600 VICTOR E. SHELFORD 
4. Tentative laws of distribution 
On this general basis tentative laws of distribution may be 
formulated. 
a. Governing the limit of geographic range. The geographic 
range of any species is limited by the fluctuation of a single factor 
(or factors) beyond the limit tolerated by that species. In non- 
migratory species the limitations are with reference to the activ- 
ity which takes place within the narrowest limits. In migratory 
species this activity limits the range only during a part of the life 
cycle. 
b. Governing distribution area. The distribution area of a 
species is the distribution of the complete environmental complex 
within which it can live as determined (1) by the activity which takes 
place within the narrowest limits and (2) by the animal’s power of 
migration. Barriers in which some one factor of the complex 
fluctuates beyond the limits of toleration of the species at all 
periods of its life-history may prevent the animal from reaching 
all the suitable habitats, but this is the result of the working of 
the laws rather than an exception, and faunistic animal geography 
begins where physiological animal geography ends. 
D. CLASSIFICATION OF ANIMAL ENVIRONMENTS 
While this is a necessary subject for discussion, it is with much 
hesitancy that I undertake it here, where brevity is necessary. 
Obviously, since our subject is physiological animal geography, 
we shall confine our attention mainly to those aspects which are 
geographic in extent in the sense that they are nearly uniform 
over a considerable area of the earth’s surface. 
If one is to understand the most elementary principle of syn- 
ecology,’ he must first recognize the distinction between local 
(edaphic, Schimper, ’03; minor and secondary, Adams, ’08), and 
climatic or geographic (extensive) environmental complexes (major, 
* Synecology is the ecology of formations. In the classification of formations 
and environments, no nomenclature has been established for the larger or cli- 
matic units. Dr. Cowles tells me that plant formations do not represent climate 
and therefore ‘climatic’ should not be used. However, every ecologist and geog- 
rapher knows the significance of ‘climatic’ and ‘local.’ The geographers object 
