Besides these active river-fish there are the little stupid Miller’s Thumbs,[51] hiding under the stones to feed on tiny animals; they are feeble relations of the gurnards which we saw walking on the bottom of the sea. Then there are the purely freshwater fish, the Pike and the large Carp family, with its many branches, the Roach, and Dace, and Gudgeon, and Minnow; and the enormous family of Cat-fish and Sheat-fish,[52] of which we have none in England, but plenty in America and other parts of the world, a family in which the fathers sometimes carry the eggs in their mouths till hatched. And last but not least among the freshwater forms is that irrepressible family of the Eels which we saw wandering in the deep sea, and which are also to be found near the shores all over the world. These fish will even travel through pipes and into cisterns; and will climb up trees so as to drop into neighbouring streams and continue their wanderings; they sleep in the mud in winter; and even after being frozen come to life again; and in the spring they go to the sea to spawn, giving rise to those shoals of young ones from three to five inches long which come in incredible numbers up the rivers in summer, making the eel-fairs,[53]-- “The silver eel, in shining volumes rolled,” so much spoken of in old books, when the eels will often climb high banks, nay, even pass over miles of dry land, closing down their narrow gill-openings, and so shutting in water to serve them as they go. All these, and many other freshwater families, show us how the fish have wandered into every possible nook of the waters, so that even in those inland salt lakes of North America and Asia into which no rivers flow fish-life is abundant; and we can only suppose that the eggs must have been carried by water-birds in their flight, or by gusts of wind, or have arrived there in ages long ago, before these lakes were cut off from the rest of the watery world. Yet some few fish besides the eels have been known to travel over land to find watery “pastures new;” the Climbing Perch[54] of India and the Doras of Tropical America will both travel many miles when their own ponds are dried, the perch breathing by the help of a special apparatus, and the doras probably shutting water into its gills; for necessity, even in fishes, proves the “mother of invention,” and in special works on fish you will find accounts of numberless strange devices and adaptations by which they manage to survive in the struggle for life.