A COMMENTARY ON LAMARCKISM 



be supposed to have the same properties and to be present in 

 the same numbers. Expose one side of the face or one arm to 

 sunhght, and it will become darker than the other, no matter 

 why. The difference of degree of pigmentation is caused by 

 differences of environmental stimuli, and by them alone. 



Differences of Class B, those which are open to a Lamarckian 

 interpretation of their origin, are commoner than is usually 

 supposed. The flexure lines of the palm of the hand provide a 

 splendid example. The bolder flexure lines are easily visible in 

 the twelve weeks' foetus, and even if the foetus is not incapable 

 of clenching its hands, it would be idle to suppose that flexure 

 lines were formed by the imprint of habitual use. Use neither 

 forms them nor keeps them in being, for the plastic surgeon 

 tells us that if skin grooved by a flexure line is displaced or 

 transplanted to positions in which it is not normally creased 

 or folded, the flexure lines will nevertheless persist. Yet ectopic 

 flexure lines can be formed by habitual creasing of the skin — 

 by frowning for example, or raising the eyebrows; and we can 

 therefore be quite confident in saying that if the palmar flexure 

 lines were not developmentally prefabricated, a very exact 

 copy of them would soon be formed in the ordinary run of 

 everyday use. Ectopic flexure lines are exactly analogous 

 to the corns and callosities that were called in evidence in our 

 earlier example. They differ from the Suborn" flexure lines 

 because they disappear with the withdrawal of the stimulus 

 that was responsible for their formation. 



In saying that flexure lines and thickened soles are of 

 developmental origin, I do not wish to deny that use within 

 an individual's own lifetime may not make flexure lines bolder 

 and soles thicker still. Why should it not be so, if folding and 

 chafing of the skin can cause the formation of ectopic flexure 

 lines or epidermal thickenings elsewhere on the body? It is 

 likely, but not certain, that the ordinary use of a joint or bone 

 completes the otherwise purely developmental differentiation 



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