e a
true vertebrate skeleton. They live underground, burrowing by means of the
ring-like folds of the skin which simulate the jointed segments of a worm's
body, and when caught they exude a viscid slime. The young have external
gills which are afterwards replaced by true lungs, and this peculiar
metamorphosis shows that they belong to the amphibia rather than to the
reptiles. The Caecilias are widely but very sparingly distributed through
all the tropical regions; a fact which may, as we have seen, be taken as an
indication of the great antiquity of the group, and that it is now verging
towards extinction. In the Seychelles Islands there appear to be three
species of these singular animals. _Cryptopsophis multiplicatus_ is
confined to the islands; _Herpele squalostoma_ is found also in Western
India and in Africa; while _Hypogeophis rostratus_ inhabits both West
Africa and South America.[107] This last is certainly one of the most
remarkable cases of the wide and discontinuous distribution of a species;
and {433} when we consider the habits of life of these animals and the
extreme slowness with which it is likely they can migrate into new areas,
we can hardly arrive at any other conclusion than that this species once
had an almost world-wide range, and that in the process of dying out it has
been left stranded, as it were, in these three remote portions of the
globe. The extreme stability and long persistence of specific form which
this implies is extraordinary, but not unprecedented, among the lower
vertebrates. The crocodiles of the Eocene period differ but slightly from
those of the present day, while a small freshwater turtle from the Pliocene
deposits of the Siwalik Hills is absolutely identical with a still living
Indian species, _Emys tectus_. The mud-fish of Australia, _Ceratodus
forsteri_ is a very ancient type, and may well have remained specifically
unchanged since early Tertiary times. It is not, therefore, incredible that
this Seychelles Caecilia may be the oldest land vertebrate now living on
the globe; dating back to the early part of the Tertiary period, when the
warm climate of the northern hemisphere in high latitudes and the union of
the Asiatic and American continents allowed of the migration of such types
over the whole northern hemisphere, from which they subsequently passed
into the southern hemisphere, maintaining themselves only in certain
limited areas, where the physical conditions were especiall