Popular Science Monthly/Volume 18/November 1880/The Evolution of Organic Form
WHAT does the story of life upon the earth teach us concerning the unfoldment of organic form? Is the human figure a chance result of an evolutionary force which might have pursued some quite different direction; or are the laws of development such as to lead inevitably toward the form of man as their highest organic product? This is a question admitting of a more definite answer than may at first thought appear, as we hope to show by a rapid survey of the various stress of the process.
And, first, it must be borne in mind that Nature's efforts at animal and plant formation have been on no contracted scale. The varying forms produced have been almost multitudinous. They exist at present in the greatest variety. But the present is only the apex of a long succession of life-epochs, each with its special organic group. We must multiply the existing forms by thousands of such epochs to obtain any adequate idea of the whole broad field of life. Plainly, then. Nature has not dealt sparsely with the subject, but has produced a most generous profusion of differing forms. Hence, narrow as is the field of the earth, there is reason to believe that the form-evolving principle has had full opportunity here to act. and that it has selected out the most favorable line of development from the many directions attempted.
Life is an incessant battle—a battle for food, and a battle for safety. The total quantity of food is limited. The powers of organic increase are unlimited. Thus a fight for food becomes necessary; a conflict in which no quarter is asked and none given. Victory inclines to the strongest and best armed. The successful combatant must have powers of defense against all Nature's attacks, and of assault against all Nature's defenses. In other words, the organism best adapted to its environment will win.
And this incessant weeding-out process is not confined to mature forms. It is constantly in action, from the germ up to maturity. There is as fierce a battle between germs as between grown animals as to which shall survive. The ill-adapted embryo perishes; the well adapted lives. Of the multitudes of young, only those survive which are best fitted to obtain food and escape peril. There is thus a succession of conditions to which the growing form must be successively adapted, and each mature form is the sole survivor of a myriad of germs which started together in the race of life. It has been sharply selected out as the best adapted.
The law of adaptation thus works vigorously throughout all embryonic development. It works as decisively on mature forms. They must be closely adapted to certain conditions of nature; but the possible variation in conditions is almost boundless. Not only in time have there been constant changes in natural conditions, and not only do they now widely vary in different localities, but even in the same locality a great variety of differing conditions simultaneously exist.
As the simple atoms of the chemical elements unite to form complex compounds, so do simple conditions unite into complex. Numerous sets of minor conditions exist together, from whose combination are formed less numerous major conditions, and from these again a single highest condition which includes all below its level. Thus each locality may possess its many sets of simpler forms, and sets of superior forms narrowing in number as they become adapted to a wider environment, until a highest or most complex form is reached, which is in physical harmony with the totality of existing conditions.
And the question of superiority and inferiority between animals is simply a question of the greater or lesser complexity of the conditions to which they are fitted, the broader or narrower field of adaptation which they occupy.
But, in this quick pressing of new forms into every nook and cranny of nature, there are certain general principles which have a controlling influence over the resulting changes in form. One consideration must always be taken into account, that of the character of organic material of protoplasm and the forms it naturally tends to assume. And a second consideration is that of the main end of animal life the absorption of aliment. From this latter it follows that the basic type of animal form is the stomach; and in viewing the field of animal development we behold only a series of stomachs, provided with various food-taking and danger-escaping appendages.
Evolution, then, means the gaining of superior powers of providing for the needs of this voracious core of all animals, the stomach; and of superior powers of escaping the voraciousness of other armed and perambulatory stomachs.
The aliment on which organisms subsist is of three kinds mineral, vegetable, and animal. The pursuit of these yields two distinct classes of organisms. Mineral food needs to undergo a high degree of chemical integration. The organisms which subsist upon it are functionally low, the forces which might have been otherwise employed being used up in the formation of protoplasm and other high organic compounds.
The conditions of alimentation limit the vegetable world to one general form. Plants do not need to seek food; food seeks them. Thus no motive powers are requisite, and they remain fixed in one position. But food seeks them in two different localities, the earth and the air. The earth constituent comes to them dissolved in water, the atmospheric constituent dissolved in air. They must, therefore, have powers of extension sufficient to occupy both these fields. This power is obtained by growth, root-extension seeking the liquid food, leaf-extension the gaseous food.
These general conditions confine plants to one generic form, a connecting link of stem between earth and air, and an extension of root-mouths into the soil and of leaf-mouths into the atmosphere. A tree is a society, or family; the main stem being the patriarch of the flock, the earth and air branches its descendants, and the leaves and rootlets its latest unisexual offspring.
