Mechanics as the science of motion (kinematics and phoronomism)—Chemistry of vital movement—Active and passive movements—Undulatory movement—Mechanism of imbibition—Autonomous and reflex movements—Will and willing—Mixed movements—Movements of growth—Direction of the vital movement—Direction of the crystallizing force—Direction of cosmic motion—Movements of protists—Amoeboid, myophenous, hydrostatic, secretory, vibratory movements: cilia and lashes—Movements of histona, metaphyta, and metazoa—Locomotion of tissue animals: ciliary motion and muscular movements—Muscles of the skin—Active and passive organs of movement—Radiata, articulata, vertebrata, mammalia—Human movements.

All things in the world are in perpetual motion. The universe is a perpetuum mobile. There is no real rest anywhere; it is always only apparent or relative. Heat itself, which constantly changes, is merely motion. In the eternal play of cosmic bodies countless suns and planets rush hither and thither in infinite space. In every chemical composition and decomposition the atoms, or smallest particles of matter, are in motion, and so are the molecules they compose. The incessant metabolism of the living substance is associated with a constant movement of its particles, with the building up and decay of plasma-molecules. But here we must disregard all these elementary kinds of movement, and be content with a brief consideration of those forms of motion which are peculiar to organic life, and a comparison of them with the corresponding motions of inorganic bodies.

The science of motion, or mechanics, is now taken in very different senses: (1) in the widest sense as a philosophy of life [generally called mechanism or mechanicism in England], equivalent to either monism or materialism; (2) in the stricter sense as the physical science of motion, or of the laws of equilibrium and movement in the whole of nature (organic and inorganic); (3) in the narrowest sense as part of physics, or dynamics, the science of moving forces (in opposition to statics, the science of equilibrium); (4) in the purely mathematical sense as a part of geometry, for the mathematical definition of magnitudes of movement; and (5) in the biological sense as phoronomy, the science of the movements of organisms in space. However, these definitions are not yet universally adopted, and there is a good deal of confusion. It would be best to follow the lead of Johannes MÜller, as we are going to do here, and restrict the name phoronomy to the science of the vital movements which are peculiar to organisms, in contrast to kinematics, the exact science of the inorganic movements of all bodies. The real material object of phoronomy is the plasm, the living matter that forms the material substratum of all active vital movements.

On our monistic principles the inner nature of organic life consists in a chemical process, and this is determined by continuous movements of the plasma-molecules and their constituent atoms. As we have already considered this metabolism in the tenth chapter, we need do no more here than point out that both the general phenomena of molecular plasma-movement and their special direction in the various species of plants and animals can be reduced in principle to chemical laws, and are subject to the same laws of mechanics as all chemical processes in organic and inorganic bodies. In this we emphasize our opposition to vitalism, which sees in the direction of plasma-movement the supernatural influence of a mystical vital force or of some ghostly "dominant" (Reinke). We agree with Ostwald, who also reduces these complex movements to the play of energy in the plasm—that is to say, in the last instance to modifications of chemical energy. In regard to the visible movements of the living things which concern us at present, we must first distinguish passive and active, and subdivide the latter into reflex and autonomous.

Many movements of the living organism which the inexpert are inclined to attribute to life itself are purely passive; they are due either to external causes which do not proceed from the living plasm, or to the physical composition of the organic but no longer living substance. Purely passive movements, which play an important part in bionomy and chorology, comprise such as the flow of water and the rush of the wind; they cause considerable changes of locality and "passive" migrations of animals and plants. Purely physical, again, is what is known as the Brownian molecular movement which we observe with a powerful microscope in the plasm of both dead and living cells. When very fine granules (for instance, of ground charcoal) are equally distributed in a liquid of a certain consistency, they are found to be in a constant shaking or dancing movement. This movement of the solid particles is passive, and is due to the shocks of the invisible molecules of the fluid which are continually impinging upon each other. In the rhizopods—the remarkable protozoa whose unicellular organism sheds so much light on the obscure wonders of life—we notice a curious streaming of the granules in the living plasm. Within the cytoplasm of the amoebÆ particles travel up and down in all directions. On the long thin plasma-threads or pseudopodia which stream out from the unicellular body of the radiolaria and thalamophora, thousands of fine particles move about, like promenaders in a street. This movement does not come from the passive granules, but from the active invisible molecules of the plasm, which are always changing their relative positions. Thus also the movements of the blood-cells which we can see with the microscope in the circulation of a young transparent fish, or in the tail of a frog-larva, are not due to the action of the blood-cells themselves, but to the flow of the blood caused by the beat of the heart.

An important factor in the life of many organisms, especially the higher plants, is the physical phenomenon called imbibition; it consists in the penetration of water between the molecules of solid bodies (drawn to them by molecular attraction), and the consequent displacement of the molecules by the fluid. In this way the volume of the solid body is increased, and movements are produced which may have the appearance of vital processes. The energy of these imbibitional bodies is notoriously very powerful; we can, for instance, split large blocks of stone by the insertion of a piece of wood dipped in water. As the cellulose membrane of plant-cells has this property of imbibition in a high degree (either in the living or the dead cell), the movements it causes are of great physiological importance. This is especially the case when the imbibition of the cell wall is one-sided, and causes a bending of the cell. In consequence of the unequal strain in the drying of many fruits, they split open and project their seeds to some distance (as do the poppy, snap-dragon, etc.). The moss-capsules also empty their spores as a result of imbibition-curving (in the teeth of the openings of the spore-cases). The hygroscopic points of the heron-bill (erodium) curl up in the dry state and stretch out when moist; hence they are used as hygrometers in the construction of meteorological huts. The so-called "resurrection plants" (anastatica, the rose of Jericho, and selaginella lepidophylla), which close up like a fist when dry, spread their leaves out flat when moistened (the leaves imbibing strongly on the inner side). There is no more real case of "resuscitation" (as many believe) in these cases than in the mythological resurrection of the body. However, these phenomena of imbibition are not active vital processes; they are independent of the living plasm, and due solely to the physical constitution of the dead cell-membranes.

