orthern land,
connecting Europe and America, for their development. We saw, however,
that this northern region was singularly warm until long after the
spread of the mammals. Other experts, impressed by the parallel
development of the mammals and the flowering plants, look to the
elevated parts of eastern North America.

Such evidence as there is seems rather to suggest that South Africa
was the cradle of the placental mammals. We shall find that many of
our mammals originated in Africa; there, too, is found to-day the most
primitive representative of the Tertiary mammals, the hyrax; and there
we find in especial abundance the remains of the mammal-like reptiles
(Theromorphs) which are regarded as their progenitors. Further search
in the unexplored geological treasures and dense forests of Africa is
needed. We may provisionally conceive the placental mammals as a group
of the South African early mammals which developed a fortunate variation
in womb-structure during the severe conditions of the early Mesozoic. In
this new structure they would have no preponderant advantage as long as
the genial Jurassic age favoured the great reptiles, and they may have
remained as small and insignificant as the Marsupials. But with the
fresh upheaval and climatic disturbance at the end of the Jurassic, and
during the Cretaceous, they spread northward, and replaced the dying
reptiles, as the Angiosperms replaced the dying cycads. When they met
the spread of the Angiosperm vegetation they would receive another great
stimulus to development.

They appear in Europe and North America in the earliest Cretaceous. The
rise of the land had connected many hitherto isolated regions, and
they seem to have poured over every bridge into all parts of the four
continents. The obscurity of their origin is richly compensated by their
intense evolutionary interest from the moment they enter the geological
record. We have seen this in the case of every important group of plants
and animals, and can easily understand it. The ancestral group was
small and local; the descendants are widely spread. While, therefore,
we discover remains of the later phases of development in our casual
cuttings and quarries, the ancestral tomb may remain for ages in some
unexplored province of the geological world. If this region is, as we
suspect, in Africa, our failure to discover it as yet is all the more
intelligible.

But these mammals of the early Tertiary are still of