we were even there. The systems of sedimentary rocks which the geologist portrays, piled one upon the other to a depth of fifty miles or more, seem like the stairway by which we have ascended, taking on some new and more developed form at each rise. What we were at the first step in Cambrian times only the Lord knows, but whatever we were, we crept up or floated up to the next rise. In the Silurian seas we may have been a trilobite for aught we know; at any rate, we were the outcome of the life impulse that begat the trilobites, but our fate was not bound up with theirs, as their race came to an end in those early geologic ages, and our stem form did not. Whether or not we were a fish in the Devonian seas, there is little doubt that we had gills, because we have the gill slits yet in our early foetal life, and it is quite certain that in some way we owe our backbones to the fishes.
When the rocks that form my native Catskills were being laid down in the Devonian waters, I fancy that my aquatic embryo was swimming about somewhere and slowly waxing strong. Up and up I climbed across the sandstone steps, across the limestone, the conglomerate, the slate, up into Carboniferous times. The upper and nether millstones of the "millstone grit" did not crush me, neither did the floods and the convulsions of Carboniferous times that buried the vast vegetable growths that resulted in our coal measures engulf or destroy me. About that time probably, I emerged from the water and became an amphibian, and maybe got my five fingers and five toes on each side.
Nor did the wholesale destruction of animal life at the end of Palaeozoic time cut off my line of descent. The monstrous reptiles of the succeeding or Mesozoic age, the petrified remains of one of which was recently found in the sandstone rocks near the river's edge under the Palisades of the Hudson, do not seem to have endangered the golden thread by which our fate hung. Still "I mount and mount." The stairs by which I climb were rent by earthquakes and volcanoes, the strata were squeezed up and overturned and folded in the great mountain-chains; the Alps, the Andes, the Himalayas, the Coast Range were born; the earth-throes must have been tremendous at times; yet I escaped it all. The huge and fearful mammals of the third or Tertiary period passed me by unharmed. Eruptions and cataclysms, the sinking of the land, the inundations of the sea, world-wide deformations of the earth's crust, fire and ice and floods, monsters of the deep and dragons of the land and the air have beset my course from the first, and yet here I am, here we all are, and apparently none the worse for the appalling dangers we have passed through.
Evolution thus makes the world over for us. It shows us in what a complex web of vital and far-reaching relations we stand. It gives us an outlook upon the past that is startling, and in some ways forbidding, yet one that ought to be stimulating and inspiring. If we look back with a shudder we should look forward with a thrill. If the past is terrible, the future is in the same degree cheering and inviting. If we came out of those lowly and groveling forms, to what heights of being may we not be carried by the impetus that brought us thus far? In fact, to what heights has it already carried us!
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.
We may think of our descent in the historic period under the image of the stream, though of a stream many times delayed and diverted, even many times diminished by wars and plagues and famine, but a stream with some sort of unity and continuity, since man became man. The stream of life is like any other stream in this respect. Divert or use up part of the water of a stream, yet what is left flows on and keeps up the continuity and identity of the stream; dip your cup into it here, and you will not get precisely the same water you would have got had none of it been diverted or used far back in its course--you get the water that was allowed to flow by.
Had there been no loss of life by war and pestilence and accidents of various kinds, the different countries would have been occupied by quite other men and women than those that fill them to-day. The course of life in every neighborhood is changed by what seem like accidental causes, as when a family is practically wiped out by some accident or dread disease. This brings new people on the scene. The farm or the business falls into other hands, and new social relations spring up, new men and women are brought together or the old ones driven apart, marriage is hastened or retarded, opportunities for family life are made or unmade, and fewer children, or more children, as the case may be, are the result. The issue of some battle hundreds or thousands of years ago may have played a part in your life and mine to-day--other races, other individuals of the race, would have been thrown together had the issue been different, and other families started, so that some one else would have been here in our stead.
But the question of hazard to the race of man in geologic time is quite a different one. Here our fate seems to hang by a single thread--a golden thread, we may call it, but, in that terrible maze of clashing forces and devouring forms of the vast geologic periods, how liable to be broken! It is not now a question of the continuity of a stream, but of the continuity of a single evolutionary process, or, as Haeckel says, the continuity of the morphological chain which stretches from the lemurs up through tailed and tailless anthropoid apes to man. If the evolutionary impulse had been checked or extinguished in the lemur--that small apelike animal that went before the true ape, the fossil remains of which have been found on this continent and the survivals of which are now found in Madagascar--would man have appeared? Again, if the race of lemurs developed from a single pair, how precarious seems our fate! In fact, if any of the transitional forms between species can be reduced to a single pair--as the forms that connect the reptiles with the mammals--our fate would seem to be in the keeping of these forms. Over this single frail bridge which escaped the floods and the tornadoes and the earthquakes of those terrible ages we must have passed. What risky business it all seems! Was it luck or law that favored us? Doubtless, if we could penetrate the mystery, we should see that there was no chance or risk in the matter. We cannot go very far in solving these great fundamental questions by applying to them the tests of our own experience, Numberless specific forms become extinct, but the impulse that begat the form does not die out. Thus, all the giant reptiles died out--the dinosaurs, the mesosaurs--but the reptilian impulse still survives. How many types of invertebrates have perished! but the invertebrate impulse still goes on. How many species of mammals have been cut off! yet the mammal impulse has steadily gone forward. These things suggest the wave that moves on but leaves the water behind. The vertebrate impulse began in wormlike forms, in the old Palaeozoic seas, and stopped not till it culminated in man. This impulse has left many forms behind it; but has this impulse itself ever been endangered? If one looks at the matter thus in an abstract instead of a concrete way, the problem of our descent becomes easier.
