266 BRITISH FRESHWATER FISHES 



families just mentioned the two first are essentially 

 northern, but the two last show their greatest variety 

 in the Indian region, which may be regarded as their 

 original home ; all four of these are represented in 

 the British Isles. 



The Grayling belongs to the Salmon family, but to 

 a genus {Thymalhis) which is strictly fluviatile and 

 does not, like the other genera, include anadromous 

 species. The Burbot is the only freshwater fish of 



Fig. 25. — Distribution of Pike family {Esocida). 



the Cod family, and is generically distinct from any 

 of the marine forms. The Miller's Thumb and 

 Planer's Lamprey are freshwater species of genera 

 which are respectively marine and anadromous, and 

 our Ten-spined Stickleback is a freshwater race of a 

 species which is marine in the far north. But the 

 evidence from their distribution and from their identity 

 with continental forms leads to the conclusion that, 

 so far as we are concerned, these are true freshwater 

 fishes which reached us from the Continent. Thus 

 they may be added to the species of the four families 

 above mentioned in the following list of the 



