CHAPTER II. 



DIMORPHISM AND DISCONTINUOUS VARIATION. 



Dimorphism in the earwig and in the crab How to distinguish be- 

 tween species and varieties, as instanced by dimorphism in 

 certain fishes, and in a marsh plant Polymorphism in plants 

 Series of Fibonacci Discontinuous variation in animals as regards 

 vertebrae, ribs, mammas, teeth, digits, and other characters 

 Homoeosis De Vries' Theory of Mutation Dimorphism may be 

 due to internal causes, or the result of divergent evolution 

 Physiological Selection Infertility between varieties. 



WE have seen that the distribution of variations about 

 their mean is in many cases quite symmetrical, whilst 

 in other cases in which it is asymmetrical it still takes 

 place according to some orderly arrangement, for which 

 a mathematical expression can be found. There is still 

 a third group of cases, however, in which the curve of 

 distribution is, as a rule, very asymmetrical, but for 

 which, even if symmetrical, no single general mathe- 

 matical expression is obtainable. A study of such 

 curves has taught us that the cause is frequently refer- 

 able to the fact that our material is not homogeneous; 

 that, in fact, we have a mixture of varying numbers of 

 two or more groups of individuals differing in mean 

 size and range of variation. For instance, Bateson* 

 measured the length of the forceps of 583 specimens of 

 the common earwig, Forficula auricularia, which had 



* " Materials for the Study of Variation," p. 41. 



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