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_.
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.
A special livery for the young is rather exceptional, but very often the
new-born is more vividly coloured than its parents, and in many black
varieties the young is similar to the typical form. Some green
Tree-Boids (_Chondropython_ and _Corallus caninus_) are not green, but
yellowish, cream-colour, or pinkish, when young, the green appearing
around the white spots, which are the remains of the ground colour, and
gradually spreading over the whole body. Conversely, the young of a
variety of the Pit-viper _Lachesis wagleri_, common in the Malay
Peninsula, is green, and the adult black and yellow. In the young of
_Grayia ornata_, a West African Water-snake, the markings of the young
are to those of the adult like positive and negative in photography, the
white bars, forked on the sides, which extend across the black back of
the former being gradually transformed into black bars on a light ground
in the latter; in such a case it is impossible to decide whether the
dark or the light parts are to be considered as the ground colour.
That the skin of many snakes contains soluble colouring matter of a
special kind is well known, green snakes, such as _Dryophis prasinus_
and _Lachesis gramineus_ staining the spirit in which they are
preserved. Chemists have not yet paid attention to this question, which
requires investigation.
Melanism is frequent in snakes, and sometimes affects all individuals in
the same locality. It seems undesirable to bestow varietal names on such
aberrations, as is so frequently done by systematists, any more than we
should in the case of albinos. Melanism may be produced in two ways: by
an extension of the black markings, which invade the whole surface, as
in the males of _Vipera berus_; or by a general darkening of the ground
colour and of the markings, as in the females of the same species. In
the latter case, the markings reappear under certain lights or after a
prolonged sojourn in spirits. Sometimes, as in _Zamenis gemonensis_, the
uniform black colour appears only as the snake approaches the adult
condition, the young having the normal livery.
Partial albinism is rare; perfect albinism, characterized by absence of
black pigment in the eye, rarer still. Cases have been observed, among
European species, in _Tropidonotus natrix_ and _tessellatus_, in
_Coluber longissimus_, and in _Coronella austriaca_.
------------------------------------------------------------------------