364 THE ZOOLOGIST. 



isolation in the differentiation of species, that special attention 

 has been paid to the peculiarities of insular forms. Here the 

 field-naturalist comes in as the helpful servant of the philosopher 

 and the systematist, by illustrating the operation of isolation in 

 the differentiation of species. I may take the typical examples 

 of two groups of oceanic islands, differing as widely as possible 

 in their position on the globe, the Sandwich Islands in the centre 

 of the Pacific, thousands of miles from the nearest Continent, and 

 the Canaries, within sight of the African coast; but agreeing in 

 this, that both are truly oceanic groups, of purely volcanic origin, 

 the ocean depths close to the Canaries, and between the different 

 islands, varying from 1500 to 2000 fathoms. In the one we may 

 study the expiring relics of an avifauna completely differentiated 

 by isolation ; in the other we have the opportunity of tracing the 

 incipient stages of the same process. 



The Sandwich Islands have long been known as possessing an 

 avifauna not surpassed in interesting peculiarity by that of New 

 Zealand or Madagascar ; in fact, it seems as though their vast dis- 

 tance from the continent had intensified the influences of isolation. 

 There is scarcely a passerine bird in its indigenous fauna which 

 can be referred to any genus known elsewhere. But, until the 

 very recent researches of Mr. Scott Wilson, and the explorations 

 of the Hon. W. Rothschild's collectors, it was not known that 

 almost every island of the group possessed one or more repre- 

 sentatives of each of these peculiar genera. Thus, every island 

 which has been thoroughly explored, and in which any extent of 

 the primeval forest remains, possesses, or has possessed, its own 

 peculiar species of Hemignathus, Himatione, Phceornis, Acrulo- 

 cercus, Loxops, Drepanis f as well as the massive-beaked finches, 

 which emulate the Geospiza of the Galapagos. Prof. Newton has 

 shown that while the greater number of these are, probably, of 

 American origin, yet the South Pacific has contributed its quota 

 to this museum of ornithological rarities, which Mr. C. B. Clarke 

 (Phil. Trans. 1892, pp. 371 — 387) very justly proposes to make 

 a distinct biological sub-region. 



That each of the islands of this group, however small, should 

 possess a flora specifically distinct, suggests thoughts of the vast 

 periods occupied in their differentiation. 



In the Canary Islands, either because they are geologically 

 more recent, or because of their proximity to the African coast, 



