=284. Permanent and temporary invasion.= The permanence of invasion depends upon the success attending ecesis, and upon the stability of the formation. It has already been noticed that under certain conditions plants may germinate and grow, and if they are perennials, even become established, 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 disappear 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 particular 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 permanently 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 succeeded in establishing themselves. Finally, those derived species which are unable to establish themselves permanently are _adventive_. _MANNER OF INVASION_