animals are descended from
several species and _become_ fertile _inter se_, then one can see
they gain fertility by becoming adapted to new conditions and
certainly domestic animals can withstand changes of climate without
loss of fertility in an astonishing manner.

{244} See Suchetet, _L'Hybridite dans la Nature_, Bruxelles, 1888,
p. 67. In _Var. under Dom._, Ed. ii. vol. II. hybrids between the
fowl and the pheasant are mentioned. I can give no information on
the other cases.

{245} _Origin_, Ed. i. p. 250, vi. p. 370.

{246} This was the position of Gaertner and of Koelreuter: see
_Origin_, Ed. i. pp. 246-7, vi. pp. 367-8.

_Causes of Sterility in Hybrids._

The difference in nature between species which causes the greater or
lesser degree of sterility in their offspring appears, according to
Herbert and Koelreuter, to be connected much less with external form,
size, or structure, than with constitutional peculiarities; by which is
meant their adaptation to different climates, food and situation, &c.:
these peculiarities of constitution probably affect the entire frame,
and no one part in particular{247}.

{247} <Note in the original.> Yet this seems introductory to the
case of the heaths and crocuses above mentioned. <Herbert observed
that crocus does not set seed if transplanted before pollination,
but that such treatment after pollination has no sterilising effect.
(_Var. under Dom._, Ed. ii. vol. II. p. 148.) On the same page is
a mention of the Ericaceae being subject to contabescence of the
anthers. For _Crinum_ see _Origin_, Ed. i. p. 250: for _Rhododenron_
and _Calceolaria_ see p. 251.>

From the foregoing facts I think we must admit that there exists a
perfect gradation in fertility between species which when crossed are
quite fertile (as in Rhododendron, Calceolaria, &c.), and indeed in an
extraordinary degree fertile (as in Crinum), and those species which
never produce offspring, but which by certain effects (as the exsertion
of the pollen-tube) evince their alliance. Hence, I conceive, we must
give up sterility, although undoubtedly in a lesser or greater degree of
very frequent occurrence, as an unfailing mark by which _species_ can be
distinguished from _races_, _i.e._ from those forms which have descended
from a common stock.

_Infertility from causes distinct from hybridisation._

Let us see