ORIGIN OF VARIATIONS. 147 



every respect except as regards its nervous system. 

 This, by necessity of its nature, bears the record of 

 the changes, and is changed by them. As the ner- 

 vous system is changed, so the rest of the organism, 

 which is dependent upon it, must eventually be 

 effected by that change. And such change is going 

 on constantly. The importance of the diversity of 

 influence upon the nervous element of living matter 

 is shown by the relatively low state of development 

 among plants, where all periodicity and alternation of 

 condition is reduced to the minimum of day and night, 

 summer and winter, and different soils and climates. 

 But while theoretically organic evolution may pro- 

 ceed indefinitely without a change of environment, 

 yet I am disposed to believe it would be like one 

 of those infinite curves which forever approaches a 

 straight line. There would come a time when the 

 evolution would be inappreciable. Practically, how- 

 ever, we know that all organisms, in the course of 

 their evolution, have been subjected to change in 

 their environment. There have been geologic and 

 climatic changes, and changes caused for each one 

 by other organisms, affecting mutually the conditions 

 of existence. All the facts of natural history tend 

 to prove that the most highly developed organisms 

 have, in their evolution, suffered the most numerous 

 and extensive changes of environment. 



