Popular Science Monthly/Volume 14/November 1878/The Differences Between Animals and Plants
THE original investigator of Nature soon learns by constant experience that descriptions or even drawings, however correct, do not exactly represent the objects themselves, but are imperfect and ideal abstractions. This is true, to a greater or less extent, of every drawing of the simplest organ or tissue, and of every description of a species or genus of animals or plants; but it is especially and most emphatically true of all attempts at definitions of the larger and more comprehensive groups of organisms.
A definition of such a group as an order or class of animals, attempting as it does to state in a few words the characteristics which are common to all the forms included, is necessarily abstract, and may not, in fact cannot, be exactly embodied in any one individual of the whole group. Then, too, certain characteristics which are exhibited by only one or two aberrant forms, and are accordingly not characteristic of the group as a whole, may be omitted from the definition, although they furnish the clew to the relationship with allied groups, and are therefore of the utmost importance. An illustration which is not drawn from the organic world may make this more evident. The fact that printed books have followed and are a perfected form of the parchment manuscripts of the middle ages is shown by the ornamental initial letters, imitations of the illuminated letters of the manuscripts, which are placed at the heads of the chapters of a few books. Notwithstanding their significance, these initial letters would not find a place in any definition or general description of a modern book.
As a consequence of this inevitable lack of agreement between natural objects and their definitions, all knowledge of Nature is of very little value unless it is based upon a direct personal acquaintance with Nature itself. How different, for instance, will be that conception of such a group as the Cœlenterate, which is formed by the study of a short, definite, verbal description, from the idea in the mind of the student whose knowledge of the group as a whole has been acquired gradually by the study and comparison of the various forms of life which it includes!
Definitions are valuable and indispensable aids to study, and, as long as their necessary lack of agreement with the reality is kept in mind, they can do no harm; but the history of science warns us to be constantly on our guard, lest distinctions which seem to be sharp and absolute, when stated in words, come to be regarded as having as real an existence in Nature as in words, and we thus come, in the words of Bacon, to exchange "things for words, reason for insanity, and the world for a fable."
The tenacity with which many thinkers still cling to a belief in the reality and absoluteness of such distinctions as those expressed by the terms "organic and inorganic", "living and dead", "animal and vegetable", "rational and instinctive", etc., is plainly the result of this tendency to attribute to Nature the exactness of words.
The terms animal and plant were not established as the result of scientific and thorough study and comparison, but were first introduced to give expression to the most superficial and obvious difference between living things.
Originally an animal was a living thing which could move and feel, and a plant one which could not; and this is still the popular view, although the scientific definitions are quite different. As soon as primitive man began to observe and to generalize and to use abstract words, one of the first generalizations which attracted his attention was that, of the bodies which were of most importance to him, and like him grew up, matured, and died, some were still more like himself in having the power to move and feel, while others lacked this power, and were fixed and insensible. We do not intend to imply that this generalization took this definite shape, but simply that it was reached and put into words at a very early period; and this is shown by the fact that in nearly all languages, and among all but the lowest races of men, this division of living things into two great groups is recognized, and definite words are employed to distinguish the animal from the vegetable organism. At a later period, when living things came to be more carefully studied, and superficial observation gave place to more exact and careful comparison, these two groups were found to have a real existence in Nature; and, as long as this study was confined to the more familiar, abundant, and easily-studied organisms, the increase of scientific knowledge only served to render the distinctness of the two groups more evident. It was soon found that all the common plants are alike in many other respects besides being fixed and insensible, and that all ordinary animals have many common characteristics, and it was found convenient to express these resemblances briefly and absolutely in definitions, and thus the terms animal and plant came to have a more and more exact and scientific value. It was seen, too, that all living things have much in common, and that the chief difference between plants and animals is the possession by the latter of the new properties of sensation and voluntary motion, added to those characteristics which they share with the plants. It is not at all strange that it was thought desirable to express this fact by a word, and that, in the same way that living things were said to differ from inorganic bodies by having, in addition to all the properties of the latter, a new and higher quality, vitality, animals were said to possess the new and higher faculties of feeling and will, in addition to all the faculties of the plant.
