The class Aves, like the class Mammalia, consists of animals with hot blood, but all birds have feathers and a number of other peculiarities of structure, as will appear later.
The next class to be adverted to is the class which includes all reptiles properly so-called—the class Reptilia.
The reptiles which exist in the world to-day may be classed in four well-marked sets, each of which has the value of an "order"—(1) crocodiles, (2) lizards, (3) serpents, and (4) tortoises. The names of these creatures alone suffice to indicate the fact that the class of reptiles presents us with an extraordinary amount of diversity of form as compared with the class of birds with which, nevertheless, reptiles have, as we shall hereafter see, very close relations. Indeed, in the diversity of kinds which it contains, the class Reptilia at the least fully equals the class Mammalia, especially if the extinct kinds are taken into consideration. The number of species of reptiles, both living and extinct, much exceeds also the number of living and extinct mammals.
To begin once more with forms which are the least strange and unknown, we may start with the little elegant and harmless lizards of our heaths and commons, which will serve as types of the order to which they belong—the order Lacertilia. That order is an extremely numerous one, containing many families, differing much in form. Our English lizards are true lizards, belonging to the typical genus Lacerta and to the typical family Lacertidæ. The rather well-known large American lizard, Iguana, is the type of another and very extensive family (almost entirely confined to America), while a nearly-allied family (Agamidæ) is an Old World group. Amongst the curious forms found in the latter family may be mentioned the frilled and moloch lizards of Australia, and those little harmless lizards of India which go by the formidable name of "flying dragons" (Draco). They are the only existing aërial reptiles—not that they can truly "fly" at all, but they are enabled to take prolonged jumps, and to sustain themselves to a considerable extent in the air by means of the extremely distensible skin of their flanks which, when extended, is supported by a peculiar solid framework hereafter to be described. Some of the largest lizards are called "monitors," and are common in Egypt; they belong to the family Monitoridæ.
In the warmest period of the year, certain lizards are found in the South of Europe, called geckos. They have a power of running, not only up walls, but across ceilings by means of a peculiar structure of their toes. They are types of a large family (Geckotidæ) widely spread over the world.
Another large family (Scincidæ) has also its type in the South of Europe in the skink (Scincus), which was formerly supposed to possess much medicinal value. This large family contains a number of species which exhibit a series of gradations in structure leading to forms which have the external aspect of serpents. One such form is the perfectly harmless slow-worm, or blind-worm, of our own country, which in spite of its scientific name, Anguis fragilis[14], is a legless lizard, and no snake.
Other lizards of a very different kind forming the family Amphisbæidæ are also legless, with the single exception of the genus Chirotes, which has a pair of anterior limbs, but no posterior ones. The name of this family is derived from the similarity of appearance presented by both ends of the body, so that either end looks as if ready to take the lead as "head."
A family of lizards familiar by name to us all from our childhood is the family of chameleons (Chameleonidæ). There are many species of chameleons, but they are found in the Old World only; they are among the most exceptional and peculiar of all lizards, but there is one form which is yet more so.
This most exceptional of lizards is one found in New Zealand, and named Sphenodon. Its external aspect would not lead the ordinary observer at all to suspect that it is so remarkable a creature as its anatomy shows it really to be.
The order Crocodilia contains, of course, the true crocodiles which are found both in the Old and New Worlds. It contains besides the alligators (which are peculiar to America), as well as the long and slender-snouted gavials which are now found only in India and Australia. At one time the number of kinds of this order was very much greater than at present, and interesting structural modifications have taken place in it during the course of ages, as will be pointed out later.