FAUNAL DIVISIONS OF EASTERN NORTH AMERICA. 209 



representing the real conditions. These large aggregates of temperature savor 

 too much of a drag-net. Furthermore, this " temperature control " theory ignores 

 several very real and vital factors in the distribution of animals, such as the 

 character of the vegetation and related soil conditions, the influence of local 

 habitat areas, throughout a region, and the fundamental fact that the species 

 which embody a fauna are mobile elements, not hard and fast fixtures in their 

 environment, and that their present phase of distribution is a part of their pn 

 history. 



In a paper read before the Delaware Valley Ornithological Club in March, 

 1909, 1 I stated my belief that " species and faunas alike are but pa sing 

 phases in the vast cosmic processes of a continent's history," and that there ifl 

 evidence, at least among birds, "that our so-called 'faunas' — Carolinian, Alle- 

 ghanian, and Canadian — in reality represent a somewhat temporary state of 

 groups of species in relation to breeding areas, and the more or less arbitrary 

 boundaries of these faunas represent our knowledge only of the present conditions 

 of distribution in a gradual and general northward movement of considerable 

 antiquity." The distribution of animal species is unquestionably closely 

 associated with the distribution of vegetation, and this in turn is influenced by 

 the climatic factors of temperature and moisture, and by soil conditions. A 

 certain type of vegetation may be said to form a unit of distribution for < rtain 

 species of animals, and these species will spread throughout the area covered by 

 this type of plant growth. During their long history they have become more 

 or less adapted to the vegetational and to the climatic conditions of t he area in 

 question and have spread with the spread of the particular kind of vegetation 

 that characterizes it. An assemblage of animal species related in this way to 

 certain broad vegetation areas constitutes a fauna. Within the limits of Mich 

 an area certain diversities occur as the result of varied topographical, climatic, 

 and soil conditions which produce certain more or less local characteristic forms 

 of vegetation. These may be regarded as habitats, and the assemblage of animal 

 species formed there as constituting a habitat group or type. Each fauna and 

 its several types must be looked upon as representing only an existing phase of 

 distribution, the reason for its present existence as a fauna or type being the 

 result of a long series of past events connected with the geological history of the 

 land during later Tertiary and Quaternary times. The influence of the last 

 glacial period, and the widespread migrations of animals introduced various 

 species and genera from outlying areas, as well as affecting the indigenous forms. 

 The components of any fauna or type may be regarded as its elements, and the 

 chief problem connected with these is as to their origin, and their historical 

 relation to the particular fauna in which they occur. 2 



Schimper has pointed out that the differentiation of the earth's vegetation 



1 Trotter, The Geological and Geographical Relations of the Land-Bird Fauna of Northeastern 

 America, The Auk, vol. XXVI, July, 1909, pp. 221-233. 



1 For an interesting discussion of the subject see a paper by Charles C. Adams on The Postglacial 

 Dispersal of the North American Biota, Biological Bulletin, vol. IX, No. 1, June, 1905, pp. 53-71. 



14 JOURN. ACAD. NAT. SCI. PHILA- VOL. XV. 



i 



