Popular Science Monthly/Volume 7/October 1875/Instinct and Intelligence
WHAT is instinct? What is its relation to intelligence? These questions form the subject of my lecture to-day.
Many persons would probably object to this subject being treated at all in a course of physiology. Many persons doubtless think that these are questions for the psychologist, and not for the physiologist. But I think you have already perceived, in the course of these lectures, how difficult, yea, impossible, it is to sharply separate these two departments. As between all other departments of science, so also between these, there is a border-land, which is common ground. The physiology of the brain is that common ground.
The precise relation of physiology to psychology it is extremely difficult to adjust. As there are two opposite errors in regard to vital force—one, the old error of regarding this force as something innate, underived, unrelated to other forces of Nature; the other, the new error of regarding it as nothing but ordinary physical and chemical forces, and thus identifying physiology with chemistry and physics—so also on this subject there are two opposite errors: one the old error of regarding mental forces as wholly unrelated to and underived from vital forces, and psychology, as wholly disconnected from physiology; the other, the new error of regarding mental phenomena as connected with the brain in the same clear and intelligible way that functions are connected with organs, and thus identifying psychology with physiology. But, as in the case of vital force, there is a truer view, viz., that which regards this force as indeed correlated with other lower forces and derived from them, but, nevertheless, as a very distinct form of force or cosmic energy, producing a very distinct and peculiar group of phenomena, the knowledge of which constitutes the science of physiology; so also, on the subject of mental force, there is a truer view which comprehends and embraces the extremes mentioned above.
Let me briefly explain my views on this subject. In recent times physiology has indeed made great, and to many startling, advances in the direction of connecting mental phenomena with brain-changes. Physiologists have established the correlation of physical and chemical with vital forces, and probably of vital with mental forces. They have proved in every act of perception the existence of a vibratory thrill passing along the nerve-cord from sense-organ to brain; and in every act of volition a similar vibratory thrill from brain to muscle; they have even determined the velocity of this vibratory thrill, and find it, to the surprise of those who identify nervous force with electricity, only about 100 feet per second. They have also established the fact of a chemical or molecular change in the brain corresponding with changes in mental states; and with great probability, also, a quantitative relation between these corresponding changes, and therefore a relation between them of cause and effect. In the near future we may do more: we may localize all the faculties and powers of the mind in different parts of the brain, each in its several place, and thus lay the foundations of a scientific phrenology. In the far-distant future we may do even much more: we may possibly connect every different kind of mental state with a different and distinctive kind of molecular or chemical change in the brain; we may find, for example, that a right-handed rotation of atoms is associated with love, and a left-handed rotation with hate. We may do all this, and much more. We may push our knowledge in this direction as far as the boldest imagination can reach, and even then we are no nearer the solution of this mystery than before. Even then it would be impossible for us to understand how brain-changes can produce even the simplest psychological phenomena such as sensation, consciousness, will. By no Tort of the mind can we conceive how molecular motion can produce sensation or consciousness. The two sets of phenomena belong to different orders—orders so different, that it is simply impossible to construe the one in terms of the other.
It is not thus with other groups of phenomena in relation to one another. The phenomena of motion, heat, gravity, light, electricity, chemical affinity—yea, also of vitality—have been, or may be, construed in terms of each other, and all in terms of molecular motion. Whether our present theories on this subject be true or not, may admit of doubt; but a true theory is at least conceivable; all these may conceivably be reduced to the same order. But no amount of knowledge nor strength of imagination will in the least degree help us to understand the mysterious causal relation between the molecular changes in the brain and the corresponding effects in the mind, or between changes in the mind and corresponding changes in the brain.
I wish to put this as clearly and as strongly as I can. Suppose, then, an infinite human knowledge—infinite in degree, but human in kind; suppose, in other words, an absolutely perfect science, such as was conceived and admirably expressed by Laplace—a science which had completely subdued its whole domain and reduced it to the greatest simplicity, so that the whole cosmos and its phenomena is expressed by a single mathematical formula, which, worked out with positive signs, would give every phenomenon which would ever occur in the future, or with negative signs every event which had ever occurred in the past. Even to such an infinitely perfect science the causal relation of molecular motion on the one hand to sensation, consciousness, thought, and emotion on the other, or vice versa, would still be utterly unintelligible. Like the essential nature of matter, or the ultimate cause of force, this relation lies outside the domain of science.
