ON THE ANATOMY OB" FI8HES. 
309 
and P. tetradactyluH are without air-bladders, while, on the contrary, such org-ans are 
present in P. indicus and P. plehtjus. Such facts certainly seem to indicate that 
total suppression may take place so rapidly as to have become completed within the 
relatively limited time that has elapsed since certain of these Fishes acquired their 
present specific characters. Without attempting to assign relative values to the 
three causes which have probably brought about the total atrophy of the air-bladder 
in these instances, the evidence seems strongly in favour of the view that, either 
singly or conjointly, their operation is calculated to lead to the speedy and complete 
suppression of a useless air-bladder, and is distinctly adverse to the retention of 
that organ, however vestigial may be the condition to which it has been reduced ; 
hence the persistence of a vestigial air-bladder in so many Siluridae, in spite of 
causes which, in most other Fishes, have sufficed to secure its total atrophy, can hardly 
be explained by the suggestion previously made by us. 
The widespread persistency of a vestigial and useless air-bladder seems to be 
peculiar to the Ostariophyseae ; in fact, we know of no species in any of the different 
families of the group in which aji air-bladder is entirely wanting. On the contrary, so 
far as our knowledge extends, this fact is altogether without parallel in all the 
remaining families of Teleostei, in which total suppression seems invariably to follow 
loss of function. There are, however, two points in connection with the air-bladder of the 
Siluridae abnormales that are not without some significance, and may possibly furnish an 
explanation of this anomaly. The first has reference to the method by which the 
reduction of the bladder has been effected. We have previously pointed out that the 
vestigial air-bladder in these Siluroids represents the anterior chamber only of the 
normally developed organ, and that the lateral compartments have almost always 
undergone total atrophy. The process of reduction has, therefore, not affected all 
parts of the air-bladder to an equal extent, and, while it has almost invariably led to 
the total suppression of all trace of lateral chambers, the anterior chamber has been 
allowed to persist in a reduced and vestigial condition ; or in other words, while 
total atrophy has taken place with respect to that part of the bladder not specially 
related to the Weberian ossicles, that portion of it which alone is directly connected 
with those ossicles invariably persists. The second point relates to the persistence of 
the now useless Weberian ossicles. The uniform retention of a fairly complete series 
of ossicles in all Siluridae abnormales is explicable as due to the absence of any j)otent 
cause for their speedy suppression. Economy of nutrition can have scarcely any effect 
in this direction, nor need the direct action of natural selection be invoked for the 
purpose, for, if useless, their persistence need not necessarily prove a source of disad- 
vantage or harm to their j^ossessor. Whatever trace of degeneration the mechanism 
presents may therefore be solely due to “ ])anmyxia,” or to “ regression towards 
mediocrity,’' and these, as we have already conjectured, are causes that operate with 
extreme slowness. The conclusion suggested to us by these facts, and one which 
seems to offer a reasonable explanation of what would otherwise be a singular anomaly. 
