CHAPTER IX 



BROOD-CARE AND LIMITATION OF FAMILIES IN 

 LOWER VERTEBRATES 



I HAVE already mentioned the prodigious fertility of many fish. 

 Most of the bony fish lay eggs in numbers that can be estimated 

 only in figures ranging from hundreds of thousands to millions. 

 Those in the ovary of a ling have been estimated at over twenty- 

 eight millions ; in a turbot of only seventeen pounds weight, nine 

 millions ; and in a cod of twenty-one pounds weight, six millions. 

 These eggs are extremely small and are discharged directly into the 

 water by the female, after which she takes no further notice of 

 them. They are known as pelagic — ^that is to say, whether they 

 are shed by fishes that spend their lives swimming either at the 

 surface or at no great depth, such as cod, whiting, hake and ling, 

 mackerel, pilchards or sprats, or by fishes which live on the mud or 

 sand of the bottom, they speedily rise to the surface and float in 

 the warmer water exposed to the light and heat of the sun. They 

 are transparent, almost invisible glassy spheres, each buoyed up 

 by a clear droplet of oil. At the usual spawning time, April or 

 May in northern waters, the whole surface of the sea is turbid with 

 the innumerable floating eggs and newly-hatched young, in the 

 favourite places for breeding, which are usually the rough waters 

 of bays, or near shoals on which the tides break. The eggs contain 

 very little food-yolk, and the tiny fish, as soon as they hatch, have 

 to begin feeding themselves. They are omnivorous, and find an 

 abundant orey in the still more innumerable young stages of various 

 crustaceans, molluscs and worms. The destruction is enormous ; 

 the larger fishes devour the smaller ; great flocks of sea-birds, 

 gulls, guillemots and gannets, scream and squabble as they gorge 

 on the larger fish ; whilst whales and dolphins come to join at the 

 feast. Other agencies share in the work of destruction ; a heavy 

 night-frost, a torrent of rain, or a storm of wind may destroy 

 millions. 



Other kinds of fish descend to the bottom of the sea and lay 



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