As the type of animal form is the stomach, so the type of plant form is the mouth. It has not yet developed into the formation of a central stomach, nor has it attained powers of digestion. It builds up protoplasm by successive steps of chemical integration. It is a laboratory for the production of chemical synthesis not of chemical analysis, as in animals.
Thus the food-taking requisite is provided for in the production of numerous leaf and root mouths, extending themselves into the two great reservoirs of food. But defense must be provided for as well. Plants are attacked by various foes. Fierce storms assail them. These can only be resisted by an inmate strength or elasticity. Wintry cold congeals their food-supply. They must therefore be capable of hibernating. Animals seek to devour them. They can only escape by inclosing themselves in a rigid armor, or by becoming unfitted for animal food.
This leads us to the most significant adaptation in plants. Their life duration is limited, and they must have powers of reproduction, the best adapted in this respect crowding out the less adapted. Obviously the seed-bearers are best fitted for survival; and of these, those bearing the most seeds, and having the best facilities for dispersing them.
But the fixed plant can not, of itself, spread its seeds beyond its own locality. It must be aided by other agencies. Many plants avail themselves of air-currents for this purpose, the seeds being provided with curious appendages to aid them in flying or rolling before the winds. In other cases the seed is surrounded by a store of palatable food, offering an inducement to the higher animals to devour it, and thus to disseminate the seeds.
Fertilization of the flowers is provided for in a similar manner. The flowers can not reach each other, and therefore enlist insects in their aid, preparing a store of food highly palatable to these air rovers, and thus having their fertilizing germs carried from flower to flower.
Such are the general agencies at work in plant-life, and producing its typical form. And thus, while protecting their vital organs by a rigid armor, plants provide for reproduction by adapting a portion of their bodies for animal food; gaining in this manner for their offspring the powers of motion which they lack themselves.
Yet all plants are not confined to this typical form, as all plants are not confined to purely inorganic aliment. Some subsist on partly or fully elaborated organic food, and these deviate from the plant and approach some of the animal types of form, which we have next to consider.
In animal life very different requirements from those presented by plants are exhibited, and the forms are essentially different. Yet their main functions are the same. All organisms are adapted to the two general purposes of food-getting and defense, to which all their other powers are subordinate.
As vegetables subsist on mineral, so animals subsist on organic food, either vegetable or animal. And this food presents another essential difference from that of plants. It exists only in the solid state, while that of plants is wholly fluid. It can not be taken by direct imbibition, like that of vegetables, but must be first rendered liquid through some digestive process, and afterward imbibed. Thus an internal stomach is necessary to all but the very lowest animals, and even these improvise temporary stomachs, which foreshadow the permanent stomach.
The animal—not being bathed in an ocean of food, which it has but to drink in at a multitude of mouths covering its whole periphery, as in the plant—must have means of drawing food to it, or organs enabling it to go in search of food. In short, it must have motive powers.
And for those creatures which are obliged to go in search of their food, it is equally requisite that they should be able to discover its locality. Sensory organs, therefore, become necessary. Consequently, the animal is superior to the plant through this possession of muscular and sense organs. It is also superior in being able to employ the energy derived from its food, not in the building up of chemical compounds, but in the force of motion and sensation.
Nor can the animal be wholly protected by armor. Some portion of it must be exposed to danger. At least those flexible limbs which aid it in food-getting are in frequent peril, and need some form of protection. The loss of them can not well be made useful to the animal, as the loss of its exposed portions is to the plants.
Evidently the animal is capable of a much wider range of form evolution than the plant. In its mobility of variation it has branched out in many directions, but the possible height attainable by each general direction of growth is limited by certain principles, which we may be able to discover.
Both herbivorous and carnivorous animals may exist in fixed and in motile forms—food-attracting and food-seeking adaptations. The fixed forms are principally or entirely water-animals, comprising the Sponges, a large section of the Polyps, the lower forms of the Echinoderms, with some divergent forms, such as the Bryozoa, the Tunicata, and the Barnacles.
These are saved the necessity of moving, by the fact of their being tenants of a liquid whose moving currents bring them food, and by being capable of themselves producing water-currents, on which food is borne to them. Their necessary movements are reduced to the motion of tentacles—current-making or food-seizing organs. No sense is requisite except touch, and therefore no higher degree of sensibility is developed. These fixed forms thus necessarily remain at the foot of the ladder of progress, being but a step above the Protozoa, or single celled animals. They may be classed, however, as superior to the internal parasites of animals, which live by imbibing elaborated animal juices, and need no motile nor sense organs.
