Page images
PDF
EPUB

Euphorbiaceæ. Additional instances of close general resemblance in habits of plants destitute of the slightest structural affinity are afforded by Haworthia, a genus of Liliaceous, and Echeveril, a genus of Crassulaceous plants, the former allied to the lilies and aloes, the latter to the stone-crops; and by Figs. 2 and 3 in our illustration, representing a Cactaceous (Rhipsalis funalis) and a Euphorbiaceous plant (Euphorbia Tirucalli), the one from tropical America, the other from South Africa, Multitudes of others might have been adduced equally striking.

If we now pass from general to special resemblances, we find ourselves entering on a still more extensive field. Granting the Darwinian or Lamarckian theory of the descent of allied forms from a common ancestor, and their gradual differentiation from one another, a wider margin of separation, as far as mere external and less important characters are concerned, appears to be allowed to near relatives in the case of plants than of animals. The same genus of plants includes frequently species much more widely divergent in habit and in all superficial features than ever occurs among animals. Hence far more play is given to a species to simulate the appearance of another species of scme very remote genus, as is often indicated in the specific names of plants : Polygonum Convolvulus, Solanum jasminioides, Osmanthus ilicifolius, &c. To such a height in even minute details is this resemblance often carried, that the most experienced botanist has sometimes referred a plant, on a too cursory examination, to a genus or even natural order with which it has no affinity whatever. Thus Sir William Hooker is said to have actually figured a Veronica as a Conifer; Kunze, a great authority on ferns, considered the curious Stangeria paradoxa, a Cycad, allied to the Conifers, as a true fern ; and Dr. Berthold Seemann speaks of having, in the Sandwich Islands, met with a variety of Solanum Nelsoni, " which looked for all the world like Thomasia solanacea of New Holland, a well-known Buettnereaceous plant of our gardens, the resemblance between these two widely-separated plants being quite as striking as that pointed out in Bates's Naturalist on the Amazons' between a certain moth and a humming-bird.” *

Less striking instances than this are familiar to all who have made plants their study. The pseudo-papilionaceous flowers of the Cape species of Polygala have deceived many a young botanist. The flowers of Mesembryanthemum remind one irresistibly of the compound capitula of Compositæ. The remarkably fern-like foliage, extending even to the dichotomouslyforked venation, of the bardy Conifer Salisburia adiantifolia, is well known to all arboriculturists. The so-called Fungus melitensis of Malta is in reality a flowering plant belonging to

• "Gardener's Chronicle," June 27, 1868.

the order Balanophoreæ. The resemblance between the true leaves of the Eucalypti, or gum-trees, and the dilated petioles or phyllodia of the Mimosa, both presenting their edges instead of their surfaces to the sky and earth, and both abundant forms of trees in Australia, is very remarkable. The development of ascidia or pitchers from the leaf-stalk or leaf itself occurs not only in the American Sarracenia and Darlingtonia and the Asiatic Nepenthes, belonging to orders at almost the opposite poles of flowering plants, but in Rosaceæ, Asclepiadaceæ, and several other natural orders. The singular irritability of the leaves of the Mimosa pudica, or sensitive plant, and other species of Leguminose, occurs again in another order of very little structural affinity, but presenting curious analogies in its foliage, the Oxalideæ, or wood-sorrel order. Dr. Hooker describes and draws, in his “ Flora Antarctica," a most singular species of Caltha (allied structurally to our marsh-marigold), whose leaves are almost an exact reproduction of those of the Dionaa muscipula, or “Venus's fly-trap.” In the collection of Mr. Saunders is a species of olive, Olea ilicifolia, and a variety of the common holly, Ilex aquifolium, var. macrocarpum, in which the resemblance is extraordinarily close, not only in the shape of the leaf and of the spiny teeth, but in the very arrangement of the principal veins, and even in the texture and colour. Pairs of leaves exhibiting as close resemblance may be composed of an Anemone (Ranunculaceae) and a Pelargonium (Geraniaceæ), a Gnaphalium (Compositæ) and a Lavandula (Labiata), an Oxalis (Oxalidea) and a Crotalaria (Leguminosæ), a Gentiana (Gentianaceae) and a Veratrum

