Biological symmetry

“ Hence it is inferable that such an organism will be divisible into similar halves by a vertical plane passing through its axis of motion—will have a bilateral symmetry. We may presume that this [185] symmetry will deviate but little from double bilateralness where the upper and under parts are not exposed to strongly-contrasted influences; while we may rationally look for single bilateral symmetry of a decided kind, in creatures having dorsal and ventral parts conversant with very unlike regions of the environment: as in all cases where the movement is over a solid surface. ”

“ If we say that the organism is pervaded by a principle of symmetry such that both growth and regrowth, whenever they take place, are constrained to follow the lines of ancestral symmetry, we are really doing little more than restating the facts without affording any real organic explanation. ”

“ The process of applying physical laws to life follows, it would seem, the reverse order of their original development. First the chemistry of organic matter was investigated, then the physical attraction of their molecules, and now their geometry is in question. So, says Professor Bateson, the 'geometrical symmetry of living things is the key to a knowledge of their regularity and the forces which cause it. In the symmetry of the dividing cell the basis of that resemblance which we call Heredity is contained'. ”

“ After rising above one-celled to more complicated organisms, we reach a class of creatures in which a radiate or wheel-like form obtains, that is, radial symmetry, as in jelly-fish, star-fish, urchins, and sea-anemones. ”

“ This kind of symmetry is suited for a more strenuous life than radial animals show; it is suited for pursuing food, for avoiding enemies, for chasing mates. And with the establishment of bilateral symmetry must be associated the establishment of head-brains, the beginning of which is to be found in some simple worm-types. ”

“ In all such creatures the body does not extend out in the form of lateral prolongations.
But in many others it does send out processes on all sides, and in various directions, as in most trees and all plants which have a definite axis of growth, so that unipolar symmetry is the predominant symmetry in the vegetable kingdom. ”

“ Altogether, then, it appears that each organism has an innate tendency to develop in a symmetrical manner, and that this tendency is controlled and subordinated by the action of external conditions, and not that this symmetry is superinduced only ab externo. In fact, that each organism has its own internal and special laws of growth and development. ”

“ If the first division of a reproductive cell set apart one mass to lie dormant for a time and ultimately to form the reproductive cells of the new generation, while the other mass, exactly of the same kind, developed directly into the new organism, then heredity would simply be a delayed case of what is called organic symmetry, the tendency of similar living material to develop in similar ways under the stimulus of similar external conditions. ”

“ Having to move about over an approximately-horizontal plane, any deviation of structure arising accidentally which leads to one side being kept always foremost, will be an advantage: greater fitness to function becoming possible in proportion as function becomes fixed. Survival of the fittest will therefore tend to establish, under such conditions, a form that keeps the same part in advance—a form in which, consequently, the original radial symmetry diverges more and more towards bilateral symmetry. ”

“ Let the growth of the system, that is to say, be continuous and uniform; it is then evident that we have the elementary conditions for the development of a simple cylindrical helix; and this “primary helix” or “genetic spiral” we can now trace, winding round and round the stem, through A, B, C, etc. But if we can trace such a helix through A, B, C, it follows from the symmetry of the system, that we have only to join A to some other leaf to trace another spiral helix, such, for instance, as A, C, E, etc. ”

“ The morphologist is accustomed to speak of a “polarity” of {167} the cell, meaning thereby a symmetry of visible structure about a particular axis. For instance, whenever we can recognise in a cell both a nucleus and a centrosome, we may consider a line drawn through the two as the morphological axis of polarity; in an epithelium cell, it is obvious that the cell is morphologically symmetrical about a median axis passing from its free surface to its attached base. ”

“ The principle of symmetry furnishes an example of an analogous evolution. To begin with, observation of Nature was able to suggest the idea of symmetry; though such observations reveal only imperfectly any regular dispositions in the aspects of animals and plants. ”

“ Differing widely nevertheless from the higher Invertebrata in their symmetry when adult, they have as larvae a marked bilateral symmetry, and the secondary development of the radial symmetry constitutes one of the most remarkable life-histories known in the animal kingdom. ”

“ Bateson attributes in great measure to the principle of 'symmetry,' or 'repetition of parts' in living things. An organism is symmetrical, and thus what happens in one of many similar organs repeats itself normally in all the others. ”

“ The symmetry of the living things is like the symmetry of the concentric waves in a pool caused by a splash. Perfect circles are made only in the imaginary case of mathematical uniformity, but the system maintains an approximate symmetry though liable to manifold deformation. ”

“ We find, therefore, that a principle which we may call the Principle of Developed and Metamorphosed Symmetry, is firmly established 475 and recognized, and familiarly and successfully applied by botanists. And it will be apparent, on reflection, that though symmetry is a notion which applies to inorganic as well as to organic things, and is, in fact, a conception of certain relations of space and position, such developement and metamorphosis as are here spoken of, are ideas entirely different from any of those to which the physical sciences have led us in our previous survey ”

“ The symmetry, in most kinds of animals, of the corresponding or homologous organs on the right and left sides of the body, is the simplest case in point; but this symmetry sometimes fails, as with rabbits having only one ear, or stags with one horn, or with many-horned sheep which sometimes carry an additional horn on one side of their heads. ”

