226 



BONY FISHES 



VII. 21 



many tales of carp coming to be fed at the ringing of a bell, and similar 

 powers of association must play a part in the life of fishes in more 

 natural situations. Bull has shown that fishes can be trained to dis- 

 criminate between very small differences of water flow, temperature, 

 salinity or pH, and no doubt it is by means of such powers that they 

 normally find a suitable habitat. 



Fig. 144. Migrations of the eels. The European species (A. anguilld) occurs along the 

 coasts outlined with lines, the American species {A. rostrata) where there are dots. 

 The curved lines show where larvae of the lengths indicated (in millimetres) are taken. 



(After Norman.) 



The migrations of fishes have attracted much attention, but are 

 still imperfectly understood. They vary from the 'catadromous' down- 

 ward migration of young animals to the sea and the reverse 'anadro- 

 mous' movement to breed, to the astounding journeys of the eels, 

 3,000 miles westwards from Europe or eastwards from America to 

 their breeding-place in the Sargasso Sea (off Bermuda) and the return 

 of the elvers to the homes of their parents (Fig. 144). No one has yet 

 discovered the factors that direct these movements, currents may play 

 a part, but can hardly be the only influence. Indeed it has been 

 suggested that European eels never complete the journey but die in 



