REVIEW OF VON GUAITA'S EXPERIMENTS IN 



BREEDING MICE. 



C. B. DAVENPORT. 



IN the two latest volumes of the BericJite der Naturfor- 

 scJicnden Gescllschaft zu Freiburg, G. von Guaita gives the 

 results of his breeding experiments. He started in 1896 with 

 fifty-five Japanese walzing mice and with numerous wJiite mice 

 belonging to a race bred by Weismann since 1888, and made 

 crossings through seven generations to 1900. His main data 

 concern the inheritance of color ; incidentally, data were got 

 on the diminution of fertility with in-and-in breeding. 



Diminution of Fertility. 



These results were gained chiefly from Weismann's breed- 

 ings. The total number of young, the number of litters, and 

 the number of young per litter are given below for each decade 

 of generations. 



ist to roth generation : 1345 young ; 219 litters ; avg. young per litter, 6.1 

 Iithto2oth " 352 " 62 " " " " " 5.6 



2ist to 29th " 124 " 29 " " " " " 4.2 



In von Guaita's breedings : 



ist and 2d generations, . . . " " " " 3.5 



3rd and 4th " ...."'" " 3.7 



5th and 6th " ....""" " 2.9 



Thus in the successive generations there is a reduction in fer- 

 tility of about thirty per cent ; and this is probably due to too 

 close breeding. 



121 



