Between England and the West Indies, about the time when one begins to recover from the first bout of sea-sickness, we come upon a certain sluggish tract of ocean, uninvaded by either Gulf Stream or arctic current, but slowly stagnating in a sort of endless eddy of its own, and known to sailors and books of physical geography as the Sargasso Sea. The sargasso or floating seaweed from which it takes its poetical name is a pretty yellow rootless alga, swimming in vast quantities on the surface of the water, and covered with tiny bladder-like bodies which at first sight might easily be mistaken for amber berries. If you drop a bucket over the ship's side and pull up a tangled mass of this beautiful seaweed, it will seem at first to be all plant alike; but, when you come to examine its tangles closely, you will find that it simply swarms with tiny crabs, fishes, and shrimps, all coloured so precisely to shade that they look exactly like the sargasso itself. Here the colour about is less uniform than in the arctic snows, but, so far as the sargasso-haunting animals are concerned, it comes pretty much to the same thing. The floating mass of weed is their whole world, and they have had to accommodate themselves to its tawny hue under pain of death, immediate and violent.

Caterpillars and butterflies often show us a further step in advance in the direction of minute imitation of ordinary surroundings. Dr. Weismann has published a very long and learned memoir, fraught with the best German erudition and prolixity, upon this highly interesting and obscure subject. As English readers, however, not unnaturally object to trudging through a stout volume on the larva of the sphinx moth, conceived in the spirit of those patriarchal ages of Hilpa and Shalum, when man lived to nine hundred and ninety-nine years, and devoted a stray century or so without stint to the work of education, I shall not refer them to Dr. Weismann's original treatise, as well translated and still further enlarged by Mr. Raphael Meldola, but will present them instead with a brief résumé, boiled down and condensed into a patent royal elixir of learning. Your caterpillar, then, runs many serious risks in early life from the annoying persistence of sundry evil-disposed birds, who insist at inconvenient times in picking him off the leaves of gooseberry bushes and other his chosen places of residence. His infant mortality, indeed, is something simply appalling, and it is only by laying the eggs that produce him in enormous quantities that his fond mother the butterfly ever succeeds in rearing on an average two of her brood to replace the imago generation just departed. Accordingly, the caterpillar has been forced by adverse circumstances to assume the most ridiculous and impossible disguises, appearing now in the shape of a leaf or stem, now as a bundle of dark-green pine needles, and now again as a bud or flower, all for the innocent purpose of concealing his whereabouts from the inquisitive gaze of the birds his enemies.

When the caterpillar lives on a plant like a grass, the ribs or veins of which run up and down longitudinally, he is usually striped or streaked with darker lines in the same direction as those on his native foliage. When, on the contrary, he lives upon broader leaves, provided with a midrib and branching veins, his stripes and streaks (not to be out of the fashion) run transversely and obliquely, at exactly the same angle as those of his wonted food-plant. Very often, if you take a green caterpillar of this sort away from his natural surroundings, you will be surprised at the conspicuousness of his pale lilac or mauve markings; surely, you will think to yourself, such very distinct variegation as that must betray him instantly to his watchful enemies. But no; if you replace him gently where you first found him, you will see that the lines exactly harmonise with the joints and shading of his native leaf: they are delicate representations of the soft shadow cast by a rib or vein, and the local colour is precisely what a painter would have had to use in order to produce the corresponding effect. The shadow of yellowish green is, of course, always purplish or lilac. It may at first sight seem surprising that a caterpillar should possess so much artistic sense and dexterity; but then the penalty for bungling or inharmonious work is so very severe as necessarily to stimulate his imitative genius. Birds are for ever hunting him down among the green leaves, and only those caterpillars which effectually deceive them by their admirable imitations can ever hope to survive and become the butterflies who hand on their larval peculiarities to after ages. Need I add that the variations are, of course, unconscious, and that accident in the first place is ultimately answerable for each fresh step in the direction of still closer simulation?

The geometric moths have brown caterpillars, which generally stand erect when at rest on the branches of trees and so resemble small twigs; and, in order that the resemblance may be the more striking, they are often covered with tiny warts which look like buds or knots upon the surface. The larva of that familiar and much-dreaded insect, the death's-head hawk-moth, feeds as a rule on the foliage of the potato, and its very varied colouring, as Sir John Lubbock has pointed out, so beautifully harmonises with the brown of the earth, the yellow and green of the leaves, and the faint purplish blue of the lurid flowers, that it can only be distinguished when the eye happens accidentally to focus itself exactly upon the spot occupied by the unobtrusive caterpillar. Other larvæ which frequent pine trees have their bodies covered with tufts of green hairs that serve to imitate the peculiar pine foliage. One queer little caterpillar, which lives upon the hoary foliage of the sea-buckthorn, has a grey-green body, just like the buckthorn leaves, relieved by a very conspicuous red spot which really represents in size and colour one of the berries that grow around it. Finally the larva of the elephant hawk-moth, which grows to a very large size, has a pair of huge spots that seem like great eyes; and direct experiment establishes the fact that small birds mistake it for a young snake, and stand in terrible awe of it accordingly, though it is in reality a perfectly harmless insect, and also, as I am credibly informed (for I cannot speak upon the point from personal experience), a very tasty and well-flavoured insect, and 'quite good to eat' too, says an eminent authority. One of these big snake-like caterpillars once frightened Mr. Bates himself on the banks of the Amazon.

