Animals made-up of parts mutually related in various ways.--What homology is.--Its various kinds.--Serial homology.--Lateral homology.--Vertical homology.--Mr. Herbert Spencer's explanations.--An internal power necessary, as shown by facts of comparative anatomy.--Of teratology.--M. St. Hilaire.--Professor Burt Wilder.--Foot-wings.--Facts of pathology.--Mr. James Paget.--Dr. William Budd.--The existence of such an internal power of individual development diminishes the improbability of an analogous law of specific origination. That concrete whole which is spoken of as "an individual" (such, _e.g._, as a bird or a lobster) is formed of a more or less complex aggregation of parts which are actually (from whatever cause or causes) grouped, together in a harmonious interdependency, and which have a multitude of complex relations amongst themselves. The mind detects a certain number of these relations as it contemplates the various component parts of an individual in one or other direction--as it follows up different lines of thought. These perceived relations, though subjective, _as relations_, have nevertheless an objective foundation as real parts, or conditions of parts, of real wholes; they are, therefore, true relations, such, _e.g._, as those between the right and left hand, between the hand and the foot, &c. The component parts of each concrete whole have also a relation of resemblance to the parts of other concrete wholes, whether of the same{156} or of different kinds, as the resemblance between the hands of two men, or that between the hand of a man and the fore-paw of a cat. Now, it is here contended that the relationships borne one to another by various component parts, imply the existence of some innate, internal condition, conveniently spoken of as a power or tendency, which is quite as mysterious as is any innate condition, power, or tendency, resulting in the orderly evolution of successive specific manifestations. These relationships, as also this developmental power, will doubtless, in a certain sense, be somewhat further explained as science advances. But the result will be merely a shifting of the inexplicability a point backwards, by the intercalation of another step between the action of the internal condition or power and its external result. In the meantime, even if by "Natural Selection" we could eliminate the puzzles of the "origin of species," yet other phenomena, not less remarkable (namely, those noticed in this chapter), would still remain unexplained and as yet inexplicable. It is not improbable that, could we arrive at the causes conditioning all the complex inter-relations between the several parts of one animal, we should at the same time obtain the key to unlock the secrets of specific origination.