rminal membrane, at an early stage; the cells ro ded. the germinal membrane consists when it is first formed. They are nearly round, and lie in simple contact with each other. But after a short time, as they grow, their shape changes, they become pressed together by the resisting capsule, and present a hexagonal appearance, as shown in Fig. 14. No one FIG. 14. Germinal membrane, at a later period; the cells flattened by pressure. doubts that this change in the form of the cells is due to the pressure arising from their increase under limit. Can we doubt, then, that the rising up of the dorsal plates is due to the same cause? in fact that it is just such a rising up as we see in the plumule of the pea? If we spread a handkerchief on a table, place the hands flat upon it a little way apart, and gradually bring them nearer to each other, we produce similar ridges. The frond of a common fern again illustrates the process. Every one has noticed how it is curled, when young (Fig. 15). It looks as if it had been rolled up. But this is not the case; it may easily be seen that it cannot be. There has not been a flat frond which could be curled up. It grows into this form, because the central part grows, while the ends are fixed. With the increase of the plant it becomes free and uncurls; but it has never curled. The curling is an appearance due to its growth. Or let us take another class of forms. The buds of plants almost always grow in the axils of the leaves. It is not hard to see a reasou for this. The axil is the interval between the leaf and the stem; a kind of vacuity or space, into which the growing tissues may most easily expand. All the rest of the surface of the stem is covered in by the hard resisting bark, but where the leaf separates this resistance is diminished. It is the joint in the armor. So, in many rapidly growing plants, if a leaf be wounded, a bud springs from the spot. The wound constitutes an artificial "axil." So, again, in "budding," a wound is made to enable the new root to grow. One reason, then, why buds come in axils surely is, that there the least resistance is offered to the expansion of the soft substance of the plant. If we turn again to the development of the bird, we shall find what is precisely analogous. Very many of the organs are formed, like buds, in axils. Fig. 16 represents the young chicken at an early period of its formation; the brain consisting then of three small lobes. Now, in the interspaces or axils between these lobes, the eye and the ear bud out. These organs grow where a free space is afforded for them, at the points of separation between the lobes which, at this early period, constitute the brain. The eye "buds out between the first and second lobes, the ear between the second and third. They are at first hollow protrusions, merely, of the substance of the brain. The attached portion, or "pedicle," lengthens and becomes rela *It is the same in all vertebrate animals, but the bird is most easily examined. FIG. 16. Diagram of the Chicken in an early stage. The double lines represent the dorsal tively smaller afterward, and constitutes respectively the optic and auditory nerve. Or, let us look at the fully developed brain of any of the higher mammalia. The surface is wrinkled up in all directions, constituting quite a maze of elevated ridges, called convolutions. Do not these recall the "dorsal plates" (Fig. 11)? Are they not evidently formed in the same way? The external layer of the brain, expanding beneath the dense resisting skull, is folded into these "convolutions" for lack of space. Surely, we have thus discovered one of the causes of the forms of living things, in the mechanical conditions under which they are developed. The chemical forces, as we have seen, are used to produce the living substance; mechanical force, in the resistance of the structures which surround the growing organism, is used to shape it into the necessary forms. This is nature's division of labor. These are the simple means employed by the Creator for bringing into being the marvels of the organic world. Chemical force stores up the power, the mechanical resistance moulds the structure. We shall see this more truly by-and-by. sumes, and may be seen to produce some of the beautiful and useful forms which it displays, we may not assume other causes until it is proved that these are insufficient. Here is a fact: the mechanical conditions under which plants and animals are developed have a power of determining their forms in the right and necessary way. The limit of this power must be learn by observation. Or, if we look at the matter in another way, the conclusion is equally evident. Let us consider for a moment the circustances of a developing plant or animal. Here is the living substance; it is a soft plastic mass increasing in size; the forces of nature are operating upon it, adding to its bulk. Around it is a more or less resisting envelope. Will it not necessarily grow in those directions in which its extension is the least resisted? The case is to a certain extent, like that of taking the copy of a medal in wax-it is a very rough comparison, but still it may help us to grasp the general idea-the plastic substance, under the pressure of the artist's hand, moulds itself into the desired