The Descent of Man

The variability or diversity of the mental faculties
in men of the same race, not to mention the
greater differences between the men of distinct
races, is so notorious that not a word need here
be said. So it is with the lower animals. All who
have had charge of menageries admit this fact,
and we see it plainly in our dogs and other
domestic animals. Brehm especially insists that
each individual monkey of those which he kept
tame in Africa had its own peculiar disposition
and temper: he mentions one baboon remarkable
for its high intelligence; and the keepers in
the Zoological Gardens pointed out to me a
monkey, belonging to the New World division,
equally remarkable for intelligence. Rengger,
also, insists on the diversity in the various mental
characters of the monkeys of the same species
which he kept in Paraguay; and this diversity,
as he adds, is partly innate, and partly the
result of the manner in which they have been
treated or educated. (8. Brehm, 'Thierleben,' B.