Now, I know that cantankerous person, the universal objector, has all along been bursting to interrupt me and declare that he himself frequently finds no end of caterpillars, and has not the slightest difficulty at all in distinguishing them with the naked eye from the leaves and plants among which they are lurking. But observe how promptly we crush and demolish this very inconvenient and disconcerting critic. The caterpillars he finds are almost all hairy ones, very conspicuous and easy to discover—'woolly bears,' and such like common and unclean creatures—and the reason they take no pains to conceal themselves from his unobservant eyes is simply this: nobody on earth wants to discover them. For either they are protectively encased in horrid hairs, which get down your throat and choke you and bother you (I speak as a bird, from the point of view of a confirmed caterpillar eater), or else they are bitter and nasty to the taste, like the larva of the spurge moth and the machaon butterfly. These are the ordinary brown and red and banded caterpillars that the critical objector finds in hundreds on his peregrinations about his own garden—commonplace things which the experienced naturalist has long since got utterly tired of. But has your rash objector ever lighted upon that rare larva which lives among the periwinkles, and exactly imitates a periwinkle petal? Has he ever discovered those deceptive creatures which pretend for all the world to be leaves of lady's-bedstraw, or dress themselves up as flowers of buttonweed? Has he ever hit upon those immoral caterpillars which wriggle through life upon the false pretence that they are only the shadows of projecting ribs on the under surface of a full-grown lime leaf? No, not he; he passes them all by without one single glance of recognition; and when the painstaking naturalist who has hunted them every one down with lens and butterfly net ventures tentatively to describe their personal appearance, he comes up smiling with his great russet woolly bear comfortably nestling upon a green cabbage leaf, and asks you in a voice of triumphant demonstration, where is the trace of concealment or disguise in that amiable but very inedible insect? Go to, Sir Critic, I will have none of you; I only use you for a metaphorical marionette to set up and knock down again, as Mr. Punch in the street show knocks down the policeman who comes to arrest him, and the grimy black personage of sulphurous antecedents who pops up with a fizz through the floor of his apartment.

Queerer still than the caterpillars which pretend to be leaves or flowers for the sake of protection are those truly diabolical and perfidious Brazilian spiders which, as Mr. Bates observed, are brilliantly coloured with crimson and purple, but 'double themselves up at the base of leaf-stalks, so as to resemble flower buds, and thus deceive the insects upon which they prey.' There is something hideously wicked and cruel in this lowest depth of imitative infamy. A flower-bud is something so innocent and childlike; and to disguise oneself as such for purposes of murder and rapine argues the final abyss of arachnoid perfidy. It reminds one of that charming and amiable young lady in Mr. Robert Louis Stevenson's 'Dynamiter,' who amused herself in moments of temporary gaiety by blowing up inhabited houses, inmates and all, out of pure lightness of heart and girlish frivolity. An Indian mantis or praying insect, a little less wicked, though no less cruel than the spiders, deceives the flies who come to his arms under the false pretence of being a quiet leaf, upon which they may light in safety for rest and refreshment. Yet another abandoned member of the same family, relying boldly upon the resources of tropical nature, gets itself up as a complete orchid, the head and fangs being moulded in the exact image of the beautiful blossom, and the arms folding treacherously around the unhappy insect which ventures to seek for honey in its deceptive jaws.

Happily, however, the tyrants and murderers do not always have things all their own way. Sometimes the inoffensive prey turn the tables upon their torturers with distinguished success. For example, Mr. Wallace noticed a kind of sand-wasp, in Borneo, much given to devouring crickets; but there was one species of cricket which exactly reproduced the features of the sand-wasps, and mixed among them on equal terms without fear of detection. Mr. Belt saw a green leaf-like locust in Nicaragua, overrun by foraging ants in search of meat for dinner, but remaining perfectly motionless all the time, and evidently mistaken by the hungry foragers for a real piece of the foliage it mimicked. So thoroughly did this innocent locust understand the necessity for remaining still, and pretending to be a leaf under all advances, that even when Mr. Belt took it up in his hands it never budged an inch, but strenuously preserved its rigid leaf-like attitude. As other insects 'sham dead,' this ingenious creature shammed vegetable.

In order to understand how cases like these begin to arise, we must remember that first of all they start of necessity from very slight and indefinite resemblances, which succeed as it were by accident in occasionally eluding the vigilance of enemies. Thus, there are stick insects which only look like long round cylinders, not obviously stick-shaped, but rudely resembling a bit of wood in outline only. These imperfectly mimetic insects may often obtain a casual immunity from attack by being mistaken for a twig by birds or lizards. There are others, again, in which natural selection has gone a step further, so as to produce upon their bodies bark-like colouring and rough patches which imitate knots, wrinkles, and leaf-buds. In these cases the protection given is far more marked, and the chances of detection are proportionately lessened. But sharp-eyed birds, with senses quickened by hunger, the true mother of invention, must learn at last to pierce such flimsy disguises, and suspect a stick insect in the most innocent-looking and apparently rigid twigs. The final step, therefore, consists in the production of that extraordinary actor, the Xeroxylus laceratus, whose formidable name means no more than 'ragged dry-stick,' and which really mimics down to the minutest particular a broken twig, overgrown with mosses, liverworts, and lichens.