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MHE subject of so-called “ Mimicry” in the animal kingdom
1 has recently attracted no small share of attention both from naturalists and from amateurs. The phenomena included in the term are indeed such as, from their singularity and their apparent marvellousness, cannot but captivate even the most dilettante student of Nature. Mr. Bates, in his “ Naturalist on the Amazons," may be said to have first introduced the subject to the notice of the general public. Mr. Trimen has recorded, in the “ Transactions of the Linnean Society," some remarkable and beautiful instances among South African Lepidoptera; and Mr Wallace, in his delightful - Malay Archipelago," has done still more to arouse the interest of even the most unobservant reader. Some of the imitations depicted in the illustrations of the latter book are, indeed, simply wonderful. The object of this singular mimicry is considered, by those most conversant with the subject, to be a certain amount of protection gained by the “mimicking” species, through its superficial resemblance, thus acquired, to another species, which enjoys, for some reason, special immunity from the attacks of enemies, or to some inanimate object. Whether this explanation is supported by a careful examination of the facts it is not now my purpose to inquire, the subject having been ably debated elsewhere. This resemblance occurs sometimes between species belonging to one family or order, as between one butterfly and another; sometimes between forms much more distantly related, as between a fly and a bee, or an ant and a spider; sometimes between animals and inorganic objects, as between a caterpillar and a twig, or an insect in the perfect condition and a decayed
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leaf. The superficial resemblance is occasionally so close, and carried into such marvellously minute details of structure, that even the eyes of practised entomologists are deceived, as it is supposed those of the natural enemies of the animal are.
Two explanations, and two only, have been offered of the origin of this “mimetism,” or “ protective resemblance”: natural selection and hybridisation. Mr. Darwin, Mr. Wallace, and Mr. Bates advocate the former view, maintaining that the resemblance is brought about by exceedingly slow gradations, each small variation in the direction of the species ultimately mimicked being perpetuated to the prejudice of the offspring which do not thus vary, by the operation of the law of “ The Survival of the Fittest.” This theory commends itself, on its first enunciation, from its beauty and simplicity, and has been eagerly adopted and zealously defended by the ultra-Darwinians who form the bulk of our rising naturalists. That this explanation is, however, not so free from difficulty as its advocates have imagined, has been shown by several recent writers, and especially by Mr. Mivart in his very able “ Genesis of Species,” although he has not offered any definite counter-hypothesis. The theory of hybridisation has found an advocate in one able and experienced naturalist, Mr. Andrew Murray, but has not met with general acceptance, and, in addition to other objections, is obviously inapplicable, at all events, to the cases of the imitation by animals of inorganic forms.
That similar curious resemblances have not hitherto been described in the vegetable kingdom, is mainly because they have not been looked for with the same zeal; and no doubt also arises partly from the much greater difficulty of preserving the outward appearance of plants than of animals. The exterior covering of most animals, and in the case of insects the whole of the body, is comparatively easily preserved, without loss of colour or form, in museums or cabinets. We have no such method of preserving the tenderer parts of plants; and, with respect to the colour and form of the natives of tropical or unexplored regions, have to trust greatly to the very unreliable fidelity of artists, very few of whom have any accurate scientific knowledge. Since, therefore, the most remarkable developments of both animal and plant life occur in the wild luxuriance of tropical countries, it is only the few who have had the good fortune to travel in those regions who have much practical opportunity of studying the phenomena we are discussing, except in the case of the few species that have been cultivated in Europe. The only work that has come under my notice in which the subject is discussed, is a little book published in 1869, by Mr. L. H. Grindon, entitled “Echoes in Plant and Flower Life," and he has avowedly not treated it in
a scientific manner, but bas collected together a large number of curious and interesting facts for others to draw their conclusions from. At the last meeting of the British Association at Edinburgh, Professor Thistleton-Dyer read a short paper with this title, but it is very far from exhausting the subject. The visitors to the soirées of the Linnean Society for the last two years have also been attracted by the collections exhibited by that munificent patron of horticulture, Mr. W. Wilson Saunders, of so-called “mimetic plants, consisting of pairs of species resembling one another in their foliage or habit to so extraordinary a degree—and yet belonging to entirely different natural orders—that even a good botanist might well be excused for passing them over as identical.*
Before alluding to the theories which have been broached on the subject, let us examine the facts which may be collected, and attempt to classify them. The resemblances among plants sufficiently close to deserve the appellation of mimicry may be classed under two heads :—those which relate to the whole habit and mode of growth, and those which refer to the development of some particular organ or part.
