INVASION 219 
rents, and wind, and slight when the movement is due to slope, growth, or 
propulsion. Disregarding the great distances over which artificial trans- 
port may operate, seeds may be carried half way across the continent in a 
week by strong-fiving birds, while the possibilities of migration by growth 
or expulsion are iimited to a few inches, or at most to a few feet per year. 
This slowness, however, is more than counterbalanced by the enormously 
greater number of disseminules, and their much greater chance of becoming 
established. 
268. The direction of migration is determinate, except in the case 
of those distributive agents which act constantly in the same direction. 
The general tendency is, of course, forward, the lines of movement radiating 
in all directions from the parent area. This is well illustrated by the opera- 
tion of winds which blow from any quarter. In the case of the constant 
winds, migration takes a more or less definite direction, the latter being 
determined to a large degree by the fruiting period of any particular species. 
In this connection, it must be kept clearly in mind that the position of new 
areas with reference to the original home of a species does not necessarily 
indicate the direction of migration, as the disseminules may have been 
carried to numerous other places in which ecesis was impossible. The local 
distribution of zoochorous species is of necessity indeterminate, though 
distant migration follows the pathways of migratory birds and animals. In 
so far as dissemination by man takes place along great commercial routes, 
or along highways, it is determinate. In ponds, lakes, and other bodies of 
standing water, migration may occur in all directions, but in ocean currents, 
streams, etc., the movement is determinate, except in the case of motile 
species. The dissemination of plants by slopes, glaciers, etc., is local and 
definite, while propulsion is in the highest degree indeterminate. Migration 
by growth is equally indefinite, with the exception that hydrotropism and 
chemotropism result in a radiate movement away from the mass, while 
propulsion throws seeds indifferently into or away from the species-mass. 
From the above it will be seen that distant migration may take place by 
means of water, wind, animals or man, and, since all these agents act in 
a more or less definite direction over great distances, that it will be in 
some degree determinate. On the other hand, local migration will as 
regularly be indeterminate, except in the case of streams and slopes. The 
direction of migration, then, is controlled by these distributive agents, and 
the limit of migration is determined by the intensity and duration of the 
agent, as well as by the character of the space through which the latter 
operates, 
