varieties, or of distinct species.
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 of Chapter_. First crosses between 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 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 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, 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 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
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