Plate VII.CATERPILLARS.
Plate VII.
CATERPILLARS.
Finally, to take the case of caterpillars, Weismann has admirably worked out the life history of many forms, and shows how the complex markings have arisen by development. Broadly, a caterpillar consists of 13 segments, the head being one. The head is often marked with darker colour, and the last segment with its clasping feet is also very frequently emphasized, as inFigs. 1 & 3, Plate VII. The spiracles are generally marked by a series of spots, and often connected by a line. Here the tendency to repetition shows itself strongly, for not only the spiracles themselves, but the corresponding points in the segments without spiracles are frequently spotted, and, moreover, these spots are frequently repeated in rows above the spiracular line. Of this,Deilephila galiiandD. Euphorbiæ,Figs. 1-5, Plate VII., are good examples.
The segmentation is also generally emphasized, as shown in all the examples on the plate, but in its simplicity inFig. 10.
Running down the centre of the back a more or less distinct line is often seen, as shown in the figures. This corresponds with the great dorsal alimentary canal lying just below the skin, and Weismann has shown that in young larvæ this line is transparent, and the green food can be seen through the skin. We have here, perhaps, a relic of the direct colouration noticed in the transparent cœlenterata.
Where larvæ possess horns either upon the head, as inApatura irisandPapilio machaon, or on the tail, as in many of the sphyngidæ, likeFigs. 1-5, Plate VII., these appendages are always emphasized in colour. As they are frequently oblique, we often find that this obliquity is continued as a slanting spot, as inD. galiiandeuphorbiæ, and sometimes repeated as a series of oblique stripes, as inFig. 4.
It must be admitted that in insects we have strong evidence of structural decoration.
CHAPTER XII.Arachnida.
T
THE Arachnida include the scorpions and spiders, and as the former are tolerably uniform in colour, our remarks will be confined to the latter.
The thorax is covered with a horny plate, while the abdomen only possesses a soft skin, and neither show any traces of segmentation. From the thorax spring four pairs of legs, and a pair of palpi, or feelers. Immediately beneath the skin of the abdomen lies the great dorsal vessel, which serves as a heart. This vessel is divided into three chambers, the general aspect of which is shown inFig. 9, Plate VIII., taken from Gegenbaur's Comparative Anatomy.[35]
From this heart the blood passes by vessels to each of the limbs, the palpi, etc., as offsets from the double-branched aorta. The shape of this dorsal vessel is peculiar, and its importance in respect to colouration will be immediately apparent.
The primary scheme of colouration in the Arachnida seems to be the distinguishing of the cephalothorax from the abdomen by a different colour. Thus, of the 272 species of British spiders represented in Blackwell's work,[36]no less than 203 have these parts differently coloured, and only 69 are of the same hue, and even in these there is often a difference of tint. So marked is this in certain cases that the two parts form vivid contrasts. Of this cases are given in the following list.
As a rule the abdomen is darker than the cephalothorax, and many species have the former red-brown and the latter black.
The legs, usually, take the colour of the cephalothorax, and are, hence, generally lighter than the abdomen, but to this there are exceptions. Where the individual legs differ in colour, the two first pairs are the darkest, and the dark hue corresponds in tint with the dark markings on the cephalothorax. The joints of the legs are in many species emphasized with dark colour, which is often repeated in bands along the limb.
The most remarkable point is, however, the pattern on the abdomen, which, though varied in all possible ways, always preserves a general character, so that we might speak with propriety of a spider-back pattern. This pattern is fairly well illustrated in the genusLycosa, but is seen to perfection, and in its simplest form inSegestria senoculata,Plate VIII., Fig. 1, and inSparassus smaragdulus,Plate VIII., Fig. 2.
This peculiar pattern is so like the dorsal-vessel that lies just beneath, that it is difficult to avoid the conclusion that we have here an actual case of the influence of internal organs on the integument, and this we believe to be the case. No matter how curious the abdominal markings may seem to be, they never so far depart from this fundamental pattern as to appear independent of it.
