232 THE RIVER-SIDE NATURALIST. 



The second genus, Thymallus, has only one British 

 representative 



The Grayling (Thymallus vulgaris). 



The third genus, Coregonus, placed by some as the 

 second, has three representatives 



The Guyniad, Schelly, or Powan (Coregonus clu- 



peoidis). 



The Vendace or Vendis (Coregonus vandesii). 

 The Pollan (Coregonus pollan). 

 The fourth genus, Osmerus, only one representative 



The Smelt (Osmerus cperlanus). 



Much difference of opinion exists as to whether many of 

 the British forms of the genus Salmo are to be considered 

 as constituting species or varieties. 



The term species in natural history is employed to 

 designate groups inferior to genera, but superior to 

 varieties. Formerly, it was supposed the species were 

 unchanging throughout the longest successions of genera- 

 tions, and would be defined as individual plants, animals, 

 &c., agreeing in their appearance and composition, their 

 similarity giving rise to the establishment of species ; 

 and individuals or species differing in circumstances arising 

 from accident in plants or animals, from soil or climate, 

 were termed varieties. 



About the end of the seventeenth century John Ray 

 limited the term species in its' natural history sense. 

 His specific characters rested more especially on constant 

 resemblance in outward form, but also on close resem- 

 blance of offspring to parent. At the same time he 

 recognised variability. 



Linnaeus's aphorism, " Tot sunt diver see, quot diver see formce 

 ab initio sunt creatce" notwithstanding Buffon's objection, 

 was in a great measure recognised by Cuvier, and held in 

 general acceptance until Darwin subverted it in 1859 on 

 the publication of the " Origin of Species." 



Darwin maintained the variability of species, the varia- 

 tion continually taking place according to the external con- 

 dition to which plants and animals are exposed. He thinks 



