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 are 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 Arenaria 
Sajanensis. Silene 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, 
