Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/359

Rh composed of three commissioners and the, all subject to the common council. A bountiful is obtained from, and the  are well supplied with. The value of as  for  was 62,000,000 in ,—the   being 2,500,000, mostly for the, which are. There is an efficient system of s under a superintendent and board of school commissioners, the value of the with their sites being estimated at 700,000. For the higher there are a, a normal school, and three  s, while the  and  have several excellent al  and s. A   belonging to the  contained 20,000  in. (undefined)  MIMICRY is the name given in biology to the advan tageous resemblance (usually protective) which one species of animal or plant often shows to another. The word was first applied in this metaphorical sense by Mr W. H. Bates, and it has since been accurately denned and limited, in its biological application, by Mr A. R. Wallace. Briefly put, the essence of the phenomenon of mimicry consists in the following relation. A certain species of plant or animal possesses some special means of defence from its enemies, such as a sting, a powerful and disagreeable odour, a nauseous taste, or a hard integument. Some other species inhabiting the same district or a part of it, and not itself provided with the same special means of defence, closely resembles the first species in all external points of form and colour, though often very different in structure and unrelated in the biological order. For example, a South- American family of butterflies, the Heliconidse, are distin guished by their very varied and beautiful colours, and their slow and weakly flight ; they might easily be captured by insectivorous birds, but their remains are never found on the ground amongst the rejected wings of other butterflies which cover the soil in many places. They also possess a strong pungent odour, which clings to the fingers for many days ; and this fact led Mr Wallace to suspect that they have a disagreeable taste, and would not there fore be eaten by birds after a single trial. Mr Belt has since experimentally proved the truth of that belief. But among the totally distinct family of the Pieridx, most of which are white, there is a genus of small butterflies, known as Leptalis, edible by birds, some species of which are white like their allies, while the greater number exactly resemble one or other of the Hdiconidx in the peculiar shape and colouring of their wings. As regards structure, the two families are widely different ; yet the resemblance of a species of one family to a species of the other is often so close that Mr Bates and Mr Wallace, experienced entomologists, frequently mistook them for one another at the time of capture, and only discovered their mistake upon nearer examination. Mr Bates observed several species or varieties of Leptalis in the Amazons valley, each of which more or less exactly copied one of the Heliconidx in its own district. Accordingly, they seem to be mistaken by birds for the uneatable insects they mimic, and so to be benefited by their resemblance. This, which may perhaps be regarded as the most typical instance of true mimicry, is also the first to which the word was applied. In considering the phenomena under review, it may be well to give first the chief observed facts, which are quite independent of any particular explanation, and then the theory which has been started to account for them by Mr Bates and Mr Wallace. Before doing so, however, true mimicry should be carefully discriminated from one or two superficially similar modes of resemblance among organic beings, whose real implications are very different. It must not be confused with mere accidental or adaptive resemblance, due either to simple chance or to similarity of external conditions. As a case of the first sort, we may adduce the real or fancied resemblance between certain orchids and flies or spiders ; as a case of the second sort, we may take certain African jEuphorbiaceae, which, growing in dry deserts, have acquired a very close likeness to the cactuses that cover the equally dry deserts of Mexico ; or again the sub-Antarctic gallinaceous bird, Chionis alba, which, living on the sea-shore, has acquired a coloration like that of the gulls, together with the legs of a wader. These resemblances, however, do not as such subserve any function. The species apparently mimicking and the species apparently mimicked either do not inhabit the same district or do not come into any definite relation with one another. The likeness is either accidental, or else it is due to similar adaptation to similar circumstances. In cases of true mimicry, on the other hand, the mimicking species derives a direct advantage from its likeness to the species mimicked ; the resemblance is deceptive ; and this is equally true whether we suppose the mimicry to be pro duced by creative design or by natural selection. On either hypothesis, however it came by its likeness, the mimicking species escapes certain enemies or obtains certain sorts of food by virtue of its resemblance to some other kind. It should also be added that the word mimicry, as applied to such cases, is used only in a metaphorical sense. It is not intended to imply any conscious or voluntary imitation by one species of the appearance or habits of another. All that is meant is the fact of an advantageous resemblance, a delusive similarity, which gives the mimicking animal or plant some extra protection or some special means of acquiring food which it would not otherwise have pos sessed but for its likeness to the creature mimicked. Taking animals first, mimicry does not occur very frequently among the higher classes. In the vertebrates it is comparatively rare, and among mammals probably only one good case has yet been adduced. This is that of Cladobates, an insectivorous genus of the Malayan region, many species of Avhich closely resemble squirrels in size, in colour, and in the bushiness and posture of the tail. It has been suggested by Mr Wallace (from whom most of the following examples have been borrowed) that Cladobates may thus be enabled to approach the insects and small birds which form its prey under the disguise of the harmless fruit-eating squirrel. In this case, as in some others, the resemblance is not protective, but is apparently useful to the animal in the quest for food. Among birds, Mr Wallace has pointed out that the general likeness of the cuckoo, a weak and defenceless group, to the hawks and gallinaceous tribe makes some approach to real mimicry. But besides such vague resem blances there are one or two very distinct cases of true mimicry in this class of vertebrates. In Australia and the Moluccas lives a genus of dull-hued honey-suckers, Tropidorhynchus, consisting of large, strong, active birds, with powerful claws and sharp beaks. They gather together in noisy flocks, and are very pugnacious, driving away crows and even hawks. In the same countries lives a group of orioles, forming the genus Mimeta ; and these, which are much weaker birds, have not the usual brilliant colouring of their allies the golden orioles, but are usually olive-green or brown. In many cases species of Mimeta closely resemble the Tropidorhynchi inhabiting the same island. For example, on the island of Bouru are found the Tropidorhynchus bouruensis and Mimeta bouruensis, the latter of which mimics the former in the particu lars thus noted by Mr Wallace : &quot; The upper and under surfaces of the two birds are exactly of the same tints of dark and light brown. The Tropidorhynchus has a large 