Thus, in the genusLycosa, which is by no means the best for the purpose, but is chosen as illustrating Gegenbaur's diagram,Plate VIII., we have the dorsal-vessel well marked inL. piscatoria,Plate VIII., Fig. 3, from which may be developed the other forms. InL. andrenivora,Plate VIII., Fig. 4, the male shows the vessel-mark attenuated posteriorly; and in the female,Fig. 5, the hinder part has become broken up into detached marks, still preserving the original shape, while the upper part remains practically unchanged. InL. allodromathe disintegration of the mark has further advanced, for in the male,Fig. 6, the upper portion has lost something of its shape, and the lower part is a series of isolated segments. This process is carried still further in the female,Fig. 8, where the upper portion is simplified, and the lower almost gone. InL. campestris,Fig. 10, the mark is reduced to a stripe, corresponding with the upper part of the vessel-mark only: and, lastly, in the maleL. agretyca,Fig. 7, this upper part is represented by two spots, though even here traces of the original form can be seen.
A simplification of marking of another sort is seen inL. rapax,Fig. 13, where the chamber-markings are almost obliterated, and merely an irregular stripe left. The stages by which this modification is arrived at are too obvious to need illustration.
In some species the lower portion of the vessel-mark is reduced to small dots, as inL. cambrica,fluviatilis,piratica, and others; and the stages are very clear. Starting with the isolated chamber-marks, as inL. allodroma,Fig. 5, we get, firstly, a set of spots, as inL. picta, which, in the female,Fig. 16, are still connected with the chamber-marks, but in the male,Fig. 17, are isolated. This leads us, by easy steps, to such forms asL. latitans,Fig. 14, which consists of a double row of spots upon dark stripes.
The intimate connection thus shown to subsist between the characteristic decoration of the abdomen of spiders, and the shape of the important dorsal organ beneath, seems to be strong evidence of effect that internal structure may have upon external decoration.[37]
The cephalothorax of spiders, being covered with a hardened membrane, does not show such evidence clearly, for it appears to be a law that the harder the covering tissue, the less does it reflect, as it were, the internal organs. The hard plates of the armadillo are thus in strong contrast to the softer skins of other animals.
Nevertheless, there does appear, occasionally, to be some trace of this kind of decoration in the cephalothorax of certain spiders, though it would be hard to prove. The blood vessels of this part (seeFig. 9), though large, are not nearly so prominent as the great dorsal vessel. The chief artery enters the cephalothorax as a straight tube, forks, and sends branches to the limbs, palpi, and eyes. In many species, notably in the genusThomisus, a furcate mark seems to shadow the forked aorta. This is best shown inT. luctuosus,Plate VIII., Fig. 11. Moreover, in this and other genera, lines frequently run to the outer pair of eyes, which alone are supplied with large arteries, seeFig. 9.
However this may be, it is certain that the entire decoration of spiders follows structural lines, and that the great dorsal vessel has been emphasized by the peculiar pattern of the abdomen.
Plate VIII.SPIDERS.
Plate VIII.
SPIDERS.
CHAPTER XIII.Colouration of Invertebrata(Continued).
O
OF the Arthropoda, including the lobsters, crabs, shrimps, etc., little can be said here, as we have not yet been able to study them with anything like completeness. Still, we find the same laws to hold good. The animals are segmented, and we find their system of colouration segmental also. Thus, in the lobsters and crabs there is no dorsal line, but the segments are separately and definitely decorated. The various organs, such as the antennæ and eyes, are picked out in colour, as may be beautifully seen in some prawns.
When we come to the Mollusca, we meet with two distinct types, so far as our subject is concerned; the naked and the shelled. In the naked molluscs, like the slugs, we have decoration applied regionally, as is shown to perfection in theNudibranchs, whose feathery gills are often the seat of some of the most vivid hues in nature.
The shell-bearing mollusca are proverbial for their beauty, but it is essential to bear in mind that the shell does not bear the same relation to the mollusc that the "shell" of a lobster does to that animal. The lobster's shell is part of its living body; it is a true exo-skeleton, whereas the shell of a mollusc is a more extraneous structure—a house built by the creature. We ought, on our view, to find no more relation between the decoration of a shell and the structure of its occupant, than we do in the decoration of a human dwelling-house to the tenant.
The shell consists of carbonate of lime, under one or both of the forms known to mineralogists as calcite and aragonite. This mineral matter is secreted by an organ called the mantle, and the edge, or lip, of the mantle is the part applied to this purpose. The edge of the mantle is the builder's hand, which lays the calcareousstones of the edifice. The shell is built up from the edge, and the action is not continuous but seasonal, hence arise the markings known as lines of growth. In some cases the mantle is expanded at times into wing-like processes, which are turned back over the shell, and deposit additional layers, thus thickening the shell.
