206 GENETICS 



beds which were laid out side by side, that is, in an 

 environment as nearly the same as possible, but with 

 half of the beds screened from insects which usually 

 transfer pollen from flower to flower. In the screened 

 half where all insects were excluded the flowers were 

 of necessity self -fertilized, while in the exposed half 

 they were presumably cross-pollinated by insects which 

 had free access to them. The seeds produced in the 

 two beds were kept separate and the experiment was 

 continued for ten years, so that at the end of that time 

 two lots of morning-glories, one self-fertilized for ten 

 generations and the other presumably cross-pollinated 

 for the same length of time, were obtained for com- 

 parison. The criterion Darwin used was the vigor of 

 the plants as shown by the length of the vine. He 

 found that the cross-pollinated plants were to the self- 

 pollinated ones as 100 to 53, and his conclusion was, 

 consequently, that cross-pollination is beneficial and 

 self-pollination is detrimental. 



Ritzema-Bos inbred rats for twenty generations. 

 For the first ten generations the average number of 

 young per litter was 7.5, while for the last ten genera- 

 tions it fell to 3.2. 



Weismann inbred mice for twenty-nine generations 

 and obtained a parallel result. For the first ten gen- 

 erations the average number per litter was 6.1, for the 

 second ten generations 5.6, and for the last nine gen- 

 erations 4.2. 



Dr. Helen King, on the other hand, practiced close 

 inbreeding with white rats for 40 generations com- 

 prising over 20,000 individuals obtained by mating 



