NATURAL HISTORY OF VERTEBRATES. 



Branch IX.— VERTEBRATA. 



A VERT 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 jiroblems con- 

 nected with man's place in nature can only be intelligently appi-oached 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 vertebi'ates 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 fol- 

 lowing 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 ques- 

 tion how and why the series of animal forms at present on tlie 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 pi'esent 

 work, to present in a connected form the evidence on which this ojjinion is based ; 

 but the description of structure and development, as well as of the fossil forms, hav- 

 ing been prepared in the light of the evolution theory, will render it an easy matter 

 for the reader to discover their bearings upon that doctrine. Thanks to the immense 

 quantities of fossil vertebrates obtained from this country, the ]ia]eontological evi- 

 dence 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 diiTerent 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. .lust 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. In the 

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