In many respects the most thoroughgoing and workmanlike experimental study of monkeys is that of Kinnaman (1902), who has reported on the study of various forms of response in P. rhesus.  He presents valuable data concerning the learning processes, sensory discrimination, reaction to number, and to tests of imitation.  His results indicate a higher level of intelligence than that discovered by Thorndike, but this is almost certainly due to difference in the species observed.  Kinnaman goes so far as to say “We have found evidence, also, of general notions and reasoning, both of low order” (p. 211).

The contribution of Hobhouse (1915) to our knowledge of the mental life of monkeys, although in a measure experimental, is based upon relatively few and unsystematic observations as contrasted with those of Thorndike and Kinnaman.  It appears, however, that Hobhouse’s experiments were admirably planned to test the ideational capacity of his subjects, and one can not find a more stimulating discussion of ideation than that contained in his “Mind in Evolution.”  The results of his tests made with a P. rhesus monkey are similar to those of Kinnaman, for almost all of them indicate the presence and importance of ideas.

Watson (1908) in tests of the imitative ability of P. rhesus saw relatively little evidence of other than extremely simple forms of ideation.  But in contrast with his results, those obtained by Haggerty (1909), in a much more extended investigation in which several species of monkey were used, obtained more numerous and convincing evidences of ideation in imitative behavior.  Although this author wholly avoids the use of psychological terms, seeking to limit himself to a strictly objective presentation of results, it is clear from an unpublished manuscript (thesis for the Doctorate of Philosophy, deposited in the Library of Harvard University) that he would attribute to monkeys simple forms of ideational experience.

Witmer (1910) reports, in confirmation of Haggerty’s results, intelligently imitative behavior in P. irus.

The work of Shepherd (1910) agrees closely, so far as evidences of ideation are concerned, with that of Thorndike.  He obviously strives for conservatism in his statements concerning the adaptive intelligence of his monkeys, all of which belonged to the species P. rhesus.  At one point he definitely states that they exhibit ideas of a low order, or something which corresponds to them.  Satisfactory evidences of reasoning he failed to obtain.

Franz’s (1907, 1911) studies of monkeys, unlike those mentioned above, have for their chief motive not the accurate description of various features of behavior but instead knowledge of the functions of various portions of the brain.  His results, therefore, although extremely interesting and of obvious value to the comparative psychologist, throw no special light upon the problem of ideation.