But, admitting this chasm which cannot be bridged—admitting the distinctness of psychology and physiology, a distinctness far greater than exists between any other two departments of science—still there can be no doubt that the changes in the brain and in the mind correspond with each other in the strictest manner. There can be no doubt that we have here two parallel series running side by side with corresponding terms, and that every change in the terms of one series is associated with a change in the corresponding terms of the other series. Whichsoever we take as cause, and whichsoever as effect, the correspondence is undoubted. This much seems certain, and this is sufficient to show that a knowledge of the terms of one series must throw light on the order of succession in the terms of the other series. In a word, physiology, as the simpler and more fundamental science, must form the only true basis of a scientific psychology.
Again, as anatomy only became scientific by becoming comparative anatomy, i. e., by the study of the structure of organisms in their relation to each other, or as connected by the law of evolution; as physiology, too, only became really scientific by becoming comparative physiology; i. e., by tracing the gradual evolution of organic functions; even so psychology can never assume the rank of a science until it becomes comparative psychology; i. e., until it adopts the comparative method, until it studies the different grades and kinds of mentality in their relation to each other, and connects them all by the law of evolution.
So much I have thought it necessary to say in order to show the importance of my subject, and its close connection with physiology. I now pass on to the subject itself.
It is well known that many of the lower animals, especially certain species of insects, perform acts perfectly adapted to accomplish results, and that without previous experience and without instruction. Often the results attained are of a very complex character; results which could not be attained by ourselves except by the exercise of high intelligence, aided by much experience. The extraordinary capacity by which these results are reached with such unerring certainty is called instinct.
I need hardly refer you to examples: You are all familiar with the wonderful instinct of the common honey-bee; their organized communities with perfect division of labor, the precision with which they make their honey-cells on perfect mathematical principles, the honors paid to their queen, their care of her eggs, their wise distribution of food to the larvæ, both its quality and quantity, and the form and size of the containing cells being varied according to the function and even to some extent determining the character of the perfect insect, whether drone, or queen, or worker. You are already familiar with all this and much more which I cannot stop to recapitulate. You are also doubtless familiar with the still more wonderfully organized communities of ants, with their queens, their domestic laborers, and their warriors. You have heard of, if you have not watched, their marauding excursions, their fierce but well-ordered battles, the triumphal return of the victors laden with spoil, and with captives whom they reduce to the condition of domestic servants. I barely mention these examples because they are familiar to all, but I must describe more fully one case of instinct from the same family, which is probably less familiar to you and yet no less wonderful: I refer to the case of the common mud-wasp or clay-dauber.
These insects do not form organized communities, and therefore have no neuters or workers, but each female works for herself. Neither does she feed her young as do bees and other wasps, but provides appropriate food in abundance, and leaves them to themselves. But the mode of building her cells and supplying the appropriate food exhibits a marvelous wisdom. She gathers wet clay in pellets about the size of a duck-shot and commences to build. Going and coming from her clay-quarry to her work, with great patience, industry, and skill, she builds two or three cells side by side, two or three inches long and about half an inch in diameter. When finished she proceeds to fill them with food. For this purpose she attacks all varieties of spiders, stings them, plunging her sting with the greatest precision directly into the principal nerve-ganglion, and, after laying an egg in the body of each, carries them off and packs them away in the cells, until these are completely full, then seals them with clay and leaves them. In due time the eggs are hatched, the larvae feed upon the spiders, until they become perfect insects, and cut their way out of the cells.
Observe, then, first, the walls of the cells must be thin and composed of porous materials, otherwise the eggs could not hatch, nor the larvæ continue to live, for want of oxygen; second, the spiders must be helpless but not dead. If they were dead they would decompose or else dry up before they could be used as food. If, on the other hand, they were alive and active, they would destroy each other, and the contained eggs, and perhaps escape from the cell. But the poison is so adjusted as to quantity, and probably as to the place of insertion, as to produce a state of complete helplessness, a deep and permanent coma: the spiders are as it were chloroformed by the poison. If touched or irritated, they move only enough to show that they are not dead. As a boy I have many times watched these wasps gathering their materials, building their cells, gathering their food-supply of spiders. I have many times broken open their finished cells and found them full of spiders in the condition described. If one desires to study spiders, he can gather more varieties in one day, by breaking open the cells of mud-wasps, than in a year in any other way.
