Assort ative Mating 



191 



of each race mate only with members of their own race ; the 

 large individuals only with other large ones ; the small in- 

 dividuals only with other small ones (Figure 52). There is 

 no crossing between the two races when they thus differ 

 considerably in size. This is a highly inconvenient fact 

 when one wishes to study heredity in such crosses ! 



Figure 52. Conjugants and non-con jugants from a culture composed 

 of a mixture of two races (k and i) of different size, of Paramecium 

 aurelia. The members of the race k are larger than those of the race 

 i; only members of the same race mate together. After Jennings, 1911. 



The same thing is observed when a culture of Paramecium 

 contains individuals of many different sizes (whether of the 

 same race, or of different races). There is a marked as- 

 sortative mating; that is, individuals of the same size tend 

 to mate, while individuals of diverse size do not readily mate 

 (Figure 53). There are some exceptions; we find a few in- 

 stances in which a small individual has mated with a larger 

 one, but such cases are rare. In general, we find that all 

 the pairs can be arranged in a rather regular series such as 

 Figure 53 shows, the two members being of about the same 

 size. 



This assortative mating takes place with respect to other 



