186 PLANT PHYSIOLOCxY AND ECOLOGY 



obtain the proof of this and of the manner in which it occurs is 

 the task of experimental evolution. 



Forms which grow for a long time in the same habitat seem to 

 fix more and more those functions and structures which are the 

 responses to it. They may be said to acquire habits which become 

 more fixed the longer the causes act. Of this tendency to fix 

 characters, there is as yet no definite and complete experimental 

 proof. Sufficient evidence to warrant its use as a working hypoth- 

 esis is found in the behavior of plants in nature and in experiment. 

 Some forms or species show little or no change when grown in 

 greatly changed conditions, while others respond to slight differ- 

 ences in habitat. The former are said to be stable, the latter are 

 termed plastic. The great majority of plants are neither extremely 

 stable nor extremely plastic. Some are more stable, others less 

 so. Some clue to this may be obtained from observations made 

 in the field, but the final test of a plant's stability must be made 

 by growing it under changed conditions. 



The amount of stability shown by a plant determines to what 

 degree evolution or change is possible for it. In other words, the 

 form and structure inherited by the plant from its ancestors not 

 only constitutes the material acted upon by evolution, but it also 

 determines how far the change may go. This is the historical or 

 ancestral factor in evolution. The change which the plant under- 

 goes is the result of a change in the habitat, which is the physical 

 factor in evolution. The amount of change or modification in 

 the plant depends upon the intensity of the change in habitat, i.e., 

 upon the stimulus as well as upon the degree of stability. Evolu- 

 tion is thus seen to be the result of two opposite tendencies, sta- 

 bility and change. When the former predominates, evolution is 

 either very slow or very slight. In extreme cases it may be 

 impossible. When the second tendency is stronger, evolution is 

 rapid, and a new form appears in response to each change in habitat. 

 In consequence, while we shall see that evolution may be brought 

 about in several ways, every instance of it is at bottom a question 

 of the relation between the antagonistic tendencies, stability and 

 change. 



202. Constant and inconstant forms. New forms that arise by 

 evolution produce offspring similar to themselves as long as the 

 offspring remain in the home. If the offspring invade new and 

 different habitats, they may retain the characters of the parent 



