178 DARWINISM AND THE PROBLEMS OF LIFE 



that the alimentary canal serves as respiratory organ 

 in this curious fish. 1 



But the regulation of rivers does even more than 

 factories in reducing the number of fishes ; it deprives 

 the fishes of their spawning-places just as the cutting 

 away of the underwood in the forest deprives the bird of 

 its nesting-place. The regulation of the rivers dams 

 them up and deepens them, and thus does away with 

 the shallows and the side-pits. It is just in these places 

 that most fishes lay their spawn, as it cannot be swept 

 away, and it receives plenty of sun. The pike likes to 

 spawn in flooded meadows. The ruffe is the only 

 exception. It wanders in troops in the spring from its 

 standing waters into the rivers, until it finds plenty of 

 reeds, amongst which it lays its eggs. 



Many fishes travel in this way. Even the heavy 

 carp turns up the stream, when it is in free water, 

 and leaps over high obstacles, in order to lay its spawn 

 in quiet water near the source of the river. The 

 minnows travel in swarms to the mountains when the 

 water becomes too hot for them, and leap over rapids 

 and weirs. Sturgeons pass from the sea into the rivers 

 in spring. But the most famous travellers are the eel 

 and the salmon. 



The reproduction of the eel has only been cleared up 

 very recently. Up to that time the most extraordinary 

 stories were in circulation about it, as no ova had ever 

 been found in it. Some maintained that the eels 

 coupled with snakes ; others that they arose from mud 



1 If the animal is taken from the water and squeezed round the 

 body, the air rushes out with a loud and plaintive noise. 



