MUSEUM OF COMPARATIVE ZOOLOGY. 189 



results from the amount of rufous pervading the plumage. Individuals 

 of Tardus Swainsoni of the rufous or bright-colored type have the dorsal 

 surface of a uniform brownish-olivaceous tint, and the sides of the head 

 and breast strongly suffused with yellowish-brown, which tint is also 

 traceable throughout the lower plumage, in the brighter color of the 

 basal brownish band on the inside of the wing-, and in the color of the 

 mouth and base of the bill. In other individuals the upper plumage 

 is of a dark olivaceous tint, without any trace of brownish, the sides 

 of the head, neck, and breast being ashen, with often no appreciable 

 tinge of ferruginous ; specimens of this type thus differing widely in 

 general aspect from those of the other. Between these extremes, of 

 which examples are not unfrequent, nor confined to any particular 

 locality or season of the year, there is every degree of intergradation, 

 specimens intermediate between the two being by far the most fre- 

 quent, and constituting the average or common form. 



Turdus Pallasi and Tv.rdus fuscescens present precisely similar vari- 

 ations. They are also seen in Turdus mustelinus, in Turdus migrato- 

 rius, in Siulia sialis, in Seiurus noveboracensi's, in many species of Dcn- 

 drceca, sparrows, and other species which I have especially investigated 

 in reference to this point, embracing examples of all the leading families 

 of birds. The ruffed grouse (Bonasa umbellus), as is well known, 

 varies in the color of the upper parts from reddish-brown to gray ; the 

 great horned owl (Bubo rirginianus) from dusky through numerous 

 shades of rufous and fulvous to nearly white, the fulvous suffusion so 

 commonly present in this species varying from ferruginous on the one 

 extreme to its complete obsolescence on the other. In such common 

 and thoroughly known species as the robin, blue-bird, etc., the true 

 character of these variations is recognized, but in groups where the 

 species are not well known, and especially in specimens from partially 

 explored regions, they are frequently regarded as of specific value, and 

 the addition of numerous nominal species is the result. 



Besides the variation in the depth of color already noticed, birds hav- 

 ing the plumage varied with streaks and spots differ exceedingly in 

 different individuals of the same species in respect to the size, shape, and 

 number of these marks, and in the general aspect of the plumage result- 

 ing from such variations. Generally, as already stated, such differences 

 correllate with the variations in the intensity of the ground color, the 

 darker or more deeply colored birds being usually those with the mark- 



