INTRODUCTORY 31 



the sea, they gradually reached a stage when sea-water became a deadly 

 poison to them. Whereas, if we imagine the original habitat to have 

 been fresh water it is plain that the breed might easily be perpetuated 

 throughout the ages, as in fact it has been. 



It is unnecessary to prove why some fish adopted the migratory 

 habit and not others of the same species, but at the same time it is 

 plausible to suppose that pursuit of food was the determining factor. 

 It is at any rate plain, if we suppose the trout to have had a fresh-water 

 origin elsewhere than in Britain, and to have become migratory, that the 

 migratory habit, having once become established, accounts for the 

 trout's wide distribution throughout the Arctic and temperate regions. 

 It was a simple matter then for it to spread from the river of its birth 

 to any other river it might chance upon in the course of its wanderings 

 in the sea. Indeed, in the passage quoted, Mr. Regan very convinc- 

 ingly shows that this is the method by which our British waters received 

 their stock of trout. From the sea they came, doubtless, to our shores, 

 but it is not unreasonable to assume that they came from rivers in other 

 parts of the Continent of Europe, or rivers elsewhere, devoid of ice, 

 where they had their origin. It seems to me, then, that equally with the 

 trout being derived from a marine sea-trout, the facts point to the sea- 

 trout being a derivative of a fresh-water trout; or, in other words, the 

 proposition may as fairly be stated that the migratory sea-trout is a 

 trout, as that the trout is a non-migratory sea-trout. 



I understand that this is a question upon which science itself has 

 come to no common agreement, although I am unaware of the precise 

 weight of authority which may be ranged on either side. Mr. George 

 A. Boulenger, F.R.S., the learned and courteous successor of Dr. 

 Giinther at the British Museum, thinks alike with Mr. Regan, who is 

 his colleague and assistant. Dr. A. Noel Paton, as quoted by Mr. 

 Calderwood, considers that " the Salmonidcs are originally fresh-water 

 fish." Mr. Calderwood is himself for a marine origin, and I cannot do 



