The Geographical Distribution of Fishes 247 



Littoral Fishes. — The shore fishes are in general the most 

 highly specialized in their respective groups, because exposed 

 to the greatest variety of selecting conditions and of competi- 

 tion. Their distribution in space is more definite than that of 

 the pelagic and bassalian types, and they may be more defi- 

 nitely assigned to geographical areas. 



Distribution of Littoral Fishes by Coast-lines. — Their distri- 

 bution is best indicated, not by realms or areas, but as form- 

 ing four parallel series corresponding to the four great north 

 and south continental outlines. Each of these series may be 

 represented as beginning at the north in the Arctic fauna, 

 practically identical in each of the four series, actually identical 

 in the two Pacific series. Passing southward, fonns are arranged 

 according to temperature. One by one in each series, the 

 Arctic types disappear; subarctic, temperate, and semi-trop- 

 ical tvpes take their places, giving way in turn to south-tem- 

 perate and Antarctic forms. The distribution of these is modi- 

 fied by barriers and by currents, yet though genera and species 

 may be dift'erent, each isotherm is represented in each series by 

 certain general types of fishes. 



Fig. 176.— Green-sided Darter, Di.pIe.non hiennimdcs Rafinesque. Clinch River. 



Family PerrirUc. 



Passing southward the two American series, the East At- 

 lantic and the East Pacific, pass on gradually through temperate 

 to Antarctic types. These are analogous to those of the Arctic, 

 and in a few cases they are generally identical. The AA'est 

 Pacific (East Asian) series is not a continuous hnc on account 

 of the presence of Australia, the East Indies, and Polynesia. 

 The irregularities of these regions make a numl:)er of subserics, 

 which break up the simphcity expressed in the idea of four 



