extremities and mouth-parts
by its aquatic and burrowing habits. As we have no complete skeletons
of these early mammals we must abstain from picturing their external
appearance. It is enough that the living Monotreme and Marsupial so
finely illustrate the transition from a reptilian to a mammalian form.
There may have been types more primitive than the Duckbill, and others
between the Duckbill and the Marsupial. It seems clear, at least,
that two main branches, the Monotremes and Marsupials, arose from the
primitive mammalian root. Whether either of these became in turn the
parent of the higher mammals we will inquire later. We must first
consider the fresh series of terrestrial disturbances which, like some
gigantic sieve, weeded out the grosser types of organisms, and cleared
the earth for a rapid and remarkable expansion of these primitive birds
and mammals.

We have attended only to a few prominent characters in tracing the line
of evolution, but it will be understood that an advance in many organs
of the body is implied in these changes. In the lower mammals the
diaphragm, or complete partition between the organs of the breast and
those of the abdomen, is developed. It is not a sudden and mysterious
growth, and its development in the embryo to-day corresponds to the
suggestion of its development which the zoologist gathers from the
animal series. The ear also is now fully developed. How far the fish
has a sense of hearing is not yet fully determined, but the amphibian
certainly has an organ for the perception of waves of sound. Parts of
the discarded gill-arches are gradually transformed into the three bones
of the mammal's internal ear; just as other parts are converted into
mouth cartilages, and as--it is believed--one of the gill clefts is
converted into the Eustachian tube. In the Monotreme and Marsupial the
ear-hole begins to be covered with a shell of cartilage; we have the
beginning of the external ear. The jaws, which are first developed
in the fish, now articulate more perfectly with the skull. Fat-glands
appear in the skin, and it is probably from a group of these that the
milk-glands are developed. The origin of the hairs is somewhat obscure.
They are not thought to be, like the bird's feathers, modifications of
the reptile's scales, but to have been evolved from other structures in
the skin, possibly under the protection of the scales.

My purpose is, however, rather to indicate the general causes of