64 



of the southern North Sea the Forth and Northumberland schools 

 have been described as being unrepresentative and insignificant. 

 This is not quite the same thing as attempting to explain the differ- 

 ence with reference to temperature and food, and I venture to 

 suggest that the explanation offered in terms of currents satisfj^ 

 all the conditions. If it be said, as it has been said, that a species 

 like the plaice migrates from one region to another to obtain a 

 particular food, examples could be given of species the schools of 

 Avhich successively replace one another along the same current 

 route. 



The general migrations of the salmon are understood and can 

 be specified, but variation occurs, as I have pointed out. The 

 length of the fresh water period of hfe varies with the conditions, 

 and even in the same river. The time of the return is also 

 subject to a deal of variation even with reference to the same 

 river, but more especially vhen different rivers are contrasted. 

 So much is this evident that it is not wise to sslj that what is true 

 of the Wye, for example, must be true of other rivers. In spite 

 of this variation, however, we would not hesitate m saying that 

 the migrations of the salmon are essentially the same throughout 

 the area of its distribution. 



I said in my work that beyond drawing attention to the 

 importance of hydrographical conditions at the limits of distri- 

 bution and the direct importance of currents I did not find it 

 necessary to introduce hydrographical considerations. I did so 

 because I did not find any proof that they play an important 

 part. I do not wish it to be inferred, however, that I have any 

 desire to depreciate the value of work in this direction ; but, if 

 we are to utilise the immense amount of material aheady collected, 

 v/e must consider each problem separately and the factors of 

 importance affecting the others will advance into their proper 

 position. 



