ASPECTS OF KINETIC EVOLUTION 279 



selection by man only the most proficient are saved. Selection, 

 by deflecting and confining the evolutionary motion of the 

 species to particular channels, conduces to the adaptive speciali- 

 zation of characters, but it is not an actuating cause of their 

 development. 



Symbasis is a primary factor in evolution, an obstacle or neg- 

 ative factor in speciation. Selection often accounts for the 

 accentuation of differences between related species, but is not on 

 this account to be reckoned as an actuating cause or principle 

 of evolution. It may explain the direction which evolution has 

 taken with reference to a particular character, but does not show 

 how the evolution has been accomplished. 



Adaptation represents the bionomic aspect of evolution, specia- 

 tion the taxonomic. Selection strengthens adaptations ; isola- 

 tion multiplies species; symbasis conducts evolution. Adapta- 

 tion and speciation have appeared to many writers as causes of 

 evolution, but in the kinetic or physiological interpretation they 

 appear only as results, quite incidental to the true evolutionary 

 process of progressive change in species. 



RELATION BETWEEN HETERISM AND SPECIATION. 



Recognition of the phenomena of heterism, the normal diver- 

 sity of the interbreeding members of specific groups, is neces- 

 sary, perhaps, to a full appreciation of the preceding distinctions 

 between evolutionary change or vital motion and the subdivi- 

 sion or multiplication of species. Although commonly treated 

 together, or even indiscriminately confused, these two processes 

 are quite distinct. They may even run counter to each other, 

 for evolutionary progress is not assisted by the subdivision of a 

 subdivision of a species, but more likely to be hindered. The 

 larger the number of interbreeding individuals the larger are 

 the possibilities that desirable variations will appear, and the 

 wider are the opportunities of a progressive utilization of a new 

 feature. The group, as a whole, will advance more rapidly 

 than if the range of transmission be narrowed by subdivision. 



Segregation permits the subordinate groups to become dif- 

 ferentiated, but it does not conduce to the advance of the whole 

 series. The newly segregated groups become capable of tax- 



Proc. Wash. Acad. Sci., January, 1907. 



