XV THE INFLUENCE OF THE ENVIRONMENT UPON MAN

1. The way has now been cleared for a consideration of the influence of the environment upon man. The relation of the ancestors of man to the environment must have been the same as that between any wild species and its environment. In the course of his history man has moved away from this position until his relation to the environment has become so different from that described for other animals and plants that we can no longer speak of a normal environment. This has not come about because man is not subject to the same laws as are other organisms. It has come about because his relation to the environment has been modified in many ways.

The conception of the normal environment involved the idea that, much as the different elements composing the environment might vary, there was some more or less clearly defined limit to their variations. In one sense this remains true for man; but the variations are so much greater in degree and in kind that there is a clear distinction between the conditions obtaining among civilized races and those obtaining among any wild species. Let us glance for a moment at what has happened. It is obvious that man has varied his surroundings in every respect, not only with regard to what we may call external circumstances, but also with regard to nutrition and to customs and habits that we sum up under the name of use. The most obvious changes in external circumstances are those connected with the spread of man to every corner of every continent. Man has become subject to every extreme of heat and cold, humidity and aridity, of barometric pressure and all that goes to make up climate. There is in addition a vast mass of artificial influences, due in the first place to various methods of protection adopted against climate. Every description of house is known, involving all degrees of access to fresh air. Dress again varies almost infinitely—the variations all being of possible influence upon the development of mental and 337physical characters. The aggregation of men into towns involves exposure to smoke, noise, and vibration among other factors. Modern industrial conditions in particular expose workers to varied surroundings. Human diet has become equally varied. The cooking of food was an innovation involving great changes in the factors which play upon the digestive organs. Innumerable animals and plants have been drawn upon by man as food. Perhaps more important than the variations in what man eats are the variations in what he drinks. Under the heading of use come all those habits such as reading, washing, smoking, and shaving. Various occupations bring with them various degrees of muscular activity or involve its reduction to next to nothing. There are various modes of riding and various ways of sitting. Lastly the prevalence of disease has introduced another factor which has a profound influence upon mental and physical characters.

This varying of the environment has come about gradually, slowly at first and with increasing speed latterly, until at the present day, of four men having their homes in the same town one may do clerical work involving no exercise, another may labour in a cotton mill where it is warm and moist, a third may perform hard physical labour in a mine in semi-darkness where the air is full of a particular kind of dust, and a fourth may work on board ship exposed to all the rigours of the Atlantic. To such differences may be added all the differences between meat-eaters and vegetarians, smokers and non-smokers, alcoholic drinkers and abstainers, and so on. Contrast the variations in the environment of modern man with the variations in the environment of any species in a state of nature and it will be apparent why it was said above that the relation of man to his environment was clearly distinguishable from that of any species in a state of nature to its environment.

We are about to consider in this chapter the influence of the environment upon the physical basis of life; we shall be concerned, in other words, with its function as the complement of the germinal constitution. The discussion will be limited to the notice of such factors as are in operation; of the possible effects of the environment there is no need to speak. From two other points of view also the environment is of importance; as a factor in selection it will be considered in Chapters XVII and XVIII; as the subject-matter, 338so to speak, upon which mental processes work, it will be considered in Chapters XIX and XX. It is important to distinguish the influence of the environment in this latter respect from the sense in which it is considered in this chapter. The failure to do so has vitiated many contributions to the subject. It is felt that the environment is somehow of great influence in social life and it is not realized that, if in the respect in which it is considered in this chapter we find reason to conclude that its influence is not great, there is another field in which its influence may be established.

When considering the influence of the environment on man, it is convenient to distinguish between physical and mental characters. Mental characters may be considered under the head of the intellect, the disposition, and the temperament. It must, however, be clearly understood that this is merely a distinction convenient for our present purpose which is not based upon, and does not imply, any fundamental distinction. Mental characters, such as the instinct of gregariousness, and physical characters, such as head form, are for our purpose merely characters resulting from the play of certain stimuli upon a certain germinal constitution.

2. It is remarkable how little on the whole is known with regard to the influence of the environment on man. For the most part we have to depend upon observations as distinguished from experiments. Though a few experiments have been made, some deliberately but others accidentally, we are for the most part in the position with regard to man that we should be with regard to other animals, had we only such observations to go on as those quoted respecting the size of marine molluscs, the variation in the coat of the tiger, and the changes in the sparrow and the periwinkle when introduced into America. Though such observations may strongly suggest certain conclusions as to the part played by the environment, no definite or precise results can be reached.

