66 A PHILOSOPHER WITH NATURE 



kept in confinement in ponds thirty and forty years. 

 They obtain a great size in such circumstances, 

 feeding voraciously in the summer, and, as is the 

 habit of eels, burying themselves in the bottom and 

 remaining in a more or less torpid state during the 

 winter. 



The mystery of the migration of eels remains. 

 It has indeed only been added to and deepened 

 by these observations. The young elvers which 

 ascend the rivers in countless millions in the spring- 

 time are already a year old. They have, indeed, 

 by the time they reach fresh water, travelled far 

 through the trackless waters of the oceans. How do 

 they find the way ? In the case of the migration of 

 birds there is always the suggestion that the young 

 birds have found the way by accompanying others 

 that have made the journey before. But here 

 this explanation fails us. The parent eels never 

 return. The little eels, which are about two or 

 three inches in length, and which have already 

 undergone metamorphosis in developing from the 

 egg in the deep sea, can obviously possess nothing 

 in the shape of memory to direct them. Yet they 

 press onwards to their destination with an unerring 

 sense. This one may readily observe if it be sought 

 to interrupt them or to place any obstacles in their 

 way. The common eel is found over the greater 

 part of Europe, and if the development of the eggs, 

 as appears to be proved, will only take place at 

 considerable depths in the ocean, it is evident that 

 journeys of immense length have to be made by the 

 parent eels in returning to their breeding-places, 

 and then by the little elvers in seeking the rivers 

 and inland waters. It seems one of the most remark- 



