XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 347 



contain indigenous mammalia or batrachia, and generally a 

 much greater variety of birds, reptiles, insects, and plants, 

 than do the oceanic islands. From these various character- 

 istics we conclude that they have all once formed parts of 

 continents, or at all events of much larger land areas, and have 

 become isolated, either by subsidence of the intervening land 

 or by the effects of long-continued marine denudation. 



Now, if we trace the thousand-fathom line around all our 

 existing continents we find that, with only two exceptions, 

 every island which can be classed as " continental " falls 

 within this line, Avhile all that lie beyond it have the un- 

 doubted characteristics of " oceanic " islands. We, therefore, 

 conclude that the thousand-fathom line marks out, approxi- 

 mately, the "continental area," — that is, the limits within 

 which continental development and change throughout kno^vn 

 geological time have gone on. There may, of course, have 

 been some extensions of land beyond this limit, while some 

 areas within it may always have been ocean ; but so far as 

 we have any direct evidence, this line may be taken to mark 

 out, approximately, the most probable boundary between the 

 "continental area," which has always consisted of land and 

 shallow sea in varying proportions, and the great oceanic 

 basins, within the limits of Avhich volcanic activity has been 

 building up numerous islands, but whose profound depths 

 have apparently undergone little change. 



Madagasmr and Neiv Zealand. 



The two exceptions just referred to are Madagascar and 

 New Zealand, and all the evidence goes to show that in these 

 cases the land connection with the nearest continental area 

 was very remote in time. The extraordinary isolation of the 

 productions of Madagascar — almost all the most characteristic 

 forms of mammalia, birds, and reptiles of Africa being 

 absent from it — renders it certain that it must have been 

 separated from that continent very early in the Tertiary, if 

 not as far back as the latter part of the Secondary period ; 

 and this extreme antiquity is indicated by a depth of 

 considerably more than a thousand fathoms in the Mozam- 

 bique Channel, though this deep portion is less than a 

 hundred miles wide between the Comoro Islands and the main- 



