384 PERMANENT HYBRIDS 



correlated with it. This gives us the minimum information with 

 the maximum certainty. It shows us the mechanism underlying 

 sex determination, but it does not show us how the mechanism 

 arose or how it operates. There are therefore an evolutionary and 

 a physiological problem to be decided. Both have to be approached 

 by the comparative method. 



The evolutionary problem has been approached by attempting a 

 unitary theory of sex determination. Correns (1928) considered 

 that such a theory may rest on the assumption that homologous 

 genes are present in all species for sex differentiation at each stage of 

 the life cycle, and that allelomorphs of those affecting one stage are 

 found in one group and affecting another stage in another group 

 (cf. also Allen, 1932). This hypothesis is of little value. The 

 assumption of genes in one organism affecting differences only 

 found in an entirely different group of organisms depends on 

 assumptions of the nature of variation for which there is no evidence. 

 Sex differentiation has certainly arisen independently in different 

 groups. The fact of its analogy of mechanism is due only to the 

 inevitable limitations of its mechanism — segregation of alternatives 

 at meiosis. 



The second attempt is based on the study of sex conditions in 

 hybrids. In these, sex seems to be determined by a balance of 

 activities working in opposite directions — to maleness and to female- 

 ness. They can be represented more or less quantitatively. The 

 one activity is determined by the X chromosome, which is present 

 in a single dose in one sex, in double dose in the other. The opposite 

 activity is determined by the autosomes (or the cytoplasm or Y 

 chromosome in Lymantria) and is in the pure species stronger than 

 the single dose of X, but weaker than the double dose. When 

 races or species are crossed this relationship does not hold, and 

 intersexes result. This formula may be taken as an approximate 

 description of the differences between races and between the effect 

 of the X and of the other chromosomes. They may be expressed 

 quantitatively and according to direction — male or female (Gold- 

 schmidt, 1934). 



The formula does not, however, decide the question as to whether 

 the effect is produced by differences in proportion or in summation, 



