124 Life and Death, Heredity and Evolution 



filaments produce small side branches, which are separated 

 off by a partition as club-shaped germ cells or reproductive 

 bodies (see Figure 36). These reproductive bodies unite 

 with corresponding bodies from another individual plant. 

 By test it is found that these reproductive bodies are not 

 indifferent as to their mates ; with reproductive bodies from 

 certain other plants they will not unite. Thorough study 

 shows that there are two classes ; that those of one class will 

 not unite with those of the same class, but will mate with 

 those of the other class. The characteristic behavior of the 

 two sexes occurs, although the two show no external differ- 

 ences (save that in some cases the "female" plants grow 

 somewhat more luxuriantly than the "male" ones). 



Such cases show that the physiological distinction of two 

 sexes may exist without any structural difference. Is this so 

 in Paramecium? The case here turns out not to be so prob- 

 able a one for sex difference as it at first seemed. It is true 

 that sometimes after mating one individual reproduces freely, 

 while the other does not. But on the other hand, sometimes 

 both individuals reproduce freely ; sometimes both reproduce 

 feebly; sometimes neither reproduces at all. That is, there 

 are great differences among the individuals that have con- 

 jugated, in respect to these (and other) matters. The ques- 

 tion therefore reduces itself to this : Are the two that have 

 mated any more unlike in these respects than are any two 

 ordinary individuals that have not mated together? For 

 example, if one multiplies strongly, is its mate more or 

 less likely than the average individual to multiply weakly 

 or not at all? 



This led Lashley and myself to investigate the matter 

 (Jennings and Lashley, 1913). If we have a large number 

 of cases it becomes a mathematical problem to determine 

 whether the two members of a pair are more unlike than 



