, in the
other, no race was evolved, because no such selection was exercised.
A race is a propagated variety; and as, by the laws of reproduction,
offspring tend to assume the parental forms, they will be more likely to
propagate a variation exhibited by both parents than that possessed by
only one.

There is no organ of the body of an animal which may not, and does not,
occasionally, vary more or less from the normal type; and there is
no variation which may not be transmitted and which, if selectively
transmitted, may not become the foundation of a race. This great truth,
sometimes forgotten by philosophers, has long been familiar to practical
agriculturists and breeders; and upon it rest all the methods of
improving the breeds of domestic animals, which, for the last century,
have been followed with so much success in England. Colour, form, size,
texture of hair or wool, proportions of various parts, strength or
weakness of constitution, tendency to fatten or to remain lean, to give
much or little milk, speed, strength, temper, intelligence, special
instincts; there is not one of these characters whose transmission is
not an every-day occurrence within the experience of cattle-breeders,
stock-farmers, horse-dealers, and dog and poultry fanciers. Nay, it
is only the other day that an eminent physiologist, Dr. Brown-Sequard,
communicated to the Royal Society his discovery that epilepsy,
artificially produced in guinea-pigs, by a means which he has
discovered, is transmitted to their offspring.

But a race, once produced, is no more a fixed and immutable entity than
the stock whence it sprang; variations arise among its members, and
as these variations are transmitted like any others, new races may be
developed out of the pre-existing one 'ad infinitum', or, at least,
within any limit at present determined. Given sufficient time and
sufficiently careful selection, and the multitude of races which
may arise from a common stock is as astonishing as are the extreme
structural differences which they may present. A remarkable example of
this is to be found in the rock-pigeon, which Dr. Darwin has, in our
opinion, satisfactorily demonstrated to be the progenitor of all our
domestic pigeons, of which there are certainly more than a hundred
well-marked races. The most noteworthy of these races are, the four
great stocks known to the "fancy" as tumblers, pouters, carriers, and
fantails; birds which not only differ most singula