Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/252

232 insect which enters the labellum is thus compelled to crawl out by one of the two narrow passages, on the sides of which the pollen-masses and stigma are placed. We hare seen that exactly the same end is gained in the case of Coryanthes by the labellum being half-filled with secreted fluid; and in the case of Pterostylis and some other Australian Orchids by the labellum being irritable, so that when touched by an entering insect it shuts up the flower, with the exception of a single narrow passage.

Homological Nature of the several Parts of the Flowers of the Orchideæ.

The theoretical structure of few flowers has been so largely discussed as that of the Orchideæ; nor is this surprising, seeing how unlike they are to common flowers; and here will be a convenient place for considering this subject. No group of organic beings can be well understood until their homologies are made out; that is, until the general pattern, or, as it has often been 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,