But, as soon as an animal obtains powers of free motion, it comes at once into contact with a much wider range of conditions and needs to gain extended powers. It is, moreover, placed under seeming disadvantages, which are really of high efficacy in its development. It possesses no stone castle of refuge, from which it has but to extend its retractile arms. It is, therefore, exposed to much greater dangers, its whole body being open to the assault of foes.
There are two general methods by which protection from these perils is gained: the first by armor; the second by activity and sensory acuteness. The armored animals are necessarily heavier, less active, and less flexible, than the unarmored. The latter depend for safety on activity and variety of motion, on quickness of sense, and on weapons of defense. They are, consequently, more highly developed than the armored, whose firm coating forms their main protective adaptation.
They also come into contact with a much wider range of natural conditions, their more extensive excursions accustoming them to more varied forms of food, adapting them to wider surface and temperature relations, and exposing them to more numerous foes. Thus they must become fitted to a wider environment, and their powers be more specialized; the naked, flexible, active animal being thus necessarily the highest in point of development.
These general views lead us to their particular application to the existing animal types. We think it can be shown that each type has had full opportunities of unfoldment, and has reached the extreme limit of its line of growth.
There are certain requirements of the animal organism to which every adaptation must conform. Underlying the stomachic type is the more primary fact that the natural form of colloid matter is the globe. Like all fluid or semi-fluid matter it tends to curve about a general center of attraction, in distinction to the angular extension of the crystal. This tendency shows itself in all parts of all animal forms, and also in these forms as wholes, the globe being departed from only through functional necessity, or from the superposition of a series of organs, each with a globular tendency, yielding, through mutual pressure, a more or less ovoid result.
In the Protozoa we have the globular form, diversified by temporary, improvised limbs, or by permanent organs. In the Metazoa variation from the globe takes place in axial directions—the fixed animals having usually but two axes of departure from the sphere; the moving animals having ordinarily three axes—a longitudinal, a vertical, and a transverse. The general result is the production of the round, flattened form of the two-axed, and the oval form of the three-axed animals; the further departure being in the production of limbs—appendages devoted to motion, or to assault and defense.
If now we take the Gastrula, the simple stomach-sac, for the primitive form of the many-celled animal, and the earliest phase of derivation from the Protozoa, it is easy to perceive that this hollow animal globe may vary in three different modes.
First, it may retain its sac-like form and stomach-opening, developing tentacles about the mouth, and radiated body divisions; thus passing from the single axis of the Gastrula to the double axis of the polyp.
Secondly, it may flatten, until it resembles a sack with the open top pressed down upon the bottom, and the sides bulging outward into a circle. If, now, radiated arms extend outward from this rounded side, we have the starfish type of organism.
Thirdly—still preserving its affiliation to the globe—it may lengthen instead of flattening. From this mode of development would come the longitudinal type of animals, the vermes, or worm-forms.
A still more primitive departure from the original Gastrula form is found in the sponge, in which the body-wall is pierced by minute apertures, through which food-bearing currents are drawn into the general internal stomach, and forced out again through the mouth. The low organization of the sponge results from the fact that it does not even require the mouth-arms of the polyp as an aid in food-getting. Its only motive apparatus is the cilia, or vibrating hair, of the Infusoria.
Of the three forms which thus seem to be the first natural variations of the Gastrula—the globular, the flattened, and the lengthened—the first two naturally rest on one extremity of the longitudinal or stomach axis; the other, or mouth extremity, being directed upward. Thus only these two extremities are exposed to diverse conditions, the one being in contact with the ground, the other with water or air. The intermediate surface is affected in but one manner. It being everywhere similarly influenced, its whole development is similar, and a radial or two-axed form results.
In the lengthened forms the longitudinal or intestinal axis naturally becomes horizontal. The two extremities of this axis develop into mouth and vent. But the intermediate portions are also differently conditioned. Vertically a lower face is in contact with the ground, an upper face with water or air. Thus ventral and dorsal surfaces are produced. Transversely, the opposite sides are similarly affected, and develop similarly. Gravitation also tends to produce a shortening of the vertical and a widening of the transverse axis. Thus the three axed animal appears, a lengthened form, with mouth-opening at the anterior-moving extremity, with diverse dorsal and ventral surfaces, with similar lateral surfaces, and with a tendency to become flattened vertically.
In this lengthened, or worm type, appears an animal form more highly conditioned than any possible two-axed form, and capable of far higher development. It is much the best adapted for rapid movement, its long, narrow shape being well calculated to overcome the frictional resistance of water or au'; while it is capable of a flexibility not possible to the compact types of animal form.
