Popular Science Monthly/Volume 18/April 1881/Man and the Vertebrate Series
By CHARLES MORRIS.
MAN stands as a connecting link between two worlds—the world of matter and that of mind. He forms the apex of the development of matter, the loftiest effort of evolution in substance. Mind, it is true, has its foundation in the regions of life below him, but all its superstructure—the towering arches and lofty pinnacles of the ideal—rests upon the human intellect. Man thus forms the gateway which Nature has placed between her two vast kingdoms of substance and thought, and in the human brain these two realms meet and merge, energy flowering into intellect, substance into soul.
But is the human form the true culmination of the development of matter? Has Nature really reached in man her acme in this direction? A deductive philosopher would perhaps answer this question in the affirmative, on the theory that Nature would not stop short of the most completely developed physical form, as the starting-point of mental evolution. He might claim that a perfect soul could only arise in a perfect body, and that, as Nature is striving toward perfection, she must lay all her foundations at the highest possible point.
But inductive science starts with no theories. It builds its theories out of facts, not its facts out of theories, and follows Nature upward from her roots, not downward from her branches. What, then, do the facts of Nature say as to the question of animal evolution? Is man truly the paragon of animals?
Unfortunately, this question opens before us a field of investigation too broad for consideration in a single article. We have already seen that Nature has exposed organic forms to an almost unlimited variety of conditions, during the long geologic ages, and has probably tried every line of development of which organic life is susceptible. By a close review of the various animal types, their advantages and deficiencies have been traced, and we think it has been shown that the vertebrate type is the one suited to the highest evolution, and the one toward which all the lower forms tend in their highest representatives.
For a complete review of organic form development the plant types should also be considered, but we must confine ourselves in this article to a consideration of the vertebrate type of animals alone.
And first, What are the causes, what the modes, what the laws, of evolution? What features in one animal constitute superiority to another animal? These questions we shall but briefly answer. There are certain requirements absolutely necessary to the continuance of animal life. One of these is a sufficient supply of food; a second, a sufficiency of oxygen; a third, proper nutritive and excretory organs; a fourth, proper reproductive powers.
The first and fourth are of most importance in this connection, for they are in constant conflict with each other. The quantity of available food is far more limited than are the possibilities of animal increase. Necessarily, then, the latter is restricted by the former. A crowding out process ensues, and only those best adapted to obtain food survive.
But an equally necessary result is an adaptation to new sources of food, one of whose earliest consequences is the production of carnivorous animals. Thus the crowding-out process becomes, in part, an eating-out process. The animals thus exposed to destruction would necessarily be at a marked disadvantage in the race for life, were not some protection provided them. For safety they need weapons of defense or means of escape. It all comes to this, then, that the strong. est, swiftest, best-armed, and most alert animals will survive, these qualities enabling herbivora to escape their foes, carnivora to overcome their prey.
But there are two ways in which this survival may be attained: one by adaptation to a few simple conditions; the other by adaptation to many and complex conditions. The wider the scope of adaptation in an animal, the greater is its functional complexity, and the higher its organic position, as compared with the more simplified tribes.
Still another requisite of the utmost importance is the principle of division of labor. No organ can do two distinct things equally well. If forced to perform two or more labors, there must be a degree of imperfection in its work, or its ability in each direction must be greatly limited. Therefore evolution is in the direction of separation of labor, each organ tending to become confined to one kind of work, to which alone it becomes adapted, but in which it produces better and wider results.
Seeking, then, for the features likely to distinguish the most highly developed animal, we may safely say that they will appear in that animal exposed to the most complex conditions, adapted to the greatest variety of food, possessed of the most fully specialized organs, capable of using its innate forces to the best advantage, and commencing its individual existence with the best start in life.
