56 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



ing examples of this enlargement among the sparrows, 

 black-birds, thrushes, crows, wrens, and warblers, in 

 the quail, the meadow-lark, the golden-winged wood- 

 pecker, etc. Generally the bill, in the slender-billed 

 forms, becomes longer, more attenuated, and more de- 

 curved (in individuals specifically the same) in pass- 

 ing from the New England States southward to Flo- 

 rida, while in those which have a short, thick, conical 

 bill there is a general increase in its size so that the 

 southern representatives of a species, as a rule, have 

 thicker and longer bills than their northern relatives, 

 though the birds themselves are smaller. There is 

 thus not only generally a relative, but often an abso- 

 lute, increase in the size of the bill in the southern 

 races. The species of the Pacific Coast and of the in- 

 terior afford similar illustrations, in some cases more 

 marked even than in any of the eastern species. More 

 rarely, but still quite frequently, is there a similar in- 

 crease in the size of the feet and claws. 



"The tail, also, affords an equally striking exam- 

 ple of the enlargement of peripheral parts southward. 

 Referring again to the birds of the Atlantic Coast, 

 many of the above-named species have the tail abso- 

 lutely longer at southern localities than at northern, 

 and quite often relatively longer. Thus while the gen- 

 eral size decreases, the length of the tail is wholly 

 maintained, or decreases less than the general size ; 

 but, in some cases, while the general size is one-tenth 

 or more smaller at the south, the tail is ten to fifteen 

 per cent, longer than in the larger northern birds. 

 Some western species are even more remarkable in 

 this respect ; and in consequence mainly of this fact 

 the southern types have been varietally separated from 

 the shorter-tailed northern forms of the same species. 



