FACTORS IN ORGANIC EVOLUTION 9II 



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 

 changes arising in or acquired by the body of an individual 

 organism as the result of surrounding change do as such 

 in any degree specifically afltect the reproductive cells. In 

 other words, we cannot at present say that " environmental 

 modifications " are transmissible. And if they are not, 

 their importance in evolution is only indirect. 



{b) Changes due to Function ( = Functional Modifications) 



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, which may be directly prompted by some change 

 in the external conditions of life. But important as these 

 functional changes 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 en- 

 vironmental 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 expression of some novelty in the protoplasmic 

 constitution of the germ cells. Then, hiding our ignor- 

 ance, we say that the variation is germinal, constitutional, 

 congenital, or blastogenic. It seems to lead to clearness if 

 we call these germinal changes and their results variations, 

 keeping the term modifications for thQse changes [{a) and (6)] 

 wrought upon the body as the result of environmental or 

 functional influences. 



But why should there be changes in the germ cells ? 

 Perhaps because living matter is very complex and un- 

 stable, and because it is of its very nature to difl'erentiate 

 and integrate and grow ; perhaps because the immediate 

 environment of the germ cells (blood, body cavity fluid, 

 sea-water, etc.) is complex and variable. Moreover, every 



