CHAP. XI 



The Elements of Structure 



111 



nineteen pairs of appendages borne by a crayfish or lobster. 

 These differ greatly in form and in function ; many of them 

 are not analogous with their neighbours, one feels and 

 another bites, one seizes and another swims, but they are 

 all homologous. So are the different forms of fore-limb, 

 the pectoral fin of a fish, the fore-leg of a frog or lizard, the 

 wing of a bird, the flipper of a whale, the fore-leg of a tiger, 

 the arm of man. But the wing of an insect is merely 

 analogous not homologous with that of a bird, while the 

 wings of bats and birds are both analogous and homo- 

 logous. 



Fig. 33. — Bones of the wing in pigeon (A), bat (B), extinct pterodactyl (C). 

 (From Chambers's Encyctop.) 



Change of Function. — Organs are not mechanisms rigidly 

 adapted for only one purpose. In most cases they have a 

 main function and several subsidiary functions, and changes 

 may take place in organs by the occasional predominance 

 of a subsidiary function over the original primary one. 

 Thus the swim- or air-bladder which grows out dorsally 

 from the food-canal of most fishes, seems usually to be a 

 hydrostatic organ ; in a few cases it helps slightly in 

 respiration, but in the double -breathing mud -fishes or 

 Dipnoi it has become a genuine lung. An unimportant 

 (allantoic) bladder at the hind end of the gut in frogs, is 

 represented in the embryos of reptiles and birds by a very 

 important respiratory (and sometimes yolk-absorbing) birth- 



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