ON THE ANATOMY OF FISHES. 
315 
animals and plants, that it is common, widely-diffused and widely-ranging species 
that vary most, and that this might be expected from the diverse physical conditions 
to which they are exposed, as well as from differences in the nature and quality of 
their living competitors in different regions. Elsewhere* he likewise alludes to the 
fact that an organ developed in an extraordinary manner implies that it is of high 
functional importance to the species, and that it may also be concluded that the 
organ has undergone a great amount of variation since it first came into existence. 
It is clear that the Siluridm furnish an admirable illustration of the truth of these 
remarks. Their extensive geographical distribution and highly varied habitat, their 
richness alike in individuals and species, and, in short, their dominant position in 
fresh waters over wide areas, are quite sufficient to account for almost infinite variation. 
On the other hand, in the air-bladder and Weberian mechanism combined we have 
an organ, which, from its highly specialized and remarkable development, must be of 
great functional importance to these Fishes. Hence, the extraordinary variations 
which this organ and its correlated skeletal structures present need occasion no 
surprise when it is obvious that we have in the character of the organ itself, and in 
the external environment of the family in which it is most highly specialized, all the 
conditions necessary for the action of natural selection in producing rapid and 
extensive modification, either in the progressive or retrogressive direction. Under 
such, conditions adequate scope would be afforded for the operation of natural 
selection in perfecting the useful organ and eliminating by atrophy and degeneration 
the useless one. Nor is it difficult to see how it is that the Weberian apparatus and 
air-bladder are more specialized in the Silurida3 than in other Ostariophysese. The 
only rivals to the Siluridae in the extent of their geographical distribution are the 
Cyprinidse, for the remaining families have but a comparatively restricted range. 
But extensive as is .their geographical distribution, the great majority of the 
Cyprinidae appear to exist under fairly uniform conditions, or, at all events, exhibit 
nothing like the diversity of habitat and habits that is so characteristic of the 
Siluridae, and hence it is that so far as the structure of the air-bladder and Weberian 
mechanism is known in the former family, it presents but little variation in the 
direction of increased specialization, although in a few genera the effects of degeneration 
are sufiiciently obvious. 
VII. — Systematic Index. 
In this Index we have adhered to Dr. Gunther’s classiBcation and order of enume- 
ration, as set forth in his “ Catalogue of the Fishes in the British Museum,” vol. 5, 
1864, p. xi. Species referred to by us which are not in that Catalogue are added at 
the end of the list of species belonging to that particular genus. The pages where 
any species is described in detail are indicated by the use of a heavier type. 
* Ibid., p. 119. 