Melanthaceae), a Grevillea (Proteaceae) and an Acacia (Legu'minosä), a carrot (Umbellifera) and a Pelargonium (Geraniaceæ), and of a Thujopsis (Coniferæ) and a Selaginella (Lycopodiaceæ); the last pair comprising a flowering and a cryptogamic plant.*

Nor are we confined to the leaf for the recurrence of the same type in widely separated families. The peculiar mode of dehiscence of the anther to allow of the escape of the pollen known as “ opening by recurved valves” occurs in the Berberidacex, in the Lauraceæ, and in a single tribe of Combretaceæ. The pollen grains covered with spiny prominences are found in Malvaceæ and in some Compositæ. But far more curious and striking than these is a remarkable recurrence in several orders of an almost identical external appearance of the fruit. Any indehiscent fruit with a broad membranous wing is called by botanists a 6 samara," of which we have instances, among our own forest-trees, in the elm, the sycamore, the maple, and the 6 keys” of the ash. Figs. 4—7 represent the form assumed

* See complete lists in “Nature,” May 26, 1870, and May 4, 1871.

by the samara in four genera, belonging to three distinct natural orders, all large shrubs or trees, natives of Brazil. A single genus of Polygalaceæ, the Securidaca, chiefly inhabitants of Tropical South America, but extending also into Africa and India, is distinguished by its remarkable winged fruit, varying somewhat in different species, one of the commonest of which is represented by Fig. 4. In Figs. 5 and 6 are delineated the similar samaroid fruits of two species belonging to different genera of the order Phytolaccaceæ, and having therefore no genetic affinity whatever with the first. Fig. 7 again is an example of the fruit of a Heteropterys, a genus of Malpighiaceæ, comprising a large number of species, also mostly Tropical American, with a few representatives in Africa. This order is again equally dissociated from both the preceding ones. It will be remarked that not only the form of the wing, but its very texture and the arrangement of the veins, are reproduced most accurately in all the species, a dissection of the fruit alone showing their essential difference in structure. So close indeed and deceptive is this resemblance when the plant is not in flower, that the very specimen of the Seguiera from which our drawing is taken, in the Berlin Herbarium, is labelled by so experienced a botanist as Klotzsch as Securidaca; and Walpers, in his “ Repertorium,” has erroneously described five species of Seguiera as Securidacas. Everyone, indeed, familiar with herbaria, will know of similar instances. It should be noted also that the samaroid fruit is not characteristic of any one of these three natural orders, but only of certain tribes or of single genera. When attention is directed to the subject, a careful search would doubtless be rewarded by the detection of a large number of instances of similar resemblance or mimetic analogy in the vegetable kingdom, as remarkable, or even more so, than those we have here instanced.

Having now chronicled a few of the facts of this curious and interesting subject, I shall be expected at least to attempt some explanation, or to start some theory respecting them. And here our real difficulty commences. Even to arrive at the recognition of any one law running through these phenomena seems, in the present state of our knowledge, impossible. In the first place I shall be found fault with for using the term “ Mimicry” in reference to the subject at all. But I must confess to being unable to see the force of the objection, and must continue to consider the series of facts as observed in the animal and vegetable kingdoms as essentially parallel. Strictly speaking, on etymological grounds, the term is open to some objection; uiunois, “an imitation; a representation by art,” implies doubtless a conscious intentional mimicry, which we can no more believe in, in the case of butterflies, than of

flowers; or at all events this hypothesis is entirely inconsistent with the theory of derelopment by Natural Selection only. There is doubtless an apparent object in the one case which we are unable to detect in the other, but this does not seem to me sufficient reason for giving a different name to the phenomenon itself.