Thus was gradually built up that conception of Nature which regards the three kingdoms—the mineral, the vegetable, and the animal—as three successive steps in an ascending series, each being supposed to have all the properties and characteristics of the groups below it, and something new and entirely different in addition. This conception admits of such brief and definite statement, and when put into words is so clear and simple, that its general acceptance is quite natural; and we need not be surprised that it only gives way very slowly in favor of a view which does not admit of the same definite and simple formulation.
We will now examine the reasons which have led modern thinkers to reject this view, and to hold that, real and actual as the differences between animals and plants are, they are by no means absolute.
The most conspicuous and superficial difference between animals and plants is the one which we have already noticed. Animals have the power of free locomotion and of independent action, which is determined within the animal; these powers are called into action by changes in the external world, and imply the existence of sensation and consciousness. In all the ordinary plants the power of locomotion is lacking, and there are no voluntary actions like those which result from the sensitiveness of the animal. This difference finds its expression in the well-known dictum of Linnæus: "Plantæ vivunt; animalia vivunt et sentiunt;" and upon it is based the classification of the functions and organs of the animal as those of vegetative and those of animal life. Those functions which are carried on independently of the will, and are not influenced directly by changes in the external world—digestion, assimilation, secretion, circulation, and reproduction, for example—are called the vegetative functions of the body; while those of relation, such as sensation and voluntary motion, are called, in contradistinction, the functions of animal life.
The difference which has led to the general acceptance and current use of these and many similar expressions is real, as far as the higher and more familiar animals and plants are concerned, but, with the growth of our knowledge of the lower forms of life, the necessity for expansion and modification of the definition of both groups becomes apparent. Many of the lower animals, such as the hydroids and sponges, as well as many highly-organized animals, like the tunicates and oysters, lack the power of locomotion; and, on the other hand, many of the lower plants are quite actively locomotive. Certain plants which are by no means low are quite sensitive to external changes, and the actions by which they respond to these changes in the Venus's-flytrap, for instance, give quite as good proofs of the existence of volition as are afforded by the actions of many of the lower animals.
Another superficial and easily-recognized distinction between animals and plants is afforded by the contrast in general form and structure; this, like the first, is real, as long as our attention is restricted to the higher forms of the two groups. In the animal we find a sharply and definitely limited body, within which is a complicated mass of viscera, while in the plant the physiological equivalents of these, the nutritive and respiratory organs, are distributed in areas of considerable extent over the diffused and indefinite outer surface of the body. In the animal the absorbent surfaces of these organs are internal; in the plant, external. In nearly all the higher animals there is a mouth opening, through which solid as well as liquid food may pass into the digestive cavity, which is furnished with specialized glandular appendages, such as the salivary glands, liver, etc. Within the digestive tract the food is elaborated and prepared for digestion and digested, and the indigestible refuse is discharged from the body through a definite anal opening. The nitrogenous products of decomposition are excreted from the body, usually in solution, by definite urinary organs. There is a muscular pulsating heart, by which the nutritive fluid or blood is propelled through blood-vessels with definite walls, and respiration is effected almost entirely by definite limited organs which are usually internal. The animal also has internal reproductive organs, as well as a nervous system and organs of sensation.
In the plants those organs which exist at all are present in a much simpler form. The roots absorb nutritive matter through their surfaces, usually as a fluid, and the surfaces of the leaves are the respiratory organs, absorbing and giving oil gases. The complicated system of internal organs, so characteristic of the animal, is entirely wanting in the plant, and the internal substance of the latter is made up of a comparatively homogeneous parenchyma of cells and tubes, through which the fluids circulate. The reproductive elements are not formed in limited local internal glands, but externally, and there are no nerves or sense-organs.
This distinction is diagnostic but not perfectly characteristic; that is, we may safely classify as an animal any organism in which we find a definite, sharply-limited body, and complicated internal viscera, such as a digestive tract, respiratory organs, blood-vessels, internal reproductive organs, and a nervous system and sense-organs; while we may, with almost equal safety, refer to the vegetable kingdom an organism in which the nervous, sensory, and circulating organs are wanting, and the processes of absorption and respiration take place through the outer surface. This distinction is therefore diagnostic, for it enables us to determine, with considerable certainty, to which of the two groups a given organism is to be referred; but it is not characteristic, and cannot be made the basis of an absolute definition, for it gradually disappears as we study the lower animals and plants. As a matter of fact, each one of the peculiarities given above as distinctive of animals will be found lacking in organisms the animal nature of which is undoubted, and many animals will be found to want all of them. Even among the vertebrates the organs of respiration are greatly simplified in the lower forms. In the adult frog the skin aids the lungs in aërating the blood, and is so thin and delicate, and so richly supplied with blood-vessels, that, when moistened, it readily admits of the interchange of gases. During the breeding-season the abdomen of the female frog is so distended by eggs that there is no room for the inflation of the lungs, and respiration is entirely carried on through the skin.