In contrast with these passive movements of organisms, we have the active movements which proceed from the living plasm. In the ultimate analysis, it is true, these may be reduced to the action of physical laws just as well as the passive movements. But the causes of them are not so clear and obvious; they are connected with the complicated chemical molecular processes of the living plasm, of the physical regularity of which we are now fully convinced, though their complicated mechanism is not yet understood. We may divide into two groups the many different movements, which are called vital in this stricter sense, and were formerly regarded as evidences of the presence of a mystic vital force, according as the stimulus—the sensation of which is caused by the movement—is directly perceptible or not. In the first case, we have stimulated (or reflex or paratonic) movements, and in the second voluntary (autonomous or spontaneous) movements. As the will appears to be free in the latter, they have been left out of consideration by many physiologists, and handed over to the treatment of the metaphysical psychologist. On our monistic principles this is a grave error; nor is it improved when "psychonomism" appeals to a false theory of knowledge. On the contrary, the conscious will (and conscious sensation) is itself a physical and chemical process like unconscious and involuntary movement (and unconscious feeling). They are both equally subject to the law of substance. However, only the external stimuli which cause reflex movements are known to us to any great extent and experimentally recognizable; the internal stimuli, which affect the will, are mostly unknown, and are not directly accessible to investigation. They are determined by the complicated structure of the psychoplasm, which has been gradually acquired by phylogenetic processes in the course of millions of years.

The great problem of the will and its freedom—the seventh and last of Dubois-Reymond's world-riddles—has been dealt with fully in the Riddle (chapter vii.). But as we still meet with the most glaring contradictions and confusion in regard to this difficult psychological question, I must touch upon it briefly once more. In the first place, I would remind the reader that it is best to restrict the name "will" to the purposive and conscious movements in the central nervous system of man and the higher animals, and to give the name of impulses (tropisms) to the corresponding unconscious processes in the psychoplasm of the lower animals, as well as of the plants and protists. For it is only the complicated mechanism of the advanced brain structure in the higher animals, in conjunction with the differentiated sense-organs on the one side and the muscles on the other, that accomplishes the purposive and deliberate actions which we are accustomed to call acts of will.

But the distinction between voluntary (autonomous) and involuntary (reflex) movements is as difficult to carry out in practice as it is clear in theory. We can easily see that the two forms of movement pass into each other without any sharp boundary (like conscious and unconscious sensation). The same action, which seems at first a conscious act of the will (for instance, in walking, speaking, etc.), may be repeated the next moment as an unconscious reflex action. Again, there are many important mixed or instinctive movements, the impulse to which comes partly from internal and partly from external stimuli. To this class belong especially the movements of growth.

Every natural body that grows increases its extent, fills a larger part of space, and so causes certain movements of its particles; this is equally true of inorganic crystals and the living organism. But there are important differences between the growth in the two cases. In the first place, crystals grow by the external apposition of fresh matter, while cells grow by the intussusception of fresh particles within the plasm (cf. chapter x.). In the second case, in growth, which determines the whole shape of the organism, two important factors always co-operate, the inner stimulus, which depends on the specific chemical constitution of the species, and is transmitted by heredity, and the external stimulus which is due to the direct action of light, heat, gravity, and other physical conditions of the environment, and is determined by adaptation (phototaxis, thermotaxis, geotropism, etc.).

A peculiar property of many vital movements (but by no means all) is the definite direction they exhibit; these are generally called purposive movements. For the teleologist they afford one of the chief and most welcome proofs of the dualistic theory of the older and the modern vitalism. Baer, especially, has laid stress on the purposiveness of all vital movement. It has been given a more precise expression recently by Reinke. His "dominants" are "intelligent directive forces," essentially different from all forms of energy or natural forces, and not subject to the law of substance. These metaphysical "vital spirits" are much the same as the immortal soul of dualistic psychology or the divine emanations of ancient theosophy. They are supposed not only to regulate the special development and construction of every species of animal and plant, and direct it to a predetermined end, but also to control all the various movements of the organism and its organs down to the cells. These "hyperenergetic forces" are equivalent to the "organizing principle" and the "unconscious will" of Edward Hartmann, the "arranging and controlling protoplasmic forces" of Hanstein and others. All these metaphysical, supernatural, and teleological ideas, like the older mystic notion of a special vital force, rest on a perversion of judgment by the apparent freedom of will and purposiveness of organization in the higher organisms. These thinkers overlook the fact that this purposiveness can be traced phylogenetically to simple physical movements in the lower organisms. Moreover, they overlook or deny the definite direction of inorganic forms of energy, though this is just as clear in the origin of a crystal as in the composition of the whole world-structure, in the direction of the mind as in the orbit of a planet. Hence it is important to bear in mind always these two forms of mechanical energy, and emphasize their identity with the direction of vital movement.

