What can we say in regard to these fundamental vital powers of the active tissues?  Firstly, we must notice that many of the processes which we now understand were formerly classed as vital, and we only retain under this term those which are not yet explained.  This, of course, suggests to us that perhaps we may some day find an explanation for all the so-called vital powers by the application of simple physical forces.  Is it a fact that the only significance to the term vital is that we have not yet been able to explain these processes to our entire satisfaction?  Is the difference between what we have called the secondary processes and the primary ones only one of degree?  Is there a probability that the actions which we now call vital will some day be as readily understood as those which have already been explained?

Is there any method by which we can approach these fundamental problems of muscle action, heart beat, gland secretion, etc.?  Evidently, if this is to be done, it must be by resolving the body into its simple units and studying these units.  Our study thus far has been a study of the machinery of the body as a whole; but we have found that the various parts of the machine are themselves active, that apart from the action of the general machine as a whole, the separate parts have vital powers.  We must, therefore, get rid of this complicated machinery, which confuses the problem, and see if we can find the fundamental units which show these properties, unencumbered by the secondary machinery which has hitherto attracted our attention.  We must turn now to the problem connected with protoplasm and the living cell, since here, if anywhere, can we find the life substance reduced to its lowest terms.

CHAPTER II.

THE CELL AND PROTOPLASM.

==Vital Properties.==—­We have seen that the general activities of the body are intelligible according to chemical and mechanical laws, provided we can assume as their foundation the simple vital properties of living phenomena.  We must now approach closer to the centre of the problem, and ask whether we can trace these fundamental properties to their source and find an explanation of them.

In the first place, what are these properties?  The vital powers are varied, and lie at the basis of every form of living activity.  When we free them from complications, however, they may all be reduced to four.  These are:  (1) Irritability, or the property possessed by living matter of reacting when stimulated. (2) Movement, or the power of contracting when stimulated. (3) Metabolism, or the power of absorbing extraneous food and producing in it certain chemical changes, which either convert it into more living tissue or break it to pieces to liberate the inclosed energy. (4) Reproduction, or the power of producing new individuals.  From these four simple vital activities all other vital