218 THE FORMATION 
involucre that the latter serves as a sort of mortar for projection, when the 
stem of the plant is bent to one side by any force, such as the wind or an 
animal. It will be noticed that two separate agents are actually concerned in 
dissemination of this sort. 
Frequently, two or more agents will act upon the same disseminule, 
usually in succession. The possibility of such combinations in nature is 
large, but actual cases seem to be infrequent, except where the activities of 
man enter into the question. Some parts, moreover, such as awned inflor- 
escences, are carried almost equally well by wind or animals, and may often 
be disseminated by the cooperation of these two agents. The wind also 
often blows seeds and fruits into streams by which they are carried away, 
but here again, parts adapted to wind-dissemination are injured as a rule 
by immersion in water, and the number of plants capable of being scattered 
by the successive action of wind and water is small. 
In the present state of our knowledge of migration, it is impossible to 
establish any definite correspondence between dissemination-contrivance, 
agent, and habitat. As a general rule, plants growing in or near the water, 
in so far as they 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 exposed to the upper air currents, are generally wind-distrib- 
uted. There is then a fair degree of correspondence, inasmuch as most 
hydrophytes are hydrochorous, most hylophytes, zoochorous, and the ma- 
jority of poophytes and xerophytes, anemochorous. Definite conclusions 
can be reached, however, only by the statistical study of representative 
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. Jn the former, the direction is more or 
less determinate, and migration takes place year by year, i. e., it is contin- 
uous, while in the latter dissemination is largely an accidental affair, inde- 
terminate in direction, and recurring only at indefinite intervals. The 
effective conversion of migration into invasion is greatest when the move- 
ment is continuous, and least when it is discontinuous, since, in the latter, 
species are usually carried not only out of their particular 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-cur- 
