Nature of Sexual Diversity 133 



that the half nucleus nearest the surface of separation of 

 the two mates is always the one that moves over into the 

 other mate; on the contrary, sometimes one of the nuclei 

 farther from this surface is the one that thus becomes the 

 migratory half nucleus. 1 Further, the actual smaller size of 

 the migratory half nucleus, observed in Didinium by Prandtl 

 (1906), and in Paramecium by Calkins and Cull (1907), is 

 held to imply an intrinsic difference between the two. Ac- 

 cording to this way of looking at the matter, one of the half 

 nuclei is male, the other female, in virtue of their diverse 

 chemical make-up. Such a view goes with the general theory 

 that sex in a matter of fundamental physiological diversity, 

 not a mere name for certain external peculiarities. 



Keeping these two contrasted opinions in mind, certain 

 facts as to what occurs in these organisms are of much 

 interest. The theory of the need for periodic unions ; the 

 theory of rejuvenescence through such unions, is based, 

 as we have seen, on the notion that the two nuclei have 

 been developing in opposite directions, one toward "male- 

 ness," the other toward "femaleness," till an unbalanced con- 

 dition is reached; union is then required to restore the bal- 

 ance. 



The situation in Paramecium and other infusoria seems 

 almost to reduce this idea to an absurdity as an explanation 

 of a necessity for periodic mating. For here the two nuclei 

 that are assumed to have been developing in opposite direc- 

 tions, the "male" and "female'* half nuclei, have in fact 

 been developing continuously together, in the same micro- 

 nucleus of the same individual! It is only immediately be- 

 fore the union of "male"' and "female" parts in the mating 

 that the united "male" and "female" parts have separated. 



1 Calkins and Cull, 1907, p. 393; Prandtl, 1906, p. 246; Collin, 1909, 

 p. 359, etc. 



