Part 14

With the Common Frog, popularly classed as a Reptile, we commence acquaintance with the zoological class Batrachia, creatures that begin life at a much lower stage of development and have to pass through a fish-like larval form before attaining to any likeness to their parents. The Reptiles get through these developmental stages whilst they are still in the egg; they never have water-breathing organs. The Batrachians or Amphibians are clothed with soft skin which is not protected by armour plates or scales as seen in the Lizards and Snakes, but through which they are able to oxygenate the blood. The Frogs, Toads, and Newts constitute a class intermediate in structure and development between the Fishes and the Reptiles. Our native species represent the two orders--Ecaudata (tailless), including the Frogs and Toads; and Caudata (tailed) comprising the Newts.

Everybody knows the Frog as well as they know any of the backboned animals, and every youngster even is familiar with the main facts of its development, from the jelly masses of eggs in the pond early in spring, through the tadpole stage to the attainment of four legs and wonderful leaping powers. It is common knowledge that he has a moist, smooth skin (the supersensitive erroneously say "slimy") of yellowish ground colour overlaid with streaks and spots of brown. There is a big patch of brown behind each eye, and the long hind legs have cross-bars of the same colour. The ground tint of the Frog varies in different individuals according to the situation in which we may find them; for the pigment cells of the skin expand and contract under the influence of varying intensities of light reflected from the surroundings, causing colour changes much after the manner of those of the Chameleon, though less marked.

The Frog's forelimbs are very short compared with the hind pair, and the four moderate-sized fingers are not connected by webs; whereas, the hind limbs have their several bones lengthened, and the abnormal lengthening of those of the ankle gives the legs the appearance of having a supplementary joint. The leg is one and a half times the length of head and body. The foot has five long toes connected for half their length by a "web" of skin which constitutes a very efficient paddle when the Frog is in the water. Of these hind toes the fourth is considerably longer than the long third and fifth.

The Frog's head is as broad as it is long, the muzzle rounded, and the horizontal gape of the mouth extends back beyond the eye. The prominent eyes are perched up on the forehead, and have a fine golden iris and a horizontal pupil. The Frog differs from the Snakes and agrees with the Lizards in having eyelids; he has also, like the Birds, an additional lid--the nictitating membrane. There is a row of delicate teeth along the upper jaw, but none on the lower; there are others on the palate. The deeply notched tongue is attached by its base to the front part of the mouth, the tip far in towards the throat; in use it has to be suddenly turned over so that the tip is projected far beyond the muzzle. The large circular depression behind and below the eye is the drum of the Frog's ear.

The Frog has no neck, the base of his skull coming close to the collar-bones, and there are only a few pairs of very short apologies for ribs between the shoulders and the long pelvis which produces that steep incline at the rear of his back. He is clothed entirely with a smooth, soft skin, which is kept moist by the action of minute mucous glands distributed all over the body. A row of these glands of larger size forms a pale line running back from the eye on either side. The skin plays an important part in the oxygenation of the Frog's blood; and the experimental physiologists have shown that a Frog deprived of its lungs can carry on its respiration for a lengthened period through the skin alone. Owing to the absence of ribs he has to fill his lungs by swallowing air.

The male is less portly than the female, and he is further distinguished by having two pads on the first finger which in the breeding season become large rough cushions enabling him to hold his mate. In his throat there is a pair of vocal sacs enabling him to produce his love songs, and when these are in use their inflation causes a distension of the skin of the throat; but without these adjuncts the female manages to give answering croakings. When these duets are sung under water they produce some curious effects.