The force of gravitation which is at work in crystal-formation in the simple chemical body exhibits just as definite a direction as that which appears in the plasm in cell-construction. In this and other respects the comparison of the cell with the crystal, which was made even by the founders of the cell-theory, Schleiden and Schwann, in 1838, is thoroughly justified, though it is not correct in some other aspects. When the crystal is formed in the mother-water, the homogeneous particles of the chemical substance arrange themselves in a perfectly definite direction and order, so that mathematical planes of symmetry and axes arise within, and definite angles at the surface. On the strength of this, modern crystallography distinguishes six different systems of crystals. But, in different conditions, the same substance may crystallize in two or even three different systems (dimorphism and trimorphism of the crystal); thus, for instance, carbonate of lime crystallizes as calcspar in the hexagonal, and as arragonite in the rhombic system. If Reinke would be consistent, he ought to postulate a "dominant" for every crystal, to control the order and direction of the particles in its formation. He makes the curious statement (in 1899) that direction "is not a measurable magnitude" like energy, and so is not subject, like it, to the law of substance. We can mathematically determine the direction of the constructive force in the crystal just as well as in the cell.

If we comprise under the head of cosmokinesis the whole of the movements of the heavenly bodies in space, we cannot deny that they have a definite direction in detail, although our knowledge of this is still very incomplete. We can calculate the distances and speeds and movements of the planets round the sun with mathematical accuracy; and we gather from our astronomical observations and calculations that a similar regularity prevails in the movements of the other countless bodies in infinite space. But we do not know either the first impulse to these complex movements or their final goal. We can only conclude from the great discoveries of modern physics, supported by spectrum analysis and celestial photography, that the universal law of substance on the one side and the law of evolution on the other control the gigantic movements of the heavenly bodies just as they do the living swarm of tiny organisms that have inhabited our little planet for millions of years. Reinke ought, consistently, to admire the cosmic intelligence of the Supreme Being in these movements of the cosmic masses and its emanations, the "dominants," in the actual direction of their movements, as much as he does in the plasma-flow in the tiny organism.

The manifold gradation of vital movement which we find everywhere in the higher organisms is not without expression even in the protist realm. In this respect the chromacea, the simplest forms of vegetal monera, and the bacteria, which we regard as corresponding animal forms, developed from the former by metasitism, are of great interest. As microscopic scrutiny fails to detect any purposive organization in these unnucleated cells, and it is impossible to discover different organs in their homogeneous plasma-body, we have to look upon their movements as direct effects of their chemical molecular structure. But the same must be said also of a number of nucleated cells, both among the protophyta and the protozoa; only in this case the structure is less simple, in so far as both the nucleus itself and the surrounding cell-body exhibit, in indirect division, complicated movements in the plasm (caryokinesis). Apart from these, however, there is nothing to be seen in many unicellular beings (e.g., paulotomea, or calcocytea) that we need call "vital movement." On the border between the organic and inorganic worlds we have, as regards movement, the simplest forms of the chromacea, chroococcacea. We can see no vital movement in these structureless particles of plasm except slight changes of form, which occur when they multiply by cleavage. The internal molecular movements of the living matter, which effect their simple plasmodomous metabolism and growth, lie beyond our vision. The reproduction itself, in its simplest form of self-cleavage, seems to be merely a redundant growth, exceeding the limit of individual size for the homogeneous plasma-globule (cf. chapters ix. and x.).

The great majority of the protists have the appearance of real, nucleated cells. Hence we have to distinguish two different forms of movement in the unicellular organism—the inner movement in the caryoplasm of the nucleus and the outer in the cytoplasm of the cell-body; the two enter into close mutual relations during the remarkable process of partial resolution of the nucleus (caryolysis). In this modification and partial dissolution of their constituents we observe, during indirect cell-division, certain complicated movements (the significance of which is as yet entirely unknown), that are accomplished by both the granules of chromatin and the threads of achromin, and which are comprised under the head of nuclear movements (caryokinesis). It has lately been attempted to explain them on purely physical principles. The same may be said of the internal flow of the plasm which we find in the plasmodia of the amoebÆ and mycetozoa, and in the endoplasm of many of the protophyta and protozoa.

The slow displacement of the molecules of plasm which is at the bottom of these plasma-movements also causes a variety of external changes of form in simple naked cells. Variable processes like folds or fingers (the "fold-feet," lobopodia) appear on their surface. As they are best observed in the common amoebÆ (naked nucleated cells of the simplest kind), they are called amoeboid movements. With these is connected the variable movement of the larger rhizopods, the radiolaria and thalamophora, in which hundreds of fine threads radiate from the surface of the naked plasma-body. A number of recent experts on the rhizopods, such as BÜtschli, Richard Hertwig, Rhumbler, and others, have attempted to trace to purely physical causes this varying formation of pseudopodia, and their branching and net-like structure (without definite direction).

