78 WHAT EVOLUTION IS 



day life that it needs no special illus- 

 tration and no one denies it. 



Another principle that Lamarck 

 advanced was the principle of effort, 

 that in order to accomplish an end an 

 animal must make an effort, must 

 exert itself. If it did not so do its 

 effective powers would diminish. This 

 is an element of a psychological na- 

 ture; it has a certain vague and in- 

 tangible side not involved in the 

 principle of use and disuse, for in- 

 stance. It nevertheless plays no un- 

 important part in Lamarck's general 

 hypothesis. 



The scheme advanced by Lamarck, 

 and briefly outlined in the preceding 

 paragraphs, carries with it the im- 

 pression of great naturalness. Every- 

 one knows that activity or lack of 

 activity modifies an organ and, grant- 

 ing that the changes thus produced 

 are handed on generation after gen- 



