202 LIFE : OUTLINES OF GENERAL BIOLOGY 



abundance of the small crustaceans called Copepods, and the 

 deasity of the marine population of microscopic Peridinid Infu- 

 sorians. Ix'sides the still more minute Diatoms, which form a very 

 im|)ortant part of the stock of the sea-soup. If one link in the nutri- 

 tive chain Ls weakened, say the Diatom link by lack of sunshine, 

 the result may be felt at Billingsgate. 



It is said tliat a pound of cod's flesh involves the cod's consumption 

 of ten pounds of large whelk or buckie, and that a pound of this 

 muscular Gastero[XKl demands for its construction ten pounds of 

 sea-worms; and that a pound of worms is in turn the reincarnation 

 of ten pounds of microscopic organisms or organic particles. Thus in 

 eating a pound of cod's steak for dinner the hungry man is devouring 

 a thousand pounds of transmogrified sea-dust; and the world is full 

 of these cycles of reincarnation or re-embodiment. This illustrates 

 the idea of nutritive chains, which contribute essentially to the 

 Balance of Nature. 



This large biological idea of nutritive chains finds many illustra- 

 tions that affect practical interests. Thus it has been noticed in some 

 parts of Britain, e.g. the garden of ^^oray, that the habits of 

 Herring Gulls have changed very much for the worse during the 

 last generation. They have become less markedly fish-eaters and 

 very hungrily vegetarian. They sit on the "stooks" in the harvest 

 fields and gorge themselves with corn. They work up the rows of 

 turnips, scooping out one after another, and pecking at more than 

 they devour, thus opening the way to fungi and threadworms. 

 How has this change come about ? Part of the answer is that Herring 

 Gulls have become much more numerous. This may be because the 

 natural enemies of the young, such as Sea Eagle and Peregrine 

 l-'alcon, have disappeared or become rare; or because the eggs are 

 not collected so systematically as in former days; or because some 

 measure of protection has been extended to the adult birds. And if 

 it be objected that there are plenty of fish in the sea for all the gulls, 

 the answer is that gulls do not dive and are therefore restricted to 

 fishes swimming near the surface. It is quite possible that around 

 some parts of Britain there are not enough of these to meet the 

 demands of tlie increased numbers of Herring Gulls. Hence the 

 change of diet. 



PiAGiKS oi- .\nimals. The sound Natural History objection 

 to any rapid elimination of any type of animal is that it may 

 result in a disastrous disturbance of a long-established balance. 

 To some extent it is good sense to connect the increased multiplica- 

 tion of voles and the like with the destruction of the natural checks 

 to their increase, such as hawks and owls, weasels and stoats. This 

 is not the whole story, for climatic cycles have their influence; but 

 it is one factor in the mischief. And it cuts both ways, for it has 

 Ix^en shown that the supply of fox skins in Hudson Bay Territory 