With this preliminary we may proceed to a closer investigation. Vertebrate animals occupy every kingdom of nature—the sea, the land, and the air—but not under equally advantageous conditions. The inhabitants of the sea, for instance, are exposed to decided disadvantages, and lack certain important incentives to development. Their vital activity is necessarily much below that of land-animals, from the limited quantity of oxygen obtainable by water-breathers as compared with air-breathers. Their sensory acuteness, also, is less developed. Light comes to them dimmed, sound comes to them dulled, the water which surrounds their food must blunt the senses of taste and smell, and there is little besides water contact to develop touch.
Also the conditions surrounding attack and escape are here greatly simplified. The water presents no lurking-places, no ambush, except to the inactive dwellers upon the sea-bottom. And, by relieving its inhabitants from the effects of gravitation, it renders rapid motion easy, with slight muscular exertion. Thus the easiest, most natural, and most effective means of assault and defense is by swift powers of swimming.
The same principle holds good in the case of the dwellers of the air. There is here no lying in wait for prey nor hiding from assault. Flight is the most effective and most ready means of attack and escape, and the only one available in the fields of open air. Thus purely water and purely air animals fail to develop a variety of resources in this particular.
Moreover, as fish are at a disadvantage from their imperfect oxidation, so are birds at a disadvantage from the effects of gravitation. The weight of fishes is almost or entirely supported; that of land animals partly supported; that of birds almost unsupported. Gravitation in them, then, must be mainly overcome by muscular exertion. Thus a large proportion of their life-force is exhausted by the effort to sustain themselves against the constant downward pull of gravity. This places them at a disadvantage with animals capable of using their forces for more varied purposes.
Indeed, we find, both in fishes and in birds, a tendency to avail themselves to some extent of the advantages which the land-surface gives. And those that most display this tendency comprise the species of most varied resources, and with the greatest degree of functional adaptation. Many species of birds, in fact, have found safety so much more assured on the land, that they have lost, first their instinct, and then, in some cases, their power of flight, and have become, virtually, surface-animals.
We may descend to the invertebrate world for one marked instance of this. The ants are acknowledged to be the highest of all insects in functional development. Their eyes, for instance, are more simplified than those of other insects; and, if we class mental attributes as nerve-functions, their claim is indisputable. Yet they have forsaken the air and taken to the earth in preference, the females casting off their wings as if in scorn, after a temporary use of them.
Of all the fields of life for the varied display and functional separation of the animal forces, we are thus brought to the land-surface as obviously the best. Here an abundant supply of oxygen assures vital activity; the firm ground largely supports the weight, and releases the muscular powers for employment in other directions; the abundance and variety of vegetable food sustains vast numbers of animals; the great diversity of conditions causes wide specific variation; while the simplification of modes of assault and defense in water and air is replaced here by the most varied motions, such as creeping, running, leaping, climbing, etc., by a diversity of ambushes and hiding-places, and by the necessary adaptation of carnivora to numerous modes of attack, and of herbivora to as numerous modes of escape.
In fact, the specific variations in birds spring from their relations to the surface. Here, as a rule, they seek their food. Here their mental powers arise. Here they develop other functions than flight, other organs than wing-muscles. And it may be that the variations in the fish type spring principally, in like manner, from their relations to the sea-bottom, and the adaptations of carnivorous fish to the powers of escape of forms thus varied.
Thus in the process of animal evolution we reach the dwellers upon the land as the most developed, and the best situated for further development. Those land-animals that retreat to the other fields of life retrograde in consequence. The whales have gone back in their functional development until, in many points, they are affiliated with the fish. The bats have sunk toward the bird-level. So the lowest in function of land-animals are those which retain close affinities with the air and water life of their ancestors, or have but lately migrated to the land. The wingless birds are low in mind and in animal function. The amphibia rest at the lowest level of land-animals. The reptiles are but a step above them.
