INTERNAL CAUSES OF VARIATION 



199 



Suppose it were the purpose of man to develop a poppy with 

 fewer, or with more, than the natural number of bands, say 

 seven or seventeen. Under what disadvantage he would work 

 as long as the fertility remained relatively low ! and he would 

 never succeed unless he separated the plantings from the more 

 prolific type. This is genetic selection. 



Breeders are constantly operating against the drag of infertility 

 without knowing it, and are as often wondering why better 

 results do not follow, especially when only approved mating 

 is practiced. Consider the mathematics involved in, say, three 

 lines of descent of different degrees of prolificacy. For the 

 sake of simplicity in illustration let us suppose three cows were 

 living in a herd together. One of these cows raises two calves 

 and becomes barren ; another raises four before she ceases to 

 breed, and another six. For the sake of further simplicity let us 

 suppose that one half the calves are females, and that each 

 daughter descendant exactly repeats the performance of her 

 dam and then becomes barren. How will the account stand in 

 a few generations ? l 



CUMULATIVE EFFECTS OF FERTILITY AS SHOWN BY THE RELATIVE 



NUMBER OF FEMALE DESCENDANTS OF Cows OF VARIOUS 



DEGREES OF FERTILITY 



This tabular presentation shows that after five generations 

 of this kind of breeding there would be but one fertile cow of 

 the first order in the herd, 2 while if all had been kept there 

 would be 32 of the second order and 243 of the third. What an 



1 There is no longer any doubt that fertility is an inheritable character. 



2 There might be any number of living barren ones if the strain happens to be 

 a favorite and is long-lived. 



