Origin of Species Variorum

→ ever lived, have come to 1859 1860 1861 1866

arrived at 1869 1872

→ of 1859 1860 1861 1866

to the action of 1869 1872

→ generally unknown differences 1859 1860 1861 1866

differences, generally of an unknown nature, 1869 1872

→ them becoming inarched 1859 1860 1861 1866

their inarching 1869 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

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

↑ 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.

→ The sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their 1859 1860 1861

With hybrids, in which the reproductive system is in an imperfect condition, and in which this system as well as the 1866

In the case of hybrids, it perhaps depends on their 1869

→ disturbed 1859 1860 1861

has been disturbed 1866

having been disturbed 1869

→ species, seems 1859 1860 1861

forms, the sterility apparently is 1866

forms; the sterility being 1869

The sterility is of all degrees, and is often so slight that the

~~two~~

two

most careful experimentalists

~~who~~

who

have →ever lived, have come to diametrically opposite conclusions in ranking forms by this test. The sterility is innately variable in individuals of the same species, and is eminently susceptible →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

hybrids

hybrid

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 →generally unknown differences 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

their

them

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 →them becoming inarched in our forests.

The sterility of first crosses →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. ↑ →The sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their whole organisation →disturbed by being compounded

from

two distinct →species, seems closely allied to that

~~sterility~~

sterility

which so