1 88 Experimental Zoology 



Experiments with Mice and Rats 



Von Guaita ^ 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 fertihty. In the following table the 

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

 young in a Htter 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 2>-6 



5th and 6th 2.9 



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

 has been a reduction in fertihty of about 30 per cent. 



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 htter is shown 

 in the following table : — 



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

 scarcely any dechne 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 : — 



Berichte der N aturjarschenden Gesellschaft zu Freiburg. 1900. 



