302 HABIT AND INTELLIGENCE. [ciiAr. 



only organs witli wliicli they are homologous are the leaf- 

 like expansions that arise from the openings of the tracheae 

 of some aquatic larvce ; though it is very difficult to guess 

 by what kind of a process leaf-like branchise came to be 

 used as wings. The difficulty is not so much to under- 

 stand how the wing-form was produced (for branchiae may 

 be produced in one form almost as easily as in any other), 

 as to understand how the habit of flight could originate ; 

 for, as a rule, there are no sudden transitions in nature. 

 But- it is to be remembered that motor habits are more 

 variable than formative ones. 



of air- The next transformation that I have to mention is not 



Vertebrafa '^^^^ ^^^^ °^ ^^^'^ most important, but one of the best under- 

 stood, in the whole organic creation. I mean the transi- 

 tion from the swim-bladder of fishes to the lungs of the 

 air-breathing Vertebrata. I have spoken of this several 

 times already ; but I have now to notice the significant 



Origin of fact that all the Batrachia, through which class the traiisi- 

 tion takes place, are inhabitants of fresh water.^ Presh 

 water is liable to be dried up ; and I believe the Batra- 

 chiaus to be descended from fishes, which, when water was 

 failing them, acquired the power of breathing air by means 

 of the swim-bladder,'^ This woiild be almost incredible, 

 were it not for the general fact of the inconstancy of the 

 form and position of the respiratory organs, and the special 

 fact of the preservation of the Perennibranchiate order, 

 wdiich presents a series of connecting forms. And what 

 still further strengthens the case, is the remarkable set of 



The facts lately discovered concerning the axolotl. This animal 



is a Batrachian, and has till now been regarded as a Peren- 



^ Except the land-newt, or land salamander, and perhaps one or two 

 other species, which are not aquatic at all. 



2 See Spencer's Principles hi Biology, vol. i. p. 395, and vol. ii. p. 325. 

 I must say, however, that I think his attempt to account for the origin 

 of the swim-bladder is quite unsuccessful. He thinlis it arose from fishes 

 acquiring the habit of swallowing bubbles of air, at a time when the supply 

 of oxygen in the water was failing in consequence of the increasing heat 

 of the weather. This appears to be possible only in shallow fresh waters, 

 and I do not think tlifre is the slightest reason for believing cod and other 

 sea-fisli that have swim-bladders to be descended from fresh- water species. 



