384 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



have also recorded the existence of larger feet in many of the smaller 

 North American mammalia at the southward than at the northward 

 among individuals of the same species, especially among the wild 

 mice, in some of the squirrels, the opossum, and raccoon, as well as in 

 other species. 



In birds the enlargement of peripheral parts,- especially of the bill, 

 claws, and tail, is far more obvious and more general than in mam- 

 mals. The bill is particularly susceptible to variation in this regard, 

 in many instances being very much larger, among individuals of 

 unquestionably the same species, at the southward than at the north- 

 ward. This accords with the general fact that all the ornithic types 

 in which the bill is remarkably enlarged occur in the intertropical 

 regions. The southward enlargement of the bill within specific 

 groups may be illustrated by reference to almost any group of North 

 American birds, or to those of any portion of the continent. As in 

 other features of geographical variation, the greatest ditfererices in 

 the size of the bill are met with among species having the widest dis- 

 tribution in latitude. Among the species inhabiting eastern North 

 America we find several striking examples of this enlargement among 

 the sparrows, blackbirds, thrushes, crows, wrens, and warblers; in 

 the quail, the meadow lark, the golden-w^inged woodpecker, etc. Gen- 

 erally the bill in the slender-billed forms becomes longer, more atten- 

 uated, and more decurved (in individuals specifically the same) in 

 passing from the New England States southward to Florida, while 

 in those which have a short, thick, conical bill there is a general in- 

 crease 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 gen- 

 erally a relative, but often an absolute, increase in the size of the bill 

 in the southern races. The species of the Pacific coast and of the 

 interior afford similar illustrations, in some cases more marked even 

 than in any of the eastern species. More rarely, but still quite fre- 

 quently, is there a similar increase in the size of the feet and claws. 



The tail, also, affords an equally striking example of the enlarge- 

 ment of peripheral parts southw^ard. Referring again to the birds of 

 the Atlantic coast, many of the above-named species have the tail 

 absolutely longer at southern localities than at northern, and quite 

 often relatively longer. Thus, while the general size decreases, the 

 length of the tail is wholly maintained, or decreases less than the gen- 

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

 more smaller at the south the tail is 10 to 15 per cent longer than in 

 the larger northern birds. Some western s^Decies are even more re- 



being annually cast and renewed, and are thus entirely different physiologically 

 from the horns of bovines, which retain a high degree of vitality throughout th'j 

 life of the iuiimul. 