In all the forms of life hitherto considered the colouring matter is deposited, or formed, in the substance of the organ, or epidermal covering, but in the mollusca this is not the case. The colouring matter is entirely upon the surface, and is, as it were, stencilled on to the colourless shell. This is precisely analogous to the colouring of the shells of birds' eggs. They, too, are calcareous envelopes, and the colouring matter is applied to the outside, as anyone can see by rubbing a coloured egg. In some eggs several layers of colouring matter are superimposed.
In no case does the external decoration of molluscan shells follow the structure lines of the animal, but it does follow the shape of the mantle. The secreting edge may be smooth, as inMactra, regularly puckered, as in mostPectens, puckered at certain points, as inTrigonia, or thrown into long folds, as inSpondylus. In each of these cases the shell naturally takes the form of the mantle. It is smooth inMactra, regularly ribbed inPecten, tubercled inTrigonia, and spined inSpondylus. Where the inside of the shell is coloured as in some Pectens, regional decoration at once appears and the paleal lines, and muscular impressions are bounded or mapped out with colour.
It is a significant fact that smooth bivalves are not so ornate as rugose ones, and that the ridges, spines, and tubercles of the latter are the seats of the most prominent colour.
Similar remarks apply to univalve shells, which are wound on an imaginary vertical axis. They may be smooth, as inConusandOliva, rugose, as inCerithium, or spined, as inMurex. The structure of these shells being more complex than that of bivalves, we find, as a rule, they are more lavishly ornamented, and the prominent parts of the shell, and especially the borders, are the seat of strongest colour. In some cases, as in adult Cowries (Cypræa), the mantle is reflexed so as to meet along the median line, where we see the darkest colour.
The rule amongst spiral shells is to possess spiral and marginal decoration, and this is what we should expect. The Nautilus repeats in the red-brown markings of its shell, the shape of the septa whichdivide the chambers, though, as is often the case, they are generally more numerous than the septa.
The naked Cephalopoda, or cuttle-fishes, often possess a distinct dorsal stripe, and when our views were first brought before the Zoological Society, this fact was cited as an objection. To us it seems one of the strongest of favourable cases, for these animals possess a sort of backbone—the well-known cuttle-bone—and hence they have a dorsal line.
Some shells, asMargarita catenata, have a chain-pattern, and in this case the action of the pigment cells takes place at regular and short intervals. Others, asMactra stultorum, the stencilling forms a series of lines and spots, generally enlarging into rays.
The whole subject of the decoration of shells deserves much more time than we have been able to give to it as yet.
CHAPTER XIV.Colouration of Vertebrata.
T
THE vertebrata, as their name implies, are distinguished by the possession of an internal skeleton, of which the backbone is the most essential part, and the general, but not universal, possession of limbs or appendages.
Consequently we find that the dorsal and ventral surfaces are almost invariably coloured differently, and the dorsal is the darker in the great majority of instances. Generally the spine is marked by a more or less defined central line, and hence this system of colouration may be termed axial, because it is in the direction of the axes, or applied about the axes.
Fishes.Where fishes have not been modified out of their original form, as are the soles, plaice, and other flat fish, we find the dorsal region darker than the ventral, and even here the under surfaces are the lightest. Even in cases like the Char,Fig. 1, Plate IX., where vivid colour is applied to the abdomen, the dorsum is the darker. The dorsum is often marked by a more or less well-defined dark band, as in the mackerel and perch,Fig. 2, Plate IX.There are sometimes parallel bands at right angles to the above, as in the perch and mackerel; and this is a common feature, and apparently a very old one, as we find it in the young of fishes whose adults are without these rib-like marks, such as the trout and pike.
It is only necessary to inspect any drawings of fishes to see that their colouration is on a definite principle, although rather erratic. Important functional parts, like the gills, fins, and tail, are generally marked in colour more or less distinctly, as may be seen, for instance, in our common fresh-water fishes, like the roach and perch. The line of mucus-secreting glands running along the sides is usually marked by a dark line. These facts point distinctly to structural decoration.
Plate IX.CHAR and PERCH.
Plate IX.
CHAR and PERCH.
