Origin of Species Variorum

→ Chapter .— 1859 1861

Chapter — . 1860

Chapter . 1866 1869

Chapter . 1872

→ 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

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~~

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.