strate, far less to imitate, in the brain of the chick, any structural equivalent to its experience, we may be told that no one expects complete inductive proof of any scientific generalization; that he who refuses to admit that all water consists of H2O until chemists have decomposed every drop of water in the ocean is lacking in good sense; and that it is equally unreasonable to demand the artificial imitation of all the responses of living things before we admit that all response is mechanical.

To this we must answer that no great harm can be done if the chemist interprets the admission that we have not the slightest reason to doubt that every drop of water is decomposable into hydrogen and oxygen as an assertion that all water is so decomposable, since, for all the ordinary purposes of chemistry, the negative admission and the positive assertion may be treated as if they were synonymous. The case is very different when the subject under consideration is not chemistry, but the nature of knowledge, for we are about to enter a field where we may easily lose our way unless we distinguish inference from perception, to the best of our ability. The utmost the physiologist is warranted in asserting is that, for all one knows to the contrary, every response may be mechanical; and I think all thoughtful students must so far agree with him as to admit that belief that any of the responsive actions of living beings are not mechanical is highly unwise and precarious, in view of the condition and prospects of modern physiology; although we must, in my opinion, also admit that not one single vital response has as yet been completely analyzed, or resolved, from beginning to end, into phenomena of matter and motion; for I am myself unable to discover, in the present status of biology, any demonstration of error in the assertion that life is different from matter and motion.

However this may be, we know, by evidence which no one can question, that many actions are attended by memory, and by conscious experience, and by volition and reason and a sense of moral responsibility. Many beneficial responses are known to be judicious and reasonable, and many voluntary acts are known to be right or wrong.

As these convictions seem, at first sight, to be contradictory to the opinion that, for all we know to the contrary, all response may

be mechanical, we must ask whether this contradiction is real or only apparent. As this question has, in one form or another, vexed the mind of man for untold ages, no one would be so bold as to attempt a final answer in few words; but I hope all who follow me to the end may find reason to ask themselves whether the contradiction may not, after all, be a matter of words rather than a real difficulty, for I shall try to review, at one time and another, some of the evidence which has convinced many thoughtful men that this apparently insoluble puzzle has arisen from an erroneous and unscientific conception of the meaning of the mechanism of nature. This evidence seems so clear and conclusive that I cannot see how any one who has mastered it can find any contradiction between anything we find in our nature and the ultimate reduction of all nature, including all the phenomena of life and of mind, to mechanical principles; for most students of the principles of science agree that natural knowledge is no more than the discovery of the order of nature; although a moment's thought is enough to show that the fact that events do take place in order is no reason why they should, or even why they should take place at all. Order is no explanation, but a thing to be explained.

The proof that there is no necessary antagonism between mechanical explanations of human life and belief in volition and duty and moral responsibility seems to me to be very simple and easy to understand. If the subject takes us into deep waters, this is because we are compelled to examine the reason why the impression that these things are antagonistic has so widely prevailed; for the view of the matter to which I hope to call your attention is, in itself, by no means difficult or obscure.

Science is still in its infancy, and we know so little that I have no sympathy with those who discount the possibilities of future discovery and assert that life is merely a question of matter and motion, although I know no reason why this should not, some day, be proved, nor am I able to see why any should find this admission alarming.

However this may be, I am convinced that they stand on treacherous ground, who base positive opinions on negative evidence, and believe that anything in our nature is inconsistent with mechanics.

Conscience, the last acquired faculty," says Maudsley, "is the first to suffer when disease invades the mental organization. One of the first symptoms of insanity - one which declares itself before there is any intellectual derangement, before the person's friends suspect even that he is becoming insane-is a deadening or complete perversion of the moral sense. In extreme cases it is observed that the modest man becomes presumptuous and exacting, the chaste man lewd and obscene, the honest man a thief, and the truthful man an unblushing liar. Short of this, however, there is an observable impairment of the finer moral feelings—a something different, which the nearest friends do not fail to feel, although they cannot always describe it. Now these signs of moral perversion are really the first symptoms of a mental derangement which may, in its further course, go through all the degrees of intellectual disorder, and end in destruction of mind, with visible destruction of the nervecells which minister to mind. Is the end, then, dependent upon organization, and is the beginning not?"

