Page:Natural History Review (1861).djvu/218

206 our figures speak, as an individual rather than as a specific peculiarity, we are compelled to assign greater importance to the curtailment in downward growth to which they, as well as other similar figures, testify. A line drawn along the edge of the cerebral hemisphere in Fig. 1, where that hemisphere overlies the cerebellum, will be seen to be much less nearly horizontal than a line is which holds the same relation in a human brain. It seems as if the cerebellum had encroached upon the cerebral lobes which roofed it over.

The same figure shows that a similar stunting has befallen the upward growth of both the frontal and posterior lobes, a line bounding the superior edge of the hemispheres from D forwards to A, describing a much more even curve than is usual in man.

Less ambiguously does the vertical direction of the fissures of Sylvius, F, and of the convolution 6, 6, 6'; parallel with, and immediately below the lower lip, 7, 7, 7, of that fissure, speak of diminished relative anteroposterior growth of the frontal lobes.

The greater relative thickness of the nerves is well seen in Fig. 2.

These nine points of greater or less discrepancy between the human and the Simious brain may be arranged under our first head: they consist, in the ape, of diminution in downward, lateral, upward, and anteroposterior growth, first, of the posterior; secondly, of the frontal lobes; and to these, based on consideration of diminution, we have to add the ninth, based upon a consideration of increase, that, viz., of the size of the nerves. What is the value of these points as differentiating characteristics? Two canons may be laid down, to assist us in estimating the value of such characteristics as means for settling the relative rank of rival organisms. The first of these may be thus expressed:—If certain structures, or certain relations of certain structures, are found to exist in animals confessedly lower in the scale of life than those which are the subjects of comparison, the presence of such structures, or of such relations of structures, cannot by itself he held to be a mark of serial elevation. Cumulatively it may have weight, absolutely it can have none. The second canon is but a converse of the first; and, expressed in similar language, it may run thus:—If certain structures, or certain relations of certain structures, are found to exist in animals confessedly higher in the scale of life than those which are the subjects of comparison, such structures, or such relations of such structures, cannot by themselves be held to be marks of serial degradation. Cumulatively, they may be of weight; absolutely, they are not. These canons have been, perhaps necessarily, expressed in complex language; in themselves, however, they are sufficiently simple and self-evident, and, being so, are compatible with either view of the origin of species.

The first of these canons we have already applied, in our comparison of the overlapped cerebellum of the lower monkeys with the partially unoverlapped cerebellum of our orang. The even curve described by the boundary line of the superior surfaces of the Irishwoman's brain, as given by M. Gratiolet in the first plate appended to his often-quoted work, and the anteriorly and posteriorly tapering ends of the