PASSIVE MIGRATION. 277 



fore justified in supj)osing that either active or passive migra- 

 tion is one of the chief general conditions for the existence of a 

 species as such. For instance, all sedentary animals, as Corals, 

 are just as dependent on migration as parasites; if we suppose 

 that one single species of coral suddenly lost the capacity of re- 

 production by eggs and swimming larviB, and only preserved 

 th3 power of reproduction hy buds, it would, no doubt, be able 

 to extend itself, like a. tree, in the spot it lived on, but it must 

 nevertheless presently die out if the external- conditions of its 

 existence were considerably changed — as, for instance, by sub- 

 marine volcanic action. Conseqviently the only means by 

 which such a species can maintain itself as such, is the power 

 possessed by its lai'vje of migrating and transporting the species 

 beyond the original limits of its dwelling-place, so that changes 

 which might lead to lis annihilation in its place of origin may 

 not exterminate it, or at any rate not everywhere at once. The 

 same is the case for the greater number of animals ; the tendency 

 to migrate is an important — perhaps even the most important — 

 means employed by Nature to pi'eserve a newly oi-iginated 

 species from destruction. 



This migration, which would seem to be indispensable to 

 the great majority of animals, may be either passive or active. 

 To the latter category belong the migrations of birds, of certain 

 mammals, of many fishes, insects, &c., while many animals 

 living in the sea are, on the contrary, passively migratory, as 

 Salpaj, Medusae, Pyrosomse, &c. These are borne involuntarily 

 by currents, while the former determine the direction of their 

 wanderings by their own choice. In all cases of passive migra- 

 tion it is self evident that the existence and distribution of the 

 species will depend on the strength and direction of currents; 

 but even in the journeys made by those creatures whose 

 migrations are voluntary, the same influence is often more or 

 less recognisable. 



The efiects of cui'rents in water or in the aii- may be of two 

 difierent kinds. In the first place they may serve as means for 

 the distribution of the species; secondly as limits hindering their 

 extension. We shall now consider the eil'ects of currents in 

 each of these directions; but at the same time it must never be 



