INVAS[ON 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 



