HEREDITY 



but in the last year of the experiment it had 

 fallen to 3.2, and many pairs were found to 

 be completely sterile. Diminution in size also 

 attended the inbreeding, at the end amounting 

 in the case of males to between 8 and 20 %. 



Experiments made by Weismann confirm 

 those of Bos as regards the falling off in fer- 

 tility due to inbreeding. For eight years Weis- 

 mann bred a colony of mice started from nine 

 individuals, six females and three males. 

 The experiment covered 29 generations. In 

 the first 10 generations the average number of 

 young to a litter was 6.1; in the next 10 gen- 

 erations, it was 5.6; and in the last 9 genera- 

 tions, it had fallen to 4.2. But sweeping 

 generalizations cannot be drawn from these 

 cases. Each species of animal must probably 

 be tested for itself before we shall know what 

 the exact effects of inbreeding are in that case. 

 In guinea-pigs, a polydactylous race built up 

 by the closest inbreeding out of individuals all 

 descended from one and the same individual 

 has now been in existence for ten years. It 

 consists of one of the largest and most vigor- 

 ous strains of guinea-pigs that I have ever 



148 



