It is clear that Darwin does not think that the sudden and large
variations that sometimes occur furnish the basis for natural selection,
and the final statement in the last citation (which was added in later
editions of the “Origin of Species”), to the effect that if such
monstrous variations appeared as single or occasional variations they
would be lost by intercrossing implies that, in general, single
variations would likewise be lost unless they appeared in a sufficient
number of individuals to maintain themselves against the swamping
effects of intercrossing.
It is necessary to quote again, in order to show that, in some cases at
least, Darwin believed selection plays little or no part in the origin
and maintenance of certain peculiarities that are of no use to the
species. “There is one point connected with individual differences,
which is extremely perplexing: I refer to those genera which have been
called protean or ‘polymorphic,’ in which the species present an
inordinate amount of variation. With respect to many of these forms,
hardly two naturalists agree, whether to rank them as species or as
varieties. We may instance Rubus, Rosa, and Hieracium amongst plants,
several genera of insects and of Brachiopod shells. In most polymorphic
genera some of the species have fixed and definite characters. Genera
which are polymorphic in one country seem to be, with a few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is independent of the
conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or
disservice to the species, and which consequently have not been seized
on by selection to act on and accumulate, in the same manner as man
accumulates in any given direction individual differences in his
domesticated productions. These individual differences generally affect
what naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the
most experienced naturalist would be surprised at the number of cases of
variability, even in important parts of structure, which he could
collect on good authority, as I have collected, during a course of
years.”
Public-domain text, read in full here on John Shaqi.
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