IN DETAIL 293 



the physical factors and the species both demonstrate that it is the forest, 

 and not its parts, which corresponds to a distinct physical entity, the hab- 

 itat. This test of a formation is superfluous in a great many cases, where 

 the physiognomy of the contiguous areas is conclusive evidence of their 

 difference. It is evident also that remote regions which are floristically dis- 

 tinct, such as the prairies and the steppes, may possess areas physically al- 

 most identical and yet be covered by different formations. This point is 

 further discussed under classification. 



The existing confusion in the matter of formations is due to two causes. 

 The first arises from the fact that much ecological work has been hasty. 

 Little or no attention has been given to development, and in consequence 

 rudimentary and transitory stages of succession have often beoi described 

 as formations. Mixed areas in particular have caused trouble. In the sec- 

 ond place, there has been a marked tendency to minimize the need of thor- 

 oughness and training by calling every slightly different area a formation. 

 A failure to recognize the primary value of alternation has also contributed 

 materially to this. Alternating facies, and principal species, when separated 

 from each other, have often been mistaken for formations. This is a danger 

 that must be fully appreciated and guarded against. In practically all re- 

 gions, the same formation is represented by numerous scattered areas, all 

 showing greater or less differences arising from alternation. This is espe- 

 cially true of thickly populated regions where virgin areas are rare. The 

 fact that twenty-five miles intervene to-day between two small stretches of 

 primitive prairie is permitted to unduly emphasize their differences. It re- 

 quires the study of a number of such examples to counteract this tendency, 

 and to cause one to see clearly that they must have been at one time merely 

 so many bits of the prairie formation. 



In this connection, the lichen and moss groups which are found on rocks 

 constitute an interesting problem. It is clear that Peltigera and Cladonia, 

 which grow on the forest floor, and Ez'crnia, Rainalina, and Physcia, which 

 are found on the trees, are merely constituent species of the forest forma- 

 tion. The same is true of Cladonia, Urceolaria, and Parmelia, which are 

 found among the sedges and grasses of alpine meadows. The physical con- 

 ditions are essentially those of the formation, and the lichens themselves 

 are more or less peculiar to it. This is particularly true of the forest, in 

 which the two strata, bark and moist shaded soil, are present because of the 

 trees. In the case of granitic rocks, the circumstances are very different. 

 The species of lichens found on the rocks are not peculiar to the formation, 

 but they also occur elsewhere. In the forest, Parmelia, Placodium, Physcia, 

 Rinodina, Urceolaria, Lecanora, Lecidca, etc., occur on the rocks. In the 

 alpine meadows, the rock groups are composed of Parmelia, Gyrophora, 



