EVOLUTION AS A NATURAL PROCESS 139 



As a consequence of the many-sided struggle for 

 existence, the interrelations of a series of species will 

 approach a condition of equilibrium in an area where 

 the natural circumstances remain relatively undisturbed 

 for a long time. For example, among the field-mice of 

 one generation, just as many individuals will survive as 

 will be able to find food and to escape hereditary foes 

 such as cats and snakes and owls. The number of owls, 

 in their turn, will be determined by the number of 

 available mice and other food organisms, as well as 

 by the severity of the adverse circumstances that cause 

 elimination of the less fit among the fledglings brought 

 into the world. The vital chain of connections is 

 sometimes astonishingly long and intricate. One re- 

 markable illustration is given by Fiske, as an elabora- 

 tion of an example cited by Darwin. He points out 

 that the fine quality of the traditional roast beef of Eng- 

 land is directly determined by the number of elderly 

 spinsters in that country. The chain of circumstances 

 is as follows : the quality of the clover fields, furnish- 

 ing the best food for cattle, depends largely upon the 

 visits to the clover-blossoms by wild bees, that accom- 

 plish the fertilization of the flowers by carrying pollen 

 upon their bodies from one plant to another. Field- 

 mice devour the young in the nests of these bees, so if 

 there are few field-mice there will be many bees, and 

 consequently better grazing for the cattle. The number 

 of field-mice will vary according to the abundance 

 of cats, and so the number of these domestic animals will 

 exert an influence upon the whole foregoing chain of 

 forms. But, as Fiske points out, cats are the favorite 

 companions of elderly spinsters ; therefore, if there are 



