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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
this cause alone, for long before its total disappearance the nutritive requirements of 
the organ must become so infinitesimally small that no positive or obvious disadvantage 
could result from its retention in a diminutive condition. When extrinsic muscles in 
connection with the air-bladder become useless, economy of nutrition would, no 
doubt, soon lead to the total suppression of such highly vascular structures, and 
hence the complete atrophy of the compressor and tensor tripodis muscles of certain 
Pimelodinae with rudimentary air-bladders is, on the contrary, easy to understand. 
Then again, while it is by no means difficult to see that the persistence of a 
fully developed air-bladder in purely “ ground ” Fishes might not only be useless, but 
even positively harmful, when the animal habitually rests on the bottom when not in 
motion by the exercise of its fins, it is at the same time difficult to appreciate the 
disadvantage of retaining an air-bladder so small as to be utterly without influence in 
its locomotor functions, or in any way capable of imparting undue buoyancy to the 
animal. Hence it seems to us, of the three causes tending to produce degeneration, 
the only ones likely to have much effect in reducing the air-bladder beyond a certain 
minimum size are “ panmyxia ” and “ regression towards mediocrity,” and it may 
therefore be conjectured that the antagonistic force of heredity has so far retarded the 
operation of these causes as to have secured the pei'sistence of a small vestigial 
air-bladder in all existing Siluridae abnormales. A difficulty in the way of this 
explanation of the invariable retention of a small vestigial and useless air-bladder 
in these Siluroids instead of its total suppression is the fact that in so many Teleostei, 
where the air-bladder has presumably become useless from some cause or other, the 
total disappearance of the organ follows almost as a matter of course. In certain 
families, Physostomi and Physoclisti, the air-bladder is totally absent. The Scopelidm 
and Symbranchidse are examples of this in the former group, and the Pleuronectidae 
and Lophiidae in the latter ; and if the line of argument adopted above in the special 
case of the Siluridae has any force, it would be reasonable to anticipate that it would 
also apply to the former, and that in these families a small vestigial, even if 
useless, air-bladder would be very generally present, which, as a matter of fact, is 
never the case. The suppression of an organ so physiologically important as the air- 
bladder in entire families of Teleostei is, of course, suggestive that the operation of 
causes tending to produce this result has probably extended over a far wider period of 
time than has been the case with genera or species only of those Siluroids in which 
merely a partial reduction has taken place, and that given sufficient time, and the 
accumulative action of such causes, the same ultimate result would be arrived at in the 
latter as in the former. But in opposition to this suggestion must be placed the fact 
that in certain Physoclist families {e.cj., Scombridoe and Polyneraidm) the air-bladder is 
absent in some species of the same genus while present in others. Thus, on the 
authority of Day,* it is stated that Scomber colias possesses an air-bladder, while 
S. scomber has none ; similarly among the Asiatic species of Poli/nemus, P. paradiseus 
* “The Burbot (Lota vulgaris) and Air-Bladder of Fishe.s,” ‘ Cottoswold Nat. Field Club,’ 1880. 