In the latter class, it is true, there are many which have long broken loose from all connection with a water habitation. Most notable among these are the serpents. But the latter, from their habit of seeking safety in concealment alone, have retrograded functionally, their limbs disappearing, and their bodies being extended prone upon the earth. This location, while an undoubted advantage as affording concealment, is a disadvantage in development. The prone condition of the serpent has caused its limbs to disappear, as useless. Its ribs have taken the place of limbs, and the body has extended in length sufficiently to increase the number of these imperfect limbs, and thus render them more available for motion. Its sensory organs have necessarily become less acute from the disadvantages of position. Gravitation is largely overcome, by the full support of the body upon the earth. But it is replaced by a friction which is equally disadvantageous.
Thus, if we would seek the type of land-animal most likely to develop functionally and mentally, we must look in an opposite direction to that taken by the serpent. The protection that is gained by concealment is a retrograde form. The animals that advance most rapidly are those that are least protected, either by powers of concealment, of flight, or of muscular vigor.
For what does development mean, in its true sense, but extended experience of nature? The greater and more varied the experiences attained by any animal, the best suited it is to cope with nature. And if this variety of experiences extend through the life of a species, it becomes adapted to escape from more varied descriptions of peril and to obtain food in greater variety and quantity.
Thus, in seeking the form of animal most fairly planted in the path of true development, we must look for one capable of attaining to wide experience of nature, and adapted to evade perilous or to take advantage of beneficial conditions of the most diverse character.
These considerations lead us to a conception of the description of animal most likely to appear as a development from the low amphibian and reptilian forms. It must have sensory organs acutely adapted to all of Nature's most active forces, as light, heat, sound, and contact in its several kinds. It must be elevated above the surface sufficiently to give it the widest range of vision and hearing, these most important of the perceptive powers being specially active at the highest elevation of the body. It must rest upon the earth in such a way as to reduce friction to a minimum, so far as is consistent with proper support; and its powers of flight, of concealment, and of physical strength, must be sufficiently reduced to force it to seek other sources of safety, to adapt its organs to more varied functions, and to develop new features of mental activity.
What will be the form and the conditions of exposure to and defense from danger, of this most highly developed animal, adapted to gain the widest experiences, and to the greatest organic division of labor? It must, of necessity, display in its evolution every intermediate gradation of form upward, from that of the lowest vertebrate. Its form must be founded upon that natural to the fish.
Now, the fish naturally and inevitably assumes the horizontal posture from the requirements of its mode of life. It has developed fins as organs of movements. The fin is, in fact, as closely adapted to the fish-form as the oar is to the boat-form. The vertebral fins are reduced in number to four. There is a decided advantage in this reduction, in the saving of muscular exertion needed to move the fin, and also of the weight of extra muscles and fins. But a smaller number than four would be a disadvantage in the varied movements requisite to safety.
The fish-body is necessarily narrow and long, so as best to avoid friction. For its most effective movement it must be properly balanced, its two sides being alike in form and equal in weight. This produces bilateral symmetry of form, and perhaps also of organs, equal weight being most easily and completely attained by a reproduction of organs on the opposite sides of the body, but flexibility and full control of such a long, narrow body could not be gained except by a separate power of motion at each extremity and at each side. Therefore adaptation tends to produce in it four fins, and four only—an anterior and a posterior one on each side.
The fish-form governs that of the amphibian and of the reptile. The fins become limbs, which are variously modified. In some amphibians, as in the frog, the limbs become different in functions, the hind-limbs being adapted to a leaping motion, the fore-limbs to support, A differentiation of another kind takes place, eventually, in the reptile stock, the fore-limbs becoming organs of aerial support. They become wings, and we obtain the bird type, with fore-limbs adapted to aërial, hind-limbs to solid support.
These, however, are but aberrations from the general path of development, which is toward a continuance of the horizontal position of the fish-body: the fins become four similar supporting limbs.
