EXTERNAL INFLUENCES AS CAUSES OF VARIATION 287 



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FIG. 30 illustrates the ability of a highly developed part to perform an extremely 

 unusual service. The above letter was written with the feet instead of with 

 the hands 



It is as futile to attempt to decide whether internal or exter- 

 nal circumstances are more helpful in development as it is to 

 attempt to say whether food or heat is more essential to life. 

 Both are absolutely necessary, and it suffices present purposes 

 if the student understands that external conditions, even to the 

 matter of exercise, are fundamentally essential to full develop- 

 ment, and that the "limiting element" may be found external 

 to the organism as well as internal. We shall not be able to 

 assert how much is due to each separately, nor can we determine 

 the coefficient to be assigned to use and to disuse, but we are 

 safe in resting assured that for many parts development is fairly 

 proportional to exercise, at least within the limits of inheritance. 



