338 PHYSIOLOGICAL DEVELOPMENT. 



though its air-sacs are not morphological equivalents of those 

 above described, yet they equally well illustrate the relation 

 between such organs and the environing condition. Still 

 more significant is the fact that the Lepidosiren, or " mud- 

 fish " as it is called from its habits, though it is a true fish 

 nevertheless has lungs. But it is among the Amphibia that 

 we see most conspicuously this relation between the develop- 

 ment of air-breathing organs, and the peculiarities of the 

 habitats. Pools, more or less dissipated annually, and so 

 rendered uninhabitable by most fishes, are very generally 

 peopled by these transitional types. Just as we see, too, 

 that in various climates and in various kinds of shallow 

 waters, the supplementary aerial respiration is needful in dif- 

 ferent degrees; so do we find among the Amphibia many 

 stages in the substitution of the one respiration for the other. 

 The facts, then, are such as give to the hypothesis a vrai- 

 semblance greater than could have been expected. 



The relative effects of direct and indirect equilibration in 

 establishing this further heterogeneity, must, as in many 

 other cases, remain undecided. The habit of taking in bub- 

 bles is scarcely interpretable as a result of spontaneous varia- 

 tion: we must regard it as arising accidentally during the 

 effort to obtain the most aerated water; as being persevered 

 in because of the relief obtained ; and as growing by repetition 

 into a tendency bequeathed to offspring, and by them, or 

 some of them, increased and transmitted. The formation of 

 the first slight modifications of the alimentary canal favour- 

 ing the lodgment of bubbles, is not to be thus explained. Some 

 favourable variation in the shape of the passage must here 

 have been the initial step. ' But the gradual increase of this 

 structural modification by the survival of individuals in 

 which it is carried furthest, will, I think, be all along aided 

 by immediate adaptation. The part of the alimentary canal 

 previously kept from the air, but now habitually in contact 

 with the air, must be in some degree modified by the action 

 of the air ; and the directly-produced modification, increasing 



