STRUCTURE OF ORGANISMS. 29 



Or, as in many of the lower multicellular algae and in 

 Monocystis, a parasitic protozoon found in the earth- 

 worm, two individuals may each form a set of similar 

 gametes without visible sexual differences, which may 

 fertilise each other. Again, two similar whole individuals 

 may fuse, and subsequently give rise to new cells ; a 

 common form of reproduction among fresh-water algae. 

 The infusorian Paramecium merely exchanges cytoplasmic 

 and nuclear material with another individual during a 

 temporary union or conjugation. 



Although these various simpler modes of fertilisation 

 do not in all probability represent actual phylogenetic 

 stages in the evolution of sex, yet they give us some 

 notion of how sex may have become developed. For its 

 very first origin one must perhaps go back to that early 

 time in the history of protoplasm before definite forms 

 had become differentiated, and when it might have been 

 beneficial for two masses of protoplasmic material, differ- 

 ing slightly in composition, to mix and combine their 

 properties together. 



But whatever may have been its origin, fertilisation 

 now gives a necessary stimulus to development, and 

 brings about some sort of rejuvenation. Even among 

 those plants and animals which propagate freely asexu- 

 ally recourse is had sooner or later to sexual reproduction. 

 It would seem, as Biitschli maintained, that prolonged 

 reproduction by fission in the protozoa in some way 

 exhausts their vitality. Certain it is that under con- 

 ditions which would lead to their death if unable to con- 

 jugate, they can be seen to start with renewed vigour on 

 a fresh career of growth and multiplication after conju- 

 gation has taken place. 



It has already been mentioned that in multicellular 

 organisms active growth occurs among the less differen- 

 tiated cells which have retained a primitive richness in 

 protoplasm, an embryonic character. Now the germ- 

 cells, usually produced in enormous numbers, are derived 



