THE EVOLUTION OF SEX 5 



in their historical aspects, but some of their most 

 modern aspects may well arrest our attention. 



In the simplest cases, as shown by some of the one- 

 celled organisms, two individuals fuse into a single 

 one (Fig. 1) ; in other related organisms the two in- 

 dividuals that fuse may be unequal in size. Some- 

 times we speak of these as male and female, but 

 it is questionable whether we should apply to these 

 unicellular types the same names that we use for the 



Fig. 1. — Union of two individuals (Stephanosphcera pluviaZis) to 

 form a single individual. (After Doflein.) 



many-celled forms where the word sex applies to the 

 soma or body, and not to the germ cells. 



One of the best known cases of conjugation is that 

 of paramcecium. Under certain conditions two in- 

 dividuals unite and partially fuse together. An in- 

 terchange of certain bodies, the micronuclei, then takes 

 place, as shown in Fig. 2, and in diagram, Fig. 3. The 

 two conjugating paramcecia next separate, and each 

 begins a new cycle of divisions. Here each individual 

 may be said to have fertilized the other. The process 

 recalls what takes place in hermaphroditic animals of 

 higher groups in the sense that sperm from one indi- 

 vidual fertilizes eggs of the other. 



We owe to Maupas the inauguration of an epoch- 

 making series of studies based on phenomena like this 

 in paramcecium. 



