Mr. Taylor has, however, called attention to an important principle which underlies the various patterns or ornamental markings of animals—namely, that diversified coloration follows the chief lines of structure, and changes at points, such as the joints, where function changes. He says, "If we take highly decorated species—that is, animals marked by alternate dark or 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; 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 emphasised in colour. In spotted animals the greatest length of the spot is generally in the direction of the largest development of the skeleton."
This structural decoration is well seen in many insects. In caterpillars, similar spots and markings are repeated in each segment, except where modified for some form of protection. In butterflies, the spots and bands usually have reference to the form of the wing and the arrangement of the nervures; and there is much evidence to show that the primitive markings are always spots in the cells, or between the nervures, or at the junctions of nervures, the extension and coalescence of these spots forming borders, bands, or blotches, which have become modified in infinitely varied ways for protection, warning, or recognition. Even in birds, the distribution of colours and markings follows generally the same law. The crown of the head, the throat, the ear-coverts, and the eyes have usually distinct tints in all highly coloured birds; the region of the furcula has often a distinct patch of colour, as have the pectoral muscles, the uropygium or root of the tail, and the under tail-coverts.[130]
Mr. Tylor was of opinion the primitive form of ornamentation consisted of spots, the confluence of these in certain directions forming lines or bands; and, these again, sometimes coalescing into blotches, or into more or less uniform tints covering a large portion of the surface of the body. The young lion and tiger are both spotted; and in the Java hog (Sus vittatus) very young animals are banded, but have spots over the shoulders and thighs. These spots run into stripes as the animal grows older; then the stripes expand, and at last, meeting together, the adult animal becomes of a uniform dark brown colour. So many of the species of deer are spotted when young, that Darwin concludes the ancestral form, from which all deer are derived, must have been spotted. Pigs and tapirs are banded or spotted when young; an imported young specimen of Tapirus Bairdi was covered with white spots in longitudinal rows, here and there forming short stripes.[131] Even the horse, which Darwin supposes to be descended from a striped animal, is often spotted, as in dappled horses; and great numbers show a tendency to spottiness, especially on the haunches.
Ocelli may also be developed from spots, or from bars, as pointed out by Mr. Darwin. Spots are an ordinary form of marking in disease, and these spots sometimes run together, forming blotches. There is evidence that colour markings are in some way dependent on nerve distribution. In the disease known as frontal herpes, an eruption occurs which corresponds exactly to the distribution of the ophthalmic division of the fifth cranial nerve, mapping out all its little branches even to the one which goes to the tip of the nose. In a Hindoo suffering from herpes the pigment was destroyed in the arm along the course of the ulnar nerve, with its branches along both sides of one finger and the half of another. In the leg the sciatic and scaphenous nerves were partly mapped out, giving to the patient the appearance of an anatomical diagram.[132]
These facts are very interesting, because they help to explain the general dependence of marking on structure which has been already pointed out. For, as the nerves everywhere follow the muscles, and these are attached to the various bones, we see how it happens, that the tracts in which distinct developments of colour appear, should so often be marked out by the chief divisions of the bony structure in vertebrates, and by the segments in the annulosa. There is, however, another correspondence of even greater interest and importance. Brilliant colours usually appear just in proportion to the development of tegumentary appendages. Among birds the most brilliant colours are possessed by those which have developed frills, crests, and elongated tails like the humming-birds; immense tail-coverts like the peacock; enormously expanded wing-feathers, as in the argus-pheasant; or magnificent plumes from the region of the coracoids in many of the birds of paradise. It is to be noted, also, that all these accessory plumes spring from parts of the body which, in other species, are distinguished by patches of colour; so that we may probably impute the development of colour and of accessory plumage to the same fundamental cause.
Among insects, the most brilliant and varied coloration occurs in the butterflies and moths, groups in which the wing-membranes have received their greatest expansion, and whose specialisation has been carried furthest in the marvellous scaly covering which is the seat of the colour. It is suggestive, that the only other group in which functional wings are much coloured is that of the dragonflies, where the membrane is exceedingly expanded. In like manner, the colours of beetles, though greatly inferior to those of the lepidoptera, occur in a group in which the anterior pair of wings has been thickened and modified in order to protect the vital parts, and in which these wing-covers (elytra), in the course of development in the different groups, must have undergone great changes, and have been the seat of very active growth.
The Origin of Accessory Plumes.