form by extending where the resistance is the least. There is no possibility of its doing otherwise. The case is as demonstrable as a proposition in Euclid. And it is equally so in respect to the growing plant or animal; under the pressure arising from the increase of its mass, it will mold itself by extending where the resitance is the least. But the process, of course, is much more complex than in this simple illustration. Perpetual changes and modifications are taking place, and especially in this respect, that every step in the development has i.s share in determining all that follow. Every newlyformed part or organ, each minutest fold, becomes at once a factor in the process. Thus it is, of course, that from seeds, all of them so much alike, their widest diversity being apparently trivial, the infinite variety of vegetable form arises. The slightest incipient diversities are continually reproduced and multiplied, like a slight error in the beginning of a long calculation; and thus very trivial differences of form or structure be tween two seeds may generate an absolute unlikeness in the resulting plant. But the true evidence of this law of living form is that which every one may find for himself. Every part of every creature, in which the means of its formation can be traced, will furnish it. If the bud of any flower be opened at an early stage, it will be seen how the petals grow into shape, modeled by the enclosing calyx; how the stamens are For the question arises, how far this refer- leaves that have not been able to unfold, and ence to mechanical conditions may be car- the anthers exactly fill the cavity of the bud, ried. Evidently that cause is operative, but receiving thence their form. Or if the pod is it the only one? In answer to this ques- of the common pea be opened at various tion, we may say, first, that, since the me- periods, the formation of the pea within it chanical conditions present during its forma- may be traced, under the influence of the tion do, to a certain extent, determine the like conditions; the plumule growing between structure which the growing organism as- the cotyledons when their expansion is re sisted, and being itself a bud formed in an, axil. Everywhere may be discerned more or less clearly a plastic expanding tissue, modeled by the varying resistances it meets. In individual instances, no observer has been able to ignore this fact. "I fear," says Mr. Ruskin, discussing the formation of the branches of trees by fibers descending from the leaves-"I fear the reader would have no patience with me, if I asked him to examine, in longitudinal section, the lines of the descending currents of wood, as they eddy into the increased single river. Of course, it is just what would take place if two strong streams, filling each a cylindrical pipe, ran together into one larger cylinder, with a central rod passing up every tube. But as this central rod increases, and at the same time the supply of the stream from above, every added leaf contributing its little current, the eddies of wood about the fork become intensely curious and interesting; of which thus much the reader may observe in a moment, by gathering a branch of any tree (laburnum shows it better, I think, than most), that the two meeting currents, first wrinkling a little, then rise in a low wave in the hollow of the fork, and flow over at the side, making their way to diffuse themselves round the stem (as in Fig. 17). Seen laterally the bough bulges out below of the branches of many trees, and of the shells which adorn the coast, are striking examples merely of a universal law. But the spiral is the direction which a body moving under resistance ever tends to take, as may be well seen by watching a bubble rising in water, or a moderately heavy body sinking through it. They will rise or sink in manifestly spiral curves. Growth under resistance is the chief cause of the spiral form assumed by living things. Parts which grow freely show it well;-the horns of animals, or the roots of seeds when made to germinate in water (as shown before in Fig. 8). The expanding tissue, compressed by its own resisting external coat, wreathes itself into spiral curves. A similar result may be attained artificially by winding a thread around a leaf bud on a tree, so as to impede its expansion; it will curve itself into a spiral as it grows. The formation of the heart is an interesting illustration of the law of spiral growth. That organ originates in a mass of pulsating cells, which, gradually becoming hollow, gives the first form of the heart in a straight tube, more or less subdivided, and terminating at each extremity in blood-vessels. This is the permanent form of the heart in many animals. Fig 18 represents the heart of an FIG. 17. the fork rather curiously and awkwardly, especially if more than two boughs meet at the same place, growing in one plane. If the reader is interested in the subject, he will find strangely complicated and wonderful arrangements of stream when smaller boughs meet larger." The reader will perceive how exactly this description and figure illustrate the principle. But no enumeration of instances could do justice to the evidence, or have any other effect than that of making the unlimited seem scanty. The proof is