We may first consider some evidence bearing upon the influence of particular factors such as exercise and climate. We may disregard those rare cases of extreme modifications deliberately induced, such as the distortion of the shape of the head or of the feet by pressure. These extreme modifications approximate to mutilations such as amputation of the finger-joints or the knocking out of the incisor teeth.

339It is well known that exercise has considerable effect upon the development of the muscles. It is also well known that there is a clearly defined point for each individual beyond which exercise will produce no further effect. Though exercise has a considerable effect upon other organs, especially certain internal organs, it is probable, not only that it has more effect upon the muscles than upon any other system of organs, but that the effect upon the muscles is relatively at least as great as, if not greater than, the effect upon any organ of any other factor that we shall consider except disease. Owing to differences in the amount of exercise the bodily development of man in a modern community varies very considerably; owing to such modifications clerical workers differ from blacksmiths, whether or not there are also germinal differences. So too owing to differences in habits men of one race differ from men of another race. Darwin, for instance, refers to the thin legs and thick arms of the Payagua Indians who spend a large part of their lives in canoes.^[1555] It is certain that all such physical differences are not due to germinal differences and that environment in the shape of use is in part the cause. The position is similar with regard to certain peculiarities observable among races that have adopted unusual modes of squatting.

Little is known as to the effects of use upon mental characters. To some degree no doubt the intellect is developed by use, though, perhaps, the degree to which this is so is apt to be exaggerated in popular estimation owing to the fact that modes of thinking acquired through education add to the efficiency of the faculty, which result is mistaken for the effects of use. That the effects of exercise are not great is shown by the experience of the Workers’ Education Association. This institution gives advanced courses to men often of middle age who have received but little intellectual training in early life. It does not appear that the strength of their intellects is much less than it would have been had they received a university training. It seems at any rate certain that the differences in the amount of use now obtaining between the various professions and classes in England have less effect generally upon mental than upon physical characters. Given, for example, two men of equal intellect, one of whom received the best educational training of the day, and the other of whom received the training given to the working classes, it does not 340seem that at the end of the training there would be anything like the same difference as regards strength of intellect between them as there would be between two men, one of whom received an athletic training and the other of whom did not.

3. Physical changes are not infrequently observed to follow upon changes in the environment, though, when these changes are complex, it is often impossible to say with which features of the environment the changes are connected. Thus ‘the Anthropological Committee of the British Association long ago showed the beneficent effects of the Factory Acts, which rescued young children from the hardships of daily toil. Boys of nine years in 1873 had a height and weight equivalent to the height and weight of boys of ten years old in 1833.’^[1556]

It is known that stature has increased in certain European countries during the last century. Soren Hansen gives the following figures for Denmark:^[1557]

1852 to 1856 165·42 cm. average height.

1879 „ 1888 167·78 „ „ „

1891 „ 1900 168·43 „ „ „

1904 „ 1905 169·11 „ „ „

Similarly the stature of the Dutch has increased from 165·5 cm. in 1866 to 167 cm. in 1883 and to 168 cm. in 1899. A number of reasons has been suggested to account for this marked increase, the higher standard of living, the decrease in the incidence of disease which is known to inhibit growth, and the smaller number of children in a family. This last factor is certainly of importance at the present day. From some observations made in an English manufacturing town Ewart concludes that when children are born at a longer interval than two years they are on an average three inches taller and three pounds heavier than children born at a shorter interval.^[1558] It is a fair assumption that in families, where the income is small, the fewer the children the more favourable would be the environmental conditions.

The physical inferiority of people living in poorly endowed surroundings is often thought to be at least in part due to modifications induced by hard conditions. Thus physical inferiority is noticeable in districts in Europe that are markedly poorly endowed, as for instance in the area between Limoges and 341Périgueux in France. So far as this is so, the causes may have to be sought in climatic differences as well as in other differences more directly comparable with those which exist between well- and ill-treated children in the same country.

