260 DARWINISM TO-DAY. 



Heermann, in 1853, recorded the species from 'San Francisco.' But 

 now, I think, the bird is unknown for several miles on either side 

 of the Gate. Doubtless this barrier accounts in part for the 

 origin of the distinct form barlozvi within so short a distance. . . . 



"As has become a generally accepted idea, the young plumages of 

 birds, if different at all from those of the adults, present a gener- 

 alised type of coloration ; or, to express it in another way, the 

 young more nearly resemble recent ancestral conditions. The 

 familiar examples of the spotted, thrush-like plumage of the young 

 robin and the streaked, sparrow-like plumage of young towhees 

 and j uncos are cases in point. Accepting this phylogenetic signifi- 

 cance of ontogeny, we find the chickadees giving some interesting 

 illustrations. 



"Although the adult of barlowi has the sides pure smoke-gray, 

 the juvenal plumage possesses pale-rusty sides. This points 

 towards a rusty-sided ancestor like neglectus. This also agrees 

 perfectly with the distributional evidence, of origin. The adult of 

 neglectus has pale-rusty sides; the young also has rusty sides, but 

 somewhat darker than in the corresponding age of barlowi, and 

 moreover is more nearly like the juvenal plumage of rufescens. 

 But the sides in adult rufescens are deep brown, almost chestnut, 

 while the young has much paler, merely dark-rusty sides. And 

 what is most significant is that the young of rufescens and hud- 

 sonicus are much nearer alike than are the adults, the former hav- 

 ing only very slightly darker rusty on the flanks. The young of 

 hudsonicus in respect to intensity of browns almost exactly equal 

 the adults of the same species, showing that the present coloration 

 is of very long standing, and offering further evidence that hudson- 

 icus is nearest the common stock form of all the chickadees under 

 consideration. Juvenal characters, resembling ancestral conditions, 

 lag behind the newer acquired adult characters. 



"To repeat: The young of barlowi has the sides paler rusty than 

 neglectus, neglectus slightly paler than rufescens, but rufescens has 

 the sides slightly more rusty than hudsonicus, a sequence which 

 accords well with the present theories of origin." 



9 Kellogg, V. L., "New Mallophaga, I," Contrib. to Biol. from the 

 Hopkins Seaside Laboratory of Leland Stanford Jr. University, 

 1806. 



10 Smith, Jas. P., "Studies for Students: Geological Study of 

 Migration of Marine Invertebrates," Journal of Geology, Vol. Ill, 

 pp. 481-495, 1895. 



11 Any selective breeding or segregation produced by other means 

 than the separation of groups of individuals by actual topographic, 

 or geographic barriers may be called biologic or sexual isolation. 



