Sea-horses.
You could hardly find a better rough test of relative development in the animal (or vegetable) world than the number of young produced and the care bestowed upon them. The fewer the offspring, the higher the type. Very low animals turn out thousands of eggs with reckless profusion; but they let them look after themselves, or be devoured by enemies, as chance will have it. The higher you go in the scale of being, the smaller the families, but the greater amount of pains expended upon the rearing and upbringing of the young. Large broods mean low organization; small broods imply higher types and more care in the nurture and education of the offspring. Primitive kinds produce eggs wholesale, on the off chance that some two or three among them may perhaps survive an infant mortality of ninety-nine per cent, so as to replace their parents. Advanced kinds produce half a dozen young, or less, but bring a large proportion of these on an average up to years of discretion.
Without taking into account insects and such other “small deer,”—to quote Shakespeare’s expression,—this fundamental principle of population will become at once apparent if we examine merely familiar instances of back-boned or vertebrate animals. The lowest vertebrates are clearly the fishes: and the true fishes have almost invariably gigantic families. A single cod, for example, is said to produce, roughly speaking, nine million eggs at a birth (I cannot pretend I have checked this calculation); but supposing they were only a million, and that one-tenth of those eggs alone ever came to maturity, there would still be a hundred thousand codfish in the sea this year for every pair that swam in it last year: and these would increase to a hundred thousand times that number next year; and so on, till in four or five years’ time the whole sea would be but one solid mass of closely-packed cod-banks. We can see for ourselves that nothing of the sort actually occurs—practically speaking, there are about the same number of cod one year as another. In spite of this enormous birth-rate, therefore, the cod population is not increasing—it is at a standstill. What does that imply? Why, that taking one brood and one year with another, only a pair of cod, roughly speaking, survive to maturity out of each eight or nine million eggs. The mother cod lays its millions, in order that two may arrive at the period of spawning. All the rest get devoured as eggs, or snapped up as young fry, or else die of starvation, or are otherwise unaccounted for. It seems to us a wasteful way of replenishing the earth: but it is nature’s way; we can only bow respectfully to her final decision.
Frogs and other amphibians stand higher in the scale of life than fish; they have acquired legs in place of fins, and lungs instead of gills; they can hop about on shore with perfect freedom. Now, frogs still produce a great deal of spawn, as every one knows: but the eggs in each brood are numbered in their case by hundreds, or at most by a thousand or two, not by millions as with many fishes. The spawn hatches out as a rule in ponds, and we have all seen the little black tadpoles crowding the edges of the water in such innumerable masses that one would suppose the frogs to be developed from them must cover the length and breadth of England. Yet what becomes of them all? Hundreds are destroyed in the early tadpole stage—eaten up or starved, or crowded out for want of air and space and water: a few alone survive or develop four legs, and absorb their tails and hop on shore as tiny froglings. Even then the massacre of the innocents continues. Only a tithe of those which succeeded in quitting their native pond ever return to it full grown, to spawn in due time, and become the parents of further generations.
Lizards and other reptiles make an obvious advance on the frog type; they lay relatively few eggs, but they begin to care for their young. The family is not here abandoned at birth, as among frogs, but is frequently tended and fed and overlooked by the mother. In birds we have a still higher development of the same marked parental tendency; only three or four eggs are laid each year, as a rule, and on these eggs the mother sits, while both parents feed the callow nestlings till such time as they are able to take care of themselves and pick up their own living. Among mammals, which stand undoubtedly at the head of created nature, the lower types, like mice and rabbits, have frequent broods of many young at a time; but the more advanced groups, such as the horses, cows, deer, and elephants, have usually one foal or calf at a birth, and seldom produce more than a couple. Moreover, in all these higher cases alike, the young are fed with milk by the mother, and so spared the trouble of providing for themselves in their early days, like the young codfish or the baby tadpole. Starvation at the outset is reduced to a minimum.
It is interesting to note, too, that anticipations of higher types, so to speak, often occur among lower races. An animal here and there among the simpler forms hits upon some device essentially similar to that of some higher group with which it is really quite unrelated. For example, those who have read my account of the common earwig (given in the sixth chapter of “Flashlights on Nature”) will recollect how that lowly insect sits on her eggs much as a hen does, and brings up her brood of callow grubs as if they were chickens. In much the same way, anticipations of the mammalian type occur pretty frequently among lower animals. Our commonest English lizard, for example, which frequents moors and sandhills, does not lay or deposit its eggs at all, but hatches them out in its own body, and so apparently brings them forth alive: while among snakes, the same habit occurs in the adder or viper. The very name viper, indeed, is a corruption of vivipara, the snake which produces living young. Still more closely do some birds resemble mammals in the habit of secreting a sort of milk for the sustenance of their nestlings. Most people think the phrase “pigeon’s milk” is much like the phrase “the horse-marines,” a burlesque name for an absurd and impossible monstrosity. But it is nothing of the sort: it answers to a real fact in the economy of certain doves, which eat grain or seeds, grind and digest it in their own gizzards into a fine soft pulp or porridge, and then feed their young with it from their crops and beaks. This is thus a sort of bird-like imitation of milk. Only the cow or the goat takes grass or leaves, chews, swallows, and digests them, and manufactures from them in her own body that much more nutritive substance, milk, with which all mammals feed their infant offspring.
Now, after this rather long preamble, I am going to show you in this present article a few other examples of special care taken of the young in certain quarters where it might be least expected. Fish are not creatures from which we look for marked domestic virtues: yet we may find them there abundantly. Let us begin with that familiar friend of our childhood, the common English stickleback.