[47]
_The Swamping Effects of Intercrossing._
This supposed insuperable difficulty was first advanced in an article in
the _North British Review_ in 1867, and much attention has been
attracted to it by the acknowledgment of Mr. Darwin that it proved to
him that "single variations," or what are usually termed "sports," could
very rarely, if ever, be perpetuated in a state of nature, as he had at
first thought might occasionally be the case. But he had always
considered that the chief part, and latterly the whole, of the materials
with which natural selection works, was afforded by individual
variations, or that amount of ever fluctuating variability which exists
in all organisms and in all their parts. Other writers have urged the
same objection, even as against individual variability, apparently in
total ignorance of its amount and range; and quite recently Professor
G.J. Romanes has adduced it as one of the difficulties which can alone
be overcome by his theory of physiological selection. He urges, that the
same variation does not occur simultaneously in a number of individuals
inhabiting the same area, and that it is mere assumption to say it does;
while he admits that "if the assumption were granted there would be an
end of the present difficulty; for if a sufficient number of individuals
were thus simultaneously and similarly modified, there need be no longer
any danger of the variety becoming swamped by intercrossing.
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