When the pairing season arrives--quite early, usually about the middle of March, but sometimes in February--all the Frogs that have just come out of hibernation select their mates. Any pool of water will do, however transient, and they often make mistakes in this matter, their egg-masses being left high and dry when the waters dry up. The eggs are deposited in a mass of a thousand to two thousand at the bottom of the water, and at first they are only about a tenth of an inch in diameter, but the gelatinous covering absorbs so much water that they swell up to a third of an inch. There is a corresponding lightening of the mass, which floats to the surface and is available for observation. Each of the little jelly-spheres is seen to have a black centre--the egg proper--with a white spot on the lower side. If the spring is an average one, in about four weeks' time the black specks will have developed into brown larvæ or tadpoles, and having escaped from the egg these will be clinging to the remains of the jelly mass by means of a pair of suckers on the underside of the head. There are at present no indications of limbs--head, body and tail, like those of a fish, merge one into another. Even the gills are not yet developed, though what we may term the buds of them are seen on the bars separating the slits behind the head on each side. These buds soon expand into gill-plumes through which the blood circulates, taking up oxygen from the water that passes between them. There is as yet no mouth, but this will soon open, and horny plates on its jaws will enable the tadpole to crop soft vegetable matter, upon which it subsists chiefly. Later on, the gill-plumes will be hidden by a flap which grows over them. The full series of stages in this development may easily be watched by keeping a few tadpoles in a glass of water with a little growing pond weed.

Ultimately, the limbs appear. Though all four develop simultaneously, the hind pair _appear_ first, because the forelimbs are at first hidden by the flap which grew over the gills. After the disappearance of the gill-plumes, proper lungs are developed inside the body, and the animal changes from a fish-like water-breather to an air-breather, in preparation for a life on land. When all the legs are well out the form of the tadpole soon changes to that of the Frog, except that it has a long tail. You may read in some books that the tail is shed, but this is a mistake that no one could make who has watched day by day the evolution of the Frog from the tadpole. The tail is _absorbed_; it gets smaller daily, until finally the hind body is rounded off and there is nothing left to indicate that it once ended in a tail. Ultimately the Frog may attain a length of head and body equal to four inches.

Mr. E. S. Goodrich, F.R.S., has recently demonstrated that eggs obtained from a female Frog by dissection can be fertilised by the leucocytes or colourless corpuscles of the blood. He exhibited a fatherless Frog, so obtained, before the Linnean Society in November, 1918.

When all the tadpoles have become real little Frogs, with their legs sufficiently firm to enable them to indulge in hopping exercises, they still for a time venture no further than the very shallow water at the extreme edge of the pond, where they can walk partially submerged. Then one day there comes a heavy summer rain storm--a deluge on a small scale. Every little Frog then appears to hear the word "Go!" for with one impulse they all scramble out of the pond into the jungle of wet grass, they know not whither. If there is a road near, that is the place in which to form an idea of their prodigious numbers. The few wayfarers who may be hurrying along that road, looking for possible shelter from the pitiless rain, and seeing the Frogs hopping along much as the raindrops bounce, are quite prepared to declare that they came down from the clouds with the rain. Many persons who in the ordinary affairs of life would be regarded as reliable witnesses have testified that this is what happens. To them it seems a much more reasonable explanation of this sudden appearance--they term it a phenomenon--than the naturalist's statement that the Frogs had been waiting in the pond for the psychical moment to arrive for their dispersion--the time when the reeking herbage of many acres around would offer the safest conditions for their tender bodies to embark on the great adventure of life, their distribution over wide areas where they could carry out their proper function, the control of any inordinate increase in the insect population. For months they will crawl and hop invisibly among the lush grass and journey through the dense herbage of hedgebottoms and spinneys. Some will come under fences even into our gardens, to help us in an unequal warfare in which the gardener is always defeated by the insect, whether the bigger combatant admits it or not. Their food consists entirely of insects, slugs, and worms. In turn the Frog constitutes the food of many larger animals, including fishes, birds, snakes and weasels. The winter is spent embedded in mud at the pond-bottom, or in damp holes in the earth.

The Common Frog is distributed widely all over Britain, but is only of local occurrence in Ireland. Abroad it ranges over Central and Northern Europe as far as Sweden and Norway, and eastward to Mid-Asia.

*Edible Frog* (_Rana esculenta_, Linn.).

