kull and
beak varies enormously, so that the skulls of the Short-faced tumbler
and some of the Carriers differ more than any wild pigeons, even those
classed in distinct genera. The breadth and number of the ribs vary, as
well as the processes on them; the number of the vertebrae and the
length of the sternum also vary; and the perforations in the sternum
vary in size and shape. The oil gland varies in development, and is
sometimes absent. The number of the wing-feathers varies, and those of
the tail to an enormous extent. The proportions of the leg and feet and
the number of the scutellae also vary. The eggs also vary somewhat in
size and shape; and the amount of downy clothing on the young bird, when
first hatched, differs very considerably. Finally, the attitude of the
body, the manner of walking, the mode of flight, and the voice, all
exhibit modifications of the most remarkable kind.[35]

_Acclimatisation_.

A very important kind of variation is that constitutional change termed
acclimatisation, which enables any organism to become gradually adapted
to a different climate from the parent stock. As closely allied species
often inhabit different countries possessing very different climates, we
should expect to find cases illustrating this change among our
domesticated animals and cultivated plants. A few examples will
therefore be adduced showing that such constitutional variation does
occur.

Among animals the cases are not numerous, because no systematic attempt
has been made to select varieties for this special quality. It has,
however, been observed that, though no European dogs thrive well in
India, the Newfoundland dog, originating from a severe climate, can
hardly be kept alive. A better case, perhaps, is furnished by merino
sheep, which, when imported directly from England, do not thrive, while
those which have been bred in the intermediate climate of the Cape of
Good Hope do much better. When geese were first introduced into Bogota,
they laid few eggs at long intervals, and few of the young survived. By
degrees, however, the fecundity improved, and in about twenty years
became equal to what it is in Europe. According to Garcilaso, when fowls
were first introduced into Peru they were not fertile, whereas now they
are as much so as in Europe.

Plants furnish much more important evidence. Our nurserymen distinguish
in their catalogues varieties of fruit-trees which are more or less
hardy, and this is esp