Although the fertility of varieties when intercrossed, and of their mongrel
offspring, has been asserted by so many authors to be universal, this
cannot be considered as quite correct after the facts given on the high
authority of Gartner and Kolreuter. Most of the varieties which have been
experimented on have been produced under domestication; and as
domestication (I do not mean mere confinement) almost certainly tends to
eliminate that sterility which, judging from analogy, would have affected
the parent-species if intercrossed, we ought not to expect that
domestication would likewise induce sterility in their modified descendants
when crossed. This elimination of sterility apparently follows from the
same cause which allows our domestic animals to breed freely under
diversified circumstances; and this again apparently follows from their
having been gradually accustomed to frequent changes in their conditions of
life.
A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring.
On the one side, there is good reason to believe that slight changes in the
conditions of life give vigour and fertility to all organic beings. We
know also that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number of their
offspring, and certainly gives to them increased size and vigour.
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