INTRODUCTION 



Very often structures originally devoted to purely physiological 

 purposes become in particular speck's adapted to ecological ends, and 

 accordingly undergo an appropriate change of form. This statement 

 applies more especially to those ecological adaptations which control 

 the relations between plants and animals. It would he a serious 

 misconception to regard such adaptations as ecological in essence and 

 origin. Take the case of the latex-tubes in the involucral bracts of 

 Lactuca; these structures stand in open communication with short 

 hairs, which exude an abundance of protective latex if they are injured 

 (e.g. by insects). Here the laticiferous system has undoubtedly under- 

 gone a secondary specialisation which has nothing to do with its 

 primary physiological function of conducting plastic materials. The 

 same point is illustrated by the case of oxalic acid. This substance 

 is a very common bye-product of vegetable metabolism in general and 

 of protein-synthesis in particular. The free acid has a very poisonous 

 action upon protoplasm, and is therefore rendered harmless by conver- 

 sion into the almost insoluble crystalline calcium oxalate. Now, the 

 crystals in question are often shaped like minute needles or darts, and 

 are thus well adapted to provide " mechanical " protection against the 

 attacks of insects and snails. Many other metabolic bye-products 

 may thus secondarily assume an ecological importance. Physiological 

 anatomy takes every adaptation into account, whether the latter be 

 physiological or ecological in nature. But whenever the function of a 

 particular structure has to be determined, full consideration should first 

 he given to all the possibilities of physiological adaptation. Ecological 

 adaptation should not be thought of unless it is found impracticable 

 to bring the structure into relation with any genuinely physiological 

 function, that is to say, with a process which constitutes an integral 

 part of the internal vital activity of the plant. It is perhaps advisable 

 at this stage once more to insist upon the fact that the two forms of 

 adaptation cannot always be sharply separated from one another. 1 



The recognition of the fact that plants exhibit adaptation as regards 

 their internal structure, is quite independent of the various sugges- 

 tions and hypotheses which have been put forward in order to 

 explain the origin of this adaptation. With respect to the latter 

 point, one may follow Darwin in attributing the "purposeful" charac- 

 ter of adaptation to the action of natural selection; or one may share 

 the belief of Lamarck and Niigeli in the existence of a direct 

 accommodation which takes place without the intervention of natural 

 selection; or, finally, one may hold that the biologist is altogether 

 unable to fiame a satisfactory explanation of the purposeful aspect 

 of nature, and thus relegate the question to the field of meta- 

 physics. 



