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 Ave trjice the thousand-fathom line around all our 
existing continents Ave find that, Avith only tAvo exceptions, 
e\'ery island which can be classed as “ continental ” falls 
within this line, while 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 Avithin 
Avhich continental development and change throughout knoAvn 
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 
Ave have any direct evidence, this line may be taken to mark 
out, approximately, the most probable boundary betAveen the 
“ continental area,” which has always consisted of land and 
shallow sea in varying proportions, and the great oceanic 
basins, Avithin the limits of which volcanic activity has been 
building up numerous islands, but AA T hose profound depths 
have apparently undergone little change. 
Madagascar and Neiv Zealand. 
The tAvo exceptions just referred to are Madagascar and 
NeAV Zealand, and all the evidence goes to shoAv that in these 
cases the land connection Avith 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 Avide betAveen the Comoro Islands and the main- 
