SECT. II. OF LIVING BEINGS, AND OF ORGANIZATION IN GENERAL. Life-Its definition-Death-Organization-Generation-Spontaneous Generation-ReproductionSpecies-Varieties-Permanence of Species-Pre-existence of Germs. LIFE, being the most important of all the properties of created existence, stands first in the scale of characters. It has always been considered the most general principle of division; and, by universal consent, natural objects have been arranged into two immense divisions, ORGANIC beings [comprising animals and plants], and INORGANIC beings [comprising minerals]. The word Life is used under two significations which are often confounded. It may be applied merely as a general term to express, with brevity, the various phenomena peculiar to living beings; or it may signify the cause of these phenomena. It is in the latter sense that the terms vital principle, or principle of life, are employed; being, in this respect, perfectly analogous to the terms gravity, heat, attraction, and electricity, which are used in the general sciences under a twofold signification, the one physical,-the other metaphysical. But, it is with the phenomena alone, or the physical sense of these terms, that Natural Philosophy has any concern. The knowledge of causes is removed far beyond the reach of human reason; and, by neglecting to discriminate between these two senses, ancient philosophers before Lord Bacon, and too many modern ones since his time, have fallen into endless discussions, and obscured the light of real science. Yet, it is difficult, upon a subject so interesting as life, in which we all feel deeply concerned, to restrain curiosity within the bounds of reason and philosophy. A recent anonymous writer asks, “Who has not put to himself the question, 'What is life?' Who would not receive a clear and just solution of the inquiry, with a feeling of interest, far beyond that afforded by the successful result of any ordinary scientific investigation? We can comprehend part of the mechanism by which life acts; we feel its result. We see that mechanism to be so delicate, so complicated, so fragile, so easily set wrong, while our interest is so deep that it should act well, and permanently well, that the exquisiteness of adjustment, the skill of contrivance, and the completeness with which the intended result is secured all subjects of distinct and interesting investigation-only increase the earnestness of our wish, that we could see beyond the mechanism, and understand that, which it is permitted us to know only by examining its phenomena. "We do not commonly consider how much is given us in life, the daily enjoyment of the boon renders us insensible to the variety and plenitude of its richness. We shall become more sensible of it upon contemplating the various tissues of organic particles that have been formed; the number of properties that are attached to each; the number of organs that are constituted by their aggregation and arrangement; the number of functions that are exercised by those organs; and the number of adjustments by which all are combined, harmonized, and made effectual to the production of one grand result. It is then we perceive how many things must exist, how many relations must be established, how many actions must be performed, how many combinations of actions must be secured, before there can be sensation and motion, thought and happiness." Many attempts have been made to account for the vital principle, but hitherto all these have proved abortive. It is possible, that various functions of the animal frame may hereafter be discovered to proceed from mechanical or from chemical laws; but, we believe, that the ultimate springs of the phenomena of life will ever remain concealed from human knowledge. In order to form a just idea of the essential conditions of life, we must first examine those beings which are the most simple in the scale of creation; and we shall readily perceive that these vital conditions consist, in a power possessed by certain bodies, for a period of time only, of existing in a determinate form; of continually drawing into their composition a part of the surrounding substances; and of returning back, to the influence of the general laws of matter, certain portions of their own materials. These phenomena are exhibited by the conferva rivularis, a small bundle of green filaments, finer than hair, found in rivulets and stagnant pools. Being without root or leaves, it is simply attached by a broad surface to the margin of the water. While life exists, it increases in size and weight, throws out filaments like branches, assimilates the particles of water, and of other inorganic substances around it, into vegetable matter, and lays them down in an oblong cellular form. In animals and plants, nutrition is the effect of an internal power; their growth is a development from within. In minerals, on the contrary, growth goes on by the external deposition of successive strata or layers; whilst organized bodies, by means of their vital power, grow and increase by the assimilation of different substances. The stalactite, once supposed to be an exception, is now proved to be subject to the ordinary laws of inorganic matter. Thus life may be compared to a whirlpool of variable rapidity and intricacy, drawing in particles of the same kind, and always in the same direction; but where the same individual particles are alternately entering and departing. The form of living bodies seems, therefore, to be more essentially their own, than the matter of which they are composed. The matter forming the bones of animals has been ascertained to undergo a very considerable change in a few days; and from this fact the probability of a corresponding change in the other parts of