602 VICTOR E. SHELFORD 
E. THE ANIMAL FORMATION 
Animals select their habitats, probably by trial and error, as 
is indicated by the making of additional holes and parts of holes 
by the tiger beetles only to reject them without laying eggs. 
The simple fact of selection is, we believe, very familiar to all 
naturalists. 
A given environmental complex is selected by a number of 
species. All of the animals of a given habitat constitute what is 
known as an animal formation (Warming, (’09); Clements, (’05) ; 
Schimper, (03); Adams, (’08); Grisebach, (’48, fide Clements). 
It follows that there is a certain physiological or ecological 
similarity and ecological equivalence in the forms that thus select 
the same or similar complexes. It follows® that the animals of 
different deserts, different deciduous forests, different steppes, 
etc., are ecologically and physiologically similar or ecologically 
equivalent if the deserts, the forests, and the steppes, etc., are 
similar (Adams ’05). 
Tentatively, formations may be characterized in general phys- 
iological or ecological terms (mores). The characterizations of 
plant formations have thus far been largely based on growth-form. 
Attempts to find structural similarity among animals of similar 
habitats, while not failing in particular cases, have led to no 
good results or generalizations (Ritter, 09). The great difficulty 
with this point of view is that it must, because of the great diffi- 
culty of investigation, remain for a long time largely a matter of 
speculation. The attempts which have been made are based on 
natural selection speculations or Lamarckian speculations. 
It should be noted further that the relations of a given group 
of animals to their habitat and to each other is more complex 
than that of the plants which are commonly treated in this manner. 
The conspicuous plants of a given environmental complex, except 
in the tropical forests, are usually rooted in a single plane which 
greatly simplifies the relations of plants to their environments. 
9 A term is needed to cover such characters. The term mores (Latin), ‘cus- 
toms,’ ‘behavior,’ ‘habits’ is suggested as best covering the need. It stands 
opposed to form and forms; thus steppe mores meaning the behavior of character- 
istic steppe animals or an animal or animals with characteristic steppe behavior. 
