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

BOOK 3

layers by the fossils found in them—when, for instance, they are distorted in mountains, or when only a single fossil-bearing layer can be found, or when new quarters of the globe are explored for fossils; and this, they proclaim, is arguing in a vicious circle.

As a matter of fact, however, the paleontologist does not fake his results with the naiveté which these assertions ascribe to him. He first of all examines the earth’s crust In some region, like the south-eastern half of England, or the Bad Lands of Wyoming, where the strata lie plainly one above the other and are not crumpled or distorted. He collects series of fossils from as many as possible of such undisturbed layers, examines them, determines to what group they belong, and whether there exist any others of exactly the same species from other layers. By so doing he can provide for each main layer and for each kind of deposit—sandstone, shale, clay, or limestone—within each main layer, a list of fossil species which are not found in any other layers, and of others which are especially abundant in the layer, or very scarce or absent therein. It is only after he has thus dated his fossils by reference to an undisturbed succession of deposits that he uses them to help him in dating layers whose age cannot be determined by these straightforward methods.

The only really serious difficulty in dating rocks is the problem of relating the rocksystems of distant regions of the globe. The succession of rocks in China may be known, and also the succession in Western Europe ; but the evidence may be lacking which ‘enables us to say exactly how the one should be fitted against the other. This may lead to uncertainty in points of detail ; but even in such cases it is always possible to date within geologically moderate limits, and every scrap of new knowledge helps to narrow those limits down. Here again the difficulty is not one of principle.

The essential fact is this: fossils are not used to date rocks of doubtful age until they themselves have been dated by their position in rocks whose order is not doubtful but obvious. And the proof of the correctness of the method is that its results are coherent and intelligible. We do not find ammonites appearing haphazard in the earth’s crust, but in a definite set of its layers ; mammals do not appear and disappear sporadically through geological time, but come on the scene when they are expected, and from then onwards show a steady development ; fourtoed and one-toed horses are never found in the same stratum, and so forth. If each

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THE SCIENCE OF LIFE

CHAPTER 2

fossil is a word in the Book of Life’s history, then a century ago these words made only a few scattered but promising sentences ; to-day, thanks to unceasing exploration, they have fallen into place, and have told man a new and clearer story of life’s past and life’s destiny.

§ 3

A Sample Section in the History of Life: the Evolution of Horses

And now let us take one of the better preserved sections from this vast, confused autobiography which life has written in the rocks. It is a section of which we shall give a considerable amount of detail and rather a bothersome multiplicity of generic names. The reader is under no obligation to remember these names, but they have to be “ produced in court ”’ for the purposes of our proof.

It is loudly argued by many Creationists and semi-Creationists that there is no fullyworked-out pedigree of any existing forms of life, and that there is nothing to dispose of the view that at irregular intervals creative forces intervene in the evoluting process and make life take a convulsive stride forward. This, however, is not the case. So far from there being no well-worked-out pedigree, in which the successive forms in some group of animals are seen visibly modified and differentiated, there are now several such family trees in existence. We are giving here the past record of the existing horses. They have been evolved from a small, four-ioed Eocene mammal, and every step in the process is traceable. (Let us warn the reader that the time-diagram—Fig. 122—is necessary to the reading of what follows.)

It is worth noting that the earliest known three-toed fossil horse was described as recently as 1860, the year after Darwin published the Origin of Species, and that it was not till about 1870 that any serious attempt was or could be made to establish the horse’s ancestry from fossils. Many startling finds were made in the ’seventies and “eighties presenting the story in rough outline. But it has been the patient accumulation of specimens since then which has filled in the details and made it convincing. As the fossil-bearing rocks have been more intensively explored, link after link has been brought to light, until now we are able to reconstruct an almost unbroken chain of change extending for over forty million years.

The existing horses constitute a very distinct family of animals. No other vertebrates have but one toe to each foot ; and no