146 INITIATIVE IN EVOLUTION 



and trunk differ much, in respect of the qualities of their epidermis, 

 from the dorsal. The skin over the former is softer, thinner and 

 more flexible than the latter, which is in adult life thick, hard and 

 with larger openings of the sebaceous glands. As the two main 

 layers of the skin are so closely united it is impossible to state any 

 general rule as to the parts played in this manufacture by the 

 epidermis and dermis respectively. Altogether the skin from 

 the dorsal surfaces of mammals provides a much denser fabric than 

 the latter, and different qualities of leather are obtained from different 

 regions. Corresponding differences of texture are found on the 

 extensor and flexor surfaces of the limbs, especially on the hands 

 and feet. In the course of his long evolution from a hairy stock, 

 whether simian as we thought yesterday, or a lower one as Professor 

 Woods Jones suggests to-day, these dorsal surfaces of neck, trunk 

 and extensor surfaces of limbs have been exposed through countless 

 generations of men to vastly more stimuli of friction, pressure, 

 and response, than those of the ventral and flexor regions. As 

 man's hairy covering diminished, through some mysterious and 

 at present unrecognised cause, these stimuli became increasingly 

 potent in producing a tissue denser than that of the more protected 

 ventral parts where all forms of these stimuli are slight. I do not 

 claim that this was a phenomenon that began with man, for in a 

 measure it was present in those forms which preceded him, and 

 in many related mammals under the cover of their hairy covering. 

 When we remember, or conceive what a large portion of 

 each of his 24-hours even in his earliest form throughout life man 

 must have spent, as he still does, in lying on his back or sides, 

 and in sitting with his back against a supporting object, and with 

 his gluteal and ischial regions pressed hard against whatever 

 seat he has selected in cave or drawing-room, we need not travel 

 far in thought to understand how great has been the preponderance 

 of stimuli from friction and pressure on the dorsal and extensor 

 surfaces over those on the ventral and flexor — and here comes in our 

 familiar " total experience " with stimulus and response spread 

 over a vast stretch of time. It must be borne in mind that from 

 the facts of the case a very large number of individual men and 

 women were exposed to similar, but not the same stimuli at each 

 stage of the process involved. It is matter of common knowledge 

 that not only on the palm and sole of man, but on regions where 

 the skin is not specialised in that remarkable manner that is found 

 in those regions, but also in others, that incieased pressure and 

 friction will very soon cause a harder and thicker growth of epi- 

 dermis, as on the skin over a projecting bone in club-foot, over the 



