THE BASIS OF ADAPTATION 251 



countries has given us illustrations in numerous cases. Rabbits 

 introduced into Australia years ago have become a nuisance. 

 The English sparrow has been equally successful in North America. 

 Such insects as the San Jose scale, gipsy moth and Japanese 

 beetle have been even more serious. The first named threatened 

 to wipe out the citrus fruit industry of the west until it was 

 traced to its oriental home, whence natural enemies were secured 

 to hold it in check. The others have cost millions in the eastern 

 United States, and are still a problem. 



The production of an excessive number of offspring is essential 

 to the maintenance of the food supply of all organisms, since 

 those which are destroyed are either killed for food or become 

 food incidentally. It is also closely linked with the food supply 

 of the species concerned, for the persistence of an abnormal number 

 of individuals might well carry the needs of the species beyond 

 the limits of the available food supply and necessitate adjustments 

 of some kind. 



Response of Organisms. The response of organisms to changes 

 in any of the above factors might be accomplished by change of 

 location. The power of locomotion is developed in some animals 

 to such a high degree, however, that it may well be a primary 

 factor in evolution itself, since a species may gradually extend 

 its distribution from an original center into regions which make 

 different demands upon it. 



The Interaction of Factors. The entire series of factors interact 

 in a complex way, for the operation of one may well bring another 

 into action. Geological changes alone are beyond the influence 

 of the others. 



Migration. Whenever environmental conditions change, two 

 courses are open to the organism. It may either migrate from the 

 region in which it lives in search of more favorable surroundings, 

 or it may remain and carry on its existence by taking advantage 

 of conditions which were formerly of no importance to it. Either 

 of these courses depends to some extent upon its latitude of 

 response, but the latter especially. To migrate may carry the 

 animal into an environment but little different from that to 

 which it is accustomed. To remain in a changing environment 

 demands change inevitably. The two responses are in some 

 degree associated, for the former is certain to contribute to the 

 adaptive modification of some organisms, and the latter is equally 



