ON DYNAMIC INFLUENCES IN EVOLUTION. O 



rather the presence of two strictl}' similar beings, could it be 

 shown, would border on the miraculous. 



The question which demands an answer is, how are the 

 small necessarj' 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. lyight, 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. 



