anyone who bears in mind that, even under existing circumstances, the tendency of the increase of population is to overtake that of the means of subsistence, and considers to how great an extent any considerable prolongation of the period of human life must, in the course of a few generations, multiply the number of human beings on the surface of the globe.

INTRODUCTORY LECTURE

DELIVERED IN THE

THEATRE OF THE ROYAL COLLEGE OF SURGEONS IN LONDON

ON THE 8TH OF MAY 1820.

Ir is the office of that professorship, to which I have had the honour of being appointed, to explain the organisation and the functions, not of a single animal, but of the animal creation generally; to demonstrate, as far as our imperfect knowledge will enable me to do so, the laws which regulate the phenomena of life, and the changes which matter undergoes, and the forms which it assumes, when it becomes associated with this mysterious and active principle. I undertake the task which has been allotted me, with a mixed sentiment of distrust and confidence. I cannot but be aware how difficult and how extensive is the science of which I am about to treat; and I am also conscious of the imperfect nature of my own qualifications. At the same time I feel well assured of the indulgence which I shall meet with from the liberality of those who compose my audience, when they consider that the subject of Comparative Anatomy and Physiology is one which might well occupy the undivided attention of an active mind during a long series of years; and that the knowledge of it, which it can fall to my lot to possess, is no more than can be acquired by an individual who joins the pursuit of science with that of an arduous profession.

It is undoubtedly to be regretted, and it may be a matter of just surprise, that examples are so rare of persons who have devoted themselves to these curious and important researches; and that they should have been almost wholly abandoned to those whose thoughts are occupied, and whose leisure is invaded, by other avocations. It must be owned, however, that there is no class of mankind to whom this department of knowledge can present such powerful attractions as to the members of our own profession. To understand the natural organisation and the natural functions of living bodies seems to be a necessary preliminary step to the knowledge of those changes which disease induces; and to what source can we so confidently look for any essential improvements, which may be made hereafter in medicine and surgery, as to the more enlightened notions, and more comprehensive views, which may be derived from a further cultivation of anatomy and physiology? Besides, the dignity of our profession, and its rank in society, depend in a great degree on its scientific character; and those who are anxious to uphold it in the estimation of others would do well to bear in mind, that, whenever its connection with science is dissolved, it must sink to the level of meaner occupations.

Anatomy and physiology ought to be regarded as inseparable from each other. The study of the former would be uninteresting and useless if pursued alone; and that of the latter would lead only to vague and absurd opinions, were it not founded on the basis of anatomical structure. But the one is much more readily brought to a certain degree of perfection than the other. An extensive acquaintance with the organisation of living bodies can scarcely fail to be attained by diligent dissections; but when we extend our inquiries further into the functions of the parts which anatomy displays, we find ourselves engaged in a complicated and difficult investigation, in which much laborious research may be rewarded by no more than a scanty addition to our previous knowledge.

I need not inform you how little progress the ancient philoso

phers had made in these sciences. After the revival of letters, anatomy was cultivated with much success; physiology was enriched with the discovery of the circulation of the blood: but, with this exception, it cannot be said that any very striking improvement was made in the latter science previously to the middle of the last century, when Baron Haller and Mr. Hunter set the example of a more philosophical mode of inquiry, by referring the phenomena of life to peculiar laws, instead of explaining them, as had been done before, by the mechanical and chemical laws, which operate on dead matter.

It is true that matter, when endowed with life, does not lose those properties which belong to it in its inorganic form. The living flesh is incapable of resisting the action of intense heat, or the operation of caustics. The light is refracted by the humours of the eye, as by a lens of glass or crystal. The body gravitates to the earth. The blood flows in the vessels according to the laws of hydrostatics, and undergoes a chemical change in the lungs. But these properties are in combination with others; and the changes, which are consequent to death, show not that they are suspended, but that they are modified and counteracted by the influence of another principle.

That the laws of life ought to be regarded as different from those which govern the changes of inorganic matter, must be acknowledged by any one who considers the essential difference which exists in the nature of the two classes of phenomena. It is the office of science first to obtain a knowledge of individual facts, and, afterwards, to reduce those facts to general principles. In proportion as the generalisation is more complete, and as the number of general principles is diminished, so is science rendered more perfect. Perhaps beings of superior intelligence, and possessed of a greater range of observation, may contemplate all the immense variety of mechanical, chemical, and vital phenomena around us, as dependent on the influence of one great general law, impressed on all matter, but variously modified by the various circumstances under which it operates. But with our limited

capacities we must be content with humbler views. The mechanical properties of matter must be regarded as distinct from the chemical; and neither of these, according to any right method of philosophising, can be resorted to for the purpose of explaining what is peculiar to animal, or even vegetable life.

The properties which are impressed on living matter are not only different from the properties of inorganic bodies, but they possess among themselves very different characters. The contractile power of muscles bears as little resemblance to nervous sensibility as it does to chemical attraction; and the propagation of volition from the brain to distant organs, as far as we are capable of perceiving, approaches as nearly to the force of gravity as it does to the function of secretion. Yet it is supposed that the vital properties, dissimilar as they are, have a certain connecting link; that they possess something in common; that they depend on a common principle; which we may denominate the principle of life. In this conclusion we are justified by the circumstance of our finding them uniformly connected with each other. They begin to exist, they cease to exist, at the same instant; and the same supply of scarlet blood, which imparts irritability to the muscles, is the source of sensibility in the nervous system, and maintains the various glandular secretions. The term, 'principle of life,' if used in a sober and restricted sense, expresses not an invention of the human mind, but something which has a real existence. We know it not indeed in its simple form, nor whether it be substance or quality; but we know it by the effects which it produces: in short, we have a knowledge of it similar to that which we possess of gravitation, electricity, or magnetism.

In general we see life combined with action, and living beings present an endless multitude of phenomena in perpetual and rapid succession. Life, however, may exist independent of any action which is evident to the senses. The egg continues unaltered, and giving no sign of an active principle within it for days and weeks; but its vitality is demonstrated by its resisting putrefaction; and when subjected to the influence of a higher tempera