Chapter IV: Part I: Amphibia (3)

Embracing of the two sexes is the universal rule with the Anura, the male creeping on to the back of the female and clasping her firmly with the arms and hands either in the inguinal region, higher up, or under the armpits. See the numerous statements in the systematic part. This often extremely forcible, pressing embrace seems to be necessary, although the females can deposit the eggs without the help of the male, but in such cases the expulsion takes place at irregular intervals instead of at one time. When the eggs appear at last, and this happens in many species many hours, or even some days, after the beginning of the embrace, the male voids the contents of its seminal vesicles over them. Fertilisation is consequently external, with the possible exception of _Pipa_, _q.v._ p. 152.

DEPOSITION OF THE EGGS AND NURSING HABITS.–The majority of the Amphibia are oviparous, but some Apoda and Urodela are viviparous. It is unnecessary to call the latter condition ovo-viviparous, since this is really a distinction without a difference.

Viviparous forms:–amongst Urodela; _Salamandra maculosa_, the young burst the egg-membrane in the act of being born, and are provided with long gills; _S. atra_, the young undergo their whole development and metamorphosis within the uterus (see p. 119); _Spelerpes fuscus_, the young are likewise born in the perfect condition: amongst Apoda; _Typhlonectes compressicauda_ and _Dermophis thomensis_.

The oviparous Apoda, at least _Ichthyophis_ and _Hypogeophis_, and a few of the Urodela, as _Desmognathus_ and _Amphiuma_, take care of their eggs by coiling themselves around them in a hole underground.

Nursing habits are very common amongst the Anura. {56}Boulenger[28] has summarised the various conditions concerning the deposition and care that is taken of the eggs, in the following list, in which more recent discoveries have been interpolated.

I. The ovum is small, and the larva leaves it in a comparatively early
embryonic condition.

A. The eggs are laid in the water:–

_a._ Without further care or preparations: probably the majority of
Anura; all European forms, except _Alytes_.

_b._ The eggs are laid in a specially walled-in part of the pond:
_Hyla faber_.

B. The eggs are deposited out of the water:–

_a._ In holes, or under grass, near the banks of pools. The larvae
are liberated and washed into the water by the next heavy rain:
_Leptodactylus ocellatus_, _L. mystacinus_, _Paludicola gracilis_,
_Pseudophryne australis_ and _P. bibroni_.

_b._ On leaves above the water, the larvae dropping down when leaving
the egg: _Chiromantis rufescens_, _Phyllomedusa iheringi_, _Ph.
hypochondrialis_.

II. The yolk is very large and the young undergoes the whole or part of
the metamorphosis within the egg; at any rate the larva does not assume
an independent existence until after the loss of the gills.

A. The eggs are deposited in damp situations, or on leaves. The young
escape as:–

_a._ Tadpoles: _Arthroleptis seychellensis_, _Rhacophorus schlegeli_,
_Rh. maculatus_.

_b._ Perfect, air-breathing frogs: _Rana opisthodon_, _Hylodes
martinicensis_, _Hyla nebulosa_.

B. The eggs are carried by a parent.

_a._ By the male:–

α. Round the legs; the young leaves the egg in the tadpole stage:
_Alytes_.

β. In the enlarged vocal sacs; the young leave in the perfect
state: _Rhinoderma_.

_b._ By the female:–

α. Attached to the belly: _Rhacophorus reticulatus_.

β. Attached to the back; the young complete their metamorphosis
within the egg: _Pipa_.

γ. In a dorsal pouch which the young leave as tadpoles: _Nototrema
marsupiatum_;–or in the perfect state: _Nototrema testudineum_, _N.
cornutum_, _N. oviferum_, _N. fissipes_, and _Hyla goeldii_.

THE DEVELOPMENT AND METAMORPHOSIS of many species have been described in the systematic part. The following is a short general account of some of the more important features. Metamorphosis in the Apoda and Urodela is restricted chiefly to the reduction of the gills, the closing of the clefts, and the loss of the {57}gill-chamber and the finny margins of the tail; but the change from the tadpole to the final Anurous animal implies an almost entire reorganisation.

In the earliest condition the embryo consists of a large head and body, while the tail is still absent. Behind the beginnings of the future mouth appears a transverse crescentic fold, with the convexity looking backwards, which develops into the paired or unpaired _adhesive apparatus_. This consists of large complex glands, developed in the Malpighian layer, originally covered by the cuticula, which soon disappears, whereupon the sticky secretion enables the larva to attach itself to the gelatinous mantle of the egg, later on to weeds or other objects in the water. The name of suckers, often applied to this apparatus, conveys a wrong idea, there being neither muscles nor any suctorial function. The shape of this organ undergoes many changes during the early life of the individual, and differs much in the various genera, affording thereby diagnostic characters.[29] At first a crescent, it divides into a right and a left oval or disc, which either remain asunder and behind the mouth (_Rana_, _Bufo_), or they move forwards to the corners of the mouth (_Hyla_) or further back, and unite again more or less completely, as in _Discoglossus_ and _Bombinator_. It is mostly of short duration, and disappears by the time that the larva, by the proper development of the gills and the tail and the functional mouth, changes into the tadpole. But in a few species these discs transform themselves into an elaborate ventral disc. Such an organ persists throughout the greater part of the tadpole-stage in certain Oriental species of _Rana_, all of which, when adult, possess fully webbed toes and {58}strongly dilated discs on the fingers and toes, e.g. _Rana whiteheadi_, _R. natatrix_, and _R. cavitympanum_ of Borneo, _R. jerboa_ of Java (this larva was originally described and figured as that of _Rhacophorus reinwardti_), and _R. afghana_ of the Himalayan system. These tadpoles, at least those of _R. jerboa_, are further remarkable for having the "spiracular" opening very far back on the left side, nearer to the base of the tail than to the snout, so as to be well out of the way when the creature has attached itself by the adhesive disc.

