See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1861
1869
1872

Chapter . 1866 1869
Chapter .— 1859 1861
Chapter — . 1860
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

in the union of individuals of the same variety, or of different varieties, or of distinct species.
Independently of
Laying aside
the question of fertility and sterility, in all other respects there seems to be a general and close similarity in the offspring of crossed species, and of crossed varieties. If we look at species as having been specially created, and at varieties as 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
Summary
of
of
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
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 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