External and Internal Factors in Evolution 311 
In regard to the song-sparrow (Melospiza), Coues writes: 
“The type of the genus is the familiar and beloved song- 
sparrow, a bird of constant characters in the east, but in the 
west is split into numerous geographical races, some of them 
looking so different from typical fasciata that they have been 
considered as distinct species, and even placed in other 
genera. This differentiation affects not only their color, but 
the size, relative proportions of parts, and particularly the 
shape of the bill; and it is sometimes so great, as in the case 
of M. cinerea, that less dissimilar looking birds are commonly 
assigned to different genera. Nevertheless the gradation is 
complete, and affected by imperceptible degrees... . The 
several degrees of likeness and unlikeness may be thrown 
into true relief better by some such expressions as the follow- 
ing, than by formal antithetical phrases: (1) The common 
eastern bird commonly modified in the interior into the duller 
colored (2) fallax. This in the Pacific watershed, more de- 
cidedly modified by deeper coloration, — broader black 
streaks in (3) hkermanni, with its diminutive local race 
(4) samuelis, and more ruddy shades in (5) guttata north- 
ward, increasing in intensity with increased size in (6) rafina. 
Then the remarkable (7) czzerea, insulated much further 
apart than any of the others. A former American school 
would probably have made four ‘ Eee species,’ (1) fasciata, 
(2) samuelis, (3) vafina, (4) cinerea.’ 
Somewhat similar relations are found in three other gen- 
era of finches. Thus Passerella is “imperfectly differen- 
tiated”; Junco is represented by one eastern species, but in 
the west the stock splits up into numerous forms, “all of 
which intergrade with each other and with the eastern bird. 
Almost all late writers have taken a hand at Junco, shuffling 
them about in the vain attempt to decide which are ‘species’ 
and which ‘ varieties.’ All are either or both, as we may 
elect to consider them.” In the distribution of the genus 
Pipilo similar relations are found. There is an eastern form 
