70 THE SALMON. 



versa ; and further, that the two fish are found not only 

 together, but bearing numerically a certain rough propor- 

 tion to each other. In the earlier stages of the great parr 

 controversy, as already mentioned, a similar fact that 

 where salmon were present, parr were present, and 

 wherever either were absent, so were the others played 

 a great part, forming the chief weapon of the supporters 

 of the theory that the parr is the fry of the salmon, but 

 was not held conclusive till afterwards confirmed by Mr. 

 Shaw's and subsequent experiments. Neither in the 

 present case can the fact of grilse and salmon being 

 always both present or both absent be held as conclu- 

 sively proving a connexion, though it supplies a very 

 strong presumption ; and the supplementary evidence, 

 though attainable, is not yet forthcoming in a complete 

 shape. But we are not altogether without evidence 

 additional to the fact of the two fish being always co- 

 inhabitants. 



The difticulty we have here to meet is the fact that 

 the presence of the salmon and grilse in this or that 

 river might be explainable merely by the facts that 

 they are both migratory fish, and that the rivers are 

 accessible or inaccessible to both alike. But salmon and 

 grilse are riot the only migratory fish ;; and what if \ve 

 can show that other migratory fish of the same genus 

 abound in some rivers, and are almost unknown in 

 others equally accessible, whilst the same thing is never 

 seen in the case of salmon and grilse ? that is, there 

 are no rivers almost destitute of grilse and abounding 

 in salmon, or the opposite. Mr. Mackenzie says, " The 

 Tweed bull-trout, commonly known as the ' black-tail/ 