Consequently, from the primitive types of animal form we have so far considered, we find two general lines of development. The first of these is a tendency in the compact types to become lengthened in form, to lose their protective armor, and to assume the free-moving condition—their most advanced genera being thus constituted. On the other hand, a retrograding tendency shows itself in certain sections of the lengthened animal type, compact forms appearing. And, significantly, these envelop themselves in shelly or horny armor for protection, become sluggish in motion, and fail to develop the acute sensitiveness and other advanced powers of the naked worms.
Such being the primitive and secondary form-evolving tendencies, the production of organs of motion is in strict accordance therewith. In the radiated polyps the limbs appear as head-organs. These are so ill-adapted to the production of free motion, that the solitary Polyps have not developed into this condition, except in the case of the Medusæ or jelly-fish. And these in no instance seek to swim by aid of their arms, a slow movement being gained by umbrella-like contractions of their radiate body disk.
In the Echinoderm family—one of the reversions from the longitudinal type—the shortening of the intestinal axis has brought it into such close relations with the radiate type that the difference is only clearly distinguishable in its embryo stage of existence. Its intestinal axis has become vertical, and it has gained radiated limbs—not head, but side limbs. By the aid of these the free-moving Echinoderms manage to progress slowly, but they depend more particularly on their armor for protection.
In the Mollusk family another form of reversion from the primitive longitudinal or three-axed form—the conditions of existence necessitate other motive organs. This family of animals, instead of clothing itself in a dermal armor like the Echinoderms, produces a limy covering, a movable house to which it is not anatomically connected, and which principally differs from the stone mansion of the polyp in being movable. Within this house the mollusk preserves his three axed form; having no such strong inducement to yield it as has the Echinoderm. But his contact with exterior nature is but a head and foot contact. He therefore develops head-limbs—tentacular organs—while his slow progress is gained by alternate expansions and contractions of a muscular portion of the ventral surface.
Thus the lower types of animal form are forced, by the necessities of their environment, to evolve certain general anatomical conditions, which, as we shall hereafter see, act as a fatal drag on their subsequent efforts to occupy the higher fields of life.
The worm type has, from the beginning, a marked advantage over them. The creeping forms of this type would naturally tend to develop moving organs at their points of contact with the ground, yielding ventral limbs, extended along the body. Breathing organs might appear on the dorsal surface, in contact with the water; or at the mouth, where inflowing currents would yield the fullest water contact.
In the swimming worms a somewhat different process of limb development would naturally arise. Here, for the freest degree of motion, some form of fin must replace the limb of the creeping worm. There is reason to believe that fins first arose as lateral extensions of the flattened body. This general fin—under the late theory of limb development—in time lost its continuity, and broke up into four separate sections, whence arose the four limbs of the future Vertebrates.
The possession of such longitudinal body-limbs or fins gives much greater rapidity of motion to the worm type than is possible to the head-limbed or radiate body-limbed types. As a consequence of their motive facility they remain naked, rapidity of motion and keenness of sense giving them powers of attack and escape not needed by the tentacled and armored forms.
In fact, the advantage of the longitudinal extension is so patent that we find all the lower types making efforts to attain it, and in this manner reaching their highest limit of progression. This constitutes the next step in the evolution of animal form, and one which presents some exceedingly curious phases.
The phases here referred to are not displayed by the mollusks or the Echinoderms. We shall therefore first speak of their simpler mode of attaining their highest development.
In the mollusk it is attained by a lengthening of the compact body. while the shell becomes internal instead of external. It continues to be useful as a basis of muscular attachment, but no longer as a defensive armor.
The whole development of the mollusks, from the lowest bivalve to the highest univalve form, has tended to the production of head-limbs, and a compact, bag-like body. In their naked state their evolution is limited by this hereditary constitution. Two modes of motion are possessed, the swimming and the creeping. For use in the first there is a fin-like expansion of the body, which enables them 16 move with much rapidity, while backward motion is gained by expulsion of water from between the arm-membranes. But the body continues rigid, and is at a disadvantage as compared with the flexible worm type.
Creeping motion is gained by a development of sucking-disks upon the arms, which serve for a slow dragging of the body, turned head downward, and also as an efficient agent in the capture of game.
This highest mollusk, the cuttle-fish, is utterly unfitted for a land residence despite its acute sense-organs. The ink-bag, which enables it to conceal itself in the water, would be of no use to it on land; its tail-fins or its radiated head-arms could not be changed into efficient organs of land-motion; it would, therefore, be at a great disadvantage as compared with the body-limbed, flexible-framed vertebrates^ Thus the highest development of the mollusk type is unsuited by its defective constitution to a land residence, and can only progress to the limited extent permitted by the restrictions of a water residence.
