GEOGRAPHICAL DISTRIBUTION 1041 



Dispersal and Migration. 



It will help US to realize the dependence of organisms on their 

 environment if we distinguish two modes of distribution, the in- 

 voluntary and the voluntary. The former may be called dispersal; 

 the latter, migration. 



Dispersal is the distribution of animals and plants by causes not 

 primarily involving the activities of these organisms, as the carrying 

 of the seeds of plants by the wind, the carrying of marine or fresh 

 water planktonic or meroplanktonic organisms by currents and the 

 like. Migration, on the contrary, is accomplished by active move- 

 ments in search of food, or to escape from enemies, and is confined 

 chiefly to animals, though stolonal propagation of plants may also be 

 classed as a species of migration. Migrants are composed of the 

 nekton and the vagrant benthos, while dispersants comprise the holo- 

 plankton, the epiplankton, and the meroplankton. Sedentary 

 benthonic organisms cannot migrate, but they may be carried by 

 a floating sub-stratum, and their mero-planktonic young may be 

 dispersed and thus settle in other districts. Migrating organisms 

 require a continuity of the conditions of existence in space, such 

 as continuous shores for the marine vagrant littoral benthos, or a 

 more or less uniform medium for pelagic migrants. The involun- 

 tary dispersal of organisms, on the other hand, may often go on in 

 spite of barriers which migrants could not surmount. Thus, as 

 already pointed out, the meroplanktonic young of the vagrant 

 benthos may have a much wider distribution than could ever be 

 brought about by the migration of the adults, who are often re- 

 stricted by barriers not present in the pelagic district. Fresh-water 

 molluscs, for example, are dispersed widely in streams, lakes, and 

 ponds which are discontinuous, and to which the adults could not 

 migrate. Again, plants may, by the dispersal of their seeds, sur- 

 mount streams or other barriers, which root or stolonal migration, 

 could never bring about. The wide dispersal of cocoanut palms, 

 by flotation of the nut, is a good example. 



Littorina littorea may be instanced as a type which has mi- 

 grated down the Atlantic coast within the space of a few years. 

 Originally introduced from England, it characterized Halifax har- 

 bor in 1852 and slowly made its way southward and northward. 

 In 1855 it was noted at Bathurst, Bay of Chaleur; in 1868 on the 

 coast of Maine, appearing at Portland in 1870. At Salem, Massa- 

 chusetts, it was first noted in 1872, and at Barnstable, Cape Cod, 

 in 1875. It was rare at Woods Hole in 1875, common in 1876, and 



