Nature of Sexual Diversity 127 



general validity, it of course requires one to hold that the 

 two chromosomes which unite at their mating within the cell 

 are sexually diverse. 



But it hardly appears possible to argue on general 

 physical grounds that attraction and union imply diversity ; 

 two masses of substance of identical physical and chemical 

 constitution show the attraction called gravitation; they 

 tend to come together and unite. Adhesion occurs between 

 bodies of like character, and in general it is not clear that 

 all attraction of bodies must be due to chemical or physical 

 diversities between them. 3 It appears therefore that we 

 should look for further evidence before holding that union 

 is itself an evidence of sex diversity. If the two things 

 that unite are characteristically diverse, the difference must 

 show itself in other ways, so that we must rely on other 

 tests to determine whether the diversity exists. Is there 

 any evidence that the two chromosomes that unite in mat- 

 ing are sexually diverse? 



In Vorticella and its relatives, as we have seen, the two 

 individuals that mate are diverse (Figures 33 and 34). But 

 curiously, these two the "male" and "female" are formed, 

 in some species at least, by the division of a single ordinary 

 fixed individual into a male and a female individual. The 

 ordinary fixed individuals before this division do not mate, 



tween the two mating cells is necessary a priori, and theoretically re- 

 quired. For it would not be intelligible that two identical cells coming 

 in contact should affect one another, stimulate one another out of the 

 state of repose and cause the complicated sexual processes to begin" 

 (p. 376). 



1 It is doubtless possible that all physical attraction between bodies is 

 due to their make-up of the two opposite kinds of electrons, so that 

 in this sense attraction may be due to diversity. But this is diversity 

 within the bodies: the two masses, as such, may have the same consti- 

 tution (including these diversities) and still attract. That is, the attrac- 

 tion of the two cells or individuals that mate does not a priori imply 

 any greater diversity between them than that between two leaden bullets 

 that gravitate toward one another. 



