RECENT DISCOVERIES IN HEREDITY. 207 



cross-breeding between the two, followed by selection for pure indi- 

 viduals, will within two generations give the desired combination of 

 characters in individuals which will breed true. This process of selec- 

 tion is simplest when the characters to be combined are recessive in 

 nature, but individual breeding-tests become necessary when dominant 

 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- (Fig. 1) with a short-eared albino (Fig. 2). The off- 

 spring will be pigmented half-lops (Fig. 3). 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 pigmented lops. The young will again all be pigmented, but will 

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

 three-quarter lops may be obtained in the next generation. By con- 

 tinuing this process of back-crossing with the lop-eared stock, and select- 

 ing the albino offspring obtained, the lop-eared character may be 

 steadily improved in the albinos until it is practically as good as in the 

 original lop-eared stock. The rate of improvement possible can be 

 readily calculated. The albino young will be : 



After 2 generations, one half lops, 



After 4 generations, three fourths lops, 



After 6 generations, seven eighths lops, 



After 8 generations, fifteen sixteenths lops, 



After 10 generations, thirty-one thirty-seconds lops, etc. 



This will be the result on the hypothesis that no secondary variation 

 occurs in the lop-eared character. If, however, variation is induced 

 by the cross-breeding, then it is possible that the desired end may be 

 reached sooner, or that an even better lop may be obtained in the 

 albino cross-breds, than that of the original pigmented stock. 



Latent characters are an important element in practical breeding. 

 Sometimes they greatly aid the breeder's work ; sometimes they impede 

 it. If a stock contains undesirable latent characters which are brought 

 into activity by cross-breeding, these latent characters will have to be 

 eliminated, or a new stock tried. 



Since cross-breeding is likely to modify characters even when these 

 conform to the laws of alternative inheritance, and is certain to modify 

 them when they give blended inheritance, it should be practised with 

 extreme caution, and only by the breeder who has a definite end in view 

 and a fairly clear idea of how he is going to attain it. 



The purity of standard breeds should be carefully guarded, and 



