The Geographical Distribution of Fishes 101 
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, forms are arranged 
according to temperature. One by one in each series, the 
Arctic types disappear; subarctic, temperate, and semi-trop- 
ical types 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 different, each isotherm is represented in each series by 
certain general types of fishes. 
Fig. 73.—Green-sided Darter, Diplesion blennioides Rafinesque. Clinch River. 
Family Percide. 
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 West 
Pacific (East Asian) series is not a continuous line on account 
of the presence of Australia, the East Indies, and Polynesia. 
The irregularities of these regions make a number of subseries, 
which break up the: simplicity expressed in the idea of four 
