THE RECORDS OF A NATURALIST ON THE AMAZONS.*
'THOSE to whom the death of Henry Bates suggests a re- perusal of his great work, will probably be slow to disagree with the judgment passed by the late Charles Darwin, who de- clared it the best book of natural-history travels ever written. If not the best—and with such rivals as his companion Mr. Wallace's "Malay Peninsula," and Mr. Belt's "Naturalist in Nicaragua," or "The Voyage of the 'Beagle,'" by Darwin himself, it is difficult to lay down an order of merit—it must in any case remain among the best.
Though he might at any time have returned to his native country, the author lingered among the woods and waters of the Amazons Valley, till his love for the tropical forest grew into a passion ; and when leaving it for ever, he recorded his fixed opinion, that "although humanity can reach an advanced state of culture only by battling with the inclemencies of Nature in high latitudes, it is under the Equator alone that the perfect race of the future will obtain complete fruition of man's beautiful heritage, the earth." Eleven years spent in a region for which he claimed that the "balanced forces of Nature there produce a climate and land surface which seem typical of mundane order and beauty," unremitting industry and observa- tion, and a constructive mind, which could distinguish among the bewildering analogies of multiform nature a uniform and controlling law, were among the qualifications which con- tributed to the excellence of his first book, and of the subsequent treatise in which he illustrated, and gave the first probable explanation of, his discoveries of the strange phenomena • Records al a Naturalist on the Amazons. By H. W. Bates. Illustrated. London : John Murray. of the mimicry of living creatures by opposite or wholly different species.
Among the most beautiful features of the forest were myriads of brilliant butterflies, of the family called Heliconidm. "The pathways in the woods near the towns are often quite alive," he writes, "with the multitudes which fly among the lower trees, in their bright dresses of orange, blue, yellow, red, and black." But the extraordinary fact which attracted his attention was not so much the numbers or the beauty of these gorgeous insects, as that, among the sixty-four species which he discovered, a great number were the objects of imitation in external appearance, shapes, and colours by other butterflies of quite different kinds, and were always accompanied in the particular parts of the forest in which they lived by these counterfeits. He soon concluded that the Heliconida were the models, and not the copies,—first, because they did not differ from the common type of their own family, which the copyists were forced to do, and secondly, because the counterfeits were in fewer numbers and a weaker race. "These imitative resemblances," he wrote, "show a minute and palpably inten- tional likeness which is perfectly staggering." In some, the resemblance was carried out even to the colours of the antennw and the spots upon the abdomen ; and the imitation was so close, especially when the creatures were on the wing, that he constantly caught the counterfeits instead of the models. He further found that these strange mimicries were not confined to the butterflies. On the banks of the Amazons were "cuckoo- flies," wearing the exact livery of working-bees in order to rob them of their honey; and, by a strange inversion of the general order which he subsequently discovered, insects were found imi- tating the very predaceous creatures from which it is their object to be concealed,—the Scaptura crickets mimicking exactly the large sand-wasps which are constantly searching for these crickets with which to provision their nests. For the strangest and most suggestive instance of this mimetic analogy, we will quote the naturalist's own description :—" A very large caterpillar stretched itself from the foliage of a tree which I was examining, and startled me by its resemblance to a small snake. The first three segments behind the head were dilateable at the will of the insect, and had on each side a large black pupillated spot which resembled the eye of the snake. It was a poisonous, or viperine species mimicked, and not an innocuous snake. This was proved by the imitation of keeled scales on the crown, which was produced by the recum- bent feet as the caterpillar threw itself backward. I carried off the caterpillar, and alarmed every one in the village where I was then living to whom I showed it." Con- fronted with these strange instances of mimetic analogy, the naturalist was not slow to form a theory both of its purpose and of the means by which such extraordinary and highly specialised changes to a new form or colour, could be produced. Speaking generally, he declared the phenomena to be instances of adaptation, either of the whole outward dress or of special parts, having in view the welfare of the creature that possesses them ; and in the particular instance of the butterflies, that the change in the copyist had for its object the protection of the copy by assuming the appearance of the model, as a pigeon might expect to gain protection by assuming the plumage of a hawk. It may be asked, what