IN-BREEDING 231 



work toward any particular goal depend largely on in- 

 breeding, knowingly or unknowingly." 1 



215. Weismann's and Von Guaita's experiments. 

 Weismann 2 in-bred mice for twenty-nine generations. As 

 shown in the following table, there was a constant and 

 fairly uniform decrease in fertility from the first to the 

 last generation : 



1 to 10 generations ; 1345 young ; 219 litters ; avg. per litter 



6.1 



11 to 20 generations; 252 young; 62 litters; avg. per litter 



5.6 



21 to 29 generations; 124 young; 29 litters; avg. per litter 



4.2 



The average number of young to a litter decreased from 

 6.1 in the first ten generations to 4.2 in the last ten genera- 

 tions. Whether this decrease is due directly to the specific 

 action of in-breeding on the quality of fertility, or whether 

 it simply represents an intensification of an innate tend- 

 ency to low fertility which existed in this particular strain, 

 it is not possible to determine. Von Guaita, working 

 with the same strain of mice and beginning with the 

 last generation (29th) bred by Weismann, obtained the 

 following results: 



1st and 2d generations, avg. per litter, 3.5 

 3d and 4th generations, avg. per litter, 3.6 

 5th and 6th generations, avg. per litter, 2.9 



There is here a clear loss of fertility from an average 

 of 6.1 to a litter to 2.9 to a litter in 35 generations, trace- 

 able to in-breeding. 



1 Georg Wilsdorf, "Tierzuchtung," 1912. 



2 "Berichte der Naturforschenden Gesellschaft zu Freiburg," 

 1900. 



See Morgan, "Experimental Zoology," p. 188. 



