168 SEA FISHERIES 



between the marine fauna of the coasts of Spain and 

 Languedoc. We see, therefore, that important geological 

 upheavals have by no means modified the primitive 

 habitat of certain species, which have remained faithful 

 to them. 



Plankton is carried by the ocean currents, but fish 

 always swim against the currents. They have only to 

 open their mouths as they swim in order to obtain 

 abundant nourishment ; and the more they swim the 

 more food they obtain and the more surely they sustain 

 their strength. The extent of the movement of masses 

 of fish is thus a function of the extent of their alimentary 

 needs. It is at the moment of spawning an operation 

 which weakens them that the need of nourishment is 

 greatest, and at the same time the movements are most 

 extensive. The concordance of the two phenomena is of 

 very ancient origin. It has given birth, in the course of 

 the ages, to an invincible hereditary instinct, the result 

 of which is migration. Like the casual movements of 

 fish, the annual migrations are strictly limited. The 

 herring of the Channel and those of the Dogger migrate 

 separately. The spring herring of the North Sea confine 

 themselves to local movements ; they are almost seden- 

 tary, and remain all the year round in the neighbourhood 

 of the coast. The mackerel of the Channel do not leave 

 the Channel after spawning ; when, in the winter, they 

 disappear from the surface, they plunge into some twenty- 

 five or thirty fathoms and simply take refuge on the 

 bottom, gathered in enormous compact shoals, covering 

 the sandy belts inshore like a carpet. 



II 



All living creatures are dependent upon their environ- 

 ment. The greater portion of modern biological research 



