EVOLUTION OF THE SOMA 205 



filamentous forms have sexually differentiated gametes, 

 and these in most cases show a wide differentiation 

 between sperms and eggs, comparable with that seen 

 in Volvox. 



Sexual differentiation of gametes is found in various 

 different groups among the algse, evidently inde- 

 pendently evolved along separate lines of descent. 

 One may say that there is an inevitable tendency 

 towards sexual differentiation, and towards the fixa- 

 tion of the extreme form in which the sperms are 

 most widely different from the eggs. This high 

 specialisation of the gametes, as already shown, is 

 certainly the most efficient mechanism for the pro- 

 duction of vigorous new individuals through conjugation. 



The " Soma " in Plants and Animals. — ^The evolu- 

 tion of a soma or mortal body is beautifully illustrated 

 in the Pandorina-Eudorina-Pkodorina-Volvox series. 

 This evolution consists essentially in the separation of 

 the vegetative and reproductive functions. In the 

 lower forms of the series all the cells of the body dis- 

 charge both functions, in the higher some cells dis- 

 charge the vegetative, others the reproductive func- 

 tion. Directly we have any cells limited to the vege- 

 tative functions, we have a soma or mortal " body " 

 by the very fact that these cells can no longer reproduce 

 the species. 



It must be noted, however, that this particular series 

 of organisms is not in the direct line of evolution of 

 any of the higher organisms. Volvox is the culmi- 

 nation of its own line of descent. It is probable that 

 no further increase in size or complication is possible 

 to the motile coenobiate form of organism. The soma 

 has been evolved on many other lines of descent' from 

 unicellular organisms. This particular line is chosen 



