ON DYNAMIC INFLUENCES IN EVOLUTION. 5 



rather the presence of two strictly similar beings, could it be 

 shown, would border on the miraculous. 



The question which demands an answer is, how are the 

 small necessary and admitted differences stimulated to develop 

 into the obvious differences which are recognized by systematic 

 biologists ? 



To this I would answer that the reactions of the organism 

 against the physical forces and mechanical properties of its 

 environment are abundantly sufficient, if we are granted a sim 

 ple organism, with a tendency to grow, to begin with ; time 

 for the operation of the forces ; and the principle of the sur 

 vival of the fittest. 



It is often assumed in discussing variation that the possi 

 bility of variation is equal in every direction. A considera 

 tion 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 

 never can be true. Under any conditions which would permit 

 it, the resulting organic forms would all be sub-spherical, and 

 would have to pass their existence in constant rotation. 



The moment that any one of them came to rest it would be 

 gin 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 or 

 ganism, friction and resistence of the circumambient medium 

 would confirm the inequality and put individuals of its kind at 

 a disadvantage when they varied toward the original shape. 

 Flexure of an elongated body would mechanically institute 

 changes analogous to^segmentation, as pointed out by Spencer. 

 Any organic mass possessed of mechanical continuity must de 

 velop surface tension and initiate a superficial film. 



