ON THE RELATIONS OF MAN WITH THE LOWER ANIMALS 485 
circumference, as Greding has stated; usually it is bent on itself, 
arched forwards and outwards, sometimes narrow and long, sometimes 
broader. Very frequently it is smooth, at other times it exhibits 
many fissures and small enlargements, especially posteriorly ; or it 
may be divided by a longitudinal cleft into two halves, the upper of 
which is almost always larger than the lower. Its dimensions are by 
no means directly proportional to the development of the posterior 
lobe. In the same subject it may be very distinct upon the one side, 
and yet be hardly perceptible upon the other. For the rest I can 
certify that, in spite of Meckel’s! assertion to the contrary, it is not 
always present. My own observations agree with those of Wenzel, 
who, among fifty-one subjects that he examined with express refer- 
ence to this point, found three in which the hippocampus was absent 
upon both sides and two in which every trace of it was absent upon 
one side only.” 
To allow a structural character totally absent in six percent. of the 
members of any group to stand as part of the definition of that group, 
considered as a sub-class, would be a very hazardous proceeding. But, 
is it true that the hippocampus minor is altogether absent in the highest 
apes? I suspect that Tiedemann is responsible for the not unfrequently 
admitted doctrine that it is; for, in the “ Icones” he writes :— 
“Pedes hippocampi minores vel ungues, vel calcaria avis, que a 
posteriore corporis callosi margine tanquam processus duo medullares 
proficiscuntur, inque fundo cornu posterioris plicas graciles et retro- 
flexas formant, in cerebro simiarum desunt; nec in cerebro aliorum 
ameexaminatorum mammalium occurrunt. omen? ergo propriz sunt.” 
However, the citation from the Memoir of Schrceder 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— 
“T 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 :— 
“TI thought it best for my own satisfaction and yours, to open the 
1 Dr. Hein here adds: ‘* What Meckel says is that he himself never failed to find the 
hippocampus minor, but that he by no means wishes to throw doubts on Wenzel’s state- 
ments ;” and on reference to Meckel’s work, I find this to be quite correct. 