It is more difficult to do this in the case of the most highly differentiated of the protozoa, the infusoria. With these the free movement of the unicellular protozoon is farther advanced through the formation of permanent hairlike processes (long single lashes in the flagellata, and a number of short lashes in the ciliata) on the cell-surface and the movement of these by contraction and expansion, like the limbs, tentacles, and bones of the higher animals. The apparent spontaneity and various modulation in the ever-changing movements of these cell-feet is, in many of the infusoria, so like the autonomous voluntary movements in the metazoa that several experts on the infusoria have been moved on this account to ascribe individual (and even conscious) souls to them. Hence the difference between the various kinds of living movement is already very considerable before we leave the kingdom of the protists. On the one hand, the lowest monera (chromacea) join on directly to inorganic phenomena. On the other hand, the highly differentiated infusoria (ciliata) show so great a resemblance to the higher animals in their differentiated and autonomous movements that they have been credited with the possession of "free-will." There is no such thing as a sharp division.

In a large section of the higher protozoa differentiated organs of movement are developed, which may be compared to the muscles of the metazoa. In the cytoplasm threadlike, contractile structures are formed, and these have, like the muscular fibres of the metazoa, the power to contract and expand again in definite directions. These myophÆna or myonema form, in many of the infusoria, both ciliata and flagellata, a special thin layer of parallel or crossed fibres underneath the exoplasm or the hyaline skin-layer of the cell. The metabolic body of the infusorium may be altered in various ways by the autonomous contraction of these. Special instances of these myophÆna are the myophrisca of the acantharia—contractile threads which surround the radial needles of these radiolaria like a crown. They are found in their outer gelatine envelope, the calymma, and by their contraction extend it, and so lessen the specific gravity.

Many of the aquatic protophyta and protozoa have the power of autonomous and independent locomotion, and this often has the appearance of being voluntary. Among the simplest fresh-water protozoa are the arcellina or thecolobosa (difflugia, arcella), little rhizopods that are distinguished from the naked amoebÆ by the possession of a firm envelope. They usually creep about in the slime at the bottom, but in certain circumstances rise to the surface of the water. As Wilhelm Engelmann has shown, they accomplish this hydrostatic movement by means of a small vesicle of carbonic acid, which expands their unicellular body like an air-balloon; the specific weight of the cell-body, which is of itself heavier than water, is sufficiently lowered by this. The same method is followed by the pretty radiolaria which live floating (as plankton) at various depths of the sea. Their unicellular (originally globular) body is divided by a membrane into a firm inner central capsule and a soft outer gelatine covering. The latter, known as the calymma, is traversed by a number of water-vesicles or vacuoles. As a result of an osmotic process, carbonic acid may be secreted or pure water (without the salt of the sea-water) be imbibed in these vacuoles; by this means the specific gravity of the cell is lessened, and it rises to the surface. When it desires to make itself heavier and sink, the vacuoles discharge their lighter contents. These hydrostatic movements of the radiolaria (for which the myophrisca, still more complicated structures, have been developed in the acantharia) attain by simple means the same end that is accomplished in the siphonophora and fishes by air-filled and voluntarily contractile swimming-bladders.

Numbers of the unicellulars alter their position very characteristically by secreting a thick mucus at one side of their body and fastening this to the ground. If the secretion continues, a longish jelly-like stalk is produced by which the cell slowly pushes itself along, like a boat with a rowing-pole. This secretory locomotion is found, among the protophyta, in the desmidiacea and diatomes, and in some of the gregarinÆ and rhizopods among the protozoa. The peculiar rolling movements of the oscillaria (threadlike chains of blueish-green unnucleated cells, closely related to the chromacea) are also effected by the secretion of mucus. On the other hand, it is probable that the sliding movements of many of the diatomes are due to fine processes (vibratory hairs?) in the plasm, which proceed either out of the seams (raphe) of the bivalvular silicious shells or through the fine pores in them.

Especially important in the easy and rapid locomotion of many unicellulars is the formation of fine hairlike processes at the surface of the body; in the broadest sense, they are called vibratory hairs. If only a few whiplike threads are formed, they are called whips (flagella); if many short ones, lashes (cilia). Flagelliform movement is found in some of the bacteria, but especially in the mastigophorous "whip-infusoria," in the mastigota among the protophyta, and the flagellata among the protozoa. As a rule, we have in these cases one or two (rarely more) long and very thin whip-shaped processes, starting from one pole of the long axis of the oval, round, or long cell-body. These whips (flagella) are set in vibratory motion (apparently often voluntary) in various ways, and serve not only for swimming or creeping, but also for feeling and securing food. Similar whip-cells (cellulÆ flagellatÆ) are also found very commonly in the body of tissue-animals, usually packed together in an extensive layer at the inner or outer surface (ciliated epithelium). If single cells are released from the group, they may live independently for some time, continuing their movements and resembling free infusoria. The same may be said of the travelling spores of many of the algÆ, and of the most remarkable of all ciliated cells—the spermia or spermatozoa of plants and animals.

As a rule they are cone-shaped, having an oval or pear-shaped (though often also rod-shaped) head, which tapers into a long and thin thread. When their lively movements were first noticed in the male seminal fluid (each drop of which contains millions of them) two hundred years ago, they were thought to be real independent animalcules, like the infusoria, and so obtained their name of seed-animals (spermatozoa). It was a long time (sixty years ago) before we learned that they are detached glandular cells, which have the function of fertilizing the ovum. It was discovered at the same time that similar vibratory cells are found in many of the plants (algÆ, mosses, and ferns). Many of the latter (for instance, the spermatozoids of the cycadea) have, instead of a few long whips, a number of short lashes (cilia), and resemble the more highly developed ciliated infusoria (ciliata).

