364 FISHES 



ichthyologists, but their permanence is now rejected by the best 

 authorities. Trout and salmon have both been introduced into 

 Tasmania and New Zealand. The former have become estab- 

 lished and in many cases have become very much larger fish, 

 differing considerably from their European ancestors, while the 

 true salmon having descended to the sea as they reached the 

 smolt stage, have always failed to return. The salmon, Salvw 

 sa/ar, is a true species, distinguished from the various forms of 

 trout by having less than thirteen scales in a transverse series 

 from the posterior border of the adipose fin to the lateral line. 

 The various varieties of char, on the other hand, of which almost 

 every lake in Great Britain and Ireland has one, are not species 

 but varieties of Salmo alpinus, which is migratory and occurs in 

 the north of Europe ; the " ombre chevalier" of the Swiss lakes, 

 and the saebling of the lakes of Austria and the Bavarian Alps 

 are also varieties of the same species. 



Discontinuous variations or mutations are best known among 

 the Pleuronectidae or flat-fishes. One of these mutations is 

 of a kind which occurs also in other asymmetrical animals, and 

 consists in a complete reversal of the usual asymmetry. It has 

 already been mentioned that some species of flat-fishes are 

 normally right-sided and others left-sided, but it frequently 

 happens that a left-sided specimen of a species normally right- 

 sided is found, or vice versa. In the flounder, which is normally 

 right-sided, reversed specimens are very common. In these 

 specimens the eyes and colour are on the left side instead of the 

 right, but there is nothing else abnormal about them. At Ply- 

 mouth left-sided flounders are almost as numerous as right- 

 sided, and the fishermen sometimes state that one kind are 

 males and the other females, but there is no truth in this. It 

 is a curious and important fact that the position of the internal 

 organs is not affected by the asymmetry of the fish, whether in 

 species which are normally left-sided like the turbot or brill, 

 or abnormally so as in reversed specimens of the flounder. In 

 fact, in all flat-fishes the liver is on the left side and the coils of 

 the intestine on the right, and the adaptations of the eyes, the 

 mouth, the fins, and the colour have left the internal organs 

 unaffected. Among the other asymmetrical animals in which 

 reversed individuals occur may be mentioned the spiral-shelled 

 molluscs called Gastropods : in these the spiral shell is usually 



