PAPERS OX BIOLOGY AND AGRICULTURE 79 



or to what extent the reverse migration has taken place. 

 In other words, our knowledge of the flora of these two 

 countries is not yet sufficient to enable any one to say 

 what floral elements are common to the two countries 

 or what the proportion of occurrence of floral elements 

 in one country is to that of the other. Presumably there 

 has been a northward trend of tropical vegetation since 

 the glacial period. In this connection it may be of inter- 

 est to site a few cases of specific plant distribution. 



In Colombia one of the most common types of vege- 

 tation is to be found on the paramo or dry ridges. This 

 type of growth consists largely of Compositae of a shrub- 

 by or suffruticose habit; and it includes several species 

 of Eupatorium and Senecio. One of the common plants 

 of the paramo of Colombia is Senecio vaccinioides AVedd. 

 Curiously enough either the same thing or a very closely 

 related species, described as Senecio fir mi pes Greenm., 

 occurs on the Vueltas and on the Cerro de la Muerte of 

 the southern Cordillera in Costa Eica at an altitude of 

 3100 meters or about 10000 feet. Only two stations are 

 known for this plant in Costa Eica, and it has never been 

 reported from Panama. The Senecio vaccinioides is very 

 common in Colombia and whether the two things are 

 conspecific or not, it is fair to assume that the Costa 

 Eican form has descended from the South American 

 type, and probably represents a northern migration 

 which has taken place since the glacial times. Certain 

 other natural groups of Senecio, consisting of several 

 little known trailing or climbing species, are represented 

 both in South America and Central America. The af- 

 finities or relationships of these species are such as to 

 show clearly a South American origin; and the present 

 distribution of these species is such as to indicate a 

 northern migration from the Andean region of Ecuador 

 and Colombia into Central America. In at least one 

 instance this northward migration has extended to that 

 great elevated mountain region of Orizaba in Southern 

 Mexico. In most cases, however, these plants do not oc- 

 cur north of the Southern Cordilleras in Costa Eica. 



May I mention one more specific example? I found 

 growing, and apparently indigenous, on the great moun- 