In those marine and aquatic animals of small size whose bodies are not covered by impervious shells, respiration may take place, as in plants, over the whole surface of the body; and in many groups, the larger and protected members of which are furnished with highly specialized respiratory organs, these organs may be entirely wanting in the smaller naked forms. This is the case, for instance, with many of the naked mollusks.
In place of a closed system of vessels for the circulation of the blood, this fluid may, in parts of its course, find its way through the spaces among the viscera, and in the Tunicata a pulsating heart keeps in motion a hæmal fluid, which is nowhere confined by distinct blood vessels, but fills and circulates through all the intra-visceral spaces of the body. In most mollusks, again, themovements of the various parts of the soft body aid the heart in maintaining the circulation; and in the Polyzoa both vessels and heart are lacking, and the movements of the blood are due to muscular contractions, aided in many cases by cilia.
The salivary glands are frequently wanting in marine animals; the water swallowed with the food taking the place of saliva. The liver and other appendages to the digestive tract are wanting in many of the lower animals, and a parasitic life may entirely do away with the need for them, even in an animal which belongs to a highly-specialized group. The ordinary gasteropod mollusks have digestive organs which are almost as highly specialized as those of a vertebrate; but "entoconcha," a strange parasitic gasteropod, has no jaws or teeth, no salivary glands or liver, no anus, or any other specialized appendage to the digestive cavity, which is a simple pouch, which is not divided into œsophagus, stomach, and intestine, and the animal lives inside the body cavity of a holothurian, attached to the wall of its stomach, and is' nourished by the fluids which it sucks from the digestive cavity of its host. In the sponges and hydroids there is no digestive cavity distinct from the body cavity, and the food is received directly into the latter. Even the mouth is wanting in many parasitic worms, and the liquid food is absorbed through the outer surface of the body, precisely as in plants, and some parasites put forth root-like processes which penetrate the tissues of the host and absorb its juices.
The nervous system and organs of special sense are wanting in many of the lower animals, and in the fresh-water hydra the reproductive organs are not formed in specialized internal glands at definite points, but resemble those of plants in making their appearance at many points upon the surface of the body.
Many of the lower animals resemble plants in their mode of growth, as well as in simplicity of structure; the colonies or clusters of the compound hydroids, and the coral-making polyps, are very plant-like, and, as they also lack the power of locomotion, their animal nature was for a long time disputed, and they were classified as plants.
However much the body of one of the higher animals differs in general form and structure from that of one of the higher plants, it is plain, from the facts which we have pointed out, that the two groups cannot be absolutely and arbitrarily separated upon this basis, although the general value of the distinction is obvious.
Histology furnishes another difference which is quite general but not universal. There are well-marked and pretty constant contrasts between the cells and tissues of the one group and those of the other, and these contrasts furnish what is, perhaps, the most constant morphological distinction. The constituent cells of the tissues of the plant retain their original form and individuality, while, in the animal, they undergo the greatest modifications, and their individuality is usually entirely lost. A vegetable tissue or organ may easily be shown to be made up of a mass of nearly similar cells, each of which is independent and sharply defined; while the tissues of animals present the greatest differences in structure and appearance, and sharply-defined individual cells are seldom to be seen. The cause of this difference in the appearance of the tissues is a difference in the cells themselves. The protoplasmic contents of the vegetable cell are inclosed by a thick, strong outer membrane or cellulose wall; while the outer surface of the animal cell is usually only a little more dense than the protoplasmic contents, and does not usually form a distinct cell-wall. The vegetable cell may, however, be destitute of the cellulose wall; and, on the other hand, many animal tissues—cartilage, for instance—resemble the tissues of plants in being made up of independent cells, each of which has an outer layer.
