RULES AND RATIOS OF INHERITANCE 23 1 



themselves, without genetic variation, and the same is true 

 for the individuals of lowest grade. 



This cessation of the effectiveness of selection after a 

 certain grade was reached was unexpected, and gave origin 

 to much discussion. But after it became clear that in such 

 cases multiple factors are at work, in other words that 

 the various grades depend on the number of certain types 

 of genes that are present, the matter was cleared up. Selec- 

 tion is effective so long as it deals with the intermediate 

 grades, which are of various different genetic constitutions, 

 and are frequently heterozygotic. But as soon as the homo- 

 zygotic individuals AABBCC, etc., or aabbcc, etc., are 

 reached, no further progress can be made, since these can 

 produce only offspring having the same constitution as 

 themselves. 



On the whole It is fully demonstrated that so-called blend- 

 ing inheritance is inheritance resulting from gene differ- 

 ences, such as yield ordinary Mendelian inheritance; but in 

 blending inheritance the parents differ in several or many 

 pairs of genes, each gene having a quantitative effect. 



