326 PHYSIOLOGICAL DEVELOPMENT. 



mentioned, which swallows air, frequents small waters liable 

 to be consideraoly warmed ; and the Cuckia, an anomalous 

 eel shaped fish, which has vascular air-sacs opening out at the 

 back of the mouth, "is generally found lurking in holes 

 and crevices, on the muddy banks of marshes or slow-moving 

 rivers." Still more significant is the fact that the Lcpidosiren, 

 or " mud-fish " as it is called from its habits, is the only true 

 fish that has lungs. But it is among the Amphibia that we see 

 most conspicuously this relation between the development 

 of air-breathing organs, and the peculiarities of the habitats. 

 Pools, more or less dissipated annually, and so rendered unin- 

 Habitable 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 supple- 

 mentary aerial respiration is needful in different degrees ; so 

 do we find among the Amphibia -many stages in the substi- 

 tution of the one respiration for the other. The facts, then, 

 are such as give to the hypothesis a vraiscmblance 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 bubbles is 

 scarcely interpretable as a result of spontaneous variation : 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 favouring 

 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 



