INVASION 



235 



hand, since a species may often be carried far from its geographical area, 

 it is frequently of great significance in distribution. 



283. Complete and partial invasion. When the movement of invaders 

 into a formation is so great that the original occupants arc finally driven out, 

 the invasion may be termed complete. Such invasion is found regularly in 

 the case of many ruderal formations, and is typical of the later stages of 

 many successions. It is ordinarily the result of continuous invasion. If 



Fig. 59. Continuous invasion into a new area; mats of Arcnaria 

 sajanensis. Silcne acaulis and Sieversia turbinata invading an alpine 

 gravel slide. 



the number of invaders is sufficiently small that they may be adopted into 

 the formation without radically changing the latter, the invasion is partial. 

 This is doubtless true of the greater number of invasions, though these are 

 regularly much less striking and important than instances of complete 

 invasion. 



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. 



