LITTORAL MARINE FAUNZ. 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 Giinther* and Gill ¢ 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 tropical faunas are much more closely 
related to one another than they are to the faunas along the same reach of 
shore 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 primary [terrestrial 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 
faunze 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 from the dispersal 
of the larve 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 larve, belong- 
ing to tropical and subtropical genera, such as Ocypode 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, 1880. 
+ The Nation, XXV. 43, 1877; Proc. Biolog. Soc. Washington, IT. 32-36, 1885. 
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