200 The Outline of Science 



in the water, and swimming gently. These open-sea young eels 

 are known as Leptocephali, a name given to them before their 

 real nature was proved. They gradually become shorter, and the 

 shape changes from knife-blade-like to cylindrical. During this 

 change they fast, and the weight of their delicate body decreases. 

 They turn into glass-eels, about 2^ inches long, like a knitting- 

 needle in girth. They begin to move towards the distant shores 

 and rivers, and they may be a year and a half old before they 

 reach their destination and go up-stream as elvers. Those that 

 ascend the rivers of the Eastern Baltic must have journeyed three 

 thousand miles. It is certain that no eel ever matures or spawns 

 in fresh water. It is practically certain that all the young eels 

 ascending the rivers of North Europe have come in from the 

 Atlantic, some of them perhaps from the Azores or further out 

 still. It is interesting to inquire how the young eels circumvent 

 the Falls of the Rhine and get into Lake Constance, or how their 

 kindred on the other side of the Atlantic overcome the obstacle of 

 Niagara ; but it is more important to lay emphasis on the variety 

 of habitats which this fish is trying the deep waters, the open 

 sea, the shore, the river, thejDond, and even, it may be, a little 

 taste of solid earth. It seems highly probable that the common 

 eel is a deep-water marine fish which has learned to colonise the 

 freshwaters. It has been adventurous and it has succeeded. The 

 only shadow on the story of achievement is that there seems to be 

 no return from the spawning. There is little doubt that death is 

 the nemesis of their reproduction. In any case, no adult eel ever 

 comes back from the deep sea. We are minded of Goethe's hard 

 saying: "Death is Nature's expert advice to get plenty of life." 



4 

 Forming New Habits 



There is a well-known mudfish of Australia, Neoceratodus 

 by name, which has turned its swim-bladder into a lung and comes 

 to the surface to spout. It expels vitiated air with considerable 



