Chap. vii. Theory of Natural Selection. 185 



mandible are serrated with teeth much more prominent, coarser, 

 and sharper than in the duck. The common goose does not sift the 

 water, but uses its beak exclusively for tearing or cutting herbage, 

 for which purpose it is so well fitted, that it can crop grass closer 

 than almost any other animal. There are other species of geese, as 

 I hear from Mr. Bartlett, in which the lamella are less developed 

 tban in the common goose. 



We thus see that a member of the duck family, with a beak 

 constructed like that of the common goose and adapted solely for 

 grazing, or even a member with a beak having less well-developed 

 lamellae, might be converted by small changes into a species like 

 the Egyptian goose, — this into one like the common duck, — and, 

 lastly, into one like the shoveller, provided with a beak almost 

 exclusively adapted for sifting the water ; for this bird could hardly 

 use any part of its beak, except the hooked tip, for seizing or tearing 

 solid food. The beak of a goose, as I may add, mfght also be con- 

 verted by small changes into one provided with prominent, recurved 

 teeth, like those of the Merganser (a member of the same family), 

 serving for the widely different purpose of securing live fish. 



Beturning to the whales. The Hyperoodon bidens is destitute of 

 true teeth in an efficient condition, but its palate is roughened, 

 according to Lacepede, with small, unequal, hard points of horn. 

 There is, therefore, nothing improbable in supposing that some 

 early Cetacean form was provided with similar points of horn on the 

 palate, but rather more regularly placed, and which, like the knobs 

 on the beak of the goose, aided it in seizing or tearing its food. If 

 so, it will hardly be denied that the points might have been con- 

 verted through variation and natural selection into lamella} as well- 

 developed as those of the Egyptian goose, in which case they would 

 have been used both for seizing objects and for sifting the water ; 

 then into lamellae like those of the domestic duck ; and so onwards, 

 until they became as well constructed as those of the shoveller, in 

 which case they would have served exclusively as a sifting appa- 

 ratus. From this stage, in which the lamellae would be two-thirds 

 of the length of the plates of baleen in the Balamoptera rostrata, 

 gradations, which may be observed in still-existing Cetaceans, lead 

 us onwards to the enormous plates of baleen in the Greenland 

 whale. Nor is there the least reason to doubt that each step in 

 this scale might have been as serviceable to certain ancient Ceta- 

 ceans, with the functions of the parts slowly changing during the 

 progress of development, as are the gradations in the beaks of 

 the different existing members of the duck-family. We should 

 bear in mind that each species of duck is subjected to a severe 



