PRINCIPLES OF PALEONTOLOGIC CORRELATION 689 



same conditions existed then as in the present Mediterranean, 

 except that the extent was vastly greater. 



And even on opposite sides of great north and south oceans 

 there are usually many species in common ; the Atlantic shore 

 American fauna has many European species, and the Pacific 

 shore harbors some from Asiatic waters. Their passage was 

 affected in most cases along continental borders that have since 

 been obliterated by subsidence and erosion. We have an abun- 

 dance of geologic and biologic evidence that just such changes 

 have taken place in comparatively recent time, for example, the 

 dismemberment of the old Antillean continent since Tertiary 

 time. Also in the Indian Ocean there existed a continent in late 

 Paleozoic and early Mesozoic time, connecting Australia with 

 Asia; and Wallace 1 has shown that even since the Tertiary, 

 Australia has been connected with many of the now separated 

 islands of the Indian Ocean, although cut off even then from 

 Asia. 



The occurrence of identical or very closely related species 

 in widely separated localities is good evidence of migration from 

 one of these localities to the other, or from a third region to 

 both. In many cases faunas even appear unheralded by local 

 ancestors ; these are exotic, having been brought in by migra- 

 tion from outside regions. In the chapter on paleontologic 

 zones many of these exotic faunas have been enumerated, and 

 the general statement made that their appearance invariably 

 coincides with a time of shifting of the boundaries of land and 

 sea, and consequent opening of new connections. 



Colonies. — It is often noticed that species or faunas are inter- 

 mittent in occurrence, especially when the character of the sedi- 

 ments is shifting. When sands are being deposited in shallow 

 waters certain animals find their favorite habitat there, and when 

 subsidence cuts off the clastic sediments and the waters become 

 clear, other animals hold sway. Such faunal changes are due 

 to the facies of sedimentation, but both sorts lived in the same 



1 Geographical Distribution of Animals, Vol. II, The Australian Region, pp. 387 

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