and that the grains slowly disappeared from the lower parts of the mass, leaving the elastic threads naked and ready to cohere into a true caudicle.
As the caudicle, whether longer or shorter in the several species, plays an important part in the fertilisation of the flower, it apparently might have been formed from a nascent condition, as in Epipactis, by the continued preservation of varying increments in its length, each beneficial in relation to other changes in the structure of the flower. But we may conclude from the facts given, that this has not been the sole means,—that the caudicle owes much of its length to the abortion of the lower pollen-grains. That it has subsequently in some cases been largely increased in length by natural selection, is highly probable; for in Bonatea speciosa the caudicle is actually more than thrice as long as the elongated mass of pollen-grains; and it is improbable that so lengthy a mass of slightly cohering grains should have existed, as an insect could not have safely transported and applied to the stigma a pollen-mass of this shape and size.
We have hitherto considered the gradations in the state of the same organ. To any one with much more knowledge than I possess, it would be an interesting subject to trace, in this great and closely-connected order, the gradations, as far as possible, between the several species and groups of species. To make a perfect gradation, all the extinct forms which have ever existed, along many lines of descent converging to the common progenitor of the order, would have to be called into life. It is due to their absence, and to the consequent wide gaps in the series, that we are enabled to divide the existing species into definable groups, such as genera, families, and tribes. If there had been no extinction, there would still have been great lines, or branches, of special development,—the Vandee, for instance, would still, as a great body, have been distinguishable from the great body of the Ophrea; but ancient and intermediate forms, very different probably from their present descendants, would have rendered it utterly impossible to separate by distinct characters the one great body from the other.
I will venture on only a few remarks. Cypripedium, in having three stigmas developed, and therefore in not having a rostellum, in having two fertile anthers with a large rudiment of a third, and in the state of its pollen, seems a remnant of the order whilst in a simpler condition. Apostasia is a related genus, placed by Brown amongst Orchids, 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 Orchids, but they probably serve to show the state of the order in ancient times, when none of the forms had become so widely differentiated from each other and from other plants, as are the existing Orchids, especially the Vandee and Ophrea; and when, consequently, the order made a nearer approach in all its characters, than at present, to such allied groups as the Marantacee.
With respect to other Orchids, we can see that an ancient form, like one of the Pleurothallidæ, some of which have waxy pollen-masses with a minute caudicle, might give rise, by the entire abortion of the caudicle, to the Dendrobide, and by an increase of the caudicle to the Epidendrea. 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 Ophrea, which the Epidendrea do to the Vandeæ. In certain genera of the Neotteæ we have the pollen-grains cemented into packets, tied together by elastic threads, which project and form a nascent caudicle. But this caudicle does not protrude from the lower end of the pollinium as in the Ophrea, nor does it always protrude from the extreme upper end in the Neottee; so that we can see that a transition in this respect would be 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 Ophrea. An ancient form, combining most of the characters, but in a less developed state, of Goodyera, Epipactis, and Spiranthes, would, by further slight modifications, give rise to the whole tribe of the Ophreæ.
Hardly any question in Natural History is more vague and difficult to decide than what forms within any large group ought to be considered as the highest;[1] for all are excellently adapted to their conditions of life. If we look to traces of successive modification, with differentiation of parts and consequent complexity of structure, as the standard of comparison, the Ophreæ and Vandeæ will stand the highest. Are we to lay much stress on the size and beauty of the flower, and on the size of the whole plant? if so, the Vandeæ are pre-eminent. They have, also, rather more complex pollinia, with the pollen-masses often reduced to two. The rostellum, on the other hand, in the Ophreæ, has apparently been more modified from its primordial stigmatic nature than in the Vandeæ. In all the Ophreæ the anthers of the inner whorl are almost enterily—suppressed,the auricles—mere rudiments of rudiments—being retained: these anthers have, therefore, undergone a greater amount of modification; but can such suppression be considered as a sign of highness? I should doubt whether any member of the Orchidean order has been more profoundly modified in its whole structure than Bonatea speciosa, one of the Ophreæ. So again, within this tribe, nothing can be more perfect than the contrivances in Orchis pyramidalis for its fertilisation. Yet an ill-defined feeling tells me to rank the
- ↑ The fullest and the most able discussion on this difficult subject is by Professor H. G. Bronn in his 'Entwickelungs-Gesetze der Organischen Welt, ' 1858. I have read the French translation published in 1861, Supplement, 'Comptes Rendus,' tom. ii. p. 520 et seq. This great work was crowned by the French Academy of Sciences.
