The most numerous examples of this class, are those birds of the temperate regions which in winter migrate, either wholly or partially, into adjacent warmer countries. This migration most likely began subsequent to the Miocene period, during that gradual refrigeration of the temperate zones which culminated in the glacial epoch, and which still continues in a mitigated form. Most of the genera, and many even of the species of birds which migrate southwards in winter, have therefore, most likely, always been inhabitants of our present Palæarctic and Nearctic regions; permanent residents during warm epochs, but only able now to maintain their existence by migration in winter. Such groups belong truly to the temperate zones, and the test of this is the fact of their not having any, or very few, representatives, which are permanent residents in the adjacent tropical regions. When there are such representative species, we do not claim them as peculiar to the Northern regions. Bearing in mind these various considerations, it will be found that we have been very moderate in our estimate of the number of genera {186}that may fairly be considered as exclusively or characteristically Palæarctic. _Reptiles and Amphibia._--The Palæarctic region possesses, in proportion to its limited reptilian fauna, a full proportion of peculiar types. We have for instance two genera of snakes, _Rhinechis_ and _Halys_; seven of lizards, _Trigonophis_, _Psammodromus_, _Hyalosaurus_, _Scincus_, _Ophiomorus_, _Megalochilus_, and _Phrynocephalus_; eight of tailed batrachians, _Proteus_, _Salamandra_, _Seiranota_, _Chioglossa_, _Hynobius_, _Onychodactylus_, _Geotriton_, and _Sieboldia_; and eight of tail-less batrachians, _Bombinator_, _Pelobates_, _Didocus_, _Alytes_, _Pelodytes_, _Discoglossus_, _Laprissa_, and _Latonia_. The distribution of these and other Palæarctic genera will be found in our second vol. chap. xix. _Freshwater Fish._--About twenty genera of freshwater fishes are wholly confined to this region, and constitute a feature which ought not to be overlooked in estimating its claim to the rank of a separate primary division of the earth. They belong to the following families:--Percidæ (three genera), _Acerina_, _Percarina_, _Aspro_; Comephoridæ (one genus), _Comephorus_, found only in Lake Baikal; Salmonidæ (three genera), _Brachymystax_, _Luciotrutta_, and _Plecoglossus_; Cyprinodontidæ (one genus), _Tellia_, found only in Alpine pools on the Atlas Mountains; Cyprinidæ (thirteen genera), _Cyprinus_, _Carassus_, _Paraphoxinus_, _Tinca_, _Achilognathus_, _Rhodeus_, _Chondrostoma_, _Pseudoperilampus_, _Ochetebius_, _Aspius_, _Alburnus_, _Misgurnus_, and _Nemachilus_.