Cross-breeding, accordingly, is a two-edged sword which must be handled carefully. It can be used by the breeder to combine in one race characters found separately in different races, but care must be exercised if it is desired to keep those characters unmodified. If modification of characters is desired at the same time as new combinations, then cross-breeding becomes doubly advantageous, for it is a means of inducing variability in characters, as, for example, in the intensity of pigmentation and in the length of hair, quite apart from the formation of new groupings of characters. Sometimes it causes a complex character to break up into simpler units, as the agouti coat of the wild guinea-pig into segregated black and yellow, or total pigmentation into
a definite series of pigmented spots. In other cases it operates by bringing into activity characters which have previously been latent in one or other of the parental forms. Compare Fig. 3 with Figs. 1 and 2. Black pigmentation was latent in the albino parent (Fig. 2) and was brought into full activity by a cross with a brown-pigmented animal.
Now, what bearing, we may ask, have these theoretical matters on the practical work of the breeder? They show (1) that a race of animals is for practical purposes a group of characters separately heritable, and (2) that the breeder who desires in any way to modify a character found in this group, or to add a new character to the group, should first consider carefully how the character in question is inherited.
If the character is alternative in heredity to some other character,
