These creatures may be roughly divided into two groups, the one consisting of the simple corals, which only live together in very small numbers, and the other of the reef-builders, which live in vast colonies, and build up ma.s.ses of coral of enormous size. The latter are by far the more interesting, and the way in which they build up immense banks of coral is very wonderful indeed.
Remember, first of all, that these animals multiply in two different ways--sometimes by eggs, and sometimes by little buds, so to speak, which grow out of the body of the parent. The polyps which hatch out from eggs swim about for some little time quite freely. But after a few days they fasten themselves down to the surface of a submerged rock, and after that they never move again. Other polyps soon come and settle down by them, and before very long there will be thousands upon thousands of the little animals all growing, as it were, close together, and all gradually building up coral underneath and round the margins of their bodies.
When they reach their full size they begin to multiply by "budding."
Baby polyps sprout out all over their bodies, and these, instead of swimming about for a few days like those which are hatched from eggs, remain fixed where they are for the whole of their lives. Then they, in their turn, begin to deposit coral, and as they have nowhere else to put it they place it on the bodies of their parents, which before very long are completely covered in. Now, you see, there is a second layer of coral on the top of the first. Then in due course of time a third layer is formed upon the second, and a fourth layer upon the third, each generation being built in by the one that comes after it, till at last the coral bank rises above the surface of the water. Then the work has to stop; for these little creatures cannot live unless the waves can constantly break over them. But although the bank cannot be raised higher it can still be extended on all sides; and so the little polyps go working on, year after year, till at last the results of their labor are almost too wonderful to realize.
CORAL BANKS
These coral banks take three different forms.
First, there are the fringing reefs. These are great banks of coral surrounding the sh.o.r.es of a tropical island, or running for long distances on the coasts of the mainland. The island of Mauritius, for example, is entirely surrounded by a fringing reef. These reefs often spread out for miles into the sea, and they are only broken here and there by narrow pa.s.sages, where some river or stream is flowing out.
For the polyps cannot live in fresh water.
Next, there are barrier reefs. These are great walls of coral at a distance from the sh.o.r.e, with deep water between the two. For the polyps are unable to work at a greater depth than about thirty fathoms, or one hundred and eighty feet, below the surface; and it often happens that while there is deep water close to the sh.o.r.es of a tropical island, there is shallow water farther out. In such a case the polyps have to build out at sea, instead of close into the land, and there is a kind of moat between the coral bank and the sh.o.r.e. In this case the bank is called a barrier reef, and sometimes it is of enormous size. The Great Barrier Reef, for instance, runs for no less than 1250 miles along the northeast coast of Australia.
Then, thirdly, there are coral islands, or atolls. There are thousands of these wonderful islands in the Pacific and the Indian oceans, and others are still being slowly pushed up out of the sea. They always take the form of more or less circular rings, in the center of which is a lake of sea-water called a lagoon. The coral bank of which they consist is seldom more than a few hundred feet wide, but sometimes the islands are very large indeed. The biggest of all is ninety miles long and sixty miles broad, while several others are not very much smaller. Soon after they rise to the surface of the sea a kind of soil is deposited upon them, made up partly of powdered coral, ground up by the action of the waves, and partly of decaying vegetable matter which has been flung up on them. Then sea-birds bring mud upon their feet from the mainland, or from another island at a distance, and leave some of it behind them when they settle down to rest; and in that mud are seeds of plants, which soon begin to sprout and grow. So in a very few years the island is covered with low vegetation. Then one day, perhaps, a floating cocoanut is flung up, and that, too, takes root and grows, so that in course of time there is a palm-tree. Other palm-trees, of course, follow; and the result is that the first glimpse which a traveler gets of a coral island is nearly always that of a row of palm-trees upon the horizon.
The simple corals live in almost all parts of the ocean. Some of them are occasionally dredged up off our coasts, and can live in very cold water. But the reef-builders are only found in warm seas, and are never found working far outside the boundaries of the tropics.
How wonderful it seems that tiny creatures such as these polyps, which really do not appear to be much more than little lumps of living jelly, should be able to build up these vast ma.s.ses of coral from out of the depths of the sea! One cannot help wondering what the results of their work will be if the world should last for a few thousand years longer.
It would really seem that by that time the tropical seas will be choked up with coral islands, and the lagoons inside them will be filled up with coral too; so that not merely islands, but continents, will have been raised from the ocean by some of the smallest and weakest and most insignificant of all living animals!
[Ill.u.s.tration: NORTH AMERICAN SEED-EATING SONG-BIRDS
1. Scarlet Tanager, or Black-winged Redbird. 2. Song Sparrow.
3. Baltimore Oriole. 4. Rose-breasted Grosbeak. 5. Cowbird.
6. Cardinal Grosbeak. 7. Purple Finch. 8. Indigo Finch.
All are adult males.]
