248 THE CONTINUITY OF THE GERM-PLASM, ETC. 



.the process of conjugation may be considered as a proof that this 

 view as to the secondary importance of sexual differentiation is 

 the true one. At the present time there can hardly be any hesita- 

 tion in accepting- the view that conjugation is the sexual repro- 

 duction of unicellular organisms. In these the two conjugating 

 cells are almost always identical in appearance, and there is no 

 evidence in favour of the assumption that they are not also identical 

 in molecular structure, at least so far as one individual of the 

 same species may be identical with another. But there are also 

 forms in which the conjugating cells are distinctly differentiated 

 into male and female, and these are connected with the former by 

 a gradual transition : thus in Pandorina, a genus of Volvocineae, we 

 are unable to make out any differences between the conjugating 

 cells, while large egg-cells and minute sperm-cells exist in the 

 closely allied Volvox. If we must suppose that the conjugation of 

 two entirely identical Infusoria has the same physiological effect as 

 the union of two sexual cells in higher animals and plants, we can- 

 not escape the conclusion that the process is essentially the same 

 throughout : and that therefore the differences, which are perhaps 

 already indicated in Pandorina 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 arc 

 only adaptations which secure the meeting of the two kinds of 

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

 peculiar conditions under which fertilization takes place. 



germ-cells becoming fixed, and withdrawing one of their two flagella, while the male 

 cells continue to swarm. But even this slight degree of differentiation requires the 

 supposition of internal molecular differentiation. 



