104 



THE FACTORS 



[Part I 



hoiuever, change with the area 1 . In one area the silica-form, in another 

 the lime-form, is better adapted to local conditions, whilst in a third area 

 both forms may be able to maintain themselves in the struggle for existence. 

 Accordingly, one and the same species is calciphobons in the first area, 

 calciphilous in the second, and indifferent in the third. 



An instructive example of the dissimilar physiological properties of 

 the lime-form and the silica-form of the same species of plant is afforded 

 by Pinus uncinata. This pine in its lime-form, at least in Switzerland 

 and Bavaria, prefers dry gravel, whilst in its silica-form it avoids stony 



dry spots and occurs only 

 on moorlands. In other 

 regions, with a different cli- 

 mate, probably both forms 

 would behave in another 

 manner. 



Nageli and Christ have 

 proved, in the case of 

 closely allied species of 

 Gentiana, Achillea, and 

 Rhododendron, that closely 

 allied plant-organisms be- 

 have very differently as re- 

 gards the chemical quality 

 of their substratum. Thus, 

 in Switzerland, Gentiana 

 acaulis is calciphilous, where- 

 asthecloselyallied Gentiana 

 excisa (Fig. 54), which is 

 usually considered as a 

 mere variety of the former, 

 is calciphobous ; neither of 

 them is quite exclusive 

 in its choice. Similar pairs, although less like, are Achillea atrata and 

 A. moschata (Fig. 55), Rhododendron hirsutum and R. ferrugineum, 

 Androsace pubescens and A. glacialis, Juncus Hostii and J. trifidus, 

 of which pairs the species first named is calciphilous. Kerner has drawn 

 up a long list of such parallel species 2 . 



The parallel forms are usually confined to their respective soils in the 

 regions where both occur, but are indifferent as regards their choice of 

 soil, wherever one of them is absent. Nageli 3 has ingeniously indicated 



1 The remarkably great susceptibility of a plant-organism to insignificant external in- 

 fluences follows from Wettstein's brilliant researches on Gentiana and Euphrasia. 



2 Kerner, I. s Nageli, op. cit. 



Fig. 54. 1. Gentiana excisa, Presl. Calciphobons. 2. Gen- 

 tiana acanlis, L. ex p. Calciphilous. Two-thirds of natural 

 size. 



