170 Outline of Genetics 



into the foregoing scheme, since species boundaries are more often 

 matters of personal opinion than indices of crossabihty. 



The theory of heterozygosis claims that hybrid vigor 

 appears in proportion to the number of factors in which 

 the parents of the cross differ. This claim should be 

 considered briefly. Is heterozygosis really an explana- 

 tion of the phenomenon of hybrid vigor? It seems 

 obvious that it is not. It was known that hybrids were 

 vigorous because they were hybrids. Heterozygosis 

 states that hybrids are vigorous to the degree that their 

 parents differed in hereditary factors; in other words, 

 this is merely a statement that hybrids are vigorous 

 because they are hybrids, with the addition that the 

 more hybrid a hybrid is the more vigorous it is. It 

 follows, therefore, that heterozygosis is not an explana- 

 tion of hybrid vigor, but merely a restatement of the 

 phenomenon in Mendehan terms, with the additional 

 idea that there may be various degrees of hybrid vigor. 

 It is not the intention to discredit heterozygosis as a 

 valuable conception, but to point out that it is not a real 

 explanation, merely a more intelligent statement of facts. 



Furthermore, heterozygosis is rather unsatisfactory 

 in another way. It locks the door on any hope of origi- 

 nating pure strains having as much vigor as first genera- 

 tion hybrids. 



For these reasons it would seem desirable to seek an 

 explanation of hybrid vigor along other lines. Such an 

 explanation may be developed from the following con- 

 siderations. 



In nature a "struggle for existence" occurs among 

 species and individuals. There must occur also a struggle 

 for existence among unit characters. If a unit char- 



