84 THE ECOLOGY OF ANIMALS 



part of a natural cycle in population. The measures 

 were of negligible importance and merely masked the 

 real reasons for natural recovery in fish such as cod. 

 They were subsequently diverted with more success 

 to inland fresh waters. During the great plague of 

 London the authorities ordered all dogs and cats to 

 be destroyed, as these were thought to be carriers, 

 whereas they were in fact keeping down the real 

 carriers — rats (Bell, 1924). Florence Nightingale 

 ordered all windows in Indian hospitals to be thrown 

 wide open, which would have produced remarkable 

 results in a mosquito-ridden country that has over a 

 million deaths from malaria per year. These examples 

 are chosen from older historical accounts, but may 

 of course still be matched anywhere at the present 

 day. 



With accurate information, practical policies can 

 be formulated and nebulous theories can be given a 

 proper perspective. An example may be given. In 

 New Zealand the introduced trout were found to be 

 getting smaller and smaller. The theories put for- 

 ward by anglers were as follows : slower growth- 

 rates caused by depletion of natural foods, which in 

 turn were said to be due to silting of rivers through 

 flooding due to cutting of forests, also due to intro- 

 duction of foreign insectivorous birds which ate land 

 insects so that they no longer fell into the rivers for 

 trout to eat. There were other theories as well. 

 Percival (1932) carried out a careful quantitative 

 analysis of the situation, partly by measuring growth - 

 rates and partly by ecological surveys of river faunas 

 available as fish food. He found that all the theories 

 were incorrect, and that tliere was a single reason 

 for smaller trout, namely, that too many were being 

 caught, and that they had not time or opportunity 

 to grow into large ones. 



In many realms of economic biology, however, 

 the problem is not so much how to persuade authori- 

 ties to undertake ecological investigations, as to 



