o survive when they do fall into suitable conditions, that
we can follow their development much more easily than that of the birds.
We find a number of strange patriarchal beasts entering the scene in the
early Eocene, and spreading into a great variety of forms in the genial
conditions of the Oligocene and Miocene. As some of these forms advance,
we begin to descry in them the features, remote and shadowy at first, of
the horse, the deer, the elephant, the whale, the tiger, and our other
familiar mammals. In some instances we can trace the evolution with a
wonderful fullness, considering the remoteness of the period and
the conditions of preservation. Then, one by one, the abortive, the
inelastic, the ill-fitted types are destroyed by changing conditions or
powerful carnivores, and the field is left to the mammals which filled
it when man in turn began his destructive career.

The first point of interest is the origin of these Tertiary mammals.
Their distinctive advantage over the mammals of the Mesozoic Era was-the
possession by the mother of a placenta (the "after-birth" of the higher
mammals), or structure in the womb by which the blood-vessels of the
mother are brought into such association with those of the foetus that
her blood passes into its arteries, and it is fully developed within the
warm shelter of her womb. The mammals of the Mesozoic had been small and
primitive animals, rarely larger than a rat, and never rising above the
marsupial stage in organisation. They not only continued to exist, and
give rise to their modern representatives (the opossum, etc.) during
the Tertiary Era, but they shared the general prosperity. In Australia,
where they were protected from the higher carnivorous mammals, they
gave rise to huge elephant-like wombats (Diprotodon), with skulls two
or three feet in length. Over the earth generally, however, they were
superseded by the placental mammals, which suddenly break into the
geological record in the early Tertiary, and spread with great vigour
and rapidity over the four continents.

Were they a progressive offshoot from the Mesozoic Marsupials, or
Monotremes, or do they represent a separate stock from the primitive
half-reptile and half-mammal family? The point is disputed; nor does the
scantiness of the record permit us to tell the place of their origin.
The placental structure would be so great an advantage in a cold and
unfavourable environment that some writers look to the n