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Show 40 30 XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 349 have extended during any portion of tho Tertiary and Secondary periods, we shall obtain a foundation of inestimable value for our inquiries illto those migrations of animals :tnd plants during past ages which have resulted in their present peculi: tritios of distribution. \Ve soc, for instance, thnt the South American and African continents have always been separated by nearly as wide an ocen,n as at present, and that whateYer . imilarities there may he in their productions must be clue to the similar forms having been derived from a common mjgin in one of tho great northern continents. The radical clifforonco between the higher forms of life of the two continents accords perfectly with their perm:tnont separation. If there had been any direct connection between them during Tertiary time., we should hardly have found the deep-seated ditforences between the Quadrumana of the two regions~no family oven being common to both ; nor tho p culiar 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 thoro has been no near approach to a land connection during tho 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 earlio t Palroozoic 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 . ·outhern latitudes (of which there is not a particle of evidence) will have been necessary to account for all tho intormigmtion that has occurred between the two continents. If, on the other hand, we remember how long mu t have been the route, and how diverse must always have been the condition. hetween the more northern and the more southern portion. of the American and Euro-Asiatic continents, we shall not be |