74 INBREEDING AND OUTBREEDING 



is theory only ; in reality the influence of one factor may 

 be somewhat different from that of another factor. 



tf=l 121 



AT = 2 14641 



N = 3 1 6 15 20 15 6 1 



tf = 4 1 8 28 56 70 56 28 8 1 



N = 5 1 10 45 120 210 252 210 120 45 10 1 



Let us now note a few of the practical difficulties in 

 interpreting results that may follow this method of in- 

 heritance. In the theoretical example that we have used 

 for the sake of clearness, it was assumed that there were 

 no non-heritable fluctuations due to environment. Unfor- 

 tunately this is not the case in nature. Fluctuations are 

 everywhere present. They would obscure the classes to 

 which individuals belong even if these class differences 

 were quite large. And since they are usually small, the 

 change of individual form due to environmental causes 

 makes it impossible to separate an F 2 population into the 

 true classes to which they belong gametically. Nor is this 

 the whole trouble. If the table showing the expected re- 

 sults with two pairs of size characters is examined, it is 

 found that not all the individuals that belong to a par- 

 ticular size class have the same zygotic formula. For this 

 reason one cannot pick out zygotes of a certain size and 

 expect them to breed the same. Their potentialities are 

 likely to be different. Furthermore, practical breeding 

 results are undoubtedly complicated by cases of correla- 

 tion. This correlation need not be ganietic, though such 

 cases in all likelihood do occur ; it may be merely physi- 

 ological. For example, a maize plant might have the 

 gametic possibilities of small plant size and large ear 

 size, but it would be foolish to expect that a plant capable 



