odified. Thus
pigeon-fanciers have gone on selecting pouters for length of body, and we
have seen that their vertebrae are generally increased in number, and their
ribs in breadth. Tumblers have been selected for their small bodies, and
their ribs and primary wing-feathers are generally lessened in number.
Fantails have been selected for their large, widely-expanded tails, with
numerous tail-feathers, and the caudal vertebrae are increased in size and
number. Carriers have been selected for length of beak, and their tongues
have become longer, but not in strict accordance with the length of beak.
In this latter breed and in others having large feet, the number of the
scutellae on the toes is greater than in the breeds with small feet. Many
similar cases could be given. In Germany it has been observed that the
period of gestation is longer in large-sized than in small-sized breeds of
cattle. With our highly-improved animals of all kinds the period of
maturity has advanced, both with respect to the full growth of the body and
the period of reproduction; and, in correspondence with this, the teeth are
now developed earlier than formerly, so that, {322} to the surprise of
agriculturists, the ancient rules for judging the age of an animal by the
state of its teeth are no longer trustworthy.[806]

_Correlated Variation of Homologous Parts._--Parts which are homologous
tend to vary in the same manner; and this is what might have been expected,
for such parts are identical in form and structure during an early period
of embryonic development, and are exposed in the egg or womb to similar
conditions. The symmetry, in most kinds of animals, of the corresponding or
homologous organs on the right and left sides of the body, is the simplest
case in point; but this symmetry sometimes fails, as with rabbits having
only one ear, or stags with one horn, or with many-horned sheep which
sometimes carry an additional horn on one side of their heads. With flowers
which have regular corollas, the petals generally vary in the same manner,
as we see in the same complicated and elegant pattern, on the flowers of
the Chinese pink; but with irregular flowers, though the petals are of
course homologous, this symmetry often fails, as with the varieties of the
_Antirrhinum_ or snapdragon, or that variety of the kidney-bean (_Phaseolus
multiflorus_) which has a white standard-petal.

In the vertebrata the front and hind limbs are homologous, and they