910 



THEORY OF EVOLUTION 



qualitative variations in chemical composition, such as the 

 appearance of a new pigment. 



Again, variations occur which may be called continuous ^ 

 being merely minute increments or diminutions of certain 

 parental or specific characters. These are related to one 

 another much in the same way as are the successive stages 

 in the continuous growth of an individual. 



But other variations occur which deserve to be called 

 discontinuous. For, without the appearance of transitional 

 stages, marked variations crop up, reaching with apparent 

 suddenness to what must be called new^ and may withal 

 exhibit a measure of perfectness. These discontinuous 

 variations may be large or small ; they are handed on as 

 Mendelian characters ; they are often called mutations. 



That both kinds of variations occur is a fact of life ; 

 the possibility of both is probably a primary quality of 

 organisms ; but we are only beginning to know the relative 

 frequency of the two kinds and their respective limits. 



Primary or originative factors. — What causes varia- 

 tion ? This is the fundamental question, but it is the least 

 answerable. 



It is, indeed, an axiom or a truism that changes in any 

 animate system are evoked by changes in the larger system 

 of which the organism forms a part. In other words, the 

 stimulus to organic change must always be ultimately 

 traceable to the environment ; but this is implied in our 

 conception of living matter, and does not help us to under- 

 stand the immediate conditions which lead to the change. 



In the absence of sufficiently precise data, we can do 

 little more than point out various possibilities : — 



{a) Changes due to Environment ( = Environmental 



Modifications) 



There is abundant proof that changes in surrounding 

 pressure, in the chemical composition of the medium, in 

 food-supply, in heat, light, etc., may be followed by changes 

 in the organism upon which these influences play. Changes 

 in the body of the organism follow changes in the environ- 

 ment. But (i) it is difficult to discriminate between 

 changes which may be spoken of as the direct results of 

 environmental influence, and those to which the organism 



