Tree-frogs have, of course, in most circumstances much greater difficulty in getting at water than pond-frogs; and this is especially true in certain tropical or desert districts. Hence most of the frogs which inhabit such regions have had to find out or invent some ingenious plan for passing through the tadpole stage with a minimum of moisture. The devices they have hit upon are very curious. Some of them make use of the little pools collected at the bases of huge tropical leaf-stalks, like those of the banana plant; others dispense with the aid of water altogether, and glue their new-laid eggs to their own backs, where the fry pass through the tadpole stage with the slimy mucus which surrounds them. Nature always discovers such cunning schemes to get over apparent difficulties in her way: and the tree-frogs have solved the problem for themselves in half a dozen manners in different localities. Oddest of all, perhaps, is the dodge invented by “Darwin’s frog,” a Chilean species, in which the male swallows the eggs as soon as laid, and gulps them into the throat-pouch beneath his capacious neck: there they hatch out and pass through their tadpole stage: and when at last they arrive at frogly maturity, they escape into the world through the mouth of their father.
No. 8. The Surinam Toad.
The Surinam toad, represented in [No. 8], is also the possessor of one of the strangest nurseries known to science. It lives in the dense tropical forests of Guiana and Brazil, and is a true water-haunter. But at the breeding season the female undergoes a curious change of integument. The skin on her back grows pulpy, soft, and jelly-like. She lays her eggs in the water: but as soon as she has laid them, her lord and master plasters them on to her impressionable back with his feet, so as to secure them from all assaults of enemies. Every egg is pressed separately into a bed of the soft skin, which soon closes over it automatically, thus burying each in a little cell or niche, where it undergoes its further development. The tadpoles pass through their larval stage within the cell, and then hop out, in the four-legged condition. As soon as they have gone off to shift for themselves, the mother toad finds herself with a ragged and honeycombed skin, which must be very uncomfortable. So she rubs the remnant of it off against stones or the bark of trees, and re-develops a similar back afresh at the next breeding season.
Almost never do we find a device in nature which occurs once only. The unique hardly exists: nature is a great copyist. At least two animals of wholly unlike kinds are all but sure to hit independently upon the self-same mechanism. So it is not surprising to learn that a cat-fish has invented an exactly similar mode of carrying its young to that adopted by the Surinam toad: only, here it is on the under surface, not the upper one, that the spawn is plastered. The eggs of this cat-fish, whose scientific name is Aspredo, are pressed into the skin below the body, and so borne about by the mother till they hatch. This is the second instance of which I spoke above, where the female fish herself assumes the care of her offspring, instead of leaving it entirely to her excellent partner.
Higher up in the scale of life we get many instances which show various stages in the same progressive development towards greater care for the safety and education of the young. Among the larger lizards, for example, a distinct advance may be traced between the comparatively uncivilized American alligator and his near ally, the much more cultivated African crocodile. On the banks of the Mississippi, the alligator lays a hundred eggs or thereabouts, which she deposits in a nest near the water’s edge, and then covers them up with leaves and other decaying vegetable matter. The fermentation of these leaves produces heat and so does for the alligator’s eggs what sitting does for those of hens and other birds: the mother deputes her maternal functions, so to speak, to a festering heap of decomposing plant-refuse. Nevertheless, she loiters about all the time, like Miriam round the ark which contained Moses, to see what happens; and when the eggs hatch out, she leads her little ones down to the river, and there makes alligators of them. This is a simple and relatively low stage in the nursery arrangements of the big lizards.
The African crocodile, on the other hand, goes a stage higher. It lays only about thirty eggs, but these it buries in warm sand, and then lies on top of them at night, both to protect them from attack and to keep them warm during the cooler hours. In short, it sits upon them. When the young crocodiles within the eggs are ready to hatch, they utter an acute cry. The mother then digs down to the eggs, and lays them freely on the surface, so that the little reptiles may have space to work their way out unimpeded. This they do by biting at the shell with a specially developed tooth; at the end of two hours’ nibbling they are free, and are led down to the water by their affectionate parent. In these two cases we see the beginnings of the instinct of hatching, which in birds, the next in order in the scale of being, has become almost universal.
I say almost universal, because even among birds there are a few kinds which have not to this day progressed beyond the alligator level. Australia is the happy hunting-ground of the zoölogist in search of antiquated forms, elsewhere extinct, and several Australian birds, such as the brush-turkeys, still treat their eggs essentially on the alligator method. The cock birds heap up huge mounds of earth and decaying vegetable matter, as much as would represent several cartloads of mould; and in this natural hot-bed the hens lay their eggs, burying each separately with a good stock of leaves around it. The heat of the sun and the fermenting mould hatch them out between them; to expedite the process, the birds uncover the eggs during the warmer part of the day, expose them to the sun, and bury them again in the hot-beds towards evening. Several intermediate steps may also be found between this early stage of communal nesting by proxy and the true hatching instinct; a good one is supplied by the ostrich, which partially buries its eggs in hot sand, but sits on them at intervals, both father and mother birds taking shares by turn in the duties of incubation.
The vast subject which I have thus lightly skimmed is not without interest, again, from its human implications. Savages as a rule produce enormous families; but then, the infant mortality in savage tribes is proportionally great. Among civilized races, families are smaller, and deaths in infancy are far less numerous. The higher the class or the natural grade of a stock, the larger as a rule the proportion of children safely reared to the adult age. The goal towards which humanity is slowly moving would thus seem to be one where families in most cases will be relatively small—perhaps not more on an average than three to a household—but where most or all of the children brought into the world will be safely reared to full maturity. This is already becoming the rule in certain favored ranks of European society.