Now, such actions performed by man would show high intelligence and much experience; and yet we cannot attribute such intelligence to these insects, because their actions in other directions and under other and new conditions exhibit but a very small amount of intelligence; we are compelled to attribute these wise actions to another and somewhat different faculty, which by way of distinction we call instinct. Let, us then, contrast these two faculties (if they may be so called) and show their distinctive features:
1. Intelligence works by experience, and is wholly dependent on individual experience for the wisdom of its actions.—Wisdom in this case is a product of two factors, intelligence and individual experience. Intelligence alone produces nothing. Experience alone is equally valueless. With a given intelligence the product will vary as the experience, with a given amount of experience the product will vary as the intelligence. Thus intelligence works by experience to attain wise results. On the contrary, instinct is wholly independent of individual experience. The young bee or mud-wasp, untaught, works at once without hesitation, with the greatest precision and in the wisest manner, to accomplish the most marvelous results. Like the reflex function of the nervous system, and like the still lower organic functions of secretion, excretion, circulation, respiration, etc., the wisdom and precision of its actions seem to be the result of structure, though unlike these the actions are not removed from the sphere of consciousness and will, if we call it intelligence; then it is not individual intelligence but cosmic intelligence, or the laws of Nature working through inherited brain-structure to produce wise results.
2. Intelligence belongs to the individual, and is therefore variable, i. e., different in different individuals, and also improvable in the life of the individual by experience.—Instinct belongs to the species, and is therefore the same in all individuals and unimprovable with age and experience. It is true that close observation would probably detect a slight difference in the skill of different bees, and slight improvement with age, in some more than others, but this must be accredited to the individual, not to the inherited element, i. e., to the small margin of intelligence which undoubtedly exists in these animals.
3. Instinct in its sphere is far more perfect and unerring than intelligence.—It makes no mistakes, because determined by structure, not by imperfect knowledge.
In a word, intelligent conduct is self-determined and becomes wise by individual experience. Instinctive conduct is predetermined in wisdom by brain-structure. The former is free, the latter is to a large extent automatic; the one is like the voluntary locomotion of the higher animals, free to turn whither it likes, but liable to mistakes and stumblings and hurtful falls; the other like the motion of an engine laid upon a track which bears it swiftly and surely to its destined goal.
I have given, thus far, only a few very conspicuous examples of instinct, in order the more clearly to contrast this faculty with intelligence. I might have added many other conspicuous and well recognized examples, such as the migrations and nest-making of birds, the dam-building of beavers, etc. But we would have a very imperfect notion of the wideness of the operation of instinct if we confined ourselves to these conspicuous cases. Instinct in a less remarkable degree is universal among animals, including man himself. But what is universal in the popular mind creates no surprise, attracts no attention, and seems to need no explanation; yet it is these very universal and therefore unobserved phenomena which are the most instructive to science. Not only the action of bees and ants and wasps, not only the migrations and the nest-making of birds and the dam-building of beavers, must be accredited to instinct, but also all complex voluntary motions which are performed without experience. Such, for example, in many animals are the acts of running, swimming, flying, walking, and standing, etc. Yes, even the simple act of standing or walking is really a marvelous feat in balancing—requiring the nice adjustment and perfect coördination of perhaps a hundred different muscles. Even the simple voluntary act of sight (looking) requires the most exquisite adjustment of the optic axes, the lenses, and the iris. These complex actions are acquired by us by experience., though there is doubtless also, even in us, a large inherited element, an inherited capacity by which we acquire them with comparative facility. But the new-born ruminant quadruped or gallinaceous bird stands and walks and uses the eyes at once, without experience. The power to coördinate these muscles, and to accomplish these complex and difficult actions, is wholly inherited, not acquired.
Thus defined, intelligence and instinct are not mutually exclusive, as some seem to suppose: the one is not simply a characteristic of man and the other of animals, but they coexist in varying relative proportions throughout the animal kingdom. As a broad general fact, in going down the animal scale we find that instinct varies inversely as intelligence. The accompanying diagram expresses in a general way this relation. If the line a b represents the animal scale from man down to the lowest radiate, and upon this as absciss we erect ordinates representing the degrees of intelligence, then by connecting these ordinates we develop what might be called the line of intelligence. This line, as is seen, rapidly descends from the higher to the lower races of man, then makes a sudden fall from the lowest races of men to the highest species of monkeys, and thence gradually descends until the ordinates of intelligence become insensible, though they probably still exist down to the lowest point, b. If in a similar way we construct a line of instinct, it would probably rise as we pass down through the lower races of men and through the animal scale, reaching its maximum about the middle of the series among insects, and again declining to the end. If, however, we were to construct a third line representing the relative amounts of these two, i. e., the proportion of instinct to intelligence, it would probably be a continuously rising curve something like the dotted line in the diagram. The quotient of instinct divided by intelligence, of acquired wisdom divided by inherited wisdom, constantly increases as we go down the scale.
