Last of all, it may be well to stop a moment to show the cogency of this evidence. If it were a question of the origin of some structure not only useful (for all structures selected by Nature must be useful) but the _best imaginable_, like the eye or the ear, for example; then, if _we examined only the highest form_ or _the finished article_, there are two ways in which it is possible to explain the adaptive structure. We may either suppose that it was made at once out of hand, by some intelligent contriver; or else that it was slowly made by a process of evolution, becoming more and more perfect by a selection of only the most perfect from generation to generation. But in the case of the six aortic arches of the lizard, we are shut up to the one explanation only, viz., by slow process of evolution. One arch is all that is necessary, as is plainly shown by the use of only one in the more perfect circulation of birds and mammals. If the thing were done out of hand, unconditioned by the previous structure in fishes, to have made six was surely but a bungling piece of work. [Illustration: FIG. 45.--Fish-brain. A, side view; B, top view.] [Illustration: FIG. 46.--Reptile-brain. A, side view; B, top view.] [Illustration: FIG. 47.--Bird-brain. A, side view; B, top view.] [Illustration: FIG. 48.--Mammal-brain. A, top view; B, side view.] [Illustration: FIG. 49.--Man’s brain. A, side view; B, top view.] =3. Vertebrate Brain.=--Another excellent example is the structure of the vertebrate brain. The brain of an average fish is represented in Fig. 45. It consists of four or five swellings, or ganglia, strung along, one beyond another. Commencing behind, these are, first, the medulla, _m_; then the cerebellum, _cb_; then the optic lobes, _ol_; then the cerebrum and thalamus combined, _cr_; and last, the olfactive lobes, _of_. Of these, it will be observed, the optic lobe is the largest in the brain of the fish (Fig. 45). In the brain of the reptile (Fig. 46) we have the same serial arrangement, of the same parts, only that the cerebrum has now become the dominant part instead of the optic lobes. In the average bird (Fig. 47) the cerebrum has grown so large that it extends backward, and partly covers the optic lobes. In the lower mammals (marsupials), the brain is much the same in this respect, as in birds--i. e., the cerebrum only partly covers the optic lobes, so that, looked at from above, the whole series of ganglia are still visible. But in the average mammal (Fig. 48) the cerebrum is so enlarged that it covers entirely the optic lobes and encroaches on the cerebellum behind and the olfactive lobes in front. In some monkeys, indeed, the cerebellum is nearly or even quite covered. Finally, in man (Fig. 49), the cerebrum has grown so enormously that it covers every other part and completely conceals them from view when the brain is looked at from above. In front it not only covers but has grown far