228 THE ZOOLOGIST. 



lives in the Mediterranean and on the African coast and islands 

 and another in the West Indies. We can hardly believe that these 

 creatures could easily traverse the whole Atlantic. The hypo- 

 thesis of a former barrier of land between Africa and America, 

 which we know is supported by other facts of distribution,* would 

 alone explain the difficulty. 



On the other hand, in the Pacific we find no such break 

 between the north and south. The aquatic mammals of Noto- 

 pelagia have evidently had free access to the whole Pacific for a 

 long period and have well availed themselves of this facility. 



Again, while the great Southern Ocean exhibits a considerable 

 uniformity of marine mammalian life, we see the Northern waters 

 divided into two distinctly recognizable regions by the interposed 

 masses of land. All these facts, with the one exception of the 

 supposed Atlantic Barrier, would tend in favour of the now 

 generally accepted doctrine that the principal masses of land and 

 water are not of modern origin, but have existed mainly in their 

 present shapes throughout all ages. 



ToPOMORPHS OF THE SlX SeA-ReGIONS. 



(1) Akc r atlantis.— Ha Uchcerus. Cystophora. — Hyperoodon. 



Delph inapterus. Monodon. 



(2) Mesatlantis. — Monachus. — Manatus. — Inia. Pontoporia. 



(3) Indopelagia. — Halicore. — Platanista. 



(4) Arctirenia. — Otaria. — Rhytina. Rhachianectes. 



(5) Mesirenia. — Otaria. Macrorhinus. 



(6) Notopelagia. — Ogmorhinus. Lobodon. Leptonychotes. 



Ommatophoca. Otaria. — Neobalcena. Berardius. 



* Cf. Wallace, Geogr. Distrib. i. 156. 



