202 AMERICAN JOURNAL OF BOTANY 



genus to migrate into new environments beyond its original home; migration 

 has brought it to its present distribution and is doubtless still continuing: 

 structural evolution, favored by geographic isolation, has differentiated 

 the present species, but is very little correlated with their physiology or 

 ecology, although proceeding simultaneously with their migration. A 

 single structural tendency appears to be general and possibly orthogenetic, 

 that of the shortening and branching of the cymes. 



Summary 



i. Three sections of the genus are represented in North America by a 

 single species each; one section is represented by 120 species. 



2. In this section the chief differentiation of groups rests on the structure 

 of the inflorescence; minor differentiation of species-groups is based on the 

 achenes, the involucre, the pappus, and the character of the pubescence. 



3. Of the two subsections, one is chiefly Antillean, the other continental, 

 while both are developed in continental South America. 



4. Characters which are held to represent primitive conditions in one 

 group may indicate advanced evolution in another, and such characters 

 have no apparent correlation with environment. 



5. In every case, those groups which appear to be the simplest in mor- 

 phological structure occur to the south, while the more complex groups 

 appear progressively farther to the north. In most groups the same state- 

 ment holds for the individual species. 



6. The geographical arrangement of the species-groups and species 

 follows well-known migration routes and supports the conclusion that 

 evolution and migration have proceeded together. 



New York Botanical Garden 



