304 ANIMAL LIFE 
ence increases with time and with distance so long as the 
separation is maintained. Hence new species and new 
fauna or aggregations of species are produced wherever 
free diffusion is checked by any kind of barrier. 
161. Faunal areas of the sea.—In like manner, we may 
divide the oceans into faunal areas or zones, according to 
the distribution of its animals. For this purpose the fishes 
probably furnish the best indications, although results very 
similar are obtained when we consider the mollusks or the 
crustacea. The fresh-water fishes are not considered here, 
as in regard to their faunal areas they agree with the land 
animals of the same regions. Perhaps the most important 
basis for primary divisions is found in the separation from 
the localized shore-fishes of the cosmopolitan pelagic species, 
and the scarcely less widely distributed bassalian species or 
fishes of the deep sea. 
The pelagic fishes are those which inhabit the open sea, 
swimming near the surface, and often in great schools. 
Such forms are mainly confined to the warmer waters. 
They are for the most part predatory fishes, strong swim- 
mers, and many of the species are found in all warm seas. 
Most species have special homing waters, to which they 
repair in the spawning season. Often there will be special 
regions to which they never resort, either for breeding or 
for food. At other times a certain species will appear in 
numbers in regions where it has hitherto been unknown. 
For example, the frigate-mackerel (Auzis thazard), homing 
in the East Indies and the Mediterranean, appeared in 
great numbers in 1880 off the coast of New England. Typ- 
ical pelagic fishes are the mackerel, tunny, dolphin, flying- 
fish, opah, and some species of shark. This group shades 
off by degrees into the ordinary shore-fish, some being partly 
pelagic, venturing out for short distances, and some are 
pelagic for part of the year only. To the free-swimming 
forms of classes of animals lower than fishes, found in the 
open ocean, the name Plankton is applied. 
