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-flying birds, while the possibilities of migration by growth 

 or expulsion are limited 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 wdiich 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 w^as 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 wdiich the latter 

 operates. 



