DEFINITION OF SEX 17 



condition of this differentiation is an environmental difference, 

 but a prior condition is a genetic difference. 



The special conditions of haplo-diploid sex differentiation will be 

 considered later. 



Since sexual differentiation necessarily restricts the possibility of 

 fusion to gametes of opposite character and therefore often of 

 different origin, other types of differentiation having a restrictive 

 effect on fusion are often classified with it. Thus in certain Protista, 

 Fungi, and Algae, fusion is restricted to cells having a different 

 genetic constitution. This is described as " heterothallism " in the 

 Phycomycetes and Basidiomycetes, and " physiological anisogamy " 

 in the Chlorophyceae and " relative sexuality " in Ectocarpus 

 (Hartmann, 1929). Such behaviour is analogous in genetic cause 

 and contraceptive effect with self-sterihty in the flowering plants ; 

 all these kinds of incompatibility demand fusion of gametes 

 that are genetically different. The genetical types of gametes are 

 usually of several different kinds, and the system is not alternative. 

 Sexual differentiation, on the other hand, is strictly alternative, 

 and its essential property is the provision of gametes so differentiated 

 as to secure readier fusion and more successful development of the 

 product. Sexual differentiation demands the fusion of gametes 

 that are morphologically different. 



IncompatibiUty and sexual differentiation are therefore distinct 

 in their genetical basis and in their physiological function. When 

 sexual differentiation comes secondarily to be determined by 

 genetic differentiation, the two agree in securing, wholly or partly, 

 the fusion of genetically distinct gametes, i.e., hybridisation, in a 

 broad sense. The importance of distinguishing them, however, 

 remains, for incompatibility may be superimposed on sexual 

 differentiation of gametes in hermaphrodites and serve the end of 

 encouraging hybridisation where genetic determination of sex has 

 failed to do so. 



To conclude, botanists and zoologists who take too little account 

 of one another's work have introduced a confusion into the use of 

 the word sex. This confusion can be avoided by using the word 

 with a consistent genetic meaning in all plants and animals. It 

 will be applied to the two following relationships : — 



