342 VICTOR E. SHELFORD. 



among animals as the separation of plants into herbs, shrubs, 

 and trees, has among plants. They merely represent, in the 

 main, the different taxonomic groups primarily capable of occupy- 

 ing different strata or the like. Here and there an aberrant 

 member has become adjusted in size or growth form to another 

 stratum. Climatic grassland formations (Shelford, 'n 3 ) may 

 perhaps be characterized by the absence of arboreally adapted 

 animals but even here we may find exceptions, for Didelphys azore 

 lives in grassland (Hudson, '03) but is unmistakably "adapted" 

 to an arboreal life. Again in the rain forest of New Guinea, 

 we find a tree kangaroo which is poorly adapted to arboreal 

 life (Wallace, '69, p. 386). 



When one brings together all the motile animals of a given 

 stratum, in a forest or other type of habitat, and examines them 

 with a view to classifying and generalizing concerning them, 

 from the point of view of structure, he finds himself confronted 

 with a hodge-podge of the so called "primitive," "adapted" and 

 "unadapted," without the possibility of making any generaliza- 

 tions concerning them or of classifying them into structural 

 ecological groups. 



Among motile animals, structure must be considered in con- 

 nection with activities, reactions, and general physiology as a 

 limiting factor to be taken into account in many cases. It is 

 often important in considering the ecological equivalence (Shel- 

 ford, 'n 3 ) of similar communities or of the animals of the same 

 community. Ecologically there is rarely reason for considering the 

 structure of motile animals separate from activity. Accordingly 

 it seems best to reject separate consideration of adaptation and to 

 treat all questions of the structure of motile animals as structures 

 playing a role in the physiology and activity of the organism. This 

 includes all the important aspects without raising the question 

 of adaptation or of the origin and genetic significance of such 

 structures concerned. Even the relations of the animals to the 

 strata are to be considered as primarily physiological relations 

 to differences in physical conditions, such as were brought to 

 attention in the preceding paper. Many animals invade two 

 or more strata and are usually to be classed primarily in the 

 stratum in which they breed. On account of this invasion of 