There are in some fishes, like the John Dory, curious eye-like dark spots, which we cannot refer to a structural origin, though a better acquaintance with the class might reveal such significance.
The Amphibia have not been well studied by us, and we must leave them with the remark that they seem to bear out the view of structural decoration, as is seen in our English newts. Some are, however, modified out of all easy recognition.
Reptiles.Among the reptiles, the snakes,Fig. 4, may be selected for illustration. Snakes are practically little more than elongated backbones, and are peculiar from the absence of limbs. The colouring matter does not reside so much in the scales as in the skin beneath, so that the sloughs do not illustrate the decoration. Hence, we might expect to find here a direct effect of morphological emphasis.
The ornamentation of snakes is very similar throughout the class, both in water and land snakes; as may be seen by Sir W. Fayrer's work on Venomous Snakes. This ornamentation is of a vertebral pattern, placed along the dorsal surface, with cross lines, which may represent ribs.
Where the ribs are wanting, as in the neck, the pattern changes, and we get merely longitudinal markings.
In the Python,Fig. 4, there are, near the central line, numerous round spots, which apparently emphasize the neural processes. There are diagonal markings on some species which illustrate the development of colour-spots already alluded to.
This snake-pattern is very singular and striking. The markings are fewer in number than the vertebræ, yet their true vertebral character is most obvious.
In Snakes, again, we find the dorsal region is darker than the ventral.
In the Lizards there are patches of colour placed axially, while each patch covers a number of scales.
Birds.Birds have their whole economy modified to subserve their great functional peculiarity of flight.
Immense muscles are required for the downward stroke of the wing, and to give attachment to these thesternum has a strongly developed keel. To bring the centre of gravity low, even the muscles which raise the wing are attached to the sternum, or breastbone,instead of to the dorsal region, as might be expected; and to brace the wings back a strong furculum—the merry-thought—is attached. The breast, then, is the seat of the greatest functional activity in birds, and, consequently, we find in a vast number of birds that the breast is the seat of vivid colour.
As many birds are modified for protective purposes, the brightest species were selected to test our views, namely, the Birds of Paradise (Paradisea), Humming Birds (Trochilidæ), and Sun Birds (Nectarinidæ). In these birds it is clear that colour has had full sway, untramelled by any necessity for modification.
Nothing is more striking than the mapping out of the surface of these birds into regions of colour, and these regions are always bounded by structural lines.
Take, for instance,Paradisea regia. In this bird we find the following regions mapped in colour:—
A beautiful ruff emphasizes the pectoral muscles, and the tail appendages emphasize the share-like caudal vertebræ.
If we turn to the other species of this genus, we find inP. Papuanathe claret breast suddenly change to green at the furculum; and similar changes take place inP. speciosa, while inP. WallaceiandWilsonithis region is decorated with a wonderful apron of metallic green.
The region of the furculum is equally well marked in the Toucans and Sun-birds.
If now we observe the back of a bird, and view the skeleton with the wings at rest, we shall find it falls into three morphological tracts. First, the shoulder, or scapular track; second, the thigh, or pelvic; third, the tail, or caudal region; and in all these birds the several tracts are beautifully marked by sudden and contrasted change of colour. InP. Wilsoniall the tracts are brilliant red, but they are separated by jet-black borders. InNectarinea chloropygiathe scapular region is red, the pelvic yellow, and the caudal green.
Plate X.SUN BIRDS.
Plate X.
SUN BIRDS.
InP. Wilsoniwe have a wonderful example of morphological emphasis. The head is bare of feathers, and coloured blue, except along the sutures of the skull, where lines of tiny black feathers map out the various bones.
But morphological emphasis exists everywhere in birds. The wing-primaries, which attach to the hand, are frequently differently decorated from the secondaries, which feathers spring from the ulna; and the spur-feathers of the thumb, or pollux, are different in shape, and often in colour, from the others, as every fly-fisher who has used woodcock spur-feathers knows full well. The wing-coverts and tail-coverts are frequently mapped in colour; and the brain case is marked by coloured crests. The eye and ear are marked by lines and stripes; and so we might go on throughout the whole bird. We may remark that these very tracts are most valuable for the description and detection of species, and among ornithologists receive special names.
