SEX DETERMINATION 233 



nature of the sexes is unknowable, but it is difficult to be content with 

 a conception of such a mystical kind. 



On the other hand, one cannot avoid some hypothesis of underlying 

 complexes, on which the alternative sex-differentiation in any given 

 species depends, since, for instance in fish, one finds the choice between 

 the two types of differentiation, with all their complicated effects, made 

 by a single gene, or even by external conditions. We must suppose that 

 the complexes, Hke all other attributes of living organisms, have been 

 evolved, and they must therefore have been, originally at least, of the 

 nature of genes or complexes of genes. 



It may seem remarkable that two sets of genes should have been 

 evolved, which control rather complicated developmental reactions and 

 give rise to compUcated organs, but such that quite trivial factors, such 

 as slight variations in the environment, can cause one set to be active 

 and the other inactive. Yet this is exactly what we must postulate for 

 phenotypic sex determination. The strangeness of the idea disappears, 

 however, if we remember that this is exactly what always happens with 

 all characters, though usually the differential conditions occur within 

 one animal body instead of between different animals. For instance, the 

 cells of the ectoderm of a gastrula contain two complexes of genes, or 

 two ways of reacting, one of which causes the development of the central 

 nervous system, the other that of skin; and the choice between them 

 is made by a single simple condition, the presence or absence of the 

 evocator. Probably in all such systems, it is only one master reaction 

 which is directly affected by the deciding stimulus; thus in an organism 

 whose sex can be determined phenotypically by the pH of the medium 

 (e.g. Bonellia viridts), it is unlikely that each of the gene reactions in 

 the sex-development is itself affected by the acidity, but more probable 

 that the direa effect is confined to some master reaction which then 

 affects certain other reactions and so on through a whole hierarchy of 

 secondary and tertiary effects. We know examples both of genes which 

 do not have any effect except in combination with certain other genes 

 (e.g. modifiers), and of genes whose activity is dependent on the external 

 conditions. If we regard A and G as symbolizing two alternative types 

 of reaction of the genotype, based on genes of this kind, they become 

 less difficult to accept than they would be if we had to regard them as 

 inherent tendencies of Hving matter. 



It also becomes clearer why we rarely find more than two sexual 

 types, since it would obviously be more difficult to build up three or 

 four alternative methods of reaction than two. In multipolar sexuality. 



