254 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



bone) and in the horse also to one, but with two rudi- 

 ments of others (splints). But in all cases we can trace 

 all the steps of gradation. This is well shown in rumi- 

 nants. In the deer, besides the two functional toes, 

 there are two rudiments — hoofed and supplied with 

 bones — but useless ; in the cow these are further re- 

 duced to mere wartlike rudiments ; in the goat and 

 sheep they disappear entirely. 



6. We need hardly call attention to the extreme 

 modification of the hand in the bird's wing, of the whole 

 fore limb in the whale and in the mole, and especially in 

 this last to the elongation of the point of the elbow to 

 increase its power of digging (Fig. 162, B) and to the 

 still more extreme modification in the case of the pec- 

 toral fin of fishes, and the enormous elongation of one 

 finger of the hand in the extinct flying reptile — the 

 pterosaurs of the Mesozoic (Fig. 161, D). 



It is well to observe, too, how the same part may be 

 differently modified for the same function. This is well 

 illustrated by the different devices used for flight in 

 birds, in mammals (bat), and in reptiles (pterodactyls). In 

 the bird the hand is shortened and consolidated, and the 

 flat plane is formed by the addition of quill feathers. In 

 the bat the hand is elongated, and a web is stretched be- 

 tween the greatly lengthened palm bones and fingers, 

 leaving only one finger, the thumb, free and clawed. In 

 the flying reptile one finger is enormously elongated and 

 strengthened, and the web is stretched from the point of 

 this to the hind limb, leaving the three other fingers 

 free and clawed. It is a most significant fact, however, 

 that these several devices are not accomplished at once, 

 but by a gradual process through successive generations. 

 In the earliest bird (the Arch&ojiteryx) (Fig. 161, C) of 

 the Jurassic the finger bones are not consolidated nor 

 feathered, but are all four free and clawed. 



