Although it sometimes happens that a definite system of markings prevails throughout a genus, such as the annulate form in the South American Elaps, this is far from being universally the case; many closely allied species, or individuals of the same species, may be distinguished by very different patterns. Even on the same individual we may find two opposite types of markings without any transition, as in two Central American species of widely different genera, Polyodontophis annulatus and Zamenis mexicanus, in which the anterior part of the body is annulate or barred, and the rest longitudinally striped.
It is also a remarkable fact that very often the two sides of the body are not alike in their markings, appearing as if formed of the union, on the median line, of the right and left halves of two individuals. Thus it may happen, in annulate forms, that some of the annuli are broken exactly in the mid-dorsal and mid-ventral lines, and that the halves do not correspond in number on the two sides. In the handsome South American Lachesis alternatus, which derives its specific name from the two series of large C-shaped, dark, light-edged markings which adorn its back, these markings are not always alternating, as is the rule; but some may lie opposite to each other and back to back, this being due to the fact that the numbers of the markings do not correspond on the two sides. In one specimen I count twenty-four of these markings on the left side, and twenty-seven on the right. This shows that great importance cannot be attached to the number of the markings, for systematic purposes. In fact, in some Coral-snakes, Elaps fulvius for instance, the number of annuli may vary from twelve to fifty-two, with every gradation between the extremes. The bilateral asymmetry to which we have alluded produces the chess-board arrangement of the ventral spots in many snakes.
Among the markings which call for investigation as to their meaning, we must allude to the presence, in some Colubrids, of a small, light, dark-edged spot, or of a pair of light dots close together, in the middle of the parietal shields or on each side of the suture between these shields, which correspond in their position to the parietal organ of many Lizards. May not this marking be in some way correlated with sensory organs, like the apical pits on the scales of the body? And what is the explanation of such bizarre signs as the spectacle or the eye-spot on the hood of the Indian Cobra? At present it is as inexplicable as the lugubrious emblem on the thorax of the Death’s-head Moth. It cannot be suggested that it is a warning mark intended to terrify intruders, for when the Cobra is at rest the hood is folded, and the characteristic marking is not displayed; whilst as soon as it is aroused, and the hood expanded, it faces its enemy in such a way that the spectacle, or ocellus, is not to be seen.
First among the most brilliantly coloured snakes, of which there are many, stand the Coral-snakes, Elaps, of America, mostly annulate with red, yellow or white, and black. This striking coloration obtains also in diverse harmless snakes inhabiting the same part of the world, and this coincidence has been adduced in favour of the theory of mimicry, correlated with that of natural selection, which accounts for the resemblance as being of advantage to a harmless species, which is thus mistaken for one notorious for its deadly poison, and advertised as such by its brilliant colours (warning coloration). But other poisonous and much more dangerous snakes are not, as a rule, endowed with brilliant colours. It is true that these also may have their mimics: the Krait, Bungarus cæruleus, and Lycodon aulicus, in India, the Pit-viper, Ancistrodon himalayanus, and Psammodynastes pulverulentus, in the Himalayas and Assam, are good examples of such cases. On the other hand, there are equally striking instances of what one would regard as mimics if they only occurred together; thus, there is no better case of general resemblance between a poisonous and a harmless snake than we find in the Indian Cobra and the Coluber corais of tropical America, where Cobras are absent, or between a Viper and the Boid Enygrus asper, from New Guinea, where no Vipers exist.
Without attempting to offer any suggestion to account for the similarity of markings which prevails in certain parts of the world, attention may be drawn to the predominance of longitudinal dark and light stripes in the Indo-Malayan representatives of the American Elaps, shared by many innocuous snakes of similar form inhabiting the same region, and to the striped tails common to various Colubrids of Madagascar, as if the snakes of a district had agreed to conform to certain fashions in dress.
It is further noteworthy, in relation to the theory of warning coloration, that many Uropeltids, innocent burrowing creatures living underground or concealed under stones or rotting tree-trunks in the forests of Southern India and Ceylon, hardly ever showing themselves in daylight, are among the most striking for their bright yellow or red and black markings. We may point out at the same time the very marked resemblance in form and coloration between the Uropeltid Melanophidium bilineatum, and the Apodal Batrachian Ichthyophis glutinosus, both occurring together in Southern India.
The colour of snakes often harmonizes with their surroundings. Thus, many Tree-snakes, Boid, Colubrid, or Viperid, are of a bright green, like the foliage in which they are concealed. On the other hand, other Tree-snakes are not green, or only some specimens are green, as in the genera Dendraspis and Dispholidus. Desert-snakes are of the yellowish or reddish colour of the sand or rock on which they live, and in species whose range extends over different districts the desert individuals are paler, without or with less distinct markings, as compared to their fellows among other surroundings. In addition to their markings, some snakes are adorned with a metallic iridescent gloss, due to a fine striation of the scales.
The iris is often metallic, gold, bronze, or copper-red, and the black streaks of the head sometimes extend over it.
Although, unlike many lizards, snakes are unable to rapidly alter their colours, some produce a semblance of this phenomenon when inflating their neck or body; this is due to the presence of dark and light markings or of a bright pigment in the interstitial skin, which is not seen when the scales overlap. Thus, in the Indian Tree-snake Dryophis mycterizans the skin between the green or brown scales in the anterior part of the body is black and white, producing a striped pattern when the neck is inflated; the skin of the same region is bright vermilion in the Malay Tropidonotus subminiatus; many more examples could be quoted. The spectacle marking on the hood of the Indian Cobra involves the scales as well as the interstitial skin.
As a rule there are no sexual differences in colour. Yet these are so marked in our Common Adder that the sex of a specimen can nearly always be recognized by the coloration. This is, however, the exception, even in the genus to which the Adder belongs. A nuptial dress is unknown in snakes.