iii. p. 161).]
[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]
[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.
215.]
[Footnote 50: In Mr. Gulick's last paper (_Journal of Linn. Soc. Zool._,
vol. xx. pp. 189-274) he discusses the various forms of isolation above
referred to, under no less than thirty-eight different divisions and
subdivisions, with an elaborate terminology, and he argues that these
will frequently bring about divergent evolution without any change in
the environment or any action of natural selection. The discussion of
the problem here given will, I believe, sufficiently expose the fallacy
of his contention; but his illustration of the varied and often
recondite modes by which practical isolation may be brought about, may
help to remove one of the popular difficulties in the way of the action
of natural selection in the origination of species.]
CHAPTER VII
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING
Statement of the problem--Extreme susceptibility of the
reproductive functions--Reciprocal crosses--Individual
differences in respect to cross-fertilisation--Dimorphism and
trimorphism among plants--Cases of the fertility of hybrids and
of the infertility of mongrels--The effects of close
interbreeding--Mr.
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