iv] NUMBERS AND DENSITY OF SPECIES 69 



have not established themselves here. Many fishes 

 make periodical migrations for breeding purposes 

 and in enormous numbers. The herring and the cod 

 and their kindred collect on certain breeding grounds. 

 The tunny, which has a world-wide distribution, passes 

 through the Straits of Gibraltar into the Mediter- 

 ranean Sea in the month of May there to breed, and 

 in the month of July it returns to the ocean. 



Many, known as anadromoits, ascending, fishes, 

 feed and mature in the sea, but ascend the rivers as 

 the impulse of reproduction grows strong, e.g. the 

 salmon, sturgeon, the shads and the mayfish of the 

 Rhine. 



Few fishes are katadromous, i.e. their usual 

 habitat is in rivers and lakes, whence they descend 

 into the deep sea for breeding purposes. The com- 

 mon eel is the classical example. 



CHAPTER IV 



NUMBERS AND DENSITY OF SPECIES 



EVERYBODY knows that some kinds of animals 

 are common and others are rare. Why this should be 

 so is easy to understand when the rare species has a 

 very limited distribution, but there are also birds and 