Mr. Darwin supposes, that these have in almost every case been developed by the preference of female birds for such males as possessed them in a higher degree than others; but this theory does not account for the fact that these plumes usually appear in a few definite parts of the body. We require some cause to initiate the development in one part rather than in another. Now, the view that colour has arisen over surfaces where muscular and nervous development is considerable, and the fact that it appears especially upon the accessory or highly developed plumes, leads us to inquire whether the same cause has not primarily determined the development of these plumes. The immense tuft of golden plumage in the best known birds of paradise (Paradisea apoda and P. minor) springs from a very small area on the side of the breast. Mr. Frank E. Beddard, who has kindly examined a specimen for me, says that "this area lies upon the pectoral muscles, and near to the point where the fibres of the muscle converge towards their attachment to the humerus. The plumes arise, therefore, close to the most powerful muscle of the body, and near to where the activities of that muscle would be at a maximum. Furthermore, the area of attachment of the plumes is just above the point where the arteries and nerves for the supply of the pectoral muscles, and neighbouring regions, leave the interior of the body. The area of attachment of the plume is, also, as you say in your letter, just above the junction of the coracoid and sternum." Ornamental plumes of considerable size rise from the same part in many other species of paradise birds, sometimes extending laterally in front, so as to form breast shields. They also occur in many humming-birds, and in some sun-birds and honey-suckers; and in all these cases there is a wonderful amount of activity and rapid movement, indicating a surplus of vitality, which is able to manifest itself in the development of these accessory plumes.[133]
In a quite distinct set of birds, the gallinaceae, we find the ornamental plumage usually arising from very different parts, in the form of elongated tail-feathers or tail-coverts, and of ruffs or hackles from the neck. Here the wings are comparatively little used, the most constant activities depending on the legs, since the gallinaceae are pre-eminently walking, running, and scratching birds. Now the magnificent train of the peacock—the grandest development of accessory plumes in this order—springs from an oval or circular area, about three inches in diameter, just above the base of the tail, and, therefore, situated over the lower part of the spinal column near the insertion of the powerful muscles which move the hind limbs and elevate the tail. The very frequent presence of neck-ruffs or breast-shields in the males of birds with accessory plumes may be partly due to selection, because they must serve as a protection in their mutual combats, just as does the lion's or the horse's mane. The enormously lengthened plumes of the bird of paradise and of the peacock can, however, have no such use, but must be rather injurious than beneficial in the bird's ordinary life. The fact that they have been developed to so great an extent in a few species is an indication of such perfect adaptation to the conditions of existence, such complete success in the battle for life, that there is, in the adult male at all events, a surplus of strength, vitality, and growth-power which is able to expend itself in this way without injury. That such is the case is shown by the great abundance of most of the species which possess these wonderful superfluities of plumage. Birds of paradise are among the commonest birds in New Guinea, and their loud voices can be often heard when the birds themselves are invisible in the depths of the forest; while Indian sportsmen have described the peafowl as being so abundant, that from twelve to fifteen hundred have been seen within an hour at one spot; and they range over the whole country from the Himalayas to Ceylon. Why, in allied species, the development of accessory plumes has taken different forms, we are unable to say, except that it may be due to that individual variability which has served as the starting-point for so much of what seems to us strange in form, or fantastic in colour, both in the animal and vegetable world.
Development of Accessory Plumes and their Display.
If we have found a vera causafor the origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of the integument where muscular and nervous action are greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to the very strongest and most energetic the parentage of the next generation. And, as all the evidence goes to show that, so far as female birds exercise any choice, it is of "the most vigorous, defiant, and mettlesome male," this form of sexual selection will act in the same direction, and help to carry on the process of plume development to its culmination. That culmination will be reached when the excessive length or abundance of the plumes begins to be injurious to the bearer of them; and it may be this check to the further lengthening of the peacock's train that has led to the broadening of the feathers at the ends, and the consequent production of the magnificent eye-spots which now form its crowning ornament.
The display of these plumes will result from the same causes which led to their production. Just in proportion as the feathers themselves increased in length and abundance, the skin-muscles which serve to elevate them would increase also; and the nervous development as well as the supply of blood to these parts being at a maximum, the erection of the plumes would become a habit at all periods of nervous or sexual excitement. The display of the plumes, like the existence of the plumes themselves, would be the chief external indication of the maturity and vigour of the male, and would, therefore, be necessarily attractive to the female. We have, thus, no reason for imputing to her any of those aesthetic emotions which are excited in us, by the beauty of form, colour, and pattern of these plumes; or the still more improbable aesthetic tastes, which would cause her to choose her mate on account of minute differences in their forms, colours, or patterns.
