92 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
lying dilate extensions in all directions, indicating also the percentage 
of dilution in the various localities. 
In the case of transition faunas between humid and arid provinces, 
the dividing line runs nearly north and south, and maj r then be termed a 
meridian of transition. This distinction is necessary in the case of cer¬ 
tain faunas which present both a parallel and a meridian of transition. A 
parallel of transition may run very irregularly, according to the topog¬ 
raphy of the country, but it will be borne in mind that it always tends 
to run at a right angle to the axis of spread. The latter term may be 
used to denote the main line along which forms from two zones or prov¬ 
inces approach each other until they meet and intermix. A meridian of 
transition likewise tends to run at a right angle to the axis of spread of 
a mixed humid and arid fauna. If 50 per cent of each fauna prevails 
for any distance in one direction, then the various center's of transition 
can be taken to determine the parallel or meridian of transition. 
There remains only to notice the principal changes in Dr. Merriam’s 
last map (PI. 14, vol. 6, Nat. Geog. Mag., December, 1894), from his 
third biogeographic map (Annual Report Dept. Agric. for 1893, map 
printed December, 1893). They are three: The introduction of the 
Ti'opical along the lower Colorado valley, which has been already dis¬ 
cussed; the extension of the Transition along the Pacific coast strip, 
which is the happy result of Dr. Merriam’s excellent investigations on 
the laws of temperature control, and marks a long stride in the correct 
interpretation of the fauna and flora of the Pacific slope; and the ex¬ 
tension of the Tropical across the peninsula of Florida. The latter I do 
not agree with, believing, as already said, that it is properly restricted 
to the narrow coast region of the peninsula south of the latitude of Bis¬ 
cay ne Bay. 
Note. —The question often arises among zoologists as to what class of 
animals is of the most importance for the determination of faunal 
regions. Now faunal and floral regions must necessarily coincide very 
closely. ' If we wish to obtain biogeographic data for economic use in re¬ 
lation to the raising of crops, we naturally turn to the flora for the exact 
and careful limitation of regions. In such a case we can not go astray 
by taking certain characteristic plants as criteria to the limits of prov¬ 
inces, zones, and regions, for many plants are obviously more sensitive 
to climatic influences than are animals. The mesquite and many other 
plants can not survive too much humidity, while others perish in arid 
regions unless irrigated. The date palm can withstand a moderately 
hard frost, while the cocoanut palm can withstand none. Therefore, if 
by these means we mark off life divisions, it follows, that we must be ap¬ 
proximating the fauna at the same time, as the latter conforms in general 
to the same meteorologic conditions. 
