PARENT AND OFFSPRING 113 



Coming next to the fishes, we see that among them 

 the care bestowed on the young varies greatly. 

 Females of many species deposit over 100,000 eggs 

 each. The cod, pilchard, and herring, for example, 

 are extremely fertile, and if they were not destroyed 

 in masses they would soon cover the whole sea. Any 

 species, if not kept in check up to a certain point, 

 would soon people the whole world, making the 

 earth uninhabitable for any other species. 



Once the eggs are laid, the female fish troubles no 

 more about them, leaving them to the destructive 

 forces that surround them on all sides. On the 

 whole, as the numbers of the species tend to remain 

 constant, only two of the eggs laid by each female 

 need reach maturity in order to replace the parents. 



There occur two kinds of eggs in fish. In some 

 species the eggs, which are discharged directly 

 into the water, always rise to the surface and float 

 in the warmer water exposed to the light and sun. 

 These eggs are known as pelagic; they contain very 

 little food-yolk, and the little fish, as soon as they 

 are hatched, have to begin feeding themselves. Out 

 of the enormous number of eggs laid only a few 

 escape destruction. The females of other species 

 descend to the bottom and lay their eggs there. 

 These are called submerged eggs ; they are larger than 

 the pelagic eggs and heavier than sea-water. Some 

 choice is shown in the selection of a ground for de- 

 positing such eggs; they adhere by various means to 

 seaweed or stones. Being safer from destruction, 

 they survive in greater numbers, and thus a lesser 

 number is needed. 



A certain amount of brood-care is to be found 



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