p as showing us approximately how far
beyond their present limits our continents may have extended during any
portion of the Tertiary and Secondary periods, we shall obtain a
foundation of inestimable value for our inquiries into those migrations
of animals and plants during past ages which have resulted in their
present peculiarities of distribution. We see, for instance, that the
South American and African continents have always been separated by
nearly as wide an ocean as at present, and that whatever similarities
there may be in their productions must be due to the similar forms
having been derived from a common origin in one of the great northern
continents. The radical difference between the higher forms of life of
the two continents accords perfectly with their permanent separation. If
there had been any direct connection between them during Tertiary times,
we should hardly have found the deep-seated differences between the
Quadrumana of the two regions--no family even being common to both; nor
the peculiar Insectivora of the one continent, and the equally peculiar
Edentata of the other. The very numerous families of birds quite
peculiar to one or other of these continents, many of which, by their
structural isolation and varied development of generic and specific
forms, indicate a high antiquity, equally suggest that there has been no
near approach to a land connection during the same epoch.

Looking to the two great northern continents, we see indications of a
possible connection between them both in the North Atlantic and the
North Pacific oceans; and when we remember that from middle Tertiary
times backward--so far as we know continuously to the earliest
Palaeozoic epoch--a temperate and equable climate, with abundant woody
vegetation, prevailed up to and within the arctic circle, we see what
facilities may have been afforded for migration from one continent to
the other, sometimes between America and Europe, sometimes between
America and Asia. Admitting these highly probable connections, no
bridging of the Atlantic in more southern latitudes (of which there is
not a particle of evidence) will have been necessary to account for all
the intermigration that has occurred between the two continents. If, on
the other hand, we remember how long must have been the route, and how
diverse must always have been the conditions between the more northern
and the more southern portions of the American and Euro-Asiatic
cont