three categories. They are either (1), functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand and those of assimilation on the other. Or (2), they are functions which subserve the process of re production, which is essentially the detachment of a part endowed with the power of developing into an independent whole. Or (3), they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, becomes a source of molar motion. The first may be termed susten- tative, the second generative, and the third correlative
functions.Of these three classes of functions the first two only can be said to be invariably present in living beings, all of which are nourished, grow, and multiply. But there are some forms of life, such as many Fungi, which are not known to possess any powers of changing their form; in which the protoplasm exhibits no movements, and reacts upon no stimulus ; and in which any influence which the different parts of the body exert upon one another must be transmitted indirectly from molecule to molecule of the common mass. In most of the lowest plants, however, and in all animals yet known, the body either constantly or temporarily changes its form, either with or without the application of a special stimulus, and thereby modifies the relations of its parts to one another, and of the whole to surrounding bodies ; while, in all the higher animals, the different parts of the body are able to affect, and be affected by, one another, by means of a special tissue, termed nerve. Molar motion is effected on a large scale by means of another special tissue, muscle; and the organism is brought into relation with surrounding bodies by means of a third kind of special tissue that of the sensory organs by means of which the forces exerted by surrounding bodies are transmuted into affections of nerve.
In the lowest forms of life, the functions which have been enumerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body; and the like is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated cell, which constitutes the starting point of their development. But the first pro cess in that development is the division of the germ into a number of morphological units or blastomeres, which, eventually, give rise to cells; and as each of these pos sesses the same physiological functions as the germ itself, it follows that each morphological unit is also a physio logical unit, and the multicellular mass is strictly a com pound organism, made up of a multitude of physiologically independent cells. The physiological activities manifested by the complex whole represent the sum, or rather the resultant, of the separate and independent physiological activities resident in each of the simpler constituents of that whole.
The morphological changes which the cells undergo in the course of the further development of the organism do not affect their individuality ; and, notwithstanding the modification and confluence of its constituent cells, the adult organism, however complex, is still an aggregate of morphological units. Nor is it less an aggregate of physio logical units, each of which retains its fundamental inde pendence, though that independence becomes restricted in various ways.
Each cell, or that element of a tissue which proceeds from the modification of a cell, must needs retain its sustentative functions so long as it grows or maintains a condition of equilibrium ; but the most completely meta morphosed cells show no trace of the generative function, and many exhibit no correlative functions. On the other hand, those cells of the adult organism which are the un- metamorphosed derivatives of the germ, exhibit all the primary functions, not only nourishing themselves and growing, but multiplying, and frequently showing more or less marked movements.
Organs are parts of the body which perform particular functions. In strictness, perhaps, it is not quite right to speak of organs of sustentation or generation, each of these functions being necessarily performed by the morphological unit which is nourished or reproduced. What are called the organs of these functions are the apparatuses by which certain operations, subsidiary to sustentation and genera tion, are carried on.
Thus, in the case of the sustentative functions, all those organs may be said to contribute to this function which are concerned in bringing nutriment within reach of the ultimate cells, or in removing waste matter from them ; while in the case of the generative function, all those organs contribute to the function which produce the cells from which germs are given off ; or help in the evacuation, or fertilization, or development of these germs.
On the other hand, the correlative functions, so long as they are exerted by a simple undifferentiated morphological unit or cell, are of the simplest character, consisting of those modifications of position which can be effected by mere changes in the form or arrangement of the parts of the protoplasm, or of those prolongations of the proto plasm which are called pseudopodia or cilia. But, in the higher animals and plants, the movements of the organism and of its parts are brought about by the change of the form of certain tissues, the property of which is to shorten in one direction when exposed to certain stimuli. Such tissues are termed contractile; and, in their most fully developed condition, muscular. The stimulus by which this contraction is naturally brought about is a molecular change, either in the substance of the contractile tissue itself, or in some other part of the body ; in which latter case, the motion which is set up in that part of the body must be propagated to the contractile tissue through the intermediate substance of the body. In plants there seems to be no question that parts which retain a hardly modified cellular structure may serve as channels for the transmis sion of this molecular motion ; whether the same is true of animals is not certain. But, in all the more complex animals, a peculiar fibrous tissue nerve serves as the agent by which contractile tissue is affected by changes occurring elsewhere, and by which contractions thus ini tiated are co-ordinated and brought into harmonious com bination. While the sustentative functions in the higher forms of life are still, as in the lower, fundamentally de pendent upon the powers inherent in all the physiological units which make up the body, the correlative functions are, in the former, deputed to two sets of specially modified units, whick constitute the muscular and the nervous tissues.
as physiological machines, they are found to differ as machines of human construction do. In the lower forms, the mechanism, though perfectly well adapted to do the work for which it is required, is rough, simple, and weak ; while, in the higher, it is finished, complicated, and powerful. Considered as machines, there is the same sort of difference between a polype and a horse as there is between a distaff and a spinning-jenny. In the progress from the lower to the higher organism, there is a gradual differentiation of organs and of functions. Each function is separated into many parts, which are severally entrusted to distinct organs. To use the striking phrase of Milne-Edwards, in passing from low to high organisms, there is a division of physiological labour. And exactly the same process is
observable in the development of any of the higher organ-
