Orthoplasy with Correlated Variation 205 



holding to the perch as long as possible with the other. 

 So constant and uniform an act is this — as also with the 

 companion bird of the same species — that it may, I think, 

 be considered a real and very useful adaptation. Further, 

 the advantage of having this utility subserved in this way 

 instead of by a blunt beak is, that the sharp point of the 

 beak, while no longer a hinderance to such a use of the 

 mandible, — as by penetrating the ground, catching in 

 the texture of any material he may be resting upon, etc., — 

 is, nevertheless, not lost ; and it is of very great service in 

 biting, rending, breaking nuts, etc. 



Now, if we admit this utility — that of a sort of third 

 foot — in the upper mandible, the question arises how the 

 adaptation may have been acquired. In answer to this, 

 we may cite the further character — in these parrots — 

 that the upper mandible is somewhat loosely attached, the 

 muscles allowing rather free movement, as is the case with 

 many other birds; and it is in connection with the relative 

 flexibility of the upper mandible, in its relation to the 

 head of the bird, that the curvature of the beak may have 

 been acquired by a gradual operation of natural selection. 

 If the mandible was at first straight, and if there were 

 variations in the relative flexibility and range of the 

 muscles by which it is attached, the use of the beak 

 more or less clumsily for descending would be possible to 

 some of the more flexible parrots. This muscular accom- 

 modation would screen variations in the direction of the 

 curved beak; for, the more curved, the better could the 

 function be performed. This process would continue from 

 generation to generation, until the adaptation attained, by 

 variation and natural selection, the degree of perfection it 

 has in the present-day parrots. 



