CHAP, xxxiv OECOLOGICAL CLASSES 133 



tribution. In this sense we must interpret Schimper's division of forma- 

 tions into climatic and edaphic. As an example, it may be mentioned 

 that Whitford and Cowles 1 interpret coniferous forest in the eastern 

 United States as an edaphic, xerophytic formation occurring in the 

 district where deciduous forest prevails ; but in the entirely different 

 climate near the Pacific coast of the United States coniferous forest is 

 dominant, while the deciduous trees give rise to edaphic, mesophilous 

 forest lining the watercourses. 



The occurrence of the same species in different formations renders 

 explanation difficult ; Cowles, no doubt correctly, states that a species 

 4 in general can grow in the largest number of formations at its centre 

 of distribution, since there the climatic conditions favour it most highly. 

 In other regions, especially near its areal limits, it can grow only in those 

 formations which resemble most closely in an edaphic way the climatic 

 features at the distribution centre.' Cowles also contends that in many 

 cases a species can occupy very different soils (for instance, clay or dune- 

 sand) only when the atmospheric conditions are the same, and that 

 conversely on one and the same soil very different vegetation may prevail 

 when the atmospheric conditions are changed. An exception is provided 

 by humus soil, for there are many species to which humus is absolutely 

 essential. As an example showing that a species may inhabit entirely 

 different kinds of soil, we may quote C. Schroter's 2 statement in reference 

 to the mountain-pine. He writes : ' How fundamentally diverse are 

 the habitats of the mountain-pine which can grow on the dry, loose, 

 calcareous talus of a hot southern slope, and on high-moors, which are 

 mainly composed of bog-mosses and are subalpine bogs dripping with 

 water but poor in mineral matter. The former soil is poor in humus, 

 rich in mineral matter, and dry ; the latter is a substratum rich in humus, 

 poor in mineral matter, and always saturated with water. Common to 

 both is only one character poverty in assimilable nitrogen.' 3 



When endeavouring to arrange all land-plants, omitting marsh- 

 plants, into comprehensive groups, we meet with, first, some communities 

 that are evidently influenced in the main by the physical and chemical 

 characters of soil which determine the amount of water therein ; secondly, 

 other communities in which extreme climatic conditions and fluctuations, 

 seasonal distribution of rain, and the like, decide the amount of water 

 in soil and character of vegetation. In accordance with these facts, 

 land-plants may be ranged into groups, though in a very uncertain 

 manner. The prevailing vagueness in this grouping is due to the fact 

 that oecology is only in its infancy, and that very few detailed investigations 

 of plant-communities have been conducted, the published descriptions of 

 vegetation being nearly always one-sided and floristic, as well as very 

 incomplete and unsatisfactory from an oecological standpoint. It is 

 to be hoped that Clements's remarks, in his Research Methods in Oecology 

 will stimulate detailed research and will banish the ignorance to which 

 he alludes in the following terms : ' Our knowledge of soil-factors is in an 



1 Cowles, 1 901 a; Whitford, 1905. 



2 Schroter, 1904. See also P. E. Muller, 1887, 1871. 



3 Also may be added the character of dryness physical in the first, physio- 

 logical in the second. See p. 134. 



