188 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 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 
1 to ro generation: 1345 young; 219 litters;  aveg. per litter 6.1 
Ir to 20 generation: 252 young; 62 litters;  aveg. per litter 5.6 
21 to 29 generation: 124 young; 2g litters;  aveg. per litter 4.2 
Von Guaita’s data, beginning with the mice of the last pemeration of 
Weismann’s stock, are as follows: — 
ist and 2d generations . : : . : . . : - 3.5 
gdand 4th. : Sos . . : : ‘ c . 3.6 
sth and 6th . : ia ol : : . . : ‘ + 29 
Comparing the first and the last averages, it is seen that there 
has been a reduction in fertility 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 litter is shown 
in the following table: — 
1887 1888 1889 1890 1891 1892 
74 14 14 633 age 33 
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: — 
' Berichte der Naturfarschenden Gesellschaft zu Freiburg. 1900. 
