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.
I was most pleased with his demonstration as to the supposed instincts
of young birds and lambs, showing clearly that the former at all events
are not due to inherited experience, as Darwin thought. The whole book,
too, is pervaded by such a true love of nature and such a perception of
its marvels and mysteries as to be unique in my experience.
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