INTRODUCTION. xix 



The swim-bladder does not exist in all fishes ; in the Lampreys it is entirely absent ; it 

 does not exist in the Sharks, Dog-fish, Rays, and the Chimera. There is a swim-bladder and 

 an air-duct in the Eel, Herring, Salmon, Pike, Carp, Silurus, with their allies ; in the order 

 Anacanthini, as in the Ophidium, Cod, and Plaice, the air-bladder when present has no 

 pneumatic duct; in the Acaiithopto-ys^il or spiny-finned Fishes, as in the Perch and Miller's 

 Thumb amongst fresh-water species, the air-bladder, where it exists, is without a pneumatic duct, 

 the same is the case with the orders Plcctognathi, File-fish, Trunk-fish, Globe-fish, and LopJio- 

 bra7ichii. Pipe-fishes and Sea-horses. In the sub-class Dipnoi, or "Double-breathers," as in 

 the Protopterus annedens of Tropical Africa, and the Lcpidosiren paradoxa of Brazil, we meet 

 with a most interesting form of air-bladder : it is double, cellular, and lung-like, and is 

 provided with an air-duct, glottis, and pulmonary vein. 



What then are the functions of the swim-bladder and air-duct, when present ? What 

 does their presence appear to indicate ? Will it serve to throw any gleam of light on that 

 most interesting subject, the origin of species? There can be no doubt that "under all its 

 diversities of structure and function, the homology of the swim-bladder with the lungs is 

 clearly traceable." True, there is nothing at all in the simple cylindrical closed air-bladder 

 of a Perch, with its shining silvery fibrous tunic, to remind one of the cellular structure of 

 the lung of an amphibian ; but it must be remembered that there are numerous gradations, 

 leading by various transitions, from a single cavity up to a highly complex cellular organ, 

 which both in structure and function is indisputably a lung. The fishes which most closely 

 resemble the amphibians are the Proioptei-iis anuectcns, or Mud-fish of Tropical Africa, and the 

 Lepidodrcn paradoxa, of the river Amazon and its tributaries. In Protopterus, we find these 

 traditions complete, for here we see a double lung-like air-bladder, with air-duct, glottis and 

 pulmonary vein, and this respiratory apparatus, be it remembered, is at certain periods 

 functionally identical with the lungs of air-breathing vertebrates; for Protopterus, after the rains 

 have ceased to flood the river Gambia, finds itself left behind in the mud of the retreating 

 waters ; the scorching rays of a tropical sun compel the fish to burrow in the mud, in which 

 it forms a kind of cocoon of hard-baked clay. How is the fish to live in this changed 

 locality? As an inhabitant of the water, the respiration was effected by means of gills alone, as in 

 ordinary fishes ; but how is the circulation to be maintained now that it is a terrestrial animal; 

 Professor Owen tells us in clear and distinct terms. "Whatever amount of respiration was 

 requisite to maintain life during the dry months is effected in the pulmonary air-bladders ; its 

 short and wide duct or trachea, the oesophageal origin of which is kept open by a laryngeal 

 cartilage, introduces the air directly into the bladders ; the blood transmitted through the 

 branchial arches to the pulmonary arteries, is distributed by their ramifications over the cellular 

 surface of the air-bladders, and is returned arterialised by the pulmonary veins. A mixed 

 venous and arterial blood is thence distributed to the system and again to the air-bladders." 

 — [Anat. of Vert., i. p. 498.) When the Protopterus resumes its piscine nature on the return of 

 the water, the branchial circulation again comes into play. In this fish, therefore, we have 

 an instance of an animal which is a fish at certain periods of its existence, and an amphibian 

 at others; and I believe with Darwin that natural selection has operated "in converting a 

 swim-bladder into a true lung, used exclusively for respiration." 



It surely is quite conceivable that under changed conditions, acting for a lengthened 

 period, the Salamandroid Protopterus — which some naturalists of note maintain to be more 

 allied to amphibia than to fish — might gradually convert its ichthycic characters into amphibian 

 ones, just as I believe it has converted, not only the swim-bladder and pneumatic duct into 

 an air-breathing lung, trachea and glottis, but also two pairs of the gills of the branchial 



oxygen. A priori, one might almost as much expect oxygen to be exhaled from the lungs, in respiration, as to be 

 separated from the blood by secretion in the air-bladder; and had we not the authority of so accurate an observer 

 as M. Biot, we might be led to suspect that the statement of its being so was founded on error." — {Physiol. Res. p. 271.) 



