Thus, the more complete the sterility of
the hybrids the more rapidly will they die out and leave the two parent
forms pure. Hence it will follow that, if there is greater infertility
between the two forms in one part of the area than the other, these
forms will be kept more pure wherever this greater infertility prevails,
will therefore have an advantage at each recurring period of severe
struggle for existence, and will thus ultimately supplant the less
infertile or completely fertile forms that may exist in other portions
of the area. It thus appears that, in such a case as here supposed,
natural selection would preserve those portions of the two breeds which
were most infertile with each other, or whose hybrid offspring were most
infertile; and would, therefore, if variations in fertility continued to
arise, tend to increase that infertility. It must particularly be noted
that this effect would result, not by the preservation of the infertile
variations on account of their infertility, but by the inferiority of
the hybrid offspring, both as being fewer in numbers, less able to
continue their race, and less adapted to the conditions of existence
than either of the pure forms.
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