Sexual Diversity m Protozoa 125 



usual ; there are definite methods for solving such a problem. 

 Using a large number of experiments, we found, rather un- 

 expectedly, that the two individuals that have mated are 

 more alike in all these respects, not less alike. If one mem- 

 ber of a pair dies, its mate is more likely to die than are 

 the mates of individuals that live. If one member multi- 

 plies feebly, its mate is likely to multiply feebly, too. If one 

 member multiplies vigorously, its mate is likely to multiply 

 vigorously also. All these relations are quite the opposite 

 of what would be expected if there are pronounced sex dif- 

 ferences between the members of pairs. But from an en- 

 tirely different point of view, they are what might well 

 be expected. The two mates have exchanged parts, so that 

 after conjugation each is, as it were, half composed of 

 material from the other (this is strictly true of the nuclei), 

 so that it is natural that they should become alike. This 

 brings into vie^ another result of mating; one of the first 

 importance. Mating causes the progeny of the two indi- 

 viduals that mate to be alike. This point we shall take up 

 later; here we shall look farther into the question of sex 

 differences. 



We have therefore no positive evidence of a difference of 

 sex between the two members of a pair of Paramecia. On 

 the other hand, perhaps it can hardly be maintained that 

 such differences are disproved. If all we hold is that some 

 difference in degree of "maleness" or "femaleness" is what 

 produces mating, it becomes extremely difficult to test 

 whether this holds or not. In this connection there are a 

 number of facts, some of them very curious, that require 

 consideration. 



In many lower organisms, germ cells are formed by the 

 division of a single cell, and so far as anyone can see, these 

 are all alike. Further, so far as anyone can determine, any 



