THE INFANCY OF FISHES 215 



a time, yet this fish is certainly as abundant as the ling : 

 and both are predaceous species. 



How great are the dangers to which floating eggs are 

 exposed has been shown, among others, by Professor 

 G. O. Sars, who has recorded cases where cods' eggs have 

 been blown ashore and thrown up on to the beach, a 

 glistening line at high-water mark. But the " demersal " 

 eggs, that is to say those deposited at the bottom of the 

 sea, when laid in numbers large enough to attract attention, 

 suffer a no less heavy toll. Thus spawning herrings, 

 whose eggs adhere to stones and rocks at the sea-bottom, 

 are followed by shoals of haddocks all greedily contending 

 for the newly shed spawn. And to these natural enemies 

 must now be added man himself, who, with the deadly 

 trawl-net, sweeps away tons of eggs yearly. 



Let us now briefly survey some of the more striking 

 cases of parental guardianship, either of the eggs or young, 

 among fishes. 



The common stickleback of our ponds and streams 

 affords an admirable illustration of this phase of fish life. 

 In this species the eggs are deposited in a nest, and the 

 whole task of guarding them is undertaken by the male, 

 his labours being with the construction of a nest. This 

 is formed of bits of weed collected with much discrimination 

 and held together by a cement formed by his kidneys. 

 Having prepared the nursery, he sets out to find a female 

 heavy with eggs, and by dint of much persuasion induces 

 her to deposit her burden in his nest. Having laid, she 

 forces her way out of the nest by wriggling through the 

 wall opposite the entrance, and thereby furnishes a channel 

 through which a continuous supply of fresh, cool water 

 can be driven, thus keeping the eggs bathed. Next day 

 he persuades her to add still further to the deposit, and 



