Letters, Announcements, ^c. 483 



and G. macrorhyncha, we have differences not only of colour, 

 but of structure. These differences are most marked in the 

 form of the bill. Now to take the two former first, G. are- 

 nicola has a very long bill, G. isahellina a very short one ; 

 the former resorts exclusively to the deep loose sandy tracts, 

 the latter haunts the hard and rocky districts. It is manifest 

 that a bird whose food has to be sought for in deep sand 

 derives a great advantage from any elongation, however slight, 

 of its bill. The other, who feeds among stones and rocks, 

 requires strength rather than length. We know that even in 

 the type species, the size of the bill varies in individuals, in 

 the Lark as well as in the Snipe. Now, in the Desert, the 

 shorter-billed varieties would undergo comparative difficulty 

 in finding food where it was not abundant, and consequently 

 would not be in such vigorous condition as their longer- billed 

 relatives. In the breeding-season, therefore, they would have 

 fewer eggs and a weaker pi'ogeny. Often, as we know, a 

 weakly bird will abstain from matrimony altogether. The 

 natural result of these causes would be that in course of time 

 the longer-billed variety would steadily predominate over the 

 shorter, and in a few centuries they would be the sole exist- 

 ing race, their shorter-billed fellows dying out until that race 

 was extinct. The converse will hold good of the stout-billed 

 and weaker-billed varieties in a rocky district. 



" Here are only two causes enumerated which might serve 

 to create as it were a new species from an old one, yet they 

 are perfectly natural causes, and such as, I think, must have 

 occurred, and are possibly occurring still. We know so very 

 little of the causes which in the majority of cases make 

 species rare or common, that there may be hundreds of others 

 at work, some even more powerful than these, which go to 

 perpetuate and eliminate certain forms ' according to natural 

 means of selection.'' But even these superficial causes appear 

 suificient to explain the marked features of the Desert races 

 which frequently approach so very closely the typical form, 

 and yet possess such invariably distinctive characteristics, 

 that naturalists seem agreed to elevate them to the rank of 

 species. The differences in size may be yet more simply 



