[42] The Natural History Review for July, 1865, contained an article on the doctrine of morphological composition set forth in the foregoing Chaps. I. to III. In this article, which unites exposition and criticism in a way that is unhappily not common with reviewers, it is suggested that the spiral structure may be caused by natural selection. When this article appeared, the foregoing five pages were standing over in type, as surplus from No. 14, issued in June, 1865.
[43] A verifying comment on this paragraph runs as follows:—“In the Hypotricha Infusoria, which creep over solid surfaces, there is a differentiation between ventral and dorsal surface and an approach to bilateral symmetry. The ventral surface is provided with movable cilia, the dorsal with immobile setæ.”
[44] Criticisms on the above passage have shown the need for naming sundry complications. These complications chiefly, if not wholly, arise from changes in modes of life—changes from the locomotive to the stationary, and from the stationary to the locomotive. Referring to my statement that (ignoring the spherical) the radial type is the lowest and must be taken as antecedent to the bilateral type, it is alleged that all existing “radial animals above Protozoa are probably derived from free-swimming, bilaterally-symmetrical animals.” If this is intended to include the planulæ of the hydroid polyps, then it seems rather a straining of the evidence. These locomotive embryos, described as severally having the structure of a gastrula with a closed mouth, can be said to show bilateralness only because the first two tentacles make their appearance on opposite sides of the mouth—a bilateralness which lasts only till two other tentacles make their appearance in a plane at right angles, so giving the radial structure. I think the criticism applies only to cases furnished by Echinoderms. The larvæ of these creatures have bilaterally-symmetrical structures, which they retain as long as they swim about and which such of them as fix themselves lose by becoming similarly related to conditions all round: the radial structure being retained by those types which, becoming subsequently detached, move about miscellaneously. But, as happens in some of the Sea-urchins and still more among the Holothurians, the structure is again made bilaterally-symmetrical by a locomotive life pursued with one end foremost. Should it be contended that the conditions and the forms are reciprocally influential—that either may initiate the other, it still remains unquestionable that ordinarily the conditions are the antecedents, as is so abundantly shown by plants.
[45] Should it be proved that the Ascidian is a degraded vertebrate, then the argument will be strengthened; since loss of bilateral symmetry has gone along with change to asymmetrical conditions.
[46] A critical comment made on this sentence runs as follows:—“The aërial roots of most epiphytic orchids contain chlorophyll in their cortex throughout their length, but the cortex being covered by a ‘velamen’ of air-containing cells which break up and reflect incident light, the green colour is not visible through this opaque coat. When moistened the cells of the velamen take up water and the green colour immediately shows through. Such roots do not however possess stomata. The roots of certain species of Angræcum, however, contain the whole of the assimilating tissue of the plant.”
[47] The current doctrine that chlorophyll is the special substance concerned in vegetal assimilation, either as an agent or as an incidental product, must be taken with considerable qualification. Besides the fact that among the Algæ there are many red and brown kinds which thrive; and besides the fact that among the lower Archegoniates there are species which are purple or chocolate-coloured; there is the fact that Phænogams are not all green. We have the Copper-Beech, we have the black-purple Coleus Verschaffeltii, and we have the red variety of Cabbage, which seems to flourish as well as the other varieties. Chlorophyll, then, must be regarded simply as the most general of the colouring matters found in those parts of plants in which assimilation is being effected by the agency of light. Though it is always present along with the red and brown pigments, yet there is much evidence to show that these are the actual assimilative pigments.
[48] This seems as fit a place as any for noting the fact, that the greater part of what we call beauty in the organic world, is in some way dependent on the sexual relation. It is not only so with the colours and odours of flowers. It is so, too, with the brilliant plumage of birds; and it is probable that the colours of the more conspicuous insects are in part similarly determined. The remarkable circumstance is, that these characteristics, which have originated by furthering the production of the best offspring, while they are naturally those which render the organisms possessing them attractive to one another, directly or indirectly, should also be those which are so generally attractive to us—those without which the fields and woods would lose half their charm. It is interesting, too, to observe how the conception of human beauty is in a considerable degree thus originated. And the trite observation that the element of beauty which grows out of the sexual relation is so predominant in æsthetic products—in music, in the drama, in fiction, in poetry—gains a new meaning when we see how deep down in organic nature this connexion extends.
[49] Students of vegetal physiology, familiar with the controversies respecting sundry points dealt with in this chapter, will probably be surprised to find taken for granted in it, propositions which they have habitually regarded as open to doubt. Hence it seems needful to say that the conclusions here set forth, have resulted from investigations undertaken for the purpose of forming opinions on several unsettled questions which I had to treat, but which I could find in books no adequate data for treating. The details of these investigations, and the entire argument of which this chapter is partly an abstract, will be found in Appendix C.
[50] To this implied inference it is objected that “excess of nutritive material does not necessarily lead to correspondingly increased growth.” My reply is that a concomitant factor is activity of the tissue, and that in its absence growth is not to be expected.
[51] In recent years (since 1890) Prof. Wilhelm Roux, in essays on functional adaptation, has set forth some views akin to the foregoing in respect to the general belief they imply, though differing in respect of the physiological processes he indicates. The following relevant passage has been translated for me from an article of his in the Real-Encyclopädie der gesammten Heilkunde:—“A more complete theory of functional adaptation by the author is founded on the assumption that the ‘functional’ stimulus, or ‘the act of exercising the function’ (in muscles and glands), and especially, in the case of bones, the concussion and tension caused by stress and strain, exert a ‘trophic’ stimulus on the cells, in consequence of which, and along with an increased absorption of nutriment, they grow and eventually increase (or the osteoblasts at the point of greater stimulus form more bone); while, conversely, with continued inactivity, by absence of these stimuli the nourishment of the cell declines so that the waste is insufficiently replaced (or otherwise that the bone-substance gradually loses its power of resistance to the osteoblasts formed as a result of inactivity”).