The Sandwich Islands are the only group among the smaller islands which are tenanted by a Siluroid, a species of Arius, which is closely allied to Central American species, and, therefore, probably immigrated from Tropical America.
II. Northern Zone.
The boundaries of the Northern Zone coincide in the main with the northern limit of the Equatorial Zone; but at three different points they overlap the latter, as has been already indicated. This happens in, and east of, Syria, where the mixed faunæ of the Jordan and the rivers of Mesopotamia demand the inclusion of this territory into the Northern Zone as well as the Equatorial; in the island of Formosa, where a Salmonoid and several Japanese Cyprinoids flourish; and in Central America, where a Lepidosteus, a Cyprinoid (Sclerognathus meridionalis), and an Amiurus (A. meridionalis) represent the North American fauna in the midst of a host of tropical forms.
A separate Arctic Zone does not exist for Freshwater fishes; ichthyic life becomes extinct towards the pole as soon as the fresh water remains frozen throughout the year, or thaws for a few weeks only; and the few fishes which extend into high latitudes, in which lakes are open for two or three months in the year, belong to types in no wise differing from those of the more temperate south. The highest latitude at which fishes have been obtained is 82° lat. N., whence the late Arctic Expedition brought back specimens of Charr (Salmo arcturus and Salmo naresii).
The ichthyological features of this zone are well marked: the Chondrosteous Ganoids or Sturgeons, and the families of Salmonidæ and Esocidæ are limited to, and characteristic of, it; Cyprinoids flourish with the Salmonoids, both families preponderating in numbers over the others, whilst the Siluroids are few in number and in variety.
The two regions in which this zone is divided are very closely related to one another, and their affinity is not unlike that which obtains between the sub-regions of the Southern Zone. The subjoined list will show their close agreement with regard to families as well as species. Several of the latter are common to both, viz.—Acipenser sturio, A. maculatus, Perca fluviatilis, Gastrosteus pungitius, Salmo salar, Esox lucius, Lota vulgaris, Petromyzon marinus, P. fluviatilis, and P. branchialis; and all recent investigations have resulted in giving additional evidence of the affinity, and not of the diversity of the two regions.
In Europe and temperate Asia, as well as in North America, mountain ranges elevated beyond the line of perpetual snow would seem to offer physical conditions favourable for the development of a distinct alpine fauna. But this is not the case, because the difference of climate between the mountain districts and the lowlands is much less in this zone than in the Equatorial. Consequently the alpine freshwater fishes do not essentially differ from those of the plains; they are principally Salmonoids; and in Asia, besides, mountain-barbels and Loaches. Salmo orientalis was found by Griffith to abound in the tributaries of the Bamean river at an altitude of about 11,000 feet.
| Europo-Asiatic. | N. American. | |||
| Acipenseridæ— | ||||
| Acipenser | 9 | species. | 12 | species. |
| Scaphirhynchus | 2 | „ | 1 | „ |
| Polyodon | 1 | „ | 1 | „ |
| Lepidosteidæ | 0 | „ | 3 | „ |
| Amiidæ | 0 | „ | 1 | „ |
| Percina [Cosmopol.] | 10 | „ | 30 | „ |
| Grystina [Australia, New Zealand] | 0 | „ | 2 | „ |
| Centrarchina | 0 | „ | 26 | „ |
| Aphredoderidæ | 0 | „ | 1 | „ |
| Cottidæ [partly marine]— | ||||
| Cottus | 3 | „ | 8 | „ |
| Ptyonotus | 0 | „ | 1 | „ |
| Gastrosteidæ | 5 | „ | 5 | „ |
| Comephoridæ | 1 | „ | 0 | „ |
| Gadidæ [marine]— | ||||
| Lota | 1 | „ | 1 | „ |
| Siluridæ— | ||||
| Silurina [India, Africa] | 5 | „ | 0 | „ |
| Bagrina | 2 | „ | 0 | „ |
| Amiurina | 1 | „ | 17 | „ |
| Salmonidæ | 90 | „ | 45 | „ |
| Percopsidæ | 0 | „ | 1 | „ |
| Esocidæ | 1 | „ | 7 | „ |
| Umbridæ | 1 | „ | 1 | „ |
| Cyprinodontidæ Carnivoræ [India, Africa, Neotrop.] | 9 | „ | 30 | „ |
| Heteropygii | 0 | „ | 2 | „ |
| Cyprinidæ— | ||||
| Catostomina | 1 | „ | 25 | „ |
| Cyprinina [India, Africa] | 80 | „ | 30 | „ |
| Leuciscina | 60 | „ | 70 | „ |
| Rhodeina | 10 | „ | 0 | „ |
| Abramidina [India, Africa] | 44 | „ | 10 | „ |
| Cobitidina [India] | 20 | „ | 0 | „ |
| Hyodontidæ | 0 | „ | 1 | „ |
| Petromyzontidæ [Southern Zone] | 4 | „ | 8 | „ |
| 360 | species. | 339 | species. | |
A. The Europo-Asiatic (Palæarctic) Region.—Its western and southern boundaries coincide with those of the Northern Zone, so that only those which divide it from North America have to be indicated. Behring’s Strait and the Kamtschatka Sea have been conventionally taken as the boundary, but this is shown to be artificial by the fact that the animals of both coasts, as far as they are known at present, are not sufficiently distinct to be referred to two distinct regions. As to the freshwater fishes those of North-western America and of Kamtschatka are but imperfectly known, but there can be little doubt that the same agreement exists between them as is the case with other classes of animals. The Japanese islands exhibit a decided Palæarctic fish-fauna, which includes Barbus and Cobitioids, forms strange to the North American fauna. A slight influx of tropical forms is perceived in the south of Japan, where two Bagrina (Pseudobagrus aurantiacus and Liocassis longirostris) have established themselves for a considerable period, for both are peculiar to the island, and have not been found elsewhere.
In the east, as well as in the west, the distinction between the Europo-Asiatic and North American regions disappears almost entirely the farther we advance towards the north. Of four species of the genus Salmo known from Iceland, one (S. salar) is common to both regions, two are European (S. fario and S. alpinus), and one is a peculiarly Icelandic race (S. nivalis). As far as we know the Salmonoids of Greenland and Baffin’s Land they are all most closely allied to European species, though they may be distinguished as local races.