As co-operating causes in the production of accessory ornamental plumes, I have elsewhere suggested[134] that crests and other erectile feathers may have been useful in making the bird more formidable in appearance, and thus serving to frighten away enemies; while long tail or wing feathers might serve to distract the aim of a bird of prey. But though this might be of some use in the earlier stages of their development, it is probably of little importance compared with the vigour and pugnacity of which the plumes are the indication, and which enable most of their possessors to defend themselves against the enemies which are dangerous to weaker and more timid birds. Even the tiny humming-birds are said to attack birds of prey that approach too near to their nests.
The Effect of Female Preference will be Neutralised by Natural Selection.
The various facts and arguments now briefly set forth, afford an explanation of the phenomena of male ornament, as being due to the general laws of growth and development, and make it unnecessary to call to our aid so hypothetical a cause as the cumulative action of female preference. There remains, however, a general argument, arising from the action of natural selection itself, which renders it almost inconceivable that female preference could have been effective in the way suggested; while the same argument strongly supports the view here set forth. Natural selection, as we have seen in our earlier chapters, acts perpetually and on an enormous scale in weeding out the "unfit" at every stage of existence, and preserving only those which are in all respects the very best. Each year, only a small percentage of young birds survive to take the place of the old birds which die; and the survivors will be those which are best able to maintain existence from the egg onwards, an important factor being that their parents should be well able to feed and protect them, while they themselves must in turn be equally able to feed and protect their own offspring. Now this extremely rigid action of natural selection must render any attempt to select mere ornament utterly nugatory, unless the most ornamented always coincide with "the fittest" in every other respect; while, if they do so coincide, then any selection of ornament is altogether superfluous. If the most brightly coloured and fullest plumaged males are not the most healthy and vigorous, have not the best instincts for the proper construction and concealment of the nest, and for the care and protection of the young, they are certainly not the fittest, and will not survive, or be the parents of survivors. If, on the other hand, there is generally this correlation—if, as has been here argued, ornament is the natural product and direct outcome of superabundant health and vigour, then no other mode of selection is needed to account for the presence of such ornament. The action of natural selection does not indeed disprove the existence of female selection of ornament as ornament, but it renders it entirely ineffective; and as the direct evidence for any such female selection is almost nil, while the objections to it are certainly weighty, there can be no longer any reason for upholding a theory which was provisionally useful in calling attention to a most curious and suggestive body of facts, but which is now no longer tenable. The term "sexual selection" must, therefore, be restricted to the direct results of male struggle and combat. This is really a form of natural selection, and is a matter of direct observation; while its results are as clearly deducible as those of any of the other modes in which selection acts. And if this restriction of the term is needful in the case of the higher animals it is much more so with the lower. In butterflies the weeding out by natural selection takes place to an enormous extent in the egg, larva, and pupa states; and perhaps not more than one in a hundred of the eggs laid produces a perfect insect which lives to breed. Here, then, the impotence of female selection, if it exist, must be complete; for, unless the most brilliantly coloured males are those which produce the best protected eggs, larvae, and pupae, and unless the particular eggs, larvae, and pupae, which are able to survive, are those which produce the most brilliantly coloured butterflies, any choice the female might make must be completely swamped. If, on the other hand, there is this correlation between colour development and perfect adaptation to conditions in all stages, then this development will necessarily proceed by the agency of natural selection and the general laws which determine the production of colour and of ornamental appendages.[135]
General Laws of Animal Coloration.
The condensed account which has now been given of the phenomena of colour in the animal world will sufficiently show the wonderful complexity and extreme interest of the subject; while it affords an admirable illustration of the importance of the great principle of utility, and of the effect of the theories of natural selection and development in giving a new interest to the most familiar facts of nature. Much yet remains to be done, both in the observation of new facts as to the relations between the colours of animals and their habits or economy, and, more especially, in the elucidation of the laws of growth which determine changes of colour in the various groups; but so much is already known that we are able, with some confidence, to formulate the general principles which have brought about all the beauty and variety of colour which everywhere delight us in our contemplation of animated nature. A brief statement of these principles will fitly conclude our exposition of the subject.
1. Colour may be looked upon as a necessary result of the highly complex chemical constitution of animal tissues and fluids. The blood, the bile, the bones, the fat, and other tissues have characteristic, and often brilliant colours, which we cannot suppose to have been determined for any special purpose, as colours, since they are usually concealed. The external organs, with their various appendages and integuments, would, by the same general laws, naturally give rise to a greater variety of colour.
2. We find it to be the fact that colour increases in variety and intensity as external structures and dermal appendages become more differentiated and developed. It is on scales, hair, and especially on the more highly specialised feathers, that colour is most varied and beautiful; while among insects colour is most fully developed in those whose wing membranes are most expanded, and, as in the lepidoptera, are clothed with highly specialised scales. Here, too, we find an additional mode of colour production in transparent lamellae or in fine surface striae which, by the laws of interference, produce the wonderful metallic hues of so many birds and insects.
