GEOGRAPHICAL DISTEIBUTION. 203 



Kiang, the Amazons, the Congo, etc. This is evidently the 

 commencement of a change in a iish's habits, and, indeed, not 

 a few of such fishes have actually taken up their permanent 

 residence in fresh waters (as species of Ambassis, Apogon 

 Dules, Therapon, Scisena, Blennius, Gobius, Atherina, Mugil, 

 Myxus, Heniirhamphus, Clupea, Anguilla, Tetrodon, Trygon)-: 

 all forms originally marine. 



On the other hand, we find fishes belonging to fresh- 

 water genera descending rivers and sojourning in the sea for 

 a more or less limited period ; but these instances are much 

 less in number than those in which the reverse obtains. We 

 may mention species of Salmo (the Common Trout, the 

 Northern Charr), and Siluroids (as Arius, Plotosus). Gore- 

 gonus, a genus so characteristic of the inland lakes of Europe, 

 Northern Asia, and North America, nevertheless offers some 

 instances of species wandering by the effluents iato the sea, 

 and taking up their residence in salt water, apparently by 

 preference, as Coregonus oxyrhynchus. But of all the Fresh- 

 water families none exhibit so great a capability of surviving 

 the change from fresh into salt water, as the Gastrosieidce 

 (Stickle-backs), of the northern Hemisphere, and the equally 

 diminutive Cyprinodontidm of the tropics ; not only do they 

 enter into, and live freely in, the sea, but many species of the 

 latter family inhabit inland waters, which, not having an 

 outlet, have become briny, or impregnated with a larger pro- 

 portion of salts than pure sea water. During the voyage of 

 the " Challenger '' a species, of Fundulus, F. nigrofasciatus, 

 which inhabits the fresh and brackish waters of the Atlantic 

 States of North America, was obtained, with Scopehds and 

 other pelagic forms, in the tow-net, midway between St. 

 Thomas and Teneriffe. 



Some fishes annually or periodically ascend rivers for the 

 purpose of spawning, passing the rest of the year in the sea, 

 as Sturgeons, many Salmonoids, some Clupeoids, Lampreys, 



