Origin and Development of the Compositev. 9 
The Paths of Migration. 
If the centre of origin is taken as the Bolivian region (see PI. I, 
Fig. 29) the study of the widespread species indicates clearly that the 
path of migration has been southward along the Andes to Fuegia 
and northwards along the same range towards Central America and 
the Cordilleran system from Panama to Alaska. Throughout the 
rest of the world the path of migration is commonly along the 
mountain ranges, usually above 3,000 ft., frequently above 6,000 ft. 
South America. Extending in the southern direction into Chili 
we get S. Bridgesii, Hook. A connection is traced between Chili 
and the Argentine and between Chili and Patagonia. There are 
two species, 5. Smithii, D.C. and S.candidans, D.C. which are of 
interest. The former extends from southern Chili into the 
Magellan and Fuegian regions and to the Falkland Islands. The 
latter occurs in Fuegia and the Palklands. The Falkland Islands 
are about 300 miles from the nearest land, but, instead of raising a 
very hypothetical land-bridge to explain the distribution, we can 
point with some certainty to the westerly trade winds and remind 
the reader of the fact demonstrated in the previous chapter that 
the minimum wind for dispersal to any distance of an average fruit 
of Senecio vulgaris is 1.5 m.p.h. In the region in question we have 
a wind of the necessary velocity in the proper direction and some¬ 
times though not always the air has the necessary low relative 
humidity. 
Extending northwards we have 5. medullosus, S.B. from Bolivia 
to Colombia, 5. superbus, S.B. from Peru to Colombia, and 5. 
rhizocephalns, Turcz. from Ecuador to Colombia, while other 
species with smaller ranges connect other parts of the Andine path. 
South America to North America. The connection between the 
two halves of the American continent is made by two species, S. 
decompositus, Hieron. and S. formosus, H.B.K. The latter occurs 
in Bolivia, Colombia and Mexico, the former in Venezuela, Colombia 
and Mexico. Greenman (100) also mentions S. ledifolius, D.C. as 
connecting North and South America. 
Central America to North America —The bridging species 
between these two regions are 5. lanicaulis, Greenm. and S. 
Aschenbornianus, Schau., the latter oecurriug at levels over 6000 ft. 
on the Central American mountains. 
North America. The widespread species in this region usually 
belong to the Cordilleran flora, and there is a considerable affiinity 
between Texas, Arizona, California and Mexico, so much so that 
