EVIDENCE FROM TAILS, LIMBS, 6- LUNGS OF ANIMALS. 115 



animals may be contained a veritable romance of science. For 

 certainly no more startling or unlikely supposition than that of the 

 common nature of the arm, the wing, the fin, the paddle, and the 

 limb, could well be broached. Yet, as the context may have shown, 

 the facts of life bear out the romance with which even a technical 

 but interesting study may be shown to be invested ; and the truths 

 of comparative anatomy are thus shown to be stranger indeed than 

 the creative fictions of former years. 



If the tails of fishes may be literally deemed " ends " in 

 the most literal sense, there yet remain one or two cases of 

 "odd" structures in fishes and in other animals, the investigation 

 of which may serve to strengthen those conclusions respecting 

 the validity of the development theory at which we have already 

 arrived. One of the most peculiar structures found in fishes is the 

 " air-bladder," " swimming-bladder," or " sound," as it is variously 

 called. From the walls of the swimming-bladder of the sturgeons 

 the well-known "isinglass" is prepared. The air-bladder exhibits 

 exceedingly diverse forms in the class of fishes, and in truth presents 

 the upholders of the " special creation " theory with one of the most 

 unsatisfactory of subjects in respect of the eccentricity of its nature 

 and distribution in the fish class. Thus, no traces of an air-bladder 

 are discernible in the lowest fishes the lancelets and lampreys before 

 alluded to. It is well represented in many common fishes, but certain 

 of the latter as, for example, the flounders and other flat-fishes 

 want it altogether, whilst the sharks, rays, and dog-fishes possess the 

 merest rudiment of this organ. The special-creation theory affords 

 no explanation of the anomaly of one fish possessing an air-bladder, 

 whilst in certain of its near neighbours this structure is entirely absent. 

 But the difficulty of the one theory of creation is, as we shall 

 presently see, the triumph of the other. Even amongst ordinary 

 fishes the air-bladder varies very much in form. In the cod and 

 perch, for instance, the air-bladder is simply a closed sac or bag 

 filled with gas. In the carp (Fig. 52), on the other hand, this organ 

 (B, c) communicates with the throat 

 (E) by means of a duct or tube (D); 

 and in this fish, as well as in the 

 roach, the air-bladder lies in curious 

 relation to the internal ear, and 



probably serves some important FIG. S 2.-AiR-BLADDER OF CARP. 

 function, such as that of increasing 

 the resonance of sound. In the herring, the air-bladder appears to 

 be placed in communication with the stomach ; and in other fishes 

 (Corvina, Fig. 53, c, and Johnius, Fig. 53, b) this structure is of com- 

 plicated form, and is divided into a large number of ramifications and 

 processes. It is interesting to note that whatever may be the nature 