4. The influence of climate upon man has long attracted attention. Statistical evidence has been produced showing that the shape of the head—a characteristic usually considered not to be in any way susceptible to such differences in the environment as occur between one climate and another—is modified by climate. Boas produced figures showing that the cephalic index (a measure obtained by calculating the relation of breadth of the head to the length which is put at 100) of Sicilians born in America to be 80, while that of Sicilians born in Sicily is 78, and of Hebrews born in America to be 81, while that of Hebrews born in Eastern Europe, whence the immigrants came, is 83. It would appear that the Hebrews who are broad-headed in Europe become narrower-headed in America, and that the Sicilians who are narrow-headed in Europe become broader-headed in America. There would thus seem to be an approximation in America to a cephalic index the mean of which lies between 80 and 81.^[1559] These results have been severely criticized from many points of view. Sergi claims to have shown that the results are the ‘pure effect of illusion due to the statistical methods employed by the author’.^[1560] It has also been suggested that the results are due to selection. In this connexion it is interesting to note that both Ammon and Levi obtained somewhat similar results and that they both attributed them to selection. The former, working with figures from Baden, found that the inhabitants of cities tended to become longer-headed and concluded that the short-headed type tended to die out under the conditions of city life.^[1561] Levi, working on Italian figures, came to the same conclusion and attributed it to the same cause.^[1562] Others have suggested that the methods of nursing children may affect the shape of the head and that the changed habits of the immigrants in America may account for the changes in the shape of the head. Though it has certainly not been proved that the environment can in the manner suggested change the shape of the head, the question cannot be regarded as settled.

342Observations have been made on the cephalic index of Jews which may perhaps be held to lend support to the views of Boas. Huntington gives the following table:^[1563]

Country. Cephalic Index of Jews. Cephalic Index of other Races. Difference

Caucasus 87·5 87·4 0·1

Galicia 83·6 84·4 0·8

Baden 83·5 84·1 0·6

Little Russia 82·9 83·2 0·3

Turin 82·4 84·9 2·5

Lithuania 81·7 80·6 1·1

Russian Poland 81·9 80·9 1·0

White Russia 80·9 82·5 1·6

It would appear that there is a tendency for the cephalic index of Jews to vary as does that of the surrounding types. Fishberg has attributed this fact to intermarriage^[1564] and this may be in part at least the explanation, though in the present state of our knowledge the possible influence of the environment cannot be altogether excluded.

The uncertainty surrounding the matter is in fact a good example of the state of our knowledge regarding many similar problems affecting man.^[1565] Boas himself, it may be noticed, only believes in a ‘strictly limited plasticity’^[1566] of head form; it is indeed evident that this and other physical characters which distinguish the races of man are for the most part of the nature of mutations and not of modifications. Europeans who migrate to tropical climates and the inhabitants of tropical climates who come to live in Europe retain the greater part of their distinguishing physical characters.

The manner and degree in which tropical climates influence Europeans is a matter of considerable interest. There is as yet but little known on the subject. Sir Patrick Manson, writing in 1907, said that ‘although many attempts have been made to trace and explain the effect of temperature on the physiological processes of the human body, more especially in reference to the pathological proclivities to which atmospheric heat and cold may 343conduce, it cannot be said that any important conclusions have been attained.... But though we may not be able to indicate precisely the way in which our bodies are physiologically affected by extremes of atmospheric temperature, especially prolonged high temperature, our sensations, the loss of physical and mental energy, the modification of physical characteristics undergone by white races when placed for several generations in tropical conditions, and the dark skins of all tropical races indicate that the white races on first arrival are not in all respects adapted for tropical conditions, that they are somehow prejudicially affected thereby, and that while living in tropical countries they are more open to certain pathological risks than are the natives of those countries.’^[1567] Since the year in which Sir Patrick Manson wrote further information has been obtained on this subject. Our knowledge is, however, still scanty. With regard to actual facts it is known that the pulse, rate of breathing, and temperature of the body do not vary when measured in Europe and when measured among white men under tropical conditions. The number of red blood-corpuscles and the amount of haemoglobin in the blood is the same and metabolism is not less intense. On the other hand the rate by which a nervous impulse travels along a nerve decreases in Europeans the longer they live in the tropics. The muscles and the connective tissue become more elastic. The well-known pallor of Europeans living in the tropics is due to a thickening and softening of the epidermis, which becomes opaque.^[1568]

It is very difficult to arrive at any conclusions as to what the effects of a tropical climate upon Europeans really are. It is necessary to discount the effect of tropical diseases and of the habits and customs of Europeans living in the tropics. On the whole it is probable that the popular notion of the considerable and generally injurious nature of the modifications undergone by Europeans living in the tropics is exaggerated. It is said that, for example, in Java, when sanitary conditions are good and reasonable habits adopted, the death-rate among European children is less than in Europe. Nevertheless a tropical climate does have an injurious effect upon Europeans. There is no doubt that Europeans in the tropics are more irritable and in general 344more highly strung than in their native land. Clearly in some fashion the nervous tone is injuriously affected by residence in tropical climates. In a similar manner nervous tone is affected by many elements in the environment in civilized countries, such as vibration, noise, and so on, leaving aside the effect of food, drink, and disease. To the importance of disease in this respect we shall return later. As regards noise, vibration, and so on, but little is known—their influence being possibly considerably greater than is usually suspected.

