Amphibian

“ The tadpole, on the contrary, goes on swimming about freely, and keeping the use of its eyes, till at last a pair of hind legs and then a pair of fore legs begin to bud out from its side, and its tail fades away, and its gills disappear, and air-breathing lungs take their place, and it boldly hops on shore a fully evolved tailless amphibian. ”

“ The successive ascendancies of the Worm, the Mollusk, the Crustacean, the Fish, the Reptile, and the Mammal, are followed triumphantly by the ascendancy of the Children of the Ape.
A large part of the life of the earth has remained steadfastly where it was cradled, beneath the waves. But more restless portions have left the sea and crept forth upon the land, or swarmed into the air. One migration, the most numerous, is represented by the insects. Another, the most enterprising, was the amphibian. After ages of evolution the amphibian branch divided. ”

“ To the uninformed it may seem that no very great advance is made when the reptile is evolved from the Amphibian. In reality the change implies a profound modification of the frame and life of the vertebrate. Partly, we may suppose, on account of the purification of the air, partly on account of the decrease in water surface, the gills are now entirely discarded. ”

“ As life advances, and the young amphibian ranges the waters, the terror of his insect contemporaries, not only are the nutritious particles supplied by its prey, by the addition of which to its frame, growth takes place, laid down, each in its proper spot, and in such due proportion to the rest, as to reproduce the form, the colour, and the size, characteristic of the parental stock; but even the wonderful powers of reproducing lost parts possessed by these animals are controlled by the same governing tendency. ”

“ The amphibians were the earliest developed four-legged animals and are represented by such living forms as the toads, frogs, and salamanders. Amphibians are cold-blooded animals that primarily breathe by lungs and spend most of their life on land, but during their early stages of development they live in the water where they breathe by means of gills. ”

“ The fish-form governs that of the amphibian and of the reptile. The fins become limbs, which are variously modified. In some amphibians, as in the frog, the limbs become different in functions, the hind-limbs being adapted to a leaping motion, the fore-limbs to support, A differentiation of another kind takes place, eventually, in the reptile stock, the fore-limbs becoming organs of aerial support. They become wings, and we obtain the bird type, with fore-limbs adapted to aërial, hind-limbs to solid support. ”

“ The broad paddle of the fin is now useless, and its main stem is converted into a jointed, bony limb, with a five-toed foot, spreading into a paddle, at the end. But the legs are still feeble, sprawling supports, letting the heavy body down almost to the ground. The Amphibian is an imperfect, but necessary, stage in evolution. ”

“ We call such creatures amphibious, which means that they are able to live with equal ease on the land and in the water. The first frog who crosses your path can tell you all about the pleasures of the double existence of the amphibian. ”

“ Let us now carry our imagination to the two kinds of existence which enter into the life of the adult amphibian. The frog has a certain set of experiences which arise out of its life in the water and another set of experiences which arise out of its life on dry land. ”

“ The reptiles differ from the Amphibians through acceleration of development, passing through the gill-bearing stages within the egg. The birds bear feathers instead of scales, and the mammals nourish their young by means of glandular secretions. Through a reptile-amphibian ancestry the birds and mammals may trace back their descent from palæozoic Crossopterygians. In the very young embryo of all higher vertebrates traces of double-breathing persist in all species, in the form of rudimentary gill-slits. ”

“ The reptiles have in fact completed the conquest of the land which was undertaken by the previous group, and many of them, living as they do in dry and hot deserts, are as independent of the water as any form of animal life. Thus whereas the amphibian has a thin skin, which is kept moist by the secretions of numerous skin glands, the reptile has a body-covering of scales, which form an effective protection against a too rapid loss of moisture. ”

“ From the above analysis of ecological distribution we may see that the rainforest provides a variety of habitats for amphibians and that these habitats are suitable for amphibian life throughout the year. ”

“ Crouched at the opening of a hole, into which they have crept back, they sleep with the mouth open, the upper jaw perpendicularly erect, so as to lie in wait for their prey. To these amphibians it is but sport to launch themselves in its pursuit, either by swimming through the waters propelled by their tails or running along the bank with a speed no man can equal. ”

“ But however curious the phenomena attending the development of the tadpoles of the Amphibian Reptiles may be to the observer who merely watches the changes perceptible from day to day in their external form, they acquire tenfold interest to the physiologist who traces the progressive evolution of their internal viscera ”

“ You will remember that the amphibians are distinguished from the true reptiles by having to pass through a tadpole stage before they obtain their perfect form. A good example is the frog, which in one kind or another exists in all parts of the earth except the very coldest. No doubt, you have often seen great masses of its jelly-like spawn floating on the surface of ponds early in the spring; and you must have wondered how such small creatures as frogs could possibly lay such enormous batches of eggs. ”

“ They are oviparous, hatched by the heat of the sun from eggs, covered with a soft, glutinous membrane, which the mother had laid in the water, and develop through tadpole stages. In the early period of their existence they are fishlike in their structure, breathing by means of gills and a two-chambered heart; in the later stages of their development they acquire lungs and a heart of three chambers. A true amphibian possesses at once both lungs and gills. ”

“ FISHES still held to the Devonian types, with the exception that the strange ostracoderms now had perished.
AMPHIBIANS. The vertebrates had now followed the arthropods and the mollusks upon the land, and had evolved a higher type adapted to the new environment. Amphibians—the class to which frogs and salamanders belong—now appear, with lungs for breathing air and with limbs for locomotion on the land. Most of the Carboniferous amphibians were shaped like the salamander, with weak limbs adapted more for crawling than for carrying the body well above the ground. ”

