192 Life and Death, Heredity and Evolution 



characters also. It has been demonstrated by careful study 

 that it occurs in Paramecium with reference to rate of fis- 

 sion; the two animals which mate have on the whole similar 

 rates of fission. It appears clear that the mating is between 

 animals of similar physiological characteristics. 



Such assortative mating has been shown to occur with 

 respect to size in certain other infusoria, noticeably, 

 Blepharisma (Watters, 1912) and Anoplophrya (Collin, 

 1909). Assortative mating is common, too, in higher ani- 

 mals and man. It is well known how strong a reluctance 



Figure 53. Pairs from a single race of Paramecium aurelia, illus- 

 trating assortative mating; individuals alike in size mate together. The 

 lines A-C and B-B X are parallel. After Jennings, 1911. 



there is in man for strikingly different races to mate; a re- 

 luctance that is reenforced by all sorts of social and legal 

 regulations (which regulations, of course, are manifestations 

 of the biological characteristics of the organisms). In the 

 case of blacks and whites among human beings, for example, 

 an observer from Mars, examining in the United States the 

 two stocks objectively, would find that in the overwhelming 

 majority of cases white is mated with white, black with 

 black, although some exceptions occur. 



In higher animals this assortative mating manifests itself 

 in details also, as it does in Paramecium; study shows, for 

 example, that on the whole tall persons tend to mate with 

 tall, short with short. Although detailed studies have been 



