950 ECOLOGY 



not such reactions happen to be useful or harmful. If a new structure 

 arises, it must be through some chemical or physical influence within 

 the plant or in its environment. The adaptation theory, in implying 

 that a plant responds only in an advantageous direction and in advan- 

 tageous amount, endows the plant with a power of choice, and almost 

 imagines it to survey the situation and to determine upon a course of 

 action. It implies the possession of an inherent power to contravene the 

 ordinary laws of nature. It presupposes a vital mechanism that holds 

 adaptations in readiness for conditions that have not as yet occurred. 

 And yet man himself possesses no such power of adaptation ; he cannot 

 " by taking thought add one cubit to his stature," though he can (as a 

 plant cannot) study the laws of nature and place himself in such con- 

 ditions as to facilitate desired reactions. 



The theory of fortuitous variation. The preceding considerations 

 appear to show that protoplasm is not inherently adaptive. Disad- 

 vantageous structures (such as the food layers of galls or the enlarged 

 conductive tracts of parasitized plants) or indifferent structures (such 

 as cork wings and many hairs and spines) are quite as normal expres- 

 sions of protoplasmic behavior as are the more numerous advantageous 

 structures. The theory of fortuitous variation, which is based upon the 

 laws of chance, postulates that newly developing structures are of all 

 kinds : some advantageous, some disadvantageous, and some indifferent. 

 The supporters of this theory are aligned in two general schools; the 

 one school holds that new structures arise chiefly through the influence 

 of external factors, while the other holds that factors residing within the 

 plant itself are more important. Many investigators maintain that 

 external factors are more important in some instances and internal fac- 

 tors in others; this composite view, embracing opinions of the two 

 opposing schools, seems best able to explain the facts as they are now 

 understood. 



In the first place, variations are of frequent occurrence, though their 

 supposed rarity once was given as an argument against the theory of 

 evolution. Scarcely any species or any structure that has been studied 

 carefully has been found to be invariable, and in some cases the amount 

 of divergence from a supposed type is enormous, Not only do the indi- 

 viduals of a species as found in nature often differ from each other in 

 many particulars, but the same is true of individuals whose ancestry 

 is known to be identical. An excellent illustration of such variation is 

 seen in water cultures of various seedlings; while in such cultures maize 



