PASSIVE MIGRATION. 357 
they attach themselves by buds, and bring forth new 
colonies of individuals of their species. 
Influential as these active migrations of most animals 
and many plants are, yet alone they would by no 
means be sufficient to explain the chorology of organisms. 
Passive migrations have ever been by far the more import- 
ant, and of far greater influence, in the case of most plants 
and in that of many animals. Such passive changes of 
locality are produced by extremely numerous causes. Air 
and water in their eternal motion, wind and waves with 
their manifold currents, play the chief part. The wind in 
all places and at all times raises light organisms, small 
animals and plants, but especially their young germs, animal 
eggs and plant seeds, and carries them far over land and 
seas. Where they fall into the water they are seized by 
currents or waves and carried to other places. It is well 
known, from numerous examples, how far in many cases 
trunks of trees, hard shelled fruits, and other not readily 
perishable portions of plants are carried away from their 
original home by the course of rivers and by the currents 
of the sea. Trunks of palm trees from the West Indies are 
brought by the Gulf Stream to the British and Norwegian 
coasts. All large rivers bring down driftwood from the 
mountains, and frequently alpine plants are carried from their 
home at the source of the river into the plains, and even 
further, down to the sea. Frequently numerous inhabitants 
live between the roots of the plants thus carried down, and 
between the branches of the trees thus washed away there 
are various inhabitants which have to take part in the 
passive migration. The bark of the tree is covered with 
mosses, lichens, and parasitic insects. Other insects, spiders, 
