March. i893. CLASSIFICATION OF ISLANDS. 189 



oceanic and continental — which are defined by Dr. A. R. Wallace in 

 the following terms : — 



Oceanic islands are " of volcanic or coralline origin, usually far 

 from continents, and always separated from them by very deep sea, 

 entirely without indigenous land mammalia or amphibia, but with a 

 fair number of birds and insects, and usually with some reptiles." 



" Continental islands are always more varied in their geological 

 formation, containing both ancient and recent stratified rocks. They 

 are rarely very remote from a continent, and they always contain 

 some land mammals and amphibia, as well as representatives of the 

 other classes and orders in considerable variety." ^ 



As general definitions framed for the purpose of describing the 

 conditions which have governed and limited the geographical 

 distribution of animals, these sentences are doubtless sufficiently 

 accurate, especially as Dr. Wallace admits there are some islands 

 which do not come very clearly under either of these categories ; but 

 he proceeds to lay down a canon the truth of which is by no means so 

 apparent. He says : — " The total absence of warm-blooded terres- 

 trial animals in an island otherwise well suited to maintain them, is 

 held to prove that such island is no mere fragment of any existing 

 or submerged continent, but one that has been actually produced in 

 mid-ocean. It is true that if a continental island were to be com- 

 pletely submerged for a single day, and then again elevated, its higher 

 terrestrial animals would be all destroyed, and if it were situated at a 

 considerable distance from land, it would be reduced to the same 

 zoological condition as an oceanic island ; but such a complete sub- 

 mergence and re-elevation appears never to have taken place, for 

 there is no single island on the globe which has the physical and 

 geological features of a continental, combined with the zoological 

 features of an oceanic island." 



Seeing how little we yet know of the geology of distant islands, 

 this is a statement which further knowledge may at aiiy time disprove, 

 and there is, even now, good reason to believe that it is contrary to 

 facts. If this assertion can be proved to be incorrect, I shall claim 

 to reverse Dr. Wallace's argument, and to maintain that inasmuch as 

 an island does exist which combines the geological features of a con- 

 tinental island with the zoological features of an oceanic one, then we 

 may assume that the submergence and re-elevation of a continental 

 island can take place, and, consequently, the absence of mammals in an 

 island cannot be held to prove that it has never been united to a 

 continent. 



Considering the many subsidences and upheavals which are 

 known to have occurred along the borders of continental areas since the 

 beginning of Tertiary time, it would indeed be strange if some tracts, 

 isolated by subsidence, had not been completely submerged for a 

 time, and afterwards raised afresh from the sea. The West Indian 

 1 " Island Life," by A. R. Wallace, second edition, 1892, p. 243. 



