CHANGES DUE TO ENVIRONMENT. 809 
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 mew, 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 :— 
(2) 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 (1) 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 
