‘PHYSIOLOGICAL ANIMAL GEOGRAPHY 603 
Animals, on the other hand, have different habitats which are not 
related to one plane, and so must be separated into similar groups 
for purposes of the comparison of one formation with another. For 
example, the animals which burrow into the ground in a given 
environmental complex cannot be compared with those that live 
in trees in another, but must be compared with subterranean 
forms. Accordingly, for comparison, animals must be separated 
into: (a) burrowing forms, (b) ground forms, (c) arboreal forms, 
etc. 
1. Classification of animal formations based on environmental 
relations 
a. Principles of classification. We have noted that all of the 
animals of a given environmental unit constitute a formation, 
and that environmental units are classified into climatic or geog- 
raphic (extensive) and local. The groups of animals which occupy 
the climatic or geographic environments may be called ‘climatic 
or geographic animal formations.’ The groups of animals which 
occupy the local environments are called local formations (soci- 
eties, or associations). 
If one is to study the relation of animal physiology and behav- 
ior to the environmental conditions, in so far as this can be done 
by field study, these distinctions must be kept clearly in mind. 
For example, in dealing with animals of the great North American 
steppe area, to treat together all forms found here (as is common 
practice) would lead to endless confusion from our point of view. 
The forms which belong to the water (aquatic), those that live 
in the timber along the ravines, in the sand areas, are forms 
belonging to local formations. Those that occupy the plains 
proper belong to the steppe formation. Some forms may belong 
to both, in which case the facts should be taken into account.?° 
10 An animal should be associated: first, with the breeding conditions; second, 
with the feeding conditions; third, with the conditions affording shelter. Calvert. 
(08) attempted to find correlation between the distribution of Odonata and vege- 
tation zones with negative results. Aside from the reasons given by the author, 
it should be noted that Odonata breed in the water and, excepting forms breeding 
in water holding plants, belong to local conditions, and no correlation was to have 
been expected. Correlation of the distribution and species is, however, not essen- 
tial to our point of view. 