Now, this distribution of colour is the more remarkable inasmuch as the feathers which cover the surface—the contour feathers—are not evenly distributed over the body, but are confined to certain limited tracts, as shown by Nitzsch; and though these tracts have a morphological origin, they are rendered quite subsidiary to the colouration, which affects the whole bird, and not these regions in particular. In fact, the colouration is dependent upon the regions on which the feathers lie, and not upon the area from which they spring. In other words, we seem to have in birds evidence of the direct action of underlying parts upon the surface.
In more obscurely coloured birds, and those which seem to be evenly spotted, close examination shows that even here the decoration is not uniform, but the sizes and axes of the spots change slightly as they occupy different regions; as may be seen in Woodpeckers and Guinea-fowl.
Although the same tone of colour may prevail throughout the plumage, as in the Argus Pheasant, great variety is obtained by the fusion of spots into stripes. A symmetrical effect is produced by the grouping of unsymmetrical feathers; as is so often seen in plants, where irregular branches and leaves produce a regular contour.
Sometimes, especially on the breast and back, the feathers of one region seem to unite so as to form one tract, so far as colour is concerned. Thus, if inP. Wilsonithe black borders of the dorsal regions were suppressed, all three areas would be of one hue. Thisseems to have been the case in the breast region of Humming Birds, where only the throat is highly coloured. In the Toucans the breast and throat regions are often marked with colour; but sometimes the hue is the same and the boundaries of the regions marked with a band of another colour; if this boundary band be increased, the regions do not seem so well shown, for the boundary becomes as broad as the area; yet, in all these cases the dependence upon regional decoration is manifest. No doubt the few uniformly coloured birds were derived from species which were once variously hued; the gradation of colour being lost in transmission.
Mammalia.The axial decoration of the mammalia is very definite, and nearly all species have a dorsal tract marked with colour. The dark bands on the back of the horse, ox, and ass, are cases in point. In nearly every case the dorsal is darker than the ventral surface.
If we take highly decorated species, that is, animals marked by alternate dark and light bands, or spots, such as the zebra, some deer, or the carnivora, we find, first, that the region of the spinal column is marked by a dark stripe (Figs.9&16); secondly, that the regions of the appendages, or limbs, are differently marked; thirdly, that the flanks are striped, or spotted, along or between the regions of the lines of the ribs; fourthly, that the shoulder and hip regions are marked by curved lines; fifthly, that the pattern changes, and the direction of the lines, or spots, at the head, neck, and every joint of the limbs; and lastly, that the tips of the ears, nose, tail, and feet, and the eye are emphasized in colour. In spotted animals the greatest length of the spot is generally in the direction of the largest development of the skeleton.
This morphological arrangement can be traced even when the decoration has been modified. Thus, in the carnivora we have the lion and puma, which live in open country, with plain skins, the tiger with stripes, an inhabitant of the jungle, and the leopard, ocelot, and jaguar with spots, inhabiting the forests.
But the lion has a dark dorsal stripe, and the nose, etc., are emphasized in colour, and, moreover, the lion has probably lost its marked decoration for protective purposes, for young lions are spotted. The tiger's stripes start from the vertebræ, and still follow the lines of the ribs. In the tiger the decoration changes at the neck, and on the head, and the cervical vertebræ are often indicated by seven stripes. SeeFig. 5.
The markings over the vertebræ are not in continuous lines, as in many mammals, but form a series of vertebra-like spots. This plan of decoration is continued even on the tail, which is coloured more on the upper than on the lower surface.
The spotted cats have their spot-groups arranged on the flanks in the direction of the ribs, at the shoulder and haunch in curves, at the neck in another pattern, on the back of the head in another; and the pattern changes as each limb-joint is reached, the spots decreasing in size as the distance is greater from the spine. See Figs.9-15.
There is in tigers, and the cat-tribe generally, a dark stripe over the dental nerve; and the zygoma, or cheek-bone, is often marked by colour. Even the supraorbital nerve is shown in the forehead, and there are dark rings round the ears. In dissecting an ocelot at the Zoological Gardens in 1883, a forked line was found immediately over the fork of the jugular vein.
The colouration in these animals seems often to be determined by the great nerves and nerve-centres, and the change from spots, or stripes, to wrinkled lines on the head are strikingly suggestive of the convolutions of the brain, falling, as they do, into two lateral masses, corresponding with the cerebral hemispheres, separated by a straight line, corresponding with the median fissure. This is well shown in the ocelot,Fig. 15, and in many other cats.
