I think it is one of the grandest features of Biology, that it occupies this central position in human knowledge. There is no side of the human mind which physiological study leaves uncultivated. Connected by innumerable ties with abstract science, Physiology is yet in the most intimate relation with humanity; and by teaching us that law and order, and a definite scheme of development, regulate even the strangest and wildest manifestations of individual life, she prepares the student to look for a goal even amidst the erratic wanderings of mankind, and to believe that history offers something more than an entertaining chaos--a journal of a toilsome, tragi-comic march nowhither. The preceding considerations have, I hope, served to indicate the replies which befit the two first of the questions which I set before you at starting, viz. what is the range and position of Physiological Science as a branch of knowledge, and what is its value as a means of mental discipline. Its _subject-matter_ is a large moiety of the universe--its _position_ is midway between the physico-chemical and the social sciences. Its _value_ as a branch of discipline is partly that which it has in common with all sciences--the training and strengthening of common sense; partly that which is more peculiar to itself--the great exercise which it affords to the faculties of observation and comparison; and I may add, the _exactness_ of knowledge which it requires on the part of those among its votaries who desire to extend its boundaries. If what has been said as to the position and scope of Biology be correct, our third question--What is the practical value of physiological instruction?--might, one would think, be left to answer itself. On other grounds even, were mankind deserving of the title "rational," which they arrogate to themselves, there can be no question that they would consider, as the most necessary of all branches of instruction for themselves and for their children, that which professes to acquaint them with the conditions of the existence they prize so highly--which teaches them how to avoid disease and to cherish health, in themselves and those who are dear to them. I am addressing, I imagine, an audience of educated persons; and yet I dare venture to assert that, with the exception of those of my hearers who may chance to have received a medical education, there is not one who could tell me what is the meaning and use of an act which he performs a score of times every minute, and whose suspension would involve his immediate death;--I mean the act of breathing--or who could state in precise terms why it is that a confined atmosphere is injurious to health.