DISTRIBUTION AND MIGRATIONS 257 



barrier, and the effects of isolation continued over a long period 

 of time has led to the evolution of distinct species in the two 

 oceans. 



The vast Indo-Pacific region extends from the Red Sea and 

 the east coast of Africa eastwards through the Indian Ocean 

 and Archipelago to Northern Australia and the islands of 

 Polynesia. It includes a far greater number of genera and 

 species than the American region. The long and almost 

 unbroken coast-line provided by the huge continent of Asia 

 has enabled shore-dwelling forms to creep slowly and gradually, 

 as it were, from rock to rock and from bay to bay, and as a result 

 of this gradual extension of their geographical range some 

 species now occur both in the Red Sea and in the islands of 

 the South Seas. On the coast of West Africa the state of affairs 

 is very different to that of the Indo-Pacific, and in place of the 

 teeming and diverse fish-life of the latter region there is a 

 comparatively poor fauna. A certain number of species are 

 the same as those occurring on the Atlantic coast of America 

 and must have been able to cross the ocean at some period of 

 their history, others are identical with those found in the 

 Mediterranean, while many others exhibit a definite affinity 

 to those of the Indo-Pacific. As far as the pelagic and deep-sea 

 fishes are concerned, the Cape of Good Hope offers no barrier 

 to their dispersal, but since the isotherm of 20° C, the southern 

 limit of the Tropical Zone, cuts off the south-western part of 

 the African continent, the change of temperature provides an 

 adequate barrier to the mixing of coastal species of the two 

 faunas via the Cape (Fig. 98). Here, again, the configuration 

 of the land masses in Eocene times explains to some extent the 

 distribution of the existing forms, for during this period the 

 Mediterranean extended much farther eastwards and opened 

 into the Pacific (Fig. 97). Further, a study of the fossil fishes 

 dug up in Southern Europe reveals the presence during this 

 period of many typically Indo-Pacific genera and species, and 

 there was nothing to prevent these from ranging in the other 

 direction to West Africa. Later, the Indo-Pacific connection 

 with the Pacific closed up, but the West African fauna remains 

 as an indication of this former connection. At the present 

 time the fish fauna of the Red Sea is totally different to that of 

 the Mediterranean, but unhke the canal across the isthmus of 

 Panama, the present Suez Canal, opened in the middle of the 

 last century, probably provides a passage from one sea to the 

 other for certain species. This is only about one hundred miles 



