CO-OPEEATING INFLUENCES. 307 



2. Currents and winds as limiting the distribution of 

 species. — ^When we reflect on tlie mode in which alone -winds 

 and currents can possibly convey animals from place to 

 place, it becomes self-evident that they must very frequently 

 act also as hindrances to the distribution of species. Ships, 

 drifting ice with the boulders, erratic blocks or drift-wood 

 transported by it, trees uprooted from the land, and the leaves 

 and dust often carried to great distances by storms — all these 

 serve from time to time and with more or less frequency for the 

 transport of many kinds of animals. And since storms, winds, 

 and currents, in spite of many variations in their courses, are 

 still on the whole very constant, it necessarily follows that those 

 animals which either do not come within their range, or which 

 cannot bear transmission by such means, are eiccluded from dis- 

 tribution by these agents. Elephants could never be conveyed 

 to any distance on floating trees, as small snails can, or even 

 such mammal^ as live among their branches; wingless land 

 birds, like those of New Zealand and Madagascar, are incapable 

 of migrating to any distance ; but still, in these and all similar 

 cases, currents serve as a means of separation only because the 

 nature of the animals concerned forbids their availing them- 

 sjlves of them. Thus the action of winds and currents is 

 dependent on that of the animals or co-operates with it. 

 Even in cases where the cui-rents appear to act quite indepen- 

 dently, their influence is always dependent on other conditions 

 which may be associated with them. Thus, for instance, many 

 animals are extremely susceptible to variations of temperature' ; 

 consequently, if any warm- water animals are borne by a current 

 from the region of warm seas into a cold one, they must in all 

 probability perish very soon. Here, then, the currents might 

 have, acted as promoting and aiding distribution; but this 

 result was completely neutralised by the contemporaneous 

 action of a diminution of warmth. In the same way very often 

 some animal may be carried by the wind from one island to 

 another without any favourable issue ; for its establishment in 

 the new locality depends, as we know, not merely on its safe 

 arrival there, but also on the creature's finding in its new home 



