GENERAL RELATIONS IN INHERITANCE 293 



with a high yield. Such "hybrid vigor" occurs in many culti- 

 vated plants, and likewise in domesticated animals. But it is 

 not limited to organisms under cultivation. It occurs in many 

 cases in the classic animal for study of heredity, the fruit-fly. 

 Two individuals with short imperfect wings or none, so that 

 they cannot fly, the defect however due to different gene 

 pairs in each case, give when mated together offspring with 

 strong perfect functional wings. This type of relation is 

 shown in many characteristics of Drosophila. It is bound to 

 occur in any organism in which there exist many gene modifi- 

 cations. This is in a high degree the case with man, so that 

 such results play a large role in man. 



This method of action is one of the most important that 

 occurs in genetics. It is not something that is confined to 

 large defects, but is at work continually in connection with 

 all sorts of characteristics, slight or marked. Human parents 

 who are stupid and lack industry and ambition may produce 

 children that have none of these defects, in consequence of 

 the contribution by the parents of supplementary genes, in 

 the way illustrated in figure 60. It can hardly be doubted 

 that this is in considerable measure the origin of individuals 

 of marked superiority, individuals of genius. They result 

 from an unusual combination of genes formed by the contri- 

 bution of supplementary genes by the two parents. Such 

 unusual combinations may arise from two parents that are 

 themselves not personally superior. This accounts for the 

 not infrequent occurrence of exceptional persons, showing 

 characteristics that make them distinguished, in common- 

 place families. 



G. Production of inferior individuals by recombinations. — 

 Results the reverse of those just described also follow from 

 recombination of genes. By the taking apart of superior 

 combinations and the making of new ones in which several 

 or many defective genes come together in the same pairs, 

 inferior, defective Individuals may be produced (figure 61). 



