THE UNIQUENESS OF THE INDIVIDUAL 



argued that they have arisen in the way that has just been 

 proposed? The answer is a very simple and obvious one: they 

 are character differences having the distinctive property that, 

 although they are in fact ''laid on** by development, they could 

 in any event have been fashioned in a?i individuaVs own lifetime 

 merely as a response to differences of use. 



Consider, for example, the difference between the character- 

 istically thick, richly stratified and mitotically active epidermis 

 on the sole or heel of the foot and the thinner and more delicate 

 epidermis that covers the greater part of the rest of the body. 

 The difference is at least in large part of purely developmental 

 origin, i.e. of the same sort as that which distinguishes epi- 

 dermal cells from pancreatic or thyroid cells. It does not arise, 

 as we are at first tempted to think, because of the chronic 

 chafing and general mechanical stress that soles of feet are 

 obliged to put up with (although such stimuli can certainly 

 exaggerate the difference). Both the human being and the 

 guinea-pig are born with a thicker epidermis on the sole of the 

 foot than elsewhere on the body.* Such a difference is therefore 

 developmentally prefabricated; it could not have arisen as an 

 adaptive response in utero because the foetus treads water in 

 so far as it treads at all. 



The argument may be reinforced by experimental proof. If 

 the difference between trunk and sole-of-foot epidermis arose 

 merely because the latter is habitually trodden upon and other- 

 wise abused, while the former is not, then sole-of-foot epidermis 

 should revert to the condition of relatively delicate and 

 quiescent body skin after transplantation to a protected posi- 

 tion elsewhere on the body. Billingham and I (1948, a, ^), have 

 done this experiment on the guinea-pig, and find that sole-of- 

 foot skin conserves its distinctive thickness, stratification and 

 mitotic activity even two years after its transplantation to a 



* [A fact well known to Darwin, and commented upon by others since: see 

 C. H. Waddington's The Evolution of Adaptations in Endeavour, 12, 1953.] 



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