TRIGGER MECHANISM 389 



D. miranda will cross with D. pseudo-obscura, but has an unpaired 

 chromosome in the male corresponding to two in the female. 

 Together with this " X^ " chromosome there persist a pair of 

 chromosomes which although equal, nevertheless associate by the 

 reciprocal chiasmata characteristic of the sex chromosomes in the 

 related species. Dobzhansky (1935 b) concludes that this is a 

 second pair of sex-determining chromosomes [X'^Y) whose segrega- 

 tion is correlated by an unknown means with that of X^, so that 

 only two kinds of progeny are produced : X^X^X^X^, females, and 

 X^YX'^, males. It is, however, possible that X^ and Y are the 

 same chromosome, derived as to their pairing segments from X 

 and y of the other species and therefore continuing to pair by 

 reciprocal chiasmata, but no longer taking part in sex-segregation 

 (D., 1936 a). Whatever the precise mechanism, however, D. 

 miranda shows the origin of a new system of sex-segregation in the 

 Diptera on the foundations of the old one. 



The experiments on Zea show how sex-differentiation can arise 

 in a hermaphrodite. The experiment on Lebistes further shows 

 how one gene may determine sex by its segregation, while others 

 which formerly did so and are equally essential in establishing the 

 system, being homozygous, no longer pull the trigger by 

 their segregation. The first lies in an incipient sex chromosome, 

 the rest lie in a former sex chromosome which is now an autosome. 

 This relationship of the autosomes and sex chromosomes to sex- 

 differentiation is true of every stage in their evolution. It follows 

 that the autosomes govern the sex-determination from the beginning, 

 although only special genetical experiments can show their influence. 



(vii) The Differentiation of the Sex Chromosomes. The quantita- 

 tive theory of sex, derived from the study of gene differences in 

 hybrids, and the proportion theory derived from the study of 

 proportion differences in polysomic forms answer different questions. 

 The one shows how gene differences arise (and act), and the other 

 shows how they interact. Both provide the basis on which a 

 unitary theory may rest. The cytological evidence, on the other 

 hand, reveals a third point of view — the mechanical one, which 

 enables us to put the different types of sex determination in an 

 evolutionary series. This can now be considered. 