That the nerves can affect the skin has already been pointed out inChapter VI., in the case of herpes, and that it can affect colour is shown in the Hindoo described in the same place.
So marked, indeed, is this emphasis of sensitive parts that every hair of the movable feelers of a cat is shown by colour to be different in function from the hairs of the neck, or from the stationary mass of hair from which the single longer hair starts.
In the Badger,Fig. 16, there is a bulge-shaped mass of coloured hair near the dorsal and lumbar regions, but it is axially placed. The shoulder and loins are well marked, although in a different manner from other species. In some species of deer, and other mammalia, there are white or coloured lines parallel to the spine, and also, as in birds, spots coalesce and form lines, and lines break up into spots.
The great anteater has what at first seems an exceptional marking on the shoulder, but a careful examination of the fine specimen which died at the Zoological Gardens in 1883, we were struck with the abnormal character of the scapula, and we must remember that,as Wallace and Darwin have pointed out, all abnormal changes of the teeth are correlated with changes in the hair. Moreover the muscles of the shoulder region are so enormously developed as to render this otherwise defenceless animal so formidable that even the jaguar avoids an embrace which tightens to a death-grip. This region is, therefore, precisely the one we should expect to be strongly emphasized. This being the case, we have really no exception in this creature.
Certain mammals are banded horizontally along their sides, thus losing most of their axial decoration, and this is well shown among the Viverridæ, and smaller rodents. Now, however conspicuous such animals may appear in collections, they are in their native haunts very difficult to detect. In all cases there is a marked dorsal line; and we suggest that the mature decoration is due to a suppression of the axial decoration for protective purposes, and a repetition of the dorsal decoration according to the law before enunciated. Indeed, in one case we were able to trace this pretty clearly, in the beautiful series ofSus vittatusin the museum at Leyden. This pig, an inhabitant of Java, when mature is a dark brown animal, but in the very young state it is clearly marked in yellow and brown, with a dark dorsal stripe, and spots, taking the line of the ribs, and over the shoulder and thigh. As the animal grows older, the spots run into stripes, and it becomes as clearly banded horizontally as the viverridæ. Finally the dark bands increase in width, until they unite, and the creature becomes almost uniformly brown.
We have not been able to see young specimens of the viverridæ, but a similar change may there occur, or it may have occurred in former times. We must also remember that these creatures are long-bodied, like the weasels, and hence they may have a tendency to produce long stripes.
In the case of our domestic animals, especially the oxen, the decoration seems often to have become irregular, but even here the emphasis of the extremities is generally clearly made out, and that of the limbs can often be traced. In horses this is better shown, and dappled varieties often well illustrate the points. Most horses at some time show traces of spots.
Sufficient has now been said to point out the laws we believe to have regulated the decoration of the animal kingdom. The full working out of the question must be left to the future, but it is hoped that a solid groundwork has been laid down.
Plate XI.LEAVES.
Plate XI.
LEAVES.
CHAPTER XV.The Colouration of Plants.
T
THE general structure of plants is so simple in comparison with that of animals that our remarks upon this sub-kingdom need only be short.
With regard to leaves, especially such as are brightly coloured, like the Begonias,Caladiums, Coleus, and Anœchtochilus,Plate XI., the colour follows pretty closely the lines of structure. We have border decoration, marking out the vein-pattern of the border; the veins are frequently the seat of vivid colour, and when decolouration takes place, as in variegated plants, we find it running along the interspaces of the veins. These facts are too patent to need much illustration; for our zonale geraniums, ribbon grasses, and beautiful-leaved plants generally, are now so common that everyone knows their character. When decay sets in, and oxidation gives rise to the vivid hues of autumn, we find the tints taking structural lines, as is well shown in dying vine and horse-chestnut leaves,Fig. 1, Plate XI. This shows us that there is a structural possibility of acquiring regional colouration.
We must remember, too, that the negative colouration of these dying leaves is of very much the same character as the positive colouration of flowers, for flowers are modified leaves, and their hues are due to the oxidation of the valuable chlorophyll.
In leaves the tendency of spots to elongate in the direction of the leaf is very marked, as may be well seen in Begonia.Fig. 17, drawn to illustrate another point, shows this partly. When leaves are unsymmetrical, like the begonias, the pattern is unsymmetrical also.
