They dream. They dream of Djelfa. They dream of the tents of the tribe, the fires of the families, the mountains of the Ouled Naïl. They dream of the mountains all green.

For it has rained in Djelfa. It has rained everywhere everywhere but in the Mzab. The winter has spent itself in one last thundering blow. At Berrian, only a finger width on the map to the north, all the barrages are full to breaking and all the fields in the oasis are ponds. Laghouat is an island in a lake.

The Jew who has to do with diligences puts away his coffee and holds up his hands.

"Two days. Two weeks. Only God (the God of the Christian, the Mohammedan, and the Jew) can say."

Out of doors the moon, grown lazy in service, is just rising over Beni-Isguen hill. Its light runs up the sky, but even in the light the stars burn on. There is no veil. Heaven is glass.

Another dry year has come to the Mzab. “In-cha-'llah! If God wills!"

WHEN

NE of the many ways in which

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animals have answered back to the difficulties ever besetting them has been to take advantage of some sort of disguise. It has often paid them to look like something else, and so we find many examples of "masking," or "camouflaging," of protective resemblance to something innocent, and even of a faking which, whatever may be its inner aspect, is from the outside uncommonly near impersonation. Just as a sniper may make up like a shattered tree or a dead horse, just as a ship may be "camouflaged" into the likeness of a wreck or a rock, so, many different kinds of animals are advantageously disguised, or have their real nature in some way or other concealed.

It is difficult to draw a strict line between coverings which make for physical protection or for comfort, and coverings that serve for concealment. The young stages of caddis-flies which creep hungrily about among the stones in the bed of the stream have encasements of small pieces of stick or of tiny pebbles, and they look innocent enough. The leafeating caterpillars, called "bagworms," carry about a protective "over-all" made of pieces of leaf, stick, bark, and debris. In many cases this bag makes them most elusively inconspicuous. When they pass into the resting chrysalis stage they often hang from the branches like cones or dry fruits. Some allied forms make cases deceptively like the shells of small snails. The cocoons or pupa cases of certain insects are like pods; the large, substantial structure made by the caterpillar of the puss-moth is often extraordinarily like the corner which has been chosen as a retreat; the North American bagworm, another remarkable caterpillar, spins its cocoon on

leaves and twigs, and so adjusts the surrounding parts, sometimes killing them first, that, although the cocoon is exposed, it has a high degree of invisibility. To bring about withering artificially, on the shoot of an orange tree, for instance, is certainly a very remarkable device. But one does not mean by this word to suggest that the caterpillar is aware of what it has actually achieved.

Just as lichens grow on trees, so there are many water plants and sedentary animals which anchor themselves on creatures like crabs, and cloak their real nature. Thestriking photograph of Fig. 1 shows a crab with a quite extraordinary agglomeration of animals on its back. To appreciate the subtlety of the disguise, however, one must see the animals in their natural colors and in their own rock pool. One rubs one's eyes, as they say, when what looks like a bunch of seaweed suddenly starts on an exploring excursion. Now, it must be admitted that it is very difficult to draw a firm line between cases where the incrusting animals have simply settled down on their bearers as they might on a stone or on a piece of rock, and cases where the bearers derive real benefit from the association, and are perhaps dimly aware of the fact. This is true all through animate nature, that different kinds of associations between living creatures grade into one another. Let us take an example. Some trees. have neutral or indifferent molds about their roots; in certain cases this becomes an important partnership, which may indeed go too far, when the big partner begins to depend too much on the activity of the self-effacing partner underground. These things are an allegory. Many plants are attacked by parasitic

bacteria; in peas and clovers and the like this becomes a valuable co-operation, by means of which the leguminous plants are somehow able to utilize the free nitrogen in the atmosphere.

