lxxxvih MIGRATIONS AND FOOD. 



that eleven species of fishes from the Mediterranean have penetrated via the 

 Suez Canal into the Red Sea, and twenty-five have reached at least half-way 

 from the Bed Sea through the same canal. 



The following species identical with British forms have been taken in the 

 seas of Australia, but not in the intermediate intertropical region : Oaranx 

 trachurus, Zeus faber, Scicena aquila, Argentina sphyrcena, Engraulis 

 encrasicholus (variety), Clupea sprattus, Conger vulgaris, Orthagoriscus 

 mola, Galeus canis, Acanthias vulgaris (variety), Echinorhinus spinosus, 

 Rhina squatina. 



It has been abundantly shown that fishes of colder climes do not unfre- 

 quently migrate towards warmer regions, but when doing so keep further 

 out from shore in deeper waters and colder areas. Although it seems 

 difficult to admit that fishes with a physoclistous form of air-bladder could 

 migrate in the cold water under the tropical sea, and thus, as it were, 

 reappear in the southern temperate ocean, still in nature we do see identical 

 forms in these two widely separated places. But in their younger stage 

 all fishes with air-bladders must have the connecting tube with the 

 alimentary canal pervious, and it is not so difficult to consider that in some 

 it might abnormally remain patent through life, giving them the same facility 

 of evacuating it as seen in physostomous forms, while M. Regnard's experi- 

 ments already alluded to (p. lxxxv) show what a large amount of pressure 

 fishes will bear without fatal effect, while in nature they would gradually 

 accommodate themselves to such changes. 



MIGKATIONS AND FOOD. 



Most fishes are subject to migrations, which are either seasonal, which is 

 consequent upon climatic changes, or for the purpose of reaching a suitable 

 spot for the continuation of their race (page lix), or diurnal or nocturnal, to 

 obtain food for the nourishment of each individual. This latter is mostly 

 dependent on temperature, certain conditions of the water, and the supply 

 of sustenance. An irregular appearance of migratory shoals of marine 

 forms, as the mackerel, anchovy, herring, pilchard, or sprat, may be 

 consequent upon their pursuing some peculiarly abundant and desirable 

 food, or else be caused by their having been alarmed by some undesirable 

 enemy, for fear may cause shoals to vary their direction ; while I have 

 observed that in the Indian seas, when from any reason the sardines have 

 remained away, voracious forms which prey upon them were likewise 

 absent. 



Temperature evidently exercises a considerable influence, and Baird 



