The science of life : fully illustrated in tone and line and including many diagrams

THE EVOLUTION OF MAN

but slowly, laboriously, by gradual and often devious ways. And having been evolved, man, we are bound to suppose, must still go on along the lines of biological change. So far as our scientific data go, we are bound to believe his present lordship is a precarious one. He may become extinct, like the great beasts of the Age of Reptiles; nothing in his past or in his structure assures us against that; he may linger on in subordination to some new type evolved from another line of life, as the crocodiles and turtles and other reptiles of to-day are subordinated to us mammals ; or he may become transformed age by age into something wholly and unrecognizably new, something more powerful or more specialized, as the Eohippus was changed into the horse. In any case let us remember this simple fact—an elementary corollary of Evolution, but never seriously considered before Darwin’s time—that there is not the slightest reason for supposing that the powers, intellectual, spiritual, and emotional, which we human beings happen to possess, are the highest of which this planet is capable. Our amphibian ancestor, which certainly had no more brains than a frog, could give rise to descendants with the brains of men. There is no reason whatever for supposing that another such stride, and yet further strides in mental possibility, may notoccur. The one sure thing of which the spectacle of Evolution convinces us is that things will not remain as they are. And since there is a vague persuasion very widely diffused at present that in a few million years this planet of ours will ‘‘ freeze up” and the evolution of life cease, we will conclude this chapter upon the fact of Evolution with a brief note upon Man’s place in Time.

8 4 Man's Place in Time

Evolution, we now perceive, is a present reality, a going concern. There is no sign in man’s incomplete being that it has culminated or is in any way arrested. So that a vividly interesting question opens before us: How long can the evolution of life go on? To that question it is now possible to give a tentative answer.

The dating of fossils by rocks, as we have explained it in an earlier chapter, can be at best only a relative dating ; it tells us that one kind of animal or plant lived and died before a second kind and after a third ; but it tells us nothing as to any absolute date in years. Yet the reader will have

remarked that we have been giving the absolute age of different rocks with very considerable assurance : Early Cenozoic fifty million years ago, and so forth. It is time to explain how this is possible.

It is possible because, as the work of recent years has shown, some of the rocks of the earth’s crust contain what we may call, without any fantastic exaggeration, geological clocks. They contain tumekeepers that were set chemically ticking when the rock was first formed, and have gone on ticking at the same rate, without once needing to be wound or regulated, ever since. These clocks are what are known as the radio-active elements, radium, uranium, thorium, and others, which exist in certain minerals. The work of Becquerel on uranium, which speedily led to the discovery of radium by Madame Curie in 1898, was the starting-point for a series of researches which have altered all our ideas on the constitution of matter, and, incidentally, given earth-history a measurable chronology. These radio-active elements, as everybody knows nowadays, shoot out particles of matter from their atoms, and in so doing transform themselves into different elements. They do this in such a way as to become effectual chronometers.

The particles given off are sometimes electrically charged atoms of the light gas, helium (which is a stable element and shows no further change), sometimes the far tinier electrons, the spinning bricks of which the atoms, save for their cores, are built. Uranium is the parent of radium. After shooting off three atoms of helium and _ several electrons, it becomes radium. Radium then continues the process of change; it first discharges a gas, radium emanation, and finally, after five helium atoms have been shot away, becomes plain lead. This, having stable atoms, does not change any more ; the clock, so far as the lead atom is concerned, has run down, and we know no means by which it can be wound up again. The lead thus produced, though in_ its chemical behaviour quite normal, differs from ordinary lead in having an atomic weight of 206 instead of 207-2; we can call it uranium-lead. The whole process may be summed up by an equation : 1 atom of uranium = I atom of uranium-lead + 8 atoms of helium + energy.

Thorium, another radio-active element, goes through a similar series of ass formations, and after shooting out six atoms of helium, also ends up_as lead. This thorium-lead, however, differs from

255