INTRODUCTORY 



37 



reason to adopt at once Mr. Regan's view that there is only one species 

 of trout involved of which some unknown proportion in the Loch 

 Lomond basin is migratory in its instincts and some unknown propor- 

 tion non-migratory at any given point of time. 



That the native trout of Loch Lomond descend to the Clyde estuary 

 is proved by the fact already mentioned that they are sometimes caught 

 m the estuary salmon nets, and I gave one instance of a trout taken in 

 the loch which, having sea-lice on it, may be presumed to have so 

 descended and then returned. If the objection is made that the mere 

 fact of the migratory and non-migratory trout being distinguishable, 

 even with difficulty, is an argument for, and not against, there being 

 two distinct species or races involved, it may be answered that it has 

 never been pretended that trout habituated to a salt-water environment 

 do not differ greatly from trout habituated to a fresh-water environment 

 m their exterior aspect. 



But it may be said that, after all, it is not enough to prove that one 

 sea-trout ceased to migrate and that one trout acquired the habit of 

 going to the sea ; what has to be proved is that a particular non- 

 migratory trout at some time adopted the migratory habit which there- 

 after persisted in itself and its descendants, or, on the other hand, that 

 any particular migratory trout ceased to migrate and thenceforth with 

 its descendants remained satisfied with its fresh-water environment. 

 Such proof would no doubt be final, but I do not think that Mr. Regan's 

 six points of evidence are conclusive. It seems to me, however, that 

 taken together they give grounds for a strong presumption that there 

 is no real distinction in structure or other material respect between 

 certain of the migratory and all of the non-migratory trout, and that some 

 of the offspring of any pair of either class may at any time develop the 

 migratory instinct in an extreme degree while the rest may remain 

 content with their environment. In the chapters which follow I think 



