whether the primordial
parent of the genus was striped, the appearance of the stripes can only
hypothetically be attributed to reversion. But most persons, after
considering the many undoubted cases of variously coloured marks
reappearing by reversion in crossed pigeons, fowls, ducks, &c., will come
to the same conclusion with respect to the horse-genus; and in this case we
must admit that the progenitor of the group was striped on the legs,
shoulders, face, and probably over the whole body, like a zebra. If we
reject this view, the frequent and almost regular appearance of stripes in
the several foregoing hybrids is left without any explanation.

* * * * *

It would appear that with crossed animals a similar tendency to the
recovery of lost characters holds good even with instincts. There are some
breeds of fowls which are called "everlasting layers," because they have
lost the instinct of incubation; and so rare is it for them to incubate
that I have seen notices published in works on poultry, when hens of such
breeds have taken to sit.[103] Yet the aboriginal species was of course a
good incubator; for with birds in a state of nature hardly any {44}
instinct is so strong as this. Now, so many cases have been recorded of the
crossed offspring from two races, neither of which are incubators, becoming
first-rate sitters, that the reappearance of this instinct must be
attributed to reversion from crossing. One author goes so far as to say,
"that a cross between two non-sitting varieties almost invariably produces
a mongrel that becomes broody, and sits with remarkable steadiness."[104]
Another author, after giving a striking example, remarks that the fact can
be explained only on the principle that "two negatives make a positive." It
cannot, however, be maintained that hens produced from a cross between two
non-sitting breeds invariably recover their lost instinct, any more than
that crossed fowls or pigeons invariably recover the red or blue plumage of
their prototypes. I raised several chickens from a Polish hen by a Spanish
cock,--breeds which do not incubate,--and none of the young hens at first
recovered their instinct, and this appeared to afford a well-marked
exception to the foregoing rule; but one of these hens, the only one which
was preserved, in the third year sat well on her eggs and reared a brood of
chickens. So that here we have the appearance with advancing age of a
primit