Chap. V] 



THE SOIL 



105 



this dissimilar relation by means of the example of Achillea atrata and 

 A. moschata. 



Achillea moschata excludes A. atrata from a silicious soil and is itself excluded by 

 the latter from a calcareous soil. On the other hand, either of them grows equally 

 well in the company of A. Millefolium. Evidently the two first-mentioned plants, as 



y- 



Fig. 55. 1. Achillea atrata. Calciphilous. 2. Achillea moschata. Calciphobous. Natural size. 



they are externally extremely alike, make analogous demands on the environment. 

 A. Millefolium, however, which systematically is further removed from either, does 

 not compete with them, because it is dependent on other conditions of existence. 

 If either of the two species be absent, the other becomes indifferent as regards its 

 choice of soil. 

 ' In Bernina-Heuthal (Upper Engadine), A. moschata, A. atrata, and A. Millefolium, 