Adaptation from this point takes one general direction, that of the reduction of weight to the lowest point consistent with the proper exercise of the nutritive functions, and the requisite strength of bone and vigor of muscle. The general shape attained by the body is governed by several mutually assisting or opposing functions. The requirement of speed needs that it shall be narrow and long, so as best to avoid the resistance of the air. But too great elongation would be a disadvantage, since the mid regions of the body would remain without support by the limbs, and could only sustain themselves by muscles acting to oppose gravity. These muscles would add to the weight, and would form additional consumers of force. The best form of the body, then, is one sufficiently narrowed to partly avoid aërial resistance, but not so elongated as to diminish its proper support upon the limbs. The chief aberrations from these requirements are those of the prone serpents, and, among mammals, of the weasel family, whose mode of life requires a very narrow body, so that digestive space can only be gained by its elongation. We may return here to the serpents to say that their elongation is probably due in great part to the same cause. Their mode of concealment and of motion requires great narrowness of body, so that space for the nutritive functions can only be gained by elongation. The requirement of narrowness is, in fact, so great that there is not sufficient room for the larger organs to be bilaterally reproduced, therefore respiration is confined to a single lung, the other being atrophied.
But, with a comparatively few exceptions, caused by highly specialized life-habits, the land vertebrates possess the general form requisite for the fullest adaptation to these three conditions—that of reduced resistance to motion, proper support, and the necessary room for the exercise of the nutritive functions.
The weight of the body is reduced to the lowest possible limit consistent with its general size and the exercise of its functions. The limbs, for instance, take no part in the nutritive function, and are reduced in size to the weight of bone requisite for support and of muscle necessary to move the body. The head is just large enough to hold the brain, the organs of special sense, and the mouth with its organs. The neck is narrowed to a mere covering to the œsophagus, the vertebræ, and the muscles of head-support. Its necessary flexibility is one reason for this, but the cutting off of extra weight wherever it can be spared is another.
Thus, as a general result of these evolutionary principles, we obtain a body of horizontal shape, supported upon four limbs, and reduced in weight and in elongation so as to give it the best possible control of its motions, and the greatest agility consistent with its necessary gravity.
The necessity of quick knowledge of danger and of ready escape from it have had other equally important results. The organs of sight and hearing has been placed at an elevated point, so as best to perceive distant danger, while the limbs of the herbivora have also elongated so as to lift the body to a wider outlook. This elongation of the limbs is also an important adjunct to rapid flight, so that there are two forces at work upon its development.
In the carnivora, on the contrary, the crouching, springing habits have tended to shorten the limbs, and to adapt them to vigorous leaps instead of to rapid running movements; to a life in ambush instead of to a life in action.
But the particular features of the body of the land vertebrate are as closely a result of natural requisites as are its general features. It forms, in a large sense, a digesting and assimilating machine, the force derived from assimilated food being largely applied to the muscular and nervous functions needful to obtain new food, or to avoid danger. Another portion of this force is applied to excretory and reproductive functions. That is all. There is none of the human employment of force in abstract mental conception. All mentality in undomesticated animals is employed in the art of self-preservation.
The organs arise as direct consequences of these necessities. We may view them as partly governed in position and character by their descent from the fish type; yet such a controlling agency hardly appears, so exactly are the various organs adapted to the life-needs of land-animals.
The digestive function is alike in all animals, and its evolution has ever been in one line, namely, a division of labor so as to secure more perfected results. In the mammalia this separation seems complete. From the masticating teeth to the salivary solvents, the stomach and intestinal digestions, and the intestinal absorption, every distinct portion of the process has gained its separate organ, adapted only to that one duty. The ensuing circulation is similarly specialized, being divided into the blood and the lymphatic circulations, the latter taking up normally the products of digestion and the nutritive products of the waste of the tissues; the former applying these to the nutrition of the tissues.
A third organic requisite is that of oxidation. This has been variously performed in animals. In primitive life it acts through the outer skin. In higher forms a portion of this skin becomes adapted to that purpose, as exterior branchiæ. In the Ascidiæ it is performed by the anterior portion of the intestine. In all vertebrates it is an intestinal function, the forward portion of the intestinal tube becoming the gill of the fish. A sac-like ingrowth from the intestine—the swimming bladder of the fish—becomes the lung of the air-breather. It eventually separates into two portions, and adapts itself more perfectly to a function which it may have partly performed in the fish.
