es or variety, when they
live mingled together and are not exposed to any cause inducing excessive
variability. The prevention of free crossing, and the intentional matching
of individual animals, are the corner-stones of the breeder's art. No man
in his senses would expect to improve or modify a breed in any particular
manner, or keep an old breed true and distinct, unless he separated his
animals. The killing of inferior animals in each generation comes to the
{86} same thing as their separation. In savage and semi-civilised
countries, where the inhabitants have not the means of separating their
animals, more than a single breed of the same species rarely or never
exists. In former times, even in a country so civilised as North America,
there were no distinct races of sheep, for all had been mingled
together.[180] The celebrated agriculturist Marshall[181] remarks that
"sheep that are kept within fences, as well as shepherded flocks in open
countries, have generally a similarity, if not a uniformity, of character
in the individuals of each flock;" for they breed freely together, and are
prevented from crossing with other kinds; whereas in the unenclosed parts
of England the unshepherded sheep, even of the same flock, are far from
true or uniform, owing to various breeds having mingled and crossed. We
have seen that the half-wild cattle in the several British parks are
uniform in character in each; but in the different parks, from not having
mingled and crossed during many generations, they differ in a slight
degree.

We cannot doubt that the extraordinary number of varieties and
sub-varieties of the pigeon, amounting to at least one hundred and fifty,
is partly due to their remaining, differently from other domesticated
birds, paired for life when once matched. On the other hand, breeds of cats
imported into this country soon disappear, for their nocturnal and rambling
habits render it hardly possible to prevent free crossing. Rengger[182]
gives an interesting case with respect to the cat in Paraguay: in all the
distant parts of the kingdom it has assumed, apparently from the effects of
the climate, a peculiar character, but near the capital this change has
been prevented, owing, as he asserts, to the native animal frequently
crossing with cats imported from Europe. In all cases like the foregoing,
the effects of an occasional cross will be augmented by the increased
vigour and fertility of the crossed offspring, of wh