Such, then, is the nature of instinct, and such its general relations to intelligence. But the most important question still remains. How was this wondrous faculty acquired? Whence did it come? How is it derived? In a word, what is the true theory of the origin of instinct?
The Origin of Instinct.—The old theology disposes of the above question, as she does so many others, in the most summary way. According to her, instincts are not acquired or derived at all. They are miraculously given in perfection to the first individuals of the species, to each species its several kind. But this explanation cannot satisfy Science. It simply places the question beyond her domain..To science Nature is a continuous chain, and her mission is to recover every link. To her a true explanation of any phenomenon consists in connecting it with other phenomena most nearly allied to it. A scientific explanation or theory of instinct must connect it with intelligence on the one hand and the lower phenomena of the nervous system on the other—must show how all these several capacities are evolved the one from the other—must bring them all under the universal law of evolution.
This, it is admitted, is no easy task. The wonderful instincts of some animals have always been regarded as one of the greatest objections to the theory of evolution. The origin of instinct is reckoned one of the hardest nuts for evolutionists to crack. The subject is indeed an obscure one, but recently some light begins to break. The task is indeed a hard one, but I believe we begin to understand in what direction, at least, we must work. The question is yet far from solved—we are yet in much perplexity, but I think we hold the thread which must eventually lead us out of this labyrinth! I have thought much for many years on this subject, and I now give you the views which have gradually grown up in my mind. Others, I observe, perhaps nearly all evolutionists, are thinking in much the same direction, but I have not yet seen any distinct presentation of the subject.
The movements of the animal body, you will remember, are divided into two great groups, the voluntary and the involuntary or reflex. But between these extremes there are undoubtedly many intermediate terms connecting them. Thus is it in all our science, and still more in our systematic teaching of science. Our distinctions are far more trenchant than the distinctions in Nature. It must and ought to be so, for we must get firm hold of the types first, and then we are prepared to study the intermediate gradations. Of the intermediate terms in this case there are two which are quite distinct. Including the extremes, therefore, we have four kinds of animal movements:
1. The perfect voluntary movements.—These require the full, constant, and immediate exercise of the will; and, when the movement is complex, requiring in addition the whole thought and attention fixed, often painfully fixed, on the movement. In this category are nearly all movements when accomplished for the first time.
2. Habitual movements.—These are semi-volitional. They are removed from thoughtful attention, from immediate and painful effort of the will. A general superintendence only of the will is necessary. When any thing goes wrong the mind takes cognizance and corrects it by direct act of the will, and the movement falls, for the time being, into the first category; but otherwise the thoughts and attention may be directed to something else. These are, therefore, to some extent, automatic. Such are, in man at least, the movements in walking, flying, swimming, speaking, playing on a musical instrument, etc. These were, in all cases, at first movements of the first kind, but fell into the second category by repetition. They are acquired, therefore, wholly by individual experience.
3. Instinctive movements or acts.—These are still farther removed from the category of the first group. They are removed, not only from thoughtful attention, but also from individual experience. If we compare them with habitual acts, they are inherited habits. They are evidently the result of inherited brain-structure, but they are not yet wholly removed from the sphere of consciousness and will. Such are the actions of bees and other insects already described.
4. Lastly, Reflex movements.—These are wholly automatic. They are wholly removed not only from thoughtful attention and individual experience, but also from consciousness and will. These are therefore the extreme type of movements determined with the greatest precision by inherited structure of the nervous centres. Such are the movements of the heart, the stomach, the intestines, etc.
Now, of these four kinds of acts, 1 and 2 and 3 are evidently formed the one from another, i. e., 2 from 1 and 3 from 2. The fourth I cannot account for in a similar way, for it must have preceded all the others. And this convinces me that there is yet a higher philosophy on this subject which I have not reached.
