Vrotectim Mimicry as Uvideuce for the Validity of the Theory of Natural 

 Selection. By Edward B. Poulton, M.A., F.B.S., Hope Professor of 

 Zoology, Oxford. ** 



(An abt^tract of the paper read before JSection D of the British Association at 

 Toronto on Monday, August 23, 1897. Reprinted from the 'Report' of the 

 Meeting, pages 692-4.) 



Several suggestions have been put forward to account for the superficial 

 resemblances between animals, especially insects, occupying- the same geographical 

 area. It has been suggested, and indeed strongly maintained, that food, climate, 

 or some other chemical or physical influence of the locality may have supplied 

 the cause. On the other hand, many naturalists consider that the facts cannot be 

 interpreted by any of these suggested causes, and only receive an intelligible and 

 probable explanation in the theory of natural selection. This theory supposes 

 that the resemblance is advantageous in the struggle for existence, the weaker 

 forms being shielded by their resemblance to the strong and well-defended species 

 (mimicry of H. W. Bates), or the latter gaining by a resemblance which enables 

 their local enemies more easily — and thus with a smaller waste of life — to recognise 

 and avoid them (mimicry of Fritz Miiller). The present paper directs attention 

 to certain facts commonly associated with mimetic resemblance which receive a 

 ready explanation upon the theory of natural selection as the efficient cause, but, 

 on the other hand, constitute a serious difficulty in the way of any other theories 

 as yet brought forward. 



Natural selection, as is well known, acts upon any variations, whatever they 

 may be, which are in the advantageous direction, and are at the same time not 

 injurious in themselves. When the end to be gained (in this case the attainment 

 of a superficial resemblance) is common to a variety of distantly related species 

 possessing entirely different constitutional tendencies, we may feel confident that 

 an approach brought about by natural selection will be by extremely diverse paths 

 of variation. Under natural selection we might predict that such a common end 

 would be reached by great diversity of means, while under the other hypotheses 

 mentioned above a result of the kind is inexplicable. Hence the facts of the case 

 should act as a convenient test between these rival suggestions. 



First as to colour. AVe know but little of the chemical nature of the pigments 

 made use of in mimetic resemblance. One case, however, has been investigated 

 by Gowland Hopkins — viz., the bright tints by which certain S. American Pierincs 

 have come to resemble HeliconincB and Ithomiin<2 in the same locality. Gowland 

 Hopkins has shown that these close resemblances in colour and pattern are 

 produced by pigments wliich are characteristic of the PieritKe, and of an entirely 

 different chemical nature from those of their models. 



Another very interesting case is that of resemblance to ants. Ants are 

 mimicked more or less closely by a great variety of insects and by spiders. In 

 some cases we find the resemblance brought about by actual alterations in the 

 shape of the body (spiders and many insects), which is modified into a superficial 

 resemblance to the Hymeuopteron. In an Acridian — Mynnecophana fallax — the 

 shape of an ant is, as it were, painted in black pigment upon the body of the 

 insect, which is elsewhere light in colour and, as it is believed, inconspicuous in 

 the natural environment. In a certain group of Homoptera — the Membracidae — 

 some of the S. American species closely resemble ants. The Membracidfe are 

 characterised by an enormous growth from the dorsal part of the first thoracic 

 segment (pronotum), which spreads backwards and covers the insect like a shield. 

 In these insects the form of an ant is moulded in the shield beneath which the 

 unmodified body of the insect is concealed. These facts are only explicable by- 

 supposing that some great advantage is to be gained by resembling an ant, and 



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