158 



Outline of Genetics 



answers this question in the following general way. 

 ''The decrease in size and vigor which accompanies self- 

 fertilization is greatest in the first generation and becomes 

 less and less in each succeeding generation, until a con- 

 dition is reached in which there is (presumably) no more 

 loss of vigor." The facts involved in this statement 

 may be represented in fig. 25. In this figure, it can be 

 seen clearly that the great loss of vigor comes immediately 

 after self-fertilization again begins. After that, self- 

 fertilization brings additional loss in vigor, but this loss 



Parent 

 Races 



F, 



F, 



F, Fg etc. 



Fig. 25. — Illustrating status of hybrid vigor in Fi and later genera- 

 tions. Vigor represented by height of rectangles. 



is less with each succeeding generation. It is as though 

 a very definite limit were being approached and each 

 generation goes down one-half of the remaining distance 

 toward that Hmit. Just why and in what way this limit 

 is approached will be considered later in connection with 

 the work of East and Jones. 



3. ''A cross between sibs (sister and brother) within 

 a self-fertilized family shows little or no improvement 

 over self-fertilization in the same family." This, it 

 will be noticed, is simply carrying a little further the 

 point that Darwin originally discovered. We realize 



