BARRIERS 61 



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 vegeta- 

 tion, 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 numerical 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 deter- 

 mining 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 vegetation the probable consti- 

 tution of a particular future stage. 



Prom 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 moun- 

 tains 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. 



Wildenow (1811:98) supposed that each great mountain 

 range had its own peculiar species, which had wandered 

 down into valleys and lowlands and had produced a corres- 

 ponding flora. Upon this basis, wholly erroneous as it was, 

 he established a floral region to correspond to each principal 

 mountain range. Brisseau-Mitbel {t&i5 : 582) defined indigenous 

 plants as those which are natural to the soil in which they 

 grow, and exotics as those not natural to the country inhabit- 

 ated; but apparently he referred only to cultivated plants. 

 DeCandolIe (J820j5J) used indigenous in the sense of native 