3. There are indications of a progressive change of colour, perhaps in some definite order, accompanying the development of tissues or appendages. Thus spots spread and fuse into bands, and when a lateral or centrifugal expansion has occurred—as in the termination of the peacocks' train feathers, the outer web of the secondary quills of the Argus pheasant, or the broad and rounded wings of many butterflies—into variously shaded or coloured ocelli. The fact that we find gradations of colour in many of the more extensive groups, from comparatively dull or simple to brilliant and varied hues, is an indication of some such law of development, due probably to progressive local segregation in the tissues of identical chemical or organic molecules, and dependent on laws of growth yet to be investigated.
4. The colours thus produced, and subject to much individual variation, have been modified in innumerable ways for the benefit of each species. The most general modification has been in such directions as to favour concealment when at rest in the usual surroundings of the species, sometimes carried on by successive steps till it has resulted in the most minute imitation of some inanimate object or exact mimicry of some other animal. In other cases bright colours or striking contrasts have been preserved, to serve as a warning of inedibility or of dangerous powers of attack. Most frequent of all has been the specialisation of each distinct form by some tint or marking for purposes of easy recognition, especially in the case of gregarious animals whose safety largely depends upon association and mutual defence.
5. As a general rule the colours of the two sexes are alike; but in the higher animals there appears a tendency to deeper or more intense colouring in the male, due probably to his greater vigour and excitability. In many groups in which this superabundant vitality is at a maximum, the development of dermal appendages and brilliant colours has gone on increasing till it has resulted in a great diversity between the sexes; and in most of these cases there is evidence to show that natural selection has caused the female to retain the primitive and more sober colours of the group for purposes of protection.
The general principles of colour development now sketched out enable us to give some rational explanation of the wonderful amount of brilliant colour which occurs among tropical animals. Looking on colour as a normal product of organisation, which has either been allowed free play, or has been checked and modified for the benefit of the species, we can see at once that the luxuriant and perennial vegetation of the tropics, by affording much more constant means of concealment, has rendered brilliant colour less hurtful there than in the temperate and colder regions. Again, this perennial vegetation supplies abundance of both vegetable and insect food throughout the year, and thus a greater abundance and greater variety of the forms of life are rendered possible, than where recurrent seasons of cold and scarcity reduce the possibilities of life to a minimum. Geology furnishes us with another reason, in the fact, that throughout the tertiary period tropical conditions prevailed far into the temperate regions, so that the possibilities of colour development were still greater than they are at the present time. The tropics, therefore, present to us the results of animal development in a much larger area and under more favourable conditions than prevail to-day. We see in them samples of the productions of an earlier and a better world, from an animal point of view; and this probably gives a greater variety and a finer display of colour than would have been produced, had conditions always been what they are now. The temperate zones, on the other hand, have recently suffered the effects of a glacial period of extreme severity, with the result that almost the only gay coloured birds they now possess are summer visitors from tropical or sub-tropical lands. It is to the unbroken and almost unchecked course of development from remote geological times that has prevailed in the tropics, favoured by abundant food and perennial shelter, that we owe such superb developments as the frills and crests and jewelled shields of the humming-birds, the golden plumes of the birds of paradise, and the resplendent train of the peacock. This last exhibits to us the culmination of that marvel and mystery of animal colour which is so well expressed by a poet-artist in the following lines. The marvel will ever remain to the sympathetic student of nature, but I venture to hope that in the preceding chapters I have succeeded in lifting—if only by one of its corners—the veil of mystery which has for long shrouded this department of nature.
On a Peacock's Feather.
[118] Darwin's Descent of Man, p. 271.
[119] Darwin's Descent of Man, p. 294, and footnote.
[120] Nature, 1871, p. 489.
[121] Darwin in Nature, 1880, p. 237.
[122] See the author's Contributions to Natural Selection, chap. vii. in which these facts were first brought forward.
[123] On this point see the author's Contributions to Natural Selection, chap. v. i.
[124] Seebohm's History of British Birds, vol. ii., introduction, p. xiii.
[125] For details see Darwin's Descent of Man, chap. xii.
[126] Descent of Man, pp. 417, 418, 420.
[127] Notes of a Naturalist on the Challenger.
[128] Descent of Man, pp. 401, 402.
[129] Coloration in Animals and Plants, London, 1886.
[130] Coloration of Animals, Pl. X, p. 90; and Pls. II, III, and IV, pp. 30, 40, 42.
[131] See coloured Fig. in Proc. Zool. Soc., 1871, p. 626.