The mouth of the tadpoles of Anura is furnished with horny armaments, substitutes for teeth. Their development and that of the mouth in general has been well described by Gutzeit.[30] In the young larvae of _Rana temporaria_, one or two days after hatching, a shallow groove appears above the conspicuous pair of adhesive organs. The groove becomes rhombic in outline, and when the mouth has been formed in its centre, the jaws appear in the median corners of the rhombus. The epidermis then rises like a circular wall around the jaws, and divides into an upper and lower lip; furrows appear on them, and between these various papillae and comb-like transverse plates of teeth. The papillae are possibly tactile organs, but although nerves enter them, nerve-endings of a sensory nature have not yet been discovered. On the fourth day the jaws become black, by the tenth day horny teeth have appeared upon all the plates of the mouth-armature, and on the seventeenth day the mouth-apparatus has reached the configuration typical of the tadpole, which is now about 14 mm. long. The number of horny teeth in _R. temporaria_ amounts to about 640. These teeth are not cuticular products, but cornified cells; they are very small, and consist each of one horny cell, which is shaped like a nightcap, the apex of which is curved back and serrated. The little teeth are shed continuously, {59}the renewal taking place by successive cells growing into the bases of the older series. The shape and size differ much in the various genera and species. The comb-like plates, composed of those teeth which surround the lips, seem to be used chiefly for the fixing or hooking of the food, while those which compose the horny beak proper, the armature of the jaws, are used like the radulae of snails. These beaks are likewise composed of a great number of individual teeth, closely packed together in several rows, but the teeth themselves are simple and not serrated.

In _Hyla arborea_ there are in all about 560 teeth. The development of the mouth does not begin before the eleventh day; the horny teeth break through, and the jaws get black edges, on the eighteenth. In _Pelobates fuscus_ the number of horny teeth is increased to about 1100. In _Borborocoetes taeniatus_ the horny teeth form series of five bells, which fit into each other like the joints of a rattlesnake's tail.

One of the most extraordinary kinds of tadpoles is that of _Megalophrys montana_.[31] Mr. Annandale (Skeat Expedition) found it at Bukit Besar, Malay Peninsula, from 2000 to 3000 feet above the level of the sea. The tadpoles (Fig. 11) were found in the beginning of the month of May 1899 in sandy streams and in pools of rain-water; they floated in a vertical position, the peculiar membranous funnel-shaped expansion of the lips acting as surface-floats. The inside of the funnel is beset with radiating series of little horny teeth, and the whole apparatus is possibly used for scraping the under-surface of the leaves of water-plants in search of food. Total length of the tadpoles 1 inch.[32]

The gills, the formation of the operculum, and the modifications of the branchial arterial arches have been described fully on p. 43; those of the hyo-branchial skeleton on p. 31. Fusion of the opercular fold with the skin of the neck, across the branchial region, causes the head to become confluent with the trunk (cf. Fig. 9, 3, p. 57). The body becomes oval, more or less globular, and the alimentary canal is greatly elongated and stowed away in the shape of a neat, very regular spiral, shining through {60}the ventral wall of the body; the anus opens at the end of a somewhat protruding tube, either in the median line, just in front of the ventral fin (Discoglossidae, _Pelobates_, _Bufo_), or it assumes an asymmetrical position by turning to the right side (_Hyla_, _Rana_).

Although both pairs of limbs begin to bud simultaneously, or the fore-limbs even earlier, the hind-limbs are hurried on, and appear first, long before the fore-limbs. The latter lie ready beneath the skin of the gill-chamber, and the right always breaks through the skin, while the left does the same in the Mediogyrinidae, while in the Laevogyrinidae it is generally pushed through the left-sided spiracular opening, immediately behind the outer gills. According to Barfurth the right limb appears, in about 80 per cent. of _Rana esculenta_, from two to eight hours before the left.

{61}Meanwhile the lungs are being developed, and the tadpole occasionally rises to the surface to breathe air. The gills, which, as has been explained elsewhere, are less ancestral than they are larval organs, degenerate, and all the organs are modified for the coming terrestrial life. The fins of the tail are absorbed, the horny armature of the mouth and lips is shed in pieces and makes room for the true teeth, the eyes receive lids, and the whole cranium, especially the apparatus of the jaws, undergoes the final modifications–widening and lengthening of the mouth, arresting of the mento-Meckelian cartilages, elongation of the Meckelian cartilages or lower jaw proper, shifting backwards of the suspensorium, and lengthening of its orbital process to form the pterygo-palatine bridge.

The tadpole ceases to feed, the whole intestinal canal is voided of its contents, and by "histolysis" is thoroughly rebuilt, becoming wider and shrinking to about one-sixth of its original length,–undoing thereby the spiral–preparatory for the coarser food, which consists of insects, worms, and other strictly animal, living matter. Hitherto the tadpoles have lived on "mud," confervae, Diatoms, rotting vegetable and animal matter. The anal tube collapses, becomes ultimately absorbed, and a new vent is formed at and below the root of the tail.

Barfurth[33] has made interesting observations and experiments with regard to the absorption of the tail and other organs which disappear during the metamorphosis. This is retarded by low temperature; it is accelerated by rest and freedom from mechanical disturbances, as, for instance, concussion of the water. Hunger shortens or hurries on the last stages of metamorphosis, the absorption of the tail taking place in four instead of five days. Amputation of the tail has no retarding influence; it is followed at once by regeneration, although the tadpole may be on the verge of reducing the tail. Whilst hungering the whole organism draws upon its available store of material, naturally first upon those parts which sooner or later are to become superfluous. This applies eminently to the tail, which represents a considerable amount of "edible" matter, and also to that portion of the skin which still covers the fore-limbs. The elements of the cutis are resorbed, thereby thinning the skin; and consequently the limbs break through earlier in fasting than in well-fed {62}specimens. Nature herself seems to apply hunger as an accelerator. Mlle. von Chauvin found that the larvae of Urodela normally fast during the transformation, and according to Barfurth the larvae of _Rana temporaria_ eat less after their hind-limbs are fully developed. This is, however, also preparatory for the reorganisation of the gut, which has to be more or less empty during the shortening process.

The loss of the tail is not due to a sudden dropping off of this organ–a crude but by no means uncommon belief–but is brought about by a very gradual process of resorbtion. When the fore-limbs begin to break through the skin, the tip of the tail shrinks and becomes black, owing to an increase, or rather concentration, of the pigment cells. The reduction proceeds from the tip forwards until on about the fifth day there remains only a short, conical, black stump. From the beginning of this process of reduction the tail is scarcely used for locomotion, the tadpole rowing with its legs, or it crawls and hops about, although the tail may still be 20 mm. long. The cells of the epidermis atrophy, shrink, and peel off, while those of the cutis, blood-vessels, nerves, muscles, and chorda dorsalis become disintegrated, often undergoing fatty degeneration. The leucocytes eat up the débris and other dissolved tissue, and carry it away through the lymphatic vessels, to be used as new building material in the rest of the animal.

