ASPECTS OF KINETIC EVOLUTION 299 



stable equilibrium, so that evolutionary motion appears as the 

 result of forces external to the organism. Differences among 

 the individuals of a species are ascribed to environmental 

 causes ; without such disturbing influences the species is 

 thought to remain stationary and uniform. Darwin and many 

 others have believed in spontaneous variations, but it has been 

 argued that such must be ' swamped ' in the general average by 

 intercrossing, so that without the external influence of selection 

 there could be no progressive change. 



Darwin himself admitted that in the domestic animals ' man 

 does not cause variability and cannot even prevent it,' but on 

 the same page he made the contradictory statement that * the 

 initial variation is caused by slight changes in the conditions of 

 life,' and this has served as the cardinal principle of those who 

 have claimed to be Darwinists, while rejecting the wider per- 

 ception cited above. Again in the same work (p. 79) Darwin is 

 ready to admit that ' a somewhat complex, though apparently 

 useless, structure may be suddenly developed without the result 

 of selection.' 1 



Saltatory Theories. — That variations can be preserved by 

 selection, and are frequently so preserved among domesticated 

 animals and plants, cannot, of course, be doubted, but the diffi- 

 culty of believing that natural conditions would provide the 

 necessary selection or segregation at the right junctures has 

 led many biologists to look with favor upon the idea that new 

 species have not arisen by imperceptibly gradual changes, as 

 Darwin supposed, but by a succession of leaps, as it were. 

 This view is defended by reference to the so-called ' sports' or 

 very pronounced variations occurring among domestic plants 

 and animals. 



Mr. Francis Galton has compared the organism to a polygo- 

 nal body which comes to rest at a point considerably in advance 

 of its former position when its equilibrium has been sufficiently 

 disturbed. Professor De Vries has adopted the saltatory view, 

 as a result of his studies of what he calls mutations, or pro- 

 nounced and readily transmissible variations of domestic plants. 



1 The Variation of Animals and Plants under Domestication, p. 3, New York, 

 1897. 



