ich fact evidence will
hereafter be given; for this will lead to the mongrels increasing more
rapidly than the pure parent-breeds.

{87}

When distinct breeds are allowed to cross freely, the result will be a
heterogenous body; for instance, the dogs in Paraguay are far from uniform,
and can no longer be affiliated to their parent-races.[183] The character
which a crossed body of animals will ultimately assume must depend on
several contingencies,--namely, on the relative numbers 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[184] 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 exceeds the other greatly in number, the
latter will soon be wholly, or almost wholly, absorbed and lost.[185] Thus
European pigs and dogs have been largely introduced into the islands of the
Pacific Ocean, and the native races have been absorbed and lost in the
course of about fifty or sixty years;[186] but the imported races no doubt
were favoured. Rats may be considered as semi-domesticated animals. Some
snake-rats (_Mus alexandrinus_) escaped in the Zoological Gardens of
London, "and for a long time afterwards the keepers frequently caught
cross-bred rats, at first half-breds, afterwards with less and less of the
character of the snake-rat, till at length all traces of it
disappeared."[187] On the other hand, {88} in some parts of London,
especially near the docks, where fresh rats are frequently imported, an
endless variety of intermediate forms may be found between the brown,
black, and snake rat, which are all three usually ranked as dist