The subject to which I have to beg your attention during the ensuing hour is "The Relation of Physiological Science to other branches of Knowledge." Had circumstances permitted of the delivery, in their strict logical order, of that series of discourses of which the present lecture is a member, I should have preceded my friend and colleague Mr. Henfrey, who addressed you on Monday last; but while, for the sake of that order, I must beg you to suppose that this discussion of the Educational bearings of Biology in general _does_ precede that of Special Zoology and Botany, I am rejoiced to be able to take advantage of the light thus already thrown upon the tendency and methods of Physiological Science. Regarding Physiological Science, then, in its widest sense--as the equivalent of _Biology_--the Science of Individual Life--we have to consider in succession: 1. Its position and scope as a branch of knowledge. 2. Its value as a means of mental discipline. 3. Its worth as practical information. And lastly, 4. At what period it may best be made a branch of Education. Our conclusions on the first of these heads must depend, of course, upon the nature of the subject-matter of Biology; and I think a few preliminary considerations will place before you in a clear light the vast difference which exists between the living bodies with which Physiological science is concerned, and the remainder of the universe;--between the phaenomena of Number and Space, of Physical and of Chemical force, on the one hand, and those of Life on the other. The mathematician, the physicist, and the chemist contemplate things in a condition of rest; they look upon a state of equilibrium as that to which all bodies normally tend. The mathematician does not suppose that a quantity will alter, or that a given point in space will change its direction with regard to another point, spontaneously. And it is the same with the physicist. When Newton saw the apple fall, he concluded at once that the act of falling was not the result of any power inherent in the apple, but that it was the result of the action of something else on the apple. In a similar manner, all physical force is regarded as the disturbance of an equilibrium to which things tended before its exertion,--to which they will tend again after its cessation. The chemist equally regards chemical change in a as the effect of the action of something external to the body changed. A chemical compound once formed would persist for ever, if no alteration took place in surrounding conditions. But to the student of Life the aspect of nature is reversed. Here, incessant, and, so far as we know, spontaneous change is the rule, rest the exception--the anomaly to be accounted for. Living things have no inertia, and tend to no equilibrium. Permit me, however, to give more force and clearness to these somewhat abstract considerations, by an illustration or two.