We have now come to a class of vertebrates that in their manner of life, and presumably in their history, connect the dwellers in the waters with those on the lands of the globe. Dr. Gamble cites examples from various groups of animals to show that adaptation to a terrestrial existence is an advance on that requisite for aquatic life, and that the critical point in the evolution of the vertebrate phylum was passed when its members migrated from water to land. "When we come to land animals," he says, "the problem of weight has to be considered before that of locomotion. The lateral undulations of the body, even when aided by unjointed paddles, or fins, are not sufficient to insure rapid movement on land. Hence a system of levers has to be evolved, partly to support the body, and partly to propel it. The use of joints becomes a necessity, and we find that all active terrestrial animals, except snakes, have jointed limbs. The critical point in the history of this phylum is passed when its members migrated from water to the land. The step was taken by the ancestors of the Amphibia (that is, the frogs, toads, and salamanders). In them the breast fins of the fish have become the jointed forelegs, the pelvic fins have become the hind legs."

How this great change from the fish fin to the five-fingered hand occurred is, at present, just as obscure as the mode of conversion of the arms of reptiles into the wings of birds. The answer can only be supplied by further discoveries in the geological history of the order, and though this history can be traced back to the time of the Coal Measures, we find the earliest Amphibia as sharply marked off from the fishes by their feet as they are to-day. These forefathers (subclass Stegocephalia) are the earliest known four-footed animals, and their fossil skeletons are found from the Carboniferous up to the Trias, after which the race disappears. They had the general form of newts, and many were only a few inches in length. That some of these, at least, were terrestrial in habit is shown by the fact that they are often found in stumps and hollow logs of sigillarias and other fossil trees of the coal beds, especially in Nova Scotia. But there were also species several feet in length, with formidable teeth, which were no doubt carnivorous and predatory, so that it was well for the little ones to seek places of safety. These stegocephalians were unmistakably amphibians, with two condyles supporting the skull, but their skeleton contains many features that suggest reptilian anatomy, and it is agreed that the reptiles sprang from this stock. The peculiar feature of this group is that their flattish heads were covered by a broad shield of bony plates (ossified skin); and similar armor protected their bellies, and in a few cases the back also.

Geological formations furnish no ancestral connection between the Stegocephalia and modern salamanders; but the limbless, wormlike, burrowing and blind cÆcilians of the tropics exhibit certain stegocephalian characteristics, especially a scaly skin, which put them into a division (Apoda) by themselves. The remainder of the class, that is, Amphibia (also called Batrachia) in general, have a soft, moist, naked skin, and are naturally divisible into two orders:

Modern amphibians in general are animals fitted for life both on land and in water. All are born from eggs hatched in water, and the young, at first in a larval form unlike the adult condition, have external gills adapted to breathing in that element; but in most cases they lose their gills, and as adults acquire lungs for breathing air. This metamorphosis of the young, comparable to that of the nymph-producing insects, is the especial characteristic of the class. The skeleton is of the vertebrate model, but in the Urodela is largely cartilaginous. The skin is smooth, soft, moist, and covered only with a filmy coat of horny texture that is molted from time to time as the animal needs room to grow. The skin abounds in sense organs about the head and along the sides of the body—an inheritance from the lateral line of fishes—which are most active in the larvÆ, and disappear altogether with age in most frogs and toads, although they revive in salamanders in the breeding season.

The skin also contains many mucus glands and other larger glands, especially on the back. These emit under provocation a poisonous liquid that is fatal to small animals, and very irritating to the eyes, nose, and throat of larger ones. Most, if not all, Amphibia, says Dr. Gadow, are more or less poisonous, and it is significant that many of the most poisonous exhibit a very conspicuous yellow or orange upon a dark ground, which is so widespread a sign of poison. There is no venom in their bite—in fact, their teeth are too small, although numerous, to let anyone fear their biting. The skin is heavily laden with pigment, and this is displayed in many amphibians in striking patterns of bright coloring. Certain groups possess in a high degree the power of altering their colors to conform to their surroundings.