[132] A. Tylor's Coloration, p. 40; and Photograph in Hutchinson's Illustrations of Clinical Surgery, quoted by Tylor.
[133] For activity and pugnacity of humming-birds, see Tropical Nature, pp. 130, 213.
[134] Tropical Nature, p. 209. In Chapter V of this work the views here advocated were first set forth, and the reader is referred there for further details.
[135] The Rev. O. Pickard-Cambridge, who has devoted himself to the study of spiders, has kindly sent me the following extract from a letter, written in 1869, in which he states his views on this question:—
"I myself doubt that particular application of the Darwinian theory which attributes male peculiarities of form, structure, colour, and ornament to female appetency or predilection. There is, it seems to me, undoubtedly something in the male organisation of a special, and sexual nature, which, of its own vital force, develops the remarkable male peculiarities so commonly seen, and of no imaginable use to that sex. In as far as these peculiarities show a great vital power, they point out to us the finest and strongest individuals of the sex, and show us which of them would most certainly appropriate to themselves the best and greatest number of females, and leave behind them the strongest and greatest number of progeny. And here would come in, as it appears to me, the proper application of Darwin's theory of Natural Selection; for the possessors of greatest vital power being those most frequently produced and reproduced, the external signs of it would go on developing in an ever-increasing exaggeration, only to be checked where it became really detrimental in some respect or other to the individual."
This passage, giving the independent views of a close observer—one, moreover, who has studied the species of an extensive group of animals both in the field and in the laboratory—very nearly accords with my own conclusions above given; and, so far as the matured opinions of a competent naturalist have any weight, afford them an important support.
THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
The general colour relations of plants—Colours of fruits—The meaning of nuts—Edible or attractive fruits—The colours of flowers—Modes of securing cross-fertilisation—The interpretation of the facts—Summary of additional facts bearing on insect fertilisation—Fertilisation of flowers by birds—Self-fertilisation of flowers—Difficulties and contradictions—Intercrossing not necessarily advantageous—Supposed evil results of close interbreeding—How the struggle for existence acts among flowers—Flowers the product of insect agency—Concluding remarks on colour in nature.
The colours of plants are both less definite and less complex than are those of animals, and their interpretation on the principle of utility is, on the whole, more direct and more easy. Yet here, too, we find that in our investigation of the uses of the various colours of fruits and flowers, we are introduced to some of the most obscure recesses of nature's workshop, and are confronted with problems of the deepest interest and of the utmost complexity.
So much has been written on this interesting subject since Mr. Darwin first called attention to it, and its main facts have become so generally known by means of lectures, articles, and popular books, that I shall give here a mere outline sketch, for the purpose of leading up to a discussion of some of the more fundamental problems which arise out of the facts, and which have hitherto received less attention than they deserve.
The General Colour Relations of Plants.
The green colour of the foliage of leafy plants is due to the existence of a substance called chlorophyll, which is almost universally developed in the leaves under the action of light. It is subject to definite chemical changes during the processes of growth and of decay, and it is owing to these changes that we have the delicate tints of spring foliage, and the more varied, intense, and gorgeous hues of autumn. But these all belong to the class of intrinsic or normal colours, due to the chemical constitution of the organism; as colours they are unadaptive, and appear to have no more relation to the wellbeing of the plants themselves than have the colours of gems and minerals. We may also include in the same category those algae and fungi which have bright colours—the "red snow" of the arctic regions, the red, green, or purple seaweeds, the brilliant scarlet, yellow, white, or black agarics, and other fungi. All these colours are probably the direct results of chemical composition or molecular structure, and, being thus normal products of the vegetable organism, need no special explanation from our present point of view; and the same remark will apply to the varied tints of the bark of trunks, branches, and twigs, which are often of various shades of brown and green, or even vivid reds or yellows.
There are, however, a few cases in which the need of protection, which we have found to be so important an agency in modifying the colours of animals, has also determined those of some of the smaller members of the vegetable kingdom. Dr. Burchell found a mesembryanthomum in South Africa like a curiously shaped pebble, closely resembling the stones among which it grew;[136] and Mr. J.P. Mansel Weale states that in the same country one of the Asclepiadeae has tubers growing above ground among stones which they exactly resemble, and that, when not in leaf, they are for this reason quite invisible.[137] It is clear that such resemblances must be highly useful to these plants, inhabiting an arid country abounding in herbivorous mammalia, which, in times of drought or scarcity, will devour everything in the shape of a fleshy stem or tuber.
