THE GROWTH OF GENES 



those in the other species and by their individual dominance rela- 

 tions. There is, therefore, little room for doubt that in the evolution 

 o{ Paratcttix tlie group of genes responsible for its important property 

 of polymorphism has been gradually aggregated into a more con- 

 venient single unit. Whether it has been done by inversions or by 

 some other method of restriction we do not yet know. 



Polymorphism can exist only because the heterozygote has some 

 special value. With heterostyly and sex, for example, this value 

 consists in their property of segregation which is essential for their 

 use in the control of breeding. With the patterns of grouse-locusts 

 it has been shown by Fisher to consist in their greater viability. But 

 the greater viability cannot well depend on the colour genes them- 

 selves. It must depend on accessory and invisible differences as it 

 does with the inversion heterozygotes in Drosophila. Thus some of 

 the complexity of the developing gene complex, or super-gene, is 

 cryptic or submerged. 



Super-Genes 



The gradual submergence of the variability determined by super- 

 genes may be illustrated by a series of examples. In many animals 

 one sex alone is polymorphic. In the guppy fish, Lehistes reticulatus, 

 there are a great number of gene differences on the sex chromosomes. 

 Some of these genes lie in the pairing segments ; others lie in the 

 differential segment of the X chromosome and mostly show 

 complete linkage with one another and of course with the sex- 

 determining complex. These gene differences are expressed only 

 in the male, which is consequently polymorphic like the grouse- 

 locust in colour and pattern. In the female they are submerged. 



The super-genes of Paratettix and Lehistes give no indication of 

 their origin beyond the fact that their components lie in the same 

 chromosomes. In Drosophila we have a super-gene, wliich is similar 

 in some respects, but which must be more ancient for it is common 

 to a group of species which have a second arm to their X chromo- 

 somes. And in all of the species it is held together by a visible 

 inversion in the second arm of this chromosome. This super-gene 

 is Sex-Ratio. It is a gene, like those we have earher examined in 

 plants, having a controlling influence on the breeding system and 

 subordinate only to tlie X — Y segregation itself. 



336 



