2 HEREDITY [ch. 



anything mysterious or unexplained ; tbey have a 

 deep human and practical importance, for not only 

 do they affect us all individually, but upon their 

 solution depends, to an extent as yet only dimly 

 realised, the answer to some of our most pressing 

 social questions ; and finally they lie at the very root 

 of all theories of organic evolution, so that they form " 

 as it were the basis of philosophical biology. The 

 relation of the study of Heredity and Variation to 

 sociology must be left to a later chapter, but before 

 proceeding further we must shortly consider its bearing 

 on theories of evolution. 



The fact of organic evolution is admitted by all 

 schools of biology, but about the causes of the pro- 

 cess and the manner in which it takes place there is 

 still wide diversity of opinion. To some of the more 

 important theories of evolution it will be necessary 

 to refer again later, but however great may be the 

 difference of opinion with regard to them, all biologists 

 agree that evolution depends ultimately on Variation 

 and Heredity. Darwin called his great book The 

 Origin of Species because the unit step, so to speak, 

 on the scale of evolution is the transition from one 

 species to another. But if a species A is to give rise 

 to a species B, in the first place some individuals of A 

 must vary in the direction of B, and then the variation 

 must be inherited, for otherwise no permanent change 

 can take place. The differences with regard to the cause 



