e last chapter that one
of two French breeds of sheep yielded up its character, when crossed with
Merinos, very much slower than the other; and the common German sheep
referred to by Fleischmann may present an analogous case. But in all cases
there will be during many subsequent generations more or less liability to
reversion, and it is this fact which has probably led authors to maintain
that a score or more of generations are requisite for one race to absorb
another. In considering the final result of the commingling of two or more
breeds, we must not forget that the act of crossing in itself tends to
bring back long-lost characters not proper to the immediate parent-forms.

With respect to the influence of the conditions of life on any two breeds
which are allowed to cross freely, unless both are indigenous and have long
been accustomed to the country where they live, they will, in all
probability, be unequally affected by the conditions, and this will modify
the result. Even with indigenous breeds, it will rarely or never occur that
both are equally well adapted to the surrounding circumstances; more
especially when permitted to roam freely, and not carefully tended, as will
generally be the case with breeds allowed to cross. As a consequence of
this, natural selection will to a certain extent come into action, and the
best fitted will survive, and this will aid in determining the ultimate
character of the commingled body.

How long a time it would require before such a crossed body of animals
would assume within a limited area a uniform character no one can say; that
they would ultimately become uniform from free intercrossing, and from the
survival of the fittest, we may feel assured; but the character thus
acquired would rarely or never, as we may infer from the several previous
{90} considerations, be exactly intermediate between that of the two
parent-breeds. With respect to the very slight differences by which the
individuals of the same sub-variety, or even of allied varieties, are
characterised, it is obvious that free crossing would soon obliterate such
small distinctions. The formation of new varieties, independently of
selection, would also thus be prevented; except when the same variation
continually recurred from the action of some strongly predisposing cause.
Hence we may conclude that free crossing has in all cases played an
important part in giving to all the members of the same domestic race, and
of