and rivers communicating with the Danube—­though some of these lakes and morasses are wonderfully fitted for them, and though they are found abundantly in the same countries, in lakes and rivers connected with the ocean and the Mediterranean.  Yet, when brought into confined water in the Danube, they fatten and thrive there.  As to the instinct which leads young eels to seek fresh water, it is difficult to reason; probably they prefer warmth, and, swimming at the surface in the early summer, find the lighter water warmer, and likewise containing more insects, and so pursue the courses of fresh water, as the waters from the land, at this season, become warmer than those from the sea.  Mr. J. Couch, in the Linnaean Transactions, says the little eels, according to his observation, are produced within reach of the tide, and climb round falls to reach fresh water from the sea.  I have sometimes seen them in spring, swimming in immense shoals in the Atlantic, in Mount Bay, making their way to the mouths of small brooks and rivers.  When the cold water from the autumnal flood begins to swell the rivers, this fish tries to return to the sea; but numbers of the smaller ones hide themselves during the winter in the mud, and many of them form, as it were, masses together.  Various authors have recorded the migration of eels in a singular way; such as Dr. Plot, who, in his History of Staffordshire, says they pass in the night across meadows from one pond to another; and Mr. Arderon, in the Philosophical Transactions, gives a distinct account of small eels rising up the flood-gates and posts of the water-works of the city of Norwich; and they made their way to the water above, though the boards were smooth planed, and five or six feet perpendicular.  He says, when they first rose out of the water upon the dry board, they rested a little—­which seemed to be till their slime was thrown out, and sufficiently glutinous—­and then they rose up the perpendicular ascent with the same facility as if they had been moving on a plane surface.—­There can, I think, be no doubt that they are assisted by their small scales, which, placed like those of serpents, must facilitate their progressive motion; these scales have been microscopically observed by Lewenhoeck.  Eels migrate from the salt water of different sizes, but I believe never when they are above a foot long—­and the great mass of them are only from two and a half to four inches.  They feed, grow, and fatten in fresh water.  In small rivers they seldom become very large; but in large, deep lakes they become as thick as a man’s arm, or even leg; and all those of a considerable size attempt to return to the sea in October or November, probably when they experience the cold of the first autumnal rains.  Those that are not of the largest size, as I said before, pass the winter in the deepest parts of the mud of rivers and lakes, and do not seem to eat much, and remain, I believe, almost torpid.  Their increase is not certainly known in any given time, but must depend