770 THEORY OF EVOLUTION. 



method of analysis, endeavouring to explain the facts in 

 their lowest terras. 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, 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 (i) the general drift of the most important suggestions 

 which have been made towards their solution. 



The two great problems before the evolutionist are : — 



(i) AVhat 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 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. 



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 



