We find united in this class animals which, at first, seem out of place, such as the whale and the bat; and, from their external appearance alone, the former would appear to belong to the fishes, and the latter to birds. Yet, on studying their structure, we find that, not only do these animals merit a place in the class which they occupy, because they possess the distinctive characters of mammals; but, still further, their internal structure is analogous to that of man and of the other individuals of this class.
Notwithstanding this similarity of structure, the whale is not without some points of difference from its neighbours the horse and the dog; therefore, in order to place each of these animals in a position suitable to it, mammals are divided into secondary groups called orders. The first of these orders includes, under the name primates, man and apes. The latter contain animals which approach birds in certain characters of their organism, forming a link between the latter and mammals.
We find, in studying the regions of the body in some of the vertebrates, that, while they present differences from the corresponding regions of the human body, they also offer most striking analogies. We can, for example, recognise the upper limb of man in the anterior one of quadrupeds, in the wing of the bat, in the paddle of the seal, etc. It is, so to speak, those variations of a great plan which give such a charm to the study of comparative anatomy.
The division of classes into orders, which we have just mentioned, being still too general, it was found necessary to establish subdivisions—more and more specialized—to which the names families, genera, species, and varieties were given.
[2] Mathias Duval, ‘Précis of Anatomy for the Use of Artists’: Paris, 1881. ‘Artistic Anatomy of the Human Body,’ third edition, plates by Dr. Fau, text with figures by Édouard Cuyer: Paris, 1896. ‘Artistic Anatomy of Man,’ by J. C. L. Sparkes, second edition, text with 50 plates: Baillière, Tindall and Cox, London, 1900.