Ancistrodon halys.
CHAPTER III
COLORATION
In dealing with the coloration, we have first to distinguish between the colour and the markings. The former is very often highly variable among snakes of the same species, to say nothing of the changes which may take place with age or with the condition of the individuals, whether before or after exuviation; it is not unusual to find among specimens from the same locality a great range of variation, from greyish-white to brown, or red, or black, as, for instance, in our Common Viper. The latter afford more important characters, and often furnish valuable indications for the distinction of species; but even the disposition of the markings is subject to great individual variations, more likely to mislead than to help the inexperienced student in the discrimination of species. It is therefore always advisable to resort in the first instance to structural characters for the purpose of specific identification, and to fall back on coloration only as a means of confirmation. If we were to be guided by colour and markings alone, how could we believe that an adult four-lined Coluber quatuorlineatus is of the same species as the handsomely spotted Coluber sauromates; and yet, if we compare the young of these two snakes we find them to be absolutely identical in their markings, and, in the absence of any structural differences, we are forced to conclude that they only represent two forms of the same species, of which the latter is the more primitive.
It is nevertheless a fact that, with a few exceptions, the markings, however variable they may be, are reducible to certain fundamental patterns to which the innumerable variations may be traced back, and their derivation followed and scientifically explained. Let us consider, for instance, another species of Coluber, highly variable in its markings: C. leopardinus, of which the typical form, so called from having been the first described and named, is not by any means to be regarded as the most primitive.
First, we must take for granted that the markings of all such snakes, whether consisting of spots, stripes, or bars, start from a regular arrangement, which may be theoretically represented by four paired longitudinal series on the head and body: (1) Dorsal series (D); (2) Dorso-lateral (DL); (3) Lateral (L); (4) Ventro-lateral (VL). The first starts from the middle line of the head, and is continued along the spine; the second occupies the space between the first and third, which originates at the tip of the snout, passes through the eye, and is continued on the temple and along the side of the body; the fourth follows the lower lip, and extends along each side of the belly. Bearing this in mind, we find that the variety of C. leopardinus named schwoederi, with a vertebral series of paired spots, is to be regarded as the most primitive, from which we can derive, on the one hand, the true leopardinus by imagining a transverse fusion of the spots of series D into a single row, some of the spots often actually revealing, in their biscuit shape, their dual origin; whilst, on the other hand, confluence of the paired spots of the same series into two longitudinal stripes produces the variety named quadrilineatus (see [Plate VII].). In this particular instance, the paired series D has fused into a single streak on the head, and the series L appears to have departed from its primitive course to extend on the upper surface of the head, both in front of and behind the eye.
Many snakes show an interocular band extending from lip to lip, through the eyes, across the interorbital region. In others the lateral stripe L may bifurcate in front of the eye, an upper branch extending across the snout, through transverse fusion of series D and DL, and it may also bifurcate in like manner on the temporal region, fusing with the corresponding marking on the other side to form a W-shaped figure. The pattern of markings on the upper surface of the head is, however, often very complicated, and hence difficult of explanation.
As a second example of the derivation of patterns, we may mention Vipera aspis, which varies enormously as to its mid-dorsal markings, forming, in different individuals or even on different parts of the body, single or paired spots, a zigzag band, or transverse bars; all these are derived from the paired spots of series D. Each pair of spots may fuse and form transversely oval or elliptical spots or bars, or the spots may assume an alternate disposition from which, through confluence, the zigzag or sinuous band results. Thus, spotted and striped patterns may be traced to a common origin, however fundamental the difference between them appears at first sight. If the elements of the four series, D, DL, L, and VL, unite transversely with each other, and also with the spots on the ventral surface, we obtain ringed forms such as the Coral-snakes. That the black nuchal collar of our common Grass-snake is actually formed by the fusion of the spots of three originally distinct series has been proved by tracing the development of the markings in the embryo.
In various species a pair of light streaks extends along the back, bordering the D area, without interfering with the other markings, as we see, among European snakes, in some specimens of Tropidonotus natrix and viperinus, and Vipera berus.