MUSEUM OF COMPARATIVE ZOOLOGY. 187 



of the soft parts no account can as yet be given. Geographical varia- 

 tion has an equally universal range, but is most strikingly exhibited in 

 the color, in size, and in the form of the bill. Individual variation will 

 be first considered, and subsequently geographical variation. In each 

 case each prominent phase of variation will be more or less fully de- 

 scribed. 



1. Individual Variation. 



Individual Variation in Color. — In birds of whatever age, two lines 

 of variation from the average or medium type of coloration are readily 

 distinguishable, the variation depending essentially on differences in the 

 depth or intensity of the general tint. On the one hand, individual 

 variation in color results from a greater than the average amount of 

 coloring matter in the integuments ; on the other hand, from an amount 

 less thau the average amount. The difference in this respect between 

 the extremes of a series of fifty or one hundred specimens of any spe- 

 cies, collected in course at a single locality, and nearly at the same 

 season of the year, is often as great as occurs between truly distinct 

 species. But the difference is here solely one of intensity of color, 

 while in allied species there is almost always an appreciable variation 

 in the style of coloration. In individual variation the differences usually 

 extend alike to all parts of the integuments ; that is, if the plumage 

 of the upper surface of the body is brighter or paler than usual, 

 the same difference extends to the plumage of the lower surface of the 

 body, and also to the bill and the feet. This is noticeable not only in 

 species that have the color in uniform masses, differing in tint on differ- 

 ent regions of the body, as in the robin (Tardus migratoriw), the blue- 

 bird (Sialiasialis), the Maryland yellow-throat (Geotlth/pis trichas), the 

 mocking-bird (Mimus polyglotti/s), and species generally of that type 

 of coloration, but also in spotted bird*, as in the various spotted species 

 of Fringillidce, Tardus, Dendrozca, etc., where the plumage on certain 

 regions of the body is marked with numerous streaks and .-pots differing 

 from the ground color, in which case the intensity of the color of the 

 markings correllates in its variations with that of the ground color. 

 Closely allied species, on the contrary, usually vary more or less, 

 not only in respect to the ground color, but also to a greater or less 

 degree in the style of the markings. In illustration of this point the 

 familiar group of the small, spotted-breasted wood-thru -lies of Eastern 



