SEX 



79 



sense, the distinction made is a valid and convenient 

 one. 



3. Many naturalists have discussed the question, 

 why should the two sexes exist ? The answer is not 

 simple, yet from the fact that sexuality is such a wide- 

 spread phenomenon, we cannot doubt that it has a 

 meaning in relation to the preservation of life. At first 

 it appears that reproduction depends upon sexuality, 



Why do 

 sexes exist? 



3 4 



Drawings by R. Weber 



FIG. 17. The rose scale (Aulacaspis roses), an extreme case of sexual dimorphism, 

 i, Adult male, with 2 wings, 2 antennae, 6 legs, but no mouth. 2, Adult female; 

 legs, antennae, and wings not developed, but mouth (on under side) well devel- 

 oped. This female never leaves the scale (3). The scale shows the cast skin of the 

 larva at A , the second cast skin at B, and the white adult scale covering the female. 

 4 shows the scales on a rose branch, twice natural size. 



