But it would be superfluous to discuss this question in detail: for with
plants we have conclusive evidence that the sterility of crossed species
must be due to some principle, quite independent of natural selection.
Both Gartner and Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when crossed yield fewer
and fewer seeds, to species which never produce a single seed, but yet are
affected by the pollen of certain other species, for the germen swells. It
is here manifestly impossible to select the more sterile individuals, which
have already ceased to yield seeds; so that this acme of sterility, when
the germen alone is effected, cannot have been gained through selection;
and from the laws governing the various grades of sterility being so
uniform throughout the animal and vegetable kingdoms, we may infer that the
cause, whatever it may be, is the same or nearly the same in all cases.
We will now look a little closer at the probable nature of the differences
between species which induce sterility in first crosses and in hybrids. In
the case of first crosses, the greater or less difficulty in effecting a
union and in obtaining offspring apparently depends on several distinct
causes. There must sometimes be a physical impossibility in the male
element reaching the ovule, as would be the case with a plant having a
pistil too long for the pollen-tubes to reach the ovarium.
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