ASPECTS OF KINETIC EVOLUTION 3O9 



tionary changes must be gradual, and some writers have dwelt 

 upon the imperceptible slowness of evolutionary progress. The 

 mutation theory of Professor De Vries adopts the other extreme, 

 in holding that evolutionary motion is abruptly discontinuons, 

 the individual organism leaping, as it were, from one species to 

 another without any steps or gradations. From the kinetic 

 standpoint, mutations like those studied by Professor De Vries 

 are interpreted as abnormal and degenerative phenomena, but 

 the fact is recognized that the individuals of many species in 

 nature have very recognized differences, so that the steps of 

 evolutionary progress may not always be infinitesimally grad- 

 ual. There are indications that prepotent new characters may 

 often transform a species or variety in a comparatively short 

 period of time. 



CONTINUITY OF EVOLUTIONARY MOTION. 



Theories which ascribe organic changes to selection or to en- 

 vironmental causes imply that progress is limited to the charac- 

 ters which happen at the time to have environmental significance. 

 In this view evolutionary motion, though gradual, must be de- 

 scribed as occasional, rather than as continuous. After a 

 period of selective development a species might cease, for a time, 

 to be affected by selection and remain stationary, or might even 

 retrograde, as claimed by Weismann and others. 



In the mutation theory the idea of occasional change is car- 

 ried still farther, so that evolutionary motion would need to be 

 described as intermittent and occurring only at rare intervals. 

 This is the type of evolutionary theory which comes nearest to 

 the older doctrine of separate creation of species. It represents 

 species as arising from single individuals, and denies gradual 

 or continuous progress. It declares that evolutionary motion 

 is saltatory or discontinuous ; that there are sudden changes or 

 jumps from one species into another. Such an evolution could 

 not be described as taking place in species, but between them, 

 the species themselves being essentially stationary except when 

 acted upon by special " forces." Whether the forces are exter- 

 nal or internal is a matter of opinion which subdivides saltatory 

 evolutionists into two subordinate schools. 



