era with the races
of domestic ducks, poultry, and pigeons; and so again with plants,
compare the cabbages, almonds, peaches and nectarines, &c. with the
species of many genera. St Hilaire has even remarked that there is a
greater difference in size between races, as in dogs (for he believes
all have descended from one stock), than between the species of any one
genus; nor is this surprising, considering that amount of food and
consequently of growth is the element of change over which man has most
power. I may refer to a former statement, that breeders believe the
growth of one part or strong action of one function causes a decrease in
other parts; for this seems in some degree analogous to the law of
"organic compensation{260}," which many naturalists believe holds good.
To give an instance of this law of compensation,--those species of
Carnivora which have the canine teeth greatly developed have certain
molar teeth deficient; or again, in that division of the Crustaceans in
which the tail is much developed, the thorax is little so, and the
converse. The points of difference between different races is often
strikingly analogous to that between species of the same genus: trifling
spots or marks of colour{261} (as the bars on pigeons' wings) are often
preserved in races of plants and animals, precisely in the same manner
as similar trifling characters often pervade all the species of a genus,
and even of a family. Flowers in varying their colours often become
veined and spotted and the leaves become divided like true species: it
is known that the varieties of the same plant never have red, blue and
yellow flowers, though the hyacinth makes a very near approach to an
exception{262}; and different species of the same genus seldom, though
sometimes they have flowers of these three colours. Dun-coloured horses
having a dark stripe down their backs, and certain domestic asses having
transverse bars on their legs, afford striking examples of a variation
analogous in character to the distinctive marks of other species of the
same genus.

{260} The law of compensation is discussed in the _Origin_, Ed. i.
p. 147, vi. p. 182.

{261} <Note in original.> Boitard and Corbie on outer edging red in
tail of bird,--so bars on wing, white or black or brown, or white
edged with black or <illegible>: analogous to marks running through
genera but with different colours. Tail coloured in pigeons.

{262}