THE MIGRATIONS OF FISHES. 215 



throng in the bay, attack the Med usee, and consume them. Mackerel 

 feed upon minute crustaceans, chiefly copepods, which swarm on the 

 surface of the water and often cover it, and follow them hither and 

 thither as they are carried about by the currents. 



The number of fish which lead an irregular vagabond life is not 

 inconsiderable. Foremost among them are the sharks, which singly 

 or in small companies will follow a ship for days at a time in order to 

 snap up whatever may be thrown overboard from it. They are often 

 accompanied by the pilot-fish, which has a peculiarly strong sense for 

 food of all kinds, and directs the shark, is protected by him, and gets 

 a share of the spoil. Other formidable fish, unsocial in their habits, 

 being scattered over the ocean, are less accessible to science. 



When more than the usual number of fish go up to spawn, the 

 number of fish pursuing them is likely to be also increased. It some- 

 times happens thus, that species of fish which have not commonly fol- 

 lowed, the schools are attracted to them by the extraordinary abun- 

 dance of food, and find their way to places where they were before 

 unknown. Many fish are found in opposite quarters of the globe. 

 The TraelmriLS tracJmrus, of the mackerel family, inhabits South Amer- 

 ican and Australasian as well as British waters. The sprat, common 

 in the North European seas, has been discovered near the coasts of 

 Tasmania, and thus lives at diametrically opposite points, while it has 

 never been observed in the intervening seas. Inasmuch as migrations 

 may often lead to a permanent enlargement of the domain of certain 

 species, a knowledge of the laws and circumstances by which they are 

 influenced has an important bearing on the study of the geographical 

 distribution of species. 



Migrations may also be performed under the influence of circum- 

 stances not connected with reproduction or the search for food. It is 

 not certain whether fish are ever' driven from their homes by a cooling 

 of the water. Removals from such a cause would not take place in 

 large masses, and might easily escape observation. As a rule, fish are 

 not sensitive to changes of temperature, and can endure the greatest 

 diversities provided they have food enough. Certain tropical fishes 

 have a remarkable faculty of performing journeys by land. The climb- 

 ing fish and an ophiocephalus of the East Indies and the Doras costa- 

 tus of South America are able, when the ponds and swamps in which 

 they live are di'ied up, to travel for several hours over the land to 

 find places affording more water. The eel has been said to travel 

 for considerable distances from one pond to another. It is certain 

 that eels are able to live for a considerable time out of the water, 

 and, though the fact has not been scientifically established, there is no 

 reason to doubt that they can travel. The stickleback is often found 

 in pools wholly unconnected with other waters. It may be that the 

 eggs of the fish have been carried on the feet of waterfowl, or that 

 the wanderers have found their way to such places during the rains of 



