CHAP, xi THE ELEMENTS OF STRUCTURE 209 



logous not homologous with that of a bird, while the 

 wings of bats and birds are both analogous and homo- 

 logous. 



When two animals not nearly related to one another 



i/ 



live in similar conditions they often show a superficial 

 resemblance, being similarly adapted to the circum- 

 stances of the case. Such resemblance is technically 

 called convergence or hoinoplasty. It is due to similar 



FIG. 57. BONES OF 



THE WING IN PIGEON 

 PTERODACTYL (C). 



(A), BAT (B), EXTINCT 



(From Chambers's Encyclop.) 



The student should notice in A the two free -wrist-bones or carpals ; 

 the fused carpo-metacarpus ; the thumb with one joint, in a line with the 

 more slender of the fore-arm bones the radius ; the first finger with two 

 joints ; the second finger with one joint. The thicker of the fore-arm 

 bones is the ulna ; it is an incomplete splint in the bat's wing. 



In B the palm-bones (metacarpals) of the fingers are long and free ; the 

 fingers have only two joints or phalanges ; claws are confined to the thumb 

 and first finger, or to the thumb only. It is instructive to become familiar 

 with the ditlerences, at almost every point, between the bird's wing and 

 the bat's wing. Yet they are thoroughly homologous as well as analogous. 



In C the thumb is drawn much too large. It is usually unrepresented 

 and is at most minute. The enormously elongated outermost or ulnar 

 digit corresponds to our little finger. 



adaptations of parts and does not indicate, as homology 

 does, any blood-relationship. A burrowing amphibian 

 (a Csecilian), a burrowing lizard (an Amphisbaenid or a 

 slow-worm), and a burrowing snake (a Typhlopid) are 

 very like one another externally, but that is because they 

 are similarly adapted to burrowing. They are not at 

 all like one another internally. Similarly a Cetacean 

 shows a slight superficial convergence to a fish. Or, to 



15 



