EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 43 



eyelids. In horses the membrane is well developed, containing cartilage. 

 When the eye is strongly retracted the membrane extends across it for 

 about an inch. The membrane is variably developed in other mammals 

 and in lower vertebrates. 



The tails commonly possessed by both wild and domestic mammals are 

 familiar to everyone. A chain of vertebrae continuous with those compris- 

 ing the remainder of the vertebral column forms the skeletal axis of the 

 tail, attaching just behind the pelvic girdle (the bones to which the hind 

 limbs articulate). In man a much reduced string of vertebrae, partly 

 fused together, arises at this same point and curves forward, instead of 

 extending out into an external tail. This structure, called the coccyx, is 

 clearly homologous to a group of reduced tail vertebrae (Fig. 11.5, p. 

 230). 



Everyone who has watched a horse on days when biting flies were both- 

 ersome is familiar with the way in which this animal can twitch certain 

 areas of the skin. All observers of horses will recall also the manner in 

 which the animal can move and turn its ears the better to hear sounds 

 coming from difi'erent directions. Although we do not have these capabili- 

 ties we commonly have rudimentary muscles connected with skin and ears. 

 Generally these organs do not function, though some individuals can 

 demonstrate ability to move the scalp or "wiggle" the ears. 



Although the list of vestigial organs in man is long the above sample will 

 suffice. We must not create the impression, however, that vestiges are the 

 exclusive attribute of man. It may safely be stated that every speciahzed 

 animal retains some rudimentary structures in its anatomy. Snakes, for ex- 

 ample, are noted for lack of limbs, yet a few, such as boas and pythons, 

 possess in appropriate position in the body tiny bones which seem to repre- 

 sent the last vestiges of pelvic girdle and hind limbs. Similarly, whales 

 have no hind limbs, yet in the position where hind limbs if present would 

 occur small bones are found which seem to represent rudiments of pelvic 

 girdle and hind limbs (Fig. 3.3). 



Vestigial structures in the leg of the horse have already been mentioned 

 (p. 23): the splint bones representing the rudimentary metacarpals of 

 digits II and IV. These vestiges are slender bones of variable development 

 partly fused to the cannon bone (metacarpal of digit III) supporting the 

 hoof. The lower end of each splint bone is bluntly pointed and without 

 connection to other bones. 



Birds are characteristically flying animals, yet a few are flighdess. One 

 of these, the kiwi of New Zealand, possesses useless vestiges of wings 

 supported by tiny replicas of the usual bones of a bird wing (Fig. 3.1 ). 

 Feathers covering the body conceal these rudimentary wings from view. 



