C. H. TYLER TOWNSEND-BIO-GEOGRAPHY OF TEXAS. 
85 
about the 23rd degree of latitude, where it slopes off obliquely to the 
northeast, striking the Soto la Marina river and following it to the sea. 
From here northward up the coast it gradually merges into the Ausfro- 
riparian. 
Texas.— From the Soto la Marina river in Tamaulipas, the Lower 
Sonoran follows the proximity of the seacoast northward to near the 
Nueces river in Texas, and includes the lower Rio Grande valley to some¬ 
what above the mouth of the Pecos. The Austroriparian should be con¬ 
sidered as having its main southwestern termination in the Nueces valley, 
but a considerable touch of it reappears in the lower Rio Grande valley; 
and this occasional reappearance of the Austroriparian in valleys of 
streams near the coast undoubtedly obtains still farther south, gradually 
becoming fainter until it dies out altogether. The Lower Sonoran ex¬ 
tends northward from the Nueces valley region, includes the region 
around San Antonio, and reaches still north to the Colorado river region 
of Texas. 
TROPICAL. 
Mexico. —On the west coast of Mexico the Tropical region ends at 
Guaymas, where cocoanut palms grow well. From Guaymas south it 
occupies only a very narrow strip along the coast, widening only as it 
gets well southward. The valleys from Cuautla to Jojutla, on the Inter- 
oceanic Railway, south of the City of Mexico, are tropical in the main. 
Royal palms grow in the plaza of Jojutla. The deep barrancas to the 
west of Guadalajara are also mainly tropical. 
On the humid east and southeast slopes of the territories of the high 
plateau region of Mexico, from the latitude of Tehuacan to that of Jalapa, 
the Tropical reaches up to the 3000 and 4000 feet line, or even higher. 
It follows northward in this way to Cardenas, and then northeasterly to 
the Soto la Marina river. 
It will be well to note here the relation between the biogeographic zones 
and the three principal life regions of Mexico, long ago recognized and 
given distinctive names by the inhabitants. The Tierra Fria or Terre- 
Fraude (above 7000 or 8000 feet) comprises the Boreal Transition , and 
Upper Sonoran zones; the Tierra Templada or Terre-Temperee (about 
4000 to 7000 feet) comprises when arid Lower Sonoran , and when 
humid corresponds to the Austroriparian , though the Tropical then enters 
it largely in its lower extent. The Tierra Caliente or Terre-Chaude (sea 
level up to 3000 or 4000 feet) comprises the Tropical. 
Texas. —The Tropical region ( neotropical ) does not actually enter 
Texas at all. A good touch of its fauna reaches the lower Rio Grande 
valley, and even the Nueces valley, but the percentage is not large 