Formation of Habits.—We are all familiar with this process. A movement or series of movements at first painfully difficult, and requiring the whole thought and attention, by repetition become so easy and semi-automatic that attention is no longer necessary. The most remarkable examples of these, such as walking, speaking, and the like, probably belong partly to the third category; the capacity for these is partly inherited. Playing on a musical instrument is therefore a better example. We all know the painful attention necessary at first, and the ease and rapidity of the most complex movements attained by practice. Now, by what means, anatomical or physiological, do these at first difficult movements become by repetition easy? The answer in general terms seems to be this: Every volitional act is attended with a change in the brain, which, however, is slight, liable to be effaced by subsequent changes, and therefore evanescent. If the same act, however, be repeated many times, the change becomes deep and permanent—becomes petrified in brain-structure; and this structure, whatever be its character or its seat, determines the appropriate acts with precision. It is as if every volitional act produced a faint line, liable to be erased, on the tablet of the brain; by running over the same lines many times, these are deepened into grooves and finally into ruts, and motion in these becomes easy and certain because the ruts guide the motion instead of the will. Thus repetition produces structure and structure determines habit.
Formation of Instincts.—The structure produced by repetition of voluntary acts, and which, as we have seen, determines habits, by the law of inheritance is transmitted in a slight degree to the next generation. I say in a slight degree only, because inheritance is from the whole line of ancestry and not from the immediate parents alone. The inheritance from the immediate parents is greater, it is true, than from any one of the series of previous generations, but infinitely less than the sum of inheritances from all previous generations. The structure may be regarded, therefore, as transmitted in an almost effaced condition. If the same acts are not repeated, the lines of structure are soon wholly effaced by new lines running across the tablet in all directions; but if they are repeated the same lines are deepened with greater ease and certainty than before; the structure becomes still more decided, the habit still more fixed. This more deeply-engraved structure is again partially transmitted to be again strengthened in the next generation—the engraved plate is retouched and the lines deepened. Thus with every generation the sum of inheritance becomes greater because from a greater number of preceding generations; with every generation the effacement by transmission is less, and the deepening by repetition is greater, until finally a highly-differentiated structure is formed, and perfectly transmitted—a structure with lines so deep as to determine the direction of conduct with the greatest certainty. Then habit becomes instinct. The individual no longer forms the structure, but inherits it ready formed. The actions are no longer learned by practice, they are already predetermined by the inherited structure.
We see illustrations of this process in the artificial formation—the deliberate manufacture—of instincts in domestic animals by human training and human selection. We know that the instincts of the pointer and the shepherd's dog have been formed in this way. The great ancestor of all the pointers, before he was a pointer, was trained with much coaxing and many beatings to do certain things. The result was doubtless any thing but satisfactory. Still a habit was formed, and, as we must believe, a corresponding brain-structure. The pups of this dog were again trained, still with difficulty, but with less difficulty than before, because the habit-structure was partially inherited. The best-trained of this generation are selected, and their pups again trained. The process is still easier, because the habit-structure is more completely inherited, and the result is more satisfactory, because the structure is more decided. Thus the improvement goes on from generation to generation, until finally, in the purest bloods, i. e., those having the longest line of well-trained ancestry, without mixture with effacing bloods, little or no training at all is required; the habit-structure is almost perfectly transmitted. Perhaps in this case transmitted habit never becomes perfect instinct; probably the best-blooded pups still require training. But this is because the process has not been continued long enough, the breeding has not been true enough, and the selection careful enough.
Now, if pointers or shepherds' dogs should become wild, their instincts would quickly be destroyed by natural selection, because they are not useful, but, on the contrary, hurtful, in the wild state. But, suppose they were useful in the struggle for life, then the habit thus acquired would be transmitted, and become strengthened with every generation, until it would become as perfectly fixed and invariable as any, even the most perfect instinct.
Now, it is precisely in this way that the wonderful instincts of bees and ants and the wonderful instinctive coördination of muscles in ruminants and gallinaceous birds have been formed, except that in these cases natural training and natural selection have operated instead or human training and human selection. The great ancestor of all the bees, before the distinctive characters of the bee yet existed, was doubtless destitute of the wonderful instincts which we now find. These have been gradually formed and improved from generation to generation through many hundred thousands of years.
It is difficult to imagine, much more to express, all the steps of this process. I will, therefore, illustrate it in the following manner: We have seen that wise conduct is a product of intelligence and experience. Evidently, therefore, great wisdom may be attained even with small intelligence, if only the experience be proportionally great. Wisdom increases with experience without limit, if only the plasticity of the brain, or its capacity to receive and retain impressions, remain unimpaired. Now, suppose a number of the ancestors of the bees many hundred thousand years ago, before these specific instincts were developed; suppose, further, that these individual insects had continued to live from that time to this, and retained their brain-plasticity unimpaired. Even with the smallest modicum of intelligence, such instincts would, by experience, slowly improve their habits from year to year, from century to century, from millennium to millennium, until they would reach a surprising skill in accomplishing the most complex results. This would be habit, not instinct. The habit so long forming, so useful, and therefore so invariable, would of course be embodied in a very decided brain-structure. Now, precisely the same result is far more perfectly reached by the experience of many generations transmitted and accumulated by the law of inheritance. I say more perfectly, because of the natural selection of only the fittest in each generation.