Barfurth asks very properly, Why do these tissues degenerate and die? Because the vasomotor nerve-fibres cease to regulate the circulation. And why does this trophic influence of the central nervous system stop? Because the function of the tail becomes superfluous through the appearance of the fore-limbs. The tail is doomed, and degenerates like any other organ without a function. The whole process is, of course, a recapitulation of ancestral, phylogenetic evolution.

{63}CHAPTER III

NEOTENY–REGENERATION–TEMPERATURE–GEOGRAPHICAL DISTRIBUTION

NEOTENY.–It has long been known that the larvae of the Spotted Salamander occasionally attain the size of 80 mm. or about 3 inches, whilst the majority undergo metamorphosis when they are only 40 mm. long. Again, larvae of _Triton_ have been found, in the months of April and May, 80 to 90 mm. long, still with functional gills, but with the sexual organs fully developed. De Filippi[34] found in one locality in Lombardy, besides a few normal fully metamorphosed specimens of only 30 mm. in length, more than forty specimens, which, although they had attained full size, about 55 mm., and were sexually mature, still retained their gills. According to him such gill-breathing, otherwise mature specimens, occur constantly in a small lake in the Val Formazzo, on the Italian slope of the Alps, in the province of Ossola. Later Duméril[35] astonished the world by his account of the metamorphosis of the Mexican gill-breathing Axolotl into an entirely lung-breathing and terrestrial creature, hitherto called _Amblystoma_, and supposed to be not only a different species, but to belong to a different family from the Axolotl, which was known as _Siredon axolotl_ s. _pisciforme_, and naturally classed with the Perennibranchiata.

This discovery led to a series of observations and experiments, chiefly conducted by Marie von Chauvin, instigated thereto by Koelliker and by Camerano.[36] It was then found that many, if not most of the European Amphibia, both Urodela and Anura, {64}occasionally postpone their metamorphosis, and also that such Urodela sometimes become adult for all practical purposes, but retain their gills.

This retardation, the retention of larval characters beyond the normal period, was called _Neotenie_ by Kollmann[37] (νέος, young; τείνω, extend, stretch). He distinguished further between:–I. Partial Neoteny, namely, simple retardation of the metamorphosis beyond the normal period, for instance, the wintering of tadpoles of _Pelobates fuscus_, _Bombinator pachypus_, _Pelodytes punctatus_, _Alytes obstetricans_, _Hyla arborea_, _Rana esculenta_, _R. temporaria_, _Bufo vulgaris_, and _B. viridis_: II. Total Neoteny, where the animal retains its gills, but becomes sexually mature; hitherto observed in Urodela only, e.g. _Triton vulgaris_, _T. alpestris_, _T. cristatus_, _T. boscai_, _T. waltli_ and _Amblystoma_. Intermediate stages between these two categories are not uncommon.

A satisfactory explanation of the meaning of neoteny is beset with difficulties. Some authorities look upon the phenomenon simply as the result of adaptation to the surroundings, which make it advantageous for the creature to retain its larval features. Others think that the surroundings somehow or other retard or prevent the assumption of the adult characters. Undoubtedly there are many cases in which larvae have been reared in water-holes with steep walls, so that they could not change from aquatic to terrestrial life, and it stands to reason that abnormally forced and prolonged use of the gills and of the tail may stimulate these organs into further growth at the expense of the limbs and other organs which are intended for terrestrial life. But not unfrequently typical neotenic and overgrown specimens occur side by side with others which have completed their metamorphosis, and the same is true of larvae of newts which were reared, for experimental purposes, under exactly the same conditions–for instance, in a high-walled glass vessel.

Weismann tried to explain neoteny as cases of reversion to atavistic ancestral conditions, but this idea is based upon an assumption which is probably wrong. His idea necessitates the supposition that all the Amphibia were originally gill-breathing, aquatic, and limbless animals, and that every feature seen in a larva must necessarily indicate an ancestral phylogenetic stage. It is, on the contrary, much more probable that {65}the external gills of the Urodela have been developed in adaptation to their embryonic and larval, essentially aquatic, life. Consequently the possession of such gills would be a secondary, and not, strictly speaking, an atavistic feature. Normal loss of these gills, exclusively pulmonary respiration, and preponderating terrestrial life characterise the final adult Amphibian. These cases of neoteny are therefore instances of more or less complete retardation, or of the retention, of partially larval conditions.

The whole problem is, however, by no means simple. _Salamandra atra_ has become viviparous, and the whole metamorphosis takes place within the uterus; in fact, the young have an embryonic, but no larval period, if by the latter we understand the free swimming and still imperfect stage. Similarly, various Anura–for instance, _Hylodes martinicensis_–pass rapidly through their metamorphosis, and have suppressed the stage of free swimming tadpoles. On the other hand, in many newts, the duration of the larval period is much prolonged, and moreover is very subject to individual variation. In the Axolotl this larval period is continued until and after sexual maturity is reached. The extreme condition would then be represented by the Perennibranchiate genera. It may seem reasonable to look upon these as the youngest members of the Urodela, and the loss of the maxillae in the Sirenidae and Proteidae supports this idea. But it so happens that the majority of the most neotenic genera are more primitive in the composition of the skull and the vertebral column than the typically terrestrial and rapidly metamorphosing genera. Witness the amphicoelous vertebrae, the completeness of the pterygoids, the separate nature of the palatine bones, and the separate splenials, as mentioned in detail in the description of their skull.

We have therefore to conclude, first, that the various Perennibranchiate genera do not form a natural group, but are a heterogeneous assembly; secondly, that they have become Perennibranchiate at a phylogenetically old stage–in fact, that they are the oldest, and not the newest, members of the present Urodela. At the same time, it would be erroneous to suppose that the first Urodela were aquatic creatures, provided with a finny tail, with small, ill-developed lungs, and with epidermal sense organs. All these features are, on the contrary, directly correlated with aquatic life, and are larval acquisitions, not ancestral {66}reminiscences. It would be equally wrong to allude to the absence of lungs in many newts as a piscine and therefore ancestral feature. The development of the typical pentadactyloid limb, the connexion of the pelvic girdle with the vertebral column, the development of the lungs, and absolute suppression of internal gills point without doubt to terrestrial creatures. What then, may we ask, were the first Amphibia like? and how about the external gills? They were undoubtedly akin to the less specialised Lepospondylous Stegocephali, in particular the gill-less Microsauri, and the various stages may perhaps be reconstructed as follows:–

(1) Terrestrial, with two pairs of pentadactyloid limbs; breathing by lungs only; with a fully developed apparatus of five pairs of gill-arches, which during the embryonic life perhaps still carried internal gills; with or without several pairs of gill-clefts. Reduction of the dermal armour and of the cutaneous scutes had taken place.

