58 



THE EVOLUTION OF SEX. 



which by continuous division a fresh colony is then developed. 

 In these transition forms there are thus reproductive cells of 

 slight distinctness, but as yet obviously no sexual organs. 



Volvox, a loose colony of cells, with some set apart for reproduction, after Kirchner. 



When we pass to the sponges, we find colonies consisting of 

 myriads of cells, among which there is a considerable division of 

 labour. An outer layer (or ectoderm) usually consisting of much 

 subordinated cells, an inner layer (or endoderm) of predominantly 

 active and well-nourished cells, a middle layer of heterogeneous 

 constituents, can always be distinguished. Every average infu- 

 sorian is as good as its neighbours, so far as reproduction of new 

 individuals by division is concerned ; in the colonial protozoa, 

 the units that are set adrift are very little different from their 

 fellows that remain behind ; but this ceases to be true when we 

 pass to colonies where considerable division of labour has been 

 established. It is certainly true that even a tiny fragment of 

 sponge, cut off from the larger mass, may, if it contain sufficient 

 samples of the body, and if the conditions be favourable, repro- 

 duce a new individual. Cultivators of bath sponges habitually 

 take advantage of this fact. But the sponge starts its new 