An interesting feature of the amphibians is that power of repairing mutilations of the body and replacing lost parts which is so well known in worms, hydroids, and other lowly creatures, and is termed "regeneration." This ability is most active in young specimens. Tadpoles frequently have their tails bitten off, whereupon new ones grow quickly. Salamanders fight bitterly, tearing off each other's gills and limbs, and turtles and fishes frequently bite off their tails. New tails are generated speedily, and usually in good and effective form, although they contain no regenerated caudal vertebrÆ, but only a rod of cartilage. The ability to rebuild lost parts is much less among the frogs.

Another notable fact is that here for the first time we meet with a voice organ, and a real voice expressing emotions, although in an extremely limited way. This is most noticeable in the tree frogs, which are the most advanced of the Amphibia in organization.

NEWTS AND SALAMANDERS

The Urodela are represented throughout the whole northern hemisphere except in desert regions, as far in North America as southern Canada, and also southward to Panama; and in the Old World, northward to the line of very cold winters and southward to the Mediterranean and Indo-China. In the main, however, our genera are different from those of Europe and Asia.

The largest and best known of American urodeles is a member of the family ProteidÆ and genus Necturus, and is widely known as "water dog" or "mud puppy," because of the doglike shape of its head. It is a brown, robust creature, sometimes two feet long, with bushy gills, retained throughout its life, springing from open gill clefts in three bright red tufts on each side of the head. It inhabits cold, rapid streams, hiding under stones by day, and moving about at night in search of crawfish, worms, insect larvÆ, frogs, etc., and dodging hungry snapping turtles. But little smaller, and even more ugly in appearance, is the "hellbender," representing the family AmphiumidÆ. These blackish creatures are to be found in mountainous regions, and hide during the day under loose rocks. By the time they are about three years old their gills have been absorbed, and their lungs are in service, so that they are compelled to rise to the surface occasionally for drafts of air. They hunt at night for food, preferring crawfish and, fishermen say, fish eggs. The breeding habits of this animal have only lately become known, and Mr. B. G. Smith, who has made a special investigation of them, says that the breeding season begins (in Pennsylvania) in August, when hellbenders of both sexes come out more freely from their rock shelters and roam about, frequently in small companies. The small number of eggs produced are hidden in a pocket under a loose stone; and the young, which are more like tadpoles than the form of their parents, breathe by gills which do not completely disappear until the animals have reached nearly their maturity.

Otherwise our salamanders are small species found in brooks, ponds, and wet woods, and often getting into cellars and wells. Uninformed persons think them to be lizards, and foolishly fear them, but except for the irritation of the hands that may follow rough handling they are utterly harmless to man or his property, and serve him by devouring great quantities of insects and worms.

A common species in damp, neglected woodlands is the little red-backed fellow that is so light and leaping in its movements when disturbed, even throwing off its tail in its panic of fear. It is more terrestrial than most, laying its few eggs in rotting wood instead of going into the water for that purpose; and the young carry gills but a few days. This red-backed Plethodon must not be confused with the small newts, bright vermilion with a row of glowing spots along the sides, that are found in woods in summer. They are young specimens of Diemyctylus viridescens, which is common all over the eastern part of the United States and southern Ontario. The parents are green, and wholly aquatic in habits. The larvÆ have gills and swim about until early autumn, by which time their gills have been gradually absorbed, and they go ashore, where their coats change in color from a mottled green to scarlet. This red condition and their residence on land continue until the autumn of the third, or the spring of the fourth year of their lives, when they become sexually mature, resume a greenish dress, go back to the water, and pass the rest of their lives there.

Mention can be made of only one more species—the black, yellow-spotted "tiger triton," which is the most widely spread and often seen of our terrestrial salamanders. It is especially noteworthy because of the extraordinary condition of suspended development exhibited by its larva, the famous edible axolotl of Mexican lakes, which, while still retaining larval gills and aquatic habits, grows nearly or quite to the size of its parents—three to four inches—and becomes capable of breeding. Similar cases are known in certain lakes in southern Europe; and it appears that this arrested development, together with natural growth of body, occurs occasionally in many other amphibians. The condition is termed "neotony," but the biological explanation of it is not clear.