by Grant Allen
Equally significant are the changes in habit or mode of life between fruit-eating and flower-feeding classes. Thus, a large number of hymenopterous insects live upon honey extracted from flowers; but the omnivorous wasps, as we all know, have taken to surreptitious feasting upon the sugary juices of peaches, pears, and nectarines. In like manner, I have often noted lepidopterous species, whose natural food consists of the nectar in summer blossoms, feeding greedily upon fallen fruit. Mr. W. M. Gabb captured the lovely Morphos of Nicaragua by baiting with a piece of over-ripe banana; and the Rev. J. G. Wood, a most trustworthy recorder in all that concerns the habits of animals, notices on one occasion having seen whole hordes of the Red Admiral butterfly (Vanessa atalanta) darkening the ground where a number of egg-plums lay beneath their parent tree. So, too, amongst birds; while most of them take their sugary food in the form of fruit or seeds, a few kinds, like humming-birds and sun-birds, live largely off the nectar of flowers, mixed with the insects which frequent them. Mr. Webber, an American naturalist, tried the experiment of taming the pretty little ruby-throats, which he fed on syrup alone, and though he found that they could not thrive without a fair proportion of insects as well, he also discovered that they showed a decided partiality for the taste of sugar. “Some which had been thus tamed and set free returned the following year, and at once flew straight to the remembered little cup of sweets.” In certain instances, we find the interchange of habit taking place within the limits of a single tribe. Thus, the true parrots live almost entirely off sweet fruits, but their congeners the lories are nectar-eaters. These facts, once more, we may correlate with well-known human habits; as when we see children, whose taste for sweets is derived from frugivorous ancestors, sucking the juices of honeysuckle and clover, or stealing the honey-bag from our domesticated hive-bees. Indeed, we could have no more significant symbol of the community of nature here pointed out than the fact that we keep these same bees to gather honey for us from the nectaries of flowers.
Conversely, whatever parts of a plant would be injured by the interference of animals, secrete a bitter or acrid juice, which acts deleteriously upon the nerves of taste. Thus, as I have already pointed out, the pericarp, which in fruits-proper is provided with sugary secretions, in nuts is commonly stored with a nauseous principle as a deterrent to animal foes. Again, those fruits which have a sweet pulp generally guard against the loss of their actual seeds by filling them with a bitter substance, of which prussic acid often forms a leading constituent: cases occur in the peach-stone, the apple-pip, and the seeds of oranges or mangoes. That animals as a rule dislike bitter substances is a matter of common observation; and experiments which I have conducted on a small number of insects and birds have always resulted in marks either of indifference or of positive distaste. Thus we see that both in their likes and dislikes a great community of taste runs through all the flower-feeding and fruit-eating animals.
If, now, we turn to the carnivores and carrion-feeders, we shall find a totally different set of sympathies and antipathies. It is true that many dogs and flesh-eating flies love sugar; but they also love numerous other bodies which several of the former class of creatures would never touch. Fresh meat, or still worse, putrid flesh, does not appeal at all to the senses of bees and parrots. Man, of course, forms an intermediate link, a frugivorous animal who has partially adopted carnivorous tastes. Hence we have a certain liking for the flavour of roast beef and turkey; some of us eat high game and caviare; and savages even prefer meat in an advanced stage of decomposition. But these are mere surface-tastes, while the deeper-seated ancestral habits come out strongly in our children and our unsophisticated adults. The liking for strong-tasted meats and half-putrid preparations has to be slowly acquired; whereas the love for sugar, for honey, for fruits, for all sweet things, is born with us into the world, and taken in with our first draught of mother’s milk. And in this connection it is worth while to note that the natural food of the human infant contains 62.3 parts of sugar in 1000, while that of a herbivorous calf possesses only 45.6, so that it becomes necessary in giving cow’s milk to babies to sweeten it considerably up to the proper point.
