EXTERNAL INFLUENCES AS CAUSES OF VARIATION 289 



When one kidney is removed from either man, rabbit, or 

 dog, the other becomes enlarged and the total amount of urea 

 excreted is unchanged, and this is true even if the removal is 

 made at maturity, after the parts have reached their probable 

 full development. 



This is allied to the fact that the full amount of urea is 

 excreted at once upon the removal of one kidney, proving that 

 its fellow is able to increase its labor even before hypertrophy, 

 and showing that under normal conditions the kidney is not fully 

 worked. There is first an increased flow of blood, then increased 

 excretion, then increased size. The same is true of the salivary 

 glands, the mammae of the female, the testes of the male, and 

 quite likely of paired organs generally. 



When the spleen is removed the " lymphatic glands of other 

 parts of the body become enlarged." 1 Another kind of compen- 

 sating development is the well-known increase of one faculty 

 when another is extinct, as the hearing and the touch of the 

 blind. In this instance, as in learning to write with the feet, 

 the part is not only developed and trained to its utmost, but 

 the undivided attention is fixed upon the matter in hand. 



The student who bestows careful study upon the relation of 

 the individual to his environment will arrive at three definite 

 conclusions : 



1. The impulse to development and its chief directive forces 

 are within. 



2. But the possibilities of that development, in kind as well 

 as in degree, lie very largely in surrounding conditions and 

 entirely external to the organism. 



3. These surrounding conditions, therefore, while not logically 

 causes of variation, since they cannot bring about a development 

 whose tendency does not already exist, are yet the limiting ele- 

 ments to all development, and many of these conditions are 

 chemical and physical forces able to exert strongly directive 

 influences upon growth capable of differentiation in more than 

 one direction. On this point see also the chapter on " Relative 

 Stability and Instability of Living Matter." 



1 Morgan, Regeneration, p. 118. 



