DERIVATION OF THE FLORA AND FAUNA 339 



early Pleistocene, some before Pliocene — when no land existed where stands the 

 Hawaiian chain, if geologists are right. UsiNGER (^/j), describing the distribu- 

 tion of Heteroptera, also looks for convenient stepping-stones. 



Divergent opinio7is on the means of transport. 



The dispersal agents universally recognized as important are air currents, 

 especially monsoon and trade winds, and cyclonic storms, ocean currents, birds, 

 and last not least man, who with his domestic animals and goods has become 

 more and more important, whether he brings plants, seeds and animals to extend 

 their range — and many of them become naturalized — or carries diaspores on his 

 body and his belongings unaware. It is as a rule not difficult to find out where 

 we have to do with human action, but we shall limit ourselves here to a discus- 

 sion of natural factors of distribution. This subject has been treated by innumer- 

 able writers in biology and a wealth of material was compiled by RiDLEY (20^), 

 a firm believer in the great value of all kinds of dispersal mechanisms, some of 

 which are, of course, very, wonderful. WULFF (2^1), in his chapter "Natural factors 

 for distribution" is more critical. Of animals only the birds deserve to be men- 

 tioned, but the plumage is no good for carrying diaspores any considerable dis- 

 tance, especially over the sea, and the extreme marine birds, the strongest flyers, 

 have no contact with land outside their own breeding-places. WULFF reduces the 

 part taken by birds to almost nothing, but I believe that we have good reason 

 to count with the migratory birds in certain cases. Water transport is responsible 

 for the diffusion of litoral halophytes but rarely for migrations of inland species. 

 Wind is important only on short distances, at least for seed-plants; special devices 

 do not help very much. Even Ridley concluded that winged or plumed diaspores 

 are not carried very far; spore-plants are more easily spread. Wulff remarks 

 that according to Bentham Leguminosae and Labiatae hold their ground just 

 as well if not better than Compositae. Altogether, if dispersal by natural factors 

 had the significance ascribed to it, the vegetation of the globe, within a certain 

 climatic zone, would be homogeneous and the sporeplants at least ought to be 

 cosmopolitan, but they are "localized in definite areas, their distribution paralleling 

 that of flowering plants" (p. 128). GoOD observes [log) that we have no proofs 

 that species equipped with special dispersal mechanisms are more widely distri- 

 buted than others. That certain plants with such devices show very wide areas 

 whereas others without them are rare and local means nothing, because the reverse 

 is also true; climatic and edaphic factors should always be taken into account. 



The relative value of the dispersal agents is put to the test when we deal 

 with oceanic islands. Setchell (21'/) was inclined to give considerable credit to 

 migratory birds. The occurrence of identical species of flov.ering plants in Arizona 

 and Argentina and in California and Chile could be explained by bird transport, 

 and bipolarity had originated in the same way. Bird transport helped him to 

 understand the remarkable disjunct areas of arborescent Compositae and Lobe- 

 liaceae; their birth place was in the Antarctic, and they had been carried by 

 birds to New Zealand, Australia, Malaysia, Polynesia, Hawaii, South America and 

 the high African mountains — we can add Saint Helena, Juan Fernandez and Des- 



