Page:The New International Encyclopædia 1st ed. v. 13.djvu/588

* MIMICRY. 528 colors permit them to be recognized and avoided, as obnoxious. If any other butterflies in the same region were to become indistinguishable from the Heliconida; they would profit by a cor- MIMICRY. MIMICKY IN IN8KCTS. a. A li'iit-likc uraxxt'opP iPI'yl- lium sitTifoliurn) : h, a iiiantiB (a cantliopMl whU'li U'etlH on inst'cts foiinii aiiionff (Ir.v Icavt's. atid ia beiiellUHl by re[*f inblilifj Ihem. MIMICBV IN BUTTEKFLfes. a, Metbona psiiJii (Heliconidse),- ^, Lrptalis orlse (Pie- lidie). responding iniiiiunity from attack. The theory assumes that some" of the PieridiS happened at the start to resemble the primitive HelieoiiidiP and received a partial immunity as a result: that such PieridiE alone survived and produced de- scendants of like character ; and that a selection of the most heli- c 11 n i d-1 i k e of tlicse followed, r.y a continua- tion (if this proc- ess the pierids and the other Ij e )) i d o ]) t c r a gained their present close re- semblance to the Heliconidae. The theory is a broad one, an<l accounts for cases of mim- icry in other groups of Lepidoptera as well as in otlier orders of animals. It is not necessaiy. how- ever, to go to South America for examples of mim- icry. In North America, as well as in Asia and Africa, occurs the genus Danais, which is also a protected form. Our common American species Aiwsid iilcj-ippua is closely mimicked by Unsil- archin disiiiinix, a buttcrllv of rather remote alhnities. Three genera of Danaidic in trojiical Asia. Kuphra. Danais. and Hestia. are verv dif- ferent, but are all protected. In each genus cer- tain species are mimicked with extraordinary nc- curai'.v by sju'cics of the genus Papilio. Hut mimii'ry is not conlini'd to the Lepidoptera. Especially widl protected wasps and bees have many imitators, and there are cases of mimicry even in vertebrates. JIuLLER'.s TiiKORY Of MiMlCBY. In 1870 Fritz oHiiUer. as the result of many years' obsenat ion in Southern Hra/.il, proposed a mudification of and addition to the foregoing ex))lanation of Rates. Bates himself, when first describing the cases ho observed, bad suggested that they might he due to some forms of parallel variation depend- ent on climatic influences, and Wallace {Island Life. p. 2.>5) adduced other cases of coincident lo- cal niodilications of color, which did not appear to be cx]dicablc by any form of mimicry, iliiller's theory is founded on the assum]>tion that insect- eating birds only learn when young and by ex- perience to distinguish the edible from the inedible butterflies, and in doing so necessarily sacrifice a certain number of distasteful butter- flies. " Now," says Miiller. "if two distasteful species are sufficiently alike to be mistaken for one another, the experience aciiuired at the ex- pense of one of them will likewise benefit the other: both species together will only have to contribute the same number of victims which each of them would have to furnish if they were ditl'crent. If both species are equally connnon, then both will derive the same benefit from their resemblance — each will save half the number of victims which it has to furnish to the inexjieri- cnce of its foes. But if one species is commoner than the other, then the benefit is incqually di- vided, and the proportional advantage for each of the two species which arises from their re- .semblance is a» the Sfjunre of their relative num- bers." Wallace, who fully accepts Miiller's theory, in his statement of the theory {Par- uinism, p. 2.5.3) adds: "But if the two species are very une(pial in numbers, the benefit will be comparatively slight for the more aliundant spe- cies, but very great for the rare one. To the ' latter it ma.v make all the difTerence between safety and destruction." The facts of mimicry are very remarkable: as to causes there is milch ditlerenee of opinion. The theory of Bates is acceiited by many — also that of Jliiller. Others, like Eimer. Klwcs. and Piepers, deny that the mimicry is due to natural selection, but rather to definitely directed evolu- tion, the result of "outward influences such as climate, nutriment, etc.. acting on a given con- stitution." Others, rejecting the Miillerian theory, accept Bates's facts, but ascribe more to the in- fluence of the local environment, such as the action of light, heat, dryness or moisture, etc., yet allowing that in the end natural selection may act as a preservative agent. The objections to the Miillerian theory are the following: Neither Bates nor Wallace himself, though each lived for several years and collected Imttcrfiies in the American tropics, ever actually saw a bird chase and devour a but- terfly, although insectivorinis birds are said by them to be abundant in Brazil and the Kastcrii Archipelago. Piepers. Pryor. Skertchley. and other tropical naturalists of long and intelligent experience agree that very rarelv has any bird been seen even to chase a butterfly: while .Tudd concludes from an examinatiim of stomachs of insectivorous birds that none of the .-Vmerican birds feeds upon butterflies "during any mcmth of the year to the extent nf one-tenth of 1 per cent, of its food." Ornitli(d(igi-<ts confirm this abstinence from eating biitfcrllies. Frnm these and numerous other cases it ajipcars that butter- flies enjo.v a peculiar innnunity from the attacks of birds. It is sometimes the case that if i-^ dillicult to tell which is the model and which the mimic. On the Solomon Islands a dark brown Eu]ihea and a llaiiais. both inedible, were a<'Companied by a llypnlininas butterfly, also inedible, all three genera being avoided by birds both in the larva and imago stages. The fact, says Packard, that