138 Life and Death, Heredity and Evolution 



other infusoria, both mates of a pair play the part of both 

 "male" and "female," so that diversity of sex cannot be 

 given as ground for the union of the two individuals. Fur- 

 ther, there is no clear evidence that the two uniting half 

 nuclei are diverse in any generally characteristic way ; indeed 

 on the whole the facts perhaps agree best with the view that 

 there is no underlying sexual difference in the uniting half 

 nuclei; that indeed in many of these lower organisms there 

 is no such thing as sex diversity. 



And this too appears the natural conclusion with rela- 

 tion to that ultimate act of mating, the union of the chromo- 

 somes. In many organisms this does not occur at once after 

 the mating of the individuals or the germ cells (though in 

 some, particularly in the flies, it does). Usually after the 

 germ cells unite, the chromosomes remain without mating for 

 many cell generations, through which the fertilized egg de- 

 velops into the body of a new individual. It is only when 

 the germ cells of this new individual are ripening for their 

 next mating that the chromosomes within each cell mate. 

 At this time they form a set of structures that in some or- 

 ganisms show many diversities of size and form (Figure 29). 

 But the mating is not between the most diverse individuals. 

 On the contrary, we find as a rule that each chromosome has 

 found as its mate one precisely like itself, so that the 

 group was really composed of sets of two individuals of the 

 same size and form, and it is these two that mate. Excep- 

 tionally two chromosomes of diverse size or form may mate, 

 but this is only when a similar mate is lacking, and there is 

 evidence that such mating between chromosomes of diverse 

 size or form is not so intimate as that between chromosomes 

 of similar structure. All the indications are that in this ul- 

 timate act of mating the union is Between structures that are 



