There is no relation of homology, however remote, between the wings of a bat and of a beetle, and these two animals (as will shortly appear) have the parts and organs of their bodies so fundamentally different, that it is doubtful whether any definite relations of homology can be established between them.
A special term has been devoted to signify a resemblance between two parts in two different animals and plants, which resemblance has been induced by or is directly related to their common needs, and the similarity of external influences. This term is “homoplasy,” and structures which may thus be supposed to have grown alike in obedience to the influence of similar external causes acting on similar innate powers have been called Homoplasts.
Such, then, are the more general conditions as to structure and figure which living creatures present, and (as has been said) with great differences as to the amount of possible variation, most kinds have a definite limit as to size. It remains only to make general observations on the colours of living creatures.
But a few years ago, hardly any few general remarks of really scientific interest and value could have been made respecting the varied hues and markings which organisms present. No rational relation was even suspected to exist between the colours of plants and the busy insect life which swarms about their blossoms or about the varied colours of birds, and the details of their habits and modes of existence.
It was known, of course, that Arctic foxes and hares became white in winter, and that each benefited by its change, and suffered from the change of the other; the snow tint which enabled the hare to escape also facilitating the unobserved approach of the fox. It was also known that many desert animals were of the colour of the sandy plain they wandered over, and that tree-snakes and tree-frogs were often green. But it seemed incredible that the varied shades or bright adornments of the living world should each and all be governed by rigid laws, generally connected with the welfare of the organisms so furnished. Here, if anywhere, the reign of utilitarianism in Nature appeared to be at an end, and creative fancy to have full play, regardless but of the harmony and beauty thus revealed to appreciating eyes. The labours and fruitful thoughts of Bates and Wallace have, however, opened up a wide field for most interesting inquiry. They have made it evident that in many instances the most direct utility accompanies colour both in animals and plants. The colours of flowers serve to attract insects and birds, by the visits of which they are fertilized or their fertility is greatly augmented. It is this relation between attractiveness and insect fertilization which explains the absence of colour from the flowers of plants which are fertilized only by the wind, such as the fir trees before-mentioned, oaks, beeches, nettles, sedges, and many others. It also explains the conspicuousness of the flowers of many oceanic islands, such as those of the Galapagos archipelago. But it also explains, as Mr. Wallace has pointed out, the remarkable beauty of Alpine flowers, by their need of attracting insects from a distance, the conspicuous patches of bright colour serving thus to attract wandering butterflies upwards from the valleys.
But more remarkable still is the explanation given to the semblance borne by the colours of some creatures to those of others of quite a different kind, as of some moths to bees, and some harmless flies to wasps. For now it is clear that by this mimicry they escape the attacks of many enemies, who avoid such apparently dangerous forms. On the other hand, the bright liveries of such offensive creatures are highly useful to the wearers, for such tints act as a warning to enemies, and so save them from their being pounced on by creatures which might fatally wound them, though unable to swallow them. But the beautiful liveries of such powerful predatory kinds as tigers and leopards do not serve as warnings. They serve their wearers, however, none the less, though it is by aiding their concealment, and so allowing their prey to approach them unsuspectingly to fatal nearness. For the vertical stripes of the tiger resemble the vertical shadows of the grasses of the jungle amongst which it lurks, as the scattered spots of the leopard agree with the scattered spots of shadow amongst the foliage of trees on the boughs of which it lies in wait. But to say more on this head would be to anticipate remarks to come, when the relations of living beings to one another are under consideration, and the subject is too extensive to be here treated in full. Moreover, it must be noted that such relations do not by any means serve to explain all the phenomena of organic colour. Direct action is in some curious way exerted upon many organisms, by surrounding tints, and similarly different geographical districts and varieties of locality affect directly the colour of both animals and plants, but these questions will be fully treated of under the head of the relations of animals to the physical world. Suffice it here to note that the phenomena of colour no less than the phenomena of form are in harmony with (whether or not the result of) the active agencies of all environing conditions. But colour of some kind is a universal attribute of all material things. Though apparently most irregularly distributed through the world of life, yet order underlies the seeming confusion. Of certain large groups certain tints are characteristic, as has already been remarked with respect to the great order to which the dandelion belongs. But the same remark may be made of various others, as, for example, of the order Cruciferæ (to which the wallflower and turnip belong), the flowers of which are generally white, pink, or yellow, while the gentians, again, are noteworthy for exhibiting pure colours.
But the colours which predominate in the whole mass of living creatures of all kinds are tints of green, brown, or reddish-yellow. Bright colours, such as blue, scarlet, crimson, gold, or silver are exceptional, and the colour blue is especially rare. The borrowed radiance of the inorganic world, in the form of metallic brightness, is especially a characteristic of those living gems, the humming birds; but not a few other animals also exhibit it. Thus, of birds more or less gifted with metallic radiance, though in a less degree than humming birds, may be mentioned the sunbirds, the trogons, and the beautiful family of pheasants; and many insects and many fishes shine with metallic tints.
Brightness of this kind (though the leaves of a few plants have a coppery lustre) is unknown in the world of plants, in which shades of green are overwhelmingly predominant, and are universally present, except in a few exceptional forms, notably the fungi.[98]
Various aquatic animals belonging to very different groups agree in possessing a perfectly glass-like transparency. Amongst them are fish which live in the ocean; for example, the Teleostean[99] fish (Leptocephalus), also mollusca of all kinds, including even perfectly transparent cuttle fishes.[100] There are also glass-like crustaceans,[101] and also planarians[102] and sea anemones.[103] Plants, however, never present this character, although by it they might, as well as animals, escape being preyed upon.
Most fishes which inhabit the deep sea are of a dull black colour, though some are white, and the majority of all deep-sea animals, considered as a whole, are more or less decidedly coloured, many brightly so.[104]