ASPECTS OF KINETIC EVOLUTION 321 



new character may depend as much or more upon its fitting 

 into and supplementing the complex of existing characters as 

 upon any direct utility from the environmental standpoint. 

 Evolution, in other words, may be viewed as an aspect of the 

 physiological process of interbreeding by which the vitality of 

 organisms is sustained. 



NATURAL SELECTION AS AN EVOLUTIONARY FACTOR. 



The preponderance attained by the selection theory has prob- 

 ably been due, in large measure, to its logical simplicity and 

 consistency in holding that selection is the positive, efficient 

 factor or actuating principle of evolution. The unbiological 

 public has accepted this interpretation of the causes of evo- 

 lutionary motion with practical unanimity, but among biologists 

 themselves there has always been a wide appreciation that the 

 facts did not warrant the definite generalization which Darwin 

 himself carefully avoided, but which his friends made for him 

 and christened with his name. 



All other suggestions of methods of evolution are the result 

 of more or less definite perceptions of the inadequacy of natural 

 selection as an evolutionary cause. No amendment of natural 

 selection has the logical consistency of the original, nor has any 

 gained a comparable popularity in the scientific world. The 

 mistake has been made, if the present diagnosis is correct, in 

 attempting to modify or repair the hypothesis of selection as an 

 evolutionary cause. 



Under the kinetic theory selection appears as a negative fac- 

 tor only ; its power is to inhibit motion, not to cause it. It is 

 not improbable that selection, by closing other avenues of 

 change, can induce more rapid progress in a particular direction, 

 but such an effect of accleration would not prove that selection 

 can cause evolutionary motion ; it would indicate that a certain 

 amount of change necessarily takes place as the result of causes 

 inherent in the species. A variation eliminated by selection 

 does not help to maintain the needful diversity of descent, and 

 this may make the surviving variations the more effective for 

 inducing adaptive specializations. Selection, by thus restricting 

 the field of change, may be able to focus the evolution upon one 



