In other cases the course of the process of selection has been such that, though the general colouring has not been changed, variations have appeared in particular regions of the body—spots or stripes which accumulated through the ages and co-operated to form an increasingly diverse and complex colour-scheme, such a 'marking' of the animal as we may observe to-day, especially in butterflies, but also in birds.
It is a fine corroboration of the origin of bright colours through sexual selection that, even in those groups of the animal kingdom which are in general sexually monomorphic, there always occur some species in which male and female are quite different, and a host of species in which both sexes are alike in the main, yet with differences in certain minor points. Among the parrots similarity of colouring prevails as a general rule, but in New Guinea there lives a parrot the female of which is a gorgeous blood-red and the male a beautiful light-green; minor differences occur in many species, for instance, the female of the horned parrot (Cyanorhamphus cornutus Gm.) lacks the two long black and red feathers on the head, that of the grass-parakeet (Melopsittacus undulatus) is a slightly paler green and has not the beautiful blue spots on the cheeks which the male possesses. Innumerable similar instances might be cited, serving to show that all these distinguishing characters of the males have been acquired step by step and piece by piece, and are slowly and independently transferred to the females—if, indeed, at all.
In yet another way the correctness of the Darwinian theory of sexual selection may be deduced from the markings and coloration of birds and butterflies.
It has frequently struck me, during the long period in which I have been studying brightly coloured birds and butterflies, that those colour-patterns which are referable to sexual selection are much simpler than those which must be referred to species-selection, especially in the case of what we call 'sympathetic coloration.' How crude is the decorative pattern of most parrots, notwithstanding all the brilliance of their colour. Large tracts of the body are red, others green, yellow, blue, and occasionally one finds a red and blue striped feather collar, a head which is red above and yellow underneath, but it is seldom that the colours vary enough in a small space to give rise to a delicate decorative pattern. The gayest of parrots are the Brush Tongues (Trichoglossus), and even among them subtlety of coloration does not go further than the combination of three colours on one of the long tail-feathers, or the production of a double band round the neck, and so forth. If we compare with this the complex markings of the inconspicuously coloured females of the pheasants, of the partridges, or that of the upper surface of the many birds in mingled grey, blackish-brown and white, which resemble the ground or the dried leaves when they crouch, we find that the colour-pattern in these cases is infinitely finer and more complex.
This seems to me quite intelligible when we remember, on the one hand, that species-selection must operate far more intensively than sexual selection, and that in the production of a protective colouring it is a question of deceiving the eye of a sharp-sighted enemy, while the aim of sexual selection is to secure the approval of others of the same species. As long as the enemy on the search for prey perceives the difference between the markings of its victim and those of the surroundings, so long will the gradual and steady improvement of the protective coloration continue, so long will new shades and new lines be added. We can thus understand how there would be gradually reached a complexity of marking to which sexual selection can never attain, or at least only in regard to a few specially favourable points. The eye-spots on the train feathers of the Argus pheasant and the peacock are such points, and these occur among polygamous birds in which sexual selection must be very intense; they are placed, too, on a part of the body, the wheel-shaped train, which is peculiarly suited for communicating the excitement of the male to the female, and must therefore be especially influenced by the latter. In general, however, we may say on a priori grounds that the intensity of species-selection is greater than that of sexual selection, because the former ceaselessly and pitilessly eliminates the less perfect, while the claims of the latter are in any case less imperative, and are also often mollified by a variety of chance circumstances.
