shall
see in the following chapters, mainly depends on changed conditions of
life, but is governed by infinitely complex, and, to a great extent,
unknown laws. Domestication, even when long continued, occasionally causes
but a small amount of variability, as in the case of the goose and turkey.
The slight differences, however, which characterise each individual animal
and plant would in most, probably in all cases, suffice for the production
of distinct races through careful and prolonged selection. We see what
selection, though acting on mere individual differences, can effect when
families of cattle, sheep, {234} pigeons, &c., of the same race, have been
separately bred during a number of years by different men without any wish
on their part to modify the breed. We see the same fact in the difference
between hounds bred for hunting in different districts,[573] and in many
other such cases.

In order that selection should produce any result, it is manifest that the
crossing of distinct races must be prevented; hence facility in pairing, as
with the pigeon, is highly favourable for the work; and difficulty in
pairing, as with cats, prevents the formation of distinct breeds. On nearly
the same principle the cattle of the small island of Jersey have been
improved in their milking qualities "with a rapidity that could not have
been obtained in a widely extended country like France."[574] Although free
crossing is a danger on the one side which every one can see, too close
interbreeding is a hidden danger on the other side. Unfavourable conditions
of life overrule the power of selection. Our improved heavy breeds of
cattle and sheep could not have been formed on mountainous pastures; nor
could dray-horses have been raised on a barren and inhospitable land, such
as the Falkland islands, where even the light horses of La Plata rapidly
decrease in size. Nor could the wool of sheep have been much increased in
length within the Tropics; yet selection has kept Merino sheep nearly true
under diversified and unfavourable conditions of life. The power of
selection is so great, that breeds of the dog, sheep, and poultry, of the
largest and least size, long and short beaked pigeons, and other breeds
with opposite characters, have had their characteristic qualities
augmented, though treated in every way alike, being exposed to the same
climate and fed on the same food. Selection, however, is either checked or
favoured by the effects of