STRUCTURAL ADAPTATIONS 373 



locomotion, and the hind-limbs adapted to support the body 

 in terrestrial locomotion. In considering at the commence- 

 ment of this chapter some of the more striking changes of 

 shape which birds present, this adumbration of a bird, or 

 schematic bird, must have been, unconsciously at least, always 

 present in the reader's mind, forming the standard of com- 

 parison. And similarly, in the further continuation of this 

 theme, we shall discuss adaptation in this restricted sense — 

 variations from the type. 



Inasmuch as flight may be considered as one of the most 

 striking attributes of birds, we may well commence this more 

 detailed study of adaptation with an account of its effects on 

 the wing, and on the structures associated therewith. The 

 wing of the bird is an absolutely unique organ ; as to its origin 

 we can say nothing with certainty ; speculations on this head 

 are few, but they will be referred to later. Of its main features 

 we have already treated in the introductory portion of this 

 volume (p. 13), and we shall therefore refer here only to the 

 modifications which are to be observed therein. 



First of all, let us consider the wing as an organ of flight, 

 and examine (i) the changes which are to be observed in the 

 skeleton thereof, and (2) the changes in the contour of the 

 wing as determined by the length of the flight feathers or 

 remiges. 



The modifications of the skeleton are mainly confined to the 

 wrist and the hand. Unfortunately, we have no record of the 

 earlier stages of the evolution of the wing, but there can be no 

 doubt but that what we now know as the wing was originally a 

 five-fingered organ. We meet with it for the first time in the 

 skeleton of the Jurassic ArchcEopieiyx, and here it has already 

 acquired its peculiar tridactyle form. The arm and forearm 

 differ in no respect from the same bones in modern birds, nor 

 can the carpus be said to offer any essential points of difference- 

 But, assuming the fossil remains of the only two specimens of 

 ArchcBopteryx so far discovered to be skeletons of adult birds, 

 it is important to notice that this complex of wrist bones 

 remained free throughout life, while in modern birds this indi- 

 viduality is retained only for the first few weeks of life, as a 

 comparison of the accompanying illustrations will show. So, 

 too, with the bones of the middle hand or metacarpus, though 



