It is to be hoped that the growing recognition of the worth of comparative and genetic studies will lead to investigations of the mental make-up of other species of monkeys, and to the careful overhauling of the work done so far, including these rather fragmentary studies of mine. Work with three monkeys of one species, especially when no general body of phenomena, such as one has at hand in the case of domestic animals, can be used as a means of comparison, must necessarily be of limited application in all its details and of insecure application even in its general features. What I shall say concerning the advance in the mental development of the monkeys over that of other mammals may then be in strictness true of only my three subjects, and it may be left to the judgment of individuals to extend my conclusions as far as seems to them likely. To me it seems fairly likely that the very general mental traits which the research has demonstrated hold true with little variation in the monkeys in general. The monkeys represent progress in mental development from the generalized mammalian type toward man:— 1. In their sensory equipment, in the presence of focalized vision. 2. In their motor equipment, in the coördinated movements of the hand and the eye. 3. In their instincts or inherited nervous connections, in their general physical and mental activity. 4. In their method of learning or associative processes; in— _a._ Quicker formation of associations, _b._ Greater number of associations, _c._ Greater delicacy of associations, _d._ Greater complexity of associations, _e._ Greater permanence of associations. The fact of (1) is well known to comparative anatomists. Its importance in mental development is perhaps not realized, but appears constantly to a systematic student. (2) is what accounts for much of the specious appearance of human ways of thinking in the monkeys and becomes in its human extension the handy tool for much of our intellectual life. It is in great measure the prerequisite of 4 _c_. (3) accounts for the rest of such specious appearances, is at the basis of much of 4 _b_, presages the similar though extended instincts of the human being, which I believe are the leading efficient causes of human mental capacity, and is thus the great mental bond which would justify the inclusion of monkeys and man in a common group if we were to classify animals on the basis of mental characteristics.