This province is made up basically of four major vegetational 

 belts, and is tripartite in composition. It consists of three large 

 blocks of mountains and two narrow coastal strips. On the 

 mountains all the fifteen possible major vegetational belts 

 between the equator and the poles, plus various transition types 

 of growth, are represented by pronounced montane zones. 

 It forms a large V-shaped structure, lying northwest to southeast 

 with its point thrust into the isthmus of central Mexico. The 

 left-hand, or west, tine of this province is one thousand miles 

 long: the right-hand, or eastern, five hundred miles; the northern 



ends of the two tines are six hundred miles apart. The central 

 portion of this triangle is occupied by the desert and scrublands 

 of the Chihuahua Province. 



The western part of the V is composed of the great Sierra 

 Madrc Occidental and the narrow coastal plain that lies between 

 those mountains and the Pacific. These ranges stretch from just 

 south of the United States border in the state of Sonora to the 

 region of Guanajuato. The eastern part, composed of the 

 Sierra Madre Oriental, starts about Monterrey in Nuevo Leon 

 and stretches southeast to the heights of Hidalgo. Between these 

 mountains and the Gulf of Mexico there is also a narrow coastal 

 plain, which broadens out somewhat in the north where a string 

 of ancient, low mountains are spread along its surface parallel 

 to the Sierra. The third or southern part is composed of the great 

 chain of volcanic peaks and connecting ranges that reach from 

 the Pacific coast in Jalisco almost to the Gulf coast in Vera Cruz. 

 Along this range lie the famous volcanos such as Popocatepetl 

 and Mt. Orizaba. 



All three parts of this conjoined province are extremely 

 complex in structure, the individual ranges being in almost 

 every case composed of dozens of individual blocks, each with 

 its own particular features. The vegetational forms, floras, and 

 faunas of these are equally diverse, but the first retains its basic 

 pattern with absolute rigidity and formality throughout. The 

 flora is different from that of any other part of the continent 

 mainly because much of this province lies within the subtropics 

 and consequently truly tropical elements from the south are 

 introduced. The fauna, on the other hand, is almost wholly of 

 North American affiliations, for the great zoological divide 

 between North and South America follows the northern edge 

 of the truly tropical forests at base level. These forests barely 

 finger north to this province through the lowlands of Campeche 

 and Tabasco. The types of birds vary widely. 



If you approach this province from the northeast, entering 

 Mexico via Matamoras, and proceed down the fine coast road 

 toward Mexico City, you will cross first the much constricted 

 Desert Belt and then almost two hundred miles of the Southern 

 Scrub Belt (see Chapter 14), The road then winds through a 

 range of low and extremely ancient mountains, one of a long 

 string that lies on the plains from somewhat north of Monterrey 

 all the way to Tampico. These are of very old strata and have 

 been above sea level for a long time, so that they have become 

 a refuge for certain plants that are not found elsewhere and that 

 have relationships only with those of distant places like upland 

 Venezuela. Beyond these mountains, one enters a low, dry valley 

 or gutter still clothed in the typical chaparrals of the scrublands, 

 but ahead one sees the towering wall of the great Sierra Madre 

 rising abruptly to the sky. Nestled at its foot is the beautiful old 

 town of Victoria. Here, if there are no clouds on the mountains, 

 one may lie back in a deck chair and study phytogeography in 

 one easy lesson, for every major belt from the Southern Scrub 

 to the Barren Grounds of the Arctic are plastered as distinct 

 zones across the face of these mountains in regular bands — at 

 the bottom a desert band, then a north scrub chaparral, then an 

 open scrub, then a prairie, a parkland, a temperate forest, a 

 transition zone with conifers coming in, above this a true boreal 

 coniferous forest, then a stunted alpine flora leading to low 

 tundra and finally barren ground. The whole scene, however, is 

 typically "northern," 



If you drive on to the south, you mount over some small hills 

 and then descend onto a continuation of the scrub plain covered 

 with cactuses and mesquites and infested with the birds called 

 road runners. This gradually narrows into a valley between the 

 Sierras and the ancient mountains. Then, about fifty miles along 

 the road, there is a modest sign which informs you bilingually 

 that you are now crossing the Tropic of Cancer. Within a mile 

 the hills close in from either side, and you plunge into an en- 

 tirely new world. You are indeed in the tropics. 



Curiously enough, almost exactly the same thing happens a 

 few miles north of Mazatlan, when one drives down the Pacific 

 coast road. In both cases one has not changed altitude and is 

 still passing along a narrow coastal plain between high moun- 

 tains and the sea when, suddenly, desiccated scrubs with cac- 

 tuses of a desert type give way to grasses of all kinds (including 

 even bamboos in swampy areas), and bananas and other tropical 

 cultivated plants become part of the landscape. The birds too 

 change shape and color abruptly, and while the ubiquitous crows 

 are still everywhere, birds such as parrots put in an appearance. 

 Real trees line the streams, which again contain water; there 

 are little vines among the bushes; and epiphytic plants may be 

 seen growing on trees and telephone wires. 



We have here entered the northernmost outliers of the great 

 tropical Savannah Belt, but it is so compressed and pushed so 

 far north that even modest elevations suffice for trees to come 

 in and form an orchardbush scenery. The stunted mesquites and 



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