Tropie Zone: Jaliscan Region. 657 
less in the dry. The annual rainfall varies in the tropics between 2 and 
5 meters in the mountains and a few centimeters in the deserts. The light 
ntensity also seems, from measurements that have been made, to be greater. 
Notwithstanding the difficulty of accurately delimiting the tropics by climatic 
factors alone, it can be satisfactorily done by means of the plants, which 
have been influenced in their distribution and evolution by the interaction 
of the complex physiologic factors briefly mentioned above”). — The North 
American tropics comprise all of Central America, the narrow Atlantic and 
Pacific coastal plains of Mexico, the Greater Antillean, Bahaman and Bermudian 
islands with outlying southernmost peninsular Florida. Mexico has a tropic 
flora which was derived from the main mass of tropic vegetation of the pre- 
* sent Central America, but originally from the flora of the disrupted Antillean 
continent after it was joined to the land mass lying south of the Isthmus 
of Tehuantepec. When the land bridge appeared, the tropic flora moved 
"north along the Atlantic and Pacific coasts of Mexico some considerable 
distance. Two regions may be distinguished, therefore; the Jaliscan Region 
on the Pacific coast and the Gulf Region along the shores of the Gulf of 
Mexico. Then follow the Guatemalan Region and the Costa Rican Region 
of Central America. 
1. Jaliscan Region. 
Lying west of the main mass of the Cordilleras, the botanist might na- 
turally expect to find a vegetation adapted to a dry or rainless climate, but 
C. G. PRINGLE informs me that about Ameca, Etzatlan and elsewhere, there is 
abundant vegetation with plentiful rains further west and little drought in 
Summer south of Culiacan. Occasionally about Guadalajara, the country may 
be a desert, as late as May or June, but summer rains always fall there to 
bring about a good vegetation. 
This region in the Mexican state of Michoacan is one of lovely lakes, 
Such as Cuitzeo, Patzcuaro and Chapala (in Jalisco), while the Lerma and Bal- 
sas rivers drain a vast territory and empty into the Pacific Ocean. Grassy 
glades and undulating grassy hills are devoted to grazing, while outcrops of 
rugged rocks are covered with a scrubby growth of such small trees as Acacıa 
Pennatula and Ipomoea muricata. Salt marshes fringe the shores of Lake 
Cuitzeo, while under the ledges which line the shore is found Malvavıscus 
acerifolius and between the shore and the hills grow Schinus molle, Prosopis 
Juliflora and the grotesque forms of Ehretia mexicana. The banks of slow 
na 
1) Der vorliegende “Survey” ist vornehmlich der Verteilung des boreslen und des a2 
subtropischen Florenelements in N. Am. gewidmet. Er führt zwar die Einteilung in en a 
geographische Zonen und Regionen auch für den kleinen, aber sehr wichtigen Anteil er ER 
an den Tropen aus, doch gehört dessen ausführlichere Betrachtung mit ee ar Aımaa 
rika (Columbien, Venezuela, Guiana, Amazonas &e.) zusammen. Die ee a ter VI 
hat daher zu einer sehr kurzen Bearbeitung der Verteilung von den Formationen in Lhap 
und VII führen müssen. Dr 
Harshberger, Survey N.-America. 
42 
