144 On the Salmon Tribe. 



any exact knowledge of the natural outlets and dams to running 

 water along the 250 miles of the Nile Valley, from Semne to As- 

 suan, it would be idle to offer even a conjecture. These marks are 

 unquestionably very difficult to account for, in the present imperfect 

 state of our knowledge of the structure of that portion of the Nile 

 Valley; and any competent geologist, well versed in the questions 

 of physical structure involved, who may hereafter visit Nubia, would 

 have a very interesting occupation in endeavouring to solve the dif- 

 ficulty. 



7th April 1850. 



On the Salmon Tribe (Salmonidce.) 



So long as the family Salmonidce remains circumcribed as it 

 was established by Cuvier, it seems to be a type almost univer- 

 sally diffused over the globe, occurring equally in the sea and in 

 fresh-vrater, so that we are left almost without a clue to its 

 natural relations to the surrounding world. Job. Muller, 

 working out some suggestions of Prince Canino, and intro- 

 ducing among them more precise anatomical characters, had 

 no sooner sub-divided the old family of Salmonidce into his 

 SalmonidcBi Characini^ and Scopelini^ than light immediately 

 spread over this field. Limited now to such fishes as, in 

 addition to the mere general character of former Salmonid(z^ 

 have a false gill on the inner surface of the operculum, the 

 SabnonidcB appeared at once as fishes peculiar to the northern 

 temperate region, occurring in immense numbers all around 

 the Artie Sea, and running regularly up the rivers at certain 

 seasons of the year to deposit their spawTi, while some live 

 permanently in fresh water. We have thus in the true Sal- 

 monidoR actually a northern family of fishes, which, when 

 found in more temperate regions, occurs there in clear moun- 

 tain rivers, sometimes very high above the level of the sea, 

 near the limits of perpetual snow, or in deep, cold lakes. 

 That this family is adapted to the cold regions is most re- 

 markably exemplified by the fact that they all spawn late in 

 the season, at the approach of autumn or winter, when frost 

 or snow has reduced the temperature of the water in which 

 they live nearly to its lowest natural point. The embryos 