the frame is inferred. The very singular rapidity with which this change is effected was accidentally discovered. Certain animals were fed with madder (rubia tinctorum), a plant cultivated for its red dye; and in twenty-four hours all their bones were found to be deeply tinged with its colour. On continuing the same food, the colour became very deep; but upon leaving it off, the colour was completely removed in a very few days. By alternately changing the food, the bones were found to be marked with concentric rings of the red dye, according to the number of times that the change was made. These phenomena, so far surpassing any thing that could have been anticipated, are well calculated to convey an idea of the extraordinary rapidity with which the particles of the animal frame are removed, while the form remains without any apparent alteration. While this movement continues, the body wherein it takes place LIVES; when it entirely ceases, the body DIES. After death, the elements which compose the living frame, being surrendered to the influence of the ordinary chemical affinities, begin to separate; and the dissolution of the once living body speedily follows. It was, therefore, by the vital movement, that dissolution had been previously arrested, and that the elements of organized bodies were preserved in a state of temporary union. All bodies cease to live after a certain period of time, the duration of which is fixed for each species. Death appears to be a necessary effect of life; and the very exercise of the vital power gradually alters the structure of the body, so as to render its longer existence impossible. The frame undergoes a regular and continual change, as long as life remains. Its bulk first increases in certain proportions, and to certain limits, fixed for each species, and for the several organs of each individual; and then, in the course of time, many of its parts become more dense or solid. This last change appears to be the immediate cause of natural death. If different living bodies be examined with attention, we shall find them to be composed of an organic structure, which is obviously essential to such a whirlpool, as that to which we have already compared the vital action. There must not only be solid particles to maintain the forms of their bodies, but fluids to communicate the motion. They are, therefore, composed of a tissue of network, or of solid fibres and thin plates (or lamina), which contain the fluids in their interstices. It is among the fluid particles that the motion is most continuous and extensive. Foreign substances penetrate into the innermost parts of the body, and incorporate with it. They nourish the solids by interposing their particles; and, in detaching from the body its former parts, which have now become superfluous, traverse the pores of the living frame, and finally exhale under a liquid or gaseous form. During their course, the foreign substances enter into the composition of the solid framework, containing the fluids; and, by contracting, communicate a part of their motion to the liquid particles within them. This mutual action of solids and liquids-this transition of particles from the one form to the other, presupposes a great chemical affinity in their elementary constituents; and we accordingly find, that the solid parts of organized bodies are composed chiefly of such elements as are capable of being readily converted into liquids or gases. The solids would also require to be endowed with considerable powers of bending and expanding, in order to facilitate the mutual action and reaction between the solids and the fluids; and hence, this is found to be a very general characteristic of the solid parts of organized bodies. This structure, common to all living bodies, --this porous or spongy texture, whose fibres or laminæ, ever varying in flexibility, intercept liquids, ever varying in quantity-constitutes what has been termed organization; and, from the definition we have already given of the term life, it necessarily follows that none but organized bodies are capable of enjoying life. Thus we see, that organization results from a great number of arrangements, all of which are essential conditions of life; and hence it follows, that if living bodies be endowed with the power of altering even one of these conditions, to such an extent as to obstruct or arrest any of the partial movements, composing the general action, they must possess within themselves the seeds of their own destruction. Every organized body, besides the ordinary properties of its texture, possesses a form peculiar to its species; and this applies, not merely to its external arrangement in general, but even to the details of its internal structure. From this form is derived the particular direction of each of its partial movements; upon it depends the degree of intricacy in the general motion; and, in fact, it is this which constitutes the body a species, and makes it what it is. Life is always attended by organization, just as the motion of a clock ever accompanies the clock itself; and this is true, whether we use the terms in a general signification, or in their application to each particular being. We never find life, except in beings completely organized and formed to enjoy it; and natural philosophers have never yet discovered matter, either in the act of organizing itself, or of being organized, by any external cause whatever. The elements forming, in succession, part of the body, and the particles attracted into its substance, are acted upon by LIFE, in direct opposition to the ordinary chemical affinities. It is impossible, therefore, to ascribe to the chemical affinities those phenomena, which are the result of the vital principle; and there