CHAPTER XI 



EVOLUTION (reproduction AND INHERITANCE) 



S I. — Sexual Selection 



In the last chapter we saw how vital for the theory of 

 evolution were all facts bearing on the fertility in se and 

 inter se of the members of a local race. If all members 

 of a race are equally fertile and they continue to pair 

 at random, then a permanent differentiation into mutually 

 infertile sections, — an origin of species, — seems impossible. 

 We have accordingly to consider these two questions : (i) 

 Do members of a local race pair at random? (2) Are 

 members of a local race equally fertile in se and inter se. 

 If we can obtain quantitative answers to (i) or (2), or both 

 of these, then we have safe ground to advance upon in 

 considering the origin of species. The danger, however, 

 in discussing one factor of neglecting another must always 

 be borne in mind, and without attention to some of the 

 principles of regression and inheritance, — particularly that 

 of the establishment of stocks — dealt with later in this 

 chapter, we are likely to slip into obscure statements. 



Let the individuals of a local race be classified by 

 sample, say, of 1000, according to some character or 

 organ, into the frequency polygon indicated in the figure, 

 and let us suppose all individuals in se equally fertile. In 

 the diagram Aa represents on some scale the frequency 

 per thousand of an organ or character determined by a 

 on the horizontal scale, c is the modal value, and 

 Cc will represent on the vertical scale the frequency of 

 modal individuals. Now, biologists often write and some- 



