Yellow-fleshed
peaches suffer more from disease than white-fleshed kinds. In Mauritius,
white sugar-canes were attacked by a disease from which the red canes
were free. White onions and verbenas are most liable to mildew; and
red-flowered hyacinths were more injured by the cold during a severe
winter in Holland than any other kinds.[60]
These curious and inexplicable correlations of colour with
constitutional peculiarities, both in animals and plants, render it
probable that the correlation of colour with infertility, which has been
detected in several cases in plants, may also extend to animals in a
state of nature; and if so, the fact is of the highest importance as
throwing light on the origin of the infertility of many allied species.
This will be better understood after considering the facts which will be
now described.
_The Isolation of Varieties by Selective Association._
In the last chapter I have shown that the importance of geographical
isolation for the formation of new species by natural selection has been
greatly exaggerated, because the very change of conditions, which is
the initial power in starting such new forms, leads also to a local or
stational segregation of the forms acted upon.
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