I96 AMERICAN JOURNAL OF BOTANY [Vol. 10, 



several species of northwestern South America, some of which extend into 

 Central America also, and several others ranging as far north as southern 

 Mexico. Species of the third stage have crossed the narrow channel from 

 Yucatan and are now limited to western Cuba; the fourth is confined to 

 Mexico, and the fifth has four species in northern Mexico and 31 in the 

 United States and the Bahamas. 



The species-group Stellares, representing the second stage, includes the 

 commonest species of the mountains of Colombia, Central America, and 

 southern Mexico. The Colombian species V. canescens H.B.K. and V. 

 mollis H.B.K. retain the primitive character of acuminate involucral 

 scales; the former also extends north into Mexico, and the latter is doubt- 

 fully admitted into the North American flora. Of the remaining five with 

 acute to rounded scales, V. patens H.B.K. occurs in both continents, while 

 the others are strictly North American. The most advanced morphologi- 

 cally is V. morelana Gleason, which alone does not occur south of Mexico. 



The third stage includes the species-group Menthaefoliae, undoubtedly 

 closely related to the Stellares, but now isolated in western Cuba and the 

 Isle of Pines, except for a few specimens from central and eastern Cuba as 

 well, where they may have been recently introduced. 



The fourth stage includes three well-marked species-groups, which 

 are nevertheless closely related. The Umbelliformes include 9 species, 

 mostly quite closely related and in some cases separated with difficulty. 

 The simplest species (and the commonest in herbaria) have small heads, 

 with short involucres and seldom more than 15 flowers; the more advanced 

 have larger heads and taller involucres. One of these, V. Conzattii Robin- 

 son, with its abruptly rounded and mucronate involucral scales, marks a 

 transition to the group Mexicanae, with three species in the higher moun- 

 tains of southern Mexico. Here the scales are extraordinarily specialized, 

 being 3-8 mm. wide, loosely spreading, at least at the tip, and prominently 

 reticulately veined. The two closely related species of the Alamanianae 

 have also large scales but lack the reticulate venation. The general evolu- 

 tionary tendency of the series is apparently toward large heads and special- 

 ized scales, and this is correlated geographically with an ascent to higher 

 levels in the mountains. 



Passing now to the 35 species of the fifth stage of evolution, as shown 

 by the inflorescence, we find the most primitive members in the Texanae, 

 a group of four species, three in northern Mexico and one in Texas. Since 

 the inflorescence has already passed to the paniculate stage, equally char- 

 acteristic of the other species of the United States, evidence for the primitive 

 character of the group must be sought in other characters. The leaves in 

 all four species are more or less pitted beneath and the outer pappus bristles 

 are poorly differentiated from the inner in width, both of which features 

 occur also in the Umbelliformes. The most important primitive character, 

 however, lies in the involucre, and has not been mentioned before because 