In the Echinoderms a similar lengthening of the body is gained. Of the free forms, we have the flattened starfish, with the arms sometimes developed at the expense of the body, the body sometimes at the expense of the arms; the globular sea-urchin, with its ambulacral arms; and the lengthened Holothuroid. In this latter is displayed what seems almost an intelligent effort to imitate the worm type. Unlike the other Echinoderms, its intestinal axis becomes horizontal instead of vertical. Thus, like the worms, it attains dorsal and ventral surfaces, exposed to diverse conditions. As a consequence, of its five rows of ambulacral suckers, those on the dorsal surface disappear in the most advanced genera, only the three ventral rows being retained. The distinguishing radiate structure is displayed only by its circle of mouth tentacles, the food-getting organs. It also loses the calcareous outer armor of the lower Echinoderms, replacing it by a flexible, leathery skin.
But, with these several advances toward the worm type, the hereditary disadvantages of the Holothuroid act as impassable restrictions to any great development. The organs of the higher senses are wanting. It is in no way adapted to swimming, its exterior organs being quite unfitted to develop into fins. Nor are the ambulacral suckers suited to any rapid progression. An utter change in character would be necessary to adapt them to a walking or running movement. Thus this line of animal evolution has reached its ultimate at a much lower level than that attained by the Mollusca.
But, by this review of what we may, in a figurative sense, call Nature's failures in animal evolution, we begin to perceive the requisites to success. The retrograde forms, after again developing into the lengthened type, are constitutionally restricted from gaining certain structural advantages which are primitive possessions of other types. These advantages we may classify as body-limbs, adapted to walking or swimming; and an articulated body, capable of a flexibility not possible to the compact, single-sectioned animals. All the other animal types, besides those we have considered, have made an effort to attain this articulated structure, sometimes by a very curious process. The success attained in this effort is closely dependent upon the primitive structure of the articulated animal, which has placed impassable restrictions in the path of some types.
In the polyps and in the articulates the end seems to have been attained by the linking together of a colony of animals, forming a structure, originally compound, which has become simple by a division of functions between the successive sections.
In the Vertebrata alone has it been attained by the articulation of an originally single animal. The vertebrates thus possess special structural advantages denied to the other articulated forms, the compound origin of these latter curiously limiting their powers of evolution.
In this merging of societies into single animals, Nature presents us instances of every step of the process, from those in which individuality remains intact, to those in which it is subordinated to the requirements of the compound animal.
A first step in the process is displayed by the Tunicate mollusks. The Salpa—one form of these shell-less creatures—is a free-moving animal, progressing by the aid of water, which is drawn into one end of its straight intestine and expelled at the other. They exist in two conditions, the single and the compound. In the latter they unite into long chains, not organically connected, but apparently adhering by little suckers.
This primitive combination seems assumed for one advantage only, that of aiding their motion. The animals in the chain contract and expand simultaneously, the whole chain moving like one lengthened animal.
The same end is achieved in a still more curious way in the Pyrosoma, another of the Tunicata. These little creatures so group themselves as to form a hollow tube, open at one end and closed at the other. The minute animals which compose the walls of this tube have one gill-opening extended outward, the other inward. Thus they draw water from outside and discharge it into the interior of the tube. This being closed at one end, the water is necessarily driven from the other, giving to the odd, phosphorescent, living tubes a lengthwise movement through the ocean.
The Ascidians—a family of fixed Tunicates—present societies to some extent organically connected. They are grouped by a common connection of their mantles, or rise successively from a common stem, through which an organic unity is established. Yet their individuality continues; for, if one of the Ascidians has its circulation cut off, by a ligature, from the common stem, it continues to exist independently.
In the fixed polyps the subordination of character resembles that of the Ascidians. It is carried further, however. Thus, in some instances, not only is the common stem fed by the efforts of a series of individual mouths, but there seems to be a sensitive connection. If, for instance, one of the expanded animals of an Alcyonium community be touched, not only does this animal contract, but gradually the remaining animals of the community contract also.
Again, in the Hydrozoa, individual members of the community are specialized as reproductive organs, being fed through the common stem by the feeding individuals. In these cases the merging of individuality has extended much beyond the simple case of the Salpæ, certain members of a society being specialized as organs of a compound animal. These reproductive buds, however, in many cases regain their individuality in a very peculiar manner. They separate from the common stem, and continue to exist as free-swimming animals. But their specialized development has produced material modifications in their form and internal organization. They are no longer fixed polyps, but free Medusæ, retaining only a general resemblance to the polyp type, and swimming by means of contractions of their umbrella-like disk.