True mimicry is very rare in plants, though adaptation to like conditions often produces in foliage and habit a similarity that is deceiving. Euphorbias growing in deserts often closely resemble cacti. Seaside plants and high alpine plants of different orders are often much alike; and innumerable resemblances of this kind are recorded in the names of plants, as Veronica epacridea (the veronica like an epacris), Limnanthemum nymphaeoides (the limnanthemum like a nymphaea), the resembling species in each case belonging to totally distinct families. But in these cases, and in most others that have been observed, the essential features of true mimicry are absent, inasmuch as the one plant cannot be supposed to derive any benefit from its close resemblance to the other, and this is still more certain from the fact that the two species usually inhabit different localities. A few cases exist, however, in which there does seem to be the necessary accordance and utility. Mr. Mansel Weale mentions a labiate plant (Ajuga ophrydis), the only species of the genus Ajuga in South Africa, which is strikingly like an orchid of the same country; while a balsam (Impatiens capensis), also a solitary species of the genus in that country, is equally like an orchid, growing in the same locality and visited by the same insects. As both these genera of plants are specialised for insect fertilisation, and both of the plants in question are isolated species of their respective genera, we may suppose that, when they first reached South Africa they were neglected by the insects of the country; but, being both remotely like orchids in form of flower, those varieties that approached nearest to the familiar species of the country were visited by insects and cross-fertilised, and thus a closer resemblance would at length be brought about. Another case of close general resemblance, is that of our common white dead-nettle (Lamium album) to the stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this is a case of true mimicry, the dead-nettle being benefited by being mistaken by grazing animals for the stinging-nettle.[138]
It is when we come to the essential parts of plants on which their perpetuation and distribution depends, that we find colour largely utilised for a distinct purpose in flowers and fruits. In the former we find attractive colours and guiding marks to secure cross-fertilisation by insects; in the latter attractive or protective coloration, the first to attract birds or other animals when the fruits are intended to be eaten, the second to enable them to escape being eaten when it would be injurious to the species. The colour phenomena of fruits being much the most simple will be considered first.
The perpetuation and therefore the very existence of each species of flowering plant depend upon its seeds being preserved from destruction and more or less effectually dispersed over a considerable area. The dispersal is effected either mechanically or by the agency of animals. Mechanical dispersal is chiefly by means of air-currents, and large numbers of seeds are specially adapted to be so carried, either by being clothed with down or pappus, as in the well-known thistle and dandelion seeds; by having wings or other appendages, as in the sycamore, birch, and many other trees; by being thrown to a considerable distance by the splitting of the seed-vessel, and by many other curious devices.[139] Very large numbers of seeds, however, are so small and light that they can be carried enormous distances by gales of wind, more especially as most of this kind are flattened or curved, so as to expose a large surface in proportion to their weight. Those which are carried by animals have their surfaces, or that of the seed-vessel, armed with minute hooks, or some prickly covering which attaches itself to the hair of mammalia or the feathers of birds, as in the burdock, cleavers, and many other species. Others again are sticky, as in Plumbago europaea, mistletoe, and many foreign plants.
All the seeds or seed-vessels which are adapted to be dispersed in any of these ways are of dull protective tints, so that when they fall on the ground they are almost indistinguishable; besides which, they are usually small, hard, and altogether unattractive, never having any soft, juicy pulp; while the edible seeds often bear such a small proportion to the hard, dry envelopes or appendages, that few animals would care to eat them.
The Meaning of Nuts.
There is, however, another class of fruits or seeds, usually termed nuts, in which there is a large amount of edible matter, often very agreeable to the taste, and especially attractive and nourishing to a large number of animals. But when eaten, the seed is destroyed and the existence of the species endangered. It is evident, therefore, that it is by a kind of accident that these nuts are eatable; and that they are not intended to be eaten is shown by the special care nature seems to have taken to conceal or to protect them. We see that all our common nuts are green when on the tree, so as not easily to be distinguished from the leaves; but when ripe they turn brown, so that when they fall on to the ground they are equally indistinguishable among the dead leaves and twigs, or on the brown earth. Then they are almost always protected by hard coverings, as in hazel-nuts, which are concealed by the enlarged leafy involucre, and in the large tropical brazil-nuts and cocoa-nuts by such a hard and tough case as to be safe from almost every animal. Others have an external bitter rind, as in the walnut; while in the chestnuts and beech-nuts two or three fruits are enclosed in a prickly involucre.
Notwithstanding all these precautions, nuts are largely devoured by mammalia and birds; but as they are chiefly the product of trees or shrubs of considerable longevity, and are generally produced in great profusion, the perpetuation of the species is not endangered. In some cases the devourers of nuts may aid in their dispersal, as they probably now and then swallow the seed whole, or not sufficiently crushed to prevent germination; while squirrels have been observed to bury nuts, many of which are forgotten and afterwards grow in places they could not have otherwise reached.[140] Nuts, especially the larger kinds which are so well protected by their hard, nearly globular cases, have their dispersal facilitated by rolling down hill, and more especially by floating in rivers and lakes, and thus reaching other localities. During the elevation of land areas this method would be very effective, as the new land would always be at a lower level than that already covered with vegetation, and therefore in the best position for being stocked with plants from it.
