3io 



COOK 



Saltatory evolution consists of a series of abrupt lateral dis- 

 placements, each species remaining stationary and unchanged 

 from the time of its origin by mutation. No forward progress 

 of the members of interbreeding groups is provided. Mo- 

 tion takes place only in the individuals which give rise to the 

 new groups. Selection would thus have no influence upon evo- 

 lutionary motion in connection with the mutation theory. Its 

 function would be limited to the determination of the survival 

 of the new species which might prove to be adapted to their 

 environments. Motion is conceived only in simple inflexible 

 lines and not in a network of descent which can bend in adap- 

 tive directions when environmental obstacles are encountered. 



Saltatory theorists do not deny that diversity exists among 

 the members of species, but they ascribe this to the influence 

 of external conditions or to a general principle of inconstancy 

 or fluctuation, without any special evolutionary significance. 



Saltatory theories stand in most direct contrast with those 

 which ascribe continuity to the evolutionary motion of species, 

 which are thought of not as advancing by leaps or sudden trans- 

 formation of one species into another, but as going forward by 

 gradual steps, larger or smaller. Natural selection by the en- 

 vironment is thought of as changing the average and hence as 

 causing evolutionary motion. The higher groups of plants and 

 animals have so many adaptive characters that evolution by 

 natural selection has been accepted by many biologists as a 

 demonstrated fact. 



Determinant and kinetic theories agree in expecting evolu- 

 tion to be continuous, the one because the internal mechanisms 

 would continue to act, the other because the interbreeding of the 

 ever-diverse individuals of the species is being continued. 



MUTATIONS DISTINGUISHED FROM NATURAL SPECIES. 



There is a wide and fundamental difference between the kind 

 of evolutionary motion shown by mutations of inbred domesticated 

 species and that by which the progressive development of 

 natural species has been brought about. The condition of in- 

 breeding under which mutations appear has so far weakened 

 the organism that the newly modified form is recessive, that is, 



