41 8 THE GRAMMAR OF SCIENCE 



{b) Inheritance of characters. 



{c) Sexual selection, leading to differential fertility. 



By sexual selection I would understand something 

 rather more than Darwin includes by that term, namely, 

 all differential mating due to taste, habit, or circumstance, 

 which prevents a form of life from freely intercrossing. 

 If this goes on for a sufficient period, during which 

 differentiation of type is in progress, the principle of 

 correlation may account for a sufficient differentiation in 

 reproductive organs or functions to render intercrossing 

 physiologically or mechanically difficult, distasteful, or 

 even impossible, and accordingly give rise to the relative 

 or absolute mutual sterility of the differentiated types, i.e. 

 to the origin of species. 



This is not " physiological selection " or the seizing of 

 chance reproductive variations, which are mutually sterile, 

 but it seems to me the simple and natural development 

 of Darwin's own statements. In order therefore to com- 

 plete our discussion we require to. see how we can 

 quantitatively deal with these unsolved problems. Does 

 like really mate with like, and if so, how is the intensity 

 of this mating to be measured ? Is fertility actually 

 correlated with physical characters likely to form the 

 material for selection ? Can we trace any differentiation 

 of mutual fertility with differentiation in type ? 



Lastly, what are the laws according to which a differen- 

 tiation in type will be transferred to the offspring ? Are 

 the laws of inheritance such that they can provide for a 

 permanent change of type such as the Darwinian theory 

 requires ? These will be the topics of our next chapter. 



SUMMARY 



In this chapter we have learnt to look upon an organism or form of life 

 as quantitatively described by the numerical values of the types and varia- 

 bilities of its several organs, and by their interrelationships as expressed by the 

 coefficients of correlation. To test whether selection is taking place or not 

 we must test whether one or more of these constants vary between one adult 

 generation and a second. Only in this way can we separate out the effects 

 of growth and death and distinguish between periodic and secular selection. 



