The Wonder of the World 25 



young eel or elver which makes for the shore and 

 proceeds up the rivers. In .spring or early sum- 

 mer legions of these elvers pass up stream, obedient 

 to their instinct to go right ahead as long as the 

 light lasts. Before reaching such rivers as those 

 which flow into the Eastern Baltic, the young eels 

 have had a journey of some 3,000 miles, for all the 

 North European eels seem to have their cradle in 

 the Atlantic west of the Faroes, the Hebrides, 

 Ireland and Spain, where the continental plateau 

 shelves steeply down into the greater depths. 

 As the elvers pass up the streams there is, accord- 

 ing to some, a separation of the sexes; the males 

 lag behind; the females go further inland. Then 

 follows a long period of growth in slow-flowing 

 reaches of the rivers and in ponds. After some 

 years there is a return journey to the sea, and, as 

 far as we know, the individual life ends in giving 

 origin to new lives. There is never any breeding 

 in fresh water, and there seems to be no return 

 from the deep sea. 



Adaptations. — One of the most characteristic 

 features of the animate world is the all-pervading 

 fitness. It was Romanes who said, *' Wherever we 

 tap organic Nature, it seems to flow with purpose." 

 We may differ as to our interpretation, but the 

 fitness of living creatures as regards structure and 

 habits and interrelations is a fact. How well the 

 structure of bone is suited to stand strains, how 



