130 Life and Death, Heredity and Evolution 



lasts and is inherited, so that the two sexes are distinguish- 

 able even before mating occurs. There is no evidence that 

 such inheritance occurs. This notion of the origin of sex 

 of course implies that there is no underlying general 

 physiological difference that makes sex ; sex would be a mere 

 matter of the external differences. If we believe that there 

 is such a general physiological difference, then we could 

 hardly hold that these differences produced during mating 

 are sex diversities, unless we believe that they are guided 

 by and coincide with the previously existing physiological 

 differences. It appears probable that the diversities be- 

 tween the two mating individuals in Chilodon are mere 

 transient alterations, with no lasting consequences. 



So far we have dealt with the differences, or lack of 

 differences, between the two individuals that mate. But the 

 sex diversities reach to the more intimate structures ; to the 

 nuclei. In Paramecium (Figure 40), each individual pro- 

 duces a larger half nucleus that is passive, like the female; 

 a smaller half nucleus, which is active, seeking out and 

 uniting with the passive half nucleus of the other mate, 

 and thus playing the part of the male. What is the differ- 

 ence between these two half nuclei produced by a single 

 individual, the "male" and "female" half nuclei? Here we 

 have the problem of the nature of sex diversity brought to 

 a point; if we could answer this question for these two 

 half nuclei, we should know the nature of sex. 



The differences that we observe are the following (see 

 Figure 40 and Figure 49, H). The "male" half nucelus is 

 a little smaller; it is usually nearer to the surface of union 

 of the two mates ; it separates from the "female" half nucleus 

 of the same individual ; it moves toward and across the sur- 

 face of union ; it refuses to unite with the other "male" half 

 nucleus which it meets and passes on its way; but it does 