By this strange modification of the polyp form, to achieve special purposes, a new free animal form is produced, which sometimes follows its new line of development so as to yield an animal markedly distinct from its unspecialized brothers of the same community. Such is one of the many strange modes in which Nature has sought to produce new forms of animal life.
But the greatest subordination of individuality is shown in the Siphonophoræ, a family of Hydrozoa in which a distinct effort seems to be made to attain the elongated, free-swimming form, through combination. In some of these the evolution of a colony into a single animal is almost complete. A large number of individuals are connected by a common stem; but these individuals are so specialized in function as to be no longer capable of a separate existence. They have lost certain powers, and developed others, so that they are reduced to the condition of special organs of a single animal. Some act as food-catching organs, some as mouths, some as reproductive and nursing members, and, by a strange transformation, some have become bell-like organs, which, by successive contractions, expel the water, and force the whole community through the seas. These swimming bells are not unlike the Medusæ in this particular, but have become far more specialized than the Medusæ, as they possess none of the organs requisite to individual life.
In the Physalia, or Portuguese man-of-war, the connecting body is developed into a floating bladder, moving by force of the winds, and with its variously modified polyps beneath it.
Such are some of the modes adopted by Nature to produce free motion in the lower types of animal life. The animals produced by this social subordination of function are imperfect because the subordination is indefinite. There is not a single organ adapted to each function, but a variable number. And the very means by which propulsion through the water is gained renders this imperfection necessary. For, if a single individual constituted each organ, the animal would become compact, and be moved by a single contracting bell. Its powers of motion would be reduced to those of the Medusæ, and its organization retrograde toward the original compact stage.
This line of progress, with its necessarily imperfect specialization, is evidently incapable of attaining the level of the Echinoderm, much less of the mollusk.
But another line—that of the segmented animals—seems much better adapted to attain a high grade of evolution. Not but that its segments possess anatomical characters as stubborn as those of the Radiates, but that these are less restrictive to a high evolution.
It is, of course, not the usual view to consider the Articulates as the result of an original social organization. The segments, in the higher genera, are so specialized that they now exist but as organic parts of a single animal. And yet, if we consider the lower articulated worms, evidences of such an origin may be discovered.
In these lowest Articulates scarcely any difference is to be traced between the segments. The anterior, from its position, acts as a mouth, but otherwise they are as similar as the individual Salpæ. But the most significant feature is that in many cases each of them possesses the organization of an individual. Each segment still retains its separate nervous ganglion, its separate muscles, its separate limbs, frequently its separate breathing organs, and, in a partial degree, its separate circulation. These are only subordinated to the extent of being joined by connecting links, while the intestine of each becomes continuous as a common intestine.
In fact, this organic individuality is carried, in certain cases, to a yet more significant extent. The organs of special sense the most highly specialized of animal organs—are, in some instances, retained by the separate segments. There are not only existing worms with eyes at each extremity of the body, but others which possess eyes in each separate segment.
Thus we are led not alone to the conception of an original animal which became associated into the Articulates, but even to some idea of the organization of this primitive animal. If we assign to it the organs still possessed by the segments of the Annelides, we find it to have had an intestine separate from the circulation, being thus superior to the polyps. It had also simple nervous and muscular systems, and immature eyes, a chitinous armor, a water-vascular system, and possibly distinct exterior breathing organs and feet. It may, indeed, have been the primitive form from which other animal types besides the Articulates originated—through a diverse process of evolution.
It is not improbable that the Articulate condition was reached, not by a combination of free individuals, but by a continued adherence of longitudinal buds. The increase in number of segments by division is still common in Articulates. The minute fresh-water worm called the Nais, is separated into two sections by a bud which appears in the center of the body. One section develops a head and the other a tail, at the ends adjoining the bud. But the bud itself again and again divides, each division becoming a young Nais, so that finally a chain of worms is formed, all organically connected, and fed by the mouth of the anterior Nais. Eventually they separate, each becoming a free individual.
The question now arises as to how a developing force would act on such an articulated society. The highest results of evolution are reached through concentration of function. Such specialization is opposed to a continuous increase in the number of segments, and must tend to the production of a definite organism, of limited extent. The activity of this organism is increased by its gaining limbs more useful than the bristle-like setæ of the Annelides. Its range of food expands when its fore-limbs are changed into food-getting organs. Its powers of motion increase when the body is compacted, and the number of joints decreased, by a welding of several segments into one.