Secondly, let us look at the sense of smell. Here again we notice that wherever any part of a plant wishes to attract animals, it adds to its sweetness the extra allurement of perfume; and the same perfumes are, as a rule, pleasant to all flower-feeders and fruit-eaters alike. The delicate odour of a peach, a pine-apple, or a strawberry scarcely differs in kind from that of a lily, a hyacinth, or a violet. Mankind, whose tastes in this matter are derived from the tropical fruits, have equal pleasure with bees or butterflies in the dainty scent of clover and meadow-sweet. Only, as might naturally be expected, the perfumes of fruits (which we have already seen reason to believe are comparatively modern structures) are not so highly developed as the perfumes of flowers; whence arises the seeming anomaly that our olfactory nerves are more pleasurably stimulated by the stephanotis or the jasmine, which is relatively remote from our practical life, than by the apple or the pear, which is relatively essential. Of course, the explanation here is that the more powerful stimulant naturally affords the greater volume of pleasure, irrespective of its ulterior usefulness.
In this case, too, we see the essential agreement between the higher and the lower forms of vegetal-feeders. For just as our taste for sweets corresponds to the insect’s taste for honey, so our love for the perfume of flowers is absolutely identical with the pleasure which draws the butterfly towards the luscious blossoms in our English meadows. And it is worth while to observe that most of the sweet-smelling flowers appear to be quite late developments of vegetal life; a fact which harmonises well with the correspondingly late development of the bees and other highly-adapted honeysuckers. There is no tribe of plants, for example, more noticeable for their perfume than the family of Labiates, which includes the various species of mint, thyme, balm, sage, marjoram, lavender, rosemary, horehound, calamint, patchouli, hyssop, and basil. The flowers of these plants are almost all very peculiarly shaped and highly scented, and their attractiveness for bees has become proverbial — the honey of poetry is commonly “redolent of thyme.” Now the Labiates, so far as known, are tertiary plants of rather late date, which did not make their appearance on the earth until bees and other specialised honey-seekers had reached a high point of evolution. Nor should we omit to notice the fact that many of these plants are now cultivated by man for the sake of this very property; lavender to dry and use for scent, patchouli to extract an essence for the handkerchief, and mint, thyme, or sage to flavour various preparations for the table. The exactly similar cases of nutmeg, cloves, and other spices, whose perfume is famous for its diffusibility, while the mode of their dispersion by nutmeg-pigeons has become classical in the pages of Darwin and Wallace, do not need further comment.
In some few instances the pleasure of perfume has been turned into a sexual allurement, as with certain butterflies, where the two sexes exhibit a different arrangement in the nervures of the wings. “In all cases which I know,” says Fritz Müller, “this difference in neuration is connected with, and probably caused by, the development in the males of spots of peculiarly formed scales, pencils, or other contrivances, which exhale odours agreeable no doubt to their females. This is the case in the genera Mechanitis, Dircenna, in some species of Thecla, &c.” Similar instances occur in the musk-deer and other mammals, whose perfumes are used by human beings as pleasurable stimulants. Indeed, I do not think it would be too much to say that almost every substance which we employ as a native scent is derived either from a vegetal product whose natural function is the attraction of animals, or from an animal product whose natural function is the attraction of the opposite sex.
On the other hand, we find amongst the carnivores and carrion-feeders a totally different form of olfactory pleasure. Dogs, wolves, and other predatory mammals, track their prey by scent, while the smell of raw meat renders the larger cats wild with excitement. Vultures and sopilotes revel
in the hideous smell of putrid animal matter, and flies collect around dung or decaying meat. Curiously enough, too, some plants have availed themselves of this special taste, and have laid themselves out, as already noticed, to deceive carrion insects by their likeness in appearance and smell to putrescent flesh. The Sumatran Rafflesia and the South African Hydnora have large and lurid blossoms, which thus cunningly induce flies to visit them for the purpose of laying their eggs, and are accordingly fertilised by means of an organised deception. To naturally frugivorous man, the scent is, of course, simply disgusting. Yet it is worth notice that many savages, who have acquired for generations the habit of eating half-decomposed meat, positively enjoy those odours which are most distasteful to the nostrils of civilised humanity. As Kolben quaintly phrases it, in his old-fashioned style, “What you take for a stink, a Hottentot, if you will believe him, receives as the most agreeable perfume.”
