THE NUMBERS OF ANIMALS 113 



out most of the other animals, being assisted in this by the 

 hordes of opossums. They came out at night in the fields, 

 grazing like sheep, and in the summer went into the garden, 

 stripping it of fruit and vegetables." 



As Hewitt has pointed out, the converse of this is also true, 

 and great abundance is no criterion that a species is in no 

 danger of extinction. Just as an animal can increase very 

 quickly in a few years under good conditions, so on the other 

 hand it may be entirely wiped out in a few years, even though 

 it is enormously abundant. The argument that a species is 

 in no danger because it is very common, is a complete fallacy ; 

 but is very often brought forward quite honestly, especially 

 by people who have a financial interest in destroying the 

 animals. One might mention the case of whales in the 

 southern hemisphere. 



19. The examples of upsets in the normal balance of 

 numbers which we have described bring us up against the 

 question : what is the desirable density of numbers for any 

 one species (" desirable " being used in the teleological sense 

 of that density which will in the long run give the best chances 

 of survival for the species) ? The question of the desirable 

 number on a given area has received a great deal of attention 

 from people studying the ecology of human beings. It is 

 found that there is an optimum density of numbers for any 

 one place and for people with any particular standard of 

 skill. 6 To take a simple case : when a man is running a 

 farm he cannot afford to employ more than a certain number 

 of men on it, since after a certain point the income he gets 

 from the farm begins to diminish. It does not pay to put in 

 more than a certain amount of work as long as the standard 

 of skill remains the same. If a new invention or a new idea 

 opens up new lines of production, then it becomes possible 

 to employ more men with advantage, but not more than a 

 certain number. On the other hand, it does not pay the owner 

 to employ less than a certain number of men if he is to get 

 the maximum return from his outlay. If there are too few 

 men working, the maximum production is not reached. 



20. Let us see whether the idea of optimum numbers 



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