eloped in their domesticated progeny. When rudiments are
formed or left under domestication, they are the result of a sudden arrest
of development, and not of long-continued disuse with the absorption of all
superfluous parts; nevertheless they are of interest, as showing that
rudiments are the relics of organs once perfectly developed.

Corporeal, periodical, and mental habits, though the latter have been
almost passed over in this work, become changed under domestication, and
the changes are often inherited. Such changed habits in any organic being,
especially when living a free life, would often lead to the augmented or
diminished use of various organs, and consequently to their modification.
From long-continued habit, and more especially from the occasional birth of
individuals with a slightly different constitution, domestic animals and
cultivated plants become to a certain extent acclimatised, or adapted to a
climate different from that proper to the parent-species.

Through the principle of correlated variability, when one part varies other
parts vary,--either simultaneously, or one after the other. Thus an organ
modified during an early embryonic period affects other parts subsequently
developed. When an {354} organ, such as the beak, increases or decreases in
length, adjoining or correlated parts, as the tongue and the orifice of the
nostrils, tend to vary in the same manner. When the whole body increases or
decreases in size, various parts become modified; thus with pigeons the
ribs increase or decrease in number and breadth. Homologous parts, which
are identical during their early development and are exposed to similar
conditions, tend to vary in the same or in some connected manner,--as in
the case of the right and left sides of the body, of the front and hind
limbs, and even of the head and limbs. So it is with the organs of sight
and hearing; for instance, white cats with blue eyes are almost always
deaf. There is a manifest relation throughout the body between the skin and
its various appendages of hair, feathers, hoofs, horns, and teeth. In
Paraguay, horses with curly hair have hoofs like those of a mule; the wool
and the horns of sheep vary together; hairless dogs are deficient in their
teeth; men with redundant hair have abnormal teeth, either deficient or in
excess. Birds with long wing-feathers usually have long tail-feathers. When
long feathers grow from the outside of the legs and toes of pigeons, th