Thus the respiratory function has gradually moved inward to a position of safety, which is only fully attained in the air-breathers. Its position in the anterior, instead of in the mid or the posterior portion of the intestine, has another reason besides that of inheritance. This is its connection with the pulmonary circulation.
In considering the division of labor in the circulation we did not speak of its separation into two distinct portions, the nutritive and the pulmonary portion. This division of function is partly attained in fishes and reptiles, fully only in birds and mammals. And the heart, which serves as a force-pump to drive the blood through the body, becomes here a double pump, one half driving the blood through the arteries, the other half driving it to the lungs, there to become aerated.
The former circulation needs to penetrate the whole body. The latter can be fully performed with little extension of its blood-vessels. And in the labor-saving principle of organs, which hinders any excess of material or of effort, the tendency is to a curtailing of the length of this pulmonary circulation. Natural selection, therefore, acts to bring the heart and the lungs into close contiguity.
But there is another reason for the position of the heart in the anterior portion of the body. Its action as a force-pump renders it an advantage that it should be placed nearest the point where it has most work to perform. Now, the brain receives a much larger percentage of the blood than any similar portion of the body. More force, then, must be exerted by the heart in the direction of the head. If it be so placed that its labor in every direction may be equalized, it should occupy an anterior position.
In quadrupeds this need becomes still stronger, for the blood going to the head has to overcome gravity, that going to the body and limbs is largely aided by gravity. This need is, of course, strongest in man, in whom the requirements of the brain are the greatest, and in whom the upward flow is directly against gravity, the downward flow directly favored by gravity. But in all the higher animals the heart, and therefore the lungs, necessarily occupy an anterior position.
Protection of the body is, to a certain extent, secured by its being inclosed in a bony case, the ribs. It may be thought strange that the advantage to be derived from this defensive armor is not extended to the whole body. It very probably would be, were there not some active influence opposing it. This influence may be, the necessity of expansion of the ventral surface. Expansion and contraction in the chest are regular and rhythmical, and are secured by the jointed connection of the ribs with the breastbone. Expansion in the abdomen is irregular and at times excessive. Incasement in an inflexible rib-case would, therefore, prove highly disadvantageous.
Yet no flexible condition can well arise in response to expansions appearing irregularly and often at long intervals. As the ribs, therefore, could not gain, by selective adaptation, the proper motive relations to these occasional expansions, and as inflexible ribs would be a disadvantage, abdominal ribs have failed to appear.
The general characteristics of the body being thus necessarily as we find them, and the position, length, joints, and action of the limbs being inevitable results of their purpose, as the organs of animal motion, it remains to trace the origin of the head with its organs.
The head is simply the carrier of the organs of the special senses. The brain, so far as its secondary action is concerned, might very well be situated in any other portion of the body, but we think it can be shown that the location of the eyes governs that of the brain, and that the head with all its organs is an adjunct of the eyes.
These important organs necessarily occupy the most elevated part of the body. The outlook is better from this location, and the safety of the animal is thus more assured. An anterior location is also highly desirable, as otherwise the forward-moving animal would be in constant peril from obstacles in its path, and would be unable to perceive the prey it was pursuing. Also, as the delicate character and exposure of the eyes tend to limit their number, the portion of the body bearing them must be sufficiently flexible to permit vision in all directions.
All these requirements tend to the production of an anterior, elevated organ mounted on a neck of flexible movement, and as long as is consistent with easy support of the weight of the head. The ears, also, are best situated upon this head organ, and in such a position as to adapt them to catch sound from all directions.
And this position of the eyes and ears necessarily requires the brain, for its fullest effectiveness, to be likewise situated in the head. For the greatest safety follows the quickest warning of danger. But, as is well known, the nerves are slow in their conveyance of sensations. The animal, therefore, whose eyes and ears are nearest the brain becomes most quickly conscious of peril, and will outlive, as a rule, the one whose sensations have a longer distance to travel.
