422 H, S. JENNINGS AND K. S. LASHLEY 



COMPARATIVE RATE OF REPRODUCTION IN THE TWO MEMBERS 



OF PAIRS 



Method of analysis 



The second ground for holding the two members of pairs to be 

 sexually differentiated is the fact that often one of them is vigor- 

 ous and reproduces freely, while the other is weak and reproduces 

 slowly. 



Here arises a question parallel to that which we have dealt 

 with in the preceding section. Differences in vigor and rate of 

 fission are common among the exconjugants, and are of course 

 found likewise between individuals that are not members of pairs. 

 In order to be considered evidence for sexual differentiation, 

 the differences between the members of pairs must be more marked 

 than the average differences between individuals taken at random. 

 Otherwise they can only be considered instances of the variability 

 in this respect among the members of a population. 



It is true that organisms might be sexually differentiated in 

 other respects, and still the members of pairs might show in this 

 particular respect differences not greater than the average differ- 

 ences between members of the population. But in the present 

 case it is this difference alone that is the ground for the supposed 

 sexual differentiation (particularly since we have shown above 

 that the relative mortality gives no such ground). Unless there- 

 fore the difference is more marked than the average differences 

 between individuals taken at random, no ground exists for holding 

 that the two members of the pairs are sexually differentiatied. 

 The mere fact that there is variation in rate of fission, distributed 

 without relation to pairing, would certainly be no indication of 

 sexual differentiation. 



We may therefore proceed to determine whether the differences 

 in this respect between the two members of pairs are greater 

 than the average difference between individuals. 



One special point will be of interest. If the two members of 

 pairs are sexually differentiated, then the one that reproduces 

 most rapidly will have to be taken as representing the female, 

 and the other as representing the male. It will then be possible 



