318 CALIFORNIA ACADEMY OF SCIENCES. 



The vireos in their colors appear to be like undeveloped 

 warblers. The olive green, black and yellow, so char- 

 acteristic of that group is present here also, but has not 

 yet obtained the mastery over the gray tints. I believe 

 this genus shows us the effect of sexual selection which 

 has, comparatively speaking, been in operation for a 

 short time only. Although a number of species have 

 one or more geographical races, differing but little from 

 one another, the species themselves are for the most 

 very distinct, even though few of them have any par- 

 ticularly striking individual characters. In some genera 

 varieties merge into species in a gradual transition, as, 

 for instance, in Melospiza, Sphyrapicus, Zonotrichia, 

 etc., where the extreme varieties are almost or quite as 

 different as the most closely allied species, but here, as 

 a general rule, there is a considerable gap between the 

 varieties of one species and the next most closely related 

 form. This would seem to indicate that the different 

 species had diverged quite widely before attaining their 

 present distribution upon which the existing varieties de- 

 pend. The isolation which produced the different spe- 

 cies is difficult to determine, but it is a noteworthy fact 

 that to the south certain mainland and insular forms 

 pass by gradual stages from variety to species. This is 

 notably the case with V. crassirostris and V. crassirostris 

 flavescens of the Bahamas, and V. ochraceus of southern 

 Mexico, which is intermediate in color between the two 

 preceding. It would seem then that the differentiation 

 of species may have been due to geographical isolation 

 in a southern district. 



FAMILY CCEREBID.E. THE HONEY CREEPERS. 

 GENUS CERTHIOLA. THE HONEY CREEPERS. 

 (2) Adult male like female; young like some ances- 

 tral stage of the adult. 



