j66 GEORGE E. NICHOLS Vol. IV, No. 2 



tinction whatever can be drawn between the physiographic formation and the 

 association : they are one and the same thing. In other words, the vegetation 

 in its entirety can here be referred to either as an association or as a physi- 

 ographic formation. 



Again, it is often difficult to draw sharp lines between different individual 

 physiographic formations. A rugged rocky upland, for example, is occupied 

 very largely by a characteristic formation. Here and there, however, associ- 

 ated with minor variations in topography, are scattered representatives of 

 swamp and ravine formations. In regard to cases of this sort it need only 

 be said that, like other cases of overlapping or intergrading conditions, they 

 are best treated in the light of conditions which prevail in situations where 

 these complications do not exist. 47 



Quite different is the situation in cases where essential similarity in vege- 

 tation prevails over areas which are physiographically different from one 

 another. In attempting to explain such conditions it should be recalled, first 

 of all, that the ecological characteristics of any piece of vegetation are the 

 result of the combined influence of all the locally effective factors of the 

 environment. Leaving out of consideration the adaptiveness of the various 

 plants which go to build up the vegetation to a more or less wide range of 

 habitats, it might seem at first thought that if two geographic unit-areas differ 

 from one another in any appreciable degree, there ought to be a corresponding 

 difference in the vegetation. That this is not actually the case is due pri- 

 marily to the fact, already suggested, 48 that the influence of one habitat factor 

 (or set of factors) may be offset or compensated by the influence of another 

 habitat factor (or set of factors). Thus the favorable influence of humus 

 accumulation and shade may offset to such a degree the influence of physi- 

 ographic factors that two areas which are physiographically unlike may sup- 

 port essentially the same type of vegetation. Again, to take an extreme 

 example, in the humid tropics the influence of congenial climatic factors may 

 so completely offset that of physiographic factors that vegetation practically 

 everywhere, regardless of the nature of the topography and soil, tends to 

 merge into one great association or association-type. 



But one other complication need be mentioned. Generally speaking, the 

 ecological characteristics of a physiographic formation are determined by the 

 combined influence of both climate and physiography. In so far as climatic 

 factors influence the nature of the vegetation, then, the same physiographic 

 formation would hardly be expected to occur in regions of different climate. 

 Yet in some cases the influence of climatic factors is so overshadowed by that 

 of physiographic factors that the same physiographic formation develops 

 under varying climatic conditions. This condition is especially well illus- 

 trated by the salt marsh formation, which exhibits essentially the same physi- 



" See in this connection the quotation from Ganong on p. 155. 



43 P. 160; but see especially in this connection Nichols '17, pp. 315, 316. 



