234 CALIFORNIA ACADEMY OF SCIENCES. 



animals, particularly Avith regard to the land birds of 

 North America. 



GEOGRAPHICAL DISTRIBUTION AS A FACTOR IN THE EVOLU- 

 TION OF COLORS. 



Inasmuch as the geographical distribution of life 

 bears such an important relation to the evolution of 

 species, a knowledge of the past and present distribution 

 of birds must throw some light upon the subject of the 

 evolution of their colors. It will accordingly be neces- 

 sary to briefly glance at the distribution of birds in 

 North America, and to note the bearing of the question 

 upon their colors. 



The land mass which now forms Europe and Asia was, 

 according to Wallace, the original home of the primitive 

 vertebrates, whence, at a slightly later period, they 

 migrated to North America. Spreading southward over 

 South America and Australia they were yerj early cut 

 off from North America, and the fauna of Australia has 

 never since intermingled with any other. At one or 

 two successive later periods communication between 

 North and South America was established, resulting in 

 a very considerable intermingling of forms. The pres- 

 ent fauna of North America, as Dr. C. Hart Merriam 

 has shown, is derived from two centers, a northern and 

 southern."^ According to his second provisional bio- 

 geographic map of North America f the principle life 

 areas are as follows: treeless area extending across the 

 northern edge of Alaska, southeast to the southern 

 border of Hudson's Bay, and thence across a narrow 

 strip of the northern coast to the Atlantic; a Boreal area 

 occupying nearly all of British Columbia and extending 



* cf. North American Fauna, No. 3. 



t The Geographic Distribution of Life in North America, Proc. BioL 

 Soc, Wash. vii. 



