Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/77

 as far as I have seen, the labellum is either thick and fleshy or is furnished with excrescences. The labellum, for instance, of Oncidium and of Odontoglossum offers all sorts of singular protuberances. In Calanthe we have (Fig. XXIV.) a cluster of odd little spherical warts on the labellum, together with an extremely long nectary, which does not include nectar; in Eulophia viridis the short nectary is in the same condition, and the labellum is covered with longitudinal and fimbriated ridges. In some also of the Ophreæ, which have no nectary, the labellum, as in the Fly Ophrys, has two shining protuberances at its base, placed beneath the two pouches. Lindley has remarked that the use of these strange and diversified excrescences is quite unknown.

From the position, relative to the viscid disc of the pollinium, which these excrescences hold, and from the absence of nectar, it seems to me highly probable that they afford food, and thus attract either Hymenoptera or flower-feeding Coleoptera. I mention this belief because a close examination of the flowers of the Vandeæ, which in their native country have had their pollinia removed, would soon settle this point. There is no more inherent improbablity in a flower being habitually fertilised by an insect coming to feed on the labellum, than in seeds being habitually disseminated by birds attracted by the sweet pulp in which they are embedded. But I am bound to state that Dr. Percy had the thick and furrowed labellum of a Warrea analysed for me, by fermentation over mercury, and it gave no evidence of containing more saccharine matter than the other petals. On the other hand, the thick labellum of Catasetum, and even the bases of the upper petals in Mormodes ignea, had, as previously stated, a slightly sweet, rather pleasant, and nutritious taste.

We have now done with exotic Orchids. To me the study has been most interesting of these wonderful and often beautiful productions, so unlike common flowers, with all their many adaptations, with parts capable of movement, and other parts endowed with something so like, though no doubt really different from, sensibility. The flowers of Orchids, in their strange and endless diversity of shape, may be compared with the great vertebrate class of Fish, or still more appropriately with tropical Homopterous insects, which seem to us in our ignorance as if modelled by the wildest caprice. CHAPTER VII. Homologies of Orchid-flowers—The great modification which they have undergone—Gradation of organs, of the rostellum, of the pollen-masses—Formation of the caudicle—Genealogical affinities—Mechanism of the movement of the pollinia—Uses of the petals—Production of seed—Importance of trifling details of structure—Cause of the vast diversity of structure for the same general purpose—Cause of the perfection of the contrivances in Orchids Summary on insect-agency Nature abhors perpetual self-fertilisation.

THE theoretical structure of few flowers has been so largely discussed as that of Orchids; nor is this surprising, seeing how unlike they are to common flowers. No group of organic beings can be well understood until their homologies are made out; that is, until the general pattern, or, as it is often called, the ideal type, of the several members of the group is intelligible. No one member may now exist exhibiting the full pattern; but this does not make the subject less important to the naturalist,.—probably makes it more important for the full understanding of the group.

The homologies of any being, or group of beings, can be most surely made out by tracing their embryological development when that is possible; or by discovery of organs in a rudimentary condition; or by tracing, through a long series of beings, a close gradation from one part to another, until the two parts, or organs, employed for widely different functions, and most unlike each other, can be joined by a succession of short links. No instance is known of a close gradation between two organs, unless they be homologically one and the same organ.

The importance of the science of Homology rests in its giving us the key-note of the possible amount of difference in plan within any group; it allows us to class under proper heads the most diversified organs; it shows us gradations which would otherwise have been overlooked, and thus aids us in our classification; it explains many monstrosities; it leads to the detection of obscure and hidden parts, or mere vestiges of parts, and shows us the meaning of rudiments. Besides these practical uses, to the naturalist who believes in the gradual modification of organic beings, the science of Homology clears away the mist from such terms as the scheme of nature, ideal types, archetypal patterns or ideas, etc.; for these terms come to express real facts. The naturalist, thus guided, sees that all homological parts or organs, however much diversified, are modifications of one and the same ancestral organ; in tracing existing gradations he gains a clue in tracing, as far as that is possible, the probable course of modification during a long line of generations. He may feel