XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 349 
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 peculi¬ 
arities 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 indica¬ 
tions 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, some¬ 
times 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 be¬ 
tween the more northern and the more southern portions of 
the American and Euro-Asiatic continents, we shall not be 
