GREAT STEPS IN EVOLUTION 87 



we see that in the simplest forms of hfe im- 

 mortahty has not even yet been pawned for 

 love. 



The Origin of Sex. — In many of the uni- 

 cellular organisms there is a kind of sexual 

 reproduction, in the sense that two cells fuse 

 to become one, just as ovum and sperma- 

 tozoon do in higher creatures. In many 

 cases, moreover, the two cells which fuse are 

 dimorphic, as is well illustrated in the bell- 

 animalcule, Vorticella, where a small, active, 

 free-swimming (we may say male) cell unites 

 with a fixed individual of full size, which 

 may be called female. This is one line of 

 approach to the origin of sex, and it may be 

 noted that the male and female cells illus- 

 trate the antithesis we have already discussed 

 between relatively more anabolic and rela- 

 tively more katabolic types. 



The next stage in the problem is to account 

 for the familiar fact that in almost all 

 organisms with bodies there are special 

 reproductive cells, or germ-cells — ova and 

 spermatozoa — quite distinct from the or- 

 dinary body-cells. This is an economical 

 improvement on the method of starting a 

 new life by a sexual over-growth or by the 

 liberation of buds. Moreover, the peculiarity 

 of true germ-cells is that they do not share in 

 building up the "body," and that they 



