524 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



than is implied in the term 'secretion of the mantle.' 

 The original theoretic protoconch may have been so, 

 but as soon as it came into being, its development was 

 governed by the physical forces impinging upon it 

 from all sides, and through it influencing the growth 

 and structure of the soft parts beneath. The gastropod 

 shell is the result of the action and reaction between 

 the physical forces of the environment, and the evolu- 

 tionary tendencies of the organic individual.* In the 

 pelecypod we have the mechanical stresses and reac- 

 tions of one valve upon the other, added to the cate- 

 gory of influences. ' To some extent it is doubtless as 

 true that the animal is moulded by its shell, as it is 

 that the shell is shaped by the soft parts of the ani- 

 mal.'" 



Dr. Dall in an able paper on the "Dynamic In- 

 fluences in Evolution," read before the Biological So- 

 ciety of Washington, March 8, 1890, writes thus forci- 

 bly : 



"It is often assumed, in discussing variation, that 

 the possibility of variation is equal in every direction. 

 A consideration of the dynamic conditions of life show 

 that this is not the case, or, at least, if we grant its 

 theoretic truth, in practice it can never be true. Un- 

 der any conditions which would permit it, the result- 

 ing organic forms would have to pass their existence 

 in constant rotation. The moment that any one of 

 them came to rest, it would begin to be subjected to 

 unequal stresses relatively to its different parts. Light, 

 gravity, friction, opportunities for nutrition, would be 

 unequally distributed, with the result of forcing an 

 unequal growth, development, and specialization of 

 its regions. Inequality of form once established, if it 

 were a moving organism, friction and resistance of the 



