CREATION BY EVOLUTION
They were already true vertebrates, with the same main divisions of the whole body as in man, the same ground plan of the brain and spinal cord, the same type of segmentation of the vertebral column, the same general type of complex skull (to be described later); moreover, these ancient Devonian fishes and their modern representatives agree with early stages of the human foetus in the general plan of the jaws and gill arches and in the basic features of the digestive, circulatory, respiratory and reproductive systems. Hence the humble dogfish or shark, which is a relatively little modified survivor of the early vertebrates, is universally recognized by biologists as affording a true ground plan of human anatomy and physiology.
We do not know why the earliest fishes gave rise on the one hand to a line of forms that progressed to ever higher types and on the other hand to far more conservative ones, including the existing sharks and ganoid fishes, which represent, on the whole, an arrested or retarded evolution; but the fossil record of the vertebrates shows that in every geological age one finds the more conservative, less modified descendants of older stocks living contemporaneously with the far more progressive relatives. The science of comparative anatomy, in collaboration with allied branches of science, is able to decipher the evolutionary history of man from the vast mass of Nature’s documents precisely because the more conservative forms of each geological age enable us to visualize the anatomical characters of older periods and to reconstruct, with progressive approach to complete accuracy, the steps by which the older conditions have given rise to the newer ones.
By lower Devonian time the vertebrate stock had already split up into the following main groups: first, the ostracoderms, an excessively ancient group, of which nearly all the
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