CHAPTER VII. 



The Theory of Natural Selection. 



Thus far we have been considering the main 

 evidences of organic evolution considered as a fact. 

 We now enter a new field, namely, the evidences 

 which thus far have been brought to light touching 

 the causes of organic evolution considered as a pro- 

 cess. 



As was pointed out in the opening chapter, this is 

 obviously the methodical course to follow : we must 

 have some reasonable assurance that a fact is a fact 

 before we endeavour to explain it. Nevertheless, it 

 is not necessary that we should actually demonstrate 

 a fact to be a fact before we endeavour to explain it. 

 Even if we have but a reasonable presumption as to 

 its probability, we may find it well worth while to con- 

 sider its explanation; for by so doing we may obtain 

 additional evidence of the fact itself. And this because, 

 if it really is a fact, and if we hit upon the right 

 explanation of it, by proving the explanation probable, 

 we may thereby greatly increase our evidence of the fact. 

 In the very case before us, for example, the evidence 

 of evolution as a fact has from the first been largely 

 derived from testing Darwin's theory concerning its 

 method. It was this theoretical explanation of its 



