28 FAUNAL AREAS 
less closely to certain isotherms, or lines of equal temperature, but their 
eastern and western faunal subdivisions are determined by the annual 
precipitation of rain. Any factors such as altitude, exposure in relation 
to the sun, or proximity to water, which affect temperature exert a 
marked influence on the boundaries of faunas and must of course be 
considered in mapping faunal areas. 
Outside of purely tropical regions, as Merriam (’94) has shown, 
temperature is not active throughout the year, but only at certain 
seasons. Merriam has therefore formulated the following Laws of 
Temperature Control: First, “Animals and plants are restricted in 
northward distribution by the total quantity of heat during the season 
of growth and reproduction.” Second, ‘Animals and plants are re- 
stricted in southward distribution by the mean temperature of a brief 
period covering the hottest part of the year.’’ With birds, of course, 
it should be understood that the southern as well as northern limits 
of the breeding range are here referred to. 
Faunal Areas.—It is not possible for us to treat, even in outline, 
this absorbing and important subject, but for practical purposes, if 
for no other reasons, the student should become familiar with the 
boundaries of the faunal areas of North America, as well as the names 
of the birds which characterize them. This is the study of faunal 
geography, or zodgeography, as compared with that of political 
geography. 
Examination of maps showing (1) the ranges of the families, (2) the 
genera, and (3) the species of birds, reveals the fact that many families, 
genera and species are distributed, respectively, over essentially the 
same parts of the earth’s surface. It will also be found that these areas 
are occupied by other families, genera and species of animals as well 
as of plants. Such areas are therefore called natural life areas, and their 
rank conforms more or less closely to the systematic standing of the 
groups of animals inhabiting them. While the faunal terms employed 
are not always used in the same sense (see Merriam ’92, and Allen ’93), 
it may be said that families are distributed through regions, genera 
through zones, and species in faunas. 
North America, it will be observed on the cover map, here repro- 
duced by courtesy of the Biological Survey, is divided primarily into 
three Regions,* the Boreal, the Austral and the Tropical. The last, 
occupying only the southern extremities of Florida and Lower Cali- 
fornia, has in these limits no zonal subdivisions. 
Zones of the Boreal Region—The Boreal- Region includes three 
transcontinental zones, the Arctic, the Hudsonian, and the Canadian. 
The first-named extends southward to the northern limit of forests, 
and not only crosses this continent but is circumpolar; the uniform 
climatic conditions of the Arctic portions of both hemispheres, in con- 
nection with their comparative proximity, being responsible for essen- 
*For detailed consideration of the characteristics of these Regions consult 
Merriam ’98, 
