DERIVATION OF THE FLORA AND FAUNA 



343 



abroad. For those, antl the)' arc in overwhelming majority, which are not an- 

 thropochorous, the chance to cover large distances ought to be very small. How- 

 ever, Zimmerman, in his admirable introduction to the Insects of Hawaii, surveys 

 one order after the other and finds nothing that speaks against his belief in the 

 permanent isolation of the Hawaiian as well as all other oceanic islands, and 

 consequently concludes that, man-borne species excepted, all the ancestors of the 

 Hawaiian insects were carried there by natural agents. Disf)ersal with the aid of 

 birds is of slight importance, but he remarks that sea birds nest in forests on 

 the islands; I do not think that this means very much, because they are stationary 

 (comp. above, p. 337). Marine drift is probably the least successful of all meth- 

 ods, he thinks. Thus, the bulk of the insect fauna was and is wind-borne, a 

 traffic going on without interrujition. Zimmerman refers to experiments clearly 

 showing that both winged and unwinged insects are carried by air currents to 

 great heights, 14,000-15.000 feet (p. 58). These are largely abnormal conditions 

 and due to cyclonic storms, which account for the dispersal all over the mid- 

 Pacific. The result is, as expected, a disharmonious fauna, where large groups 

 common to all continents are lacking: "they have been eliminated by the selective 

 agents of oversea dispersal". It would be interesting to know why all represent- 

 atives of large and otherwise widely distributed insect groups are excluded from 

 the passenger list. Besides, would not disharmony result even if land connections 

 once existed.^ — the islands have remained isolated for a long time, perhaps 

 millions of years, while migration, favoured by climatic and edaphic changes 

 affecting the general character and composition of the vegetation has continued 

 over land on the continents. 



HiNTON {dj) who, as we have seen, was opposed against both continental 

 drift and land bridges, believed that wind-borne and raft-borne transportation 

 across the oceans must have been of common occurrence. If this be so, why did 

 no snakes, frogs or gymnosperm cones ever get aboard the rafts and arrive at 

 distant islands.'' Rafts formed by large, uprooted trees are observed in big rivers 

 like the Amazon or Orinoco, and I guess that an analysis of their composition 

 would reveal the presence of a rather varied fauna. WALLACE'S Boa constrictor 

 was referred to above (p. 335). Snakes and giant spiders are often found hidden 

 in banana trunks imported to Europe from tropical America. On the other hand, 

 the chances for the formation of substantial rafts are small within the tropical 

 Pacific, where no big rivers empty. 



What kind of invertebrates are likely to ivithstaiid tra?isoceanic migration r 



HiNTON has an answer ready: 



The chance of accidental dispersal varies according to the nature of the group. 

 Colonization of the Hawaiian and other islands, always far removed from any conti- 

 nent, provides us with absolute proof of the kind of animals and plants that can with- 

 stand long distance wind or raft transportation across the oceans. 



This sounds quite simple, but really is a very complicated problem, to 

 which sufficient attention has not been paid. The possibilities vary according to 



