INVASION 227 
a definite region, as it is only in this restricted sense that complete barriers 
have produced endemism. 
Distance, though hardly to be considered a barrier in the strict sense of 
the word, unquestionably plays an important part in determining the amount 
of invasion. The effect of distance is best seen in the case of migration, as 
it influences ecesis only in those rare cases where viability is affected. The 
importance of distance, or take the converse, of proximity, is readily ascer- 
tained by the study of any succession from denudation. It has been es- 
tablished that the contiguous vegetation furnishes 75-90 per cent of the 
constituent species of the initial formation, and in mountainous regions, 
where ruderal plants are extremely rare, the percentage is even higher. The 
reason for this is to be found not only in the fact that the adjacent species 
have a much shorter distance to go, and hence will be carried in much 
greater quantity, but also in that the species of the formations beyond must 
pass through or over the adjacent ones. In the latter case, the number of 
disseminules is relatively small on account of the distance, while invasion 
through the intermediate vegetation, if not entirely impossible, is extremely 
slow, so that plants coming in by this route reach the denuded area only 
to find it already occupied. It is as yet impossible to give a definite numeri- 
cal value to proximity in the various invasions that mark any particular 
succession. This will not be feasible until a satisfactory method has been 
found for determining a coefficient of mobility, but, this once done, it will 
be a relatively simple matter, not merely to trace the exact evolution of any 
succession of formations, but actually to ascertain from the adjacent vege- 
tation the probable constitution of a particular future stage. 
From what has been said, it follows that the primary effect of barriers 
upon vegetation is obstruction. Where the barrier is in the pathway of 
migration, however, it causes deflection of the migrant as a rule, and sets 
up migration in a new direction. This is often the case when the strong 
winds of the plains carry disseminules towards the mountains and, being 
unable to cross the range, drop them at the base, or, being deflected, carry 
them away at right angles to the original direction. The same thing happens 
when resistant fruits and seeds borne by the wind fall into streams of water, 
or into ocean currents. The direction of migration is changed, and what is 
normally a barrier serves as an agent of dissemination. 
ENDEMISM 
276. Concept. Since its first use by DeCandolle, the term endemic has 
been employed quite consistently by phytogeographers with the meaning 
of “peculiar to a certain region.’””’ Some confusion, however, has arisen 
from the fact that a few authors have made it more or less synonymous 