Thus we see that instinctive wisdom is also the result of experience, but it is ancestral, and not individual experience. Individual experience is first fixed in habit, and then habit is transmitted and petrified in instinct. In a note published in the Philosophical Magazine, April, 1871, I speak of instinct as "inherited experience." I did not then know that I had been anticipated by a few months by Hering ("Archives des Science," February, 1871), who calls instinct "inherited memory." These are but different modes of expressing the same idea. Intelligence works by individual experience treasured in memory; instinct by racial or communal experience treasured in inherited structure. But memory is evidently the result of brain-structure formed by experience; therefore also is instinct inherited memory. Again, knowledge is remembered experience; therefore is instinct also inherited knowledge. Thus experience, memory, knowledge, things which seem to us so indissolubly connected with individual identity, are also sometimes inherited.
Thus, then, the sum of experience and the mental wealth which is accumulated by experience consists of two parts, individual and inherited. In man the individual acquisition is large, and the inheritance is comparatively small. In the lower animals the individual acquisition is small, while the inheritance is large. In bees the wealth is almost wholly inheritance.
We now easily see why intelligence varies inversely as instinct—why high intelligence seems incompatible with remarkable and invariable instinct. It is because, with high intelligence, actions are so varied, in different individuals and in different generations, that it is impossible that their results should accumulate and become petrified in structure. But, in the lower animals, the conditions of life are narrow, the habits necessary for successful struggle for life run in few lines, and these lines become deepened with every generation, until they become, as it were, petrified in brain-structure.
Instinct, therefore, is accumulated experience, or knowledge of many generations fixed permanently and petrified in brain-structure. All such petrifaction arrests development, because unadaptable to new conditions. They are found, therefore, only in classes and families widely differentiated from the main stem of evolution, from the lowest animals to man. Instincts are, indeed, the flower and fruit at the end of these widely-differentiated branches, but flowering and fruiting arrest onward growth.
Now, there is also a social evolution. The organic evolution, which found its term in man, is continued by man in social evolution. It is natural, therefore, to look for the corresponding phenomenon in the higher sphere of social evolution. I believe we find it in the phenomenon of arrested civilizations, of which nearly all barbarous and semi-civilized races are examples, but the Chinese and Japanese are the most conspicuous; and also, perhaps, to some extent, in the phenomenon of dead civilizations, of which the Greek and Roman are the most conspicuous. Nations isolated and breeding true, i. e., without mixture with other nations, gradually assume fixed customs and habits which become enforced, and therefore perpetuated by law, and finally petrified in national character. The result is often marvelous development, but extremely limited. Here, again, perfect flower and fruit destroy growth. Here again, also, it occurs in a type or branch widely differentiated from the main stem of social progress. This explains one of the advantages of cross-breeding, or mixing of varieties within certain limits of national varieties, if not of races. It confers plasticity; it prevents the formation of fixed national character, and the consequent arrest of progress by petrifaction.
Let us hope, then, that the growing tree of society will always remain an excurrent; that its upshooting stem shall never lose itself in mere branches; that its terminal bud shall never fail, but always continue to grow. Its branches may flower, and fruit, and die, or cease to grow, but the trunk stretches ever upward and bears each successive flowering branch higher and still higher. Doubtless the ideal of humanity is that all right actions are spontaneously or instinctively performed, and all important truths intuitively or instinctively known; but this is and must be an unattainable ideal; for, this condition reached, how shall we any longer aspire?—the terminal bud flowering, how shall the tree continue to grow? Human nature must never petrify into instinct; inherited wealth must never supersede the necessity of individual acquirement.
- A Lecture to the Class in Comparative Physiology in the University of California.
- "Instinct in New-born Chickens." Naturalist, vol. vii., pp. 300, 377, 384.
- As we are not dealing here with measurable quantities, of course I do not use the expression "varies inversely as" in a strict mathematical sense.
- The effect of mixing varieties requires careful investigation, for it is yet very imperfectly understood. There seems little doubt, however, that there is a limit beyond which varieties do not mix with improvement.