252 CALIFORNIA ACADEMY OF SCIENCES. 



the western side developed into a new race, the dryness 

 of the climate encouraging the production of white 

 markings. Later this new species (P. maculatus arcticus), 

 spread to the south and west. Toward the west it be- 

 came successively darker as it approached the moist 

 coast district, or if there was free intercrossing for a time 

 the coast birds would mingle with the interior race tend- 

 ing to make it darker. In the interior of Mexico, how- 

 ever, the birds probably inhabit a dry arid region and 

 have consequently not become conspicuously darker than 

 the form from which they were descended. 



In the species so far considered there has been a tol- 

 erably uniform east and west isolation, but in many in- 

 stances the segregation is far more complex, there being 

 an east and west isolation of species and a north and 

 south isolation of varieties, or, conversely, a north and 

 south isolation of species and an east and west of varie- 

 ties. Both these forms are illustrated in the genus 

 Dryobates as shown in Plate XI. The different races of 

 the jD. villosus group all belong to the same species but 

 the forms east of the Rocky Mountains have the wing 

 coverts and tertials conspicuously spotted with white, 

 these marks being largely or wholly lacking in the west- 

 ern and Central American races. It will be noted that 

 this is the converse of the relations in the genus Pipilo, 

 tending to show apparently that isolation, together with 

 various indirect effects rather than the direct influence 

 of the environment has been instrumental in producing 

 the difference. The eastern and western forms bear a 

 relation to one another midway between species, such as 

 in Pipilo, and varieties such as Sphyrapicus varius and 

 nuchalis, being much more distinctly differentiated than 

 the north and south races. Indeed, in group A the dif- 

 ferences appear to be in size rather than in color, with 

 the exception of the insular form D. villosus maynardi 