The most universal and characteristic difference between animals and plants is physiological, and relates to the nature of the food and the character of the nutritive process. From comparatively simple inorganic substances, such as water, carbonic acid, and ammonia, the plant is able to build up the highly-complex protein compounds which are so characteristic of living beings. The animal feeds, in part, upon inorganic substances, such as water, and certain carbonates and phosphates, but it derives all its protein from plants, either directly, or, as in the case of the carnivorous animals, indirectly, through the aid of vegetable-feeding animals. In the body of the animal the complex protein compounds are broken down into simpler substances, and the energy thus set free is converted into the various manifestations of "vital force." The animal organism is thus a consumer of protein and a liberator of force. The vital activities of the plant depend, like those of the animal, upon the liberation of energy by the breaking down of protein compounds; but, as the formation of new protein within the body of the plant usually exceeds this consumption, plants, as manufacturers of protein, are broadly distinguished from animals. It is now known, however, that many very highly-organized flowering plants are carnivorous, and digest and make use of the protein of the animals which they capture, and it is probable that the potato-fungus and many other parasitic plants obtain all their protein ready made, like animals; and, as it is impossible to show that none of the lower animals have the power to make protein for themselves, this distinction cannot be made the basis of an absolute line between the two groups.
The difference in the process of respiration in animals and plants is well known. Animals while in a state of vital activity absorb oxygen from the air; and this is given off from their bodies, usually united with carbon, as carbonic acid. The green plants, on the contrary, absorb carbonic acid, which is separated by the chlorophyll, under the influence of sunlight, into carbon, which is appropriated by the plant, and oxygen, which is given off and may be again taken up by an animal. This difference is made use of in the arrangement of an aquarium; enough green plants being placed in the water to absorb the excess of carbonic acid given off by the animals, and to supply the oxygen for their respiration. The difference is not by any means absolute, however, since the vital changes of the plant are dependent, like those of the animal, upon oxidation, and result in the formation of carbonic acid. The colorless plants, like animals, absorb oxygen and give off carbonic acid. This is also true of green plants which are not exposed to light; but in the latter plants this process is normally masked and hidden by the opposite process already spoken of.
It is plain, from what has been said, that the separation of organisms into two great group —animals and plants—is convenient and natural, and that the distinctions between them are real but not absolute; and it is possible to define, that is give, all the characteristics which are distinctive of an animal, without implying or assuming that all animals conform with the definition to the same degree, or that no plant shares any of the characteristics. Since the lower representatives of the two groups resemble each other more closely than the higher forms, and since all positive characteristics gradually disappear as we approach the point of union or origin, we must, in order to give our definition any definiteness whatever, neglect the lowest and simplest forms, and consider only the more specialized.
As shown by the highest forms, an animal may be described morphologically as an organism made up of cells, which are usually without a cell-wall or membrane. In the adult, the individuality of these cells is usually lost, since they are united to form membranes, tissues, and fibres. In nearly all animals the tissues thus built up from cells fall into four groups—epithelium, connective tissue, muscular tissue, and nervous tissue.
The organs of the body are composed mainly of these tissues, and present the greatest diversity of structure and function; but they may be roughly arranged, according to their functions, into four groups: organs of nutrition, such as the digestive, circulatory, and respiratory organs; organs of reproduction, organs of motion, and organs of relation, such as the nervous system and sense-organs, and organs of defense or protection, such as horns or spurs. Except among the lower and simpler groups the individuals are not organically united, as in plants, into a community, although such communities as pairs, or flocks, or herds, are frequent. In such a community as a hive of bees the different individuals are specialized for the good of the whole, and are unable to exist apart; and the community is as real as in the case of a plant, although the connection is not material, but purely ideal.
