194 



PLANT PHYSIOLOGY AND ECOLOGY 



are species or not depends entirely upon the meaning given to 

 this term. For the present it is clearer to use the term ecad 

 for all new forms arising by adaptation. 



Adaptation is closely related to variation. Both are probably 

 equally due to response to the habitat, but they differ in amount 

 and direction of responce. The latter is definite in the one case, 

 indefinite in the other. Origin by adaptation, like mutation, 



Fig. 68. A floating form of a crowfoot, Ranunculus sceleratus, produced by 



artificial adaptation. 



takes place quickly, usually in a single generation. Variation 

 must work slowly through many years, in consequence of the 

 heaping up of minute differences. In this process natural selec- 

 tion is the essential factor; in adaptation while present it is much 

 less evident. Moreover, while origin Ijy variation is still a doul)tful 

 factor in evolution, it is probable that origin by adaptation is the 

 most frequent method found among plants. 



