CHAPTER XII. 

 ADAPTIVE VAKIATIONS. 



Adaptability a fundamental property of protoplasm Instances of 

 adaptive variation in plants Acclimatisation of Protozoa to high 

 temperature, to poisons, to mechanical stimuli, to saline solutions 

 Acclimatisation of fresh-water Mollusca to salt water, and of 

 various marine animals to fresh water Acclimatisation of Mam- 

 mals to vegetable poisons, and to toxins Sum total of somatic 

 variations always in direction of adaptation Somatic varia- 

 tions of importance in evolution, but they can effect little without 

 Natural .Selection Germinal Selection. 



THE question of the definiteness or indefiniteness of 

 variations has been frequently and hotly debated, but 

 there has been a singular absence of exact definitions 

 of the views actually held by the supporters of the rival 

 theories. Had such definitions been forthcoming, I 

 doubt if any fundamental differences of opinion would 

 have been found to exist at all. Take, for instance, 

 Darwin's definition of definite variations, viz. : " The 

 effects of (conditions of life). . . may be considered 

 as definite when all or nearly all the offspring of 

 individuals, exposed to certain conditions during 

 several generations, are modified in the same man- 

 ner." * There is surely nothing in this definition 

 which would not be generally admitted. As has 

 been shown at some length in several of the preced- 

 ing chapters, change in one or many of the conditions 

 * " Origin of Species," p. 6. 



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