opounded by Pallas,[239] and has been adopted
by several authors. I can find hardly any direct facts in its support; but
unfortunately no one has compared, in the case of either animals or plants,
the fertility of anciently domesticated varieties, when crossed with a
distinct species, with that of the wild parent-species when similarly
crossed. No one has compared, for instance, the fertility of _Gallus
bankiva_ and of the domesticated fowl, when crossed with a distinct species
of Gallus or Phasianus; and the {110} experiment would in all cases be
surrounded by many difficulties. Dureau de la Malle, who has so closely
studied classical literature, states[240] that in the time of the Romans
the common mule was produced with more difficulty than at the present day;
but whether this statement may be trusted I know not. A much more
important, though somewhat different, case is given by M. Groenland,[241]
namely, that plants, known from their intermediate character and sterility
to be hybrids between Aegilops and wheat, have perpetuated themselves under
culture since 1857, _with a rapid but varying increase of fertility in each
generation_. In the fourth generation the plants, still retaining their
intermediate character, had become as fertile as common cultivated wheat.

The indirect evidence in favour of the Pallasian doctrine appears to me to
be extremely strong. In the earlier chapters I have attempted to show that
our various breeds of dogs are descended from several wild species; and
this probably is the case with sheep. There can no longer be any doubt that
the Zebu or humped Indian ox belongs to a distinct species from European
cattle: the latter, moreover, are descended from two or three forms, which
may be called either species or wild races, but which co-existed in a state
of nature and kept distinct. We have good evidence that our domesticated
pigs belong to at least two specific types, _S. scrofa_ and _Indica_, which
probably lived together in a wild state in South-eastern Europe. Now, a
widely-extended analogy leads to the belief that if these several allied
species, in the wild state or when first reclaimed, had been crossed, they
would have exhibited, both in their first unions and in their hybrid
offspring, some degree of sterility. Nevertheless the several domesticated
races descended from them are now all, as far as can be ascertained,
perfectly fertile together. If this reasoning be trustworthy, and it is
a