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 



