No. 636] GUINEA PIGS AND TV l s 



41 



3-5, 7-9 and 10-14, these half bloods exceed family 35 

 by 13 per cent., 19 per cent, and 12 per cent, respectively 

 in duration of life, and exceed the other inbred families 

 by 31 per cent., 42 per cent, and 60 per cent, respectively. 

 We have here more decisive evidence of the improve- 

 ment in vigor in this respect which may follow a cross, 

 than in the comparison previously made between the 

 total inbreds and total crossbreds. The most probable 

 explanation is the same as that applied to the improve- 

 ment in fertility, weight and death rate following crosses, 

 viz., that each inbred family tends to supply dominant 

 factors, favoring vigor, which are lacking in the other. 

 Applied to the present case, this means not only that 

 family 35 possesses dominant factors for resistance, 

 lacking in the other families, but that some or all of the 

 latter may possess such factors lacking in family 35. By 

 inbreeding among the crossbreds, it should be possible to 

 develop a strain even more resistant than family 35 pro- 

 vided that the linkage relations do not interfere with the 

 fixation of the factors for resistance in one strain. 



The quarter-bloods of family 35 confirm the conclu- 

 sions derived from consideration of the half bloods. 

 They are about intermediate between the half-bloods 



