PROTBCTIVE MIMICRY AND THEORY OF NATURAL SELECTION. 99 



common end would bo 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. We 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 J'ininac have come to resemble 1 [(iuoninae 

 and It/ioiiiiinae in the same locality. Gowland Hopkins has shown that 

 these close resemblances in colour and pattern arc produced by pig- 

 ments which are characteristic of the Pierinae, 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 Hymenopteron. In 

 an Acridian — Mijrmecnjihana fallax — the shape of an ant is, as it were, 

 painted in black pigment upon the body of the insect, which is else- 

 where light in colour, and, as it is believed, inconspicuous in the 

 natural environment. In a certain group of Homoptera — the Mcm- 

 bracidae — some of the S. American species closely resemble ants. The 

 Mnnhracidac 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 that 

 very different species have attained this end, each Ijy the accumulation 

 of those variations which were rendered possible by its peculiar 

 ancestral history and present constitution— in other words, by the 

 theory of natural selection. 



A more elaborate case, which I have recently investigated, is 

 afforded by a large group of tropical American Lepidoptera — moths as 

 well as butterflies — which closely resemble certain common wide-spread 

 species of the Ithomiine genera, Metlwna and 'Thi/ridia. The appearance 

 thus produced consists of a transparent ground with a black border to 

 both wings, the fore-wing being also divided by black transverse bars 

 into three transparent areas — the hind-wing usually into two. From 

 a comparison with other species of various families, etc., not altered in 

 this direction, we know that the transparent wings are not ancestral. 

 When we investigate the manner in which transparency has been 

 attained, it is found to be by different methods in the different con- 

 stituents of the group. Amoilg the numerous genera of Ithomiinae 

 {Mithona, Tlnjridia, Diircnna, J'httrcsis, Ithoniia, etc.); the result has 

 been attained by the reduction of the scales to a very minute size, so 

 that they hardly interfere with the passage of light. This reduction 

 affects the two kinds of scales which alternate with each other in the 

 rows upon the wings of this sub-family, a common result being {r.ii.,in 

 Mrtliuna and Tlu/ridia) the alteration of the more slender scales into 

 hairs, and of the broader ones into minute bifid structures, still retain- 

 ing scale-like proportions, in spite of their extremely small size. In 

 others, again, the two kinds of scales are reduced respectively to simple 



