anything corresponding to the usual popular conception—which was also along the view of physiologists—that the body is "animated" by a specific "vital principle," or "vital force," a dominating "archæus" that exists only in the realm of organic nature? If such a principle exists, then the mechanistic hypothesis fails and the fundamental problem of biology becomes a problem sui generis.

In its bearing on man's place in nature this question is one of the most momentous with which natural science has to deal, and it has occupied the attention of thinking men in every age. I cannot trace its history, but it will be worth our while to place side by side the words of three of the great leaders of modern scientific and philosophic thought. The saying has been attributed to Descartes, "Give me matter and I will construct the world"—meaning by this the living world as well as the non-living; but Descartes specifically excepted the human mind. I do not know whether the great French philosopher actually used these particular words, but they express the essence of the mechanistic hypothesis that he adopted. Kant utterly repudiated such a conception in the following well known passage: "It is quite certain that we cannot become adequately acquainted with organized creatures and their hidden potentialities by means of the merely mechanical principles of nature, much less can we explain them; and this is so certain that we may boldly assert that it is absurd for man even to make such an attempt or to hope that a Newton may one day arise who will make the production of a blade of grass comprehensible to us according to natural laws that have not been ordered by design. Such an insight we must absolutely deny to man." Still, in another place Kant admitted that the facts of comparative anatomy give us "a ray of hope, however faint, that something may be accomplished by the aid of the principle of the mechanism of nature, without which there can be no science in general." It is interesting to turn from this to the bold and aggressive assertion of Huxley: "Living matter differs from other matter in degree and not in kind, the microcosm repeats the macrocosm; and one chain of causation connects the nebulous origin of suns and planetary systems with the protoplasmic foundations of life and organization."

Do not expect me to decide where such learned doctors disagree; but I will at this point venture on one comment which may sound the key-note of this address. Perhaps we shall find that in the long run and in the large sense Kant was right; but it is certain that to-day we know very much more about the formation of the living body, whether a blade of grass or a man, than did the naturalists of Kant's time; and for better or for worse the human mind seems to be so constituted that it will continue its efforts to explain such matters, however difficult they may seem to be. But I return to our more specific inquiry with the remark that the history of physiology in the past two hundred years has been the history of a progressive restriction of the notion of a "vital force" or "vital principle" within narrower and narrower limits, until at present it may seem to many physiologists that no room for it remains within the limits of our biological philosophy. One after another the vital activities have been shown to be in greater or less degree explicable or comprehensible considered as physico-chemical operations of various degrees of complexity. Every physiologist will maintain that we cannot name one of these activities, not even thought, that is not carried on by a physical mechanism. He will maintain further that in most cases the vital actions are not merely accompanied by physico-chemical operations but actually consist of them; and he may go so far as definitely to maintain that we have no evidence that life itself can be regarded as anything more than their sum total. He is able to bring forward cogent evidence that all modes of vital activity are carried on by means of energy that is set free in protoplasm or its products by means of definite chemical processes collectively known as metabolism. When the matter is reduced to its lowest terms, life, as thus viewed, seems to have its root in chemical change; and we can understand how an eminent German physiologist offers us a definition or characterization of life that runs: "The life-process consists in the metabolism of proteids." I ask your particular attention to this definition since I now wish to contrast with it another and very different one.

I shall introduce it to your attention by asking a very simple question. We may admit that digestion, for example, is a purely chemical operation, and one that may be exactly imitated outside the living body in a glass flask. My question is, how does it come to pass that an animal has a stomach?—and, pursuing the inquiry, how does it happen that the human stomach is practically incapable of digesting cellulose, while the stomachs of some lower animals, such as the goat, readily digest this substance? The earlier naturalists, such as Linnaeus, Cuvier or Agassiz, were ready with a reply which seemed so simple, adequate and final that the plodding modern naturalist cannot repress a feeling of envy. In their view plants and animals are made as they were originally created, each according to its kind. The biologist of to-day views the matter differently; and I shall give his answer in the form in which I now and then make it to a student who may chance to ask why an insect has six legs and a spider eight, or why a yellowbird is yellow and a bluebird blue. The answer is: "For the same reason that the elephant has a trunk." I trust that a certain rugged pedagogical virtue in this reply may atone for its lack of elegance. The elephant has a trunk, as the insect has six legs, for the reason that such is the specific nature of the animal; and we may assert with a degree of probability that amounts to practical certainty that this specific nature is the outcome of a definite evolutionary process, the nature and causes of which it is our tremendous task to determine to such extent as we may be able. But this does not yet touch the most essential side of the problem. What is most significant is that the clumsy, short-necked elephant has been endowed—"by nature," as we say—with precisely such an organ, the trunk, as he needs to compensate for his lack of flexibility and agility in other respects. If we are asked why the elephant has a trunk, we must answer because the animal needs it. But does such a reply in itself explain the fact? Evidently not. The question which science must seek to answer, is how came the elephant to have a trunk; and we do not properly answer it by saying that it has developed in the course of evolution. It has been well said that even the most complete knowledge of the genealogy of plants and animals would give us no more than an ancestral portrait-gallery. We must determine the causes and conditions that have cooperated to produce this particular result if our answer is to constitute a true scientific explanation. And evidently he who adopts the machine-theory as a general interpretation of vital phenomena must make clear to us how the machine was built before we can admit the validity of his theory, even in a single case. Our apparently simple question as to why the animal has a stomach has thus revealed to us the full magnitude of the task with which the mechanist is confronted; and it has brought us to that part of our problem that is concerned with the nature and origin of organic adaptations. Without tarrying to attempt a definition of adaptation I will only emphasize the fact that many of the great naturalists, from Aristotle onward, have recognized the purposeful or design-like quality of vital phenomena as their most essential and fundamental characteristic. Herbert