Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/2

 CHAPTER VI. Catasetida, the most remarkable of all Orchids—The mechanism by which the pollinia of Catasetum are ejected to a distance, and are transported by insects- Sensitiveness of the horns of the rostellum—Extraordinary difference in the male, female, and hermaphrodite forms of Catasetum tridentatum— Mormodes ignea, curious structure of flower; ejection of its pollinia—Cypripedium, importance of the slipper-like form of the labellum- Secretion of nectar—Advantage derived from insects being delayed in sucking the nectar—Singular excrescences on the labellum, apparently attractive to insects

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 INTRODUCTION.

THE object of the following work is to show that the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom; and, secondly, to show that these contrivances have for their main object the fertilisation of each flower. In my volume 'On the Origin of Species' I have given only general reasons for my belief that it is apparently a universal law of nature that organic beings require an occasional cross with another individual; or, which is almost the same thing, that no hermaphrodite fertilises itself for a perpetuity of generations. Having been blamed for propounding this doctrine without giving ample facts, for which I had not, in that work, sufficient space, I wish to show that I have not spoken without having gone into details.

I have been led to publish this little treatise separately, as it has become inconveniently large to be incorporated with the rest of the discussion on the same subject. And I have thought, that, as Orchids are universally acknowledged to rank amongst the most singular and most modified forms in the vegetable kingdonkingdom [sic], the facts to be presently given might lead some observers to look more curiously into the habits of our several native species. An examination of their many beautiful contrivances will exalt the whole vegetable kingdom in most persons' estimation. I fear, however, that the necessary details will be too minute and complex for any one who has not a strong taste for Natural History. This treatise affords me also an opportunity of attempting to show that the study of organic beings may be as interesting to an observer who is fully convinced that the structure of each is due to secondary laws, as to one who views every trifling detail of structure as the result of the direct interposition of the Creator.

I must premise that Christian Konrad Sprengel, in his curious and valuable work, 'Das entdeckte Geheimniss der Natur,' published in 1793, gave an excellent outline of the action of the several parts in Orchids; for he well knew the position of the stigma; and he discovered that insects were necessary to remove the pollen-masses, by pushing open the pouch and coming into contact with the enclosed sticky glands. But he overlooked many curious contrivances, a consequence, apparently, of his belief that the stigma generally receives the pollen of the same flower. Sprengel, likewise, has partially described the structure of Epipactis; but in the case of Listera he entirely misunderstood the remarkable phenomena characteristic of that genus, which has been so well described by Dr. Hooker in the 'Philosophical Transactions' for 1854. Dr. Hooker has given a full and accurate account, with drawings, of the structure of the parts, and of what takes place; but from not having attended to the agency of insects, he has not fully understood the object gained. Robert Brown, in his celebrated paper in the 'Linmean Transactions,' expresses his belief that insects are necessary for the fructification of most Orchids; but adds, that the fact of all the capsules on a dense spike not infrequently producing seed, seems hardly reconcileable with this belief: we shall hereafter see that this doubt is groundless. Many other authors have given facts and expressed their belief, more or less fully, on the necessity of insect-agency in the fertilisation of Orchids.

In the course of the following work I shall have the pleasure of expressing my deep obligation to several gentlemen for their unremitting kindness in sending me fresh specimens, without which aid this work would have been impossible. The trouble which several of my kind assistants have taken has been extraordinary: I