Hence one may see how futile is the argument of Geiger, who remarks, as illustrating the sensuous inferiority of the lower animals, that it would be useless to offer a dog a bouquet to sniff at. Of course the dog has no reason for being pleased with a perfume which has no special relation to himself or his ancestors in any way; but if we offer him a piece of meat, or set him to hunt down game, we shall find how keenly he has been provided by nature with senses to aid him in his own mode of life. In fact, we can only expect pleasure to be felt where ancestral habit has produced a corresponding sensory system. The transference of feeling whereby we are enabled to enjoy the perfume of flowers does not contradict this general principle, for it is really analogous to the transference whereby the humming-bird sipped the syrup which resembled the native nectar, or whereby we ourselves enjoy sweetmeats and cakes through our hereditary liking for fruits and berries. Honey is a more concentrated form of sugar than that which we get in strawberries or oranges, and frangipanni is a more concentrated form of perfume than that which we get in peaches and pine-apples; but they probably act in just the same way, though to a greater extent, upon the nerves involved as do the original stimulants, and consequently they need no special explanation. Very different, however, would be the case if a dog or any other animal were to feel pleasure in a stimulation derived from some object which had no kind of relation to his ancestral habits. Such an instance, one might venture to say, would be wholly inexplicable, and opposed to all the known principles of scientific psychology.
The sense of hearing, though interesting in itself through its connection with song-birds and the sexual allurements of sound amongst sundry insects, has so little relation with our present subject, that I must reluctantly pass it over here.
Lastly, then, we arrive at the sense of sight, which we must only examine with special reference to the taste for colour. And here, as before, we note at once that those portions of plants which lay themselves out to attract animals are almost without exception conspicuous for their bright colours. Entomophilous flowers, as we have already abundantly observed, have a monopoly of brilliant corollas, while fruits-proper differ from nuts in the startling vividness of their hues. These facts would go by themselves to prove that flower-feeding and fruit-eating animals find an allurement in colour. It is certainly a noticeable fact that just as the sweetness of fruits answers to the sweetness of honey, and just as the scent of fruits answers to the scent of flowers, so do the colours of fruits answer exactly to the colours of flowers. It would seem as though, in every case, nature found a single mode of modifying the nervous substance was amply sufficient (because simplest and easiest) alike for insect and reptile, for bird and ape and human being.
Some special facts help to point in the same direction. Thus we find within the limits of a single family, the Rosaceæ, a large number of fruits-proper, the plum, the apple, the hip, the haw, the strawberry, the raspberry, and the bramble, in which the pericarp or other succulent portion, besides being sweet and scented, is more or less brilliantly coloured; and again, we also find an aberrant member, the almond, whose seed is enclosed in a nut, and whose pericarp accordingly is hard, dry, and green or brownish, after the usual fashion of nuts. Or once more, we know that in most Oceanic islands there are few flying insects, and that most of the flowers are destitute of bright corollas; but Mr. Wallace has pointed out that in a few cases, where honeysucking birds frequent such islands the flowers are extremely large and handsome. This fact clearly shows that the birds in question find colour quite as attractive as do bees or butterflies.
