236 THE FORMATION 
and still ecesis be so imperfect that reproduction is impossible. Others may 
find the conditions sufficiently favorable for propagation, but unfavorable 
for the formation of flowers and fruits. Finally there are plants 
which seem to be perfectly established for a few years, only to dis- 
appear completely. The latter are examples of tentporary invasion, It is 
necessary to draw clearly the line between complete and partial invasion in 
this connection. The former is temporary in the initial or intermediate 
stages of nearly all-successions, as compared with the ultimate stages, 
though it is in a large degree permanent in comparison with the partial 
invasion of species which are able to maintain themselves for a few years. 
In a sense, there is a real distinction between the two, inasmuch as a par- 
ticular stage of succession is permanent as long as the habitat remains 
essentially the same. A critical study of the species of such stages shows, 
however, that they manifest very different degrees of permanence. Species 
which invade stable vegetation temporarily have been termed adventive by 
A. DeCandolle. Permanent invasion occurs when a species becomes per- 
manently established in a more or less stable formation. It is characteristic 
of the great majority of invaders found in the grassland and forest stages 
of successions. 
Plants which have arisen within a formation or have been a constituent 
part of it since its origin are indigenous. Contrasted with these are the 
species which have invaded the formation since it received its distinctive 
impress: these are derived. The determination of the indigenous and 
derived species of a formation or larger division is of the utmost importance, 
as it enables us to retrace the steps by which the formation has reached its 
present structure, and to reconstruct formations long since disappeared. To 
render it less difficult, it is necessary to scrutinize the derived elements 
closely, first, because it is easiest to recognize the indigenous species by 
eliminating the derived, and second, because this analysis will show ‘that 
not all derived species have entered the formation at the same time and 
from the same sources. Derived species may be termed vicine, when they 
are fully established invaders from adjacent formations or regions, and 
adventitious, when they have come from distant formations and have suc- 
ceeded in establishing themselves. Finally, those derived species which are 
unable to establish themselves permanently are adventive. 
MANNER OF INVASION 
285. Entrance into the habitat. Since the ecesis of invaders depends in 
large measure upon the occupation of the plants in possession, the method 
and degree of invasion will be determined by the presence or absence of. 
vegetation. Areas without vegetation are either originally naked or de- 
