would be undistinguishable
from a species. For comparing, on the one hand, the several species of a
genus, and on the other hand several domestic races from a common stock,
we cannot discriminate them by the amount of external difference, but
only, first, by domestic races not remaining so constant or being so
"true" as species are; and secondly by races always producing fertile
offspring when crossed. And it was then shown that a race naturally
selected--from the variation being slower--from the selection steadily
leading towards the same ends{510}, and from every new slight change in
structure being adapted (as is implied by its selection) to the new
conditions and being fully exercised, and lastly from the freedom from
occasional crosses with other species, would almost necessarily be
"truer" than a race selected by ignorant or capricious and short-lived
man. With respect to the sterility of species when crossed, it was shown
not to be a universal character, and when present to vary in degree:
sterility also was shown probably to depend less on external than on
constitutional differences. And it was shown that when individual
animals and plants are placed under new conditions, they become, without
losing their healths, as sterile, in the same manner and to the same
degree, as hybrids; and it is therefore conceivable that the cross-bred
offspring between two species, having different constitutions, might
have its constitution affected in the same peculiar manner as when an
individual animal or plant is placed under new conditions. Man in
selecting domestic races has little wish and still less power to adapt
the whole frame to new conditions; in nature, however, where each
species survives by a struggle against other species and external
nature, the result must be very different.

{510} Thus according to the author what is now known as
_orthogenesis_ is due to selection.

Races descending from the same stock were then compared with species of
the same genus, and they were found to present some striking analogies.
The offspring also of races when crossed, that is mongrels, were
compared with the cross-bred offspring of species, that is hybrids, and
they were found to resemble each other in all their characters, with the
one exception of sterility, and even this, when present, often becomes
after some generations variable in degree. The chapter was summed up,
and it was shown that no ascertained limit to the