268 A HISTORY OF FISHES 



formed permanent fresh-water colonies in various lakes of 

 Scandinavia, Switzerland, Scotland, Ireland, and the Lake 

 District of England, but, as they only thrive in deep cold water, 

 have not colonised the rivers like the Trout. In the same way 

 the White-fish of our lakes, such as the Pollan {Coregonus pollan) , 

 Vendace (C vandesius), Gwyniad (C pennantii), etc., are 

 descended from northern migratory species which were in the 

 habit of ascending rivers to spawn, as do certain Arctic species 

 to-day. There is little doubt that the Char and White-fish 

 reached the lakes that they now inhabit from the sea during 

 the glacial period, when the climate was considerably colder 

 and the range of migratory Salmonids extended much farther 

 south. When these again retreated northwards, isolated 

 colonies remained behind in the lakes, and these have continued 

 to evolve in various directions according to the nature of the 

 local conditions, many being now so distinct from their migratory 

 ancestors that they might well be regarded as separate species. 

 The fishes of the second category, spending their whole lives 

 in fresh water, include fluviatile species of genera otherwise 

 marine in habitat, or of genera normally anadromous, fresh- 

 water genera of marine families, or families and even suborders 

 which include only fresh-water fishes. The Bull-heads (Cottus), 

 for example, represent a typically marine genus, but the little 

 Miller's Thumb (C gobio) of Europe, a species common in the 

 rivers and streams of England and Wales, is entirely confined 

 to fresh water. The Grayling ( Thymallus) belongs to the family 

 Salmonidae, but the genus is strictly fluviatile and contains no 

 anadromous species. The family Brotulidae includes many 

 diverse genera found at great depths in the oceans, and the 

 Cuban Blind-fishes {cf. p. 233) are the only fresh - water 

 members. The Cods and their allies (Gadidae) represent another 

 big marine family, which includes a single fresh- water genus and 

 species, the Burbot {Lota lota). The Atherines or Sand Smelts 

 {Atherinidae) are little silvery fishes frequenting bays and 

 estuaries, many of them entering the rivers. In countries where 

 true fresh-water fishes are scarce or absent species of Sand 

 Smelts have become permanently resident in fresh water, and 

 there is a distinct tribe or sub-family of these little fishes in the 

 rivers of Australia and New Guinea, while other forms occur 

 in Madagascar. In the lakes of the Valley of Mexico there are 

 about twenty species of Atherines, most of which grow to a fair 

 size and are valued as food by the Mexicans, who know them 

 as '' Pescados blancos^ These must have entered the lakes (which 