The other modes of dispersal of seeds are so clearly adapted to their special wants, that we feel sure they must have been acquired by the process of variation and natural selection. The hooked and sticky seeds are always those of such herbaceous plants as are likely, from their size, to come in contact with the wool of sheep or the hair of cattle; while seeds of this kind never occur on forest trees, on aquatic plants, or even on very dwarf creepers or trailers. The winged seed-vessels or seeds, on the other hand, mostly belong to trees and to tall shrubs or climbers. We have, therefore, a very exact adaptation to conditions in these different modes of dispersal; while, when we come to consider individual cases, we find innumerable other adaptations, some of which the reader will find described in the little work by Sir John Lubbock already referred to.
Edible or Attractive Fruits.
It is, however, when we come to true fruits (in a popular sense) that we find varied colours evidently intended to attract animals, in order that the fruits may be eaten, while the seeds pass through the body undigested and are then in the fittest state for germination. This end has been gained in a great variety of ways, and with so many corresponding adaptations as to leave no doubt as to the value of the result. Fruits are pulpy or juicy, and usually sweet, and form the favourite food of innumerable birds and some mammals. They are always coloured so as to contrast with the foliage or surroundings, red being the most common as it is certainly the most conspicuous colour, but yellow, purple, black, or white being not uncommon. The edible portion of fruits is developed from different parts of the floral envelopes, or of the ovary, in the various orders and genera. Sometimes the calyx becomes enlarged and fleshy, as in the apple and pear tribe; more often the integuments of the ovary itself are enlarged, as in the plum, peach, grape, etc.; the receptacle is enlarged and forms the fruit of the strawberry; while the mulberry, pineapple, and fig are examples of compound fruits formed in various ways from a dense mass of flowers.
In all cases the seeds themselves are protected from injury by various devices. They are small and hard in the strawberry, raspberry, currant, etc., and are readily swallowed among the copious pulp. In the grape they are hard and bitter; in the rose (hip) disagreeably hairy; in the orange tribe very bitter; and all these have a smooth, glutinous exterior which facilitates their being swallowed. When the seeds are larger and are eatable, they are enclosed in an excessively hard and thick covering, as in the various kinds of "stone" fruit (plums, peaches, etc.), or in a very tough core, as in the apple. In the nutmeg of the Eastern Archipelago we have a curious adaptation to a single group of birds. The fruit is yellow, somewhat like an oval peach, but firm and hardly eatable. This splits open and shows the glossy black covering of the seed or nutmeg, over which spreads the bright scarlet arillus or "mace," an adventitious growth of no use to the plant except to attract attention. Large fruit pigeons pluck out this seed and swallow it entire for the sake of the mace, while the large nutmeg passes through their bodies and germinates; and this has led to the wide distribution of wild nutmegs over New Guinea and the surrounding islands.
In the restriction of bright colour to those edible fruits the eating of which is beneficial to the plant, we see the undoubted result of natural selection; and this is the more evident when we find that the colour never appears till the fruit is ripe—that is, till the seeds within it are fully matured and in the best state for germination. Some brilliantly coloured fruits are poisonous, as in our bitter-sweet (Solanum dulcamara), cuckoo-pint (Arum) and the West Indian manchineel. Many of these are, no doubt, eaten by animals to whom they are harmless; and it has been suggested that even if some animals are poisoned by them the plant is benefited, since it not only gets dispersed, but finds, in the decaying body of its victim, a rich manure heap.[141] The particular colours of fruits are not, so far as we know, of any use to them other than as regards conspicuousness, hence a tendency to any decided colour has been preserved and accumulated as serving to render the fruit easily visible among its surroundings of leaves or herbage. Out of 134 fruit-bearing plants in Mongredien's Trees and Shrubs, and Hooker's British Flora, the fruits of no less than sixty-eight, or rather more than half, are red, forty-five are black, fourteen yellow, and seven white. The great prevalence of red fruits is almost certainly due to their greater conspicuousness having favoured their dispersal, though it may also have arisen in part from the chemical changes of chlorophyll during ripening and decay producing red tints as in many fading leaves. Yet the comparative scarcity of yellow in fruits, while it is the most common tint of fading leaves, is against this supposition.
