See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1866
1869
1872

hybrids — Sterility 1859 1860 1861 1869 1872
hybrids—Sterility 1866

domestication — Laws 1859 1860 1861 1869 1872
domestication—Laws 1866

hybrids — Sterility 1859 1860 1861 1869 1872
hybrids—Sterility 1866

— Causes 1859 1860 1861
not accumulated by natural selection—Causes 1866
not accumulated by natural selection — Causes 1869 1872

hybrids — Parallelism 1859 1860 1861 1869 1872
hybrids—Parallelism 1866

of crossing — Fertility 1861
crossing — Fertility 1859 1860
of crossing—Dimorphism and trimorphism—Fertility 1866
of crossing — Dimorphism and trimorphism — Fertility 1869 1872

universal — Hybrids 1859 1860 1861 1869 1872
universal—Hybrids 1866

fertility — Summary. 1859 1860 1861 1869 1872
fertility—Summary. 1866

the quality of 1859 1860 1861 1866
OMIT 1869 1872

within the same country 1859 1860 1861 1866 1869
living together 1872

2 blocks not present in 1859 1860 1861 1866; present in 1869 1872
The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired by the continued preservation of successive, profitable degrees of sterility. It is, as I hope to show, incidental on differences in the reproductive system of the parent-species, and is not either a specially acquired or endowed quality.

hybrids 1859 1860 1861
first crosses between distinct species and hybrids 1866

of hybrids could not possibly be of any advantage to them, and therefore could not 1859 1860 1861
can hardly 1866

I hope, however, to be able 1859 1860 1861
But to this subject I shall have to return, and I hope ultimately 1866

sterility is not 1859 1860 1861
this sterility is neither 1866

and little known differences. 1861
differences. 1859 1860
and little-known differences in the reproductive systems of the parent-species. 1866

CHAPTER
IX.
VIII.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids — Sterility various in degree, not universal, affected by close
interbreeding,
inter-breeding,
removed by domestication — Laws governing the sterility of hybrids — Sterility not a special endowment, but incidental on other
differences,
differences
— Causes of the sterility of first crosses and of hybrids — Parallelism between the effects of changed conditions of life and of crossing — Fertility of varieties when crossed and of their mongrel offspring not universal — Hybrids and mongrels compared independently of their fertility — Summary.
THE view
commonly
generally
entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent
their
the
confusion.
confusion
of
of
all
all
organic
organic
forms.
forms.
This view certainly seems at first
highly probable,
probable,
for species within the same country could hardly have
been kept
kept
distinct had they been capable of
crossing
crossing
freely crossing.
freely.
The importance of the fact that hybrids are very generally sterile, has, I think, been much underrated by some late writers. On the theory of natural selection the case is especially important, inasmuch as
this
the
sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been
increased
acquired
by the continued preservation of
successive,
successive
profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired
nor
or
endowed quality, but is incidental on other acquired and little known differences.
In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been
confounded;
confounded
together;
together;
namely, the sterility of
two
two
species