My dear Mr. Poulton,--As to panmixia you have quite misunderstood my position. By the "mean condition," I do not mean the "mean" during the whole course of development 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 development of which the life of the bird depends. Many 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 survive to become the parents of the next generation 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 imperfection 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 Romanes took.--Yours faithfully, A.R. WALLACE. * * * * * TO MR. J.W. MARSHALL _Parkstone, Dorset. September 23, 1892._ My dear Marshall,--I am glad you enjoyed Mr. Hudson's book. His observations are inimitable--and his theories and suggestions, if not always the best, at least show thought on what he has observed.