SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 



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the wing was also a web of skin attached to the sides of the body and the 

 hing legs, but anteriorly it was supported by an arm with a single enor- 

 mously elongated finger, the fourth; the other fingers were small or lost. 

 To the extent that all of these wings are modified vertebrate limbs based 

 on the pentadactyl plan, they are strictly homologous. But considered as 

 wings they are merely analogous ; 

 the adaptations for flight are unlike 

 in basic design and do not indicate 

 relationship. In terms of phylogeny 

 this means that pterosaurs, birds, 

 and bats have a remote common 

 ancestry along with other terrestrial 

 vertebrates but that each group 

 independently acquired the power 

 of flight, at different times and by 

 different means. 



Similar instances of analogous 

 resemblance superposed on more 

 fundamental homologous resem- 

 blance are by no means uncommon. 

 Various desert plants have succu- 

 lent water-storing stems and a 

 protective armature of spines. The 

 stems and spines are fundamentally 

 homologous with corresponding 

 structures in other angiosperms; 

 but their special adaptations to an 

 arid environment have been in- 

 dependently developed in plants of 

 different families and demonstrate 

 only analogous resemblance. The 

 digging legs of the Australian mar- 

 supial moles and of the true placen- 

 tal moles, the paddlelike forelimbs 

 of penguins and of seals, and the 

 lobed swimming feet of different 

 groups of water birds (coots, grebes, 

 etc.) are examples of analogous modifications of fundamentally homolo- 

 gous structures. Failure to recognize the existence of such combinations 

 leads to erroneous conclusions about relationship. 



Vestigial structures. A special category of homologous resemblance 

 is that which relates to rudimentary, or vestigial, structures. Both in 

 plants and in animals it is common to find organs that perform no useful 



Fig. 24.4. Three vertebrate wings, to show 

 that structures may simultaneously exhibit 

 both homologous and analogous resem- 

 blances. Each of these wings is funda- 

 mentally a modified pentadactyl limb, and 

 to this extent all three are homologous. 

 Considered as wings they are merely 

 analogous, for the modifications for flight 

 are basically unlike and are known to have 

 been independently acquired. (Redrawn 

 from Colbert, The Dinosaur Book, by per- 

 mission American Museum of Natural 

 History.) 



