In this example of ridges developed on an abnormal situation we see what is, perhaps, an undesigned experiment as to the production of ridges by a more frequent habit of walking in captivity than would be found to occur in the wild state, for, as Lydekker says in the _Royal Natural History_, Vol. I, p. 27, “When the chimpanzee goes on all-fours, he generally supports himself on the backs of his closed fingers rather than on the palm of the hand (see Fig. 6 of the illustration on p. 15) and he goes _sometimes_ on the soles of his feet and _sometimes_ on his closed toes.” [Illustration: Fig. 70.--Left hand of chacma baboon.] I have underlined purposely this word “sometimes,” for in the instance I have described, not only the presence of the ridges and their direction on the backs of the fingers but their absence on the backs of the toes is significant, and I suggest that the _chimpanzees_ examined have not sufficiently often exposed the backs of their toes to pressure and friction for the production of ridges, whereas those on the backs of the fingers have done so. Another point worth notice is that in the oldest of the three _chimpanzees_, “Mickie,” æt six years, the greatest number of ridges is present; in “Jimmie,” æt two-and-a-half years, they were “small and ill-defined as if in process of development,” and in “Jack,” æt twenty months they were absent. This would agree at any rate with the hypothesis that the element of time and frequent repetition of stimuli enter into the causation of aberrant ridges. A similar condition, with aberrant papillary ridges, has been found on the digits of the hand of the _orang_. On the heel of adult man ridges are found surrounding it, of the average depth of one inch from the plantar surface, and in one particular case of a woman aged forty-nine, the depth of this area on each foot measured was one and a half inches from the plantar surface. The extensor surface, or back, of the little toe shows ridges when it is distorted by ill-fitting boots. In man ridges frequently appear on the radial side of the back or extensor surface of the index finger to nearly the middle line of the finger, and this is often more on the right than the left hand. CHAPTER XIX. FLEXURES OF THE PALM AND SOLE.