The cutaneous disease on the
white patches of hair on horses, the special liability of white terriers
to distemper, of white chickens to the gapes, and of silkworms which
produce yellow silk to the fungus, are not explained by it. The
analogous facts in plants also indicate a real constitutional relation
with colour, not an affection of the sense of smell and taste only.]
[Footnote 60: For all these facts, see _Animals and Plants under
Domestication_, vol. ii. pp. 335-338.]
[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.
102, 103.]
[Footnote 62: As this argument is a rather difficult one to follow,
while its theoretical importance is very great, I add here the following
briefer exposition of it, in a series of propositions; being, with a few
verbal alterations, a copy of what I wrote on the subject about twenty
years back. Some readers may find this easier to follow than the fuller
discussion in the text:--
_Can Sterility of Hybrids have been Produced by Natural
Selection?_
1. Let there be a species which has varied into _two forms_ each
adapted to certain existing conditions better than the parent
form, which they soon supplant.
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