808 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 (2) what the great problems are, 
and (4) 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, ze. 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 przmary 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 
