238 Merriam—Laws of Temperature Control. - 
controlling cause. Humidity governs details of distribution 
of numerous species of plants, reptiles and birds, and of a few 
species of mammals, within the several temperature zones. 
Thus the palmetto, the green chameleon, the chuck-wills widow 
and the ricefield mouse inhabit humid parts of the Lower Aus- 
tral zone (the Austroriparian area), while the mesquite, the 
leopard lizard, the sickle-billed thrashers and the four-toed. kan- 
garoo rats find their homes in arid parts of the same zone (the 
Lower Sonoran area). 
That humidity is less potent than temperature as a controll- 
ing factor in distribution may be shown in several ways. The 
numerical evidence I have given on ‘a previous occasion.* 
Equally convincing is the circumstance that many genera re- 
stricted to particular conditions of temperature range completely 
across the continent, inhabiting alike the humid and arid sub- 
divisions of their respective zones; but no genus restricted to 
particular conditions of humidity ranges north and south across 
the several temperature zones. 
Humidity and other secondary causes determine the presence 
or absence of particular species in particular localities within 
their appropriate zones, but temperature predetermines the pos- 
sibilities of distribution ; it fixes the limits beyond which species 
cannot pass; it defines broad transcontinental belts within which 
certain forms may thrive if other conditions permit, but outside 
of which they cannot exist, be the other conditions never so 
favorable. 
EXPLANATION OF MAps. 
The temperature maps show the isotherms that conform to the boun- 
daries of the life zones and the data on which they are based. The spots 
show the actual positions of the temperature stations. 
In map 12, showing the distribution of the sum of effective tempera- 
tures, the isotherms conform with the northern boundaries of the life 
zones (as Shown on map 14) as follows: The isotherm of 14,500° C. con- 
forms with the northern boundary of the Tropical; of 10,000° C. with 
that of the Lower Austral; 6,400° C. with that of the Upper Austral, and 
5,900° C. with that of the Transition. 
In map 13, showing the normal mean temperature of the six hottest 
consecutive weeks, the isotherms conform with the southern boundaries 
of the life zones (as shown on map 14) as follows: The isotherm of 18° 
C. with the sonthern boundary of the Boreal; of 22° C. with the south- 
ern boundary of the Transition, and 26° C. with the southern boundary 
of the Upper Austral. 
* Presidential Address, Ibid., pp. 47-49. 
