48 



GENETICS IN RELATION TO AGRICULTURE 



Multimodal Curves. Thus far we have considered only homogeneous 

 populations, which, when examined statistically, exhibit a certain degree 

 of approximation to the normal curve of variation. Populations fre- 

 quently occur, however, both in nature and among domesticated animals 

 and plants, which are found to be heterogeneous for certain characters 

 at least when subjected to statistical analysis. Graphically shown the 

 data for such a character produces a polygon with more than one mode. 

 In general such data indicate either the permanent influence of different 

 causes affecting only certain individuals or of the same cause acting 

 differently upon a portion of the population. Conditions of bimodal 

 curves are more or less familiar to all. Sexual dimorphism and certain 



FIG. 20. Bimodal polygon plotted from data on the earwig. Mean types ( X %) 

 indicated above corresponding modes. Numbers below the base line indicate length of 

 pincers in mm. (From Bateson and Johannsen.) 



differences in development which are contingent upon sex, such as height 

 of comb in fowls, obviously would result in a "notched" graph if the 

 characters were measured and the data plotted. The classic example of 

 dimorphism producing a bimodal curve is found in the length of the 

 pincers of the common earwig (Forficula auricularia) as reported by 

 Bateson. Fig. 20 illustrates the two mean types, each sketch being 

 placed directly above its corresponding modal class in the graph. Other 

 conditions commonly causing mixed populations such as would result in 

 bi- or multimodal curves are the following : 



1. Coexistence of groups of different ages; common in birds at certain 

 times of the year. 



2. Overlapping of geographical races of the same species birds, 

 mammals. 



3. Coexistence of different races of the same species, for example, 

 many grasses in the wild state and various cultivated grains contain 