I do not propose to enter very fully into this question until we have seen what light may be cast upon it by the examples collected and the inferences drawn in the succeeding chapter. But I should like to point out here that if our general theory of pleasure be well-founded, it must necessarily result that flower-feeding and fruit-eating animals should derive agreeable sensations from coloured objects. So soon as the eyes of insects or birds have become sufficiently differentiated to discriminate the pinkish or ruddy flower-cases and fruit-vessels from the green leaves around them, and to employ their nascent sense in the quest for food, so soon must the special nerves exercised and strengthened in this process receive some faint pleasure from their due stimulation. And the more developed the nerves become, the more intense must be the resulting enjoyment, till at last an ever-increasing gratification would grow up side by side with the growth of entomophilous blossoms and coloured fruit, becoming stronger and stronger day by day as the structures increased by practice in calibre and power.
Two short passages from the works of two leading evolutionists will serve to bring out in strong relief the position here assumed. Mr. A. R. Wallace thus sums up his view with regard to the nature of colour-perception in the lower animals:— “The fact that the higher vertebrates, and even some insects, distinguish what are to us diversities of colour, by no means proves that their sensations of colour bear any resemblance whatever to ours. An insect’s capacity to distinguish red from blue or yellow may be (and probably is) due to perceptions of a totally distinct nature, and quite unaccompanied by any of that sense of enjoyment or even of radical distinctness which pure colours excite in us. Mammalia and birds, whose structure and emotions are so similar to our own, do probably receive somewhat similar impressions of colour; but we have no evidence to show that they experience pleasurable emotions from colour itself, when not associated with the satisfaction of their wants or the gratification of their passions.” From the whole of this passage, with all due deference to Mr. Wallace (for most of whose work I entertain the deepest respect), I must take leave to differ toto cælo. Each of its three sentences appears to me to contain a fallacious position. For the first, the burden of proof lies distinctly with Mr. Wallace, not with his opponents; because, where the external stimulus is the same, and where a general continuity of structure exists, we are not justified in assuming a difference of sensation without some special reason; nor do I believe that so clear a thinker as the author of the “Malay Archipelago” would have assumed such a difference, were it not for that predisposition to find some effective distinction between man and the lower animals which has so often led him into questionable conclusions. For the second, it seems to me that, since the insect’s need for discriminating colour is far greater than our own, analogy would lead us to suppose that his enjoyment would be even deeper, and his sense of distinctness more marked, than in the human subject. For the third, the single instance of the oft-quoted bower-birds, who collect coloured objects to decorate their meeting places, shows that some vertebrates at least possess a liking for brilliant hues in themselves, of a truly æsthetic sort: and the behaviour of monkeys with regard to flowers and birds, or to red shawls and other strikingly-dyed articles, would seem to point in the same direction. For the rest, Mr. Darwin has gathered together a few isolated instances of disinterested love for colour in a well-known section of his “Descent of Man.” It is true that the evidence on this head is still far from satisfactory; but it must be remembered that without the assistance of language definite informatio
n as to tastes cannot be procured except with great difficulty, and that human infants only display the love for colour in the same simple ways as monkeys or bower-birds.
The second passage to which I would refer is one from our great naturalist himself. “How the sense of beauty in its simplest form,” says Mr. Darwin,— “that is, the reception of a peculiar kind of pleasure from certain colours, forms, and sounds, — was first developed in the mind of man and of the lower animals, is a very obscure subject. The same sort of difficulty is presented if we inquire how it is that certain flavours and odours give pleasure, and others displeasure. Habit in all these cases appears to have come to a certain extent into play; but there must be some fundamental cause in the constitution of the nervous system in each species.” Now this fundamental cause I believe to reside in the general law that pleasure accompanies normal stimulation when not excessive in amount; while the influence of ancestral habit, joined with natural selection, has so modified the nervous system in each case that it finds itself normally stimulated by those external agents which conduce to the general welfare of the organism, and excessively or destructively stimulated by those which conduce to its general detriment. Accordingly, I infer that in all fruit-eating and flower-feeding species, a taste for sweet flavours, delicate perfumes, and bright colours will have been slowly developed by the hereditary mode of life; and that the taste so developed will have opportunities for exerting itself in the sexual selection of bright-coloured mates.
