ADAPTATION 949 



Many phenomena are out of harmony with the theory of adaptive 

 response. Perhaps the best illustration of this is afforded by the re- 

 actions of plants that are attacked by parasites, where the conductive 

 tissues are stimulated to increased development ; obviously this is a most 

 disadvantageous reaction, and the same may be said for the increased 

 development of such tissues in xerophytes. In other cases galls are 

 formed which accumulate quantities of food that may be very useful to 

 the parasite, but certainly are harmful to the host. Nor does the adap- 

 tation theory account for the reactions of bast fibers, which develop best 

 under xerophytic conditions, but are of little if any value in checking 

 transpiration; they seem not to be stimulated by mechanical agents, 

 though their chief value is mechanical. Conductive tissues and the 

 velamen of orchid roots are most useful only when they are dead, and 

 it is not easy to see how adaptation can arise through dying, nor how 

 function here determines form, as adaptationists suppose. 



Even instances commonly cited by teleologists as demonstrating adap- 

 tation often prove fallacious when analyzed. For example, root hairs, 

 though lacking in a water medium where they are said not to be needed, 

 are present in the same species in saturated soil, where their need is no 

 more obvious ; in dry soil or in concentrated media, where they are needed 

 most of all, they fail to develop. Both in water and in dry soil, root hairs 

 are absent, not because they are not needed, but because the conditions 

 necessary for their development are absent. Similarly, palisade tissue, 

 which has been thought to develop where the plastids need protection 

 from intense sunlight, now is believed to have but httle significance in 

 this respect, since the palisade plastids of xerophytes have relatively 

 slight motility. Nor are useless structures lost because they are useless, 

 but only because the factors which induce them fail to operate. There 

 are many obviously useless structures in plants, as the stamens of Balano- 

 phora (p. 917) and the cork wings of the sweet gum and of various elms; 

 many other structures seem inconsequential, for example, many hairs 

 (including stinging hairs) and spines. If the adaptation hypothesis 

 is inadequate to explain the cases above noted, it is unnecessary else- 

 where, since there are theories of causation which account equally well 

 for the origin and survival of useless or moderately harmful structures 

 and for the origin and survival of structures that are advantageous. 



The theory of adaptive response is contrary to the current physical 

 and chemical conceptions of the behavior of matter. Plants are made 

 up of substances that react definitely to definite conditions, whether or 



