However, the citation from the Memoir of Schroeder van der Kolk and Vrolik, given above, proves that in their opinion a rudimentary hippocampus minor does exist in the Chimpanzee, and Dr. Allen Thomson adds his valuable testimony in a still more decided manner to the same effect. In the letter which I have already quoted, he says:—
"I found an eminence in the floor of the posterior cornu and towards its inner side, which I regarded as the hippocampus minor, and I found it produced exactly in the same manner as in man, by the bulging into the ventricles of a portion of the brain, by a very deep groove between the convolutions."
In another letter (the 11th of November, 1860), replying to further troublesome inquiries of mine, Dr. Thomson writes:—
"I thought it best for my own satisfaction and yours, to open the lateral ventricle from above, in a second brain which I possess. This brain, which was extracted from a young animal in Africa, was placed in rum there, and it was both much discoloured and not so well preserved as I could have wished. The appearances are, however, sufficiently distinct to enable me to confirm entirely what I think I stated to you before, viz.: 1. The prolongation of the cavity of the posterior cornu, to a considerable distance beyond the plane of the posterior edge of the corpus callosum (which, I presume, may be taken as the best measure of the position of the parts); and, 2. The existence on the inner side, and partly in the floor of that posterior cornu, of an eminence corresponding in all respects with the hippocampus minor. . . . . . . . . Just as I was setting about the examination of this point, I found an opportunity, in my dissecting-room, of looking at a fresh human brain, and I thought it might be more satisfactory to examine the two brains together. It so turned out, that the brain I cut in upon presented an example (not uncommon) of great deficiency in the extent of the posterior cornu. I think it is worth sending you a sketch of it, for it is really scarcely more developed than that of the chimpanzee in this respect."
Having now, as I trust, redeemed my pledge to prove that neither the third lobe of the cerebrum, nor the posterior cornu of the lateral ventricle, nor the hippocampus minor, are structures distinctive of and "peculiar to the genus Homo," I may leave it to the reader to decide the fate of the "sub-class Archencephala" founded upon the supposed existence of these three distinctive characters.
And here I might fairly leave the question; but, essential as I have felt it to be to my personal and scientific character to prove that my public assertions are entirely borne out by facts, I am far from desiring to deal with this important matter in a merely controversial spirit. Therefore, although the differences hitherto referred to are certainly nonexistent, I proceed to inquire whether there are any other marked and constant characters by which the human may be distinguished from the Simian brain.
Without doubt such characters are to be found; and in all probability, as in the case of any other two distinct genera, the more carefully and minutely our inquiries are carried out, the greater will be the number of these differentiæ. So far as my knowledge goes, the most prominent and important are the following:—
1 . In the anthropoid apes the brain is smaller, as compared with the nerves which proceed from it, than in man.
