736 



THE AMEBIC AN NATURALIST [Vol. L 



Averaging the 75 hoodeds may first be criticized. These 

 include all the extracted hoodeds that came from crosses 

 between the wild and the plus races. The third, fifth, 

 sixth and tenth generations of the plus race are involved. 

 It has been shown that the early generations of the plus 

 race are not lowered very much by the crosses in com- 

 parison with the tenth generation, which was consider- 

 ably modified. Therefore among these 75 extracted 

 hooded rats are some that were lowered by the crosses, 

 but more that were practically unmodified. Moreover, 

 the 263 twice extracted hooded rats came from ancestors 

 that had been selected for at least ten generations. Only 

 16 of the 75 once extracted rats had ancestors that had 

 been selected for ten generations; the others, having an- 

 cestors selected for a shorter time, would be expected to 

 give lower averages. In testing for further lowering in 

 this second cross it would seem unjustified to use an aver- 

 age including rats not lowered by the first cross or rats 

 that had not been selected for an equal number of gen- 

 erations before the crosses. Modified by the above con- 

 siderations the comparisons stand as follows : 



Extracted hoodeds from 



tenth gen. plus X wild average 3.15 



Extracted hoodeds from 



extracted hoodeds X wild average 3.33 



grade as hooded grandparent average 3.84 



The conclusion has been quoted that the cross of the 

 extracted hoodeds with wild has not carried on a further 

 reduction, but it has shown a return, "to about what the 

 selected race would have been had no crossing at all oc- 

 curred." "Will the above figures support this conclusion! 

 The cross of the extracted hoodeds with wild does indeed 

 give a higher F 2 average than the cross of the tenth gen- 

 eration, but the difference is only slight (.18). These 

 two averages are based on very different numbers. It 

 is entirely possible that a larger number of rats extracted 

 from the first cross would have had a higher average than 

 that of the rats extracted from the second cross; in such 



