106 GEOGRAPHICAL DISTRIBUTION 
The Cetacea are not limited to the ocean, or even to salt water, 
some entering large rivers for considerable distances, and others 
being exclusively fluviatile. One species of Platanista is extensively 
distributed throughout nearly the whole of the river systems of the 
Ganges, Brahmaputra, and Indus, ascending as high as there is 
water enough to swim in, but apparently never passing out to sea. 
The individuals inhabiting the Indus and the Ganges must therefore 
have been for long ages isolated without developing any definite 
distinguishing anatomical characters; for those by which the sup- 
posed P. indi was formerly separated from 2. ygangetica have been 
shown by Anderson to be of no constant value. Oreella fluminalis 
appears to be limited to the Irawaddy river, and at least two distinct 
species of Dolphin belonging to different genera are found in the 
waters of the upper Amazon. A Neomeris has been found in the 
great Chinese river, the Yang-tsi-Kiang, nearly a thousand miles 
from the sea. It is remarkable, however, that none of the great 
lakes or inland seas of the world are, according to our present 
knowledge, inhabited by Cetaceans. A regular seasonal migration 
has been observed in many of the oceanic Cetacea, especially those 
inhabiting the North Atlantic, but further observations upon this 
subject are still much needed. 
The great difference in the manner of life of the Sirenia, as 
compared with that of the Cetacea, causes a corresponding difference 
in their geographical distribution. Slow in their movements, and 
feeding exclusively upon vegetable substances, water-grasses, or fuci, 
the Sirenia are confined to rivers, estuaries, or coasts where these 
grow, and are not denizens of the open sea, although of course there 
is a possibility of accidental transport by the assistance of oceanic 
currents across considerable distances. Of the three genera exist- 
ing within historic times, one (J/anatus) is exclusively confined to 
the shores of the tropical Atlantic and the rivers entering into it, 
individuals scarcely specifically distinguishable being found both on 
the American and the African side of the ocean. The Dugong 
(Haticore) is distributed in different colonies, at present isolated, 
throughout the Indian Ocean from Arabia to North Australia. 
The Rhytina or Northern Sea-Cow was, for some time before its 
extinction, limited to a single island in the extreme north of the 
Pacific Ocean. 
The Pinnipeds, although capable of traversing long reaches of 
ocean, are less truly aquatic than the last two groups, always 
resorting to the land or to extensive ice-floes for the purpose ‘of 
breeding. The geographical range of the various species is generally 
more or less restricted, usually according to climate, as they are 
mostly inhabitants either of the Arctic or Antarctic seas and adjacent 
temperate regions, very few being found within the tropics. For this 
reason the northern and the southern species are for the most part 
