About 5000 species of birds have been classified. By Cuvier's system they are divided into six orders:-- 1. _Raptores_, or birds of prey. 2. _Passerine birds_, now generally called _Insessores_, or Perching-birds. 3. _Scansores_, or Climbing, frequently called _Zygodactyli_ or _Zygodactylous_ birds. 4. _Gallinaceæ_, now more frequently known as _Rasores_. 5. _Grallatores_, Waders, or Stilt birds. 6. _Palmipedes_, or Web-footed birds, now more generally recognised as _Natatores_, or Swimmers. It has been proposed to separate the _Brevipennes_, or short-winged birds, from the Grallatores, and erect them into a separate order. * * * * * The _Reptiles_, of which the majority possess the faculty of living upon land and in water,--whence their name of Amphibia,--never pass beyond the limits of warm and temperate climates: their blood, which has the same temperature as the medium wherein they live--whence their name of "cold-blooded animals"--does not circulate where the mean annual temperature descends below freezing-point. Yet frogs and salamanders have been met with in Greenland, and on the banks of the Mackenzie River, in North America, under 67° latitude. * * * * * Linnæus was not acquainted with more than 215 species of _Amphibia_, divided into four orders:--the Chelonians, or tortoises; the Saurians (as the lizard and crocodile); the Ophidians (serpents); and the Batrachians (frogs). In 1789, Lacépède raised the total to 303; in 1820, Merrem estimated it at 677. At present, the number of species of Reptilia classified and described amounts to 2000, and the four orders into which they are distributed are-- 1. _Ophidia_, or Serpents. 2. _Sauria_, or Lizards. 3. _Loricata_, or Crocodiles. 4. _Chelonia_, or Tortoises. According to Sching, there are 7 tortoises, 33 serpents, and 35 lizards. * * * * * _Fishes_ are the least known of those superior animals whose skeleton and vertebral column are situated in the interior of the body, and which are thence named _Vertebrata_. The richest collections, such as those of the British Museum, and those of the Museum of Natural History in Paris, which contain about 3000 species, do not represent probably more than a fourth of the existing total, including fresh-water and salt-water fish. How many rivers and streams in both hemispheres still remain to be explored! How far we are from a knowledge of the fishes which people the different strata of the great ocean. Agassiz divides this great class of vertebrated animals into the four orders of _Cycloid, Ctenoid, Placoid, and Ganoid_, according to the character of their scales. Cuvier, into _Osseous_ fishes (with true bones), and _Cartilaginous_; subdividing the former into _Acanthopterygii_ and _Malacopterygii_. * * * * *