"Note, again, the effect which a severe attack of insanity sometimes produces upon the moral nature of the individual. The person entirely recovers his reason; his intellectual faculties are as acute as ever, but his moral character is changed; he is no longer the moral man that he was; the shock has destroyed the finest part of his organization. Henceforth his life may be as different from his former life as, in an opposite direction, was the life of Saul of Tarsus from the life of Paul the Apostle to the Gentiles. An attack of epilepsy has produced the same effects, effacing the moral sense as it effaces the memory sometimes, and one of the most striking phenomena observed in asylums is the extreme change in the moral character in the epileptic which precedes and heralds the approach of his fits. A fever or an injury to the head has, in like manner, transformed the moral character."

Passing this subject by for the present, it is clear that, consciously or unconsciously, arbitrarily or naturally, freely or of necessity, every living thing responds to some part of the order of nature, and that the study of this order is part of biology; for there are many reasons, besides those we have considered, why the biologist should have peculiar interest in the principles of science. His studies bring him into intimate contact with certain conceptions which play such a

subordinate part in the other sciences that it falls to him to assert their importance, since they are so little regarded outside his circle that students in other lines often fail to catch what he has in mind. Among these are the principle of genetic continuity and the principle of fitness, with all that they imply. For all I know to the contrary, the principle of fitness may be universal, and the order of nature may be the order of fitness; and again, for all I know to the contrary, all significant resemblances between the phenomena of nature may be due to genetic continuity; but, at the present day, these principles hold no prominent place in the minds of those who deal with the not-living, and their introduction among the principles of science is due to the biologists. Now only a moment's thought is needed to discover how great are the difficulties that attend the application of these principles. What do we mean by the genetic continuity of life? How are we to interpret the facts of embryology? How many perplexing intricacies face us if we undertake, with William Harvey, "to seek the truth regarding the following difficult questions: Which and what principle is it whence motion and generation proceed? Whether is that which in the egg is cause, artificer, and principle of generation, and of all the vital and vegetative operations, conservation, nutrition, growth, innate or superadded? and whether does it inhere primarily, of itself, and as a kind of nature, or intervene by accident, as a physician in curing disease? Whether is that which transfers an egg into a pullet inherent or acquired?"

-

"In truth," says Harvey, "there is no proposition more magnificent to investigate or more useful to ascertain than this: How are all things formed by an univocal agent? How does the like ever generate its like? Why may not the thoughts, opinions, and manners now prevalent, many years hence return again, after an intermediate period of neglect?"1

As we find embryologists, two hundred and fifty years after these words were written, still vexing themselves over the question, — Whether is that which transfers an egg into a pullet inherent or acquired?-it is clear that we cannot hope for much progress in the investigation of this magnificent proposition unless we can determine what we mean by that metaphysical notion, inherent potency.

1 Harvey, "De Generatione," pp. 274-582.

"By way of escape from the metaphysical Will-o'-the-Wisps generated in the marshes of literature and theology, the serious student is sometimes bidden," says Huxley, "to betake himself to the solid ground of physical science. But the fish of immortal memory, who threw himself out of the frying-pan into the fire, was not more ill advised than the man who seeks sanctuary from philosophical persecution within the walls of the observatory or the laboratory; for metaphysical speculation follows as closely upon physical theory as black care upon the horseman."1

If, as modest biologists, we were to assert that the biological aspects of the physical sciences are the only basis for rational interest in these sciences, our good friends in physical and chemical laboratories would, no doubt, charge us with arrogance, although I think they must admit that the principles of science, as distinguished from the concrete sciences, are part of biology.

We cannot investigate response to the order of nature without asking what the order of nature is. What are the properties of things and of thought that convince us of its existence? What is this conviction worth? What are the methods by which knowledge of this order is acquired and perfected and extended? How far are these methods and instruments trustworthy? Are any limits to their application known, and, if so, how known?

To all these questions the zoologist has a peculiar right to ask answers, in addition to the right which he shares with other students of science.

"The Mind, her acts and faculties," says Berkeley, "furnish a new and distinct class of objects, from the contemplation whereof arise other notions, principles, and verities. It may therefore be pardoned if this rude essay doth, by insensible transitions, draw the reader into remote inquiries and speculations, that were not, perhaps, thought of either by him or by the author at first setting out."

Some, who believe they at least are rigorously scientific, may here feel impelled to cry out that these inquiries are not scientific, but metaphysical, and that modern men of science have nothing to do with them. For my own part, I might be disposed to agree with them if the average human mind were, on these difficult

1 Huxley, VI., p. 200.