The morphological changes which the cells undergo in the course of further develop ment of the organism do not affect their individuality: and, notwithstanding the modi fication and confluence of its constituent cells. the adult organism, however complex, is still an aggregate of,morphological units. Not less is it an aggregate of physiological units, each of which retains its fundamental independence, though that independence becomes restricted in various ways. Each cell, or that element of a tissue which pro ceeds from the modification of a cell, must retain its snstentative functions so long as it grows or maintains a condition of equilibrium; but the most completely metamor phosed cells show no trace of the generative function, and many exhibit no correla tive functions. On the other hand, those cells of the adult organism which are the unmetamorphosed 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. Perhaps It ;s not strictly right to speak of organs of sustentation or generation, each of these func tions being necessanly performed by the morphological unit which is nourished or reproduced. What are called the organs of these functions are apparatuses 1,y which certain operations subsidiary to sustentation and generation are carried on. Thus in the case of sustentative function, all those organs may be said to contribute to this function which are concerned in bringing nutriment within reach of the'ultimate or in removing waste matter from them; while in the case of the generative function, all those organs contribute to the functions which prodlice the cells from which germs are given off, or help the evacuation, or fertilization, or development of those germs. On the other hand, the correlative functions, so long as they are exerted by a simple undif ferentiated 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 arninge tnent of the parts of the protoplasm. or of those prolongations of the protoplasm which are called pseudopodia or cilia. But, in the higher animals and plants, the movements of the organism and of are brought about by the changes of the form of certain tissues, the property of which is to shorten in one direction when exposed to certain Such tissues are termed contractile, and, in their most fully developed condi tion, m-uscular. The stimulus by which this contraction is naturally brought about is a molecular change, either in the substance Of the contractile itself, or in some other part of the body; in which latter ease 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 plena there seems to be no question that parts which retain a hardly modified cellular structure may serve as channels for the transmission of this molecular motion; whether the same is true of animals is not certain. But in all the more com plex 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 initiated are co-ordinated and brought into harmonious combination. While the sustenta tive functions in the higher forms of life are still, as in the lower, fundamentally dependent 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, which Constitute the muscular and nervous tissues.
When we compare the different forms of life together as physiological machines, we lind that they differ as do Machines of human construction. In the lower forms, the mechanism, though perfectly adapted to the work to be done, is rough, simple, and weak; while in the higher forms, it is finished, complicated, and powerful. Considered
ns machines, the difference between a polyp and a horse suggests that between the distaff and the spinning-jenny. In the progress from the lower to the higher organisms there is a gradual differentiation of organs and of functions, Each function is separated into many parts, which are severally- intrusted to distinct organs—a sort of equitable division of physiological labor. And precisely the same proeess is observable in the development of anyof the higher organisms; so that, physiologically as well as morpho logically, development is a progress from the general to the special.
Conditions of Existence.—Thus far the physiological activities of living matter have been considered in themselves, and without reference to anything that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it; and the study of the influence of the condi tions of existence thus determined constitutes a most important part of physiology. The sustentative functions, for example, can be exerted only under certain conditions of temperature, pressure, and light, in certain media. and with supplies of particular kinds of nutritive matter; the sufficiency of which supplies is again greatly influenced by the competition of other organisms, which. striving to satisfy the same need, give rise to the passive for ,existence." The exercise of the correlative functions is influenced by similar conditions, and by direct conflict with other organisms, which constitute the actice "struggle for existence," and, finally, the generative functions are subject to extensive modifications, dependent partly upon what arc commonly called external conditions, and partly upm. r,t Liy .o ici,own agencies. In the lowest forms of life, the only mode of generation at present Izzow,a is the division of the body into two or more parts, each of which then grows to the size and assumes the form of the parent, and repeats the same process of multiplication. This method of multiplication by fission is properly called generation, because the parts which are separated are severally competent to give rise to individual organisms of the same nature as that from which they arose. In many of the lowest organisms the process is modified so far that, instead of the parent dividing into two equal parts, only a small portion of its substance is detached, as bud, which develops into the likeness of the tree from which it was taken. This is generation by gemmation. Generation by fission and by gcmmation are not confined to the simplest forms of life, however. Both modes of multiplication are common not only among plants, but among animals of considerable complexity. The multiplication of flowering plants by bulbs, that of animals by fission, and that of polyps by budding, are well-known examples of these modes of reproduction. In all the cases the bud or segment consists of a multitude of more or less metamorphosed cells. But, in other instances, a single cell detached from a mass of such undifferentiated cells con- twined in the parental organism is the foundation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segment—whether the process is more akin to fission or to gemination. In all these cases the development of the new being from the detached germ takes place without the influence of other liv ing matter. Common as the•process is in plants and in the lower animals, it becomes rare among the higher animals. In these the reproduction of the whole organism from a part, in the way indicated above, ceases. At most we find that the cells at the end of an amputated portion of the u.ganism are capable of reproducing the lost part; and, in the highest enimals, even this power vanishes in the adult; and, in most parts of the body, though the undifferentiated cells are capable of multiplication, their grow, not into whole organisms like that of which they form a part, but into elements of the tissues.