Among parallel veined leaves we find parallel decoration. Thus,in theCalatheaswe have dark marks running along the veins. InDracæna ferreawe have a dark green leaf, with a red border and tip, the red running downwards along the veins. This action may be continued until the leaf is all red except the mid-rib, which remains green. In long net-veined leaves we may citePavetta Borbonica, whose dark green blade has a crimson mid-rib. Of unsymmetrical leaves those in the plate may suffice.
When we come to flowers, the same general law prevails, and is generally more marked in wild than in cultivated forms, which have been much, and to some extent unnaturally, modified. Broadly speaking, when a flower is regular the decoration is alike on all the parts; the petals are alike in size, the decoration is similar in each, but where they differ in size the decoration changes. Thus, inPelargoniumswe may find all five petals alike, or the two upper petals may be longer or shorter than the lower three. In the first case each is coloured similarly, in the other the colour pattern varies with the size of the petal. The same may be seen in Rhododendron.
Where the petals are united the same law holds good. In regular flowers, like the lilies, the colouration is equal. In irregular flowers, like the snapdragon and foxglove, the decoration is irregular. In Gloxinia the petals may be either regular or irregular, and the decoration changes in concert.
A very instructive case was noticed by one of us inLamium galeobdolon, or yellow Archangel. This plant is normally a labiate with the usual irregular corolla, but we have found it regular, and in this instance the normal irregular decoration was changed to a regular pattern on each petal.
In gamopetalous flowers the line of junction of the petals is frequently marked with colour, and we know of no case in which a pattern runs deliberately across this structure line, though a blotch may spread from it.
When we remember that flowers are absolutely the result of the efforts of plants to secure the fertilizing attention of insects, and that they are supreme efforts, put forth at the expense of a great deal of vegetable energy—that they are sacrifices to the necessity for offspring—it does strike us forcibly when we see that even under these circumstances the great law of structural decoration has to be adhered to.
Plate XII.FLOWERS.
Plate XII.
FLOWERS.
CHAPTER XVI.Conclusions.
W
WE have now, more or less fully, examined into the system of colouration in the living world, and have drawn certain inferences from the facts observed.
It appears that colouration began—perhaps as a product of digestion—by the application of pigment to the organs of transparent creatures. Supposing that evolution be true—and, if we may not accept this theory there is no use in induction whatever—it must follow that even the highest animals have in the past been transparent objects. This was admirably illustrated by Prof. Ray Lankester in a lecture on the development of the eyes of certain animals, before the British Association meeting at Sheffield, in which it was shown that the eyes commenced below the surface, and were useful even then, for its "body was full of light."
Granting this, it follows that the fundamental law of decoration is a structural one. Assuming, as we do, that memory has played a most important part in evolution, it follows that all living matter has a profound experience in decorating its organs—it is knowledge just as anciently acquired, and as perfectly, as the power of digestion. This colour was produced under the influence of light—so it is even in opaque animals.
With a knowledge so far reaching, we might expect that even in opaque animals the colouring would still follow structural lines, and there should still be traces of this, more or less distinct.
This is precisely what we do find; and, moreover, we sometimes get a very fair drawing of the important hidden parts, even where least expected, as in a cat's head, a snake's body, a dragon-fly's thorax, a spider's abdomen, a bird's skull.
But if animals thus learned to paint themselves in definite patterns, we might expect that when called upon to decoratefor the sake of beautycertain parts not structurally emphatic, they would adopt well-known patterns, and hence arose the law of repetition.
But with wider experience came greater powers, and the necessity for protection arising, the well-known patterns were enlarged, till an uniform tint is produced, as in the Java pig, or some repeated at the expense of others, as in the civets. But so ingrained is the tendency to structural decoration that even where modification has reached its highest level, as in the leaf-butterflies, some trace of the plan that the new pattern was founded on is recognisable, just as the rectangular basis can be traced in the arabesque ornaments of the Alhambra.
The pointing out of this great fact has seemed to us a useful addition to the great law of evolution. It supplements it; it gives a reason why.
Could he who first saw these points have read these final pages, it would have lightened the responsibility of the one upon whom the completion of the work has fallen. But he died when the work was nearly finished. The investigation is of necessity incomplete, but nothing bears such misstatements as truth, and though specialists may demur to certain points, the fundamental arguments will probably remain intact.