are no other powers except those of life, capable of reuniting particles formerly separated. The birth of organized beings is, therefore, the greatest mystery of organic arrangements, and indeed of all nature. We see organized bodies develop themselves, but they never form themselves; on the contrary, in all those cases where we have been able to trace them to their source, they are found to derive their origin from a being of similar form, but previously developed; that is, from a parent. The offspring is termed a germ, as long as it participates in the life of its parent, and before it has an independent existence of its own. In various species differences are found to exist in the place where the germ is attached to its parent; and also, in the occasional cause which detaches it, and gives it a separate existence; but, it is a rule which holds universally, without one single exception, that the progeny must have originally formed part of a being like itself. The separation of the germ is termed generation. Many ancient, and some more recent philosophers, believed that certain organized beings could be produced without parents; and this opinion, though now completely exploded among the learned by the most convincing experiments, still maintains its ground with the ignorant. It originated, as most errors do, from hasty and inaccurate observation. Virgil gravely attempts, in a very elegant passage of the Georgics, to Explain The great discovery of the Arcadian swain; And Kircher, who lived in the seventeenth century, gives a recipe to make snakes, which, however, he does not appear to have tried. In Scotland the country people still believe that the hair-worm (Gordius aquaticus, Linn.) can be formed artificially by placing a horse's hair in water; and this unfounded opinion is, we understand, generally diffused throughout the kingdom. The mites in cheese, the blight on plants, and the maggots in meat, seem at first sight to favour the belief in spontaneous generation; but in all these cases the insects have been demonstrated to proceed from eggs, deposited instinctively by the parent, upon a substance capable of affording nutriment to her young. The popular mistakes on this subject are generally, however, concerning the lower tribes of animals. But the ancients taught that even man could be produced without a parent. The newly-formed earth was supposed to have been originally covered with a green down, like that on young birds; and, soon afterwards, men, like mushrooms, rose from the ground. Lucretius (A. C. 60) relates, that even in his time, when the earth was supposed to be too old for gen eration, "many animals were concreted out of mud by showers and sunshine." Every organized being produces others resembling it. Without this provision, all species would become extinct, since death is the necessary consequence of the continued action of life. Certain animals possess the power of reproducing some of their parts, after these have been removed. This power is termed REPRODUCTION, and it is found in various degrees of perfection, according to the species. In general, this power of renovating mutilated parts is found to exist most perfectly in the lower species of organized beings. The head of the snail (Limax, Linn.) may be cut off, and the whole organ, including its elegant telescopic eyes, will be reproduced. The claws, feet, and feelers (or antenna) of crabs and lobsters, as well as the limbs of spiders, when amputated, are completely restored by the fresh growth of new organs. When accident deprives a shark of its teeth, they are replaced with facility. If the fins of fishes be cut, they will reunite, and the rays themselves will be reproduced, provided only the small parts at their bases are left. The eyes of lizards, though possessed of an intricate apparatus of coats and humours, if removed, will be replaced by new eyes equal to the former. Even man and the higher animals possess the same power, only restricted within narrower limits. Injuries to various parts of our frame are speedily repaired, and the wounds heal. The effect of injury to a living bone is curious. A new bone is produced round the old one; which finally dies, and is absorbed or discharged. The new bone, which at first was spongy in its texture, and irregularly formed, assumes, in a few years, its natural dimensions, and all appearance of change is completely removed. Thus we see the bountiful provision of Nature, and the effect of that principle of reproduction, which restores most of the organs of the body to their natural form and action, when deranged by injury or by disease. Organized beings are developed with greater or less rapidity and perfection, according as they are placed in favourable or unfavourable circumstances. Heat, the quantity or quality of their nutriment, and other causes, exercise considerable influence 1 over them; and this influence may extend over the whole frame, or be confined only to certain organs. Hence it follows, that the resemblance between the progeny and its parents can never be perfectly exact. These minor differences among organized beings are called varieties. The different kinds of dog (Canis familiaris, Linn.), of horse (Equus caballus, Linn.), of sheep (Ovis aries, Desm.), are all varieties of the same species, and are produced by merely accidental causes, such as domestication, climate, &c. By cultivation, the sloe has been transformed into the plum, and the crab-tree into the apple-tree. The cauliflower and red cabbage, though apparently very different plants, are descended from the same parents,-the wild Brassica oleracea,—a weed growing near the sea. Mr. Herbert relates, in the Horticultural Transactions, that he succeeded in raising, from the natural seed of