While this appendix is in hand I have received from another expert, whose view is in general agreement with my own, a letter containing the following passage:—
“You will see that Dohrn’s theory was the antithesis of your own view of vertebrate structure, namely that the vertebræ were formed by the segmentation, from mechanical causes of a body originally simple. This view of yours has been confirmed by later researches, which have shown that the most primitive forms allied to the Vertebrates, possessing the essential organs, viz., gill-slits, notochord, and dorsal nerve cord, are not segmented animals, like Annelids and Crustacea, but simple animals, having at most three regions, not exactly corresponding to segments. These primitive unsegmented forms are Ascidian tadpoles, Balanoglossus, and certain other primitive forms. The embryology of Vertebrates also proves that they are originally simple and not segmented animals, especially the fact that there is originally one pronephric duct or primitive kidney.”
Nevertheless there survives a leaning towards the notion of a segmental origin of the Vertebrata. But the repetitions of organs named in support of this notion have, I think, no more relation to the genesis of the vertebrate type than the multiplication of vertebræ in a snake has relation to the genesis of the vertebral column.
APPENDIX E.
THE SHAPES AND ARRANGEMENTS OF FLOWERS.
In Part IV., Chapter X., under the title of “The Shapes of Flowers,” I have, after describing their several kinds of symmetry, as habitually related to their positions, made some remarks by way of interpretation. The truth that flowers exhibit a radial symmetry when they are so placed as to be equally affected all round by incident forces, having been exemplified, and also the truth that they assume a bilateral symmetry when they are so placed that their two sides are conditioned in ways different from the ways in which their upper and lower parts are conditioned; I have gone on to inquire (in [§ 234]) by what causes such modifications of form are produced. I have stated that, originally, I inclined to ascribe them entirely to differences in the relations of the parts to physical forces—light, heat, gravitation, etc.; but that I found sundry facts stood in the way of this interpretation. And I have said that “Mr. Darwin’s investigations into the fertilization of Orchids led me to take into account an unnoticed agency.” Continuing to recognize the physical forces as factors having some influence, I have concluded that the most important factor is the action of insects; which, aiding most the fertilization of those flowers which most facilitate their entrance, produce, in course of generations, a form of flower specially adapted to the special position.
Though still adhering to this interpretation, I have since found reason to think that the original interpretation contains a larger portion of truth than I supposed at the time when I was led thus to revise it. While staying at Mürren, in Switzerland, in 1872, I observed some modifications in a species of Gentian, which proved to me that the action of incident physical forces on flowers is, in some cases, very rapid and decided. The species furnishing this evidence was the Gentiana Asclepiadea; which I found in a copse formed of bushes that were here wide apart and there close together. In some places not near to the bushes, the individuals of the species grew vertically; in other places, partially shaded, their inclined shoots curved in such directions as to get the most light; and in other cases their shoots were led to take directions almost or quite horizontal. That, along with these modifications in the directions of their shoots, there went adjustments in the attitudes of their leaves, was a fact not specially worthy of remark; for plants placed inside the windows of houses habitually show us that leaves quickly bend themselves into attitudes giving them the greatest amounts of light. But the fact which attracted my attention was, that the flowers changed their attitudes in an equally-marked manner. The radial distribution passed into a bilateral distribution with the greatest readiness. Comparison of the annexed figures will show the character of this change.
Figure I. represents part of a vertically-growing shoot. This belonged to an individual growing unimpeded by bushes, and getting light on all sides. Here it is observable that the pairs of leaves, placed alternately in directions transverse to one another—one pair pointing, say, north and south, and the next pair pointing east and west—maintain, taking them in the aggregate, a radial distribution; and it is also observable that the alternate pairs of flowers are similarly arranged.
Figure II. is a sketch from a shoot which leaned towards one side, and of which the higher part, as it bent more and more, got its upper side more and more differently conditioned from its lower side. Here we find that not only the leaves, but also the flowers, have adjusted themselves to the changed conditions. The leaves of the lowest pair hang out in the normal way, on the opposite sides of the axis, so that a plane passing through their surfaces will cut the axis transversely; and their two axillary flower-buds, c and d, are similarly placed on opposite sides of the axis. But at the other part of the shoot, we see both that the leaves have adjusted themselves so that their planes, no longer cutting the axis transversely, keep a fit adjustment with respect to the light; and also that the flowers, no longer on opposite sides of the axis, have bent round to the upper side, as at a and b.
Figure III. shows us this re-arrangement carried still further. The shoot it represents was growing in a direction nearly horizontal, and therefore receiving the light only on one side. And here, besides seeing that the leaves have so adjusted themselves that they all lie in approximately the same plane, which is parallel to the axis instead of transverse to it, we see that the two pairs of flower-buds have both come round to the upper side of the axis. So that in this shoot, the original radial symmetry in the arrangement of leaves and flowers, is completely changed into a bilateral symmetry.
