rgument, no special kind of barring action being
contended for.

He, however, attempts to account for the modification of the sexual
products of species as compared with those of varieties, by the exposure of
the former to more uniform conditions during longer periods of time than
those to which varieties are exposed, and that as wild animals, when
captured, are often rendered sterile by captivity, so the influence of
union with another species may produce a similar effect. It seems to the
author an unwarrantable assumption that a cross with what, on the Darwinian
theory, can only be a slightly diverging descendant of a common parent,
should produce an effect equal to that of captivity, and consequent change
of habit, as well as considerable modification of food.

No clear case has been given by Mr. Darwin in which mongrel animals,
descended from the same undoubted species, have been persistently infertile
_inter se_; nor any clear case in which hybrids between animals, generally
admitted to be distinct species, have been continuously fertile _inter se_.

It is true that facts are brought forward tending to establish the
probability of the doctrine of Pallas, that species may sometimes be {126}
rendered fertile by domestication. But even if this were true, it would be
no approximation towards proving the converse, _i.e._ that races and
varieties may become sterile when wild. And whatever may be the preference
occasionally shown by certain breeds to mate with their own variety, no
sterility is recorded as resulting from unions with other varieties.
Indeed, Mr. Darwin remarks,[124] "With respect to sterility from the
crossing of domestic races, I know of no well-ascertained case with
animals. This fact (seeing the great difference in structure between some
breeds of pigeons, fowls, pigs, dogs, &c.) is extraordinary when contrasted
with the sterility, of many closely-allied natural species when crossed."

It has been alleged that the domestic and wild guinea-pig do not breed
together, but the specific identity of these forms is very problematical.
Mr. A. D. Bartlett, superintendent of the Zoological Gardens, whose
experience is so great, and observation so quick, believes them to be
decidedly distinct species.

Thus, then, it seems that a certain normal specific stability in species,
accompanied by occasional sudden and considerable modifications, might be
expected _a priori_ from what we know of crystalline inorganic