Although the Common Frog is the only species that is really native in Britain, another one--the Edible Frog, a Continental species--has been naturalised in the Eastern Counties of England since the early part of the nineteenth century, when Mr. Geo. Berney brought about 1500 specimens from France and Belgium and turned them loose in the Fens, in the neighbourhood of Stoke Ferry, where they are no longer plentiful, though they occur locally in various parts of Norfolk. A few years later (1843) Mr. Thurnall discovered the species in the Cambridgeshire Fens at Foulmire--a great distance (30 to 40 miles) from Stoke Ferry. Bell says his father had noted the presence of these Foulmire frogs, under the name of "Whaddon Organs," about the middle of the eighteenth century; so that it appeared that Mr. Berney had "taken coals to Newcastle"--in other words, had introduced the Edible Frog to a part of England where it already existed. In 1884 Dr. G.A. Boulenger discovered that the Foulmire frogs were of the Italian form of _Rana esculenta_ known as the variety _lessonæ_, which made it doubtful whether they could be travelled descendants of Mr. Berney's frogs. So it was suggested that they were a survival from an introduction by the Romans--who are always dragged in to help out doubtful cases.

The difference in the French and Italian forms is mainly one of colour, the type being a beautiful grass-green, whereas _lessonæ_ is olive-brown. But it has since transpired that _lessonæ_ is not restricted to Italy as Boulenger thought, for he has more recently discovered it in Belgium and near Paris, and it has been recorded from parts of the former Austrian and German Empires. Such differences as there are in the two forms are not fundamental, and the brown tint of the Foulmire examples may be due to their environment. Fresh importations from the Continent have been liberated in recent years in Hampshire, Surrey, Oxfordshire, and Bedfordshire.

The Edible Frog attains to a rather larger size than the Common Frog. It is usually without the dark patch extending from the eye to the shoulder, and the markings of the body--especially the bright yellow and black marblings of the hinder parts--are darker and bolder. There is usually a light yellow or green line running down the middle of the back from the muzzle to the hinder extremity. The most distinctive feature, however, is restricted to the male sex: at the hinder angle of the mouth, just below the ear, are external vocal sacs which, when the owner is inclined to be melodious, become distended with air to the size of large peas, giving him a very quaint appearance. The croak differs from that of the Common Frog, and has been described as "more of a loud snore, exactly like that of the Barn Owl;" but this probably refers to the vocal efforts of the female, for Bell says it is so loud and shrill as to have obtained for the frogs the names of "Cambridgeshire Nightingales" and "Whaddon Organs." The males continue to "sing" after the breeding season is past, particularly on warm moonlight nights, when they may be heard for over a mile when the choir consists of several hundred voices. The notes are "Brekeke, gwarr, ooaar, coarx."

To return to a description of the Edible Frog. Full-grown examples measure from two and a half to four inches of head and body; the females larger than the males. The head is more slender than in the Common Frog, and the brown eardrum is two-thirds of the diameter of the eye. The teeth on the palate form two oblique lines; and there is a pair of glandular folds behind the eye. The ground colour of the upper parts ranges from dull brown through olive to bright green, with dark brown or blackish spots on the back and larger patches of similar tint on the limbs. There is usually a bronzy-brown line along each side of the back, in addition to the central one already named. The back of the thigh is always spotted with black and white or yellow. Though the thigh of the Common Frog is barred or blotched, it never bears these additional spots. The coloration generally is much brighter where the vegetation is light than in dark swamps with sombre vegetation.

The developmental history of the Edible Frog from the egg to the loss of the tadpole tail follows much the same course as that of the common species, and it is not necessary to recapitulate it. The eggs are more numerous, one female producing from five to ten thousand. The tadpole condition lasts three or four months. Full-grown tadpoles are about two and a half inches long, of which more than an inch and a half is tail. The frog that has just got rid of his tail measures only half an inch. The young frogs are not such wanderers as their Common cousins, but remain in the vicinity of their birthplace, unless the pond dries up. They like to bask in the sun and wait till their food comes within range of their extensible tongues. They become mature between the fourth and fifth years.

This is the Frog whose hind legs are served as food in the restaurants of France and of the French quarters in London. We have not experimented with them as food, but remember that Frank Buckland, who was keen upon out-of-the-ordinary dishes, described them as "tasting more like the delicate flesh of the Rabbit than anything else I can think of." Our old friend, Miss Susan Hopley, told us that she once unwittingly partook of a much larger kind in the United States, and innocently remarked, "What a pity to kill such very young chickens!" She says she was moved to the remark by the insipidity of the dish.