The ciliary movement of the infusoria is held to be a more perfect form of vibratory movement, because the many short lashes found on them are used for different purposes, and have accordingly assumed different forms in the division of labor. Some of the cilia are used for running or swimming, others for grasping or touching, and so on. In social combinations we have the ciliated cells of the ciliated epithelium of the higher animals—for instance, in the lungs, nostrils, and oviducts of vertebrates.

In the unicellular, non-tissue forming protists, all the vital movements seem to be active functions of the plasm of the single cell; but in the histona, the multicellular tissue-forming organisms, they are the outcome of the combined movements of the many cells which compose the tissue. Careful anatomic study and experimental physiological scrutiny of the motor processes are, therefore, first directed, in the case of the histona, to clearing up the nature and activity of the special cells which compose the tissue, and then the structure and functions of the tissue itself. When we start from this point, and survey the manifold active motor phenomena of the histona as a whole, we see at once an essential agreement in the phoronomy of the two kingdoms of the metaphyta and metazoa, in the sense that at the lower stages the chemical and physical character of the motor processes can be clearly shown and can be traced to an interchange of energy in the plasm of the cells that make up the tissue. In the higher stages, however, we find striking differences, the voluntary character of many autonomous movements being very conspicuous in the higher animals, and thus the great problem of the freedom of the will is added to the purely physiological questions of stimulated movement, growth-movement, etc.

Moreover, the movements of the metazoa are much more varied and complicated than those of the metaphyta, in consequence of the higher differentiation of their sense-organs and the centralization of their nervous system. The former have generally free locomotion and the latter not. The special mechanism of the organs of movement is also very different in the two groups. In most of the metazoa the chief motor organs are the muscles, which have developed in the highest degree the power of definitely directed contraction and expansion. In most of the metaphyta, on the other hand, the chief part of the movements depend on the strain of the living plasm, or what is called the turgor or expansibility of the plant-cells. This is effected by the osmotic pressure of the internal cell-fluid and the elasticity of the cellulose wall, which is thus expanded. Nevertheless, in both cases—and in all "vital" phenomena—the real cause of the process is, in the ultimate analysis, the chemical play of energy in the active plasm.

The metaphyta, with few exceptions, are fixed in one spot for life, or only mobile for a short time when they are young. In this they resemble the lower metazoa, the sponges, polyps, corals, bryozoa, etc. They have not free locomotion. The motor phenomena which we find in them affect only special parts or organs. They are mostly reflex or paratonic, and due to external stimuli. Only a few of the higher plants exhibit autonomous or spontaneous movement, the stimulating cause of which is unknown to us, and which may be compared to the apparently voluntary actions of the higher animals. The lateral feather-leaves of an Indian butterfly flower (hedysarum gyrans) move in circles through the air, like a pair of arms swinging, without any external cause; they complete a circle in a couple of minutes. Variations in the intensity of light have no effect on them. Similar spontaneous movements of the leaves of several species of clover (trifolium) and sorrel (oxalis) are performed only in the dark, not in the light. The terminal leaf of the meadow-clover repeats its rotation, which describes more than one hundred and twenty degrees of an arc, every two to four hours. The mechanical cause of these spontaneous "variation movements" seems to lie in variations of expansibility.

Voluntary and autonomous turgescence-movements of this kind are only observed in a few of the higher plants, but stimulated movements that are accomplished by the same mechanism are very common in the vegetal world. We have, especially, the well-known "sleep," or nyktitropic movements, of many plants. Many leaves and flowers hold themselves vertically to the streaming rays of the sun. When darkness comes on they contract, and the calices of the flowers close. Many flowers are open for only a few hours a day. The mechanism of turgescence, which effects these swelling movements, consists in the co-operation of the osmotic pressure of the internal cell-fluid and the elasticity of the strained cell-membrane enclosing the cytoplasm. The strain of the outer cellulose membrane on the plasmatic primordial sac within it grows so much on the accession of osmotically active matter that the internal pressure is equal to several atmospheres, and the elastic strained membrane stretches from ten to twenty percent. When water is withdrawn again from one of these swollen or turgescent cells, the membrane contracts; the cell becomes smaller, and the tissue looser. Other stimuli besides light (heat, pressure, electricity) may produce these expansional variations, and, as a consequence of it, certain reflex movements (or paratonic variational movements). The most striking and familiar examples are the flesh-eating fly-trap (dionÆa muscipula) and the sensitive plant (mimosa pudica); their contraction is caused by mechanical stimuli, shaking, pressure, or the touching of the leaves.

Most of the higher animals have the power of free and voluntary locomotion. It is, however, wanting in some of the lower classes, which spend the greater part of their life at the bottom of the water, like plants. Hence these were formerly held to be vegetable—thus the sponges, polyps, and corals among the coelenteria. A number of classes of the coelomaria have also adopted the stationary life, such as the bryozoa and the spirobranchia among the vermalia, many mussels (oysters, etc.), the actinia among the tunicates, the sea-lilies (crinoidea) among the echinoderms, and even highly organized articulate, such as the tube-worms (tubicolÆ), among the annelids, and the crawling crabs (cirripedia), among the crustacea. All these stationary metazoa move freely in their youth, and swim about in the water as gastrulÆ, or in some other larva form. They have taken only gradually to stationary habits, and have been considerably modified, and often greatly degenerated, in consequence; for instance, in the loss of the higher sense-organs, the bones, and even of the whole head. Arnold Lang has shown this very clearly in his excellent work on the influence of stationary life on animals. The study of these retrogressive metamorphoses is very important for the theory of progressive heredity and selection; it also shows the great value of free locomotion for the higher sensitive and intellectual development of the animals and man.

