ASPECTS OF KINETIC EVOLUTION 23 1 



ing much to the moulding influence of the environment, and in 

 requiring correspondingly little of selection. Other writers have 

 gone to the opposite extreme, making little of environmental 

 factors and much of natural selection of fortuitous individual 

 variations. The latter tendency has been dominant since Weis- 

 mann showed that "acquired characters," the results of direct 

 environmental influences, are seldom or never inherited. 



In the original Darwinism and its various amended forms 

 there seems usually to have been included the tacit assumption 

 of a constant of variability. It is taken for granted that a cer- 

 tain amount of variation shall be manifested by each species, so 

 that selection by paring off the species on one side can cause it 

 to grow out on the other, and thus compel a gradual change of 

 characters. Without selection the average is thought to remain 

 stationary, and if selection be withdrawn the progress already 

 made may be lost by retrogression. Selection, in this view, is 

 the true actuating cause or principle of evolution. 



Mivart, and recently many others, have considered that both 

 the environmental variations and the minute and fluctuating indi- 

 vidual differences were alike in adequate to accomplish evolution 

 through selection, and have advocated a return toward the older 

 doctrine of special creation. They hold still to the evolutionary 

 idea that species arise one from another, but suppose that the 

 new types originate suddenly by " extraordinary births," or by 

 abrupt mutative variations, that is, by individuals which depart 

 widely from the type of the older species. The occurrence of 

 many such abrupt variations is a definitely established fact. 

 Among plants they often come true to seed, and among animals 

 they are often prepotent when bred with other members of their 

 own variety or local species. Nevertheless, it does not appear 

 that this is the method by which species originate in nature. 

 The prepotency of new variations indicates the probability that 

 old species are tranformed by this means rather than that new 

 species are abruptly originated. 



Darwin appreciated better than many of his successors in the 



field of evolutionary literature the fact that variations are of 



many kinds, of very different evolutionary significance, and due 



to many different causes. As an evolutionary pioneer it was 



Proc. Wash. Acad. Sci., December, 1906. 



