notice the tendency to "vegetative repetition;" for it may be observed in many animals which can be divided without the destruction of their lives, especially among the Radiated, and the lower Articulated tribes. Where such a repetition exists, some of the organs may be removed without permanent injury to the structure; their function being performed by those that remain. Thus it is not uncommon to meet with specimens of the common five-rayed Starfish, in which not only one or two, but even three or four, of the arms have been lost without the destruction of the animal's life; and this is the more remarkable, as the arms are not simply organs of locomotion or prehension, but contain prolongations of the stomach. In the bodies of the higher animals, however, where there are few or no such repetitions (save on the two sides of the body), and where there is consequently a greater diversity in character and function between the different organs, the mutual dependence of their actions upon one another is much greater, and the loss of a single part is much more likely to endanger the existence of the whole. Such structures are said to be more highly organized than those of the lower classes; not because the whole number of parts is greater, for it is frequently much less; but because the number of dissimilar parts, and the consequent adap tation to a variety of purposes, is much greater,-the principle of division of labor, in fact, being carried much further, a much larger class of objects being attained, and a much greater perfection in the accomplishment of them being thus provided for.

6. Keeping in view, then, what has just been stated in regard to the divisibility of a Tree or a Zoophyte into a number of parts, each capable of maintaining its own existence, we may trace a certain gradation from the condition of the Mineral body to that of the highest Animal, in regard to the character in question. Thus, the individuality of a Mineral substance may be said to reside in each molecule; that of a Plant or Zoophyte, in each complete member; and that of one of the higher Animals, in the sum of all the organs. The distinction is much greater, however, between the lowest organized fabric and any mineral body, than it is between the highest and the lowest organized structures; for, as we shall hereafter see, the highest and most complicated may be regarded as made up of an assemblage of the lowest and simplest; whose structure and actions have been so modified as to render them mutually dependent; but which yet retain a separate individuality, such as enables them to continue performing their functions when separated from the mass, so long as the proper conditions are supplied.

7. Between the very simplest Organized fabric, and every form of Mineral matter, there is a marked difference in regard to intimate structure and consistence. Inorganic substances can scarcely be regarded as possessing a structure; since (if there be no admixture of components) they are uniform and homogeneous throughout, whether existing in the solid, the liquid, or the gaseous form; being composed of similar particles, held together by attractions which affect all alike. Far different is the character of Organized structures; for in the minutest parts of these may be detected a heterogeneous composition,-a mixture of solid and fluid elements, which are so intimately combined and arranged, as to impart such peculiarities to the tissues, even in regard to their physi

cal properties, as we never encounter amongst Mineral bodies. In the latter, solidity or hardness may be looked upon as the characteristic condition; whilst in Organized structures, softness (resulting from the large proportion of fluid components) may be considered the distinctive quality, being most obvious in the parts that are most actively concerned in vital operations. This softness is evidently connected with the roundness of form characteristic of Organized fabrics, which is most evident when the tissues contain the greatest proportion of fluid; whilst the plane surfaces and angular contours of Mineral bodies are evidently due to the mode in which the solid particles are aggregated together, without any intervening spaces.

8. The greatest solidity exhibited by Organized fabrics, is found where it is desired to impart to them the simple physical property of resistance; and this is attained by the disposition of solid particles, often of a mineral character, in tissues that were originally soft and yielding. It is in this manner that the almost jelly-like substance, in which all the organs of animals originate, becomes condensed into cartilage, and that the cartilage is afterwards converted into bone; it is in the same manner, also, that the stones of fruit, and the heart-wood of timber-trees, are formed out of softer tissues. But, as we shall hereafter see, this kind of conversion, whilst it renders the tissue more solid and durable, cuts it off from any active participation in the vital operations; and thence reduces it to a state much more nearly analogous to that of mineral bodies. This resemblance is rendered more close by the fact, that the earthy deposits frequently retain a distinctly crystalline condition; so that, when they are present in large proportion, they impart a more or less crystalline aspect to the mass, and especially a crystalline mode of fracture, which is evident enough in many shells. It must not be hence concluded, however, that such substances are of an inorganic nature; all that is shown by their crystalline structure being, that the animal basis exists in comparatively small amount, and that the mode in which the mineral matter was deposited has not interfered with its crystalline aggregation.

