92 The Bird 



toral fins. Even among fossil forms there have as yet 

 been found no " missing links" in this respect. But how- 

 ever it came about, it is certain that when the fish- 

 amphibians of olden time, venturing into shallow water, 

 felt more or less solid mud under them, and tried to 

 move about upon it, their fins must have become pressed 

 downward, and before they could safely push themselves 

 about on dry land or lift their bodies clear of the ground, 

 the stiff fin-rays must have become split up into a few, 

 thick, bony rays or toes. We know that these were 

 originally five in number on all four limbs, and when- 

 ever, among living creatures, we find a lesser number, the 

 reduction has been brought about by some subsequent 

 change in the life of the animal. As yet, however, we 

 know of no direct transitions from fins to feet. 



The requirements of flight demanded a fin-like stiff- 

 ness in the wings of birds, and therefore many of the 

 smaller bones of lizards, counterparts of which we find 

 in our own wrists and hands, are in the bird fused together. 



The upper arm-bone, or humerus, corresponds exactly 

 to our bone of that name, and when we feel the two long 

 bones of our forearm and look for them in the bird, we 

 find both very plainly represented, the large one with 

 notches, where the great wing-feathers are fastened, being 

 called the ulna, and the smaller, straighter bone the 

 radius. In our wrist there are eight little bones which 

 are joined to each other so delicately that we can move 

 and turn our hand in every direction. But when a bird's 

 wing is extended, if the wrist was at all flexible, the pres- 

 sure of air on the great wing-feathers would turn the 



