44 SEX 



and variation are thus deeply one, and the 

 variations on which Natural Selection operates 

 are thus fundamentally dichotomous. 



Granted the advantage of cross-fertilisation 

 and of dimorphic sex cells, it seems clear 

 that, other things being equal, those species 

 would get on best which exhibited two dis- 

 tinct constitutions or forms, sperm-producers 

 and egg-producers. As the difference became 

 defined the expression, we maintain, of a 

 continually recurrent dichotomy it would 

 doubtless react on the sex-cells. More effec- 

 tive ova would be produced by the more 

 anabolic females, and more effective sperma- 

 tozoa by the more katabolic males. 



Returning to Volvox, we see that some of 

 the cells in the ball become large, well-fed 

 ova; others, less anabolic, fade from green 

 to yellow, divide and re-divide into many 

 minute units the spermatozoa. The large 

 cells or ova of one colony are fertilised by the 

 small cells or spermatozoa of another colony. 

 The formation of dimorphic reproductive cells 

 may be seen going on in different parts of the 

 same colony, but it often happens that the 

 spermatozoa are formed first and the ova 

 later which is like a temporal division of the 

 sexes. In other cases the eggs are formed 

 first. Or we may find Volvox balls in which 

 only ova are produced, or only sperms the 

 sexes being separate. The former seem to be 

 more vegetative and nutritive than the latter, 

 and we are at the foundation of the divergence 

 between the sexes. Occasionally, moreover, 

 ova are produced which develop partheno- 

 genetically. In fact, this primitive organism, 



