ON THE ANATOMY OF FISHES. 
307 
In the absence of any definite knowledge of the habits of these species no certain 
conclusions can be drawn as to the causes which have brought about the rudi- 
mentary state of the air-bladder, but the assumption of a “ ground ” habit may 
prove to be one of them. 
The structural variations met with in the air-bladder of difierent Siluridfe abnor- 
males require, in many instances, no special explanation, and are readily explicable as 
being due to the more or less complete degeneration of parts of a useless organ, 
combined with the usual polymorphism of vestigial structures. Such features as 
the diminutive size of the air-bladder ; the atrophy of the latei’al compartments and 
the partial solidification of the reduced anterior chamber ; the atroph}^ of the fibrous 
tracts by which the contraction or distension of the air-bladder becomes converted 
into corresponding movements of the Weberian ossicles; and the suppression of the 
ductus pneumaticus, can be easily understood as having arisen in this way. But 
there are nevertheless at least two features in connection with the air-bladder, which 
are somewhat difficult to explain. One is, its persistence even in a reduced condition, 
when as a useless organ its total suppression would seem to be the more natural 
result, and indeed is the only result so far as all other Teleostei are concerned, while 
the second difficulty is the partial or complete encapsulation of the vestigial air- 
bladder by bone. The following considerations may possibly throw some light on 
the first of these difficulties: — 
The degeneration of an organ which, from change of habit, has become more or 
less useless to its possessor may be brought about by the single or conjoint operation 
of several causes. (1) The cessation of the preservative influence of natural selection, 
or what Weismann has termed “ panmyxia,” combined with what Galton has called 
“ regression towards mediocrity,” so that the organ fails to be maintained at its 
previous high level of development. (2) Beduction in size and sinqfiification jn other 
respects of a useless organ may also be due to action of natural selection in view of 
the advantage which the animal derives from economy of nutrition. (3) The 
retention in its fully develoj)ed condition of an organ of no functional utility might 
entail some positive disadvantage to the animal when exposed to new conditions of 
life, in which case the operation of natural selection in more or less coin]fietely 
eliminating the useless structure would be certain and sj)eedy. On the assumption of 
a “ ground ” habit by a Fish, the air-bladder would tend to become useless and, sooner 
or later, vestigial, and it is highly probable that the three factors mentioned above 
may all have co-operated in initiating this result. But like so many other vestigial 
structures, and especially those which result from the retrogressive modification of 
organs of great functional importance in a large series of animals, and therefore 
probably of great antiquity, the air-bladder, even when useless, often exhibits a 
singular vitality and persistence. ^I’he initial degeneration of the air-bladder in its 
useless condition may be accounted for, in part, by the advantage of economy of 
nutrition ; but reduction in size beyond a certain limit can scarcely have been due to 
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