In many of the lower aquatic metazoa the surface of the body is covered with vibratory epithelium—that is to say, with a layer of skin-cells which bear either one long whip (flagellum) or several short lashes (cilia). Flagellated epithelium is especially found in the cnidaria and platodes; ciliated epithelium mostly in the vermalia and mollusca. As the lashing motion of these hairlike processes brings a constant stream of fresh water to the surface of the body, they first of all effect respiration through the skin. But in many of the smaller metazoa they also serve the purpose of locomotion, as in the gastrÆads, the turbellaria, the rotifera, the nemertina, and the young larvÆ of many other metazoa. The vibratory apparatus reaches its highest development in the ctenophora. The extremely delicate and soft body of these gherkin-shaped cnidaria swims slowly in the water by means of the strokes of thousands of tiny oar-blades. They are arranged in eight longitudinal rows which stretch from the mouth to the opposite pole. Each oar-blade consists of the long hair-lashes of a group of epithelial cells glued together.

The chief motor organs in the metazoa are the muscles which constitute the "flesh" of the body. Muscular tissue consists of contractile cells—that is to say, of cells with the sole property of contraction. When the muscular cell contracts, it becomes shorter and its diameter increases. This brings nearer together the two parts of the body to which its ends are attached. In the lower metazoa the muscle-cells have, as a rule, no particular structure; but in the higher animals the contractile plasm undergoes a peculiar differentiation, which has the appearance under the microscope of a transverse streaking of the long cells. On this ground a distinction is drawn between striated muscles and simple non-striated or smooth muscles. The more vigorous, rapid, and definite is the contraction of the muscle, the more marked is the streaky character, and the more pronounced the difference between the doubly refractive muscular particles from the simple refractive. The striated muscle is "the most perfect dynamo we know of" (Verworn). The normal heart of a man accomplishes every day, according to Zuntz, a work of about twenty thousand kilogrammetres—in other words, an energy that would suffice to lift to a height of one metre a weight of twenty thousand kilogrammes. In many flying insects (gnats, for instance) the flying muscles make three hundred to four hundred contractions a second.

In the lower and higher classes of the metazoa the muscle amounts to no more than a thin layer of flesh underneath the skin. This layer consists of muscular cells, which come originally from the ectoderm in the form of internal contractile processes of the skin-cells themselves, as in the polyps. In other cases the muscle-cells are developed from the connective-tissue cells of the mesoderm, the middle skin-layer, as in the ctenophora. This mesenchymic muscle is less common than epithelial muscle. In most of the askeletal vermalia the subdermal muscle divides into two layers—an outer deposit of concentric muscles and an inner layer of longitudinal muscles; in the cylindrical worms (nematodes, sagittÆ, etc.) the latter fall into four longitudinal bands, one pair of upper (dorsal) and a pair of lower (ventral) muscular bands. At those parts of the body which are especially used for locomotion the muscle is more strongly developed, as in the belly-side of the crawling worms and mollusks. This muscular surface develops into a kind of fleshy "foot" (podium); it assumes a great variety of forms in the various classes of mollusks. In most of the snails which creep on the solid ground it grows into a muscular "flat-foot" (gasteropoda); in the mussels which cut like a plough through the soft slime it forms a sharp "hatchet-foot" (pelecypoda). The keel-snails (heteropoda) swim by means of a "keel-foot," which works like the screw of a ship; the floating-snails (pteropoda) swim unsteadily (like butterflies flying) by means of a pair of head-folds, which develop from the side of the anterior foot-section. In the highest mollusks, the cuttle-fishes (cephalopoda), this fore-foot divides into four or five pairs of folds, which grow into long and very muscular "head-arms"; the numbers of strong suckers on the latter have also special muscles. In all these non-articulate mollusks and vermalia hard skeletons are either altogether wanting or (like the external shells of the mollusks) they have no functional relation to the motor muscles. It is otherwise in the higher animals, in which we find this relation to a solid jointed skeleton that becomes a passive motor apparatus.

The higher groups of the animal kingdom in which a characteristic solid skeleton is developed and forms an important starting-point for the muscles, as well as a support and protection for the whole body, are the three stems of the echinoderms, articulates, and vertebrates. All three groups are very rich in forms, and far surpass all the other stems of the animal world in the perfection of their locomotive apparatus. However, the disposition and development of the skeleton as a passive support, and the correlation of the muscles to it as active pulling-organs, differ very much in the three classes, and are the chief factors in determining their characteristic types; they show clearly (even apart from other radical differences) that the three stems have arisen independently of each other from three different roots in the vermalia-stem. In the echinoderms the calcareous skeleton is formed from chalky deposits in the corium, in the articulates from chitine secretions of the epidermis, and in the vertebrates from cartilage of an internal chord-sheath (cf. Anthropogeny, chapter xxvi.).

