2o6 POPULAR SCIENCE MONTHLY. 



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 modi- 

 fication 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 char- 

 acter to break up into simpler units, as the agouti coat of the wild 

 guinea-pig into segregated black and yellow, or total pigmentation into 



Fig. 14. An Imperfectly Rough-coated Guinea-Pig. Compare Fig. 6. 



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. 



Xoav, what bearing, we may ask, have these theoretical matters on 

 the practical work of the breeder ? They show ( 1 ) that a race of ani- 

 mals 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, 



