INBREEDING AND CROSSBREEDING 231 



Experiments made bj^ Weismann confirm those of Bos as 

 regards the falHng off in fertility due to inbreeding. For 

 eight years Weismann bred a colony of mice started from 

 nine individuals, — six females and three males. The experi- 

 ment covered twenty-nine generations. In the first ten gen- 

 erations the average number of young to a litter was 6.1; 

 in the next ten generations, it was 5.6; and in the last nine 

 generations, it had fallen to 4.2. 



But recent inbreeding experiments with rats carried on at 

 the Wistar Institute by Dr. Helen King give results quite at 

 variance with those of Bos and Weismann. She finds, as 

 was found to be the case in Drosophila, that races of large 

 size and vigor and of complete fertility may be maintained 

 under the closest inbreeding, if the more vigorous individuals 

 are selected as parents. By this means she seems to have 

 secured races of rats which are relatively immune to injurious 

 effects from inbreeding. My own experience with rats inbred 

 within lines of narrow selection for seventeen generations is 

 that races of fair vigor and fecundity can be maintained 

 under these conditions, but that when two of these inbred 

 races are crossed with each other, even though they had their 

 origin in a small common stock many generations earlier, an 

 immediate and striking increase of vigor and fecundity oc- 

 curs. This is quite similar to the result observed in the case 

 of Drosophila, and is quite in harmony with the results ob- 

 tained by Shull in maize; it indicates that by careful selection 

 races may be secured which are vigorous in spite of inbreed- 

 ing, but that nevertheless an added stimulus to growth and 

 reproduction may be secured in such cases by crossbreeding. 



In the production of pure breeds of sheep, cattle, hogs, and 

 horses inbreeding has frequently been practiced extensively, 

 and where in such cases selection has been made of the more 

 vigorous offspring as parents, it is doubtful whether any 

 diminution in size, vigor, or fertility has resulted. Never- 

 theless it very frequently happens that when two pure breeds 

 are crossed, the offspring surpass either pure race in size and 

 vigor. This is the reason for much crossbreeding in eco- 



