whole order – for example, of the batrachia – asserts itself even where in one particular species (pipa) one of its determinations is superfluous. Still less can nature allow a determination (eyes) which belongs to the type of a whole division (Vertebrata) to vanish entirely without a trace, even if it is wanting in some particular species (Mus typhlus) as superfluous; but here also it must at least indicate in a rudimentary manner what it carries out in all the others.
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 Geoffrey 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: "(
Greek characters)" (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
