iological facts which enable us to mark out these areas
with some confidence.

We have seen that there are a large number of islands which may be
classed as oceanic, because they have never formed parts of continents,
but have originated in mid-ocean, and have derived their forms of life
by migration across the sea. Their peculiarities are seen to be very
marked in comparison with those islands which there is good reason to
believe are really fragments of more extensive land areas, and are hence
termed "continental." These continental islands consist in every case of
a variety of stratified rocks of various ages, thus corresponding
closely with the usual structure of continents; although many of the
islands are small like Jersey or the Shetland Islands, or far from
continental land like the Falkland Islands or New Zealand. They all
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 characteristics 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, while all that
lie beyond it have the undoubted characteristics of "oceanic" islands.
We, therefore, conclude that the thousand-fathom line marks out,
approximately, the "continental area,"--that is, the limits within which
continental development and change throughout known 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 which
volcanic activity has been building up numerous islands, but whose
profound depths have apparently undergone little change.

_Madagascar and New Zealand._

The two exceptions just referred to are Madagascar and New Zealand, and
all the evidence goes to show that in these cases the