Origin of Species Variorum

→ 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

→ crossing — Fertility 1859 1860

of crossing — Fertility 1861

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

→ differences. 1859 1860

and little known differences. 1861

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

inter-breeding,

interbreeding,

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 →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

confusion.

~~all~~

~~organic~~

~~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

endowed quality, but is incidental on other acquired →differences.

In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been