4 BIOLOGICAL SOCIETY OF WASHINGTON. 



tary effort, no matter how seconded by habit, should be as con 

 stant and unceasing as the impacts due to ordinary mechanical 

 forces, we should expect the responses due to conscious effort 

 to be feeble in intensity and numerically feV in comparison 

 with those arising from the dynamic forces undirected by con 

 sciousness. 



The dynamics of the environment, so far as we are able to 

 understand them, in their principal features must be remark 

 ably constant. The weight and consistency of the water or air 

 which forms the surrounding medium, the character of the 

 supporting surface, the range of temperature, the supply of 

 light, the friction of adjacent bodies, the attraction of gravita 

 tion, vary within comparatively narrow limits, when consistent 

 with organic existence. We should therefore expect that 

 their influence would on the whole be conservative and tend 

 toward the preservation of the main characteristics of organ 

 isms once brought into substantial equilibrium with their 

 surroundings. 



On the other hand, owing to the very narrowness of the 

 limits within which life is possible, the dynamic variations, 

 within those limits, to which organic forms are subjected be 

 come relatively more important. It is probable that since the 

 initiation of life upon the planet no two organisms have ever 

 been subjected to exactly the same dynamic influences during 

 their development. Differences of impact necessarily imply dif 

 ferences in response, hence a certain amount of variation is the 

 inevitable result. It is absolutely impossible that any two in 

 dividuals can be or ever have been strictly similar and the ap 

 plication of a conception of exact similarity to any two actual 

 beings becomes more and more difficult as the complexity of 

 their organization is increased. 



The origin of variation therefore presents no difficulties : 



