254 CONTINUITY OF THE GERM-PLASM, ETC. 



the differences, which are perhaps already indicated in Pando- 

 riua and are very distinct in Volvox and in all higher organisms, 

 have nothing to do with the nature of the process, but are of 

 quite secondary importance. If we further take into account 

 the extremely different constitution of the two kinds of sexual 

 cells in size, appearance, membranes, motile power, and finally 

 in number, no doubt remains that these differences are only 

 adaptations which secure the meeting of the two kinds of con- 

 jugating cells : that in each species they are adaptations to the 

 peculiar conditions under which fertilization takes place. 



il 



