1 88 Experimental Zoology 



Experiments with Mice and Rats 



Von Guaita 1 has made some experiments with mice that had 

 been already inbred by Weismann for 29 generations. He in- 

 bred the animals through seven generations and found clear 

 evidence of diminishing fertility. In the following table the 

 total number of young, the number of litters, and the average 

 young in a litter are given. 



WEISMANN'S DATA 



i to 10 generation : 1345 young; 219 litters; aveg. per litter 6.1 

 ii to 20 generation: 252 young; 62 litters; aveg. per litter 5.6 



21 to 29 generation: 124 young; 29 litters; aveg. per litter 4.2 



Von Guaita's data, beginning with the mice of the last generation of 

 Weismann's stock, are as follows : 

 ist and 2d generations . . . . . ... . 3.5 



3d and 4th 3.6 



5th and 6th . . ' .. . . . . . . . 2.9 



Comparing the first and the last averages, it is seen that there 

 has been a reduction in fertility of about 30 percent. 



Ritzema Bos inbred rats for 30 generations. The family 

 started with a female albino white rat, that was paired with a 

 wild rat, and gave twelve young. A white male of different 

 parentage was bred to seven of these offspring, but later no 

 other foreign blood was introduced during the six years of the 

 experiment. Parents were bred to offspring, and sisters to 

 brothers. The average number of young per litter is shown 

 in the following table : 



1887 1888 1889 1890 1891 1892 



7* 7* 7jf 6fJ 4^ 3* 



During the first 20 generations (in the first four years) there was 

 scarcely any decline in the productiveness, as the table shows, 

 but in the following ten generations there was a marked and 

 sudden decline. The number of pairings that were sterile in- 

 creased steadily as shown in the next table : 



1 Berichte der Naturjarsch^iden Gesellschaft ZM Freiburg. 1900. 



