EVOLUTION OF THE COLORS OF BIRDS. 253 



which differs in having the lores white instead of 

 black the result of isolation. The Central American 

 form,D. villosus jardinii. is characterized by the smoky 

 brown color of the under parts which replaces the white 

 or gray of the northern race. This is probably a cli- 

 matic influence corresponding to the tropical tendency 

 towards melanism in other species. 



The lower map of Plate XII shows the distribution of 

 the barred members of the genus Dryobates the " lad- 

 der backs." They are all apparently forms of Sonoraii 

 origin, dividing into three species and a number of 

 races. The distinctions, both specific and racial, it would 

 seem, are due to isolation rather than to any direct en- 

 vironmental influence. The forms thus far considered 

 have been such as display trifling varietal or specific 

 distinctions which are due wholly or in large measure 

 to their geographical distribution either to the direct 

 influence of the environment or to the indirect influence 

 of environment together with segregation. There are 

 certain closely related species or varieties, on the other 

 hand, whose ranges are to a greater or less degree co- 

 incident, or overlap. Sometimes, as with Ammodramus 

 sanchvichensis bryanti and alaudinus it is simply in the 

 winter distribution that they are together, but in a 

 number of cases it is the breeding range which co- 

 incides. If the forms are tolerably distinct species it is 

 not impossible that they have been differentiated through 

 geographical segregation and afterwards come together, 

 but when the races are not very strongly marked, or 

 occupy a large territory in common this hypothesis will 

 hardly be a satisfactory one. The following is a list 

 of such species and varieties which occupy territory in 

 common: 



Bubo virginianus and B. mexicanus. 

 Chordeiles virginianus henry i and C. texensis. 



