282 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
of being a constant factor, varies incessantly with almost every locomotor movement 
of these usually active Fishes, while the Fish can have no power of differentiating the 
effects due to the incidence of the two pressures. It also seems reasonable so far to 
extend this conclusion as to infer that if the extent and variety of the barometrical 
variations above recorded may be regarded as fairly typical of temperate or North 
temperate regions, the application of Sagemehl’s theory to the large number of species 
of Cyprinidae which inhabit these areas must be open to very grave objections ; and it 
is at least not improbable that a substantially similar line of argument may be applied 
wdth equal force in the case of the remaining Ostariophj^seae which are of tropical or 
subtropical habitat. 
(3.) There is no clear evidence that the Ostariophysese are in any way different 
from other Fishes in being specially susceptible to atmospheric pressure variations, or 
that they possess any special capacity for anticipating impending changes in the 
weather. The only instance that we have been able to discover has reference to the 
European “ Sheat Fish ” [Silurus glanis), whicli is said to become greatly disturbed 
during the progress of thunderstorms (28a). But this isolated case certainly does not 
w’arrant the assumptioii that the air-bladder and Weberian mechanism are at all con- 
cerned in conveying the necessary stimulus to the nervous system of the Fish, inas- 
much as at least two alternatives present themselves : — (1) One suggested by 
Murray, namely, that the Fish is excited by electricity. This is extremely improb- 
able, and is rendered more so when we recall the fact that electrical currents do not 
readily pass from the atmosphere into water — especially fresh water. (2) The more 
probable alternative is that thunder, as is wmll known, causes considerable earth 
tremblings, which may be easily conceived as a possible source of uneasiness to the 
Fish. 
(4.) Even if the truth of Sagemehl’s theory be admitted, it still remains very 
difficult to imagine what advantage these Fishes could derive from their ability to 
appreciate variations in the pressure of the atmosphere and consequent impending 
changes in the w’eather, nor is it easy to conceive in what way the Fish can adjust its 
habits or its locomotor activities in accordance with varying meteorological conditions. 
It might possibly be advantageous for certain marine species of active habit to be 
able to anticipate a storm and retreat to deeper and calmer waters ; but even this 
advantage could only apply to very few of the Fishes in wdiich a Weberian mechanism 
is pj’esent, and would be altogether superfluous in the case of the fresh-water species 
which form by far the great majority of the four or five families concerned. 
If we regard Sagemehl’s barometrical theory as untenable, there remains only 
Hasse’s view that the Weberian mechanism constitutes a register of varying hydro- 
.slatic pressures, and this, in our opinion, is the only conclusion that has any foundation 
in fact. Seeing that the primary and almost universal function of the air-bladder, in 
spite of the occasional and secondary functions that may, in a relatively small number 
of Fishes, be grafted upon it, is that of acting as a hydrostatic organ, there is a strong 
