278 THE INFLUENCE OF INANIMATE SUEEOUNDINGS. 



forgotten that the results thus produced may easily be altogether 

 concealed, or even completely nullified and reversed, by the 

 simultaneous operation of other conditions of existence which 

 are inseparably united with this, the only factor we have imme- 

 diately to consider. Granting that a species, be it what it may, 

 were borne by a marine current to au uninhabited island, its exist- 

 ence would depend not merely on its safe arrival there, but also 

 on the favourable or unfavourable circumstances prevailing in 

 the island. It is beyond a doubt that very many larvse of marine 

 creatures are carried by currents to the shore or into the mouths 

 of rivers ; but most of them perish because they do not there 

 find suitable conditions of existence. The warm Mozambique 

 current carries many warm-water animals out of the Indian 

 Ocean into latitudes in which they cannot continue to live 

 because they are unable to accommodate themselves to the low 

 temperature prevailing there. In such researches we are obliged, 

 and indeed required, to separate the different influences to which 

 animals are exposed ; still we must not forget that such a 

 separation never occui'S in nature, but that, on the contrary, 

 difierent and often antagonistic forces are frequently quite in- 

 separable. 



1. Currents and Winds as a means of the difEiision of 

 species. — It is a well-known fact that regular winds, as well as 

 storms, are able to carry many flying creatures to enormous dis- 

 tances from their homes. Insects of all kinds are often caught 

 hundreds of miles from the nearest land, out on the high 

 seas; North Am^erican birds not unfrequently come across 

 the Atlantic Ocean to Scotland ; it is well-known, too, that the 

 birds of many small islands are identical with, or very nearly 

 related to, those of the nearest continent from which the winds 

 blow that usually sweep over those islands. Ocean currents act 

 in the same manner; all free-swimming or even drifting animals 

 and larvse are borne along by them, and the edge of the currents 

 often marks a very sharply defined line of limitation between two 

 quite dissimilar faunas. I shall never forget the impression 

 made on my mind, when I was sailing round the Cape of Good 

 Hope, by a sudden change in the fauna of the ocean surface from 

 this very cause. "West of the meridian of Cape Town the 



