266 Stejneger, Isolation vs. Natural Selection. [july 



Apparently then temperature, as indicative of the life zones r 

 has very little to do with the distribution and differentiation of 

 these races of woodpeckers. On the other hand, the transverse 

 lines which Mr. Jenkins has drawn on his map "separating forms 

 of equal size" in showing a gradual increase of size northward 

 point to thermal influences, although they do not present any special 

 agreement with the lines on the zonal maps. 



Nor do the east and west forms find a ready explanation in the 

 humidity or aridity of the climate, as the western form D. v. harrisi, 

 which enjoys a climate fully as humid as typical D. villosus in the 

 northeastern United States, is the one most removed from it in color 

 characters. It is true that the western forms to some extent corre- 

 spond to the humid and arid districts of the Pacific slope in as much 

 as the dark underside of the large northwestern D. v. harrisi in a 

 general way coincides with the extent of an annual rainfall of over 

 50 inches. It is also true, that the range of the small southeastern 

 D. v. auduboni falls inside the area in the east having a rainfall of 

 over 50 inches, but the latter amount of precipitation is also found 

 over larger areas inhabited by typical D. villosus. 



As we go south into Mexico and Central America we meet an 

 increasing darkening of the underside through D. v. intermedins 

 to D. v. jardinii. This darkening is plainly connected with the 

 increasing amount of the annual precipitation which exceeds 50 

 inches considerably. 



We can thus, in a general way, trace the influence of present 

 climatic conditions in the formation of some of these races, in as. 

 much as those distinguished by a darkened underside inhabit dis- 

 tricts having a yearly rainfall of 50 inches or more. Present tem- 

 perature conditions apparently play a very subordinate role, except 

 that the size of the various forms gradually increases from the 

 warmer to the colder regions, but apart from the fact that this in- 

 crease in size can only be very indirectly caused by the temperature, 

 the, difference in size is not diagnostic of the various forms, that is 

 to say, the northern individuals of each form are larger than the 

 southern ones (see Jenkin's table on p. 171, showing an average 

 greater size of D. v. hyloseopus north of 35° N. lat. than south of it), 

 so that the gradual increase in size northward is a gradual increase 

 in the size of the individuals and thus onlv indirectlv of the races. 



