CHAP. XII The Life-History of Animals 187 



(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. The problem of sexual reproduction becomes 

 less difficult when the various facts are discussed separ- 

 ately, and if you grant that there is no great difficulty 

 in understanding the liberation of special cells, and 

 no great difficulty in understanding why two different 

 kinds should in most cases have to unite if either is to 

 develop, then I do not think that the remaining fact — 

 the evolution of male and female individuals — need remain 

 obscure. 



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. From 

 such a ball of cells, units are sometimes set free which 

 divide and form new colonies. In other conditions a less 

 direct multiplication occurs. Some of the cells — apparently 

 better fed than their neighbours — become large ; others, 

 less successful, divide into many minute units. The large 

 kind of cell is fertilised by the small kind of cell, and there 

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

 tozoa respectively. In such a Volvox, two different kinds 

 of reproductive cell are made within one organism. But 

 we also find Volvox balls in which only ova are being 

 made, and others in which only spermatozoa are being • 

 made. The sexes are separate. Indeed we have in Vol- 

 vox, 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 I have stated facts ; now I shall briefly state the 

 theory by which Professor Geddes has sought to rationalise 

 these facts. 



All through the animal series, from the active Infusorians 

 and passive Gregarines, to the feverish birds and sluggish 

 reptiles, and down into the detailed contrasts between order 



