CHAPTER VII 



SEXUAL DIMORPHISM 



Meaning of the term Fatal to Wallaceism Sexual Selection 

 The law of battle Female preference Mutual Selection 

 Finn's experiments Objections to the theory of Sexual 

 Selection Wallace's explanation of sexual dimorphism 

 stated and shown to be unsatisfactory The explanation of 

 Thomson and Geddes shown to be inadequate Stolzmann's 

 theory stated and criticised Neo-Lamarckian explanation 

 of sexual dimorphism stated and criticised Some features of 

 sexual dimorphism Dissimilarity of the sexes probably arises 

 as a sudden mutation The four kinds of mutations Sexual 

 dimorphism having shown itself, Natural Selection determines 

 whether or not the organisms which display it shall survive. 



IN some species the sexes are so similar in 

 appearance that it is not possible to tell 

 by mere outward inspection to which 

 sex a given individual belongs. 

 In other species the sexes differ so widely in 

 external appearance that it is difficult to believe 

 that the male and the female belong to the same 

 species. Between these two extremes are a 

 great number of species in which the sexes are 

 more or less dissimilar. Those species in which 

 the sexes differ in appearance are said to be 

 sexually dimorphic. The phenomena of sexual 

 dimorphism are fatal to that form of Neo- 

 Darwinism which sees in natural selection an 



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