8o8 THEORY OF EVOLUTION. 



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 

 opprobrious be meant by that adjective. The common 

 denominator of the biologist is as inexpressibly marvellous 

 as the philosopher's greatest common measure if, indeed, 

 the two are not practically the same. 



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 (b) the general drift of the most important suggestions 

 which have been made towards their solution. 



The two great problems before the evolutionist are : 



(1) 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 ? 



(2) What are the directive factors which may operate 



upon given variations, determining their elimina- 

 tion 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. 



Nature of variations. We mean by variations those 

 changes in organisms which make them appreciably 

 different from their parents or from their species. 



The term of course includes not only material differ- 

 ences, but also those whose only demonstrable expression 

 is psychical. Thus an increase in maternal affection is as 

 important and real a variation as the sharpening of a 

 canine tooth. 



It may also be useful to distinguish variations in size, 

 symmetry, number of appendages, and so on, from more 

 qualitative variations in chemical composition, such as the 



