252 Dr. T. Gill on the Geographical 
The Gadoids, for example, are replaced by Notothenioids, the 
Lycodoids by peculiar genera, the Cottoids by Harpagiferoids, 
&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 types. 
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 & prior? be supposed. While, for ex- 
ample, the inhabitants of the opposite sides of converging 
continents are to a great 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—Percide (Htheostomine), 
Centrarchide, Aphredoderide, Amblyopside, Percopside, 
Hyodontide, Amide, and Lepidosteide. 
2. Peculiar to Tropical Asia—Platypteride, Helostomide, 
Osphromenide, Nandide, Luciocephalide, Ophiocephalide, 
Notopteride, Salangide, Homalopteride, and Sisoride. 
3. Peculiarto Africa—Knertide, Mormyride,Gymnarchide, 
and Polypteridee. 
4, Peculiar to tropical America—Centropomide, Polycen- 
tride, Sternopygide, Electrophoride, Hypophthalmide, Tri- 
chomycteridee, Callichthyide, Argiide, Loricariide, and As- 
predinidee. 
5. Peculiar to Australia—Gadopside, Ceratodontide. 
6. Peculiar and common to the cistropical hemisphere (that 
is, Northern America, Europe, and Northern Asia) —Gadide 
(Lotine), Cottide (Uranidee), Percidee (Percine), Gasteros- 
teide (Gasterosteine), Esocide, Umbride, Catastomide 
(America and Eastern Asia), Salmonide, Acipenseride, and 
Polyodontide (America and Eastern Asia). 
7. Peculiar and common to Europe and Asia—Cobitide. 
* 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 Cichlide send representa- 
tives into the regions of Asia near Egypt, and the Lepidosteidz have a 
representative as far southward as Panama. In the latter case, indeed, 
the question might even arise whether the Lepidosteide might not rather 
be immigrants into North America than the reverse; but a recourse to 
palzontology solves the question. 
