throng in the bay, attack the Medusæ, 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 sometimes happens thus, that species of fish which have not commonly followed, the schools are attracted to them by the extraordinary abundance of food, and find their way to places where they were before unknown. Many fish are found in opposite quarters of the globe. The Trachurus trachurus, of the mackerel family, inhabits South American 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 circumstances 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 climbing fish and an ophiocephalus of the East Indies and the Doras costatus of South America are able, when the ponds and swamps in which they live are dried 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
