uding man,
have in some rare cases grown to full size and secreted milk, perhaps
offer an analogous case. The hind feet of dogs include rudiments of a
fifth toe, and in certain large breeds these toes, though still
rudimentary, become considerably developed {318} and are furnished with
claws. In the common Hen, the spurs and comb are rudimentary, but in
certain breeds these become, independently of age or disease of the
ovaria, well developed. The stallion has canine teeth, but the mare has
only traces of the alveoli, which, as I am informed by the eminent
veterinary Mr. G. T. Brown, frequently contain minute irregular nodules
of bone. These nodules, however, sometimes become developed into
imperfect teeth, protruding through the gums and coated with enamel;
and occasionally they grow to a third or even a fourth of the length of
the canines in the stallion. With plants I do not know whether the
redevelopment of rudimentary organs occurs more frequently under
culture than under nature. Perhaps the pear-tree may be a case in
point, for when wild it bears thorns, which though useful as a
protection are formed of branches in a rudimentary condition, but, when
the tree is cultivated, the thorns are reconverted into branches.

Finally, though organs which must be classed as rudimentary frequently
occur in our domesticated animals and cultivated plants, these have
generally been formed suddenly, through an arrest of development. They
usually differ in appearance from the rudiments which so frequently
characterise natural species. In the latter, rudimentary organs have been
slowly formed through continued disuse, acting by inheritance at a
corresponding age, aided by the principle of the economy of growth, all
under the control of natural selection. With domesticated animals, on the
other hand, the principle of economy is far from coming into action, and
their organs, although often slightly reduced by disuse, are not thus
almost obliterated with mere rudiments left.

* * * * *

{319}

CHAPTER XXV.

LAWS OF VARIATION, _continued_--CORRELATED VARIABILITY.

EXPLANATION OF TERM--CORRELATION AS CONNECTED WITH
DEVELOPMENT--MODIFICATIONS CORRELATED WITH THE INCREASED OR DECREASED
SIZE OF PARTS--CORRELATED VARIATION OF HOMOLOGOUS PARTS--FEATHERED FEET
IN BIRDS ASSUMING THE STRUCTURE OF THE WINGS--CORRELATION B