a highly-manured red cowslip, a primrose, a cowslip, oxlips of the usual and other colours, a black polyanthus, a hose-in-hose cowslip, and a natural primrose, bearing its flower on a polyanthus stalk;-all these are instances of varieties, depending upon soil and situation. There is, however, no real ground for supposing that all the differences observable in organized beings are the result of accidental circumstances. Every thing hitherto advanced in favour of this opinion is purely conjectural. On the contrary, experience clearly shows, that, in the actual state of the globe, species vary only within very narrow limits; and, as far as past researches have extended, these limits are found to have been in ancient times the same as at present. The French naturalists, who visited Egypt with Bonaparte, found the bodies of the crocodile, the ibis, the dog, the cat, the bull, and the ape, which had been embalmed three thousand years ago by the Egyptians as objects of veneration, to be perfectly identical with the living species now seen in that country, even to the minutest bones and the smallest portions of their skins. The common wheat, the fruits, seeds, and other parts of twenty different species of plants, were also discovered, some of them from closed vessels in the sepulchres of the kings; and they resembled in every respect the plants now growing in the East. The human mummies, also, exactly corresponded with the men of the present day. We are, therefore, compelled to admit that certain forms have been regularly transmitted to us from the first origin of things, without having transgressed the limits assigned to them [except in a slight degree, when modified by certain accidental circumstances]. All beings, derived from the same original form, are said to constitute a species; and the varieties are, as has been stated, the accidental subdivisions of species. Generation appears to be the only means of ascertaining the limits by which varie ties are circumscribed; and we may therefore define a SPECIES to be a group or assemblage of individuals, descended one from another, or from common parents, or from others resembling them as much as they resemble each other. However rigorous this definition may appear, its application in practice to particular individuals is involved in many difficulties, especially when we are unable to make the necessary experiments. In conclusion, we shall repeat, that all living bodies are endowed with the functions of absorption [by which they draw in foreign substances]; of assimilation [by which they convert them into organized matter]; of exhalation [by which they surrender their superfluous materials]; of development [by which their parts increase in size and density]; and of generation [by which they continue the form of their species]. Birth and death are universal limits to their existence: the essential character of their structure consists in a cellular tissue or network, capable of contracting; containing in its meshes fluids or gases, ever in motion: and the bases of their chemical composition are substances, easily convertible into liquids or gases; or, into proximate principles, having great affinity for each other. Fixed forms, transmitted by generation, distinguish their species, determine the arrangement of the secondary functions assigned to each, and point out the part they are destined to perform on the great stage of the universe. These organized forms can neither produce themselves, nor change their characters. Life is never found separated from organization; and, whenever the vital spark bursts into a flame, its progress is attended by a beautifully organized body. The impenetrable mystery of the pre-existence of germs alike defies observations the most delicate, and meditations the most profound. We trace an individual to its parents, and these again to their parents. After a few generations the clue is lost, and in vain we inquire, Whence arose the first animal of the species? and what produced the first germs from which have descended the innumerable tribes of animals and plants that we see in constant succession rising around us? Whence did the species MAN arise? Philosophical inquiry fails to lead us through the labyrinth; and we feel the force of the same principle which inspired Adam, when he says, with Milton, SECT. III-DIVISION OF ORGANIZED BEINGS INTO ANIMALS AND PLANTS. Animals and Plants— Irritability—Animals possess Intestinal Canals-Circulating System—their Chemical Composition-Respiration. LIVING or organized beings have been subdivided by universal consent, from the earliest ages, into ANIMALS endowed with sensation and motion, and into PLANTS destitute of both, and reduced to the simple powers of vegetation. Some plants retract their leaves when touched; and all direct their roots towards moisture, and their flowers or leaves towards air and light. Certain parts of plants even exhibit vibrations, unassignable to any external cause. Yet, these different movements, when attentively examined, are found to possess too little resemblance to the motions of animals, to authorize us in considering them as proofs of perception and of volition. They seem to proceed from a power, possessed in general by all living substances, of contracting and expanding when stimulated, a power to which the name of irritability has been assigned. The fibres composing the heart of animals alternately expand and contract, altogether independent of the will of the animal; and thick hair will grow on the skins of some animals, when removed into a cold climate. As we neither ascribe volition nor sensation to the heart or to the hair, so we cannot attribute these qualities to the heliotrope, to the sun-flower, or to the sensitive plant. The |