Such a common cause, the conditions of whose action are so simple and uniform that we can account for, and even predict, by a process of deduction, all the phenomena which it can operate to produce, may be regarded for a time as an ultimate fact. It may still, how ever, be capable of union with other facts of a similar order, under a still more comprehen sive expression' But it is not in every de partment of science that the same facility in the attainment of general laws exists. 1Vhere the phenomena are of such a complex nature that the operation of the real cause is, as it were, masked by the influence of concurrent conditions, or where (as often happens in phy siology) the effects of the same apparent cause are totally different according to the instru ments through which it operates, it is obvious that there will be great difficulty in the first stage of the inductive process—that of the clas sification of phenomena,—so great, indeed, that it may be regarded as one of the principal obstacles to the advancement of those branches of science in which it presents itself. Of all the branches of physical science, that of me teorology is the most obscure and apparently uncertain, and bears most resemblance to phy siology. The changes which it concerns are daily and hourly occurring under our observation ; and the general laws which govern them are tolerably well ascertained ; yet the mode in which their actions are com bined is so peculiar, as hitherto to have baffled the most persevering and penetrating enquirers, in their attempts to explain or predict their operation. But no one thence feels justified in assuming the existence of any new or un known cause, capable of controlling or sub verting the influence of the rest; and such a proceeding would not be justifiable, until all their possible modes of action have been ascer tained and put aside, leaving certain residual phenomena not otherwise to be accounted for.
The peculiar difficulties which beset the in vestigation of the laws of vital action have greatly retarded our acquaintance with them, and have even led to the belief that the induc tive process is not applicable to them. These difficulties have arisen, in the first place, from the obstacles in the way of the collection of phenomena; secondly, from the peculiarly com plex nature of these phenomena ; and, thirdly, from the vague hypotheses which have pre vented them from being classed as simple facts on which generalisations are to be erected, or effects whose sources are to be ascertained, but which have clothed them in the delusive aspect of laws or causes. Until, therefore, the prin ciples of philosophical induction are thoroughly understood, the peculiar combinations in which vital phenomena present themselves to our notice, their apparent dissimilarity from the changes which we witness in the world around, and their obvious adaptation to particular ends, might lead us astray into the labyrinth of un profitable speculation with regard to the pre siding agencies by which they are governed ; and the retrospective view which we shall pre sently take will afford many examples of this error, even in recent times, and will in fact show that the legitimate objects of investiga tion, and the true mode of pursuing them, are only now beginning to be understood.
When we observe the circumstances under which vital actions occur, we perceive that at least two conditions are required for their pro duction. The first is a structure in that pecu liar state which is termed organised (see OR GANISATION); the second is a of some kind fitted to act upon it. Now this is no more than what we observe in the world around, where every action involves two con • ditions of a corresponding character. When water is changed into steam, for example, it is by the stimulus of heat. When a stone falls to the ground, it is by the attraction which the mass of the earth exercises over its own. The difference consists in the peculiarity of the actions exhibited by living beings, which are not identical with those elsewhere presented to us, and which we cannot imitate by any phy sical or chemical operations. Whilst the me chanical philosopher, then, refers to the pro-. perty of gravitation as the cause of the effect just mentioned, the physiologist refers to the capability of exhibiting vital actions, when excited by certain stimuli, as the property of the tissue which manifests them. Thus, when he witnesses the contraction of a muscle, under the stimulus of innervation or of galvanism, &c. he regards the effect as due to a property of contractility inherent in the muscle, and standing in precisely the same relation to its organic structure, as gravity to matter in ge neral. So far, however, the advance in our in quiry is more apparent than real ; since it may fairly be said that, to speak of contractility as the character of a body exhibiting contractions, is merely a change in words without absolute gain. But, having done this, we are led to inquire the conditions under which this con tractility operates ; and to analyse a number of phenomena apparently dissimilar, so as to at tain the general law of its action. In this man ner we proceed in regard to other classes of phenomena; and we shall thus acquire (when our data are sufficiently precise and extensive) a knowledge of the properties of all the tissues or organised structures which compose the living body, and of the phenomena which their single or combined operation will pro duce, under the influence of their respective stimuli.
But the physiologist will not stop here. He will seek to inquire to what these properties are due, which are so different from anything exhibited by the same matter before it had be come a part of the organised system. And, if he consider the matter in all its bearings, with a total dismissal of prejudice, he will be unable, we think, to arrive at any other conclu sion than that they are due to the act of organi sation, which, in combining the inorganic ele ments into new compounds, and giving them a peculiar structure, calls out or developes in them properties which had previously existed in a dormant state, but required these circum stances for their manifestation. To this ques tion, however, we shall presently return, when considering other views which have been en tertained respecting it. We shall now take a retrospective glance at the