148 HEREDITY AND EVOLUTION IN PLANTS 



as it is without raising and sinking continents, as Darwin 

 says, 'in a quite reckless manner.' This latter is an impor- 

 tant point, as the Compositae are almost certainly of such 

 recent origin that the possibility of land bridges is in many 

 cases quite out of the question." In fact, Small contends 

 that a "rational study of the history of the Compositae, 

 their migrations and colonizations, their paths of travel 

 and regions of concentration," is not possible without a 

 correct understanding of the conditions of wind dispersal. 

 The occurrence of a species of Senecio (a pappose- 

 fruited Composite) on the Falkland Islands, 300 miles 

 from the nearest land, and of another species on St. 

 Helena and on Prince's Island, nearly 1500 miles from the 

 nearest land, are attributed by Small respectively to the 

 westerly and the south-east trade winds. The distri- 

 bution of the family, as worked out in detail by Small 1 

 affords an instructive illustration of how geographical 

 distribution affords new evidence and confirms other 

 evidence as to the relative ages of various related groups 

 of plants, and as to the fact and course of evolution within 

 a given plant family. The immense genus, Senecio, 

 for example, according to Small, comprising over 2300 

 species, is of very wide distribution, being marked by a 

 concentration at high altitudes, which is not surprising in a 

 wind-distributed group. 2 Some of the species are wide- 

 spread, and some are local, and the group is characterized 

 by its ready response to the influence of environment; 

 to this is attributed, in large part, its great morphological 

 variation. No species covers the range of the genus. 



1 Small, James. The origin and development of the Compositae. 

 Chapter X. New Phylologisl 18 : 1-35. Jan. and Feb., 1919. 



2 It has been calculated by Ball that 25-30 per cent, of the flora of the 

 higher Andes are Composites. 



