anation with the
knowledge we actually possess. In the old arctogaeal continent, for
instance, in what is now Europe, Asia, and North America, the glacial
period made a complete, but of course explicable, change in the faunal
life of the region. At one time the continent held a rich and varied
fauna. Then a period of great cold supervened, and a different fauna
succeeded the first. The explanation of the change is obvious.

But in many other cases we cannot so much as hazard a guess at why a
given change occurred. One of the most striking instances of these
inexplicable changes is that afforded by the history of South America
towards the close of the tertiary period. For ages South America had
been an island by itself, cut off from North America at the very time
that the latter was at least occasionally in land communication with
Asia. During this time a very peculiar fauna grew up in South America,
some of the types resembling nothing now existing, while others are
recognizable as ancestral forms of the ant-eaters, sloths, and
armadillos of to-day. It was a peculiar and diversified mammalian
fauna, of, on the whole, rather small species, and without any
representatives of the animals with which man has been most familiar
during his career on this earth.

Towards the end of the tertiary period there was an upheaval of land
between this old South American island and North America, near what is
now the Isthmus of Panama, thereby making a bridge across which the
teeming animal life of the northern continent had access to this queer
southern continent. There followed an inrush of huge, or swift, or
formidable creatures which had attained their development in the
fierce competition of the arctogaeal realm. Elephants, camels, horses,
tapirs, swine, sabre-toothed tigers, big cats, wolves, bears, deer,
crowded into South America, warring each against the other incomers
and against the old long-existing forms. A riot of life followed. Not
only was the character of the South American fauna totally changed by
the invasion of these creatures from the north, which soon swarmed
over the continent, but it was also changed through the development
wrought in the old inhabitants by the severe competition to which they
were exposed. Many of the smaller or less capable types died out.
Others developed enormous bulk or complete armor protection, and
thereby saved themselves from the new beasts. In consequence, South
America soon became po