Many instances of mimicry are known to-day, but comparatively few of them have been studied in any detail. Yet a single carefully analysed case is worth dozens which are merely superficially recorded. In trying to arrive at some conception of the way in which the resemblance has come about we want to know the nature and extent of the likeness in the living as well as in the dead; the relative abundance of model and mimic; what are likely enemies and whether they could be supposed to select in the way required, whether the model is distasteful to them; whether intermediate forms occur among the mimics; how the various forms behave when bred together, etc., etc. Probably the form that from these many points of view has, up to the present, been studied with most care is that of the Swallow-tail, Papilio polytes. It is a common butterfly throughout the greater part of India and Ceylon, and closely allied forms, probably to be reckoned in the same species, reach eastwards through China as far as Hongkong. P. polytes is one of those species which exhibit polymorphism in the female sex. Three distinct forms of female are known, of which one is like the male, while the other two are very different. Indeed
for many years they were regarded as distinct species, and given definite specific names. To Wallace belongs the credit of shewing that these three forms of female are all to be regarded as wives of the same type of male[[38]]. He shewed that there were no males corresponding to two of the females; also that the same one male form was always to be found wherever any of the females occurred. As the result of breeding experiments in more recent years Wallace's conclusions have been shewn to be perfectly sound.
The male of polytes ([Pl. V], fig. 1) is a handsome blackish insect with a wing expanse of about four inches. With the exception of some yellowish-white spots along their outer margin the fore wings are entirely dark. Similar spots occur along the margin of the hind wing also, while across the middle runs a series of six yellowish-white patches producing the appearance of a broad light band. The thorax and abdomen are full black, though the black of the head is relieved by a few lighter yellowish scales. The under surface is much like the upper, the chief difference being a series of small and slightly reddish lunules running outside the light band near the margin of the hind wing ([Pl. V], fig. 1 a). In some specimens these markings are almost absent. One form of female is almost exactly like the male ([Pl. V], fig. 2), the one slight difference being that the lunules on the under surface of the hind wing are generally a trifle larger. For brevity she may be called the M form. The second form of female
differs in many respects from the male and the M female. Instead of being quite dark, the fore wings are marked by darker ribbed lines on a lighter ground[[39]] ([Pl. V], fig. 3). The hind wings shew several marked differences from those of the male. Of the series of six patches forming the cross band the outermost has nearly disappeared, and the innermost has become smaller and reddish. The middle four, on the other hand, have become deeper, reaching up towards the insertion of the wing, and are pure white. A series of red lunules occurs on the upper surface outside the white band, and the yellowish-white marginal markings tend to become red. These differences are equally well marked on the under surface ([Pl. V], fig. 3 a). The colour of the body, however, remains as in the male. From the resemblance shewn by this form to another species of Swallow-tail, Papilio aristolochiae ([Pl. V], fig. 5), we shall speak of it as the A form.
The third form of female is again very distinct from the other two. The fore wings are dark but are broken by an irregular white band running across the middle ([Pl. V], fig. 4), and there is also an irregular white patch nearer the tips of the wing. The hind wings, on the other hand, are characterised by having only red markings. The yellowish-white band of the male is much reduced and is entirely red, while the red lunules are much larger than in the A form. The under surface ([Pl. V], fig. 4 a) corresponds closely with the
upper. The body remains black as in all the other forms. This type of female bears a resemblance to Papilio hector ([Pl. V], fig. 6), and for that reason we shall speak of it as the H form. It should be added that these three forms of female are quite indistinguishable in the larval and chrysalis stages.
It was Wallace who first offered an explanation of this interesting case in terms of mimicry. According to this interpretation P. polytes is a palatable form. The larva, which feeds on citronaceous plants, and the chrysalis are both inconspicuous in their natural surroundings. They may be regarded as protectively coloured, and consequently edible and liable to persecution. The original coloration is that of the male and the M female. From this the other two forms of female have diverged in the direction of greater instead of less conspicuousness, although the presumed edibility of the insect might have led us to think that a less conspicuous coloration would have been more to its advantage. But these two females resemble the two species Papilio aristolochiae and Papilio hector, which, though placed in the same genus as P. polytes, belong to a very different section of it[[40]]. The larvae of these two species are conspicuously coloured black and red with spiny tubercles. They feed upon the poisonous Aristolochia plants. For these reasons and also from the fact that the butterflies themselves are both conspicuous and plentiful it is inferred that they are unpalatable. In short, they are the models upon
which the two polytes females that are unlike the male have been built up by natural selection.
The suggestion of mimicry in this case is supported by the fact that there is a general correspondence between the areas of distribution of model and mimic. P. hector is not found outside India and Ceylon, and the H female of P. polytes is also confined to this area. P. aristolochiae, on the other hand, has a much wider range, almost as wide indeed as that of P. polytes itself. Generally speaking the A female accompanies P. aristolochiae wherever the latter species is found. Beyond the range of P. aristolochiae, in northern China, the M female alone is said to occur. On the other hand, as the matter comes to be more closely studied exceptions are beginning to turn up. The H female, for instance, is found on the lower slopes of the Himalayas, far north of the range of P. hector, and there are indications that a careful study of the distribution in China and Japan may prove of importance.
Moreover, the investigation of a smaller area may also bring to light points of difficulty. In Ceylon, for example, P. polytes is common up to several thousand feet, while P. hector is rare at half the height to which polytes ascends. Nevertheless the H form of female is relatively just as abundant up-country where hector is rarely found as it is low down where hector is plentiful[[41]]. On the other hand, P. aristolochiae may be exceedingly abundant at altitudes where hector is scarce. Yet the A form of polytes is no more relatively abundant