C. H. TYLER TOWNSEND-BIO-GEOGRAPHY OF TEXAS. 
91 
for the southern limits of the Upper and Lower Austral. I consider the 
Tropical region to actually exist in the United States in but one locality, 
the extreme southern coast district of Florida. It runs north on the east 
coast to but little beyond Biscayne Ba} r . Wherever else it enters the 
United States, as in the extreme south of Texas, it is so greatly diluted 
as to preclude its classification with the Tropical region. If it enters the 
United States at all on the lower Colorado river, it must be still more 
dilute and possess still less claim to I'ecognition. So far as temperature 
goes, Yuma can lay no claim to being Tropical. Its mean annual appa¬ 
rent temperature is from 71° to 72° F., and it usually has more or less 
severe frosts from December to February, sometimes killing frosts. This 
cuts its total of effective apparent temperatures below 26,000° F. I am 
convinced that the isothermal marking the mean annual apparent tem¬ 
perature of 74° F., without frost, or in other words the 27,000° F. total 
of effective apparent temperature is nearer to the proper boundary of the 
Tropical , though it should doubtless be much higher here on account of 
the absence of humidity. It is evident that it is almost useless to discuss 
these isotherms in connection with life zones, until they are drawn from 
sensible temperatures. 
It is highly important to recognize all these overlapping or transition 
faunas, but they can not be correctly referred in the whole to either the 
one zone or the other. The correct way to treat them is to map them 
separately, showing on one map the limit of the zones and provinces, and 
on another the limits of the faunas. Overlapping faunas and floras pos¬ 
sess what may be termed a center of transition , where about an equal per¬ 
centage of forms occur which belong to each of the life zones or pro¬ 
vinces contributing thereto. This center of transition is bisected by what 
I shall term a parallel of transition , which runs more or less irregularly east 
and west across the transition fauna, and divides it into two portions. 
The latter fall in their respective provinces, according to what provincial 
elements preponderate in their fauna and flora. This is the only plan 
that can be adopted to define the normal limits of the larger life divi¬ 
sions. 
For example, the Tamaulipas fauna and flora, or the area of overlap¬ 
ping of Temperate and Tropical types along the coast of the Gulf of Mex¬ 
ico, as very correctly defined by Dr. Allen, extends from north of the 
Nueces river, in Texas, to south of the Panuco river in the State of Vera 
Cruz. It has its parallel of transition, as already pointed out, in the 
neighborhood of the Soto la Marina river, where the Tropical types com¬ 
prise about fifty per cent of the fauna and flora. Only the portion 
south of that parallel can be mapped with the neotropical region, or with 
the Mexican province of that region. It would, of course, be an excel¬ 
lent plan to devote a separate map to each life zone, and show its out- 
