78 INVASION 



is an annual movement from one into the other, and at the 

 same time a forward movement through each, result- 

 ing from the invaders established the preceding year. By 

 far the greater amount of invasion is of this sort, as may 

 readily be seen from the fact that migration varies inversely 

 as the distance, and ecesis may decrease even more rapidly 

 than the distance increases. The significant feature of con- 

 tinuous invasion is that an outpost may be reinforced every 

 year, thus making probable the establishment of new 

 outposts from this as a centre, and the ultimate extension of 

 the species over a wide area. The comparatively short dis- 

 tance and the regular alternation of migration and ecesis 

 render invasion of this sort very effective. An excellent 

 illustration of this is seen in transition areas and regions, 

 which are due directly to continuous and usually to mutual 

 invasion. Intermittent invasion results commonly from 

 distant carriage, though it may occur very rarely between 

 dissimilar adjacent formations, when a temporary swing in 

 the physical factors makes ecesis possible for a time. It is 

 characterised by the fact that the succession of factors 

 which have brought about the invasion is more or less acci- 

 dental, and may never recur. Intermittent invasion is 

 relatively rare, and from the small number of disseminules 

 affected, it is of little importance in modifying vegetation 

 quantitatively. On the other hand, since a species may often 

 be carried far from its geographical area, it is frequently of 

 great significance in distribution. 



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 {invasio perfecta). Such 

 invasion is found regularly in the case of many ruderal for- 

 mations, and is typical of the later stages of many suc- 

 cessions. It is ordinarily the result of continuous invasion. 

 If the number of invaders is sufBciently small that they may 

 be adopted into the formation without radically changing the 

 latter, the invasion is partial {invasio imperfecta). This is 

 doubtless true of the greater number of invasions, though 



