60 PROCEEDINGS OF THE ACADEMY OF 



in inverse ratio with the latitude of the breeding-place. 1 (Cf. 

 Allen, Bull. Mus. Comp. Zool. ii. p. 229 ) 



d. The ? is generally a little smaller than the % . 



e. Intensity of coloration varies in direct ratio with the tem- 

 perature and humidity of the breeding-place. Moisture, however, 

 intensifies color more than heat ; aridity tones down color more than 

 cold. Birds from hot dry places, therefore, are paler cseteris 

 paribus^ than birds from wet places of the same or even lower 

 temperature. (Cf. Allen, op. cit. p. 239.) 



f. The rufous coloration belongs to the category of what some 

 call " embryonic features," in the sense that it generally decreases 

 with age. Young birds are suffused with rufous to an extent 

 rarely if ever seen in the old ; this is shown both in the greater 

 extent of the color on the quills and tail, and in the modification 

 of the olivaceous by admixture, young birds being "browner" 

 than the old. 



g. In the adult state, the freshest plumage is the most oliva- 

 ceous, as the feathers wear browner with exposure. 



/;. The 3'ellow coloration increases with age to a certain extent; 

 and in the fall is at least as bright as, if not brighter than, in 

 spring, in equally mature birds. 



i. The sexes do not differ in color to a recognizable deo-ree. 2 



k. Variation unconnected with age,* sex, or season, is in inverse 

 ratio with the migration or changeable geographical distribution 

 of individuals. 



This last important generalization is well illustrated in the cases 

 of crinitus and stolidus. The former is the most extensiveby dis- 

 persed species of the genus, being found at different seasons from 

 Guatemala to New England. As its individuals are never con- 



1 But certain localities produce larger bills, in opposition to this rule, or 

 show greater variability in the size of the bill, according to influences at 

 present unknown; e. g., the large bills of the Tehuantepec Myiarchi; the 

 extremely variable bills of the Jamaican M. stolidus. 



2 An unquestionable fact, in its application to crinitus and some others. 

 Authors, however, speak of color-differences in the sexes of the South Ame- 

 rican species, ferox. I have been unable to verify such statements, and 

 think that a point of age, not of sex, is involved, younger birds having ru- 

 fous that afterward disappears. If so be it, that such sexual differences 

 really subsist in the case of ferox, then my entire characterization of that 

 species falls to the ground, and the bird cannot be specifically distinguished 

 from the variety of crinitus that I call irritabilis, beyond. 



[July 2, 