(2) Additional respiratory organs were developed by the embryo, in the shape of external gills; these were at first restricted to embryonic life (as in the existing Apoda), but were gradually used also during the aquatic life of the larva. These external gills, together with the lungs, have superseded the internal gills, of which there are now no traces either in Urodela or in Anura.

(3_a_) Some Urodeles, retaking to aquatic life, retained and further enlarged the external gills into more or less permanent organs (cf. also _Siren_, p. 136).

(3_b_) The majority of Urodela hurried through the larval, aquatic stage, and some–e.g. _Salamandra atra_–became absolutely terrestrial. The possession of unusually long external gills by this species and by the Apoda indicate that these organs are essentially embryonic, not larval, features.

REGENERATION.–Most Amphibia possess the faculty of regenerating mutilated or lost limbs. This takes place the more certainly and quickly the younger the animal. The amputation necessary to study these phenomena need not be experimental. Axolotls and other Urodelous larvae frequently maim each other fearfully, by biting off the gills or one or more limbs. The gills do not even require amputation. If the larvae are kept in stagnant water the gills often shrivel up or slough off and grow again. {67}The same applies to the larvae of viviparous species, e.g. _Salamandra atra_, which, when cut out of the uterus and put into water, soon cast off their long, tender gills and produce a stronger set. In an Axolotl,[38] two years old, a hand was cut off. After four weeks there was a conical stump; after the sixth week this stump had two points; in the eleventh week three or four fingers were discernible, and a week later the complete hand. Frequently these creatures reproduce five instead of the normal four fingers. But the more proximal the cut, the more liable is the new limb to reproduce supernumerary fingers, or even extra hands and feet. Complete regeneration of the limb, cut off in the middle of the humerus, took place within five months.

_Triton taeniatus_, adult, reproduces cut fingers within five or six weeks, and if the hand be cut above the carpus, new finger-stumps appear in about one month. Götte has observed that an adult _Proteus_ did not completely reproduce its whole leg until after eighteen months; and, according to Spallanzani, more than one year elapses before the limb, bones, and cartilages of _Triton_ regain their normal strength.

The Anura are likewise capable of regenerating their limbs, the more readily the younger the specimens. For instance, in a tadpole of _Rana temporaria_, in which the fore-limbs were still hidden, the hind-limb, cut at the middle of the thigh, reproduced nineteen days later a knee, followed by a short two-toed stump. Ultimately the whole limb became completed. The tail of tadpoles regenerates very quickly and completely, even if it be cut off shortly before the final metamorphosis, when the tail would in any case be reduced. Metamorphosed Anura have almost entirely lost this faculty, but not absolutely. I myself have kept two specimens of _Rana temporaria_, which, when already adult, had each lost a hand at the wrist. First there was only the clean-cut stump with a scar, but within a year this changed into a four-cornered stump, and two of the protuberances developed a little further, reaching a length of about 4 mm. These specimens lived for four years without further changes.

TEMPERATURE.–Amphibia, like Fishes and Reptiles, are, as a rule, classed as cold-blooded animals, in opposition to the warm-blooded Birds and Mammals. This distinction is one of degree only. The terms poikilothermous and homothermous (ποίκιλος, {68}variable; ὅμος, equable) are based upon a sounder principle, but are likewise liable to exceptions. Those creatures which, like Birds and Mammals, possess a specific temperature of their own under normal conditions, that of hibernation being excepted, are homothermous. Cold-blooded creatures have no specific temperature; they more or less assume that of their surroundings. Frogs and newts, for instance, when living in the water, naturally assume its temperature, which is, of course, many degrees lower in a cold spring than in a shallow pond warmed by the sun on a hot summer's day. The same applies to the changes from day to night. Dark-coloured tortoises basking in the sun are sometimes so hot that they are disagreeable to touch, since they possess but little mechanism for regulating their heat. The same individual cools down during a chilly night by perhaps 40° C. Anura are, however, very susceptible to heat; most of them die when their temperature rises to about 40° C. Under such conditions they die quickly when in the water, but in the air their moist skin counteracts the heat, lowering it by evaporation; otherwise it would be impossible for a tree-frog to sit in the glaring sun in a temperature of 120° F. Toads and others with drier skins seek the shade, hide under stones, or bury themselves in the coolest spots available, and many Amphibia and Reptiles aestivate in a torpid condition during the dry and hot season. Many of them can endure a surprising amount of cold, and during hibernation their temperature may sink to freezing-point. This power of endurance does not apply to all alike; tropical species can stand less than those which live in temperate and cold regions. In spite of many assertions to the contrary, it may safely be stated that none of our European frogs, toads, and newts survive being frozen hard. They may be cooled down to nearly -1° C., and they may be partially frozen into the ice. Circulation of the blood is suspended in such cooled-down frogs; their limbs may become so hard that they break like a piece of wood, but the citadel of life, the heart, must not sink much below freezing-point, and must itself not be frozen, if the animal is to have a chance of recovering. The protoplasm resists a long time, and so long as some of it is left unfrozen the rest will recover. Hibernating frogs are lost if they are reached by prolonged frost during exceptionally severe winters. Every frog will be killed in an artificial pond with a clean concrete bottom, {69}but if there is sufficient mud, with decaying vegetable matter, the creatures survive, simply because they are not absolutely frozen. A severe winter not infrequently kills off all the younger creatures, while the older and more experienced hide themselves more carefully and live to propagate the race.

GEOGRAPHICAL DISTRIBUTION.

There is a very ably written chapter on the geographical distribution of the Amphibia by Boulenger in the _Catalogue of Batrachia Gradientia_, pp. 104-118. He came to the important conclusion that the geographical distribution of the Amphibia agrees in general with that of the freshwater fishes. Günther's division into a Northern, Equatorial, and Southern zone is modified only in so far as the last two are combined into one, "Tasmania and Patagonia not differing in any point regarding their Frog Fauna from Australia and South America respectively."