It can be easily shown that the mouth and the organs of taste and smell are as necessarily confined to the head, and that their special location is closely governed by that of the eyes.
For animals have safety of two kinds to provide for, safety from foes and safety from food; external and internal perils. Poisonous or unwholesome food is quite as necessary to be avoided as dangerous foes. The animal that is best protected against this peril has the best chance to survive. So important, indeed, is this necessity, that not alone the sight and the sense of touch and of temperature are on guard against injury from such a source, but two organs of sense, smell and taste, seem specially provided for this purpose alone.
The needful action of the eyes, as food-inspectors for the body, fixes the position of the mouth at such a distance from them that they will unavoidably perceive the food. Animals are not likely to voluntarily examine their food before eating it. They must be forced to do so involuntarily. Therefore, the eyes naturally command the entrance to the mouth, at the best distance for the most acute vision.
The relative positions of eyes and mouth being thus fixed, those of the smell and taste organs follow. Odorous emanations arise from the food significant of its character. The animal becoming most conscious of them has the best chance to escape danger. These odors naturally rise upward. The nose, as the organ of smell, is therefore best situated just above the mouth, and overhanging it, the performance of its function, as the organ of respiration, causing the respired air to sweep the lips and draw in the odors arising from the food.
The organ of taste, on the contrary, is best situated on the rear portion of the tongue, since the food must be masticated by the teeth, and partly dissolved by the saliva, ere it is in condition to excite the sense of taste. It must not be placed so far back, however, as to hinder the rejection of food warned against by the nerves of taste.
The head of the animal, then, seems necessarily to be just as we find it, the seat of the special senses and of the brain; while the relative location of these sense-organs and of the mouth, the position and elevation of the head, and the narrowness and flexibility of the neck, appear to be all necessary adaptations for the most complete protection of the whole organism.
The mammalian body, then, so far as we have yet seen, appears the best adapted of animal forms to gain extended and varied experiences of nature, to become exposed to diversified perils, and to evolve the most complete division of function. This body, once attained, is closely adhered to. While displaying thousands of minor variations, adapting it to special circumstances, it retains unvaryingly all the general characteristics mentioned. This close adherence seems to show that its form and organic functions are the best of all adaptations attained by the animal world up to that height.
The mammalia have another advantage which we will but glance at here. The young of the mammal has a better start in life than that of any other type of animal. In this direction, also, there has been a constant development in the process of evolution, the young of lower animals being born in an embryo state, and needing to consume force in passing through various degrees of metamorphosis. The young of the mammal is fed by the mother through all the embryological stages, this being most fully performed in the highest mammals, so that their young commence an independent life at a stage to reach which the young of lower animals consume a considerable portion of their vital energy.
Yet, with all these advantages, the mammalian quadruped has not attained the highest position in animal development. It will be very easy to point out several defects in its organization, which detract from its powers as a living body. In the first place it only imperfectly overcomes gravity. Shortened as the body is, a considerable part of its weight is not supported by the limbs, and needs muscular support. This increases weight and uses up force. The head also is not directly supported by the limbs and needs powerful muscular support in the neck. The need of using the teeth as food-grasping instruments requires a forward extension of the head, instead of a vertical position over the fore-limbs.
In division of labor it is likewise defective. Thus its limbs have a double duty to perform—they are used both as organs of motion and as weapons. The herbivora use their hind-limbs for defense, the carnivora use all the limbs as offensive organs. The same may be said of the teeth. The carnivora use them both as weapons of attack and as organs of mastication. The herbivora are frequently supplied with heavy horns as defensive weapons, thus adding to the weight of the head.
Obviously, then, there are a variety of requirements to be supplied ere the most completely developed animal form can be attained. In what direction shall this further development proceed? How shall the above-named disadvantages be obviated? The first steps toward it are made when mammalian animals begin to differentiate the functions of their fore and hind limbs. We have noticed some forms of this differentiation. A more significant form displays itself in those animals which live in trees. Many of these, it is true, use all the limbs alike in climbing, and remain true quadrupeds upon the land-surface; others, as the monkeys, use the fore-limbs as grasping, the hind-limbs as supporting organs, and thus begin to separate them in function.
