Chap. VII.] THEOEY OF NATUEAL SELECTION. 287 



of equal length with those above, but finer ; and in being 

 thus furnished it differs conspicuously from the lower 

 jaw of a whale, which is destitute of baleen. On the 

 other hand, the extremities of these lower lamellae are 

 frayed into fine bristly points, so that they thus curiously 

 resemble the plates of baleen. In the genus Prion, a 

 member of the distinct family of the Petrels, the upper 

 mandible alone is furnished with lamellae, which are well 

 developed and project beneath the margin ; so that the 

 beak of tins bird resembles in this respect the mouth of a 

 whale. 



From the highly developed structure of the shoveller's 

 beak we may proceed (as I have learnt from information 

 and specimens sent to me by Mr. Salvin), without any 

 great break, as far as fitness for sifting is concerned, 

 through the beak of the Merganetta armata, and in some 

 respects through that of the Aix sponsa, to the beak of 

 the common duck. In this latter species, the lamellae 

 are much coarser than in the shoveller, and are firmly 

 attached to the sides of the mandible ; they are only 

 about 50 in number on each side, and do not project at 

 all beneath the margin. They are square-topped, and 

 are edged with translucent harclish tissue, as if for crush- 

 ino- food. The edges of the lower mandible are crossed 

 by numerous fine ridges, which project very little. 

 Although the beak is thus very inferior as a sifter to 

 that of the shoveller, yet this bird, as every one knows, 

 constantly uses it for this purpose. There are other 

 species, as I hear from Mr. Salvin, in which the lamellae 

 are considerably less developed than in the common 

 duck ; but I do not know whether they use their beaks 

 for sifting the water. 



Turning to another group of the same family. In the 

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