BARRO COLORADO ISLAND—GROSS 331 
May and ending the first of December, southerly winds become 
dominant, and rains are general throughout the Isthmus. During 
the time I spent in the Zone (June to September) the climate was 
very pleasing and comfortable. I do not recall that the heat was 
ever as oppressive as on some of the summer days I have experienced 
in our northern cities, and the nights were never as hot as they 
frequently are on the plains of the Middle West States. The 
evenings were usually cool, and though we seldom used blankets 
there was never a night when we were obliged to lie awake because 
of great heat. The temperature during August ranges from 71° F. 
at night to 90° F. in the daytime. The December extremes are 
69° F. to 89° F., only slightly lower than the summer temperature. 
It is this uniformity of temperature which tires and which seems 
to sap the energy and vitality of those who remain in the Tropics 
for more than a few years. To a person who goes to the Zone for 
a few months or at most a year the climate does not give the im- 
pression of being oppressive. Of course, it is necessary to adapt 
your clothing to the climate, and it is only the most ambitious 
who exert themselves physically during the hottest hours of the 
more humid days. 
In the Republic of Panama there are three life or faunal zones 
represented, the lower and upper Tropical Zones which include 
most of the country and a Temperate Zone limited to comparatively 
small areas on the tops of some of the highest mountains. In the 
canal region there is only the lower Tropical Zone, a zone which 
includes an area of high temperature in which many species of 
plants and animals range in suitable places throughout its extent 
while others are restricted to the so-called arid and humid divisions 
or else reach their greatest development there. The rainfall is much 
greater, some years twice as great, in the humid than in the arid 
division. The average rainfall for 13 years at Balboa on the Pacific 
side of the Isthmus, which is in the arid division of the lower 
Tropical Zone, is 71.67 inches whereas an average for 40 years at 
Cristobal on the northern or Atlantic side is 130.03 inches. Although 
the amount of rainfall is important the most significant difference 
between the two sides of the Isthmus representing the two divisions 
of the life zone is the comparative continuity of the supply. In the 
humid division which extends from the Atlantic Ocean nearly to the 
Continental Divide, moisture in the form of rain and fog is received 
at short intervals throughout the year, whereas in the arid division 
there are long periods of drought. As a result of these contrasting 
conditions the leaves are persistent and a luxuriant evergreen forest, 
the so-called “rain forest,” is found on the Atlantic slopes, while 
in the arid division on the Pacific side the leaves are largely de- 
ciduous, the forests turn brown, and the savannas become parched in 