This separation proceeds with extreme slowness. It is only imperfectly attained in all existing monkeys and apes. For its complete attainment the differentiation must proceed to that degree that the fore-limbs shall become useful only as grasping, the hind-limbs only as supporting organs. Necessarily, then, the pelvic joint must gradually change its direction from a right, angle to a straight line. The complete result, in fact, is only attained when the hind-limbs and the body form a vertical line, the function of support being performed entirely by these limbs, while the fore-limbs are freed for other functions, and so changed as to be specially adapted for grasping.
This desideratum is attained in man, and in man only of all the animal kingdom.
In the human form, then, we find all the advantages possessed by the mammalia as a class, together with certain important features of development not possessed by any other mammalian animal. Perhaps the most important of these is the fact that in man gravitation is overcome with a less expenditure of muscular force than in any other land animal. The whole weight of the body stands vertically above the organs of support. The muscles which in other animals act as ropes and levers of support are only called upon in man to preserve his vertical position. Evidently much less force is needed to preserve vertical equilibrium than to support horizontal weights.
The head, also, which needs muscular support in quadrupeds, in man presses directly downward upon the common center of gravity. And significantly the complete development of the brain tends to perfect this vertical position, as it yields a rounded and vertically poised Lead. The head in man has but one set of duties to perform, sensory and masticating labors. The hands bring food to it, instead of its having to seek food; therefore it has no need of the horizontal position and movements found in quadrupeds. Finally, that there shall be no weight needing muscular support, the fore-limbs hang vertically downward, being sustained by bones and tendons instead of muscles.
Support on the hind-limbs releases the fore-limbs to act as the defensive and offensive organs. For their most complete adaptation to this function the position of the shoulder-joint (like that of the pelvic joint) is changed, and the arms become lateral instead of ventral limbs. Finally, the teeth are released from duty as weapons, and are confined to their proper duty as masticating organs.
Thus only in man does the organic division of labor become complete, every function having a separate organ adapted to it alone. And the stores of force are husbanded to a degree not found in any other land-animal, the weight of the body being supported by bones instead of muscles, by adjusting instead of lifting energies.
And in regard to reproduction man surpasses all other animals, except the highest apes, in division of labor and localization of function. The young of the human race thus commence life with the best possible preparation, their vital activity being husbanded so that they enter upon their individual life-work at perhaps a higher starting-point than any other animal.
In organic differentiation, then, man seems to have readied the highest possible point. Also, in avoidance of the constant forces of gravitation and friction, he has almost achieved perfection. And he starts life with the least expenditure of force in embryological development. In all these respects he seems to have attained the utmost height of organic development.
In respect to his adaptation to the other forces of nature—his powers of sensory perception—he is also in advance of all other animals. Not only is the division of labor of the animal organs within him—the nervous, muscular, and osseous systems—complete, but his exterior sensibility to the impress of force is the most delicate of that of all animals. This is perhaps not the case in the organs of special sense, though the position of the human nose, with its nostrils directly overhanging the mouth, seems a superior adaptation to its duty in the perception of odors. But in regard to the sense of touch, not only has he a superior provision in the tactile organs of the fingers, but the naked and soft condition of the skin renders it susceptible to contact in a degree not possessed by any other animals.
Other animals, in fact, are either covered with a dense coat of hair or feathers for protection from cold, or with a thick leathery or bony skin as armor against danger. In the development of man alone has Nature pursued her most elevated path, increasing his susceptibility to exterior influence, his power of gaining sensible experience of nature, to the utmost possible degree. This is probably the true explanation of the naked condition of the human skin. His mode of life has rendered the fullest perception of nature desirable, and adaptation has consequently taken this direction, removing from his exterior surface everything opposing the utmost sensibility, and, for the same reason, hindering any undue thickening or induration of the outer skin.
