GREAT BRITAIN. 39 



their home in the colder north, from whence, at certain 

 seasons, they migrated southwards in specified lines. Of 

 late years it has been generally held that, instead of their 

 making these long annual migrations, they merely pass 

 from the shallower into the deeper waters, from whence 

 they return as spring sets in, and impelled either in their 

 search for food, or else seeking for a favourable locality in 

 which to continue their species. It is often evident that 

 temperature exercises a considerable influence on these 

 migrations, while local circumstances may occasion certain 

 forms to intermit visiting coasts which they have been 

 accustomed to yearly resort to. This is more especially 

 seen in the herrings and flat fishes, and, as a consequence, 

 in those predaceous forms which prey upon them. Various 

 conjectures have been hazarded as reasons for fishes for- 

 saking certain localities, as the firing of guns in a sea- 

 battle, burning of kelp on sea-shores, the regular running 

 of steamboats which were not previously in existence, 

 fouling of the water from various causes, and other reasons 

 which will be shortly alluded to further on. 



It is well known that strictly fresh- water fish, as rarp, 

 cannot, as a rule, live in sea-water ; thus in Norfolk it has 

 been observed that if the salt water pass up the rivers, the 

 first fish that suffer are the pike, bream, and roach. Perch, 

 however, will bear a strong admixture of salt, while eels 

 seem to resist its influence altogether. 



Marine fishes have been known to either become tem- 

 porary or permanent residents of fresh waters. This has 

 been sometimes due to their having ascended rivers or 

 entered bays, when, with an ebbing tide, they were unable 

 to return. In the Baltic, at its northern portion, certain 

 marine fishes are found, where the water is scarcely saline, 

 whereas they are absent from southern portions of that 



