The Origin of Crefns and Related Genera 45 



Crepis and Prenanthes, are confined to the Himalayan region. 

 In addition, the most primitive species which has been excluded 

 from Crepis, namely, Paleya oligocephala, occurs in this region 

 only. The existence of these species in the Himalayas, however, 

 need not be stressed, inasmuch as the origin of the Crepidinae 

 may have antedated the upheaval of that great mountain system. 

 This evidence on distribution, however, is consistent with the 

 concept that Crepis and its closest allies originated in south- 

 central Asia. 



This leads naturally to the question of whether any genetical 

 evolutionary processes can be suggested to account for speciation 

 and differentiation within these genera. It has been intimated 

 (p. 19) that our present knowledge of phylogeny in Crepis 

 makes possible a tentative hypothesis of this kind. Before pre- 

 senting this hypothesis, however, it is necessary to consider 

 briefly two fundamental genetical processes which appear to 

 have been operating in the evolution of Crepis. The first of these 

 is reduction of chromosome number from 10 to 8. If monophyle- 

 sis for the genus be assumed, it is necessary also to assume that 

 such a process actually operated during the early development 

 of the genus. Although there is no experimental evidence that 

 new species can originate in this way, yet the combined evidence 

 from comparative morphology, geographic distribution, and 

 cytology in Crepis both necessitates and supports this assump- 

 tion. It will be recalled that, although 10 is the most primitive 

 number in the genus, yet 8 is found in the larger number of 

 species and there are some rather primitive types among the 

 8-chromosome species. Furthermore, in certain groups of closely 

 related species two different chromosome numbers occur. Thus 

 the two 6-chromosome Eucrepis species are obviously related to 

 species with 8 chromosomes, and in BarJ^hausia a lo-chromo- 



