NOVEMBEE 9, 1900.] 



SCIENCE. 



715 



Mendoza southward to the Rio Colorado in 

 South America, where three species occur 

 which are sharply distinct from each other 

 and especially from Larrea Mexicana. One 

 of these South American species, L. divari- 

 eata, is described as covering great areas of 

 Cordoba and Mendoza as L. Mexicana cov- 

 ers areas in Texas, Arizona and Northern 

 Mexico. 



From these and other illustrations, it is 

 necessary to conclude that we are here deal- 

 ing with forms which were connected by a 

 remote ancestry, which flourished at a time 

 and under conditions which permitted a 

 more general distribution. We may pos- 

 sibly ascribe these condition to a certain 

 stage in the elevation of land masses along 

 the continental axis. At any rate, the fluc- 

 tuations in climatic and geological condi- 

 tions since the Tertiary Period would have 

 very different conditions of distribution and 

 relationship from those we observe now. 



On the other hand, that the same spe- 

 cies may occur in both these widely sepa- 

 rated areas, and nowhere between, indicates 

 the energy of certain agencies acting now 

 and in spite of climatic and geological bar- 

 riers, e. g., Fagonia creticd, FranJcenia grandi- 

 flora, Munroa squarrosa and the three pre- 

 viously cited species of Polygonaeew-erio- 

 gonece. 



(3) The Semi-tropical xeropMlous Jrnins of Gidf 

 Zone Distribution. 

 In discussing the Neo-tropical and semi- 

 tropical elements, attention was called to a 

 Gulf Zone distribution between extra-trop- 

 ical regions. The forms involved here are 

 the less extremely xerophytic species of the 

 warmer and less arid portions of the Lower 

 Sonoran Zone; e. g., the Rio Grande Plain 

 in Texas and Mexico below Eagle Pass. 

 Such species occur also in the xerophytic 

 areas of Colombia, Venezuela, Guiana, Bra- 

 zil, Uruguay, Paraguay and Argentine, and 

 in similar areas of the Antilles. Some are 



undoubtedly sea-coast species. The fol- 

 lowing are illustrations : 



Sida leprosa : Uruguay, Patagonia, Argen- 

 tine, Cuba, Lower Sonoran Zone (even 

 north to Washington). 



Sida hastata : Argentine, Uruguay, Mexico, 

 Texas, Arizona. 



Sida Anomala: Mattogrosso, Uruguay, 

 Argentine, Bolivia, Cuba, Florida, Texas, 

 Mexico. 



Cienfugosia sidjyJmrea: Southwest Texas, 

 Mexico, South Brazil, Paraguay. 



Spergularia plattensis : Texas to California, 

 South Brazil. 



Folygala paludosa: Brazil, Paraguay, Louis- 

 iana and Texas. 



The Amaranth- Gomphrenece are prevail- 

 ingly of the Gulf Zone distribution, especi- 

 ally Froelichia, Alternanthera and Gomphrena, 

 but in the last case, mention should be 

 made of the massing of species in Southern 

 Brazil and Argentine, and their compar- 

 ative absence northward until the Mexican 

 plateau is reached, where, again, are many 

 species, mostly distinct from the South 

 American forms. This fact would suggest 

 the propriety of including Gomphrena in the 

 category of genera like Larrea, FranJcenia, 

 Spirostachys, 3Ialvastrum,Chorizanthe, and oth- 

 ers, in which the present conditions of dis- 

 tribution and kinship point to them as rem- 

 nants of a previous general distribution over 

 territory not now adapted to their needs. 



SUMMARY. 



Reviewing the floral relations of North 

 and South America as illustrated in the 

 foregoing instances, we may say that the 

 phenomena of distribution agree fairly with 

 the record of physical conditions which 

 have succeeded each other and those which 

 still exist, and upon which we might almost 

 a priori have predicted an analogous set of 

 distribution phenomena. In this relation- 

 ship we may distinguish three categories of 

 distribution : 



(1) Those due to the conditions of hu- 



