INTRODUCTORY 17 



also designates " the common yellow trout," he writes : — " Many 

 naturalists maintain that there are different species of trout in the British 

 Islands — Loch Leven trout, Gillaroo trout, tidal trout, and many others; 

 but from a close study of all these trout for the last forty years, I have 

 come to the conclusion that there is only one species of trout in Great 

 Britain, and that in the different varieties the differences are caused by 

 the nature of the water in which they are found and by the food they 

 eat." These views are valuable as being those of a practical man, who, 

 had he studied the sea-trout as long as he had studied the trout, might 

 have simplified matters still further. It was left to Mr. Regan, as a 

 scientific man, to take the more advanced step. "Although the silvery 

 Sea-trout and non-migratory Brown Trout," he writes, " differ so much 

 in habits and appearance, there are no structural differences, and the 

 young are indistinguishable." 



I may say here that in my account of the life-history of the sea-trout 

 which follows I shall have occasion from time to time to comment on 

 the main theory that in the British Isles there is only one species of 

 trout, and I hope that such observations as I am able to make will help 

 the reader to decide for himself how far the theory is in accordance 

 with fact. Without doing so in so many words, in connection with every 

 point discussed, I will, I trust with perfect fairness and apparent 

 reason, substantiate the theory where in my opinion it is capable of 

 being substantiated, and refute it with equal candour if I find it 

 necessary to refute it. But, in the meantime, standing thus as I have 

 quoted it, the theory of which Mr. Regan has been the most recent 

 authoritative exponent is so striking in itself that it will be interesting 

 to consider the views by which he supports it. 



As a good systematist, Mr. Regan gives us first a scheme of classifi- 

 cation of our British freshwater fishes, and those who prefer order and 

 method in their approach to any subject will naturally wish it to be 

 outlined, however briefly. The reader who is deeply interested in such 