But whence come such new limbs? A consideration of their character leads us to the idea that they may proceed from a simple continuance of the budding process, acting, in this case, in a lateral direction instead of lengthwise. For the limbs are hollow, jointed segments, covered with chitine like the body-segments. They seem, indeed, to be specialized side-segments which have lost their internal organs through disuse, retaining only their chitinous armor, their muscles, and their intestinal cavity. And the successive joints of the limbs appear to be formed by a continuance of the budding process. One evidence of this is the fact that they may be reproduced by budding when broken off at the joint; and also that lateral budding again takes place at the extremity of the limbs, yielding double tarsi or pincers in the head-limbs.
Such is the character of the articulated animal; and it appears as if this persistent partial individuality of the segments must prevent that complete localization of function which seems necessary to the greatest animal development.
As we ascend to the higher members of the Articulate type, the specialization of function increases, but not sufficiently to obliterate all the individuality of the segments. The tendency in the Arthropods is toward a continuous welding of the segments. Thus, in the Crustaceans we find twenty or twenty-one segments compacted into three body sections. In the higher families these three are reduced to two; and, in the highest crabs, the abdominal section becomes so reduced that all the body functions are performed by a single section.
At the same time the chitinous armor of the segments becomes a continuous cortical armor; and the chain of nerve-ganglia is reduced to a single large ganglion, which supplies nerve-fibers to all the body.
In this manner the Crustacean reaches its most specialized condition, but only by a loss of its longitudinal extension, and a return to the compact, slow-moving, armored type of animal. Thus, its highest evolution has produced an organization antagonistic to any advanced degree of development.
Of the air-breathing Arthropods the Arachnidæ seem to be closely allied to the Crustaceans. They have the same compactness of organization; are not, as a rule, adapted to swift motion; and are inferior to the high Crustaceans from the fact that their tendency is toward development of the abdomen, instead of the head section, as in the crabs.
The insects and Myriapods do not possess the relations to the Crustaceans shown by the spiders and their allies. On the contrary, their larval form seems to indicate a separate line of descent from the worms. Different as insects and Myriapods are in their mature forms, they appear to have had a common origin—the embryo of the Myriapods passing through a stage that resembles the larval stage of insects. They seem, indeed, to have developed from their primitive form in opposite directions, the segments being multiplied in the Myriapods and reduced in the insects. The embryo of the Myriapod has at first but three pairs of legs. At a later period posterior legs bud successively from the new-formed segments. There seems to be no fixed limit to the number of segments, since they continue to increase throughout life. And their individuality is strongly declared, each segment possessing the organs necessary to a separate life, as a nerve ganglion and fibers, breathing organs, muscles, an intestine, and a vascular space. These organs, if redeveloped from their partly aborted condition, might well suffice to sustain life in separate animals.
Even in the highest of the Arthropods—the insects—this hereditary individual organization of the segments continues manifest; these organically independent members of the society stubbornly resist the cession of their primitive functions, only partly yielding to the common needs, and thus retaining a generalization of function which is repressive of any high development.
The animal best suited for progression is one which has all its functions separately concentrated. Its aeration, its circulation, its sensation must have single, localized centers, and its limbs be reduced to the smallest possible number, and separated in duty. These requisites are only fully attained in the human form. They are constitutionally prohibited to the segmented animals.
In the insects the persistent individuality of the segments is shown partly in their six legs, each pair attached to a segment; but more particularly in their generalized nervous and respiratory systems. To a great extent each segment preserves its nerve-ganglion. So, to a similar extent, each segment does its own breathing, the whole body becoming one generalized lung. The blood circulation, which is only partly confined to specialized blood-vessels, is accompanied by a general air circulation. There is nothing resembling the localized relations of these circulating systems as seen in the Vertebrates.
Such are the constitutional limitations to development in the Articulates, probably resulting from their social origin. The effort to overcome these limitations in the crabs has resulted in organic conditions opposed to a high development. How is it in the insects? In them the segments are so welded as to form three distinct body sections. In the higher insects the individuality of the segments is so reduced that the nerve-ganglia of the thoracic segments are concentrated into one ganglion, while a single head-ganglion, of large size, officiates as a brain. Their muscular force is greater in proportion than that of Vertebrates, so that they are strong, active, and enduring in bodily vigor. What natural influence is it that has restricted their development?
