LITTORAL MARINE FAUN^. 233 



into two great primary divisions, the Polar and the Tropical ; the Polar being 

 subdivided into North- and South-Polar and North- and South-Subpolar, the 

 Tropical into Indo- and Atlantico-Tropical and North- and South-Subtropical. 

 A similar mode of division is adopted by GUnther * and Gill t in treating of 

 the distribution of shore fishes. Gill in particular protests against making 

 the lay of the continents the prime factor in the distribution of littoral 

 marine animals, re-affirming what had long before been pointed out by 

 Milne Edwards and others : " The tiopical faunas are much more closely 

 related to one another than they are to the faunas along the same reach of 

 slioi'e toward the arctic or antarctic regions. This relationship is evinced 

 more or less in every class and branch of animals. . . . Consequently, the 

 marine faunas cannot be at all correlated with the primar}- [teriestrial or 

 inland] realms or regions of the globe." He then proceeds logically to 

 divide the littoral marine fauna into five primary circumterrestrial realms 

 whose boundaries are determined by the isocrymal lines. These realms are 

 the Arctic, Pararctic or North Temperate, Tropical, Notalian or South Tem- 

 perate, and Antarctic. 



It is true, as Gill well says, that the relations between the littoral marine 

 fiiunte in a longitudinal direction are traversed and complicated by relations 

 existing in a latitudinal direction. This must necessarily' result from the easy 

 routes of migration afforded by the great coast lines and fioni the dispersal 

 of the larvae of tropical species northward and southward by the deflected 

 equatorial currents. But, on the whole, the change of temperature encoun- 

 tered in passing from low to high latitudes has proved a barrier to the spread 

 of tropical littoral types northward — a more effectual barrier, it would seem^ 

 than the immense distances between the tropical shores of the different 

 continents have proved to be against the intertropical dispersal of such 

 types around the globe. Every summer myriads of delicate larvaa, belong- 

 ing to tropical and subtropical genera, such as Ocyjiodc and Calappa, are 

 borne on the warm bosom of the Gulf Stream to the southern shores of New 

 England, only to perish on the approach of the northern winter. Yet these 

 same genera are represented by flourishing colonies established on tropical 

 shores around the whole girdle of the globe. Geological evidence goes to 

 show that the tropical Atlantic and Pacific were formerly connected over the 

 region now occupied by the Isthmus of Panama, Central America, and parts 



» Introduction to the Study of Fishes, p. 259, ISSO. 



t The Nation, XXV. 43, 1877 ; Proc. Biolo,;?. Soc. Wasliiugton, II. .32-30, 1885. 



