THE WORLD'S COTTONS — HUTCHINSON 513 



It may well be, of course, that G. raimondii was formerly more 

 widely distributed, or that there were other, related species in the 

 Peruvian valleys. The pressure of the population of coastal Peru 

 on the very limited resources of water and of vegetation is such that 

 the natural flora has been virtually eliminated and other wild species 

 besides G. raimondii may have existed in former times. Nevertheless, 

 though restriction of distribution and even extinction may have af- 

 fected the situation greatly, it cannot be claimed that evidence for the 

 origin of the allopolyploid cottons in coastal Peru has increased 

 greatly since the hypothesis was put forward, and alternative theories 

 must now be examined. 



G. L. Stebbins [25] has suggested that the diploid Old World parent 

 reached the New World by way of China and Alaska, and the pos- 

 sibility of an Antarctic route has been proposed. Both the northern 

 and the southern routes are advocated on the ground that at one time 

 mesophytic temperate woodland floras existed that were common to 

 the two hemispheres. Gossypium is a genus of xerophytic perennial 

 shrubs adapted to the arid Tropics. No member of the genus would 

 grow in an ecological situation where temperate woodlands existed, 

 and until the modem development of short-term annual cottons under 

 domestication, no member of the genus would survive in a climate 

 with winter frosts. It is therefore reasonable to conclude that contact 

 between Old World and New World species did not come about by 

 migration round the Pacific, either by a northern or by a southern 

 route. 



The remaining alternative to a trans-Pacific link is a link across the 

 Atlantic, and Gerstel [9] has recently suggested that this should be 

 considered. He showed that in G. herhaceum the chromosome order 

 does not differ substantially from that in the wild African G. 

 anomahim^ and he concluded that the G. herhaceAim order is primi- 

 tive. G. arhoreum differs from the primitive order by one transloca- 

 tion, and the corresponding chromosome set in the New World 

 allopolyploids differs from that in G. lierbaceumi by two and from 

 that in G. arhoreum, by three. He therefore argued that G. arhoreum 

 is not in the main line of development from the primitive chromosome 

 pattern to the modern allopolyploids. Since G. arhoreum is the cot- 

 ton of the Indian subcontinent and of the western coasts of the Pacific 

 basin, it seemed to him unlikely that the link between the primitive 

 diploids represented by G. herhaceum and the New World cottons of 

 South and Central America was to be sought across southeast Asia. 



The difficulty of supposing that the allopolyploids arose following 

 migration of an Old World cotton across the Atlantic lies not only 

 in accounting for the ocean passage, but in also imderstanding how 

 the two parents came in contact after the Old World cottons reached 



