SALMONIDiE. 643 



distinguished under distinct specific names. Most likely they 

 have been derived, at a not very remote period, from common 

 ancestors, but the question of their specific distinctness is no 

 more affected by this consideration than the question whether 

 Salmo and Coregonus are distinct genera. "Whenever the 

 zoologist observes two forms distinguished by peculiarities of 

 organisation, such as cannot be conceived to be the effects of 

 an external or internal cause, disappearing with the disap- 

 pearance of that cause, and which forms have been propagated 

 and are being propagated uniformly through all the genera- 

 tions within the limits of our observations, and are yet most 

 probably to be propagated during the existence of mankind, 

 he is obliged to describe these forms as distinct, and they will 

 commonly be called species. 



The species of the genus Salmo are inhabitants of the 

 temperate and arctic zones of the Northern Hemisphere ; the 

 species are most abundant in the northern parts of the tem- 

 perate zone, becoming scarcer beyond the Arctic circle, and 

 in the warmer parts towards the south. The southernmost 

 points in which Salmones are found, are, on the American 

 continent, the rivers falling into the head of the Californian 

 Gulf, and in the Old World the mountain rivers of the Atlas 

 and Hindu Kush. The Salmones from those localities 

 are migratory Trout in the New World, non-migratory and 

 small in the Old. Those species which range to the highest 

 latitudes (lat. 82°) belong to the division of Charr, a group 

 which generally are more intolerant of a moderate temperature, 

 than real Trout. The genus is subdivided into 



a. Salmones — Salmon and Trout — with teeth on the body of 

 the vomer as well as its head (see Figs. 289 and 290). 



6. Salvelini — Charr — with teeth on the head of .the vomer 

 only (see Fig. 291). 



Of the host of species (the majority of which is unfortu- 



