GENERAL ANALOGIES. 237 



ducks ride deeper in the water than the swimming 

 ducks, and the divers still deeper than they. 



Then there is the difference of form. The motions 

 which a diving bird performs in the water are so 

 varied that to determine the solid of least resistance 

 with regard to them all would be no easy matter. 

 But the boat which rows fastest, keeps course or 

 turns most easily, and lives in the roughest water, is 

 an approximation. That boat is one with the two 

 ends nearly equal, and of an average length. If too 

 short it " yaws like a tub," and will not keep course, 

 and if too long it turns wide. This is the form which 

 we find in those birds which have the most complete 

 command of the water. If they go on long courses 

 after fish, as is the case with the divers, they have the 

 body elongated ; and if they search about among the 

 rocks after mollusca and Crustacea, as is the case with 

 the puffin, they have it shorter. These last, by the 

 way, have the most powerful bruising bills of any of 

 the sea birds, just as the parrots have among the land 

 ones ; and it is curious to notice that there is a con- 

 siderable resemblance both in the appearance and in 

 the harsh screaming voice, so much so that the puffin 

 is sometimes called the sea-parrot. 



The boat shape of those birds, and the backward 

 position of the feet, with the weight of muscles neces- 

 sary for moving them, are incompatible with that 

 structure which answers best for powerful flight. That, 

 in order to be performed with the least effort, requires 

 the weight to be concentrated near the centre of 

 motion in the wings. 



From these few observations, short and imperfect 

 as they are, it will perhaps be seen that there might 

 be a much more natural arrangement of birds founded 



