Origin of Species Variorum

→ Chapter . 1872

Chapter .— 1859 1861

Chapter — . 1860

Chapter . 1866 1869

→ arrived at 1869 1872

ever lived, have come to 1859 1860 1861 1866

→ to the action of 1869 1872

of 1859 1860 1861 1866

→ differences, generally of an unknown nature, 1869 1872

generally unknown differences 1859 1860 1861 1866

→ their inarching 1869 1872

them becoming inarched 1859 1860 1861 1866

→ and of their hybrid progeny has not been acquired through natural selection. In the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo. 1872

between pure species, which have their reproductive systems perfect, seems to depend on several circumstances; in some cases largely on the early death of the embryo. 1859 1860 1861

and of their hybrid progeny has not, as far as we can judge, been increased through natural selection so as to attain that high degree which is universal with species when rendered widely distinct. 1866

and of their hybrid progeny has not, as far as we can judge, been acquired through natural selection. 1869

↑ 1 blocks not present in 1859 1860 1861 1872; present in 1866 1869

With first crosses between pure species, in which the reproductive system is in a perfect condition, the sterility seems to depend on several circumstances; in some cases largely on the early death of the embryo, but this apparently depends on some imperfection in the original act of impregnation.

→ firstly, slight changes in the conditions of life add to 1872

the crossing of forms only slightly different is favourable to 1859 1860 1861

the crossing of forms only slightly differentiated favours 1866

first, the crossing of forms only slightly differentiated favours 1869

→ all organic beings; and secondly, that the crossing of forms, which have been exposed to slightly different 1872

their offspring; and that slight changes in the 1859 1860 1861 1866

their offspring, whilst close interbreeding is injurious; and secondly, that slight changes in the 1869

→ or which have varied, favours the size, 1872

are apparently favourable to the 1859 1860 1861

apparently add to the 1866 1869

→ their offspring. 1872

all organic beings. 1859 1860 1861 1866

all organic beings, whilst greater changes are often injurious. 1869

having been produced by secondary laws, this similarity would be an astonishing fact. But it harmonises perfectly with the view that there is no essential distinction between species and varieties.

Summary

→ Chapter .

First crosses between

forms

forms,

sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. The sterility is of all degrees, and is often so slight that the

two

....

most careful experimentalists

who

....

have →arrived at diametrically opposite conclusions in ranking forms by this test. The sterility is innately variable in individuals of the same species, and is eminently susceptible →to the action of favourable and unfavourable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes widely different, in reciprocal crosses between the same two species. It is not always equal in degree in a first cross and in the

hybrid

hybrids

produced from this cross.

In the same manner as in grafting trees, the capacity of one species or variety to take on another, is incidental on →differences, generally of an unknown nature, in their vegetative systems, so in crossing, the greater or less facility of one species to unite with

another,

another

is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent

them

their

crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent →their inarching in our forests.

The sterility of first crosses →and of their hybrid progeny has not been acquired through natural selection. In the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo. ↑ In the case of hybrids, it apparently depends on their whole organisation having been disturbed by being compounded from two distinct forms; the sterility being closely allied to that which so frequently affects pure species, when exposed to new and unnatural conditions of life. He who will explain these latter cases will be able to explain the sterility of hybrids. This view is

supported

strongly supported

by a parallelism of another

kind;—

kind:—

kind:

namely,

that

that,

→firstly, slight changes in the conditions of life add to the vigour and fertility of →all organic beings; and secondly, that the crossing of forms, which have been exposed to slightly different conditions of life →or which have varied, favours the size,

vigour

vigour,

and fertility of →their offspring.

But the

The

facts given on