External and Internal Factors in Evolution x\\ 



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In regard to the song-sparrow (Melospizd), 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 fascia ta 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: (i) 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) hermanni, 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) cinerea, insulated much further 

 apart than any of the others. A former American school 

 would probably have made four 'good species,' (1) fascia ta, 

 (2) samuelis, (3) rafina, (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 



