Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/303

. IX. the anther opening a little more widely, becomes specially adapted for the very different purpose of self-fertilisation, through the combined aid of the weight of the pollen-mass and the vibration of the flower when moved by the wind. Every gradation between these two states is possible,—of which we have a partial instance in O. aranifera.

Again, the elasticity of the pedicel of the pollinium in some Vandeæ is adapted to free the pollen-masses from their anther-cases; but by a further slight modification, the elasticity of the pedicel becomes specially adapted to shoot out the pollinium with considerable force so as to strike the body of the visiting insect. The great cavity in the labellum of many Vandeæ is gnawed by insects and thus attracts them; but in Mormodes ignea it is greatly reduced in size, and serves in chief part to keep the labellum in its new position on the summit of the column. From the analogy of many plants we may infer that a long spur-like nectary is primarily adapted to secrete and hold a store of nectar; but in many Orchids it has so far lost this function, that it contains fluid only in the intercellular spaces. In those Orchids in which the nectary contains both free nectar and fluid in the intercellular spaces, we can see how a transition from the one state to the other could be effected, namely, by less and less nectar being secreted from the inner membrane, with more and more retained within the intercellular spaces. Other analogous cases could be given.

Although an organ may not have been originally formed for some special purpose, if it now serves for this end, we are justified in saying that it is specially adapted for it. On the same principle, if a man were to make a machine for some special purpose, but were