FROGS AS NURSERY MAIDS AND WEATHER PROPHETS

The tree frogs are a very large family (Hylidæ) distributed all over the world, except Africa, but most of the species belong to the steamy forests of tropical America. All are of small size, have smooth skins, normally greenish, but very changeable in color to adapt the creatures to the hue of their surroundings, as a protective device; and most of them inhabit trees. To enable them to do this the toes end in expanded, padlike disks, the contraction of which, when the foot is pressed against a surface, produces one or more furrows and, in addition, causes the exudation of a little mucilaginous liquid. The foot pressed against the surface expels the air, and this fact, aided by the stickiness of the pad, enables the frog to hold on to even a vertical plane of glass. All Hylidæ have a voice, often very loud, and enhanced by membranous sacs under or on each side of the throat, or in some cases internal; this sac, when blown out may be almost as large as the creature's body, as may be seen in our common gray tree frog when "singing." This species, like most others, becomes very noisy in the evening, in cloudy weather and before rain, with its not unmusical croaking; and a similar European species is kept in confinement by some people as an interesting pet and weather prophet.

The most interesting thing about the Hylidæ is their various methods of breeding, for while most of them lay their eggs, up to a thousand in number, in the water, many produce but a few, and attach them to the body.

A large tree frog called in Brazil "ferreiro" (smith), makes a sound like a mallet slowly and regularly struck on a metal plate. This frog actually builds a nursery in the shallow edge of a pond, where a basin-shaped hollow, with a rim, is formed by the broad-handed female. Here she leaves her eggs, safe from egg-eating fishes or insects, as the rim forms a wall higher than the surface of the water. A Japanese frog makes a similar basin, then produces a liquid which she kicks into a froth, and into the midst of this the eggs are dropped, and there the hatched larvæ develop, and remain until the gradual collapse of the mud rim sets them free.

In these and similar cases the eggs and tadpoles are abandoned by the parents; but many frogs watch over and care for their young. Some carry the young in a pouch on the back, but how it is accomplished is not known. A West African species carries its eggs in its mouth; and the male of Darwin's frog, of Chile, carries the eggs in a great vocal pouch beneath its throat, which subsequently forms a nursery for the tadpoles until they emerge as young frogs.

It must be noted, however, that some of these examples belong to the related family Cystignathidæ—a very extensive family largely represented in Central and South America.

The remainder of the tailless amphibians are assembled in the numerous and widely distributed family Ranidæ, which is that of the "true" frogs. The typical subfamily, Raninæ, is cosmopolitan, except as to Australia and South America south of the Amazon basin; but some less typical forms are confined to the tropics, and include several strange species, such as the little arboreal Dendrobates frogs of Brazil, one of which is famous for furnishing in the secretion of its skin a dye that when properly applied turns the green plumage of tame parrots into yellow—a fashionable tint. These small and pretty frogs are noted for their solicitude for their young, carrying baby tadpoles on their backs—where the infants creep and become attached—from place to place, as safety or better water conditions suggest.

The North American frogs are good examples of the ranine race, and those more commonly seen are the following:

Leopard frog (Rana pipiens), green with irregular black blotches, mostly in two rows on the back; legs barred above; belly pale. Eastern specimens are more olive than bright green.

Pickerel frog (R. palustris), light brown with two rows of large, oblong, square blotches of dark brown on the back, a brown spot above each eye, and a dark line from the nostril to the eye; upper jaw white and black. Habitat, eastern United States among mountains.

Wood frog (R. sylvatica), pale reddish brown; a black band across the pointed face. This smallest of our species is to be found only in damp woods, resorting to water only in early spring to deposit its eggs; and it is almost silent.

Green, or spring frog (R. clamatans), green or bronze-brown, brighter in front, with more or less small black spots; yellowish white below. This is a rather solitary frog, living in springs and small ponds, where it utters the familiar "chung" at frequent intervals. It is distinguished by the enormous size of its eardrum.

Bullfrog (R. catesbiana), greenish, brightest on the head, and with small dark spots on its back; legs blotched; eardrum large; toes broadly webbed. Length five to eight inches, breadth four to five inches. It utters a roar not unlike that of a distant bull, and a company of them on a still summer evening will awaken the neighborhood. Bullfrogs are present throughout the eastern United States and Canada, west to the dry plains; and furnish the market with "saddles" (their hind legs) as a table delicacy when fried. These frogs may lay 12,000 eggs apiece.

