768 THEORY OF EVOLUTION. 



{a) Variations dtie to Chaiige in the Envir07iment. 

 There is abundant proof that changes in surrounding 

 pressure, in the chemical composition of the medium, in food 

 supply, in heat, light, &c., 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 

 variations which may be spoken of as the direct results of 

 environmental influence, and those to which the organism 

 was already definitely predisposed, and to which the 

 environmental change supplied only the stimulus. (2) We 

 have not at present sufficient data to enable us to state that 

 variations arising in or acquired by the body of an individual 

 organism as the result of surrounding change, do as such in 

 any degree specifically affect the reproductive cells. In 

 other words, we cannot at present say that " environmental 

 variations " are transmissible. And if they are not, their 

 importance in evolution is only indirect. 



(b) Variations due to Change in Function. 

 It is an undoubted fact that the bodily structure of an 

 animal may be changed by the increased use of certain 

 parts, or the disuse of others, in short, by some change of 

 function. This change of or in function may be directly 

 prompted by some change in the external conditions of life, 

 or it may be the expression of a deeper variation in the 

 animal's material constitution or mental character. But 

 important as these functional variations and their results 

 are to the individual, we are uncertain as to their importance 

 for the race, for we do not know to what extent (if any) the 

 results are transmissible. 



(c) Variations due to Changes in the Germ Cells. 

 In many cases of variation, particularly those which appear 

 in early life, it is not possible to suggest any environmental 

 or functional condition which may be regarded as the 

 stimulus or the cause. We are led in such cases to believe 

 that the variation in bodily structure or habit is the expres- 

 sion of some novelty in the protoplasmic constitution of the 

 germ-cells. Then, hiding our ignorance, we say that the varia- 

 tion is germinal, constitutional, congenital, or blastogenic. 



