THE STRUGGLE FOR EXISTENCE 381 



are related to their surroundings may be shown by supposing 

 a change in living places between a wolf of the Western 

 plains, a squirrel of the Central States forest, and a fish of 

 the Gulf of Mexico. None of these would survive if placed 

 in the region in which the others live. 



Man has disturbed the natural balance of life wherever 

 he has gone. When he cuts a forest, plows a field, drains a 

 swamp, or builds a city, many kinds of life are destroyed 

 either directly or because the necessary conditions of life are 

 removed. Agriculture and horticulture, in fact, are attempts 

 of man to introduce new kinds of life where they did not 

 exist. The destruction of the wheat crop by insects, of cotton 

 by the boll weevil, and of apples by the codling moth are 

 evidence that as yet man has not succeeded in maintaining 

 his artificial balance of life without great loss. 



393. Types of disturbance of nature's balance of life. The 

 common rabbit was introduced into Australia and, finding 

 favorable conditions, multiplied astonishingly. Under favor- 

 able circumstances a pair of rabbits will produce each year 

 six litters averaging five young in each. Since rabbits feed 

 upon vegetation, when present in large numbers they com- 

 peted with cattle and sheep to such an extent that the grazing 

 industry was almost destroyed in some sections of Australia. 



Professor Sidney Dickinson says of rabbits : 



The original pair might be responsible in five years for a 

 progeny of over twenty millions. That the original score that 

 were brought to the country have propagated after some such 

 ratio, no one can doubt who has seen the enormous hordes that 

 now devastate the land in certain districts. In all but the remoter 

 sections the rabbits are now fairly under control; one rabbiter 

 with a pack of dogs supervises stations where one hundred were 

 employed ten years ago. Millions have been killed by fencing in 

 the water holes and dams during a dry season, whereby they died 

 of thirst and lay in enormous piles against obstructions they had 

 frantically and vainly striven to climb, and poisoned grain and 

 fruit have killed myriads more- 