WALKS WITH A NATURALIST
Suggestions for Teacher and Pupil in Nature Study
I
SPRING
Let us suppose that we are taking four country walks together, and trying to use, in actual experience in the field, the information we have been reading. The first shall be in the spring, the second in summer, the third when autumn leaves are falling, and the last in midwinter. We will go along the field-path, follow the lane through the woods to the creek, then down the stream to the road, and so homeward.
There is plenty to be seen, this bright spring morning. The birds are very busy, of course, for they have nests to build, and eggs to lay, and little ones to take care of; so they are hard at work from the very first thing in the morning till the very last thing at night. Almost every sparrow that we see has a feather or a piece of straw in its beak, and the robin which has just flown out of that tree with a terrified squall has already finished building, and was most likely sitting upon her eggs. Yes, there is her nest, you see, right on the lowest limb, with four greenish blue eggs.
See that catbird, all lead-color, with a black cap. See her dodge into that bush just beyond us. It is just the place for her nest; and, sure enough, here it is. It is a rough affair, but she mews as pitifully at us as if it were the finest of homes, and half a dozen other birds are already screaming their sympathy. Let us just look at the eggs, and remember that they are a deep polished green, and then walk on, for the poor mother is very unhappy. We have no use for the eggs, and it would be shameful to rob her; and, besides, we should thus destroy the coming lives of four catbirds, who will be too useful as insect-hunters in our gardens to be wasted.
This path is dusty, and we notice a great many pinhole doors of the little black ants. The ants are running in and out of them, and if we should carefully dig up the ground we would find a labyrinth of narrow pa.s.sages, here and there widening into chambers, and so learn that these tiny holes are entrances to an ant-city whose streets are all subways.
Here are some larger ants--three times as big--a regular procession of them going and coming out from under that half-buried stone, winding through the gra.s.s, and then trotting up and down this tree-trunk. A lot of them go out along that low limb. Let us climb upon the fence, and try to see what it is that attracts them. Ah! This is the secret. Cl.u.s.tered thickly on the bark are hundreds of minute green creatures, smaller than pinheads. They are busily sucking the sap from the bark, and seem to interest the ants greatly, for they are stroking these bark-lice (aphids) with their feelers, and if we had a magnifying-gla.s.s we could see that they were licking up a honey-like liquid which oozes out of two short tubes on the back of each aphid.
A little distance beyond the ant"s apple-tree a young maple stretches one of its branches out to the sunlight just above our heads, where the sharp eyes which young naturalists must keep wide open when they walk abroad will notice a bird"s nest hung under the shelter of its broad outermost leaves. It is one of the loveliest nests in the world. A slim, graceful, olive-green little bird glides out from beneath the maple-leaves as we approach, perches near by and watches us silently.
Though she does not mew and scream as did the catbird, she is just as anxious, you may be sure. Be easy, dear little vireo--for we know your name--we shall not ruin your home. Let us pull the branch gently down a little. Now we can see that the nest is a round hammock, woven of grapevine bark and spider-web, and hung by its edges. It seems too fragile to hold the weight of the mother, slight as she is; and in it are three white eggs with a circle of pink and purple dots around their larger ends. But here is also a fourth egg, much larger, grayish white, and speckled all over with brown.
That is the egg of the cowbird, a sort of purple, brown-headed blackbird which you may almost always see in pastures where there are cattle. The cowbirds, like the European cuckoos, never build any nests of their own, but put their eggs into those of other birds, and leave them to be hatched. And they are very fond of choosing the nest of a vireo. One would think that the mother would notice at once that a strange egg had been placed in her nest, and would throw it out. But she never seems to do so, but sits on the cowbird egg as well as on her own, so that in course of time she hatches out three or four little vireos and one young cowbird. Then what do you think the stranger does? Why, as soon as the mother vireo goes out to look for caterpillars for food, it begins to wriggle underneath the other little birds, and soon shoves them out of the nest, one after another. Still more strange is it, that when the vireo comes back she never seems to care that her own little ones are all lying dead on the ground below, but gives all the food that they would have eaten to the cowbird. And the greedy cowbird eats it all!
Until it is fledged she feeds it in this way, and takes the greatest care of it, and even after it has left the nest and is able to fly about she will come and put caterpillars into its beak.
Look at the trunk of this tree. Why has so much of the bark fallen away from the wood? And what is this curious pattern engraved, as it were, upon the wood--a broad groove running downward, and a number of smaller grooves branching out from this on each side?
Ah! that is the work of a very odd little beetle, with a black head and reddish-brown wing-cases. About eighteen months ago, probably, a mother beetle came flying along, settled on the tree, and bored a hole through the bark, just big enough for her to pa.s.s through. Then she began to burrow downward between the bark and the wood, cutting the central groove which you see in the pattern. As she did so she kept on laying eggs, first on one side of the groove and then on the other, in the short branch-tunnels, which she cut out as she went along. In this way she laid, perhaps, eighty or ninety eggs altogether. When the last had been laid she turned round, climbed up her burrow again, pa.s.sed into the hole by which she came in, and--died in it! And by so doing she blocked up her burrow with her dead body, and so prevented centipedes and other hungry creatures from getting in and eating up her eggs.