everywhere. One general fact may be referred to-the universally spiral form of organic bodies. The most superficial glance reveals a spiral tendency as a general characteristic both of the vegetable and animal creation; but a minute examination traces it in every detail. An essentially spiral construction is manifested from the lowest rudiments of life, upward throughcut every organ of the highest and most complex animal. The beautifully spiral forms Modern Painters, vol. v., p. 46. FIG. 18. Diagram of the circulation in a winged insect. The dark central portion represents the heart; it extends nearly the whole length of the body. 1 insect. When the organ is to be developed | into a more complex form, the first step in the process is its twisting into the shape shown in Fig. 19. It is like what takes T FIG. 19. Heart of mammal at an early stage. The central expanded portion is the heart; above and below are the blood-vessels communicating with it. place when we hold a flexible rod in our hands, and gradually approximate its end. The straight tube is growing within a limited space, and therefore coils itself into a spiral form." And this fundamental form it retains throughout all its subsequent development. But if this principle is true, why has it been overlooked? and why have men fallen into a way of speaking as if living matter had some inherent tendency to grow into certain forms, or as if masses of cells could model themselves, by some faculty or power of their own, into elaborate and complex shapes? It seems a strange thing that they should have done so, and yet it may easily be accounted for. The simplicity of nature's working is too profound for man's imagination to fathom, and is revealed only to humble seeking and steadfast self-control. Never could men have guessed that through such means such results could be achieved, even by a skill they deemed divine. And if we ask why it was not examined and observed long ago, the answer is, that other causes had been invented, and men had made up their minds. There was a plastic power," a specific property," a "formative nsius," or "effort." Shall we go on with the list? Is it any wonder that men could not see a simple, commonplace fact like this-that living things grow as they cannot help growing? es. Mothers are patient, Heaven be praised; but not so patient as our great Mother. For when the young rogue, finding it is of no use to guess any more, says, in mock resig. nation, "I can't tell," the maternal indignation will sometimes flash forth. But when we, finding that the mystery of life will not yield to our hypotheses, say, "We cannot learn it; it is a mystery insoluble," no sound of impatience or rebuke escapes the calm lips of Nature. Silently as of old the great volume is spread out before us year by year. Quietly and lovingly, as at the first, her finger points us to the words, written in tender herb, and stately tree, and glowing flow. er; ever to our hearts repeating her simple admonition, "Look." She knows we shall obey her when the time is come. But we are wandering from the subject. The law that the mechanical conditions under which they grow determine the form of living things, requires, like all laws, to be seen in its relations. It does not of course, operate alone. The expanding germ is molded into its shape by the resistance it meets; but the expansion has its own laws, and does not always take place equally in all directions. For the most part, in growing organisms, the tendency to growth exists more strongly in some parts than in others; and this varying tendency depends on causes which, though they are sometimes discover. able, are not always so. Let us revert to the case of the dorsal plates before referred to (Figs. 11 and 12). If they are caused to rise up by the expansion of the germinal membrane within its unyielding capsule, it is evident that this membrane must be growing chiefly in one direction (that at right angles to their length). It is the same in almost every case, but this one instance will suffice. Now this tendency to growth in particular directions is sometimes merely apparent, and arises from these being the directions in which there is least resistance to expansion. Sometimes, however, it seems to be due to a greater intensity, in certain parts, of the forces which produce growth; as, for instance, to a local decomposition generating a greater energy of vital action in that part, according to the law explained in the previous chapters. In these cases, the local growth resembles the increased development of plants on the side which receives most light. And the causes of the greater energy of often traced back several steps; as when an increased pressure produces a local decompo sition, and this gives rise again to a new or ganizing action. And, truth to say, there is all excuse for them. Nature is a wise and patient instructress of our ignorance. She never hurries us; but is content that we should read her lesson at last, after we have exhausted all our guess-growth in one part than another, may be Has the reader ever taught a child to read, or watched the process? If so, he has seen a "great fact" in miniature; the whole history of science on a reduced scale. For will not the urchin do any conceivable thing rather than look