8o8 THEOR Y OF E VOL UTION. 



appearance of a new pigment, but this distinction is only a 

 matter of convenience, as it is only a matter of degree. 



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. 



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 (see 

 Bateson's " Materials for the Study of Variation, 1894," and 

 De Vries's "Species and Varieties," 1905). 



Primary or originative factors. What causes variation ? 

 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 



Modified tions) . 



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 



