SUMMARY OF RESULTS. 23 



5. During the progress of the original selection (thirteen successive 

 generations) variability as measured by the standard deviation was 

 somewhat diminished. 



6. Upon crossing the selected plus and minus races with each other, 

 the variability was somewhat increased in Fi and was further increa 



in F 2 . The extreme conditions (plus or minus) of the grandparents 

 rarely, if ever, recur in this generation. Only one individual among 378 

 F 2 young has been recorded in a grade as extreme as either grandparent. 



7. Hooded animals extracted in F 2 as recessives from a cross with 

 either Irish or wild rats are as a rule more variable than the selected race 

 used in making the cross. In crosses with an Irish race the minus series 

 was affected in like measure. In crosses with wild rats the variability 

 of the plus series was not appreciably affected (in two experiments it 

 was slightly reduced, and in one experiment it was slightly increased). 

 But the variability of the minus race was more than doubled by crosses 

 with wild rats. 



8. The mean of the minus race was lowered by a cross with either the 

 Irish race or with wild rats, but more extensively by the latter. The 

 mean of the plus race was lowered a very little by a cross with wild rats, 

 but considerably by a cross with the Irish race. 



DISCUSSION. 



It would be possible to suppose, as the senior author has elsewhere 

 suggested (Castle, 1912), that the Mendelian unit character involved in 

 these experiments is subject to quantitative variation and that such 

 quantitative variations have a tendency to persist from generation to 

 generation. This would account for the effectiveness and permanency 

 of selection when brought to bear upon the variations. It might also 

 form a basis for explaining the increased variability which follows cross- 

 ing, this being regarded as due to contamination in the heterozygote, but 

 there are certain other observed effects of crossing which it seems impos- 

 sible to account for on this basis. In particular it is observed that while 

 crossing the minus series makes it less minus as the hypothesis of con- 

 tamination would demand, crossing the plus series makes it less plus, the 

 opposite of what a contamination theory would demand. For we can 

 readily understand, on the basis of contamination, how a +6 gamete 

 being combined with a —2 gamete might change the latter in a plus 

 direction; but if the same +8 gamete is associated with a +4 gamete 

 we should expect it, if it has any influence at all, to make this also more 

 plus, but the observed effect is the opposite; the extracted gametes are 

 less plus in character. 



This difficulty is met by an alternative explanation, the main feature 

 of which was first suggested by our colleague, Dr. E. M. East, viz, that 

 although w r e seem to be dealing with a single unit character as evidenced 

 by the monohybrid ratios obtained, nevertheless the modifications 