But in the case of insects, in particular, we have to add that the protective colours and the decorative colours have been, so to speak, painted by different artists—the former by birds, lizards, and other persecutors endowed with well-developed eyes, the latter by the insects themselves, whose eyes can hardly possess, for objects not quite near, that acuteness of vision which the bird's eye has. Thus we find that the protective coloration of butterflies has often a very complex marking, while the same butterfly may exhibit only a rather crude though brilliant pattern on its upper surface, where the coloration is due to sexual selection. Thus the famous Kallima has on its under surface the likeness of a dry or decayed leaf composed of a number of colour-tones—quite a complex painting. But if we look at the upper surface we see a deep brown with a shimmer of steel blue as the ground-colour of the wings, and on it a broad yellow band and a white spot: that is the whole pattern. We find a similar state of things among many of the forest butterflies of Brazil, and also among our indigenous butterflies. The pattern of our gayest diurnal butterflies, the red Admiral and the tortoiseshell butterfly (Vanessa atalanta and Vanessa cardui), is somewhat crude on the upper surface, and very simple compared with the protective colouring of the under surface, which is made up of hundreds of points, spots, strokes, and lines of every shape and colour. On the other hand, the upper surface of the anterior wings in the hawk-moths and the Noctuidæ exhibits protective coloration, and is made up of curious zigzag complex lines, strokes, and spots, so that it resembles the bark of a tree or a bit of an old wooden fence—a painting, like the modern impressionist work, which, with an apparently meaningless confusion of colour splashes, conveys a perfect impression even of the details of a landscape. In the owl-moths (Noctuidæ) the wing surfaces, which are brightly coloured, are simple, almost crude, in pattern, as in the moths of the genus Catocala, with their red, blue, or yellow posterior wings, traversed by a large black band; while in the Geometer-moths, whose wings are spread out flat when at rest, the protective upper surface of all four wings is covered with a complex pattern of lines, spots, and streaks in different shades of grey, yellow, white, and black, so that it bears a deceptive resemblance to the bark of a tree or the side of a wall. For a long time I could not understand how such a definite and constant pattern could arise through natural selection if it was a case of mimicking the impression of bark or of any other irregularly covered surface, the colours of which are not mingled in exactly the same way everywhere. But now I think I understand it; for in the apparently meaningless colour-splashes of an 'impressionist' landscape the different splashes must be exactly where they are, otherwise on stepping back from the picture one would see, not a Haarlem hyacinth-field, or an avenue of poplars with their golden autumn leaves, but a mere unintelligible daub. It is the type of the colour-pattern that must be attained, and in nature this is attained very slowly, step by step, spot after spot, and therefore, obviously, no correct stroke once attained will be given up again, since, in combination with the rest, it secures the proper type of colour-pattern. Only thus, it seems to me, can we understand how apparently meaningless lines, like the figures 1840 on the under surface of Vanessa atalanta, could have become a constant characteristic of the species.
To sum up briefly, we may say that sexual selection is a much more powerful factor in transformation than we should at first be inclined to believe. It cannot, of course, have been operative in the case of plants, nor can it be taken into consideration in regard to the lower animals, for these, like the plants, do not pair, or, at any rate, do so without any possibility of choice. Animals which live on the sea-floor, or which are attached there, must simply liberate their reproductive cells into the water, and cannot secure that they unite with those of this or that individual. This is the case among sponges, corals, and Hydroid polyps. In some other classes the sense organs are too poorly developed, and the eyes in particular too imperfect to be excited in different degrees by any peculiarities in the appearance or behaviour of the males. This is what Darwin meant when he ascribed to these animals 'too imperfect senses and much too low intelligence' 'to estimate the beauty or other attractive points of the opposite sex, or to feel anything like rivalry.' Accordingly, in the Protozoa, Echinoderms, Medusæ, and Ctenophores, secondary sexual characters are entirely absent, as pairing also is.
In those worms that pair we first meet with secondary sexual characters, and from this level upwards they are never quite absent from any large group, and gradually play an increasingly important rôle.
But the significance of sexual selection lies, as we have seen, not only in the fact that one sex of a species, usually the male, is modified, but in the possibility of the transference of this modification to the females, and further, in the fact that the process of variation may start afresh at any time, and thus one variation may be developed upon or alongside of another. In this way we can explain certain complex and often fantastic forms and colourings which we could not otherwise understand; thus the extraordinary number of nearly related species in some animal groups, such as butterflies and birds, in which the differences mainly concern the colour-patterns.
Darwin has shown convincingly that a surprising number of characters in animals, from worms upwards, have their roots in sexual selection, and has pointed out the probability that this process has played an important part in the evolution of the human race also, though, in this case, all is not yet so clearly and certainly known as among animals.