Excerpt from The Riverside Natural History, Vol. 3: Lower Vertebrates
A very direct and personal interest attaches to the study of the Vertebrata, that branch of the animal kingdom to which man himself belongs, and of which he is in many respects the highest member. Many of the most interesting problems connected with man's place in nature can only be intelligently approached if the student is possessed of a certain degree of acquaintance with the structure of the animals built upon the same fundamental plan as himself. There can therefore be no more fitting introduction to the special description of structure and habits of the various groups of vertebrates than an attempt to indicate the anatomical features which are common to all, and the chief lines of modification of them. The object of the following pages will thus be to afford the reader some general stand-points, from which he may view the details embodied in the description of the separate groups, and at the same time to indicate the directions in which anatomists are at present working.
Since the appearance of Darwin's origin of species, a quarter of a century ago, a flood of light has been shed on comparative anatomy, by the labors of those anxious to test the applicability of the doctrine of evolution within the domain of their special researches. However far we may still be from the solution of the question how and why the series of animal forms at present on the earth's surface differ from those of past geological periods, the general consensus of opinion is that they have descended from them. It is impossible, within the scope of the present work, to present in a connected form the evidence on which this opinion is based; but the description of structure and development, as well as of the fossil forms, having been prepared in the light of the evolution theory, will render it an easy matter for the reader to discover their hearings upon that doctrine. Thanks to the immense quantities of fossil vertebrates obtained from this country, the paleontological evidence for the theory of evolution is more complete of information in this branch than in any other; and thus the systematist, having regard to these different sources, is enabled, by the construction of genealogical trees, to indicate near or remote kinship, and to illustrate, as far as can be done from our imperfect knowledge, the phylogeny or line of development of the various groups.
Since Lamarck, in the beginning of this century, first drew a sharp distinction between animals with and without backbones, the use of the terms vertebrate and invertebrate has become almost a part of popular language. Just as, however, the zoologist is obliged to restrict his use of the term fish to a narrower than the popular sense, so the advance of knowledge has led him to recognize affinities which oblige him to extend the term Vertebrata further than its etymology would justify. The conception embodied in that term is of animals possessed of a backbone formed of joints - the 'vertebrae' - having a certain amount of motion on each other.
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