There are, however, a few instances of coloured fruits which do not seem to be intended to be eaten; such are the colocynth plant (Cucumis colocynthus), which has a beautiful fruit the size and colour of an orange, but nauseous beyond description to the taste. It has a hard rind, and may perhaps be dispersed by being blown along the ground, the colour being an adventitious product; but it is quite possible, notwithstanding its repulsiveness to us, that it may be eaten by some animals. With regard to the fruit of another plant, Calotropis procera, there is less doubt, as it is dry and full of thin, flat-winged seeds, with fine silky filaments, eminently adapted for wind-dispersal; yet it is of a bright yellow colour, as large as an apple, and therefore very conspicuous. Here, therefore, we seem to have colour which is a mere byproduct of the organism and of no use to it; but such cases are exceedingly rare, and this rarity, when compared with the great abundance of cases in which there is an obvious purpose in the colour, adds weight to the evidence in favour of the theory of the attractive coloration of edible fruits in order that birds and other animals may assist in their dispersal. Both the above-named plants are natives of Palestine and the adjacent arid countries.[142]
The Colours of Flowers.
Flowers are much more varied in their colours than fruits, as they are more complex and more varied in form and structure; yet there is some parallelism between them in both respects. Flowers are frequently adapted to attract insects as fruits are to attract birds, the object being in the former to secure cross-fertilisation, in the latter dispersal; while just as colour is an index of the edibility of fruits which supply pulp or juice to birds, so are the colours of flowers an indication of the presence of nectar or of pollen which are devoured by insects.
The main facts and many of the details, as to the relation of insects to flowers, were discovered by Sprengel in 1793. He noticed the curious adaptation of the structure of many flowers to the particular insects which visit them; he proved that insects do cross-fertilise flowers, and he believed that this was the object of the adaptations, while the presence of nectar and pollen ensured the continuance of their visits; yet he missed discovering the use of this cross-fertilisation. Several writers at a later period obtained evidence that cross-fertilisation of plants was a benefit to them; but the wide generality of this fact and its intimate connection with the numerous and curious adaptations discovered by Sprengel, was first shown by Mr. Darwin, and has since been demonstrated by a vast mass of observations, foremost among which are his own researches on orchids, primulas, and other plants.[143]
By an elaborate series of experiments carried on for many years Mr. Darwin demonstrated the great value of cross-fertilisation in increasing the rapidity of growth, the strength and vigour of the plant, and in adding to its fertility. This effect is produced immediately, not as he expected would be the case, after several generations of crosses. He planted seeds from cross-fertilised and self-fertilised plants on two sides of the same pot exposed to exactly similar conditions, and in most cases the difference in size and vigour was amazing, while the plants from cross-fertilised parents also produced more and finer seeds. These experiments entirely confirmed the experience of breeders of animals already referred to (p. 160), and led him to enunciate his famous aphorism, "Nature abhors perpetual self-fertilisation".[144] In this principle we appear to have a sufficient reason for the various contrivances by which so many flowers secure cross-fertilisation, either constantly or occasionally. These contrivances are so numerous, so varied, and often so highly complex and extraordinary, that they have formed the subject of many elaborate treatises, and have also been amply popularised in lectures and handbooks. It will be unnecessary, therefore, to give details here, but the main facts will be summarised in order to call attention to some difficulties of the theory which seem to require further elucidation.
Modes of securing Cross-Fertilisation.
When we examine the various modes in which the cross-fertilisation of flowers is brought about, we find that some are comparatively simple in their operation and needful adjustments, others highly complex. The simple methods belong to four principal classes:—(1) By dichogamy—that is, by the anthers and the stigma becoming mature or in a fit state for fertilisation at slightly different times on the same plant. The result of this is that, as plants in different stations, on different soils, or exposed to different aspects flower earlier or later, the mature pollen of one plant can only fertilise some plant exposed to somewhat different conditions or of different constitution, whose stigma will be mature at the same time; and this difference has been shown by Darwin to be that which is adapted to secure the fullest benefit of cross-fertilisation. This occurs in Geranium pratense, Thymus serpyllum, Arum maculatum, and many others. (2) By the flower being self-sterile with its own pollen, as in the crimson flax. This absolutely prevents self-fertilisation. (3) By the stamens and anthers being so placed that the pollen cannot fall upon the stigma, while it does fall upon a visiting insect which carries it to the stigma of another flower. This effect is produced in a variety of very simple ways, and is often aided by the motion of the stamens which bend down out of the way of the stigmas before the pollen is ripe, as in Malva sylvestris (see Fig. 28). (4) By the male and female flowers being on different plants, forming the class Dioecia of Linnaeus. In these cases the pollen may be carried to the stigmas either by the wind or by the agency of insects.