As to life and organization, a recent writer remarks: It may he safely said of all living things, large enorgh to enable us to trust the evidence of microscopes, that they are optically heterogeneous. and that their different parts, especially the surface layers as contrasted with exteriors, differ physically and chemically; while in most living things, mere heterogeneity Is exchanged for a definite whereby the body is distinguished into visible parts, which possess different powers or functions. Living things which present this visible structure are said to he "organized;" and so widely does organization obtain among living beings, that "organized" and — living" are not unfrequently used as if they were terms of coextensive applicability. This is not exactly accurate, if it thereby be implied that all living things have a visible organization, as there arc numerous forms of living matter of which it cannot properly be said that they possess either a definite structure or permanently specialized organs: though doubtless the simplest particle of living matter must possess a highly complex molecular structure far beyond the reach of vision. The broad distinctions which, as a matter of fact, exist between every known form of living substance and every other component of the mate rial world, justify the separation of the biological sciences from all others. But it must not be supposed that the differences between living and non-living matter are such as to justify the assumption that the forces at work in the one are different from those which are to be met with in the other. Considered apart from the phenomena of conscious ness, the phenomena of life arc all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. It may be con venient to use the terms " vitality" and " vital force" to denote the causes of certain great groups of natural operations, as we employ "electricity" and "electrical force" to denote others; but it ceases to be proper to do so if such a name implies the absurd assumption that "electricity" and "vitality" arc entities playing the part of efficient causes of electrical or vital phenomena. A mass of living protoplasm is simply a molecular machine of great complexity, the total results of the working of which, of its vital phenomena, depend on the one band upon its construction, and on the other upon the energy supplied to it; and to speak of " vitality" as anything but the name of a series of operations. is as if one should talk of the "horology" of a desk.--(Huzley.) Other writers, objecting to this use of terms, call attention to the fact that even if the term " be thus limited in science to a series of operations, the term " life" is not thereby precluded from a larger applioffion.
Coming to the CLASSIFICATION OF THE PHENOMENA OF LIFE, we find that living matter, or protoplasm, and the products of its metamorphoses, may be regarded under four aspects: 1. It has a certain external and Internal form, the latter being usually called " structure." 2. It occupies a certain position in space and time. 3. It is the subject of the operation of certain forces, by virtue of which it undergoes internal changes, modifies external objects, and is modified by them. 4. Its form, place, and powers arc the effects of certain causes corresponding to these four aspects. Biology is separated into four chief subdivisions, which are: I. Morphology; II. Distribution; III. Physiology; IV. /Etiology.
I. MorrnoLoor. As far as living beings have form and structure they come within the province of anatomy and histofOgy, the latter being', the name for microscopic analysis of living forms. When the form and structure of a living being are not the same during its whole existence, hut undergo changes, such beings have dereZopment, and the history of development is an account of the anatomy of a living being at Successive epochs of its existence, and of the manner in which one anatomical stage passes into another. Finally, the systematic statement and.generalization of the facts of morphology, in such
a manner as to arrange living beings into groups according to their degrees of likeness, is taxonomy. The study of anatomy and development has brought out certain generali zations of wide applicability and importance.
1. Most plants and animals are aggregates of cells. Ordinary dissection by unassisted vision suffices to separate the body of any of the higher animals or plants into fabrics of different sorts, which in the samo organism always present the same general arrange ment, but in different organisms are combined in differing manner. Tho discrimination . of these comparatively few fabrics, or tissues, of which organisms are composed, was the first step toward that ultimate analysis of visible structure which has become possi ble only by recent perfection of microscopes and improved methods of preparation. Histology, which embodies the results of such analysis, shows that every tissue of a plant is composed of more or less modified structural elements, each of which is called a cell; and this cell in its simplest condition is only a mass of protoplasm, surrounded by a coat or sac called the cell-wall, which contains cellulose. lu various tissues the cells may undergo innumerable changes of form, the protoplasm may change into a nucleus with its nucleolus, n primordial utricle, and a cavity filled with watery fluid, and the cell-wall may be altered in composition or structure, or may coalesce with other cell walls. But however extensive these changes maybe, the fact remains clear that the tissues are made up of morphologically distinct units, which are the cells. Every plant, so far as traceable, may he said to commence existence as n simple cell, identical in its funda mental characteristics with the least modified of those. cells of which the whole body is composed. Cell-walls, however, are not always necessary. There are plants which spend a portion of their existence in the condition of a spheroid of protoplasm, with nothing like a wall, while at other times the protoplasmic body becomes :aclosed within a cell-wall fabricated by its superficial layer. Therefore, just as the nucleus, the primor dial utricle, and the central fluid are no essential constituents of the morphological unit of the plant, but represent results of its metamorphoses, so the cell-wall is equally unessential; and either the term "cell" must acquire a merely technical significance as the equivalent of "morphological unit," or some new term must be invente71 to deseribe the latter. Probably it would be least inconvenient to modify the signification of the term "cell." Analysis of animal tissue has led to similar difficulties in terminology. In the higher animals, however, the modifications which the cells undergo are so extensive that the fact that Lhe tissues are, as in plants, resolvable into an aggregation of morphological units, could never have been established without the aid of Lhe study of development, which proves that the animal, no less than the plaint, commences its traceable existence as a simple cell, fundamentally identical with the less modified cells which are found in'tbe tissues of the adult. Though the nucleus is almost constant among animal cells, it is not universally present; and atnong, the lowest forms of animal life the protoplasmic mass which represents the morphological unit may be, as in the lowest plants, devoid or a nucleus. In the animal the cell-wall never Las the character of a shut sac containing cellulose; and it is not-a little difficult, in many cases, to say how much of the so-called cell-wall of the animal cell answers to the primordial utriele, and bow much to the proper "cellulose cell-wall" of the vegetable cell. But it is certain that in the animal. as in the plant, neither cell wall nor nucleus are essential constituents of the cell, inasmuch as bodies which are unquestionably the equivalents of cells--true morphological units— are mere masses of protoplasm, devoid alike of cell-wall and nucleus.