PDF - Wallace Online

CHAP. XIV.] THE NEOTROPICAL REGION. 83
of interchange of the mammals or land-birds of the two
countries, no less than by the very fragmentary nature of the
resemblances that do exist. The northern element consists
almost wholly of insects ;
and is evidently due to the migration
of arctic and north temperate forms along the ridges and
plateaus of the Andes ; and most likely occurred when these
organisms were driven southward at successive cold or Glacial
periods.
A curious parallel exists between the past history and actual
zoological condition of South America and Africa. In both
we see a very ancient land-area extending into the South
Temperate zone, isolated at a very early period, and developing
only a low grade of Mammalian life ; chiefly Edentates and
Rodents on the one, Lemurs and Insectivora in the other. Later
we find an irruption into both of higher forms, including
Quadrumana, which soon acquired a large and special development
in the tropical portions of each country. Still later we
have an irruption into both of northern forms, which spread
widely over the two regions, and having become extinct in the
land from whence they came, have been long held to be the
original denizens of their adopted country. Such are the
various forms of antelopes, the giraffe, the elephant, rhinoceros,
and lion in Africa ; while in America we have deer and peccaries,
the tapir, opossums, and the puma.
On the whole, we cannot but consider that the broad outlines
of the zoological history of the Neotropical region can be traced
with some degree of certainty ; but, owing to the absence of
information as to the most important of the geological periods
—the Miocene and Eocene—we have no clue to the character of
its early fauna, or to the land connections with other countries,
which may possibly have occurred in early Tertiary times.