GEOGRAPHICAL DISTRIBUTION. 13 



when forked in the young 1 , may become truncated or rounded in the old fish. 

 The Lampreys pass through a sort of metamorphosis, by which changes equi- 

 valent to a generic modification take place when the fish is three or four years 

 old. The size of a fresh-water fish varies with its food, and many varieties 

 which are small in one locality become large where more suitable food 

 abounds. It is impossible at present to estimate the importance of the 

 changes which varieties of food may induce, for when fishes which usually 

 feed on small animals take to eating mollusca, the stomach may become 

 thickened, as in some species of Trout, and the number of pyloric appen- 

 dages appears also to vary with the food. The odour of fish would seem 

 to depend on the food, and some insects possess a power of stimulating 

 growth in the fishes which eat them, which is out of proportion to the 

 weight of the insects eaten. 



GEOGRAPHICAL DISTRIBUTION. 



Numerical statements of the distribution of species necessarily vary with 

 the size of the species, and, therefore, species are scarcely so instructive in 

 illustrating geographical distribution as genera. Thus, excluding types 

 like the Bass, Grey Mullets, Atherine, Flounders, Plaice, and Soles, there 

 are about forty-one genera of fresh-water fishes in Europe, and of these, 

 twenty-five, Perca, Acerina, Cottus, Gasterosteus, Lota, Cyprinus, Carassius, 

 Barbus, Gobio, Leuciscus, Tinea, Abramis, Alburnus, Misgurnus, Nemachilus, 

 Cobitis, Clupea, Salmo, Osmerus, Coregonus, Thymallus, Esox, Anguilla, 

 Acipenser, and Petromyzon, are represented in the British Islands, to 

 which, however, no genus is limited. 



The Acanthopterygian fishes are not so numerous in Great Britain as on 

 the Continent, and are best represented- in Austria, Prussia, and Germany. 

 Austria and Russia have largest fish fauna. Both countries are rich in 

 Sturgeons, Salmon, Herrings, and Cyprinoid fishes. By means of the annexed 

 table, which gives the geographical range of the principal species, with their 

 popular names in the chief European countries, a better idea of the distribution 

 of the fishes will be gained than can possibly be conveyed in any analytical 

 statement. For further details we must refer to the body of the work, where 

 about two hundred and fifty fishes are described. 



Weights and measurements of fishes are frequently given in the metric 

 system. A kilogramme is 2'2 pounds avoirdupois. A metre is 39*37 inches. 

 The centimetre is about four- tenths of an inch. 



