766 THEORY OF EVOLUTION. 



implicit in the beginning. In so doing, they follow the 

 method of analysis, endeavouring to explain the facts in 

 their lowest terms. But as the biologist's lowest term is 

 living matter, and as one aspect of this is, in favourable 

 conditions, known as thought, there is no reason to call the 

 evolutionist's analysis " materialistic " if anything oppro- 

 brious be meant by that adjective. The common denom- 

 inator of the biologist is as inexpressibly marvellous as the 

 philosopher's greatest common measure, if indeed, they are 

 not practically the same. 



The Two Great Problems. 



Our uncertainty in regard to the factors of evolution is so 

 great, that I cannot venture here to do more than indicate 

 (a.) what the great problems are, and (<.) the general drift 

 of the most important suggestions which have been made 

 towards their solution. 



The two great problems before the evolutionist are : 



(a.) What is the nature and origin of variations, i.e., of 

 those organic changes which make an organism 

 appreciably different from its parents or its species ? 



(&) What are the directive factors which may operate 

 upon given variations, determining their elimination 

 or their persistence, and helping towards the familiar 

 but puzzling result the existence of distinct and 

 relatively well-adapted species ? 



Secure answers to these two questions must be found in 

 reference to the present ; as our data accumulate it will be 

 more possible to argue back to the past. 



It may be convenient to speak of the factors which cause 

 variation as primary or originative, and of the factors which 

 operate upon or direct the course of variation as secondary 

 or directive. As far as practical results are concerned, the 

 two sets of factors are of equal importance. 



The Nature of Variations. 



We mean by variations those changes in organisms which 

 make them appreciably different from their parents or from 

 their species. 