The Edible Frog is found all over Europe and in Northern Asia.

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The beautiful little Tree Frog (_Hyla arborea_), of bright green colour, with expanded toe-tips which make it an expert climber, is widely distributed on the Continent, whence it is frequently introduced to our conservatories. Some of these examples turned loose years ago in the Isle of Wight have become naturalised in some parts of the island, where they have become so numerous as to arouse complaints against their noisy nocturnal croaking during the breeding season.

*Common Toad* (_Bufo vulgaris_, Laurent).

Though in general terms the Toad may be said to be of similar form to the Frog, there is no need for a very minute catalogue of differences to enable the reader to discriminate between the two. So well-known are both amphibians to sight that the majority of persons know them by their correct names on a casual glance; yet we have met many who confuse them, and for this minority it is well to give some of the Toad's points.

He has a flatter back than the Frog, the bones of the pelvis not producing so sharp an angle; and the hind legs are not so long in proportion to the body, only slightly exceeding the length of head and body, whereas in the Frog they are one and a half times that length. The Toad seems more solidly built than the Frog, with broader head, shorter limbs, and in general aspect is closer to the earth, a heavier, more grovelling creature than the vaulting Frog. This earthliness is accentuated by the texture and colour of his skin. Instead of the moist and shining, bright coloured coat of the Frog we have a dry, dull, pimply skin so strongly resembling the earth that he is frequently passed by as a lifeless clod. That is one of the Toad's strong points; and he has the patience to squat motionless for hours, tiring out any enemy that looks for movement as proof of life. He is too heavy to take a leap; instead he progresses by very short jumps on all four feet which give the impression of being accomplished only by a great effort. But he rises alertly to his full quadrupedal height when he is considering the best way to negotiate a worm.

The colour of the Toad varies a good deal according to the nature of the soil upon which he happens to live. It is usually some tint of brown or grey, but the brown may be almost red in sandpits, a rich brown or a dirty brown; the grey may be light or with an olive tinge or a sooty hue that may pass as black. As he is only active in the evenings and at night, any of these tints serve to render him inconspicuous in the general duskiness. Even his bright eyes, being coppery-red in colour, do not serve to draw attention to him.

The pimples of various size that diversify his skin are not mere ornament, though they help materially to produce the clod-resemblance. They are glands that on occasion pour out an acrid and offensive fluid that often saves the Toad when he is caught up in the jaws of some unsophisticated carnivorous beast or bird. Experience teaches such enemies to leave the Toad alone. The largest of these glands--the parotid--may be seen as an elongated, porous swelling behind the eye. The underside is whitish, the white being qualified always with an admixture of yellow, brown, or red, sometimes spotted with black.

In the matter of size: taking the head and body for length, average males measure about two and a half inches and females an inch longer. Occasionally we may meet with much larger examples, and we may safely set down such monsters as females. The male has no vocal sacs, internal or external, as in the Frogs; but both sexes can croak with several variations of tone. These sounds are emitted much more freely in the pairing season. The male develops special grasping pads on the palm and three inner fingers, at the pairing time.

After the breeding season Toads wander away from the water, and distribute their forces over field, hedgerow, wood, and gardens, wherever there is an abundance of insect life, for the quantity of food each Toad consumes is enormous. It includes beetles, caterpillars, flies, snails, worms, woodlice, and small mice. If the droppings of a Toad be examined, they will be found to consist very largely of the indigestible parts of beetles. The Toad spends the hotter part of the day concealed under the lower foliage of plants, and as many nocturnal insects seek similar situations in the daytime, he has no difficulty in enjoying a continual feast. His appetite appears to be always keen, no matter how well he has fed. Some years ago, when we were pointing to a portly female in her favourite daytime "form" in the garden, a friend expressed the opinion that she was overfed, and we remarked that you cannot overfeed a Toad. Our friend was sceptical, and undertook to provide more food than she could eat. There followed a hunt for the fattest caterpillars and the longest worms, and the Toad accepted them as readily as though she were breaking a fast. The caterpillar hunter grew tired of the business whilst the Toad was still quite fresh, and he admitted that with so elastic an integument there was no knowing what was the limit of a Toad's feeding capacity.