When we look at the evolution of life on a grand scale, nature seems to feel her way, like a blind man, groping, hesitating, trying this road and then that. In some cases the line of evolution seems to end in a cul de sac beyond which no progress is possible. The forms thus cornered soon become extinct. The mystery, the unaccountable thing, is the appearance of new characters. The slow modification or transformation of an existing character may often be traced; natural selection, or the struggle for existence, takes it in hand and adapts and perpetuates it, or else eliminates it. But the origin of certain new parts or characters--that is the secret of the evolutionary process. Thus there was a time when no animal had horns; then horns appeared. "In the great quadruped known as titanothere," says Osborn, "rudiments of horns first arise independently at certain definite parts of the skull; they arise at first alike in both sexes, or asexually; then they become sexual, or chiefly characteristic of males; then they rapidly evolve in the males while being arrested in development in the females; finally, they become in some of the animals dominant characteristics to which all others bend." Nature seems to throw out these new characters and then lets them take their chances in the clash of forces and tendencies that go on in the arena of life. If they serve a purpose or are an advantage, they remain; if not, they drop out. Nature feels her way. The horns proved of less advantage to the females than to the males; they seem a part of the plus or overflow of the male principle, like the beard in man--the badge of masculinity. The titanothere is traceable back to a hornless animal the size of a sheep, and it ended in a horned quadruped nearly as large as an elephant. It flourished in Wyoming in early Tertiary times. Nature did not seem to know what to do with horns when she first got them. She played with them like a child with a new toy. Thus she gave two pairs to several species of mammals, one pair on the nose and one pair on the top of the skull--certainly an embarrassment of weapons.
The first horns appear to have been crude, heavy, uncouth, but long before we reach our own geologic era they appear in various species of quadrupeds, and become graceful and ornamental. How beautiful they are in many of the African antelope tribe! Nature's workmanship nearly always improves with time, like that of man's, and sooner or later takes on an ornamental phase.
The early uncouth, bizarre forms seem to be the result of the excess or surplus of life. Life in remote biologic times was rank and riotous, as it is now, in a measure, in tropical lands. One reason may be that the climate of the globe during the middle period, and well into the third period, appears to have been of a tropical character. The climatic and seasonal divisions were not at all pronounced, and both animal and vegetable life took on gigantic and grotesque forms. In the ugliness of alligator and rhinoceros and hippopotamus of our day we get some hint of what early reptilian and mammalian life was like.
That Nature should have turned out better and better handiwork as the ages passed; that she either should have improved upon every model or else discarded it; that she should have progressed from the bird, half-dragon, to the sweet songsters of our day and to the superb forms of the air that we know; that evolution should have entered upon a refining and spiritualizing phase, developing larger brains and smaller bodies, is a very significant fact, and one quite beyond the range of the mechanistic conception of life.
Our own immediate line of descent leads down through the minor forms of Tertiary and Mesozoic times--forms that probably skulked and dodged about amid the terrible and gigantic creatures of those ages as the small game of to-day hide and flee from the presence of their arch-enemy, man; and that the frail line upon which the fate of the human race hung should not have been severed during the wild turmoil of those ages is, to me, a source of perpetual wonder.
III
The hazards of the future of the race must be quite different from those I have been considering. They are the hazards incident to an exceptional being upon this earth--a being that takes his fate in his own hands in a sense that no other creature does.
Man has partaken of the fruit of the Tree of Good and Evil, which all the lower orders have escaped. He knows, and knows that he knows. Will this knowledge, through the opposition in which it places him to elemental nature and the vast system of artificial things with which it has enabled him to surround himself, cut short his history upon this planet? Will Nature in the end be avenged for the secrets he has forced from her? His civilization has doubtless made him the victim of diseases to which the lower orders, and even savage man, are strangers. Will not these diseases increase as his life becomes more and more complex and artificial? Will he go on extending his mastery over Nature and refining or suppressing his natural appetites till his original hold upon life is fatally enfeebled?
It seems as though science ought to save man and prolong his stay on this planet,--it ought to bring him natural salvation, as his religion promises him supernatural salvation. But of course, man's fate is bound up with the fate of the planet and of the biological tree of which he is one of the shoots. Biology is rooted in geology. The higher forms of life did not arbitrarily appear, they flowed out of conditions that were long in maturing; they flowered in season, and the flower will fall in season. Man could not have appeared earlier than he did, nor later than he did; he came out of what went before, and he will go out with what comes after. His coming was natural, and his going will be natural. His period had a beginning, and it will have an end. Natural philosophy leads one to affirm this; but of time measured by human history he may yet have a lease of tens of thousands of years.
The hazard of the future is a question of both astronomy and geology. That there are cosmic dangers, though infinitely remote, every astronomer knows. That there are collisions between heavenly bodies is an indubitable fact, and if collisions do happen to any, allow time enough and they must happen to all. That there are geologic dangers through the shifting and crumpling of the earth's crust, every geologist knows, though probably none that could wipe out the whole race of man. The biologic dangers of the past we have outlived--the dangers that must have beset a single line of descent amid the carnival of power and the ferocity of the monster reptiles of Mesozoic times, and the wholesale extinction of species that occurred in different geologic periods.
Nothing but a cosmic catastrophe, involving the fate of the whole earth, could now exterminate the human race. It is highly improbable that this will ever happen. The race of man will go out from a slow, insensible failure, through the aging of the planet, of the conditions of life that brought man here. The evolutionary process upon a cooling world must, after the elapse of a vast period of time, lose its impetus and cease.
[The end]
John Burroughs's essay: The Hazards Of The Past