Correspondence on Biology, etc. 



do not mean the '' mean " during the whole course of de- 

 velopment of the organ, as you seem to take it. That would 

 indeed be absurd. I do mean the '' mean " of the whole 

 series of individual variations now occurring, during a 

 period sufficient to contain all or almost all the variations 

 to which the species is now subject. Take, for instance, 

 such a case as the wings of the swallow, on the full de- 

 velopment of which the life of the bird depends. Manv 

 individuals no doubt perish for lack of wing-power, due to 

 deficiency in size or form of wing, or in the muscles which 

 move it. The extreme limits of variation would be seen 

 probably if we examined every swallow that had reached 

 maturity during the last century. The average of all those 

 would perhaps be 5 or 10 per cent, below the average of 

 those that sur\'ive to become the parents of the next genera- 

 tion in any year; and what I maintain is, that panmixia 

 alone could not reduce a swallow's wings below this first 

 average. Any further reduction must be due either to 

 some form of selection or to " economy of growth " — which 

 is also, fundamentally, a form of selection. So with the 

 eyes of cave animals, panmixia could only cause an imper- 

 fection of vision equal to the average of those variations 

 which occurred, say, during a century before the animal 

 entered the cave. It could only produce more effect than 

 this if the effects of disuse are hereditary — which is a non- 

 Weismannian doctrine. I think this is also the position 

 that Eomanes took. — Yours faithfully, 



A. R. Wallace. 



To Mr. J. W. Marshall 



ParksioTie, Dorset. September 23, 1892. 



My dear Marshall, — I am glad you enjoyed Mr. Hudson's 

 book. His observations are inimitable — and his theories and 



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