Boulenger recognises–

I. The Northern zone–(1) Palaearctic, (2) North American, region.

II. The Equatorial Southern zone.

_A._ Firmisternia division = Cyprinoid division of Günther.

1. Indian region.

2. African region.

_B._ Arcifera division = Acyprinoid division of Günther.

1. Tropical American region.

2. Australian region.

In the chapter on geographical distribution in Bronn's _Thierreich, Vögel, Systematischer Theil_, p. 296 (1893), and in my _Classification of Vertebrata_ (1898), due attention had been paid to the Amphibia as well as to the other classes of Vertebrata. It will be seen in the following pages that my arrangement is well applicable to the Amphibia so far as fundamental principles are concerned.

It cannot be sufficiently emphasised that any attempt to form the various faunas of the different classes of animals into one scheme must necessarily be a _petitio principii_. The time-honoured six zoo-geographical regions established by Sclater and Wallace represent fairly well the main continental divisions: North America, South America, Africa, Australia, and the large northern continental mass of the Old World, with India as a tropical appendix. There is no correlation and no subordination {70}in this scheme. Huxley's division (1868) into NOTOGAEA and ARCTOGAEA (see p. 74) is of fundamental importance. The next improvement was the combination of the Palaearctic and Nearctic "regions" into one, an advance originally due to Professor Newton, carried out by Heilprin (1887) as the Holarctic region. I have, in 1893, substituted for it the more appropriate term _Periarctic_, meaning the whole mass of land which lies around the indifferent Arctic zone. The want of further co-ordination and subordination required the combination of the African and Oriental or Indian countries into a _Palaeotropical region_ (1893); the Ethiopian or African and the Indian or Oriental regions of Sclater and Wallace thereby assuming their proper subordinate rank of subregions.

The two primary divisions NOTOGAEA and ARCTOGAEA are fundamental. The four secondary divisions, namely the _Australian_ and _Neotropical_, _Periarctic_ and _Palaeotropical regions_, also stand the test of application to the various classes and main groups of Vertebrata; but naturally, under the present configuration of the world, the Palaeotropical region is nothing but the Southern continuation of the Eastern half of the Periarctic mass of land. This is especially obvious so far as India is concerned. There is, however, that broad belt of desert, sand, and salt-steppes, which extends from North-West Africa to Manchuria, and this belt is one of the most important physical features of the Old World. It is complicated by the system of mountain-chains which, broadly speaking, centre at the Pamirs, and radiate westwards through the Caucasus and Alps into Spain, eastwards through the Himalayas into China, and north-eastwards to Kamtschatka; interrupted by Bering's Sea, it is continued as the backbone of both Americas to Patagonia.

The tertiary divisions, the subregions, have no real existence. They depend upon the class, or even order, of animals, which we happen to study. The faunistic distribution of the Urodela is not that of the Anura, and both follow separate lines of dispersal, different from those of the various orders of Reptiles, Birds, and Mammals. This must be so. There is no doubt that the distribution of land and water was totally different in the Coal Age from what it is now. The face of the globe at the Jurassic Age can scarcely be compared with the aspect which the world has assumed in the Miocene period.

This leads to another consideration, often neglected. We {71}know that the various classes, orders, families, etc., of animals have appeared successively upon the stage. A group which arose in the Coal Age followed lines of dispersal different from one which was not evolved until Jurassic times, and post-cretaceous creatures could not avail themselves of what assisted their ancestors, and _vice versâ_. The Amphibia are bound absolutely to the land and to fresh water; transportation across salt water is not excluded, but must be accidental, and is not a case of regular "spreading." Speaking generally, the older a group, the more likely is it to be widely distributed. If it appears scattered, this may be due to extinction in intermediate countries or to submergence of former land-connexions.

There is great danger of arguing in a circle. It is one of the most difficult tasks to decide in cases of great resemblance of groups of animals between their being due to direct affinity or to heterogeneous convergence, or parallel development. It is the morphologist who is ultimately responsible for the establishment of faunistic regions, not the systematist, least of all he who accepts an elaborate classification, and then mechanically, mathematically, by lists of genera and species, maps out the world. Let us take an example. The Neotropical region and Madagascar, but not Africa, are supposed to be faunistically related to each other. In both namely occur _Boa_ and _Corallus_ amongst snakes, Dendrobatinae amongst Ranidae, and of the Insectivora _Solenodon_ in Cuba, _Centetes_ in Madagascar. More cases can no doubt be found which would strengthen this resemblance, perhaps in support of the startling view that Madagascar and South America have received part of their fauna from the famous Antarctica. But the value of the Insectivores has been disposed of by their recognition as an extremely ancient group, or as a case of convergence, and the two genera are no longer put into the same family as Centetidae. The Dendrobatinae (_Mantella_ in Madagascar, the others in South America) are decidedly not a natural group, but an instance of very recent convergence (cf. p. 272). About the members of the ancient Boidae we do not feel quite so sure.

It is therefore advisable to eliminate for zoogeographical purposes groups about which there can be any reasonable doubt, otherwise we may argue that certain genera must constitute a very old family, because they are now restricted to widely {72}separated countries, or on the strength of their distribution we may conclude that the genera in question cannot be related to each other, and do not belong to the same sub-family or family as the case may be. Such groups are the Engystomatinae and the genus _Spelerpes_; amongst reptiles the Eublepharidae, Helodermatidae, Anelytropidae, Ilysiidae, Amblycephalidae.

It is customary to represent the various regions and sub-regions as if they had boundaries as fixed as political frontiers. Such limitations are quite arbitrary, and what is of more importance, they differ in reality according to the class or order of animals with which we happen to deal. Moreover, there has been, and is probably still going on, an exchange or overlapping of faunas. Such debatable grounds are Central America and the highlands of North-western South America. The famous Wallace's line, between Borneo and Celebes, Java and Lombok, is absolutely inapplicable to the Anura. From their point of view the Austro-Malayan countries, Papuasia and Polynesia do not form a sub-region of the Australian, but rather of the Palaeotropical region. Concerning the Urodela, the division into Palae- and Ne-arctic sub-regions is unjustifiable since Eastern Asia has emphatically American affinities (cf. also p. 96). The Sahara and the rest of Northern Africa are intimately connected with Arabia, Persia, Afghanistan, and Northern India, just as equatorial Africa and Madagascar possess strong faunistic relationship with Southern India and the Malay islands.

