48 INVASION 



are modified for this purpose at all, are adapted to water- 

 carriage. Species which grow in exposed grassy or barren 

 habitats are for the most part anemochores, while those that 

 are found in the shelter of forests and thickets are usually 

 zoochorous, though the taller trees and shrubs, being ex- 

 posed to the upper air-currents, are generally wind-distri- 

 buted. There is then a fair degree of correspondence, in 

 as much as most hydrophytes are hydrochorous, most hylo- 

 phytes, zoochorous, and the majority of poophytes and 

 xerophytes, anemochorous. Definite conclusions can be 

 reached, however, only by the statistical study of represent- 

 ative formations. 



With respect to their activity, agents may be distinguished 

 as constant, as in the case of currents, streams, winds, 

 slope, growth, and propulsion, or intermittent, animals and 

 man. In the former, the direction is more or less determin- 

 ate, and migration takes place year by year, i.e., it is con- 

 tinuous, while in the latter dissemination is largely an 

 accidental affair, indeterminate in direction, and recurring 

 only at indefinite intervals. The effective conversion of 

 migration into invasion is greatest when the movement is 

 continuous, and least when it is discontinuous, since, in the 

 latter, species are usually carried not only out of their par- 

 ticular habitat but even far beyond their geographical area, 

 and the migration, instead of being an annual one with the 

 possibility of gradual adjustment, may not recur for several 

 years, or may, indeed, never take place again. The rapidity 

 of migration is greatest in the case of intermittent agents, 

 while the distance of migration is variable, being great 

 chiefly in the case of man, ocean-currents and wind, and 

 slight when the movement is due to slope, growth or pro- 

 pulsion. Disregarding the great distances over which arti- 

 ficial transport 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 



