ASPECTS OF KINETIC EVOLUTION 3 I 5 



It is commonly taken for granted by the advocates of the 

 selection hypothesis that a certain constant of variation will be 

 maintained by the species, so that the cutting off of the extremes 

 on one side will cause a still greater development on the other, 

 and thus actually move the species along. 



This idea may never have been very definitely formulated, 

 but it is obvious that many writers on selection have relied upon 

 the unexpressed assumption as affording the means by which 

 selection could produce evolutionary change in a normally 

 stationary group of organisms. 



The Darwinian doctrine of variation grafted upon the older 

 idea of stationary species resulted in the conception of a species 

 composed of variable individuals, but with a stationary specific 

 average. Experiments with domesticated varieties had shown 

 that selection could change the center of gravity or character- 

 average of a group, and this idea applied to nature at large 

 gave the hypothesis of evolution through selection. 



In arguing the inadequacy of selection, Mivart, De Vries and 

 others have taken the ground that selection could not carry the 

 specific average beyond the boundary or limit of range of 

 variation for the original group, and this is the logically correct 

 inference, unless the idea of a constant of variability be included 

 as a factor of the problem. But even this is inadequate to 

 account for the general evolutionary results, for unless the 

 further notion of a normal tendency to progressive change be 

 added, the presumption would be that the selectively reduced 

 species would attempt merely to reproduce its lost members, to 

 regain its original size and cover again the field from which it 

 has been excluded by selection. 



It may be held, therefore, that both in logic and in fact the 

 explanation of the ascertained and generally admitted data of 

 selection depends upon the recognition of a normal and spon- 

 taneous tendency of species to evolutionary change. It is this 

 tendency, this specific kinesis or law of motion, which carries 

 species into close selective contacts with their environments. 

 The species are travelling by their own motion, in spite of 

 selective obstacles, and not because environmental selection is 

 carrying them along. 



