C. H. TYLER TOWNSEND-BIO-GEOGRAPHY OF TEXAS. 
87 
Tropical.— Any coast region where cocoanut palms can survive must 
be conceded to be strictly Tropical. This tree can not withstand frost. 
It can not survive on the Gulf of Mexico coast at Brazos de Santiago, 
near the mouth of the Rio Grande. 
A preliminary examination of the Coleopterous and Dipterous faunas 
of the lower Rio Grande valley, representing not less than 500 species, 
indicates about 25 per cent of Tropical forms. The Hemiptera and other 
orders show, if anything, a less percentage; so that, so far as our present 
knowledge goes, it appears that there is about 25 per cent of Tropical 
forms of insects occurring here, divided between the Mexican and 
Antillean provinces.* The district, therefore, can not be considered 
tropical, which would require at least 50 per cent. A small percentage 
of its flora is Tropical, while some is Austroriparian, and a large part is 
Lower Sonoran. As for the Tropical forms of mammals, birds and rep¬ 
tiles which occur, such as Felis onca , Totusia, Dicotyles , Fells eyra, F. 
yaguarandi, Ortalis (penelope or chachalaca), Elaps (coral snake), etc., 
none of them is Tropical in its absolute needs. They all range out of 
the Tropical , and some of them are as much Lower Sonoran as Tropical. 
Going southwest from the Nueces river, where the fauna and flora 
attain a marked change from the Austroriparian, we find outside of val- 
le 3 ?s of streams a more or less arid extra-tropical region until we reach 
the Soto la Marina river in Tamaulipas, 150 miles south of the Rio 
Grande. Here another change occurs, that from sub-arid (or Lower So¬ 
noran) to Ti'opical. To the north of the Soto la Marina river, the summer 
nights are nearlj 7 without dew, and rains are extremely irregular and 
scant. This whole region, from the Nueces to the Soto la Marina, has to 
depend on irrigation for the raising of crops. The meteorologic condi¬ 
tions of this region are peculiar. Ordinarily, light frosts are more or less 
common throughout it in the winter. On an average, snowfalls and 
heavy frosts visit it once in about thirteen to fifteen years. Heavy wind 
storms or hurricanes come with about equal frequency. For a year or two 
after such storms, sometimes for several years, rains are more frequent, 
and by taking chances crops can be raised during such years without 
irrigation. Otherwise than this the climate is a dry one, and irrigation 
is on the whole a necessity. 
All these conditions are changed to the south of the Soto la Marina. 
The nights are heavy with dew, rains become regular, irrigation is un¬ 
necessary, and frosts are unknown. Even the whitish soil of South 
Texas and North Tamaulipas is exchanged for a black and richer one on 
* The author has in preparation a detailed paper on the insect fauna of the 
Lower Rio Grande valley, in which he will endeavor to show with more exact¬ 
ness, and from a much larger amount of material, the various bio-geographic affin¬ 
ities of that district. 