_Limiting factors of distribution._–Common salt is poison to the Amphibia; even a solution of 1 per cent prevents the development of their larvae. Consequently seas, salt lakes, and plains encrusted with saline deposits act as most efficient boundaries to normal "spreading." But undoubtedly many individuals have made long and successful voyages across the seas on floating trees. Solutions of lime are likewise detrimental to many species, and it is a general fact that limestone-terrain is poor in Amphibian life, unless, of course, sufficient accumulation of humus counteracts or prevents the calcareous impregnation of the springs and pools in meadows. _Salamandra maculosa_ is, for instance, absent in Central Germany on the Muschelkalk, but it occurs in abundance in neighbouring districts of red sandstone or granite; nor can the larvae be reared successfully in very "hard" water. On the other hand, _Proteus_ lives in the {73}subterranean waters of Carniola, where the whole country is nothing but limestone.

Cold is another powerful limiting factor. The absolute northern limit of Amphibian life coincides rather closely with the somewhat erratic line of 0° Centigrade of annual mean temperature, a little to the north of which line the ground remains permanently frozen below the surface. The surface-crust, which thaws during the summer, engenders an abundance of insects as food-supply, but its freezing down to the icy bottom makes hibernation impossible. There are, of course, some exceptions, for instance the occurrence of Urodela in the Schilka river and in the district of Lake Baikal.

Ranges of mountains are far less effective barriers than is generally supposed. In many cases the fauna is the same on either slope, and they act rather as equalising or dispersing factors, especially when they extend from north to south. Witness the Andes, owing to which Ecuador and Peru bear a great resemblance to the Central American fauna, and differ from the tropical parts of South America. The existence of an _Amblystoma_ in Siam is another instance.

The more specialised a family the more intimately is it connected with the physical features of the country. Typically arboreal frogs are dependent on the presence of trees. Some have undoubtedly spread into treeless countries and have changed into prairie-frogs, e.g. _Acris_. They come out, so to speak, as something different at the other end, and it is unlikely that these modified descendants redevelop exactly the same features as their ancestors before the migration. Baldwin Spencer[39] met with only six species of frogs in Central Australia, _Limnodynastes_, _Chiroleptes_, _Heleioporus_, and _Hyla_. They are in the main identical with certain forms found in the dry inland parts of New South Wales and Queensland. They are to be regarded as immigrants from the latter regions, which have been able in the majority of cases to adapt themselves to unfavourable climatic conditions by means of a marked development of the burrowing habit, to which in certain cases has been added a capacity for absorbing and holding water.

{74}FAUNISTIC DIVISIONS OF THE AMPHIBIA.

NOTOGÆA.–SOUTH WORLD.

* indicates Amphibia which are peculiar to the respective regions or sub-regions.

Characterised by the Cystignathidae* and by the predominance of Arcifera, which form nearly 90 per cent of the Anurous population.

I. AUSTRALIAN REGION.–Absence of Apoda and Urodela. All the Anura are arciferous, with the exception of one species of _Rana_ in the Cape York peninsula. The fauna of the Australian continent and of Tasmania consists chiefly of Cystignathidae and Hylidae (_Hyla_ and _Hylella_) and several small genera of Bufonidae (_Pseudophryne_,* _Notaden_,* and _Myobatrachus_*).

It is customary, and from the study of other Vertebrata quite justifiable, to divide the Australian region into several sub-regions, but the Amphibia lend no support to this. The only Amphibian in the Sandwich Islands is a _Bufo_, closely related to North American species. The only Amphibian in New Zealand is _Liopelma_,* one of the Discoglossidae which are otherwise confined to Europe, North-east Asia, and North-west America, and, to judge from their low organisation, had formerly a much wider distribution. New Caledonia possesses no Amphibia. The Fiji Islands are inhabited by one or two species of _Cornufer_, a genus of Ranidae. The same genus is typical of the Austro-Malayan and Papuasian islands, the fauna of which consists of _Rana_ and _Cornufer_, _Ceratobatrachus_, several genera of Engystomatinae, Hylidae, and Pelobatidae.

II. NEOTROPICAL REGION.–Characterised by Apoda, Aglossa (_Pipa_), abundance of Cystignathidae (Hemiphractinae,* Cystignathinae, and Dendrophryniscinae*), Hylidae (Hylinae and Amphignathodontinae*), numerous Bufonidae and Engystomatinae; Dendrobatinae*; the Raninae are represented by a few peculiar genera, mostly restricted to the Andesian province; the genus _Rana_ occurs there in a few species only.

Absence of Discoglossidae, Pelobatidae and Dyscophinae.

Several species of Urodela, of the genus _Spelerpes_, extend from Central America into the Andesian province, one occurs in Hayti, and _Plethodon platense_ in Argentina.

This region is by far the richest in the number of families, {75}genera and species; the total number of the latter being, according to Boulenger, about four-ninths of the known species. The region comprises South America, Central America, and the West Indian islands. Central America is naturally debatable ground; one species of _Hylodes_ and one _Engystoma_, besides about twenty Hylidae, extend into North America proper, while possibly the Raninae have entered the Neotropical region from the north. _Bufo_ is too cosmopolitan to assist our conclusions. The occurrence of four species of _Hylella_ in South America, one in Australia, and one in New Guinea indicate that this is not a natural genus.

From the point of the Amphibia the whole region can be divided into two sub-regions only: (1) The West Indian islands with Central America and the north-western Andesian province; (2) the rest of South America.

ARCTOGAEA.–NORTH WORLD.

Characterised by the absence of Cystignathidae.

I. PERIARCTIC REGION.–Characterised by the Urodela, these being almost peculiar to the region (cf. p. 96). Absence of Apoda. Presence of Discoglossidae, Pelobatidae, Bufonidae, Raninae. Few Hylinae occur.

The whole region can be subdivided into three sub-regions.

1. Western Palaearctic.–Prevalence of Salamandrinae (_Salamandra_,* _Chioglossa_,* _Salamandrina_,* _Triton_); Proteidae (_Proteus anguinus_*); _Spelerpes fuscus_.*–_Discoglossus_, _Bombinator_, _Alytes_,* _Bufo_, _Hyla arborea_, _Pelobates_,* _Pelodytes_,* _Rana_.