9. It is not to be disputed that a certain degree of homogeneity is apparently to be found in the minutest elements, into which certain Organized tissues are to be resolved. Thus, in the membranes which form the walls of Animal and Vegetable cells, the highest powers of the microscope fail in detecting any such distinction of fluid and solid components, as that which has been described as characteristic of organized structures. Nevertheless it is indubitable that such distinct components must exist; and this especially from the properties of these membranes in regard to water. For it is one of the most remarkable facts in the whole range of science, that a membrane, in which not the slightest appearance of a pore can be discovered under the highest powers of the microscope, should be capable of allowing water to pass through it; and that, too, with no inconsiderable rapidity. The change which these membranes undergo in drying, is another proof that they are not so homogeneous as they appear, and that water is an element of their structure, not merely chemically, but mechanically. The same may be said. in regard to the fibres, which form the apparently ultimate elements of

the simple fibrous tissues in Animals, and which are also met with in the interior of certain cells and vessels in Plants. These fibres would appear to be of perfectly simple structure; yet we know from the loss of fluid, and the change of properties which they undergo in drying, that water must have formed part of their substance.-It may be remarked, however, in regard to both these elementary forms of Organized tissue, that the simplicity of their function is in complete conformity with the apparent homogeneousness of their structure; for the cell-membrane is chiefly destined to act, like the porous septum in certain forms of the voltaic battery, as a boundary-wall to the contained fluid, without altogether interfering with its passage elsewhere; the forces which produce its imbibition or expulsion being probably situated, not in this pervious wall, but in the cavity which it bounds. And, in the same manner, the function of the fibrous tissues, to which allusion was just now made, is of an entirely physical character; being simply to resist strain or pressure, and yet to allow of a certain degree of yielding by their elasticity.

10. In all cases in which active vital operations are going on, we can make a very obvious distinction of the structures subservient to them, into liquid and solid parts; and it is, indeed by the continual reaction which is taking place between these, that the fabric is maintained in its normal condition. For, as we shall hereafter see, it is liable to a constant decomposition or separation into its ultimate elements; and it is consequently necessary that the matters which have undergone that disintegration should be carried off, and that they should be replaced by new particles. These processes of removal and replacement, with the various actions subservient to them, make up a large proportion of the life of all Organized beings. Now as all the alimentary matter must be reduced to the liquid form, in order that it may be conveyed to the situations in which it is required, and as all the decomposed or disintegrated matter must be reduced to the same form in order to be carried off, the intermingling or mutual penetration of solids and liquids in the minutest parts of the body is at once accounted for. We shall hereafter see that a cell or closed vesicle, formed of a membranous wall, and containing fluid, may be regarded as the simplest form of a living body, and the simplest independent part or instrument of the more complex fabrics (§ 30).

11. Organized structures are further distinguished from Inorganic masses, by the peculiarity of their chemical constitution. This peculiarity does not consist, however, in the presence of any elementary substances which are not found elsewhere; for all the elements, of which organized bodies are composed, exist abundantly in the world around. It might have been supposed that beings endowed with such remarkable powers as those of Animals and Plants,-powers which depend, as we shall hereafter see, upon the exercise of properties to which we find nothing analogous in the Mineral world,-would have had an entirely different material constitution; but a little reflection will show, that the identity of the ultimate elements of Organized structures with those of the Inorganic world, is a necessary consequence of the mode in which the former are built up. For that which the parent communicates, in giving origin to a new being, is not so much the structure itself as the power of forming that structure from the surrounding elements; and it

is by gradually drawing to itself certain of these elements, that the germ becomes developed into the complete fabric. Now, of the sixtytwo simple or elementary substances, which are known to occur in the Mineral world, only about eighteen or nineteen are found in Plants and Animals; and many of these in extremely minute proportion. Some of these appear to be merely introduced, to answer certain chemical or mechanical purposes; and the composition of the parts which possess the highest vital endowments, is for the most part simple and more uniform.

