GEOGRAPHIC DISTRIBUTION OF ANIMALS: OCEANIC ISLANDS 283 



Harmonic and Disharmonic Faunas 



Faunas of oceanic islands usually present internal evidence of not having 

 ar^en bylriigration^veriand. The animals are usually a rather haphazard 

 assemblage. Such a fauna is frequently spoken of as disharmonic, in con- 

 trast with the harmonic faunas of continents and continental islands. In 

 harmonic faunas the various habitats and means of livelihood (environ- 

 mental niches) are uniformly filled by animals, each adapted for its par- 

 ticular niche. In disharmonic faunas many environmental niches remain 

 unexploited or are filled by animals which in a harmonic fauna have dif- 

 ferent habits and means of livelihood. Under island conditions animals 

 have sometimes "improvised" means of exploiting environmental niches 

 foreign to them on continents. Examples are given later in the discussions 

 of the Galapagos and Hawaiian faunas. 



Disharmonic faunas seem, then, to indicate absence of land connection. 

 Where land connections occur, to continental islands, harmonic faunas are 

 transferred from continents to islands more or less intact, except as island 

 conditions may be unsuitable for this or that species. It is highly unhkely, 

 for example, that a land bridge would be used by one species of tree snail 

 and not by other animals inhabiting the original home of that snail. Yet 

 land bridges have been postulated to explain just such arrival of single 

 species on oceanic islands. 



Not all faunas of oceanic islands are disharmonic. Some of the larger and 

 older islands and archipelagos, like the Hawaiian Islands, have faunas 

 which are quite harmonic. In such cases, however, it is evident that the 

 harmonic fauna is not like that of any continent but is an evolutionary 

 achievement that occurred on the islands themselves. It occurs on islands 

 and archipelagos large enough to provide considerable diversity of habitat 

 and andentenough so that time has been provided for evolutionary xhange.^ 



Absence of amphibians is particularly characteristic of most oceanic is- 

 lands. Neither amphibian eggs nor adults can survive immersion in sea 

 water. Transportation on rafts or by other means which avoid contact with 

 sea water is possible and doubtless accounts for the presence of amphibians 

 on those oceanic islands that do possess them. If land bridges had occurred, 

 on the other hand, there is no reason why amphibians might not have 

 traversed them, as in fact they must have done in populating continental 

 islands, where they are usually abundant. 



Absence of mammals, particularly of larger species, is also characteristic 

 of oceanic islands. Had land bridges existed, at any time subsequent to the 

 Cretaceous period at least (p. 137), such regularly observed absence 



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