2. Eastern Palaearctic.–Amphiumidae (_Cryptobranchus_); Salamandrinae (_Triton_, _Pachytriton_,* _Tylototriton_*); Amblystomatinae.–_Bombinator_, _Bufo_, _Hyla arborea_, _Rana_, _Rhacophorus_.

3. Nearctic.–Amphiumidae (_Cryptobranchus_, _Amphiuma_*); Proteidae (_Typhlomolge_,* _Necturus_*); Sirenidae*; Amblystomatinae; most Plethodontinae; Desmognathinae.*–Discoglossidae, Pelobatidae (_Scaphiopus_*); _Bufo_; Hylidae (_Hyla_, _Acris_, _Chorophilus_); _Rana_.

II. PALAEOTROPICAL REGION.–Characterised by the presence of Apoda and by the great prevalence of Firmisternal Anura, which amount to nearly 90 per cent of the total population. {76}Absence of Urodela (except _Amblystoma persimile_*), of Cystignathidae, and practically of the Hylidae, only two of which occur in the Himalayan district. But this great chain of mountains should not be included within the region, while the outlying spurs in Upper Burma (with _Amblystoma_) are debatable ground. The subdivision of this widely extended region is beset with difficulties, chiefly on account of Madagascar and Papuasia. The fauna of Madagascar is very remarkable. All its Amphibia are Firmisternal, a mixture of African and Indian forms. The island agrees with Africa, in opposition to the Oriental countries, in no special point; all the Raninae, except _Megalixalus_, _Rappia_, and two rather common species of _Rana_, belong to different genera. Madagascar differs from Africa by the absence of Apoda, of Aglossa, and Bufonidae. On the other hand, it agrees with India or with the Malay islands, in opposition to Africa, by the possession of Dyscophinae, of the Ranine genus _Rhacophorus_, and the Engystomatine genus _Calophrynus_.

Africa and India agree with each other, and differ from Madagascar by the possession of Apoda, the genera _Bufo_ and _Nectophryne_, and by the close resemblance of several genera of Raninae.

India, the Malay islands, and Papuasia with Melanesia possess Pelobatidae (_Leptobrachium_,* _Batrachopsis_,* _Asterophrys_*), and thereby differ considerably from Africa and Madagascar. _Batrachylodes_* of the Solomon Islands has unmistakable affinities with _Phrynoderma_* of Karen, between Burma and Siam; _Oreobatrachus_* of Borneo much resembles _Phrynobatrachus_* of West Africa; and _Cornufer_, typical of the Malay and Melanesian islands, occurs also in West Africa. All these Raninae indicate that the Austro-Malayan and Melanesian islands belong to the Palaeotropical region. _Ceratobatrachus_,* type of a sub-family, is peculiar to Melanesia.

There are consequently several possible modes of subdivision, all with a different result, according to the group of Amphibia, which we may select as of leading importance, _e.g._ Apoda or Pelobatidae, or Dyscophinae and _Rhacophorus_. The Engystomatinae and Raninae are to be eliminated, since they occur in all the countries in question. We have either to leave the whole region undivided–and it is a significant fact that the {77}Indian countries possess not one sub-family of their own–or we must break it up into four provinces, not sub-regions:–

1. Ethiopian, or continental African, with Aglossa and Apoda, no
Pelobatidae, no Dyscophinae, few Bufonidae, and many Raninae.

2. Indian and Malayan, with Apoda, no Aglossa, but with Pelobatidae,
Dyscophinae, many Bufonidae and Raninae, amongst which _Rhacophorus_.

3. Malagasy, without either Apoda or Aglossa; with Firmisternal Anura
only, chiefly Dyscophinae, and _Rhacophorus_ and other Raninae.

4. Papuasian, without Apoda, Aglossa, Dyscophinae, and Bufonidae, but
with Pelobatidae and Ranidae.

DISTRIBUTION OF FAMILIES AND SUB-FAMILIES OF THE AMPHIBIA.

Aus. = Australian.
N-t. = Neotropical.
N-a. = Ne-arctic.
Pal-arct. = Palae-arctic.
Eth. = Ethiopian.
Mal. = Malagasy.
Ind. = Indian.
Pap. = Papuasian.
N.Z. = New Zealand.

+––––––––––––––––––––+––––+––––+––––+––––+––––+––––+––––+––––+––––+––––+
| |Aus.|N–t.|N–a.|Pal–arct.|Eth.|Mal.|Ind.|Pap.|N.Z.|
| | | | +––––+––––+ | | | | |
| | | | | W. | E. | | | | | |
+––––––––––––––––––––+––––+––––+––––+––––+––––+––––+––––+––––+––––+––––+
|Amphiumidae | | | + | | + | | | | | |
|Salamandridae | | + | + | + | + | | | 1 | | |
|Proteidae | | | + | + | | | | | | |
|Sirenidae | | | + | | | | | | | |
|Apoda | | + | | | | + | | + | | |
|Aglossa | | + | | | | + | | | | |
|Discoglossidae | | | | + | + | | | | | + |
|Pelobatidae | | | + | + | | | | + | + | |
|Bufonidae | + | + | + | + | + | + | | + | + | |
|Hylinae | + | + | + | 1 | 1 | | | | | |
|Amphignathodontinae | | + | | | | | | | | |
|Hemiphractinae | | + | | | | | | | | |
|Cystignathinae | + | + | 1 | | | | | | | |
|Dendrophryniscinae | | + | | | | | | | | |
|Genyophryninae | | | | | | | | | | |
|Engystomatinae | | + | 1 | | | | + | + | + | |
|Dyscophinae | | | | | | + | + | + | | |
|Dendrobatinae | | + | | | | | × | | | |
|Raninae | | + | + | + | + | + | + | + | + | |
|Ceratobatrachinae | | | | | | | | | + | |
+––––––––––––––––––––+––––+––––+––––+––––+––––+––––+––––+––––+––––+––––+

1 signifies the occurrence of only one species of an elsewhere numerous
group.
× _Mantella_, cf. p. 71 and p. 272.