protection could be secured for one butterfly simply by assuming the appearance of another butterfly I' The answer is, that the species imitated is generally one distasteful to birds or insect-eating animals, either on account of its hardness or, as in the case of the frogs in Nicaragua observed by Mr. Belt, of its disagreeable smell. "The Heliconidx," says Bates, "have a peculiar smell. I have never seen the flocks of slow- flying Heliconicke persecuted by birds or dragon-flies, to which they would have been an easy prey." The others were, there- fore, protected against these birds and dragon-flies by assuming a dress which suggested that, like the Heliconidx, they were tough and evil-smelling. It is now known that in Africa numbers of the swallow-tailed butterflies alter the well- known shape of their wings, and assume the narrow wings and exact colours of the Eupleinz, a race protected, like the Heliconidle, by unpleasant odours. But perhaps the most universally mimicked insect is the Chrysippus, a handsome, bright, tawny butterfly, with white spots on its wings, which is found in Asia, Africa, Australia, and in Greece, and is thought to be protected in the same way as the Heliccrnidae, by its " bony " structure and odour. The protection so afforded is sought by many different species both of butterflies and moths, in some of which the females mimic the colours of the Chrysippus, while the males do not ; the female of one species in Africa counterfeits the Chrysippus, while in Madagascar it retains its primitive form. A third species mimics the Chrysippus in India, but not in Australia. Such curious differences suggest the need of further careful observation of the habits of the creatures, without which such apparent contradictions cannot be explained. But it seems impossible to doubt that the conclusion at which Bates arrived as to the object of these imitations is the true one. English moths that fly by day and sit on flowers, imitate exactly the clear wings and downy-belted bodies of humble-bees. Another, the Sphecia moth, is an exact model of the hornet, and as it sits on the bark of poplar-trees in the spring, is constantly mistaken and avoided on this account. A leading entomolo- gist once declared to us that, though it was impossible that he could really be mistaken, he never handled the counterfeit hornet without reluctance. The imaginary sting saves both these creatures from human molestation. There is a common fly, which may often be seen on garden flowers, which is an almost exact imitation of the hive-bee. Another mimics the humble-bee, and so enters its burrow and lays its eggs, the larvce from which feed upon the 'arm of the bee. In the Central Hall of the Natural History Museum at South Kensington, where the statue of Darwin sits musing above the illustrations of his laws of development which are there presented to the eye in con- crete form, these counterfeit bees and moths may now be seen, with the Eupleinw butterflies and their imitators, and other strange instances of protective resemblance. Birds also appear among the list of animal counterfeits. In India, there is a small black crow, with a long forked tail. This is imitated by a weaker creature of wholly different kind, a black cuckoo. Both species are shown in the same case as the counterfeiting butterflies and mock-bees. Another cuckoo has arrived at the Zoological Gardens which imitates a ground-pheasant. In the Island of Bourn, in the Moluccas, Mr. Wallace discovered an oriole which had abandoned the gay black and yellow plumage of its family, and counterfeited exactly the colours and shape of a " honeysucker." The object was clear ; for the "honey- suckers," according to Mr. Wallace, are active and powerful birds, with strong claws, sharp beaks, and loud, bawling voices, and being plentiful and pugnacious, drive away the crows and hawks ; the oriole thus obtains protection by being mistaken for the stronger bird. In another island, a different oriole was found which imitated another kind of honeysucker.
In the absence of complete knowledge of the habits of the birds which show these mimetic analogies, it is too early to speak with confidence of the causes to which they are due. Bat in the case of the insects, it is difficult to disagree either with Bates's explanation of their object, or the confidence with which he saw in the process which pro- daces these resemblances a highly specialised instance of the law of natural selection, the selecting agents being the insectivorous creatures which gradually kill off all that do not acquire protection by mimicry ; while according to the closeness of its persecution by enemies, which seek the imitator but avoid the imitated, will be its tendency to become an exact counterfeit.
The determination of this natural law was the last contribution of Mr. Bates to the larger questions of natural history. Daring his later years he devoted himself mainly to his work as Secretary of the Royal Geographical Society, contenting himself, so far as his old pursuits went, with collecting and studying certain pet families of beetles, which collections, we may mention, were not, as asserted in a daily paper, offered to, and refused by, the authorities of the Zoological Museum.