BIOGEOGRAPHY 



might then multiply and become established in the new locality. This is 

 the basis of the rains of fishes which are occasionally reported. While such 

 reports are usually received with much well-justified skepticism, nonethe- 

 less Gudger has examined all such reports critically, and he believes that 

 at least seventy-eight recorded rains of fishes are valid. Shorebirds and 

 waterfowl may also act as agents of dispersal for fresh-water organisms. 

 For as the birds arise from the water, small organisms, eeigs, larvae, and 

 mud containing seeds are likely to cling to the feet of the birds. As the 

 flight is likely to terminate in a comparable body of fresh water, and as 

 these birds are among the most wide-ranging, it seems probable that much 

 dispersal of small organisms occurs in this way. Also, the seeds of many 

 plants retain their viability after passing through the digestive tract of a 

 bird. Thus seeds which are eaten in one pond may be discharged in a 

 quite distant pond, there to germinate. 



All of this is not to say that there is a single, world-wide fresh-water 

 flora and fauna. Discontinuities do exist among the inhabitants of fresh- 

 water systems. But they are less marked than might at first be expected, 

 and they correspond to the most ancient and imposing geographical 

 barriers. 



ISLAND LIFE 



The final category of geographical evidence, and the one which had the 

 greatest effect upon the thinking of Darwin, is that of oceanic islands and 

 their living inhabitants. Darwin observed that such islands are typically 

 poor in numbers of species present, although the success of animals and 

 plants introduced by man has proven that these islands are well adapted 

 to support a much greater variety of organisms than existed upon them 

 aboriginally. He reasoned that, if all organisms had been created in their 

 present localities, there is no reason why oceanic islands (islands, that is, 

 which are located beyond the continental shelf) should not be as richly 

 inhabited as comparable areas of the continents. Yet this is readily under- 

 standable upon his theory of migration from a common place of origin 

 for all members of any group, with subsequent modification. For relatively 

 few species could cross the great water barrier separating oceanic islands 

 from the continental centers of origin. 



Of the few species found on oceanic islands, a large number are en- 

 demic, that is, found nowhere else. Darwin found twenty-six species of 

 land birds in the Galapagos Islands ( Figure 5 ) . Of these, twenty-one and 

 possibly twenty-three are endemic. But of the eleven species of marine 

 birds, only two are endemic. This is again just what one would expect in 

 accordance with Darwin's theory. For, the occasional immigrants from the 

 distant mainland (South America) would, upon arrival in their new en- 

 vironment, compete with quite different species from their cousins on the 

 mainland, and so would be modified, eventually reaching the status of 

 new and distinct species. But the great water barrier would greatly reduce 

 the probability of these new species spreading to other localities. Yet for 

 the marine birds the barrier ought to be less formidable, and hence the 



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