When distinct breeds are allowed to cross freely, the result will be a
heterogeneous body; for instance, the dogs in Paraguay are far from uniform,
and can no longer be affiliated to their parent-races. (15/4. Rengger
'Saugethiere' etc. s. 154.) The character which a crossed body of animals will
ultimately assume must depend on several contingencies,--namely, on the
relative members of the individuals belonging to the two or more races which
are allowed to mingle; on the prepotency of one race over the other in the
transmission of character; and on the conditions of life to which they are
exposed. When two commingled breeds exist at first in nearly equal numbers,
the whole will sooner or later become intimately blended, but not so soon,
both breeds being equally favoured in all respects, as might have been
expected. The following calculation (15/5. White 'Regular Gradation in Man'
page 146.) shows that this is the case: if a colony with an equal number of
black and white men were founded, and we assume that they marry
indiscriminately, are equally prolific, and that one in thirty annually dies
and is born; then "in 65 years the number of blacks, whites, and mulattoes
would be equal. In 91 years the whites would be 1-10th, the blacks 1-10th, and
the mulattoes, or people of intermediate degrees of colour, 8-10ths of the
whole number. In three centuries not 1-100th part of the whites would exist."

When one of two mingled races exceed the other greatly in number, the latter
will soon be wholly, or almost wholly, absorbed and lost. (15/6. Dr. W.F.
Edwards in his 'Caracteres Physiolog. des Races Humaines' page 24 first called
attention to this subject, and ably discussed it.) Thus European pigs and dogs
have been largely introduced in the islands of the Pacific Ocean, and the
native races have been absorbed and lost in the course of about fifty or sixty
years (15/7. Rev. D. Tyerman and Bennett 'Journal of Voyages' 1821-1829 volume
1 page 300.); but the imported races no doubt were favoured. Rats may be
considered as semi-domesticated animals. Some snake-rats (Mus alexandrinus)