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.