Taking first the former of these classes : there are a number of facts which are familiar to every student of botany, and even to casual observers. Every one knows that to a certain extent that assemblage of characters which we call the habit of a plant becomes changed by the circumstances in which it grows. A tree in a warm genial climate becomes a dwarf shrub when exposed to the bitter cutting winds of northern latitudes; an annual in a temperate changeable climate becomes a perennial when transplanted to a tropical country where there is no alternation of summer and winter. Hence the general features which characterise what have been termed the phyto-geographical regions of the earth; the absence of trees, and the prostrate shrubs with a peculiar tortuous and compact habit of growth of the Arctic zone; the green pastures, showy flowering annual herbs, and deciduous forests of temperate latitudes; the shiny-leaved evergreen forests and profusion of splendid climbers of the tropics; and the scanty thorny or succulent vegetation of the deserts. Under peculiar conditions all plants, no matter to what class they belong, or how remote their relationship, have a tendency to assume a certain resemblance in external features. Plants growing in running water, whether flowering or flowerless, Ranunculus or Myriophyllum, Chara or Potamogeton, have the submerged leaves long and filiform,
* To the courtesy of Mr. Saunders and of his very intelligent gardener Mr. Green, who has paid special attention to this subject, we are indebted for the facility for making several of the drawings with which this paper is illustrated.
or cut into slender divisions. Maritime plants growing within reach of the salt spray are apt to become dwarf and fleshy in their habit; and the same remark applies to those which grow on exposed mountain summits, where they are liable to severe though short droughts during the brief but intense summer. In arid desert situations this feature of the vegetation is still more remarkable. Our yellow and white stone-crops, with their round juicy leaves, lovers of rocks and dry walls, are replaced, as we go farther south, by larger species of the same order, or by the similarly disposed Ficoideæ, as the pretty little Mesembryanthemum crystallinum, the ice-plant of our greenhouses, which refreshes with its cool foliage the borders of the desert in Egypt, and elsewhere in North Africa. Many orders of plants, indeed, occurring with us only as ordinary herbs or slender shrubs, are represented in those countries by genera of succulent plants, great favourites in our greenhouses, whose affinity it is hard to recognise.
One of the most remarkable features of the hotter and drier parts of America is the abundance of different forms of Cactus, so much cultivated in this country for the beauty of their flowers and the singular weird form of their trunks, which perform the functions of both stem and leaves. Having its head-quarters in Mexico, the order extends as far as the temperate latitudes of Chile and Canada, and includes, on a moderate computation, at least one thousand species. In Africa the order is entirely absent, or rather its absence is made more conspicuous by the occurrence of a single species of Rhipsalis at the Cape ; but its place is supplied by another class of plants, the Euphorbias, a genus represented in this country by several inconspicuous but familiar weeds known as Spurges. In tropical and subtropical Africa the genus assumes the habit and general appearance of the absent Cacti, though in their botanical affinities they are nearly as remote as two orders of plants can well be. Except when they are in flower, it is, indeed, difficult to believe that these African Euphorbias are not in reality Cacti ; and the resemblance is not merely a general one; particular groups, and even species, of African Euphorbia imitate particular groups or species of American Cactus in the form and habit of the stem and the arrangement of the spines, so that it is almost impossible to distinguish between them. This singular imitation is not, moreover, confined to these two families. The accompanying illustration (Fig. 1), reminding one irresistibly of a familiar Cactus, is drawn from a species of Stapelia, allied to S. hirsuta, belonging to the order Asclepiadaceæ, a near ally of the brilliant and fragrant Stephanotis and Hoya of our stoves, and equally remote, in any system of classification, alike from the Cactacea and the