Professor Thistleton-Dyer objects to the application of the term “ Mimicry” to the case of closely resembling plants, on the ground that we do not here find the imitative species occupying the same area as occurs in the animal kingdom. The instances I have given above will show, however, that his statement that “the resembling plants are hardly ever found with those they resemble " is a far too general one. Professor Dyer has made a useful suggestion in proposing the terms“homoplastic” and “ Homoplasy" (first applied by Mr. E. R. Lankester to external resemblances in the organs of animals) to the class of phenomena under discussion. The term is a good one, as

objection I have mentioned above to the use of " Mimicry.”

One explanation of Mimicry or Homoplasy in plants that has been suggested is that it is due to consanguinity or

to offer this suggestion to account for the resemblance between a Thujopsis and a Selaginella already referred to. But the value of the theory of hereditary reversion is entirely destroyed if it is strained in this manner. It is true that some botanists have traced a genealogical affinity between Conifers and the higher Cryptogams; but the relationship is at the best a very remote one; and to attribute the external facies of a Conifer to its alliance with a Lycopodium is as wild as to attempt to account for the varied colours of birds by their affinity to insects, or of snakes from their alliance with fishes. To be consistent, this theory ought to be applied to the animal kingdom also, and is a hundred times more to the purpose as an explanation of mimetism among Lepidoptera. We may compare with this unnatural straining of a theory the truly scientific manner in which Mr. Darwin applies the principle of heredity to account for the occasional occurrence of stripes on the hindquarters of the horse from its affinity with the zebra. If, however, hereditary reversion acts as remotely as has been suggested, this no more proves the horse to be related to the zebra than to the hyæna.

A certain class of general superficial resemblances may undoubtedly be attributed to the action of natural external causes, to a similarity of conditions of growth; and to these I have already sufficiently alluded. This explanation is, however, entirely inadequate in the case of the minute resemblances of species to species, either in the general habit, or in the development of some particular organ, the leaf or the fruit, such as I have attempted to describe and to represent in the illustrations. No conjunction of external circumstances will avail to account for these, whether acting through Natural Selection or any other known process.

The theory of Protective Resemblance, so seductive an explanation of similar phenomena in the animal kingdom, is also entirely inapplicable here; it is, in fact, more completely inadequate than either of the others. The only manner in which it seems possible to conceive that a species of the vegetable kingdom can benefit by resembling another species, is by presenting so close an imitation of its flowers, in appearance or odour, that it may thereby deceive insects that would otherwise pass it by into visiting it, and thus bringing about the necessary distribution of the pollen. But if such mimicry, where there is no genetic affinity, ever occurs in the flower, it is extremely rare. The only instance of such apparent imitation that occurs to me is in the case of the Bee Orchis, and perhaps one or two of its allies; and here the mimicry is not of another flower, but of the insect itself. It might well be assumed that the extraordinary resemblance of the flower of this singular plant to the body of a bee was designed to attract these insects to the flower; but, unhappily for this theory, the Bee Orchis appears to be one of the comparatively small number of plants that are independent of insect agency for the maturing of their seeds. Mr. Darwin, who has closely watched the plant, has never seen a bee or other insect alight upon its labellum ; and both he and other observers state that the construction of the pollinia seems especially contrived to secure self-fertilization, in contrast to the provisions of the larger number of species belonging to the order. The special specific resemblances, on the other hand, which I have described, are chiefly in the foliage, the fruit, and the general habit, from which it is difficult to conceive any profit to arise to the species. In many cases also the resemblance occurs between plants which are natives of countries belonging to entirely different phytogeographical regions, which can never have come into contact with one another. It is just possible that we have a curious instance of protective, or rather of beneficial resemblance in scent, in the case of the carrion-like odour of the flowers of Stapelia, which attracts blue-bottle and other flies that may assist in the distribution of the pollen.

We seem then, in attempting to discover some explanation of these phenomena, to be forced back to a view of the operations of Nature which has been too much lost sight of by modern naturalists. Darwin and Wallace's theory of Natural

« PreviousContinue »