EXTERNAL INFLUENCES AS CAUSES OF VARIATION 285 



lakes. In view of these facts Bateson remarks, as quoted by 

 Vernon, 1 " It seems almost certain that these conditions are in 

 some way the cause of the variations." 



Biological literature is full of similar examples of the char- 

 acteristic effects of varying degrees of salinity. The limits of 

 space forbid the further pursuit of a subject which might be 

 extended almost indefinitely. It may be sufficient to say that 

 the specific influence of salinity upon certain characters is, 

 beyond a doubt, well established. 



SECTION X INFLUENCE OF USE AND DISUSE UPON 

 DEVELOPMENT 



No fact is better or more generally known than that use stim- 

 ulates and disuse dwarfs the development of many organs. To 

 say that development is in proportion to use would doubtless 

 be true, roughly speaking, of certain parts, as the muscular 

 system, secreting glands, etc. It certainly would not be true 

 of many others, as hair, feathers, bony skeleton, etc., which 

 develop independently of use, and some of which, as hair and 

 feathers, involve no activity in the sense in which the term is 

 here understood. 



This discussion should be limited to the distinctively active 

 parts, and the influence of exercise or the lack of exercise upon 

 their development. Of these parts it may fairly be said that 

 perfect development is dependent upon, if not proportional to, 

 the degree of their use, especially during the earlier stages of 

 development. 



The classic illustration from Darwin, showing the leg bones 

 of the tame duck and the wing bones of the wild duck to be 

 relatively heavier ; the arm of the artisan and the body of the 

 athlete ; the training of the track horse ; the marvelous coordi- 

 nation of complicated nervous impulse and muscular response 

 in the violinist and the pianist, all these and a multitude of 

 similar facts teach clearly that individual development of 

 usable parts depends very much upon their early and continuous 

 exercise. 



1 Vernon, Variation in Animals and Plants, p. 277. 



