" And finally he
concludes (p. 386): "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 Gaeartner 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 affected, 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."
Wallace still adhered to his view (_see_ "Darwinism," 1889, p. 174,
_also_ p. 292 of "More Letters," note 1, and Letter 211, p.
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