Inter -Relations of Living Creatures 661 



But further changes were in progress. The grouse that used 

 to frequent the moor took their departure. Teal ducks arrived on 

 the scene, attracted by the marsh and the rushes. A single flock, 

 when the gullery was at its height, numbered seventy. The grouse 

 were out, the gulls and teals were in. 



Fifteen years passed, and the scene was changed. Man inter- 

 fered again, for he rapidly ousted the gulls from their tenancy 

 of the White Moss. The villagers were disappointed because the 

 coarse grass they had been wont to cut had been replaced by use- 

 less docks. The proprietor, who had been using a proportion of 

 the gulls' eggs as food for his young pheasants, was disappointed 

 because the grouse had gone. So the edict went forth against the 

 gulls. In the early summer of 1917 scarcely a gull was to be seen ; 

 the docks had almost disappeared ; the rushes were giving way to 

 rough grasses and even heather ; the teal had gone and the grouse 

 were returning. In a few years a slight imprint of man's hand 

 had set in motion a complicated cycle of changes. The story is 

 like a Darwinian diagram, and Darwin might have written the 

 sentence with which the story ends: 



If the natural processes set a-rolling by a tiny and temporary 

 interference of man can be so marked, how can imagination 

 grasp the total effects of man's influence, impressed upon the 

 world of Nature often with great power, and persisted in, 

 not for a few years, nor for a few centuries, but for thousands, 

 nay, even for tens of thousands of years. 



The practical moral of this and every other story of inter-relations 

 is that man should be very careful in his interferences with the 

 system to which he belongs. 



5 



The Weird Ways of Parasites 



When one organism lives in or on another (its "host"), gets 

 its food from it, has its life-history inextricably bound up with it 



VOL. Ill 7 



