KINDS OF INVASION 79 



these are regularly much less striking and important than 

 instances of complete invasion. 



The permanence of invasion depends upon the success at- 

 tending 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 con- 

 ditions sufficiently favorable for propagation, but unfavor- 

 able 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 ex- 

 amples of temporary invasion {invasio temporalis). 

 It is necessary to draw clearly the line between complete 

 and partial invasion in this connection. The former is tem- 

 porary in the initial or intermediate stages of nearly all suc- 

 cessions, 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 be- 

 tween 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 per- 

 manence. Species which invade stable vegetation tempor- 

 arily have been termed adventive by A. DeCandoIIe (t855t 

 643). Permanent invasion {invasio diutina) 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 

 (species indigenae). Contrasted with these are the species 

 which have invaded the formation since it received its dis- 

 tinctive impress : these are derived {species derivatae sen 

 advenae). The determination of the indigenous and derived 

 •pecies of a formation or larger division is of the utmost im- 



