the classes into orders, the orders into families, the families into genera, the genera into species, and the species into varieties. The guiding principle of anatomical resemblance determined each of these divisions. Those largest groups, which resemble each other only in having what is called the typical character in common, are brought together under the first head. Thus all the groups which agree in possessing a backbone and internal skeleton, although they differ widely in form, structure, and habitat, do nevertheless resemble each other more than they resemble the groups which have no backbone. This great division having been formed, it is seen to arrange itself in very obvious minor divisions, or Classes—the mammalia, birds, reptiles, and fishes. All mammals resemble each other more than they resemble birds; all reptiles resemble each other more than they resemble fishes (in spite of the superficial resemblance between serpents and eels or lampreys). Each Class again falls into the minor groups of Orders; and on the same principles: the monkeys being obviously distinguished from rodents, and the carnivora from the ruminating animals; and so of the rest. In each Order there are generally Families, and the Families fall into Genera, which differ from each other only in fewer and less important characters. The Genera include groups which have still fewer differences, and are called Species; and these again include groups which have only minute and unimportant differences of colour, size, and the like, and are called Sub-species, or Varieties.
Whoever looks at the immensity of the animal kingdom, and observes how intelligibly and systematically it is arranged in these various divisions, will admit that, however imperfect, the scheme is a magnificent product of human ingenuity and labour. It is not an arbitrary arrangement, like the grouping of the stars in constellations; it expresses, though obscurely, the real order of Nature. All true Classification should be to forms what laws are to phenomena: the one reducing varieties to systematic order, as the other reduces phenomena to their relation of sequence. Now if it be true that the classification expresses the real order of nature, and not simply the order which we may find convenient, there will be something more than mere resemblance indicated in the various groups; or, rather let me say, this resemblance itself is the consequence of some community in the things compared, and will therefore be the mark of some deeper cause. What is this cause? Mr. Darwin holds that "propinquity of descent—the only known cause of the similarity of organic beings—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications"[*]—"that the characters which naturalists consider as showing true affinity between any two or more species are those which have been inherited from a common parent, and in so far all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general pro-*
- DARWIN: Origin of Species, p. 414.
