[3] In further illustration, Mr. Tansley names the fact that in the genus Caulerpa we have extremely complicated forms often of considerable size produced in the same way. The various species simulate very perfectly the members of different groups among the higher plants, such as Horse-tails, Mosses, Cactuses, Conifers and the like.
[4] It may be objected that in Cladophora the separate compartments of the thallus severally contain many nuclei, making it doubtful whether they descend from uni-nucleate cells. If, however, they do not they simply illustrate another form of integration.
[5] The great mass of early ancestral types—plant and animal—consisting of soft tissues, have left no remains whatever, and we have no reason to suppose that those which left remains fell within the direct ancestral lines of any existing forms. Contrariwise, we have reason to suppose that they fell within lines of evolution out of which the lines ending in existing forms diverged. We must therefore infer that the difficulties of affiliation which arise if we contemplate divergent types now existing, would not arise if we had before us all the early intermediate types. The Mammalia differ in sundry respects from all other kinds of Vertebrata—Fishes, Reptiles, Birds; and if the absence of hair, mammæ, and two occipital condyles, in these other vertebrates were taken to imply a fundamental distinction, it might, in the absence of any known fossil links, be inferred that the Mammalia belonged to a separate phylum. But these differences are not held to negative the assumed relationship. Similarly among plants. We must not reject an hypothesis respecting a certain supposed type, because the existing types it must have been akin to present traits which it could not have had. We are justified in assuming, within limits, a hypothetical type, unlike existing types in traits of some importance. Hence results the answer to a criticism passed on the above argument, that it implies relations between the undeveloped and developed forms of the Jungermanniaceæ such as the facts do not show us. This objection is met on remembering that the types in which the supposed transition took place disappeared myriads of years ago.
[6] There is much force in the criticism passed on the above paragraph, and by implication on some preceding paragraphs, that though in plants which tend to produce compound leaves the production is largely dependent on the supply of nutriment, yet the unqualified statement of this relation as a general one, is negatived by the existence of plants which bear only simple leaves, however much high nutrition causes growth. But mostly valid though this objection is, it is probably not universally valid. I am led to say this by what occasionally occurs in flowers. The flowering stem of the Hyacinth is single; but I have seen a cultivated Hyacinth in which one of the flowers had developed into a lateral spike. Still more striking evidence was once supplied to me by Agrimony. All samples of this plant previously seen had single flowering spikes, but some years ago I met with one, extremely luxuriant, in which some flowers of the primitive spike were replaced by lateral spikes; and I am not sure that some of these, again, did not bear lateral spikes. Now if in plants which, in probably millions of cases, have their flowering stems single, excessive nutrition changes certain of their flowers into new spikes, it is a reasonable supposition that in like manner plants which are thought invariably to bear only single leaves, will, under kindred conditions, bear compound leaves.
[7] See British and Foreign Medico-Chirurgical Review for January, 1862.
[8] Schleiden, who chooses to regard as an axis that which Mr. Berkeley, with more obvious truth, calls a mid-rib, says:—“The flat stem of the Liverworts presents many varieties, consisting frequently of one simple layer of thin-walled cells, or it exhibits in its axis the elements of the ordinary stem.” This passage exemplifies the wholly gratuitous hypotheses which men will sometimes espouse, to escape hypotheses they dislike. Schleiden, with the positiveness characteristic of him, asserts the primordial distinction between axial organs and foliar organs. In the higher Archegoniates he sees an undeniable stem. In the lower Archegoniates, clearly allied to them by their fructification, there is no structure having the remotest resemblance to a stem. But to save his hypothesis, Schleiden calls that “a flat stem,” which is obviously a structure in which stem and leaf are not differentiated. He is the more to be blamed for this unphilosophical assumption, since he is merciless in his strictures on the unphilosophical assumptions of other botanists.
[9] To this interpretation it is objected that “the more-developed Jungermanniaceæ” do not appear to have arisen from the lower forms of Jungermanniaceæ—that is to say, from such lower forms as are now existing. It may, however, be contended that this fact does not exclude the interpretation given; since the higher forms may well have been evolved, not from any of the lower forms we now know, but from lower forms which have become extinct. This, indeed, is the implication of the evolutionary process as pointed out in the note to Chap. I. If then we assume some early type of intermediate structure, the explanation may not improbably hold.
[10] I am indebted to Dr. Hooker for pointing out further facts supporting this view. In his Flora Antarctica, he describes the genus Lessonia (see Fig. [37]), and especially L. ovata, as having a mode of growth simulating that of the dicotyledonous trees, not only in general form but in internal structure. The tall vertical stem thickens as it grows, by the periodical addition of layers to its periphery. That even Thallophytes should thus, under certain conditions, present a transversely-increasing axis, shows that there is nothing absolutely characteristic of Phanerogams in their habit of stem-thickening. Mr. Tansley gives me further verification by the statement that “it is also now certain that members of the Equisetineæ and Lycopodineæ, as well as some Ferns which flourished in Carboniferous times, had secondary thickening in their stems quite comparable to that of modern Dicotyledonous trees.”
[11] See note at the end of the chapter.
[12] Since this paragraph was put in type [this refers to the first edition], I have observed that in some varieties of Cineraria, as probably in other plants, a single individual furnishes all these forms of leaves—all gradations between unstipulated leaves on long petioles, and leaves that embrace the axis. It may be added that the distribution of these various forms is quite in harmony with the rationale above given.