640 FISHES. 



must be negatived for the present. Several instances of suc- 

 cessful experiments made for this purpose have been brought 

 forward ; but all these accounts are open to serious doubts, 

 inasmuch as they do not afford us sufficient proof that the 

 young fish introduced into ponds were really young migra- 

 tory Salmonoids, or that the full-grown specimens were 

 identical with those introduced, and not hybrids or non- 

 migratory Trout of a somewhat altered appearance in conse- 

 quence of the change of their locality. We have seen the 

 experiment tried at two places in South Wales, and in both 

 cases the Salmon and the pure Sewin died when not allowed 

 to return to the sea. On the other hand, hybrid fishes from 

 the Sewin and the Trout survived the experiment, and con- 

 tinued to grow in a pond perfectly shut up from communica- 

 tion with the sea. In that locality neither those hybrids nor 

 the trout spawn. 



4. Although the majority of the mature individuals of a 

 migratory species ascend a river at a certain fixed time before 

 the commencement of spawning, others enter the fresh-water 

 at a much earlier period, either singly or in small troops ; 

 and many appear to return to the sea before they reascend 

 at the time of the regular immigration. It is not improbable 

 that one and the same individual may change the salt- or 

 fresh- water several times in the year. However, this is the 

 case in certain rivers only, for instance, in those falling into 

 the Moray Firth ; in others one immigration only is known 

 to occur. The cause of the irregular ascents previous to the 

 autumnal ascents is unknown. A part, at least, of the hybrid 

 fishes retain the migratory instinct; but it is not known 

 whether sterile individuals accompany the others in their 

 migrations, 



5. It is said that the migratory species invariably return 

 to the river in wliich they are bred. Experiments have 

 shown that this is normally the case ; but a small proportion 



