Page:Popular Science Monthly Volume 82.djvu/423

Rh The first vertebrates to make their appearance on the face of the earth were fishes. They are still wonderfully well preserved as fossils in the rocks of the Devonian period ; and it is perfectly clear that, when alive, they were practically identical in structure with certain fishes now living. But we have no records of true fishes from an earlier period ; from this point downwards into the abyss of time, without warning or apparent reason, the vertebrates drop from the records, although the records themselves remain, and they contain, both after that period and for an immeasurably long time previous to it, a full, even a detailed, account of nearly every known group of invertebrates. Why do the vertebrates disappear at this point? Where did they come from? What kind of invertebrates were their ancestors? How did the anatomical structures peculiar to all vertebrates originate? Here- tofore no one has been able to give even an approximately satisfactory answer to these questions. Here indeed is a great gap in the evolution of the animal kingdom. It is not merely one link that is missing; the whole middle section, perhaps two thirds of the entire animal kingdom, is either absent, or, if present, it has not been recognized and properly located. As there is no apparent resemblance between the structural plan of any known invertebrate and that of a vertebrate, there is no way of uniting the higher animals with the lower; no way of deciding what was the great trunk line of evolution.

This is a serious defect in the very foundations of the biological sciences. While it remains we are compelled to admit that, with all our boasted schemes of classification according to genetic relationships, the whole class of vertebrates hangs in mid-air over an unknown and apparently inaccessible abyss ; that we are totally ignorant of the great creative period in the evolution of the highest type of animals; that we know nothing of the way in which the fundamental structural features of man arose; that we have no basis for the interpretation of the early stages of his embryonic development; and no clue to the initial significance of a single one of his characteristic organs, such as the mouth, notochord, skeleton, lungs, jaws, appendages, heart, thymus, thyroids, pituitary body, pineal gland, sense organs and brain!

During the generation following the appearance of Darwin's "Origin of Species," many attempts were made to bridge this great gap in our knowledge of evolution. The best known theories were defended by the most distinguished zoologists of their time ; but they were, after all, mere suggestions, and their authors were compelled to unite the nearest probable extremes by long arrays of purely imaginary animals.