All our frogs lay their eggs in water in rounded masses, not in strings, as do the toads, usually attached to some submerged stick or plant stem. The tadpoles, light in color, are very voracious, and feed on every sort of flesh that they can bite off and chew with their horny jaws. On the approach of winter the frogs—except the wood frog, which hibernates in the loam of the forest, or in some rotten stump—sink into the mud of the pond or marsh where they live, and pass the cold months in torpidity. Their food is almost exclusively insects, caught by the tongue, but the big bullfrogs seize with their mouths any small creature that comes their way.

[CHAPTER XX]
REPTILES—MONARCHS OF THE MESOZOIC WORLD

What is a reptile? It is a cold-blooded, air-breathing vertebrate, with one occipital condyle, complete right and left aortic arches, red blood and a covering of scales. The classification of the class (Reptilia) recognizes the existence of many distinct subdivisions, as follows:

Proreptilia (extinct).

Prosauria (extinct, except the tuatara).

Theromorpha (extinct).

Chelonia—Turtles; tortoises.

Dinosauria (extinct; dinosaurs).

Crocodilia—Crocodiles; alligators.

Plesiosauria (extinct).

Ichthyosauria (extinct; fish lizards).

Pterosauria (extinct; pterodactyls).

Pythonomorpha (extinct; mososaurs, etc.).

Sauria—Lizards; snakes.

This surprising diversity of groups, each so widely isolated, as is implied by separation as subclasses—divisions of almost the highest rank—shows that the class developed in favorable circumstances that stimulated enterprise, so to speak, and resulted in rapid variation of habits, terrestrial, aquatic, arboreal, and aerial, and consequently of adaptive structure. The fact that most of the subclasses are extinct also shows us that the story of the Reptilia is mainly a tale of the departed glory recorded in the archives of the rocks; and we shall hardly be able to understand living reptiles properly without knowing something of their prehistoric development into the dominance to which they rose in the Mesozoic era, which we call Age of Reptiles, and their subsequent decadence.

The first subclass covers certain most ancient skeletons and parts of skeletons that naturalists are not yet agreed are true reptiles, some considering them stegocephalian amphibians. Anyway, they indicate plainly that it was from that group of Amphibia that the variety sprang that developed into what, in time, became the distinct reptilian type. The first distinct product of this departure from the stegocephalian stock appears in the fossils of a division of the second subclass, the Prosauria (pro, "before"; saurus, a "lizard"), named Rhynchocephalia ("beakheads"), which, although lizardlike in general form, retain many amphibian characteristics of structure. Now the amazing and extremely interesting thing about this is that a representative of this earliest of true reptiles is still living—probably the premier peer among all vertebrates, reckoned by length of ancestry. This most primitive of reptiles, illustrating how hundreds of ancient species known to us only by a few bones must have appeared and acted in life, is the tuatara of New Zealand, catalogued in science as Sphenodon punctatum.

It has the shape and general appearance of a big lizard, dull in color and with a granulated rather than scaly hide, and an oddly shaped head, toothless in the adult, when the jaws become somewhat like a horny beak. Yet it is not a lizard any more than it is a crocodile or a turtle, but combines features of all three in its anatomy. Hence it is what naturalists term a synthetic or generalized race (as is the case with all very primitive creatures) out of which more and more specialized groups and species may be, and are, developed, each sorting out and strengthening some particular characteristic of structure, continuously modified by adaptations to habits and environment until a separate type results. The ribs, for example, in the tuatara are remarkable for the presence of hooklike processes that project backward from each rib over the next rib behind it; such processes occur elsewhere only in the crocodiles and the birds. Behind the breastbone are rodlike bones embedded in the muscles of the belly; they occur again in the ancient fish lizards and modern crocodiles, and probably gave rise to the under shield of the turtles. And so on.

The tuatara is verging on extinction. It has nearly disappeared from the mainland of New Zealand, but is now protected on some small adjacent islands where it dwells in burrows which it digs and then shares with petrels. During the greater part of the day the tuataras sleep; and are fond of lying in the water, being able to remain submerged for hours without breathing. They feed only upon other animals.

The third subclass (Theromorpha, "beast-shaped") comprises very ancient reptiles whose remains lie in the rocks of Permian and Triassic age, principally in South Africa, and exhibit a skull, and especially teeth, so much resembling those of carnivorous mammals (for instance, those of a dog) that at first their true nature was mistaken. These creatures have excited the most profound interest, not only because they present so many differences from the Prosauria, but also, and chiefly, because it is from their ranks that we are able to trace, with no small degree of certainty, the origin of the Mammalia.