Intelligence of Monkeys, Apes and Baboons - Psychological Study

Although monkeys do not reach the human stage of a rich life of ideas, yet they carry the animal method of learning by the selection of impulses and association of them with different sense impressions, to a point beyond that reached by any other of the lower animals. In this, too, they resemble man; for he differs from the lower animals not only in the possession of a new sort of intelligence but also in the tremendous extension of that sort which he has in common with them... The intelligence of apes, monkeys, and baboons, unfortunately, has not presented material for nearly so many observations as that of other intelligent mammals.
Useless for all purposes of labour or art, mischievous as domestic pets, and in all cases troublesome to keep, these animals have never enjoyed the improving influences of hereditary domestication, while for the same reasons observation of the intelligence of captured individuals has been comparatively scant. Still more unfortunately, these remarks apply most of all to the most man-like of the group, and the nearest existing prototypes of the human race: our knowledge of the psychology of the anthropoid apes is less than our knowledge of the psychology of any other animal.
But notwithstanding the scarcity of the material which I have to present, I think there is enough to show that the mental life of the Simiadæ is of a distinctly different type from any that we have hitherto considered, and that in their psychology, as in their anatomy, these animals approach most nearly to Homo sapiens...