istribution of others was very different from what
prevails at the present day. The problem presented by these ancient islands
is often complicated by the changes they themselves have undergone since
the period of their separation. A partial subsidence will have led to the
{412} extinction of some of the types that were originally preserved, and
may leave the ancient fauna in a very fragmentary state; while subsequent
elevations may have brought it so near to the continent that some
immigration even of mammalia may have taken place. If these elevations and
subsidences occurred several times over, though never to such an extent as
again to unite the island with the continent, it is evident that a very
complex result might be produced; for besides the relics of the ancient
fauna, we might have successive immigrations from surrounding lands
reaching down to the era of existing species. Bearing in mind these
possible changes, we shall generally be able to arrive at a fair
conjectural solution of the phenomena of distribution presented by these
ancient islands.

Undoubtedly the most interesting of such islands, and that which exhibits
their chief peculiarities in the greatest perfection, is Madagascar, and we
shall therefore enter somewhat fully into its biological and physical
history.

_Physical Features of Madagascar._--This great island is situated about 250
miles from the east coast of Africa, and extends from 12deg to 25-1/2deg S.
Lat. It is almost exactly 1,000 miles long, with an extreme width of 360
and an average width of more than 260 miles. A lofty granitic plateau, from
eighty to 160 miles wide and from 3,000 to 5,000 feet high, occupies its
central portion, on which rise peaks and domes of basalt and granite to a
height of nearly 9,000 feet; and there are also numerous extinct volcanic
cones and craters. All round the island, but especially developed on the
south and west, are plains of a few hundred feet elevation, formed of rocks
which are shown by their fossils to be of Jurassic age, or at all events to
belong to somewhere near the middle portion of the Secondary period. The
higher granitic plateau consists of bare undulating moors, while the lower
Secondary plains are more or less wooded; and there is here also a
continuous belt of dense forest, varying from six or eight to fifty miles
wide, encircling the whole island, usually at about thirty miles distance
from the coast but in the north-east coming down