CHAPTER XIV 



BRITISH FRESHWATER FISHES— THEIR ORIGIN 

 AND GEOGRAPHICAL DISTRIBUTION 



Origin of freshwater fishes — some still spend part of their 

 life in the sea. Catadromous fishes. Anadromous fishes. 

 Freshwater colonies of marine species — Char and Whitefish — 

 true freshwater fishes, belonging to freshwater families. 

 Zoogeographical divisions. Distribution of British species — 

 poverty of Irish fish-fauna — decrease in number of species 

 northwards and westwards in Britain — our species found on 

 the Continent. Recent geological history of the British Isles. 

 Establishment of colonies of Char and Whitefish. Union of 

 Britain with the Continent— our true freshwater fishes come 

 from the Rhine and the Seine— means of dispersal in the 

 British Isles. 



NO doubt the first fishes who left their original 

 home in the sea and entered fresh water did 

 so in pursuit of food, or to escape large and 

 predaceous enemies, or to seek quiet places where 

 they could breed unmolested. Some of these visitors 

 became permanent residents, and so formed freshwater 

 races and, in time, species distinct from their marine 

 allies ; in the course of ages these spread and became 

 further diversified, and so it happens that at the present 

 day there are families, and even orders, composed 

 entirely of freshwater fishes. There is no special 

 character common to freshwater fishes ; nearly every 

 order has some freshwater representatives, and they 



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