The remarkable stem of the sea-dwelling echinoderms or "prickly skins" is distinguished from all the other animal groups by a number of striking peculiarities; prominent among these are the special formation of their active and passive motor organs and the curious form of their individual development. In this ontogenesis two totally different forms appear successively—the simple astrolarva and the elaborately organized and sexually mature astrozoon. The small, free-swimming astrolarva has the general structural features of the rotatoria, and so shows, in accordance with the biogenetic law, that the original stem-form of the echinoderms (the amphoridea) belonged to this group of the vermalia. I have briefly explained these structures in the History of Creation (chapter xxii.), and more fully in my essay on the amphoridea and cystoidea (1896). The little astrolarva has no muscles, and no water-vessels or blood-vessels. It moves by means of vibratory lashes or bands, which are attached to special armlike processes at the surface. These arms are regularly developed to the right and left of the bilateral symmetrical larva (which as yet shows no trace of the five-rayed structure). By a very curious modification the small bilateral astrolarva is transformed into the totally different pentaradial astrozoon, the large sexually mature echinoderm with a pronounced five-rayed structure. (See Art-forms in Nature, plates 10, 20, 30, 40, 60, 70, 80, 90, and 95.) It has a most elaborate organization, with muscles and cuticular skeleton, blood-vessels and water-vessels, etc. A section of the astrozoa—the living crinoidea, or sea-lilies, and the extinct classes of blastoidea (sea-buds), cystoidea (sea-apples), and amphoridea (sea-urns)—grow in stationary fashion at the bottom of the sea. The other four extant classes creep about in the sea—the sea-gherkins (holothuria), the star-fish (asteridea and ophoidea), and the sea-urchins (echinidea). Their creeping motion is accomplished by two kinds of organs—water-feet and skin-muscles. The latter find their support and attachment in solid calcareous needles, which develop from chalky deposits in the corium. As these calcareous needles (which are particularly conspicuous in the sea-urchin) are set movably in special protuberances of the calcareous plates of the cuticular skeleton, and moved by little muscular needles, the echinoderms walk on them as if they were stilts. Between these, however, a number of water-feet arise from inside—thin tubes like the fingers of a glove, which are filled with water by an internal conduit-system (the so-called ambulacral system) and become stiff. These very extensible ambulacral feet, often provided with a suctorial plate at the closed outer end, serve for creeping, sucking, touching, and grasping. As these distinctive motor organs of the echinoderms—both the ambulacral feet with their complicated water-tubes and the movable needles with their joints and muscles—are found in hundreds, often in thousands, on every individual five-rayed astrozoon, we might say that the echinoderms have the most advanced and complicated motor organs of all animals. Their historical development is perfectly understood from its earliest stages, since Richard Semon found, in his ingenious pentact Æatheory (1888), the correct phylogenetic meaning of the curious embryology of the echinoderms discovered in 1845 by Johannes MÜller. I endeavored in 1896 to establish it in detail, in relation to paleontological discoveries, in the essay I have mentioned.

The large stem of the articulata (the richest in forms of all the animal stems) comprises three chief classes—the annelids, crustacea, and tracheata. All three groups agree in the essential features of their organization, especially in the external articulation or metamerism of the long bilateral body, and also in the repetition of the internal organs in each joint or segment. In each joint there is originally a knot of the ventral nervous system (the ventral marrow), a chamber of the dorsal heart, a chitine-ring of the cutaneous skeleton, and a corresponding group of muscles.

Of the three great classes of the articulates the annelids are developed directly from the vermalia, of which both the nematoda and nemertinÆ approach very closely to them. The two other and more highly organized classes, the crustacea and tracheata, are younger groups, independently evolved from two different stems of the annelids. The annelids, or "ringed-worms" (to which, e.g., the rain-worms belong), have mostly a very homogeneous articulation; their segments or metamera repeat the same structure to a great extent, especially the subdermal muscles. In a transverse section we see in every joint underneath the layer of concentric muscles a pair of dorsal and a pair of ventral muscles. Their epidermis has secreted a thin covering of chitine, in the tubular worms a leather-like or calcified tube. There are no bones in the oldest annelids; in the younger bristle-worms (polychÆta) one or two pairs of short unjointed feet (parapodia) are found in every joint.

The other two chief classes of the articulates develop long and jointed feet of very varied forms, and at the same time assume different shapes of limbs in the division of labor. This heterogeneous articulation (heteronomy) is the more pronounced the higher the whole organization. This is equally true of the aquatic, gill-breathing crustacea (crabs, etc.) and the tracheata (terrestrial animals breathing through a trachea, the myriopods, spiders, and insects). In the higher groups of both classes the number of limbs is usually not higher than fifteen to twenty; and they are distributed in three principal sections—head, breast, and posterior part of the body. The firm covering of chitine, which was delicate and thin in most of the annelids, is much thicker in most of the crustacea and tracheata, and often hardened by a calcareous deposit; it forms a solid ring of chitine in each segment, inside which the motor muscles are attached. The successive hard rings are connected by thin, mobile, intermediate rings, so that the whole body combines firmness, elasticity, and mobility in a high degree. The structure of the long jointed legs, which are fixed in pairs on each segment, is very similar. Hence the typical character of the motor organs of the crustacea lies in the circumstance that both in the body and the limbs the muscles are attached to the interior of hollow chitine tubes, and go in these from member to member.

