242 HABIT AND INTELLIGENCE. [cHAP. 



Wing of bird and the wing of the insect are analogous, because they 

 ofkis^ect ^^^ ^*^*^^ organs of flight; but they are not in any way 

 homologous, because they have no resemblance either in 

 anatomical structure, or in position relatively to the other 

 organs of the body, or in the mode of their development. 

 This is perhaps the best example that could be mentioned of 

 two organs which are analogous without being homologous. 

 Conversely, it is possible for two organs to be homo- 

 logous without being analogous : perhaps the best instance 

 Lungs and of this is that of the lungs of the air-breathing Vertebrata, 

 bladder, wliicli are now universally believed to be homologous with 

 the swim-bladder of the fish. This homology is made out, 

 partly from the fact that the lungs and the swim-bladder 

 are similarly placed with respect to the other organs of 

 the body ; and partly from the existence of a tolerably 

 perfect transitional series from fishes that have a swim- 

 bladder to Batrachians (newts, frogs, &c.) that breathe by 

 means of lungs, through the remarkable group of the 

 Perennibranchiate Batrachians. 

 Homo- This instance of the homology between swim-bladder 



respha°ory and lungs introduces us to the fact, that the homologies of 

 organs ^j^g respiratory organs of animals are peculiarly variable. 

 This is not the case among the Vertebrata, except that the 

 lungs of the air-breathing Vertebrata are not homologous 

 with the branchi^ of fishes. And among air-breathing 

 tribes of animals generally, the homologies of the respi- 

 ratory organs are tolerably constant for each tribe. But 

 among the water-breathing Invertebrata, the branchiae are 

 singularly inconstant, both as to their existence, and, where 

 they exist, as to their position and homologies. I do not 

 think this fact has received the attention it deserves. I 

 shall have to recur to it as a most important one, and of 

 great significance in accounting for that process of modi- 

 fication which, as I agree with Darwin in believing, has 

 given origin to all organic forms. 

 Analogies It is to be observed that what has been said about 



and homo- , , . i i • ^ j_ • -i • ti> 



loffies homologies and analogies between similar organs m dii- 

 witliinthe fereut species is equally true of similar organs in the same 



SltCClGS. i X </ o 



individual. For instance, the fore-legs and hind-legs of a 



