3i 6 GENERAL BIOLOGY 



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. 



"Now, what bearing, we may ask, have these theoretical 

 matters on the practical work of the breeder? They show 

 i) 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, cross-breeding between the two, followed by 

 selection for pure individuals, will within two generations 

 give the desired combination of characters in individuals 

 which will breed true. This process of selection is simplest 

 when the characters to be combined are recessive in nature, 

 but individual breeding-tests become necessary when domi- 

 nant characters are included in the combination desired. 



"If a character gives blending inheritance, it must be 

 treated in a different way. Suppose, for example, that we 

 desire to combine lop-ears in rabbits with albinism, but that 

 our lop-eared stock consists wholly of pigmented animals. 

 How shall we proceed? First, mate a pigmented lop- with 

 a short-eared albino. The offspring will be pigmented half- 

 lops. If two of these be bred together their young will all 

 be half-lops, and about one in four of them will be albinos. 

 Now these albino half-lops may be mated with pure pig- 

 mented lops. The young will again all be pigmented, but 

 will this time be three-quarter lops, and by breeding these 



