360 



THE CHANGING GENERATIONS 



function. Usually they are reduced in size, and often they have lost essen- 

 tial parts. There are glands that do not secrete, appendages that cannot 

 be moved, eyes that do not see, leaves that have no chlorophyll, and 

 flowers that produce no seed. 



The higher insects as a group are characterized by having wings. Yet 

 a great many insects have wings that are too small for flight, or crumpled 

 and useless, or reduced to small immovable pads. Sometimes the wings 



are completely lost, though the 

 body wall shows where they were 

 once attached. Most snakes have 

 no trace of limbs, but in the python 

 there are rudimentary hind legs 

 projecting as blunt spines close to 

 the vent. Whales have no external 

 sign of hind legs, but within the 

 body are a rudimentary pelvis and 

 vestiges of leg bones. Birds typically 

 have wings; but in some birds the 

 wings are too small for flight, and 

 in others only buried rudiments of 

 the shoulder girdle and wing bones 

 remain. Modern horses are one- 

 toed, but traces of two additional 

 toes exist in the form of splint 

 bones along the sides of the foot. 

 Many animals that live in the dark- 

 ness of caves have eyes that are 

 more or less reduced and often 

 totally blind. 



Man is no exception to the list 

 of creatures possessing rudimentary 

 organs. In fact, because his anat- 



Fig. 24.5. Homology and analogy. Left, the 

 African placental mole (Chrysochloris) ; 

 right, the Australian marsupial mole 

 (Notoryctes). In so far as these animals 

 are both mammals, with all that this 

 implies, they show a multitude of homolo- 

 gous resemblances. In their adaptations to 

 a burrowing existence, however, they show 

 only analogous resemblance, which does 

 not imply a close relationship. The Aus- 

 tralian mole is, in fact, more closely related 

 to kangaroos than it is to the African mole. 

 {Courtesy American Museum of Natural 

 History.) 



omy is so well-known, more ves- 

 tigial structures have been described in him than in any other animal. 

 Some authorities have recognized as many as 180; we can mention only 

 a few of the more evident ones. 



The hair on man's body is vestigial, since it is too sparse to prevent heat loss. 

 Small hair-erecting muscles are attached to the base of each hair. In hairy mam- 

 mals their homologues raise the hairs to make the fur coat thicker in cold weather ; 

 but when we feel cold, all that these muscles can do is to raise goose flesh. The 

 outer ear of man is partly vestigial. It is not a good sound-concentrating funnel 

 and cannot be turned toward the source of a sound. The wisdom teeth are becoming 

 vestigial. There is hardly room for them in the jaw; often they never erupt, and 