This may not be difficult to discover. We have seen that the too great compacting of the articulated body, as in the crabs and spiders, has proved a hindrance to development. The three sections of the insect body, each devoted to a single class of duties, has given them variety of motion, and more diversified food-getting functions. But it has otherwise worked injuriously. Rapid variations in movement require that these sections should be united by flexible joints. But these joints are articulations of an external skeleton, and can only be produced by a deep depression of this cortical armor into the regions dividing the sections. Thus the continuity of the body is almost broken at these joints. A similar relation exists between the joints of the limbs.
It seems evident from these considerations that the insect is not constituted to attain a large size. Conditions which are suitable to a small body might prove utterly unfitted to the requirements of a larger organism. Let us imagine an insect of the size of an ox; walking on its six many-jointed, hollow legs; its body composed of three almost separate sections; breathing through air-holes in its sides, its whole body but an air-tank, or lung. Even if such a growth were possible, it would obviously be at a disadvantage as compared with the Vertebrates. Whatever size it might have attained in the absence of the Vertebrata, it certainly would be unfitted to compete with these better adapted animals for the possession of the higher fields of life.
Insects thus seem restricted to a small form, contracted localities, and a narrow range of conditions. The ants, their highest form, is one of the most limited in range. It is highest in having best succeeded in adapting nature to its needs, and, in so doing, having developed a superior mentality; but it can not advance beyond the needs of its contracted environment.
In the various animal types we have considered, Nature seems to have exhausted all side-issues in her efforts to produce an animal form adapted to a high grade of evolution. The persistent individuality of the segments hinders a colony from merging into an individual capable of an advanced phase of development.
Another and simpler method remains to be considered; the direct elongation of a single individual—not the elongation of a previously organized animal, but a primary derivative, unshackled by anatomical difficulties.
For high progress in this individual, certain conditions are necessary. It must not seek safety in a coat of armor. It must save itself from danger by powers of flight and acuteness of sense. In a water residence the most effective flight is gained by swimming. Therefore our worm must become a swimming animal, its sides being flattened into swimming-flaps.
In such an individual the functions would be specialized, as they were in the individuals which became welded into the Articulate. Indeed, the Vertebrate and the Articulate may have had a single origin in this primitive organic form.
The swimming worm we are considering has no hindrances to specialization of function. His side-flaps may be reduced to local fins. His intestinal tube—not acting as a series of sectional stomachs—may become localized in function, its anterior portion acting as a lung, its posterior portion as a stomach. There are several advantages in this. The circulation is no longer exposed to danger by a perilous thinning of the outer surface into branchiæ. The food being drawn in by water-currents, oxygen is extracted from the water by the anterior intestine, and aliment by the posterior. Similarly, the nerve and muscle systems are single and specialized, and the sense organs local.
But another condition is necessary to the full adaptation of this swimming animal to its situation. Its swift motion necessitates muscular vigor, and requires some firm point of attachment for the muscles. In all the armored types the shell, or outer coating, serves for this purpose. In the naked worm there is no such exterior point of attachment, and an interior one must be developed.
Thus we have arrived at the necessity of an interior skeleton, an organic condition not displayed in all the vast field of life we have so far reviewed, except imperfectly, in the Cephalopod mollusks.
This is, at first, attained by the indurating of a dorsal layer of flesh into a cartilaginous cord, which stiffens the body while leaving it flexible, and furnishes points for muscular attachment.
Only a few instances remain of this earlier condition of the Vertebrate type. All others have disappeared. In the embryo of a Tunicate animal, the Ascidia, both the cartilaginous cord and the intestinal branchiæ appear. In its mature form it becomes a fixed animal, and loses this cord. But in the Appendicularia, a related animal, the cord is retained throughout life. It is also retained, in a more complete development, in the Lancelet Amphioxus, a creature having strong vertebrate affinities in its extended nerve-cord and its general functional system.
But one further step is required to produce the typical Vertebrate from such an original. This is the formation of joints in the cartilaginous cord, when it has become so firm as to resist the lateral movements of the body, or is hardened by deposition of carbonate of lime.
There is nothing in this like the welding of segments in the Articulate. The vertebrate joints display none of the separate vital animal functions. They yield every indication of being produced in the mode indicated, by the stress of an undulating body. The joints in the subsequent limbs resemble them in character, and seem to be formed in the same manner. The Lancelet is not jointed; it is a single individual. But the worm from which the Articulate arises is jointed, and each joint is possessed of all the vital functions.
Thus it appears that the Vertebrate animal starts in the race of life with advantages possessed by none of its competitors. It remains to trace the steps of its development.