ISLAND FAUNAS. 7 



the reptiles, insects, and land mollusks. The island of St. Helena, 

 in the South Atlantic, and the Sandwich Islands, in the North 

 Pacific, present us with similar instances of faunal specialisation, 

 and to a less extent, also, the group of the Azores. In the case of 

 these last, which lie in the course of the storm-winds, a considerable 

 intermixture has been effected with the faunas of Western Africa 

 and Europe, for we find that by far the greater number of the resi- 

 dent land-birds are inhabitants of those two continents as well. 

 The fact that there are so very few peculiar forms is proof either of 

 a recent separation of the islands from the mainland — not suflBcient 

 time having been allowed for the development of new species — or 

 of a recent or repeated peopling with old forms from the continents. 

 Even irrespective of considerations connected with the physical 

 geography or geology of the region, it w^ould naturally be inferred, 

 from the prevalence of in-blowing storm-winds, and the known fact 

 that certain birds are transported hither, that the second supposi- 

 tion is the correct one; and that this is the true explanation is 

 proved by evidence of a very positive character furnished by some 

 of the other groups of animals. Thus, the land-MoUusca, which 

 in their distribution are not so readily affected by aerial currents, 

 are eminently distinguished from those of either Europe or Africa, 

 or of any other continental land-mass, proving in their case a long- 

 protracted period of isolation. Further, there is not a single species 

 of fresh-water mollusk known in the entire group ! The Bermuda 

 Islands, which are about equally distant from the mainland, occupy 

 a nearly analogous position with respect of their fauna ; that is, par- 

 tial interchanges have been effected with the fauna of the American 

 continent. 



In all these cases, necessarily, the amount of faunal specialisation 

 will be the index of the period of isolation. Where faunal immi- 

 gration from a foreign region takes place it not only checks the 

 development of a newly-forming fauna, by infusing into it an ele- 

 ment that does not properly belong there, but also prevents in a 

 measure that variation among individuals which might otherwise 

 obtain. The case of the bobolink of the Galapagos Islands is a 

 well-known example of this kind. It alone, of about thirty species 

 of land-birds inhabiting those islands, is considered to be indis- 

 putably identical with any form occurring on the mainland ; hence 

 it is concluded that this is about the only species of South American 