Early in the following spring all the eggs hatched, and out came a number of hungry little grubs with hard, h.o.r.n.y heads and strong, sharp little jaws. Every one of these grubs at once began to make a burrow of its own, boring away at right angles to the groove made by the mother beetle, and cutting away the fibers which bind the bark to the wood. The consequence was, of course, that by the time they were fully grown quite a big piece of bark had been cut away. And very likely if we were to come and look at the tree again in two years" time we should find that the whole of the trunk had been completely stripped.
"Then these little beetles are very mischievous?" Oh, no, they are not; for they never touch a healthy tree. They only attack those trees which are sickly or diseased.
Here we are on the banks of the stream. Let us make our way home by the path which lies beside it.
Ah! Did you see that flash of blue and white and orange that went darting by, almost like a streak of many-colored light, sounding a loud rattling call as he flew? It was a kingfisher, and if we stand quite still for a minute or two, without moving so much as a finger, we shall very likely see him again. Yes, there he is, sitting on that branch overhanging the stream, and peering down into the water beneath. He is watching for little fishes, upon which he feeds. There, he has caught sight of one, and down he drops into the water, splashes about for a moment or two, and then rises with a minnow in his beak. Back he flies to his perch, slaps the little fish against the branch once or twice to kill it, jerks it up into the air, catches it head foremost as it falls, and then swallows it with one big gulp. A moment later he is peering down into the water again on the lookout for another.
That hole in the face of the steep bank across the stream is the doorway of the kingfisher"s home. If we could get there, and should try to dig it, we would find it a hard task; for from that round door a tunnel runs into the ground probably six or eight feet, and ends in a chamber where lie half a dozen pure white eggs, resting upon the bones of fishes and sc.r.a.ps of every sort, which make a very ill-smelling place for the young kingfishers to be born in; but they do not mind that.
The b.u.t.terfly that has just floated by is a small tortoise-sh.e.l.l, and it has lived through the winter, which kills nearly all of the b.u.t.terfly tribe. That is why its wings are faded and chipped, for it had six or eight weeks of active life before it hid itself away, last of all, in a hollow tree, and entered upon a six-months" slumber. Sometimes, on a warmer day than usual, these and certain other b.u.t.terflies will be roused up, and will flutter about in the sunshine, so that now and then you may capture a tortoise-sh.e.l.l even in the Christmas holidays.
The warm May sunshine is enticing out many a minute insect--gnats and flies especially. Dancing companies of small sulphur yellow and other companies of blue b.u.t.terflies whirl about one another over the rapidly growing gra.s.s.
Have you noticed among the May flowers how many are yellow? There are dandelions, and yellow violets, and the modest fivefinger low in the herbage, while above them tower great tufts of wild mustard and indigo, the b.u.t.tercups, the marsh-marigold, and many another.
The frogs and toads are less noisy than a month ago, and one sees fewer ma.s.ses and strings of eggs in the roadside ditches than in April; but in their place the pools swarm with tadpoles, and it will be well worth your while to keep watch of their growth. Try to find out what they eat, and what eats them. Observe when the tail begins to disappear, and how it is lost; when the legs begin to appear, and which pair first shows itself. You may learn a lot of interesting facts about frogs and toads before the summer is done, if you are diligent.
In this stream are a few turtles. Can you tell when and where they lay their eggs? Keep careful watch of the little sandy beaches, and perhaps you may see one digging a hole in which to bury her set of sixty or so, leaving the sun to supply a better warmth than she could give them.
May is a month of activity for snakes. They have thrown off the stiffness and drowsiness of their long winter torpidity, and, grown thin after five months of fasting, are running about in search of food.
Let the frogs and toads, the beetles and young ground-sparrows and mice--also weak from their winter trials--take heed, for the swift blacksnake or sly garter, or rapacious water-snake will seize them before they have time to squeal!
The water in the stream is still cool, but the fishes are struggling up the current, pickerel are sp.a.w.ning in the weedy shallows, and among the pebbles of the bottom a host of young creatures are beginning to grow vigorous.
None among them is more active than the larval caddis-flies, or case-worms, as anglers call them. Here is a caddis-fly now, its gauzy wings folded tentwise over its back. All its earlier life was spent in the water, and when it was a grub it lived in a very curious case, which it made by cutting up a rush into short lengths, and sticking them together by means of a kind of natural glue. When once a caddis-grub has made one of these cases it never gets out of it again, but drags it about wherever it goes. And if you try to pull it out you will find that you cannot do so without killing it. For at the end of its body it has a pair of strong little pincers, with which it holds on so firmly and so doggedly to its case that you might actually pull it in two without forcing it to loose its hold.
There are different kinds of caddis-flies, however, and the grub of one kind fastens grains of sand together to make a case, while that of another sticks two dead leaves face to face, and lives between them.
It would be an interesting task for a boy or girl to see how many different kinds of caddis-flies, judging by their cases, lived in the stream, and to keep them alive in an aquarium, and watch their behavior and changes.
II
SUMMER