THE WONDER OF LIFE 631 



knife-blade-like creature, about three inches in length, 

 with no spot of colour save in its eyes. It lives for many 

 months in this state — ^known as a Leptocephalus — expend- 

 ing energy in gentle swimming, but taking no food. It 

 subsists on itself, and becomes shorter and lighter, and 

 cylindrical instead of blade-like. It is transformed into a 

 glass-eel, about two and a half inches long, like a knitting 

 needle in girth. It begins to move towards the distant 

 shores and rivers. In some cases it may take more than 

 a year to reach the feeding ground — those that ascend the 

 rivers of the Eastern Baltic having journeyed over three 

 thousand miles. Their ranks are thinned, but large num- 

 bers succeed in finding the estuaries, and the passage of 

 millions of elvers up our rivers is one of the most remark- 

 able sights of Spring. There is a long period of feeding and 

 growing in the slow-flowing reaches of the rivers and in the 

 fish-stocked ponds. But there is never any breeding in 

 fresh water, and after some years a restlessness seizes the 

 adults as it seized the larvae — a restlessness due, however, 

 to a reproductive, not to a nutritive motive or impulse. 

 There is an excited return journey to the sea — ^they don 

 wedding garments of silver as they go and become large 

 of eye. They appear to migrate hundreds of miles, often 

 at least out into the Atlantic to the verge of the deep sea, 

 where, as far as we know, the individual life ends in giving 

 rise to new lives. In no case is there any return. 



Let us consider in particular the penultimate chapter, the 

 migration from the rivers to the distant spawning grounds. 

 Like many other fishes, the eel requires for spawning very 

 definite conditions of depth, salinity, and temperature. 

 The North Sea will not serve, for it is too shallow ; nor the 

 Arctic Ocean, for it is too cold. What can the Machine 