{78}CHAPTER IV

STEGOCEPHALI OR LABYRINTHODONTS–LISSAMPHIBIA–APODA

_SUB-CLASS I. STEGOCEPHALI OR PHRACTAMPHIBIA_

_With a considerable amount of dermal armour, especially on the head._

The earliest known terrestrial four-footed creatures occur in the Carboniferous strata of Europe and North America. They and their immediate allies, which extend through the Permian into the Upper Trias, are now comprised under the name of STEGOCEPHALI, so called because the whole of the dorsal side of the cranium is covered, or roofed over, by dermal bones (στέγος, roof; κεφαλή, head). That these creatures, of which naturally only the skeletal parts are known, were not fishes, is shown by the typically pentadactyloid limbs; but to recognise them as Amphibia, and as distinct from Reptiles, is difficult, especially if the incipient Reptilia, which have sprung from some members of this Stegocephalous stock, are taken into account. However, they possess either two occipital condyles, or none, and their vertebrae are either pseudocentrous or notocentrous, but not gastrocentrous. Moreover, the whole skeletal organisation is still so ideally generalised, that it is easy to derive directly from it the arrangement prevailing in the Apoda and Urodela.

The vertebral column always comprises a well-developed, sometimes a very long tail. The vertebrae exhibit three types, two of which are fundamentally distinct, while the third is a further development of the second.

1. _Lepospondylous and pseudocentrous._–The vertebra consists of a thin shell of bone surrounding the chorda dorsalis, and is composed of two pairs of arcualia, which meet each other, {79}forming a suture, along the lateral side of the vertebra, both partaking in the formation of a transverse process which carries the rib.

2_a_. _Temnospondylous._–The vertebra is composed of three pairs of units, which remain in a separate, unfused state. Two of them are dorsal arcualia, one of which tends to form the centrum of the vertebra, which then carries the neural arch.

2_b_. _Stereospondylous._–The three component units fuse by co-ossification into a solid, amphicoelous vertebra.

The ribs are one- or two-headed, rather strong, but short, rarely reaching half-way round the body. They occur on all the vertebrae of the trunk and on most of those of the tail. One pair of ribs connects one vertebra, the sacral, with the pelvis, of which the ilium and ischium are generally ossified, rarely also a portion of the pubic region.

The shoulder-girdle is very primitive, greatly resembling that of the Crossopterygian fishes. It consists of the following bones:–a median, rhombic, or T-shaped interclavicle, a pair of clavicles, of cleithra, of coracoids, and of scapulae. The limbs show the typical pentadactyle plan, but even in these earliest Tetrapoda the hand possesses only four fingers, with 2, 2, 3, 2 phalanges respectively. The foot has five toes, with 2, 3, 4, 4, 3, or 2, 2, 3, 4, 3 phalanges.

Many Stegocephali were possessed of a dermal armour, covering either the whole body or only the under parts. Hence the term Phractamphibia (φρακτός, armoured). The armour consists of a great number of small cutaneous scales, partly calcified, or perhaps ossified, and arranged in many more or less transverse rows. We can only surmise that these scales were covered by corresponding epidermal sheaths. The skull is ideally complete in the number of separate bones which appear on its surface. Besides the outer nares and the orbits there is always an unpaired, small, interparietal foramen. The whole temporal region is completely roofed over. The following bones are present:–nasals, frontals, parietals, supra- and latero-occipitals; lacrymals (unless fused with the jugals?), prefrontals, postfrontals, postorbitals, squamosals, and epi-(or opisth-)otics; premaxillaries, maxillaries, jugals, quadrato-jugals, and supra-temporals; quadrates, pterygoids, palatines, vomers, and an unpaired parasphenoid.–The lower jaw is composed of a pair of dentaries, {80}articulars, angulars, and splenials. The dentaries and apparently sometimes the splenials, the palatines, maxillae, and vomers carry teeth. The eyes possess a ring of sclerotic bones.

ORDER I. STEGOCEPHALI LEPOSPONDYLI.

Vertebrae pseudocentrous.

SUB-ORDER 1. BRANCHIOSAURI.–The young had several pairs of gill-arches, which, to judge from their size and from the fact that they are beset with numerous nodules, denticles, or irregular little processes like gill-rakers–seem to have been exposed to the surface and to have carried gills. In the adult the arches and gills seem to be absent.

One of the commonest genera is _Branchiosaurus_, including _Protriton_. _B. salamandroides_ of the Lower Red Sandstone of Europe is known in every stage, from larvae of 16 mm. to the full grown animal of 64 mm. in length. The whole body was {81}covered with little cutaneous scales. _Pelosaurus_ and perhaps _Melanerpeton_ are allied genera.

The following genera are small newt-like creatures of the Carboniferous age of Europe and North America. In _Keraterpeton_ of Bohemia, Ireland, and Ohio, the dermal scales were restricted to the under parts; and the ribs were rather long, reaching half way round the body. Gills have not been observed. _K. crassum_, a European species, reached more than one foot in length, two-thirds of which fall to the tail. The ventral side is covered with a most elaborate armour, which consists of about eighty chevron-shaped rows of little scale-shaped nodules. The epiotic bones end in strange processes, carrying a pair of spikes, giving the skull a "horned" appearance, hence the generic name. _Urocordylus_ is an allied genus.

SUB-ORDER 2. AISTOPODES.–Body snake-like and without any limbs, hence the name ἄιστος, unseen; ribs long, and reaching half way round the body; from Carboniferous strata in Ireland and Bohemia, with allied, or perhaps identical forms in Ohio. _Dolichosoma longissimum_ possessed more than 150 vertebrae, and was about a yard long. The epiotics end in obtuse projections, recalling those of _Keraterpeton_. These marvellous creatures had strange appendages, extending from behind the sides of the head, which were possibly the supports of external gills; since the upper end of one of the visceral arches, probably the hyoidean arch, is attached to the labyrinthic region, and from this arch starts a bony rod which carries long skeletal filaments. The body seems to have been naked.

_Ophiderpeton_ had a compound ventral shield, while the skin of the back contained granular scutes. Although the Aistopodes have, not without reason, been looked upon as greatly resembling the Coeciliae or Apoda in organisation, especially in that of the vertebral column, the total absence of any other fossils which might bridge over the enormous gulf between the Coal Age and recent times, makes the attempt to derive the Apoda from these creatures very hazardous.

ORDER II. STEGOCEPHALI TEMNOSPONDYLI.

Mostly with rather long ribs and with chiefly ventral armour.