Even from this point of view it is to some extent possible to see upon what depends that _homology_ in the skeleton primarily of mammals, and in a wider sense of all vertebrates, which has been so fully explained, especially by Richard Owen in his “_Ostéologie comparée_,” and on account of which, for example, all mammals have seven cervical vertebræ, every bone of the human hand and arm finds its analogue in the fin of the whale, the skull of the bird in the egg has exactly as many bones as that of the human fœtus, &c. All this points to a principle which is independent of teleology, but which is yet the foundation upon which teleology builds, or the already given material for its works, and just that which Geoffroy St. Hilaire has explained as the “anatomical element.” It is the _unité de plan_, the fundamental type of the higher animal world, as it were the arbitrarily chosen key upon which nature here plays. Aristotle has already correctly defined the difference between the efficient cause (_causa efficiens_) and the final cause (_causa finalis_) in these words: “Δυο τροποι της αιτιας, το οὑ ἑνεκα και το εξ αναγκης, και δει λεγοντας τυγχανειν μαλιστα μεν αμφοιν.” (_Duo sunt causæ modi: alter cujus gratia, et alter e necessitate; ac potissimum utrumque eruere oportet._) _De part. anim._, i. 1. The efficient cause is that _whereby_ something is, the final cause that _on account of which_ it is; the phenomenon to be explained has, in time, the former _behind_ it, and the latter _before_ it. Only in the case of the voluntary actions of animal beings do the two directly unite, for here the final cause, the end, appears as the motive; a motive, however, is always the true and proper _cause_ of the action, is wholly and solely its _efficient_ cause, the change preceding it which calls it forth, by virtue of which it necessarily appears, and without which it could not happen; as I have shown in my prize essay upon freedom. For whatever of a physiological nature one might wish to insert between the act of will and the corporeal movement, the _will_ always remains here confessedly that which moves, and what moves _it_ is the _motive_ coming from without, thus the _causa finalis_; which consequently appears here as _causa efficiens_. Besides, we know from what has gone before that the bodily movement is one with the act of will, for it is merely its phenomenal appearance in cerebral perception. This union of the _causa finalis_ with the efficient cause in the one phenomenon _intimately_ known to us, which accordingly remains throughout our typical phenomenon, is certainly to be firmly retained; for it leads precisely to the conclusion that at least in organised nature, the knowledge of which has throughout final causes for its clue, a _will_ is the forming power. In fact, we cannot otherwise distinctly think a final cause except as an end in view, _i.e._, a motive. Indeed, if we carefully consider the final causes in nature in order to express their