Physiologically, animals are characterized by the fact that, with few exceptions, they are able to receive solid food into a definite internal digestive cavity, in which it is digested, and then absorbed through the wall of the cavity. They absorb oxygen from the surrounding medium, air or water, and, in addition to certain inorganic substances, take into their bodies, as their proper nutritive material, complicated protein compounds, which they derive either directly or indirectly from plants. Through the oxidation of these compounds they form substances of a simpler chemical structure, such as carbonic acid, water, and ammonia, and discharge these through their bodies as waste. Since the sum of the chemical changes which take place in the animal is the breaking down of the highly-complex protein-molecules, derived from plants, into the simpler molecules of water, ammonia, and carbonic acid, it results that the potential energy thus set free is shown by the animal as "vital force," and the body of the animal is therefore a magazine or storehouse of force which may manifest itself as animal heat or light, or as sensible motion, or nervous disturbance; an animal, then, is an organism which has the power to change the potential energy of vegetable protein by oxidation into "vital force," which may manifest itself as animal heat, or, in the case of many marine animals, as light, or by peculiar disturbances of the nerves and muscles, organs which are peculiarly diagnostic of animals. The changes of the muscles result in motion, either of the animal as a whole or of the various parts in relation to each other. The structure and functions of the nervous system of one of the higher animals are so entirely different from any other phenomena, that they seem to be sui generis and peculiar; but we must not forget that there are true animals which entirely lack a nervous organization, and that in the history of each individual, as well as in the history of the animal kingdom, we may pass without any considerable break from animals with a complicated system of nerves and sense organs to animals which give no evidence of conscious life, and are no more sensitive than ordinary plants. The nervous system of an animal may be roughly described as a regulative apparatus by which the various parts of the body are brought into relation-with each other, in such a way that a disturbance or change in one part shall bring about in another part the liberation of a certain amount of energy, which shall result in change tending to bring the organism into harmony with the conditions which determined the first change. In many cases the action of the nervous system is accompanied by consciousness, and in the higher animals it has a subjective existence as intelligence and volition.
Such, briefly stated, are the most important characteristics of animals as they are manifested by the higher representatives of the group, and it is hardly necessary to call attention again to the fact that none of them furnishes a basis for the absolute separation of animals and plants, or to point out again that many of them are met with only in the higher animals, while others are not confined to animals, but are shared by some plants. The two groups are related to each other somewhat like two streams which have, their sources in the same watershed, but flow in different directions, and through regions of different characters. It is almost impossible to say whether the springs and marshes among which they rise belong to one stream or the other, and they may be connected with both; but, as we pass from this common source, the characteristics of each stream become more marked, until at last their differences, the result of the different conditions to which they have been exposed, overbalance and sink the resemblances which are due to their common source.
We must not suppose that this fact does away with the idea of the essential diversity of animals and plants, or that the distinction between them is any the less real and natural because they can be traced to a common source, and cannot be absolutely defined. As much confusion of ideas exists with reference to this point, it may not be out of place to give an illustration, drawn from another field, to show that a distinction may be real without being absolute:
A person in charge of a small library would find it easy to arrange his books under a few headings: some being devoted to history or science; others to theology or philosophy; others to fiction, poetry, and so on. In most cases the placing of a book would present no difficulty; but, as the size of the collection increased, works would be met with which, though devoted to history, were in part fictitious, and many works of fiction would be found to be historical. Novels would be met with, the aim of which is the exemplification of some psychological, physiological, or religious truth; and so with all the other departments. Most of the new books could still be arranged under headings as readily as in a smaller collection, but every increase in the size of the library would render the inosculation of the various departments of literature more apparent, and would increase the need of a catalogue with cross-references. If the librarian did not confine his attention to the books in his library, but studied the history of the growth of literature, its embryology and paleontology, he would find that departments which are now widely separated were once united, and sprang from a common source. He would find that there had been a time when all history and all theology were poetical, and all poetry historical or theological; that all romance was originally more or less historical, and that all history or science was at first more or less imaginary.
By the study of the less frequent and familiar forms of literature, and by the history of its growth, he would, like the naturalist, learn that his distinctions and classifications are only relative; of great value, indeed, but by no means absolute; but he would not therefore conclude that his groups are not real. He would not conclude that, because some novels are historical, there is no such thing as a history and no such thing as a novel, although he would perceive that they are connected by intermediate forms, and have originated from a common source.
The fact that there is no arbitrary line between the groups of natural objects, between animals and plants for instance, or between two related species of animals, does not prove that the groups themselves have not a real existence. The differences between plants and animals are real, and each group may be defined, but no definition can embrace all the forms of one group, and exclude all of the other, any more than a definition of fiction or of poetry can exclude all historical works.
The things, like the words, are real; but the definiteness of words is very different from the indefiniteness and complexity of Nature.