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 been 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 cattse 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 Evernia, Ramalina, 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, Lecidea, etc., occur on the rocks. In the 
alpine meadows, the rock groups are composed of Parmelia, Gyrophora, 