“ Without this study of symmetry it may easily happen that two different kinds of symmetry may be supposed to be alike, because they seem outwardly alike to our senses, just as forms of crystals of different systems may be confounded together for want of careful examination; the chief thing is to know the plan of symmetry in every class of plants, and the study of this is the foundation of every theory of natural affinities. ”

“ And especially when we bear in mind that many differentiated functions, as those of the senses, cannot be held complementary to any other functions in particular; it becomes manifest that the equilibrations that have to be made in an organism of much heterogeneity, are extremely complex, and do not take place between each organ and some other, but between each organ and all the others. The peculiarity of the molecular motion propagated from each organ, has to be neutralized by some counter-peculiarity in the average of the molecular motions with which it is brought into relation. ”

“ Occupied with the idea that symmetry or dissymmetry in the molecular arrangement of any chemical substance must be manifested in all its properties capable of showing the quality, he pursued his investigations till he reached the conclusion that an essential difference in properties as to symmetry exists between mineral and dead matter and matter in which life is in course of development, the former being symmetrical, the latter unsymmetrical. ”

“ The last kind of symmetry which here needs notice is that termed serial symmetry. In the creatures which exhibit it we have a body which is not only almost always bilaterally symmetrical but which is made up of a succession of similar parts, forming a series along its main or longitudinal axis. Insects, crabs, lobsters, and other allied forms give us examples of serial symmetry, but this is perhaps best seen in such animals as thousand legs and hundred legs—millipedes and centipedes. ”

“ Fluctuations in the chemical composition of the molecules of a unicellular organism (for example) must continually arise, just as fluctuations are always occurring in the number of pigment granules in a certain cell, or in the number of pigment cells in a certain region of the body, or even in the size of the various parts of the body.
All these quantitative relations are exposed to individual fluctuations in every species; and natural selection can strengthen the fluctuations of any part, and thus cause it to develope further in any given direction. ”

“ It is worthy of notice here that the lower animals which show aggregative tendencies present but imperfect indications, or none at all, of bilateral symmetry. Their bodies, like those of plants, are for the most part built up around a central axis, or they show tendencies to spiral modes of growth. ”

“ To satisfy or balance the polarities of each order of physiological units, not only a certain structure of organism, but a certain size of organism is needed; for the complexities of that adult structure in which the physiological units are equilibrated, cannot be represented within the small bulk of the embryo. In many minute organisms, where the whole mass of physiological units required for the structure is present, the very thing does take place which it is above implied ought to take place. The mass builds itself directly into the complete form. ”

“ Hence, then, there is congruity between the above interpretation and certain general truths displayed by animal organization at large. We may infer that in a way parallel to that just indicated, cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes: leaving behind the substance excited, now distinguished as cytoplasm. ”

“ For though we come perilously near to it when we contemplate the lowest orders of magnitude to which life has been attributed, yet in the case of the ordinary cell, or ordinary egg or germ which is going to develop into a complex organism, if we have no reason to assume or to believe that it comprises an intricate “mechanism,” we may be quite sure, both on direct and indirect evidence, that, like the powder in our rocket, it is very heterogeneous in its structure. ”

“ They are the gemmules of Darwin, the physiological units of Spencer, the bioblasts of Altmann, the pangenes of De Vries, the plasomes of Wiesner, the idioblasts of Hertwig, and the biophores of Weismann.
The cells of every organic species possess a proper, specific organisation, more or less complicated, and, in correspondence with this, they are composed of more or less numerous and varied organised particles.
The nucleus is a special organ of cells, which is always present. It displays a collection of numerous, peculiar, elementary living units, the idioblasts. ”

“ The splendid increase of microscopical lens power during the past decade has not only enabled us to see that there is complexity and structure in both the animal and vegetable cell, which bisects by an elaborate process; but even the minute nucleus of the minutest organisms, often not more than one tenth part of the infinitesimal body itself, is now proved to undergo profound structural changes, which precede all the great cyclic changes of the organism as a whole. The nucleus is the centre, in fact, of all the higher activities of the least and lowest infusoria ”

“ If the substance of the germ-plasm is—as we assumed—composed of heterogeneous living particles, which have dissimilar rôles in the building up of the organism, there must of necessity be among them a definite labile state of equilibrium, which cannot be disturbed without modifying in some way the structure of the organism itself which arises from the germ-plasm. ”

“ If we suppose that a vital force unifies and coordinates the organism and is its very essence, we must also suppose that this force is divisible and that a part of it—separated in reproduction—can bring about the same results as the whole. If on the contrary the forces having play in the organism are the mere result of the particular combination of the matter composing it, the reconstruction of a particular combination of molecules in the ovum is all that is necessary to set development a-going along exactly the course taken by the ovum of the parent. ”

“ On the other hand, what seem very slight changes in the medium are often responded to by important changes in the vital chemistry, and consequently in the structure of the organism. Now the nature of the organism at the time being, that is to say, its structure and the physico-chemical state of its tissues and plasmodes, is the main condition of this response; the same external agent will be powerful, or powerless, over slightly different organisms, or over the same organism at different times. ”

“ While one part of a crystal is atomically and morphologically identical with every other, and is the whole crystal “writ small,” one part of an organism is unlike another, and no part is like the whole. Hence the dependence of one organ and one tissue on another, and each on all. Yet, while every part is, so to speak, a condition of existence of every other, and the unity of the organism is but the expression of this solidarity,—wherever organized substance has been differentiated into morphological elements (cells, etc.) ”