Yet these few seeds were fertile; so that in this case the
first cross was almost sterile, though the hybrid when at length
produced was fertile. In like manner, dissimilarly coloured varieties of
Verbascum or mullein have been found by two distinct observers to be
comparatively infertile. The two pimpernels (Anagallis arvensis and A.
coerulea), classed by most botanists as varieties of one species, have
been found, after repeated trials, to be perfectly sterile when crossed.
No cases of this kind are recorded among animals; but this is not to be
wondered at, when we consider how very few experiments have been made
with natural varieties; while there is good reason for believing that
domestic varieties are exceptionally fertile, partly because one of the
conditions of domestication was fertility under changed conditions, and
also because long continued domestication is believed to have the effect
of increasing fertility and eliminating whatever sterility may exist.
This is shown by the fact that, in many cases, domestic animals are
descended from two or more distinct species.
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