﻿474 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



least 3 semiarid islands in the West Indies, is clearly derived from 

 pallida or from common parent stock. 



Of the other three species that are associated with the Abyssinian 

 and Uganda highlands, Mecidea lutzi appears to have no close relative 

 and tellinii is known only from the original description. However, the 

 third species, kristenseni, is clearly allied to the more northern and 

 and eastern pallidissima and to the South African prolixa. Surprisingly 

 enough the two remaining American species, minor (southwestern 

 United States and northern Mexico) and pampeana (Argentina), both 

 resemble prolixa more than they do pallida. 



If lines are drawn to connect the areas occupied by related species 

 (see fig. 88) one line must reach from India across North Africa 

 through the West Indies to the southwestern United States, The other 

 line may also start in India, pass south through Ethiopia, and South 

 Africa, across the South Atlantic to Argentina and north to the 

 southwestern United States. 



Having described the distribution of the genus and correlated it as 

 far as possible with relationships within the genus, there remains the 

 question of how significant this information may be and what if any 

 conclusions may be derived from it. 



The possibility that one or more of the species has been transported 

 from one desert area to another through the agency of man cannot 

 be overlooked. On two occasions living specimens of Mecidea 

 prolixa have been intercepted by quarantine inspectors at United 

 States ports, each time in shipments of grass seed from South Africa, 

 The very close relationship of indica, pallida, and major may also be 

 used as an argument for a recent dispersal of what may prove to be 

 one species. Also, if we accept the distribution as resulting from 

 environmental discontinuity follomng topographic and climatic 

 changes, we must be prepared to accept the probability that pallida 

 and its derivatives indica and major have remained virtually un- 

 changed since early Tertiary time (see Johnston, 1940). 



The existence of parallel examples of this particular pattern of 

 distribution probably constitutes the best argument in favor of a 

 historico-geological explanation. Among the plants several genera 

 are common to the arid region of southwestern United States and 

 northwestern Mexico and the deserts of Argentina. These have been 

 treated by Johnston (1940), and Cain (1944) has summarized the 

 information concerning discontinuous distribution of plant genera 

 that are known from the desert regions of North America, South 

 America, and Africa. Cain was able to cite four well-marked xero- 

 phytic genera as having representatives on all three continents. 

 Menodora, one of the four genera, was shown by Steyermark (1932) 



