WHAT EVOLUTION IS ^y 



change in habit is followed by a 

 change in structure. Thus the condi- 

 tion of the animal is modified and 

 evolution is the result. 



Such an application of the La- 

 marckian principle, as is involved in 

 the last two examples, requires what 

 may be called the indirect influence of 

 the environment in contrast wath the 

 direct influence as seen in most plants, 

 but in both direct and indirect influ- 

 ences, the environment and its changes 

 are the paramount elements. 



In addition to the general principle 

 that has just been illustrated, La- 

 marck also called attention to certain 

 subordinate principles that he believed 

 to be significant in evolution. First 

 of these was the principle of use and 

 disuse. Organs that are exercised 

 tend to increase in size, and organs 

 that are not exercised tend to shrink. 

 This is so obvious a matter in every- 



