ASPECTS OF KINETIC EVOLUTION 379 



De Vries, Gulick and Nageli have given their chief attention 

 to extreme forms of restriction, like those of Draba, Achatinclla 

 and Hieracium. Darwin kept much nearer to the consideration 

 of natural conditions, though his doctrine of selection implies 

 that evolutionary progress depends entirely upon the plan of 

 causing species to change by restricting the descent of the com- 

 ponent individuals. In the kinetic theory, it need scarcely be 

 repeated, the result of selective restriction is not evolution, but 

 specialization. The evolutionary motion would still take place 

 if the selective restrictions of descent were not imposed. 



COMBINED FORMS OF SUBSPECIFIC DIVERSITY. 



Modifications of the constitution of species by specializations 

 of heterism do not interfere with the attainment of the other form 

 of diversity by restricted descent. Thus a sexual species may 

 be partially segregated into geographical subspecies or may be 

 narrowed still further into the stenic condition of domesticated 

 varieties and breeds. Linic and clonic subdivisions of sexually 

 differentiated species do not occur, of course, among the higher 

 animals, being limited to the lower groups and to plants which 

 have the power of sexual propagation or of parthenogenetic de- 

 velopment. But even among the cultivated plants it does not 

 appear that any sexually differentiated species has been resolved 

 completely into the clonic condition. There are large numbers 

 of clonic female varieties of figs and date-palms, but the male 

 trees are usually recruited from chance seedlings, so that the 

 network of descent is not entirely destroyed. The female half 

 of the species is represented by vegetatively propagated clones, 

 but on the male side miscellaneous individual diversity remains. 



The existence of restricted subspecific groups may not inter- 

 fere in the least with the maintenance of a normal specific net- 

 work of descent. A widely distributed symbasic species may 

 have a few porric subspecies as a result of the partial isolation 

 of particular localities. Special conditions, such as an alpine 

 climate, might restrict a part of a species to linic or clonic 

 propagation while the remainder retained fully symbasic condi- 

 tions of descent. Through the fabric of broadly diversified 

 descent there may run narrowly compact strands composed of 



