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 

 whith seem to be perfectly established for a few years, only to dis- 

 appear completely. The latter are examples of temporary 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 arc 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 

 2nd degree of invasion will be determined by the presence or absence of 

 vegetation. Areas without vegetation are either originally naked or de- 



