VI. THEORIES OF ORGANIC EVOLUTION
GOETHE AND THE METAMORPHOSIS OF PARTS
When Coleridge said of Humphry Davy that he might have been the greatest poet of his time had he not chosen rather to be the greatest chemist, it is possible that the enthusiasm of the friend outweighed the caution of the critic. But however that may be, it is beyond dispute that the man who actually was the greatest poet of that time might easily have taken the very highest rank as a scientist had not the muse distracted his attention. Indeed, despite these distractions, Johann Wolfgang von Goethe achieved successes in the field of pure science that would insure permanent recognition for his name had he never written a stanza of poetry. Such is the versatility that marks the highest genius.
It was in 1790 that Goethe published the work that laid the foundations of his scientific reputation—the work on the Metamorphoses of Plants, in which he advanced the novel doctrine that all parts of the flower are modified or metamorphosed leaves.
"Every one who observes the growth of plants, even superficially," wrote Goethe, "will notice that certain external parts of them become transformed at times and go over into the forms of the contiguous parts, now completely, now to a greater or less degree. Thus, for example, the single flower is transformed into a double one when, instead of stamens, petals are developed, which are either exactly like the other petals of the corolla in form, and color or else still bear visible signs of their origin.
"When we observe that it is possible for a plant in this way to take a step backward, we shall give so much the more heed to the regular course of nature and learn the laws of transformation according to which she produces one part through another, and displays the most varying forms through the modification of one single organ.
"Let us first direct our attention to the plant at the moment when it develops out of the seed-kernel. The first organs of its upward growth are known by the name of cotyledons; they have also been called seed-leaves.
"They often appear shapeless, filled with new matter, and are just as thick as they are broad. Their vessels are unrecognizable and are hardly to be distinguished from the mass of the whole; they bear almost no resemblance to a leaf, and we could easily be misled into regarding them as special organs. Occasionally, however, they appear as real leaves, their vessels are capable of the most minute development, their similarity to the following leaves does not permit us to take them for special organs, but we recognize them instead to be the first leaves of the stalk.
"The cotyledons are mostly double, and there is an observation to be made here which will appear still more important as we proceed—that is, that the leaves of the first node are often paired, even when the following leaves of the stalk stand alternately upon it. Here we see an approximation and a joining of parts which nature afterwards separates and places at a distance from one another. It is still more remarkable when the cotyledons take the form of many little leaves gathered about an axis, and the stalk which grows gradually from their midst produces the following leaves arranged around it singly in a whorl. This may be observed very exactly in the growth of the pinus species. Here a corolla of needles forms at the same time a calyx, and we shall have occasion to remember the present case in connection with similar phenomena later.
"On the other hand, we observe that even the cotyledons which are most like a leaf when compared with the following leaves of the stalk are always more undeveloped or less developed. This is chiefly noticeable in their margin which is extremely simple and shows few traces of indentation.
"A few or many of the next following leaves are often already present in the seed, and lie enclosed between the cotyledons; in their folded state they are known by the name of plumules. Their form, as compared with the cotyledons and the following leaves, varies in different plants. Their chief point of variance, however, from the cotyledons is that they are flat, delicate, and formed like real leaves generally. They are wholly green, rest on a visible node, and can no longer deny their relationship to the following leaves of the stalk, to which, however, they are usually still inferior, in so far as that their margin is not completely developed.
"The further development, however, goes on ceaselessly in the leaf, from node to node; its midrib is elongated, and more or less additional ribs stretch out from this towards the sides. The leaves now appear notched, deeply indented, or composed of several small leaves, in which last case they seem to form complete little branches. The date-palm furnishes a striking example of such a successive transformation of the simplest leaf form. A midrib is elongated through a succession of several leaves, the single fan-shaped leaf becomes torn and diverted, and a very complicated leaf is developed, which rivals a branch in form.
"The transition to inflorescence takes place more or less rapidly. In the latter case we usually observe that the leaves of the stalk loose their different external divisions, and, on the other hand, spread out more or less in their lower parts where they are attached to the stalk. If the transition takes place rapidly, the stalk, suddenly become thinner and more elongated since the node of the last-developed leaf, shoots up and collects several leaves around an axis at its end.