ADAPTATION 241 



And he should close with a brief exposition, or even definition, 

 of Irritability. 



Literature of Irritability. The literature of this important 

 subject is fully summarized in Volume III of Pfeffer's "Physiology," 

 which brings the subject down to so recent a date as hardly to leave 

 anything later to mention, and there is the usual good synopsis in 

 Jost. There is also a very suggestive treatment of the subject by 

 Verworn, though not without some flaws so far as plants are con- 

 cerned. Some notes on the latest papers are in the Botanical Gazette, 

 43, 1907, 218, 226. Pfeffer has given a very clear popular treat- 

 ment of the several subjects in Nature, 49, 1894, 586, while a very 

 clear scientific-popular treatment of the whole subject, in a series 

 of articles by F. Darwin, is in the New Phytologist, 5, 1906, and 6, 

 1907. 



12. ADAPTATION. 



Organisms exhibit not only individual adjustments to their 

 surroundings through irritable responses, but also a race adjust- 

 ment, or adaptation, both in structure and habit, to the general 

 conditions of the surroundings. This is fixed in heredity, and 

 is generally supposed to have been acquired by the natural selec- 

 tion of variations (or mutations). It has three great leading 

 phases: (a) development of structure (including form, size, 

 color, texture) ; (b) locomotion ; (c) protection. Some phases have 

 already been noted in this book, but as a whole its study is a 

 distinct department of investigation, properly comprehended 

 under Ecology. 



