I. The Arguments from Classification. Spencer started with the fact that naturalists have utilised resemblances in structure and development as a basis for the orderly classification of organisms in groups within groups--varieties, species, genera, families, races, and so on. But "this is the arrangement which we see arises by descent, alike in individual families and among races of men." "Where it is known to take place evolution actually produces these feebly-distinguished small groups and these strongly-distinguished great groups." "The impression made by these two parallelisms, which add meaning to each other, is deepened by the third parallelism, which enforces the meaning of both--the parallelism, namely, that as, between the species, genera, orders, classes, etc., which naturalists have formed, there are transitional types; so between the groups, sub-groups, and sub-sub-groups, which we know to have been evolved, types of intermediate values exist. And these three correspondences between the known results of evolution (as in human races, domesticated animals, and cultivated plants) and the results here ascribed to evolution have further weight given to them by the fact, that the kinship of groups through their lowest members is just the kinship which the hypothesis of evolution implies." Even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strikingly display, is strongly suggestive of evolution. Freeing ourselves from pre-conceptions, we shall see good reason to think with Mr Darwin, "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 partly revealed to us by our classifications" (_Principles of Biology_, Rev. Ed. vol. i. p. 448).
II. Arguments from Embryology. Organisms may be arranged on a tree which symbolises their structural affinities and divergences. On the evolutionist interpretation this is an adumbration of the actual genealogical tree or _Stammbaum_. But when we consider the facts of embryology we find that the developing organism advances from stage to stage by steps which are more or less comparable to the various levels and branchings of the classificatory tree. There is a resemblance, sometimes a parallelism, between individual development and the grades of organisation which have or have had persistent stability as living creatures. "On the hypothesis of evolution this parallelism has a meaning--indicates that primordial kinship of all organisms, and that progressive differentiation of them which the hypothesis alleges. On any other hypothesis the parallelism is meaningless." It is true that there are nonconformities to the general law that individual development tends to recapitulate racial history, or that ontogeny tends to recapitulate phylogeny. There may be in the individual development condensations or telescopings of the presumed ancestral stages, and there may be an interpolation of developmental stages which are adaptive to peculiar conditions of juvenile life and have no historical import, but the deviations are such as may be readily interpreted on the evolution-hypothesis (_Principles of Biology_, i. pp. 450-467).