the internal structure of the brain, the large size of the optic thalami in relation to the corpora striata, and the total absence of a posterior cornu to the lateral ventricle[1]—are all characters which are perfectly obvious, and which separate the brain of the Lemur as completely from that of Pithecus or Troglodytes, as from that of man.
The description of the brain of Stenops tardigradus, by Vrolik, tells the same story even more strikingly; and the brains of Perodicticus and other Prosimiæ, exhibited in the Hunterian Museum, fully bear out the conclusion, that the vast differences noted obtain throughout the Prosimian division of the Quadrumana.
M. Gratiolet, in fact, has been so struck by the immense discrepancy between the Simiæ and Prosimiæ in cerebral structure, that he proposes to consider the latter as forming a part of the order Insectivora. In this view he is at variance with all the other zoologists; but, in order to meet all possible objections, I will, for the moment, suppose that he is right, and that the order Quadrumana should be restricted to the Simiæ. Even on this supposition, the force of my argument remains unchanged; for the brains of the lower true apes and monkeys differ far more widely from the brain of the orang than the brain of the orang differs from that of man. Not only do they differ from the orang (and to a greater degree) in most of those respects in which the orang differs from man, but they present the absolute distinction, that while the orang, like man, has two corpora candicantia, the lower apes, like the other Mammalia, have only one.
In respect of their cerebral characters, therefore, I hold it to be demonstrable that the Quadrumana differ less from man than they do from one another; and that, hence, the separation of Homo and Pithecus in distinct sub-classes, while Pithecus and Cynocephalus are retained in one order, is utterly inconsistent with the principle of any classification of the Mammalia by cerebral characters.
On a future occasion I propose to take up the question, whether, on other grounds, there is any reason for departing from the Linnean view, that man is to be regarded as a genus of the same order as that which contains the Quadrumana.
- ↑ "Cornu posterius in Simiis et Phocis brevissimum et vix conspicuum est: in cæteris mammalibus plane desideratur."—Icones, p. 54.
