Xll 



PREFACE 



thousand birds, reptiles, and plants have been systemat- 

 ically collected in various parts of North America from 

 the arctic regions to the tropics, and from the eastern to 

 the western borders of the continent. From the data 

 thus secured and from other sources, the areas occupied 

 by a considerable number of species have been mapped. 

 The study of this accumulated information has resulted 

 in definitely outlining the life zones into which North 

 America is divided, and in demonstrating that each life 

 zone, except the most northern, represents a crop zone or 

 climatic belt, to which certain crops are best adapted. 

 This close relation of the life zones to agriculture is 

 shown in maps and bulletins published by the Biological 

 Survey. 



Throughout its existence the Survey has made ex- 

 haustive studies of the food habits of the species of birds 

 and mammals for the purpose of determining their re- 

 lations to agriculture, and has conducted experiments for 

 the purpose of devising methods of protection for the 

 beneficial, and methods of destruction for the injurious 

 species. The benefits to our agricultural and grazing in- 

 terests have already been great, and as our lands become 

 more densely populated the economic results will prove 

 of still greater value. To accomplish these ends Dr. 

 Merriam has gathered about him a corps of trained field 

 naturalists, more efficient for the purpose than any similar 

 body of men in the world. 



As a nation we should be proud that the men engaged 

 in this work have been broad enough not to neglect the 

 profound scientific problems involved in it problems 

 touching the origin and dispersal of species. Darwin 

 was perplexed by what seemed a grave objection to his 

 theory, the apparent absence of transitional forms be- 

 tween species. The collections made by the Biological 



