On the Generation and Migration of Eels. 329 



Rhone makes a subterranean fall below Geneva ; and though small eels can 

 pass by moss or mount rocks, they cannot penetrate limestone rocks or 

 move against a rapid descending current of water passing as it were through 

 a pipe again : no eels mount the Danube from the Black Sea; and there 

 are none found in the great extent of lakes, swamps, and rivers, commu- 

 nicating with the Danube, — though some of those 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 {Lin. Trans. Vol. xiv. p. 70 5 ) says, the little eels, accord- 

 ing 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, the fish tries 

 to return to the sea ; but numbers of the smaller ones hide themselves 

 during the winter in the sand, 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 {Phil. 

 Trans. 1747, vol. 41, p. 395.) 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 

 2£ to 4 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 at- 

 tempt 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 part of the mud 

 of rivers and lakes, and do not seem to eat much, and remain, I believe, 



