38 A HISTORY OF BIRDS 



must turn to find that process of differentiation at work which 

 evolved the several types of land and water birds, of which our 

 bird fauna to-day are the descendants. 



What these " pro-aves " were like we can only dimly sur- 

 mise ; and all our inferences must be inspired by and based upon 

 that strange, kite-tailed form, Archaeopteryx. 



From what we know of the evolution of other types of 

 vertebrates, we may safely assume that these ancestral birds 

 were of small size, and were almost certainly also arboreal. 

 And from the unmistakable signs of the shortening of the body 

 in modern birds, the trunk, we may assume, was also relatively 

 longer, as it certainly was in Archaeopteryx. From these two 

 inferences we may conclude with some degree of probability 

 that these creatures, these birds " in the making," had substituted 

 leaping for climbing about the trees, and from this there was 

 but a short passage to leaping from tree to tree. In these 

 movements we may reasonably suppose the fore-limbs were 

 used for grasping at the end of the leap. The use of the fore- 

 limb for this work would naturally throw more work upon the 

 inner digits, 1-3, so that the process of selection would rapidly 

 tend to the increased development of these, and the gradual 

 decrease of the two outer and now useless members. Corre- 

 lated with this trend in the evolution, the axillary membrane, 

 the skin between the inner border of the upper arm and the 

 body, became drawn out into a fold, while a similar fold came 

 to extend from the shoulder to the wrist, as the fore-limb, in 

 adaptation to this new function, became more and more flexed. 

 While the fingers upon which safety now depended were 

 increasing in length, and growing more and more eflScient, they 

 were at the same time losing the power of lateral extension 

 and becoming more and more drawn together by ligament and 

 muscle, and more and more tending to become flexed upon the 

 fore-arm. And the growth in this direction was probably 

 accompanied by the development of connective tissue and mem- 

 brane along the post-axial or hinder border of the whole limb, 

 tending to increase the breadth of the limb when extended 

 preparatory to parachuting through space from one tree to 

 another, long claws being used to effect a hold at the end of 

 the leap; though to a less extent, the hind-limbs were also 

 affected by this leaping method of locomotion, resulting in the 