“ The possession of four limbs with five fingers or toes on each, and a host of identical arrangements of bones, muscles, nerves, &c., in these limbs, reveals the fact that all the four-limbed creatures or Tetrapoda represent one group which developed from some fish-like ancestor to become an amphibian. The discovery of fossilized remains of extinct animals reveals that the fishes are much older than the amphibians, and that a number of intermediate stages demonstrate the process of gradual transformation which converted one group of fishes into four-limbed, semi-terrestrial amphibians. ”

“ In many Amphibia certainly, as [85] well as in many higher vertebrates, we find less than five digits. But in all these cases it can be shown that separate digits have retrograded, and have finally been completely lost. The causes which affected the development of the five-fingered foot of the higher vertebrates in this amphibian form from the many-fingered foot (or properly fin) , must certainly be found in the adaptation to the totally altered functions which the limbs had to discharge during the transition from an exclusively aquatic life to one which was partially terrestrial. ”

“ Subaquatic or terrestrial vertebrates with a typically amphibian development of the ventral armature; ribs intercentral, as in all members of the order; skull elongate and slender or broad and obtuse; cranial and dermal elements of the pectoral girdle sculptured; lateral-line canals indicated in one genus, Saurerpeton; limbs well developed, with well-developed digits and ungual phalanges claw-like; body usually slender, broad in Saurerpeton; the ribs broad and heavy; the vertebræ relatively stout; ventral armature highly developed, reaching the height of specialization among the Microsauria ”

“ Aquatic.—This habitat includes permanent streams and rivers (Pl. 10, fig. 1) , some of which are clear and others muddy. In the rainy season temporary ponds form in depressions on the forest floor (Pl. 10, fig. 2) ; these are important as breeding sites for many species of amphibians. Aquatic members of the herpetofauna are here considered to be those species that either spend the greatest part of their lives in the water or usually retreat to water for shelter. Seven species of turtles and one crocodilian are [215] aquatic. ”

“ In the Linton cannel are buried fragments or entire individuals of all the inhabitants of this body of water which had hard parts, bones, scales, spines, or teeth, capable of preservation. Hence we get a locally complete picture of the life of the Carboniferous age, and we find it to be unexpectedly rich and varied. In that age fishes and amphibians were the highest forms of animal life, and the amphibians were comparatively newcomers on the earth's surface. ”

“ We are positive that Amphibians descended from fishes, probably of the crossopterygian kind, but the link still eludes us; we have brought the reptiles within close reach of the amphibians, but the direct link is still to be found; mammals are in close proximity to a certain order of reptiles, but the connecting form is still undiscovered; man himself is not far from the various types of anthropoid apes, but his actual connecting relationship is unknown. ”

“ Four-footed vertebrates resembling salamanders were prominent animals of the Carboniferous swamps. At first adapted to a life in water and later to land conditions, they are known as amphibians, the name being based on the ability to live in two different kinds of environment. Common living representatives of this group are the toads and 43 frogs, but these tailless forms are not known among fossils of the Paleozoic era and are almost unknown throughout the Mesozoic. The Age of Amphibians, as we apply that phrase, was definitely not an age of toads and frogs. ”

“ And this condition, in which a change in the mode of life is made by each individual in the course of its development, is the typical one. But the modern amphibians include types ranging from completely water to perfect land forms. Some, like the Austrian Olm (Fig. 86) , are gill breathers throughout their whole life. One which is normally of this type, the Axolotyl, illustrated in Fig. 87, can be made to acquire lungs and assume a land mode of life. ”

“ In early life they are fishes, with gills and two-chambered hearts. In later life they are air-breathing quadrupeds, with legs and lungs and three-chambered hearts. Here is evolution, plenty of it, and of the most tangible character. And it takes place right before the eyes. The transformation from the fish to the frog is, however, no more wonderful than the embryonic transformations of other vertebrates. It is simply more apparent, because it can be seen. The lungs of amphibians and the lower reptiles are simple sacks opening by a very short passage into the mouth. ”

“ Geological evidence also proves that the reptiles came definitely later than the amphibians, and that only one small group of very primitive amphibians shared in the progressive modifications of brain, limbs, and organs of circulation, &c., to become reptiles—creatures able to live wholly on the dry land, and capable of a wider range of activities than the Amphibia. ”

“ Henceforth, then, in trying to find out the sequence of evolution, we must leave the land and examine the nature of the animals living in the sea; the air-breathing animals which lived on the land in the Upper Silurian and Devonian times cannot have reached a stage of organization comparable with that of the fishes, seeing how easily the amphibians became dominant. ”

“ The mandible is rather heavy and is coarsely sculptured with radiating grooves and ridges. The character of the teeth can not be determined, save to say that they were present. The posterior end of the mandible projects somewhat beyond the quadrate angle of the skull.
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The interest in the present specimen is heightened by the light it throws on the characters for the separation of the Amphibia and Reptilia. The wide separation of the pterygoids by the parasphenoid is an amphibian character of undoubted value. The reduction of the parasphenoid in this specimen is noteworthy. ”

“ It is an interesting fact that for millions of years there was upon the earth no sound of life at all, only the noise of wind and wave, thunder and avalanche. Apart from the instrumental music of some insects, perhaps beginning in the Carboniferous, the first vital sounds were due to Amphibians, and theirs certainly was the first voice—surely one of the great steps in organic evolution. ”

“ From fossil forms we cannot say much in regard to soft parts; but if we consider the living representatives of the class, we may credit amphibians with such important acquisitions as fingers and toes, a three-chambered heart, true ventral lungs, a drum to the ear, a mobile tongue, and vocal cords. ”

“ We Amphibians—our Skins are not like yours. We do not wear them for the same reason you do. You are imprisoned by your Skins—they tell you how to feel, what to think. Above all, they keep you from getting ideas about non-cooperation or non-integration with Nature as a whole. ”