1892.] Contributions to the Anatomy of Fishes. 157 



anomalous abundance and diversity of fresh-water piscine life, as 

 compared with the remarkable poverty of all other groups of fresh- 

 water organisms. 



Concluding Remarks. 



The varied structural modifications met with in the air-bladder cf 

 the Siluridffl are not surprising, in view of the exceptionally diversi- 

 fied conditions under which the different species and genera live. 

 The physical conditions under which many Siluridas are capable of 

 living are almost as varied as their geographical distribution or 

 climatic range. 



Darwin has pointed out, in referring to the conditions favourable 

 to variation in 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 would be 

 exposed, as well as from differences in the nature and quality of their 

 living competitors in different regions. Further, he 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 varia- 

 tion since it first came into existence. It is clear that the Siluridae 

 furnish an admirable illustration of the truth of these remarks. 



Nor is it difficult to see how ifc is that the Weberian apparatus and 

 air-bladder are more specialised in the Siluridae than in other Ostario- 

 physese. The only rivals to the Siluridse in the extent of their 

 geographical distribution are the Cyprinidee, for the remaining 

 families have but a comparatively restricted range. But, extensive 

 as is their geographical distribution, the great majority of the 

 Cyprinidce 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 Siluridee, 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 specialisation, although in a few genera the effects of de- 

 generation are sufficiently obvious. 



