xiv THE CONTINUANCE OF LIFE 301 



(c) But the higher forms of sexual reproduction imply 

 more than the liberation of special reproductive cells, 

 more than the union of two different and mutually 

 dependent kinds of reproductive cells, they imply the 

 separation of the sexes. It is unnecessary to compli- 

 cate the problem by considering just at present the 

 frequent occurrence of hermaphroditism, e.g. in earthworm 

 and snail, where the production of male and female 

 germ-cells is combined in one animal. The main problem 

 is the evolution of male and female individuals, i.e. 

 fundamentally, of sperm-producers and egg-producers. 



2. The Evolution of Sex. If we study those interesting 

 Infusorian colonies, of which Volvox is a good type, the 

 riddle may be at least partially read. Though Protozoa, 

 they are balls of cells, in which the component units are 

 united by protoplasmic bridges and show almost no 

 division of labour. In some colonies there is asexual 

 multiplication, for certain cells divide up into clusters 

 of cells which escape from the parent and form new free- 

 swimming balls. In other conditions a less direct 



o 



multiplication occurs. Some of the cells apparently 

 better fed than their neighbours become large ; others, 

 less successful, divide into many minute units each with 

 two flagella. The large kind of cell is fertilised by the 

 small kind of cell (from another colony), and there is 

 no reason why we should not call them ova and sperma- 

 tozoa respectively. In some colonies both kinds of re- 

 productive cell are formed, in others only ova ; in others 

 only spermatozoa. There may be hermaphroditism, or 

 sometimes the colony is almost quite female, or almost 

 quite male. In some cases the ova develop partheno- 

 genetically, without being fertilised. Indeed we have in 

 Volvox, as Dr. Klein an enthusiastic investigator of 

 this form rightly says, an epitome of all the great steps 

 in the evolution of sex. 



So far we have stated facts ; let us now briefly state a 

 theory 1 which seeks to unify them. 



All through the animal series, from the active In- 



1 The Evolution of Sex, by Patrick Geddes and J. Arthur 

 Thomson, 1889. Revised Ed. 1901. 



