November 9, 1900.] 



SCIENCE. 



713 



greatly during the period of development 

 of the plant life of the present era, and 

 with profound effect in molding the pres- 

 ent conditions. As a highvsray for north- 

 south distribution of the boreal elements, 

 its eflQciency has of course varied. As at 

 present constituted, it is interrupted by a 

 stretch of moist tropical conditions for a 

 distance of some 10 degrees of latitude, 

 namely, from the southern downfall of the 

 Guatemala highland, 15° IST. , to the Colom- 

 bian Andes, 5° N,, at an altitude of some 

 12,000 feet. Practically, however, one 

 must allow for a degree of continuity even 

 over this stretch as offered by the highest 

 peaks of Costa Eica, Nicaragua and even 

 in the Panama district. An analysis of 

 the floral elements of this north-south Arc- 

 tic and Arctic- Alpine zone shows the fol- 

 lowing interesting phenomena : 



First, that the flora of the Rocky and 

 Sierra Nevada Mountains above the Transi- 

 tion zone, the Mexican Cordilleras in the 

 tierra frias (from 8,000 to 12,000 feet) , of the 

 Guatemalan tierra frias and of the tropical 

 Andes above 12,500 feet, and the extra-trop- 

 ical Andes and highlands, is one of North- 

 ern extraction, abounding in genera asso- 

 ciated with the colder zones of North Amer- 

 ica and Eur- Asia. Such, for example, are . 

 Ranunculus, Anemone, Berberis, Geranium^ 

 Spircea, Geum, Ruhus, Ribes, Saxifraga, Hy- 

 droeotyle, Gaultheria, Vaceinium, Veronica, 

 Eritriohium, Gentiana, Polemonium, Hiera- 

 dum, etc. 



Second, that while possessing very many 

 genera in common, by far the greater per 

 cent, of species in the Mexican Cordilleras 

 are endemic, as are those of the Alpine 

 Andes. This points to a long continued 

 and effective isolation of the Mexican and 

 South American Andes from each other and 

 from the Rocky Mountains. 



Third, that of Arctic- Alpine genera those 

 are most common which belong to the ele- 

 ment common to the Himalayan and East- 



Asiatic regions and the Rocky Mountains 

 from Alaska to Colorado ; that such genera 

 occur sparingly in the Mexican and trop" 

 ical Andes, and then with endemic species ; 

 that there is an increase of this element in 

 the extra- tropical Andes toward the Straits 

 of Magellan. Here is to be noted that cer- 

 tain species of the Rocky Mountain Arctic- 

 Alpine region reappear in the extra-trop- 

 ical Andes toward the southern extremity 

 of South America, being, so far as known, 

 absent from the Mexican and Tropical 

 Andes. Among these are : Gentiana pro- 

 strata, Trisetum subspicatum, Primula farinosa 

 and var. magellanica ; Draba incana = Draba 

 magellanica ; Alopecurus Alpinus = A. antarc- 

 tieus ; Saxifraga cespitosa = S. cordillerarum ; 

 Polemonium micranthum = P. antarcticum; Col- 

 lomia gracilis.* 



a. WARM TEMPERATE AND SUB-TEOPICAL 



XEEOPHILOUS ELEMENTS COMMON TO 



NORTH AND SOUTH AMERICA. 



These elements of flora common to both 

 Americas deserve special emphasis. They 

 embrace for the most part, the flora of the 

 arid regions of the western and south- 

 western states and North Mexico. This 

 flora occupies the mountain slopes of the 

 transition zone, the plains and plateaus of 

 the Upper Sonoran and the hot deserts of 

 the Lower Sonoran zones. This area has 

 been the field of development of many 

 groups peculiarly American. It is the re- 

 gion of xerophytic composites, Nyctagi- 

 nacece, Polygonacece-Eriogonece, Onagracece, 

 Amaranthacece- Gomphrenece, Malvacece, Bor- 

 raginacece-Eritrichiaece, Gilias, the Yucca 

 and Agave kinships and the Cactacece. 



When this peculiar flora was in the vigor 

 of its development and occupation of new 

 territory, the climatic conditions seem to 

 have exerted a pressure to the southward 

 which geological conditions favored, with 



*This list is taken mostly from Bugler's Entwick- 

 elungsgesohichte, II., p. 256. 



