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

→ not accumulated by natural selection — Causes 1869 1872

— Causes 1859 1860 1861

not accumulated by natural selection—Causes 1866

→ hybrids — Parallelism 1859 1860 1861 1869 1872

hybrids—Parallelism 1866

→ of crossing — Dimorphism and trimorphism — Fertility 1869 1872

crossing — Fertility 1859 1860

of crossing — Fertility 1861

of crossing—Dimorphism and trimorphism—Fertility 1866

→ universal — Hybrids 1859 1860 1861 1869 1872

universal—Hybrids 1866

→ fertility — Summary. 1859 1860 1861 1869 1872

fertility—Summary. 1866

→ OMIT 1869 1872

the quality of 1859 1860 1861 1866

→ living together 1872

within the same country 1859 1860 1861 1866 1869

→ as I shall show, by the 1872

by the continued 1869

→ is an incidental result of 1872

is, as I hope to show, incidental on 1869

↑ 3 blocks not present in 1869 1872; present in 1859 1860 1861 1866

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 the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences.

CHAPTER

VIII.

IX.

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,

→not accumulated by natural selection — Causes of the sterility of first crosses and of →hybrids — Parallelism between the effects of changed conditions of life and →of crossing — Dimorphism and trimorphism — Fertility of varieties when crossed and of their mongrel offspring not →universal — Hybrids and mongrels compared independently of their →fertility — Summary.

THE view

generally

commonly

entertained by naturalists is that species, when intercrossed, have been specially endowed with →OMIT sterility, in order to prevent

the

confusion

all

organic

forms.

This view certainly seems at first

probable,

highly probable,

for species →living together could hardly have

kept

been kept

distinct had they been capable of

crossing

....

freely.

freely crossing.

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

acquired,

→as I shall show, by the preservation of

successive,

successive

profitable degrees of sterility. It →is an incidental result of differences in the reproductive

system

systems

of the

parent-species,

and

not

either

specially

acquired

endowed

quality.

↑

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

confounded

confounded;

together;

....

namely, the sterility of

two

....

species when first crossed, and the sterility of the hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the

organs

formative organs

themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual elements which go to

from

form

the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. This distinction is important, when the cause of the sterility, which is common to the