EVIDENCE FROM TAILS, LIMBS, &> LUNGS OF ANIMALS. 119 



functions of a lung in purifying venous blood, and in returning such 

 purified blood to the heart. 



The lungs of the mud-fishes, formed thus by the gradual modi- 

 fication of an air-bladder, present us with the true origin of the 

 breathing organs of all higher vertebrates. It is interesting to note 

 that in the Climbing Perches and Ophiocephali both characteristic 

 Indian fishes we find examples of fishes which appear actually to 

 breathe air directly from the atmosphere, in addition to the air 

 respired from the water by their gills. The former fishes appear to 

 breathe out of water chiefly through a supply of moisture being 

 retained in certain curiously twisted bones of the head. The latter 

 fishes possess large cavities in the throat, air being admitted to these 

 receptacles by the mouth. Impure blood circulating in the blood- 

 vessels of these cavities is probably purified by the oxygen of the 

 inhaled air, and the essential functions of a lung are thus discharged 

 by the receptacles in question. Experiments on these fishes reveal 

 the interesting fact that, unless they are occasionally permitted to 

 gain free access to the atmosphere for the purpose of inhaling air, 

 they die suffocated. The climbing perch, indeed, is known to make 

 overland journeys, ambling along on its spiny fins in search of water, 

 and presents thus a striking exception to the truth of the universally 

 accepted apophthegm regarding the discomfort of " a fish out of 

 water." We thus discover that the process of modification in the 

 fish-class in the direction of air-breathing habits may be illustrated 

 in other ways than by development of the swimming-bladder; 

 although it must be borne in mind that the latter organ is the true 

 representative and ancestor as illustrated by Lepidosiren of the 

 lungs. 



The lungs of true air-breathers, as seen in members of the frog 

 class, may indeed (as in the Proteus and its neighbours) be actually 

 inferior in structure to those of the mud-fishes. When we consider 

 that, like the mud-fishes, the frogs and their neighbours breathe in- 

 variably by gills in early life (Fig. 55, g\ in their tadpole stage, and 

 afterwards, as represented by the frogs, discard their gills (",,) for 

 lungs, we may discern in such a series of changes in the breathing 

 apparatus the further stages through which the progenitors of the 

 higher Vertebrata passed from the fish-like type and assumed that of 

 the higher atmospheric breathers. For, as has been remarked by 

 authority in matters biological, " the tadpole is at first a fish, and 

 then a tailed amphibian, provided with both gills and lungs, before 

 it becomes a frog, because the frog was the last term in a series of 

 modifications whereby some ancient fish became a urodele (or tailed) 

 amphibian ; and the urodele amphibian became an anurous (or tail- 

 less) frog. In fact, the development of the embryo is a recapitulation 

 of the ancestral history of the species." Mr. Darwin, too, remarks 



