CLIMATES AND HABITATS. 357 



constantly reproducing the adult or cKmax organism. The formation of the 

 ice-mass in the Pleistocene modified the details of the vegetation profoundly, 

 but it only emphasized the essential relations of the zones and alternes. While 

 the resulting climatic differentiation doubtless had some part in sorting out 

 the population, its greatest effect was probably exerted in the shifting of 

 preexisting zones and climaxes. 



CLIMATES AND HABITATS. 



Relation of habitat to climate. — In the preceding chapters, no attempt has 

 been made to define or delimit the habitat concretely. While its basic 

 importance in succession has been repeatedly emphasized, this has been in 

 the general sense of a complex of controlHng factors, modified by reaction. 

 The actual delimitation of concrete areas as habitats, and hence the possi- 

 biUty of defining the term itself in a concrete fashion, must await a greater 

 increase in the amount of exact study by means of instruments. Certain 

 theoretical considerations, however, seem to suggest the final and definite form 

 of a concept of the habitat, in which our present subjective ideas wiU disappear. 

 In the discussion of the relations of habitat and sere, it was repeatedly pointed 

 out that they showed a progressive interaction. The physical factors of a 

 bare area determine the pioneers, which then react upon the habitat in such 

 a way as to form soil and modify the deficit or excess of water-content. As 

 it is thus slowly changed, the habitat modifies its selective action upon invaders 

 and a new stage of the sere gradually develops. This in turn reacts upon the 

 habitat, and the latter again modifies its selective action in consequence. 

 This progressive interplay of reaction and selection continues until the cUmax 

 is fully established, when it stops. This is due in the first place to the reaction 

 of the climax dominants, which is so controlUng that all other dominants are 

 excluded. In short, habitat and climax have reached an equilibrium for the 

 fiiTst time in the development of that particular sere. Back of this equihbrium, 

 however, is the climate. The stage at which the balance is attained and 

 development stops is fixed by the factors of the climate. The proof of this 

 is taken for granted, since it is furnished by the detailed study of every sere. 

 Moreover, it is strikingly in evidence in a mountain clisere, where each climax 

 zone, from the deciduous or evergreen forest at the base to the lichen and moss 

 communities of the highest peaks, is terminated in an earher or lower stage 

 than the preceding. An efficient change of climate would at once change the 

 habitat, this would modify the reaction of the community, and the two would 

 again interact and develop mutually until the new climatic Umit is reached 

 and equilibrixmi again results. 



In the concept here suggested the habitat and sere are regarded as related 

 processes or aspects of the same development to a climax. In short, the 

 habitat is itself regarded as marked by development in the same way that the 

 formation is. Hence it either becomes necessary to distinguish definitely 

 between developmental and climax habitats, or, better still, to harmonize fully 

 the concept of habitat and formation. In this event the habitat would become 

 the plexus of physical and biological factors which persist in a climax area as 

 long as the climate remains essentially unchanged, as measured in terms of 

 vegetation. It would have a progressive development all its own, but similar 

 in a general way to that of the climax and in the closest contact with it at all 



