Chap. VII.] THEORY OF NATURAL SELECTION. 287 



that if we were to make the head of the shoveller as 

 long as that of the Balsenoptera, the lamellae would be 

 six inches in length, — that is, two-thirds of the length 

 of the baleen in this species of whale. The lower man- 

 dible of the shoveller-duck is furnished with lamellae 

 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 curious- 

 ly 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 lamella, which are well 

 developed and project beneath the margin; so that the. 

 beak of this bird resembles in this respect the mouth of 

 a whale. 



From the highly developed structure of the shovel- 

 ler's beak we may proceed (as I have learnt from infor- 

 mation and specimens sent to me by Mr. Salvin), with- 

 out any great break, as far as fitness for sifting is con- 

 cerned, 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 hardish tissue, as if for crush- 

 ing food. The edges of the lower mandible are crossed 

 by numerous fine ridges, which project very little. Al- 

 though 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 



