Page:A Text-book of Animal Physiology.djvu/64

 there can be no perfect classification of objects; hence, any classification must be regarded but as the temporary creed of science, to be modified with the extension of knowledge. As a matter of fact, this has been the history of all zoological and other systems of arrangement. The only purpose of grouping is to simplify and extend knowledge; this being the case, it follows that a method of grouping that accomplishes this has value, though the system may be artificial that is based on resemblances which, though real and constant, are associated with differences so numerous and radical that the total amount of likeness between objects thus grouped is often less than the difference. Such a system was that of Linnæus, who classified plants according to the number of stamens, etc., they bore.

Seeing that animals which resemble each other are of common descent from some earlier form, to establish the line of descent is to determine in great part the classification. Much assistance in this direction is derived from embryology, or the history of the development of the individual (ontogeny); so that it may be said that the ontogeny indicates, though it does not actually determine, the line of descent (phylogeny); and it is owing to the importance of this truth that naturalists have in recent years given so much attention to comparative embryology.

It will be inferred that a natural system of classification must be based both on function and structure, though chiefly on the latter, since organs of very different origin may have a similar function; or, to express this otherwise, homologous structures may not be analogous; and homology gives the better basis for classification. To illustrate, the wing of a bat and a bird are both homologous and analogous; the wing of a butterfly is analogous but not homologous with these; manifestly, to classify bats and birds together would be better than to put birds and insects in the same group, thus leaving other points of relationship out of consideration.

The broadest possible division of the animal kingdom is into groups, including respectively one-celled and many-celled forms—i. e., into Protozoa and Metazoa. As the wider the grouping the less are differences considered, it follows that the more subdivided the groups the more complete is the information conveyed: thus, to say that a dog is a metazoan is to convey a certain amount of information; that it is a vertebrate, more; that it is a mammal, a good deal more, because each of the latter terms includes the former.