36 



THE AMERICAN NATURALIST [Vol. LV 



itary differences as one of the possible causes of this 

 variation. 



Hereditaey Diffeeences in Resistance 

 The average length of life in each inbred family and 

 crossbreeding experiment in lots 3-5, 7-9 and 10-14 is 

 given in Tables YII. VIII and IX together with other 



TABLE VII 



Averages of the Characteristics of the Guinea Pigs Tested for Ee- 



The days lived in lot 3 are multiplied by 3/5 to make the average about 



TABLE VIII 



2 13 ; 2.46 i 80.2 197.1 243.1 64.2 22.8 



13 16 2.81 1 91.0 257.1 308.1 63.6 1 23.9 



32 10 2.40 i 86.6 224.4 273.0 71.8 i 20.6 



data. The distribution of the deaths is given in Tables 

 X, XI and XII. As already stated, most of the lots were 

 brought to a close while a few animals were still living. 

 This introduces an element of uncertainty into the aver- 



