Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/283

. IX. is a related genus, placed by Brown amongst the Orchideæ, but by Lindley in a small distinct family. These broken groups do not indicate to us the structure of the common parent-form of all the Orchideæ, but they serve to show the probable state of the order in ancient times, when none of the forms had become so widely differentiated from one another and from other plants, as are the existing Orchids, especially the Vandeæ and Ophreæ; and when, consequently, the order made a nearer approach in all its characters, than it does at present, to such allied groups as the Marantaceæ.

With respect to other Orchids, we can see that an ancient form, like one of the sub-tribe of the Pleurothallidæ, some of which have waxy pollen-masses with a minute caudicle, might have given rise, by the entire abortion of the caudicle, to the Dendrobiæ, and by an increase of the caudicle to the Epidendreæ. Cymbidium shows us how simply a form like one of our present Epidendreæ could be modified into one of the Vandeæ. The Neotteæ stand in nearly a similar relation to the higher Ophreæ, which the Epidendreæ do to the higher Vandeæ. In certain genera of the Neotteæ we have compound pollen-grains cemented into packets and tied together by elastic threads, which project and thus form a nascent caudicle. But this caudicle does not protude from the lower end of the pollinium as in the Ophreæ, nor does it always protrude from the extreme upper end in the Neotteæ, but sometimes at an intermediate level; so that a transition in this respect is far from impossible. In Spiranthes, the back of the rostellum, lined with viscid matter, is alone removed: the front part is membranous, and ruptures like the pouch-formed rostellum of the Ophreæ. An ancient form combining most of the characters, but in a less