The vertebrates are just the reverse in structure. In their case a solid internal skeleton is formed in the longitudinal axis of the body, and the muscles are external to these supporting organs. The articulation or metamerism itself is not visible externally in the vertebrates; it is only seen in the muscular system when the non-articulated skin has been removed. Then, even in the lowest skull-less vertebrates, the acrania, the internal skeleton of which consists merely of a cylindrical, solid, and elastic axial rod (chorda), we see on each side a row of muscular plates (fifty to eighty in the amphioxus). In this case there are not pairs of limbs, and it is the same with the oldest craniate animals, the cyclostoma (myxinoida and petromyzonta). It is only with the third class of the vertebrates, the true fishes (pisces), that two pairs of lateral limbs appear—the breast-fins and belly-fins. From these, in their terrestrial descendants, the oldest amphibia of the Carboniferous Period, the two pairs of jointed legs—fore-legs (carpomela) and hind-legs (tarsomela)—are derived. These four lateral five-toed legs have a very characteristic and complicated articulation, both in the internal bony skeleton and the muscular system that encloses this and is attached to it. From the amphibia, the earliest quadrupeds, this locomotive apparatus is transmitted by heredity to their descendants, the three higher classes of the vertebrates, reptiles, birds, and mammals. As I have dealt with these important structures fully in my Anthropogeny (chapter xxvi.), and given a number of illustrations of them, I must refer the reader to that work,[8] and will only make a few observations on the mammals.

Both parts of the motor apparatus, the internal bony skeleton (the passive supporting apparatus) and the external muscular system (the active motor), exhibit a great variety of construction within the mammal class, in consequence of adaptation to the most different habits and functions. We have only to compare the running carnivora and ungulata, the leaping kangaroos and jerboas, the burrowing moles and hyperdÆi, the flying cheiroptera and bats, the fishlike swimming sirens and whales, and climbing lemures and apes. In all these and the remaining orders of the mammals the whole regular structure of the motor apparatus is strikingly adapted to the habits of life which have been formed by this adaptation itself. Nevertheless, we see that the essential character of the inner organization which distinguishes the mammals as a class is not affected by this adaptation, but constantly maintained by heredity. These recognized facts of comparative anatomy and ontogeny, and the concordant results of paleontology, prove convincingly that all living and fossil mammals, from the lowest ungulates and marsupials to the ape and man, have descended from one common stem-form, a pro-mammal, that lived in the Triassic Period; its earlier ancestors in the Permian Period were reptiles, and, in the Carboniferous Period, amphibia. Among the characters of the locomotive apparatus which are peculiar to mammals we have, on the one hand, the structure of the vertebral column and the skull, and, on the other hand, the formation of the muscles which are attached to these supporting organs. In the skull we particularly notice the formation of the lower jaw and the joint by which it is connected with the temporal bone. This joint is temporal, and so distinguished from the square joint of the other vertebrates. The latter is found in the mammals in the tympanic cavity of the middle-ear, between the hammer (the modified joint of the lower jaw, articulare) and the anvil (the original quadratum). In harmony with this remarkable modification of the maxillary joint, the corresponding muscles have naturally also undergone a considerable transformation. A distinctive muscle that is only found in the mammals and regulates their respiration is the diaphragm, which completely divides the abdominal and thoracic cavities; the various muscles, from the blending of which it has been formed, still remain separate in the other vertebrates.

The many organs by means of which our human organism accomplishes its manifold movements are just the same as in the apes, and the mechanism of their action is in no way different. The same two hundred bones, in the same order and composition, form our internal bony skeleton; the same three hundred muscles effect our movements. The differences we find in the form and size of the various muscles and bones (and which are, as is well known, also found between lower and higher races of men) are due to differences in growth in consequence of divergent adaptation. On the other hand, the complete agreement in the construction of the whole motor apparatus is explained by heredity from the common stem-form of the apes and men. The most striking difference between the movements of the two is due to man's adaptation to the erect posture, while the climbing of trees is the normal habit of the ape. However, it is unquestionable that the former is an evolution from the latter. A double parallel to this modification is seen in the jerboa among the ungulates, and in the kangaroo among the marsupials. Both these, in springing, use only the strong hinder extremities, and not the weaker fore-limbs; as a result of this their posture has become more or less erect. Among the birds we have an analogous case in the penguins (aptenodytes); as they no longer use their atrophied wings for flight, but only in swimming, they have developed an erect posture when on land.

The human will is also not specifically different from that of the ape or any other mammal; and its microscopic organs, the neurona in the brain and the muscular cells in the flesh, work with the same forms of energy, and are similarly subject to the law of substance. Hence it is immaterial for the moment whether one believes in the freedom of the will according to the antiquated creed of indeterminism, or whether one holds it to be refuted scientifically by the arguments of modern determinists; in either case the acts of the will and voluntary movements follow the same laws in man as in the ape. The high development of the function in civilized man, the ample differentiation of speech and morality, art and science—in a word, the ethical significance of the will for higher culture—is in no way discordant to this monistic and zoologically grounded conception. In the lower races these privileges of the civilized will are only found in a slight degree, and some of them are wholly wanting among the lowest races. The distance between the lowest savage and the most civilized human being is greater, in this respect also, than that which separates the savage from the anthropoid ape. However, I refer the reader to the remarks I made at the close of the seventh chapter of the Riddle on the problem of the freedom of the will and the infinite literature relating thereto. The reader who desires to go further into this subject will find it well treated in the works of Traugott Trunk (1902) and Paul RÉe (1903) [also in Dr. Stout's recent little manual of psychology and Mr. W. H. Mallock's Religion as a Credible Doctrine].

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