Such is man—the extreme upward limit of physical progress in organic nature—the one last step forward which living beings have taken after their long permanence in the quadrupedal stage. And beyond his form no physical progress seems possible, for he fulfills what we conceive to be Nature's design, viz., to husband force by the fullest avoidance of gravity and friction, to decrease weight to the lowest available point consistent with the size and strength necessary to best adapt him to surrounding conditions, and to produce the utmost susceptibility to impression of natural force—to attain a form, in fact, having the greatest excess of available energy, and best adapted to gain experiences of the conditions of nature.
But these very advantages in the human form produce certain unavoidable disadvantages. Man's adaptations to the conditions of nature are necessarily limited. Thus it might be supposed that a perfect animal would be adapted to traverse the water and the air, as well as the land. But such an adaptation would require extra organs, extra weight, and extra consumption of force in their support. Man, on the contrary, has become adapted to the highest field of life, and escaped an adaptation to inferior fields which would prove a disadvantage in the struggle for existence with land-animals.
His extreme sensitiveness to exterior influences gained by his naked skin is, of course, a sensitiveness to temperature as well as to touch. He is thus limited organically to tropical regions, and to some extent to a life in the shade—to a forest residence. In fact, he seems more limited in locality and in powers of resistance than most other animals. His unclad skin renders him acutely sensitive to extremes of heat and cold. He has no cortical defense against the attack of his animal foes. His limbs have become adapted to grasping and to support, but have lost their character as offensive weapons. Finally, his adaptation to an arboreal residence has become imperfect. He can not climb like a monkey, run like a deer, swim like an otter, mine like a mole, or crouch like a cat.
Physically, then, man is one of the most poorly protected of animals, seemingly a form not likely to survive in competition with his swift-ruining, flying, and climbing neighbors, and with his carnivorous foes, armed with tearing claws and rending teeth.
Yet in other respects he has decided advantages. One of these is a feature in which very few animals rival him, a differentiation in his adaptations to nutriment, enabling him to masticate, digest, and assimilate both vegetable and animal food. This is a decided advantage. Man is at once herbivorous and carnivorous, his field of possible food thus being doubled, and his consequent variety of adaptation to nature being likewise doubled. There is no other animal adapted to this double diet to the same degree as man. By a rather unpleasant resemblance, the hog most nearly approaches him in this respect. Yet the hog is principally a vegetable feeder, and only occasionally varies his diet to animal flesh.
A second advantage is the economy of muscular force gained by the vertical attitude. The force thus saved might have been employed in the production of an extreme agility, enabling man to escape danger by speed and alertness. It has fortunately been applied in another direction, that of the production of mental acuteness. From the time that man first employed the grasping power of his hands to seize stick or stone for defense against his foes, a process was begun which is yet far from completion. It was, in its full results, the process of mental evolution. But, for our present purpose we must give it a more narrow significance.
In all probability man, physically, is not now what he was originally. The use of the stick or stone in defense set in motion a new process of adaptation which has tended toward a physical weakness, at least in regard to conflict with wild animals.
It was, in fact, the first step in the enhancement of animal force by the employment of the vast stores of inorganic force existing throughout nature. It was the earliest inventive action, the bringing of outer nature to human aid, which has since produced such wonderful results. Muscular vigor and acuteness of sense have probably decreased as they have been thus partly replaced. For man has gained new muscles, in his use of the forms and forces of the inorganic world, and has commenced a new process of adaptation, which has already enabled him to extend his kingdom from the tropic to the frigid zone, and which promises to make him to some extent master of the fields of water and air. And his mental experience of wider and wider conditions of nature has produced a new form of physical adaptation—that signified in clothing and habitation.
But this opens a new subject, too wide to be considered here. We can only end as we began, with the assertion that the human form occupies the apex of possible organic development, and forms the true foundation for that higher mental evolution which is still growing, branching, and flowering upward.