12. The actual tissues of Plants, when entirely freed from the substances they may contain, have been found to possess a very uniform composition, and to agree in their chemical properties. The substance which forms the principal part of the thickness of the walls of the cells, vessels, &c., of which the Vegetable organism is composed, is identical with Starch in the proportion of its components; but as these are in a different state of aggregation, it is distinguished as Cellulose. It consists of 12 Carbon, 10 Hydrogen, and 10 Oxygen; or, in other words, of Carbon united to the elements of water, in the proportion of eight of the former to seven of the latter. It may be very easily converted into gum or sugar, by chemical processes, which effect the removal or the addition of the elements of water. Now there is no compound known to exist in the Inorganic world, which bears the remotest analogy to this; and we have no reason to believe that it could be produced in any other way, than by that peculiar combination of force which exists. in the growing Plant. But although Cellulose is the predominating component of the Vegetable fabric, yet it is not the most essential; for late researches have shown, that within what has been ordinarily considered as the cell-wall, is a delicate membrane, termed the "primordial utricle," which is really the original cell-wall, from the exterior of which the layer of cellulose is secreted. And it is a very interesting fact, that the composition of this membrane corresponds with that of the proper cell-walls of the Animal tissues; it being, in fact, a proteine compound (§ 13). Hence every act of Vegetable growth involves the production of this substance also, which is still more removed in its composition from ordinary Inorganic compounds.

13. The composition of the Animal tissues, when freed from the fluids they may contain, or from the solid matters which may have been deposited within them, is nearly as uniform. We may distinguish among them two chief proximate principles, which appear under various modifications in a great variety of dissimilar parts, and which seem capable of conversion into other principles by the addition or subtraction of some of their constituents. The first and most important of these, named Proteine,* consists of 40 Carbon, 31 Hydrogen, 5 Nitrogen, and 12 Oxygen; and although its composition is so complex, it appears to act like a simple body, in uniting with Oxygen, Acids, &c., in definite proportions, as well as with Sulphur and Phosphorus; with

* Although it may be doubted whether Proteine has ever been actually obtained in a separate state, yet the term may be conveniently applied to that composite base, which is united with different equivalents of sulphur and phosphorus in the albuminous compounds.

which last, indeed, it is always found combined, in the Albumen, Fibrine, &c., that are commonly regarded as the organic constituents of the Animal tissues. The second of the chief proximate principles, termed Gelatine, is largely diffused through the Animal body; but the tissues which are composed of it possess a simple fibrous structure, and a purely mechanical function; and no vital action seems to take place in them, subsequently to their first production. It consists of 13 Carbon, 10 Hydrogen, 2 Nitrogen, and 5 Oxygen; and it is principally characterized by its solubility in hot water, and by the insolubility of its compound with tannic acid.

14. We shall hereafter dwell more in detail upon the Chemical Constitution of the Animal tissues and products (CHAP. III.) These substances are only noticed here, in illustration of the general statement, that the "proximate principles" of Animal and Vegtable bodies (that is, the simplest forms to which their component structures can be reduced, without altogether separating them into their ultimate elements), are of extremely peculiar constitution; being made up of three or four elements, of which the atoms do not seem to be united two by two, or by the method of binary composition, but of which a large number are brought together to form one compound atom of ternary or quaternary composition. This compound atom, like Cyanogen, and many others derived from Organic products, acts like a simple or elementary one in its combinations with other substances.-It is worthy of remark, however, that, in this respect as in others, the Vegetable kingdom is intermediate between the Animal and the Mineral. For whilst Proteine and Gelatine are remarkable, not only for containing four elements, but for the very large number of atoms of these components which enter into the single compound atom of each; the Cellulose of Plants is much simpler in its composition, since it includes only three elements, and the numbers which represent their proportions are smaller. And further, the proportions of the components of Cellulose are themselves such as suggest the idea of simplicity in their method of combination, the union of water and carbon in the common binary method; an idea which is confirmed by the mode of its original production, which indicates a direct union of carbon with water; as well as by the fact, that the chemical difference between Cellulose and numerous other substances found in Plants, may be represented by the simple addition or subtraction of a certain number of atoms of water, and that the chemist can effect an actual conversion of the former into certain of the latter, by means which are calculated to effect such an addition or subtraction.

15. We shall hereafter see that Vegetables are intermediate between the Animal kingdom and the Inorganic world in another most important particular the nature of the chemical operations they effect; for it is their function to combine the oxygen, hydrogen, carbon, and nitrogen, of the Inorganic world into Organic Compounds; which not only serve as the materials of their own growth, but also as the food of Animals, whose existence is entirely dependent upon them, since they possess no such combining power. It is from the Water, Carbonic Acid, and Ammonia, supplied by the atmosphere and by the soil in which they are fixed, that