at the book? Does he not, with the utmost assurance, call out whatever letter comes uppermost, whatever word presents to his little imagination the slightest semblance of plausibility? He never looks until he cannot guess any more. Thus some apparent exceptions to the law of growth in the direction of least resistance receive an explanation. As, for example, that the root extends beneath the soil, and overcomes the resistance of the earth. The answer to this objection is, first, that the soft cellular condition of the growing radi cles forbids the idea that the roots force themselves into the ground; and secondly, that their growth is accounted for by the presence in the soil of the agencies which produce growth. In truth, the formation of the root affords a beautiful illustration of the CHAPTER V. LIVING FORMS.-THE LAW Of form. short, to the laws which force obeys wherever These few instances, which might be inlaw of least resistance, for it grows by insin- definitely multiplied, may suffice to make it uating itself, cell by cell, through the inter- manifest that organic forms are to be ascribed stices of the soil, winding and twisting to causes essentially the same as those which whithersoever the obstacles in its path deter-regulate the forms of inorganic bodies: in mine, and growing there most, where the nutritive materials are added to it most abundantly. As we look on the roots of a mighty tree, it appears to us as if they had thrust themselves with giant violence into the solid earth. But it is not so; they were led on gently, cell added to cell, softly as the dews descended and the loosened earth made way. Once formed, indeed, they expand with an enormous power, and it is probable that this expansion of the roots already formed may crack the surrounding soil, and help to make the interstices into which the new rootlets grow. Nor is there any good reason for assuming that the roots encounter from the soil a greater resistance to their growth than the portions of the stem meet with from other causes. We must not forget the hard external covering of the parts exposed to air and light. In some classes of palms this resistance is so great that the growth of a tree is stopped by it. A word may be said here, also, respecting the doctrine of "types," or standards, to which all living forms are referred. As a guide to the investigation of the organic world, this idea has proved itself invaluable; and the doctrine of corresponding parts in different organisms, to which it has been made subservient, constitutes, and must continue to constitute, a beautiful branch of physiological science.* But it is hardly necessary to say that no formative power is to be ascribed to those types or standards. The body needs some efficient cause to determine its form just as much, being conformed to such a type, as it would if it were not so conformed. Constancy of form proves constancy of conditions, and must do so equally upon every hypothesis. Similar to the case of the root are those in which mushrooms have been known to lift up heavy masses by their growth, sometimes raising in a single night a stone weighing many pounds. The forces which produce growth operate with enormous power. And well they may; for they are essentially the same forces those arising from the chemical properties of bodies-which in our own hands produce the most powerful effects, and are often indeed so violent in their action as to be wholly beyond our control. But it is clear that such cases as this can offer no difficulty But, in truth, neither general arguments, in respect to the laws of growth. Every one nor even an array of instances, are needed to must see that the mushroom would certainly give conclusiveness to the evidence that the not have raised the stone if that had not been forms of living bodies are mechanically dethe direction in which its expansion was re-termined. Startling as the proposition may sisted least. In this respect, the case seems seem when it is first uttered,t we no sooner precisely similar to the expansion of steam clearly grasp the conception and see what it in a boiler raising the piston. The mechanical resistance yields when the invisible inward force reaches a proportionate amount. There is, however, another class of instances to which we must refer. These are the forms which result not directly from growth but from decay, and of which the spongy pith of many plants is an example. The irregular cells and plates of which the pith consists are due to the drying up and shrinking of the pulp. In many animal structures this wasting process is accompanied by a contraction of the surrounding parts, and results in forms which can easily be traced to their causes. means, than it becomes self-evident. It is, *These corresponding parts are called "homologous:" as, for example, in plants, leaves and stamens are homologous; they correspond in their nature, although performing different offices. + The surprise with which it affects us is similar to that which we feel, or might well feel, when we reflect that our sensations of light or color, of music or of warmth, are referred to motion. The cause have in science to accept many such strange things as appears altogether inadequate to the effect. But we at least scientifically true. T |