252 Dr. T. Gill on the Oeographiccd 



Tlie Gadoids, for example, are replaced by Notothcnioids, the 

 Lycodoids by peculiar i^enera, the Cottoids by Ilaqjaj^iferoids, 

 &c. In the contrast between these antarctic and the arctic 

 forms we have evidence of the absence of any paramount 

 causal relation between temperature and structure ; and, in 

 addition to the " tropicopolitan " types, each great tropical 

 region has a number of characteristic and peculiar t}'pes. 



But the distribution of the inhabitants of the great open seas 

 and of those of the inland waters are determined by different 

 conditions, as might a priori be supposed. While, for ex- 

 ample, the inhabitants of the opposite sides of converging 

 continents are to a gi'cat extent similar, the freshwater species 

 of those continents are mostly quite dissimilar, and become 

 more and more so as we progress southward. 



There are numerous families of fishes which are represented 

 in the fresh waters — some exclusively so, others with marine 

 species. The geographical limitations and relations in space 

 of these families may be exhibited under combinations in 

 several categories*, viz. : — 



1. Peculiar to North America — Percidaj {Etheostomince) , 

 Centrarchidse, Aphredoderidte, Amblyopsidw, Percopsidse, 

 Hyodontidae, Amiidte, and Lepidosteidfe. 



2. Peculiar to Tropical Asia — Plat^^terida?, Helostomidaj, 

 Osphromenidfe, Nandids, Luciocephalidaj, Ophiocephalidae, 

 Notopterida3, Salangidffi, Homalopterida?, and Sisorid^e. 



3. Peculiarto Africa — Kneriidje, Mormyrida, Gymnarchid^e, 

 and Polypteridffi. 



4. Peculiar to tropical America — Centropomidfe, Polycen- 

 tridje, Steniopygidfe, Electrophoridse, Hypophthalmidge, Tri- 

 chomycterida}, Callichthyidaj, Argiidae, Loricariidse, and As- 

 predinida?. 



5. Peculiar to Australia — Gadopsidte, Ceratodontidai. 



6. Peculiar and common to the cistropical hemisphere (that 

 is, Northern America, Europe, and Northern Asia) — Gadidse 

 {Lotince), Cottidge {Uraniflea>), Percidfe {Percince), Gasteros- 

 teidse [Gasterosteinoi) ^ Esocidffi, Umbrid^e, Catastomidae 

 (America and Eastern Asia), Salmonida?, Acipenseridae, and 

 Polyodontidae (America and Eastern Asia). 



7. Peculiar and common to Europe and Asia — Cobitidae. 



* As might naturally be supposed, the forms assigned to the categories 

 enumerated are not always rigidly limited to the specific regions when 

 contiguous regions are contiguous : thus the Cichlidfe send representa- 

 tives into the regions of Asia near Egj-pt, and the Lepidosteidse have a 

 representative as far southward as Panama. In the latter case, indeed, 

 the question might even arise whether the Lepidosteidse might not rather 

 be immigrants into North America than the reverse ; but a recourse to 

 palaeontology solves the question. 



