THE THEORY OF DESCENT 253 



But let us study the second group of facts which 

 support the theory of descent. It is a group of evidences 

 supplied by biology itself that we meet here, there being 

 indeed some features in biology which can be said to gain 

 some light, some sort of elucidation, if the theory of descent 

 is accepted. Of course, these facts can only be such as 

 relate to specific diversities, and indeed are facts of 

 systematics ; in other words, there exists something in 

 the very nature of the system of organisms that renders 

 transformism probable. The system of animals and plants 

 is based upon a principle which might be called the 

 principle of similarities and diversities by gradation ; its 

 categories are not uniform but different in degree and 

 importance, and there are different kinds of such differences. 

 No doubt, some light would be shed upon this character 

 of the system, if we were allowed to assume that the relation 

 between similarities and diversities, which is gradual, 

 corresponded to a blood-relationship, which is gradual also. 



THE COVERT PRESUMPTION OF ALL THEORIES OF DESCENT 



We have used very neutral and somewhat figurative 

 words, in order to show what might be called the logical 

 value of the theory of descent, in order to signify its 

 value with respect to so-called " explanation." We have 

 spoken of the " light " or the " elucidation " which it brings, 

 of the " peculiarity of aspect '' which is destroyed by it. 

 We have used this terminology intentionally, for it is very 

 important to understand that a specific though hidden 

 addition is made almost unconsciously to the mere state- 

 ment of the hypothesis of descent as such, whenever this 