Similarly, while many incrusting growths on marine animals must be regarded as fortuitous and indifferent, every now and then the note of utility is struck. Many hermit-crabs which in the course of time have come to be constantly associated with the shells of periwinkles, whelks, and other mollusks, in which their soft tail is protected, have this borrowed shell covered with a growth of innocent-looking sponge or zoophyte. As hermit-crabs are voracious and combative, and must have something remotely, but really, comparable to a bad reputation among shore animals, the usefulness of an innocent-looking cloak is obvious. In our shore pools we often see a hermit-crab inside a whelk shell which is almost quite covered by a growth of a beautiful colony of polyps called Hydractinia. In some Japanese instances of this association there seems to be no mollusk shell at all, the Hydractinia by itself forming a sort of protective basket for the crustacean. species the basket is very spiny and is sold as a curiosity under the name "Igaguri-dai," or "Chestnut-burr-shell." This association of Hydractinia and hermit-crab is probably a very useful masking, but it must be confessed that there are cases where the association is positively disadvantageous to the bearer. Thus, a full-grown crab, which has ceased to molt its shell, has occasionally a burden of rock barnacles and other creatures weighing more than itself (see Fig. 1). This must be a serious handicap to an active animal. We do not know what to make of the rare case of a common lobster literally festooned on body and limbs with over a dozen long fronds of seaweed.

In one

In the Firth of Clyde in Scotland, and elsewhere, the dredge or trawl often brings up numerous specimens of a very interesting association between a hermit

crab and an orange-colored sponge called Suberites domuncula, which is never found anywhere else. The free-swimming sponge larva settles down on the shell which the hermit crab, Pagurus, has borrowed; it grows into a thick incrusting mass and surrounds the whole shell except the aperture through which the hermit protrudes and retracts himself; it grows much bigger than the mollusk shell, and this is very useful to the tenant because he has not to flit, as others of his kind have, when he becomes too big for his house. The sponge has a strong odor, and its body is crammed with flinty needles. It is probable, therefore, that to these qualities of its partner the hermit-crab may owe some protection. Experiments have shown, for instance, that some fishes will have nothing to do with the sponge, even when they are very hungry. Whether Suberites is benefited by the association it is difficult to say; perhaps it reaps some advantage from being carried about from place to place by its crustacean bearer. We have taken this case here because it seems to be on the border line between protective disguise and the mutually beneficial partnerships called commensalism, to which we shall refer later.

Stiff classifications should not be pressed when we are dealing with habits, for different devices shade into one another. Take the case of the larvæ of the beautiful lacewing flies (Chrysopa) which are of service in destroying large numbers of "green fly." Some kinds cover themselves with the sucked bodies of their victims united by waxy material. If the clothing be stripped off, the lacewing larvæ will stick on tiny fragments of paper or lichen if they can get nothing else. This looks like masking, but the utility of the investment is obscure. And what are we to make of the "cuckoospit," the frothlike, soapy material (Fig. 2) so familiar on wayside plants in early summer, which is made by the larvæ of froghoppers? It seems to protect them from almost all enemies; it also keeps them moist in the heat of the day.

Deserving a place by itself is the disguise seen in the shaggy tree sloths (Fig. 3) of Brazil and Guiana, which live an exclusively arboreal life, moving about, back downward, among the branches. The hair is rough and shaggy and on its surface there lives, strange to say, a minute green alga, like that which makes tree stems green in damp weather. The net result is to give the sloth a garment of invisibility among the green boughs. Some travelers have pointed out that the long greenish hair is quaintly like certain forms of a plant called Tillandsia, belonging to the pineapple order, which grow on South American trees and are popularly known as "vegetable horsehair." But it has not been shown that the shaggy sloths and the shaggy Tillandsia occur on the same trees, and the case of the sloth's masking is striking enough without adding any doubtful embellishment.

A deeper note is struck when an animal takes an active interest in its own disguise. It does not seem to have been proved that a hermit-crab ever picks out a live whelk or "buckie" from a shell

VOL. CXLIV.-No. 863.-82

that "strikes its fancy," though it may clean up a shell whose tenant has been half eaten by a codfish. But there is no mistaking its keen anxiety when it has to shift its quarters from a smaller to a larger house. In the case of the robbercrab (Birgus), which seems to be descended from the hermit-crab race, there is no borrowed shell, for the creature lives, except at the breeding time, far from the sea, climbing mountains and even trees, and the exposed surface of the tail is hard. Yet there is an interesting hint of a deeply rooted racial instinct in the way this quaint animal will sometimes tuck its tail under logs or into holes about the roots of trees. It has been said to make occasional use of a broken coconut, but those that are abundant on Christmas Island never carry any protective covering. Nearer home, however, there are not a few shore crabs which sometimes, at least, mask themselves. More than one naturalist has seen a crab seize a frond of seaweed, cut off a piece, nibble the end of it, and rub it against its body till it gets fixed on the projecting curved bris