5. Ellsworth Huntingdon has in recent years in a number of publications elaborated a theory according to which climate has been one of the main factors in determining where civilization shall develop and flourish. As this theory depends upon the supposed direct influence of the environment on man, it may be noticed here. He has made observations which are interpreted as showing that there are optimum climatic conditions under which the maximum energy is exhibited. These conditions arise when the average temperature of day and night together lies between 58° F. and 71° F. and when there is a certain degree of moisture. His observations, made in America, were based on the output in piece-work factories and on similar data and tend to show that not only do all European races, including the Finns, display most energy under these optimum conditions, but also the Japanese and the negroes. He then proceeds to show that where these optimum conditions prevail in the world, there to-day are to be found the highest forms of civilization. Upon these data he raises a very far-reaching theory to the effect that throughout history civilization has arisen and flourished only where there has been an approximation to these climatic conditions. To the obvious difficulty that former civilizations have often flourished in countries the climate of which at the present day is far from these optimum conditions, he replies that climate has changed, a theory which he has for some years strongly advocated.^[1569]

It may be said that there is nothing novel in the idea of optimum climatic conditions.^[1570] In the previous section it was pointed out that Europeans in tropical climates suffer from 345injurious mental disturbances. What is remarkable is that the optimum conditions for negroes should be the same as for Europeans. It will require more proof than has yet been advanced before this can be accepted. Further, the changes in climate which the theory demands have not been proved. Professor Gregory has reviewed the question and his conclusions do not support those of Huntingdon—at any rate not in such a manner as to render the theory tenable.^[1571] Nevertheless, whatever the fate of the theory in its present form may be, its enunciation has raised many interesting questions and has incidentally helped to show how little we know at present regarding the effect of the surroundings on man.

6. When men move from one climate to another they come under the influence not only of changes in temperature and moisture but also of food and sometimes of altitude and other factors about the effects of which there is a considerable amount of information. A vegetarian diet is said to produce changes in the gut; but changes in beverages are probably of far greater importance than changes in food. The effects of alcohol have been closely studied, chiefly with regard to its influence upon nervous tone. Nervous tone is affected in an important manner by many drugs, as for instance by opium, and in a lesser degree by tea and coffee. Changes in nervous tone are of such importance that its susceptibility to various influences has to be borne in mind. It is quite possible that the introduction of a new form of beverage into a country might have a perceptible effect upon the average condition of nervous tone and thus have a bearing upon the course of history.

Altitude is known to have various effects upon physical characters. The fact that the larger lung capacity of those who live at high altitudes diminishes on descent to the plains, as recorded by Darwin of the Quicha Indians, is evidence that this character is in part at least environmental.^[1572] The effect of high altitudes has lately been studied in much detail. It is known that there is among other changes an increase in the number of red blood-corpuscles in the blood.^[1573]

3467. Attempts have been made to obtain more precise information by the use of statistical methods. Thus the correlations have been measured between the state of children’s eyesight and fifteen environmental conditions. The mean of the correlations was found to be 0·04—only one reaching 0·1.^[1574] Again the association between various intellectual and physical characters and conditions, which were taken as representing a good or bad environment, has been measured. A slight association was found between intelligence in boys and few people per room and no association between eyesight, condition of glands and hearing, and bad economic and moral surroundings.^[1575] Somewhat different results have been reached by American workers, who find that by employing psychological tests a fairly well-marked difference can be detected between children in the same school whose parents belong to different social classes and who would therefore be subject to different home conditions.^[1576]

The interpretation of these results is difficult. Before any definite conclusion could be reached with regard to any one character, it would be necessary to measure the effect of every factor in the environment upon that character. As the matter stands there is strong but not conclusive evidence of the small influence of the surroundings, so long as we suppose that innate differences do not exist between the subjects measured. But if innate differences exist, then the interpretation of the absence of any marked degree of correlation must be that the common elements of the environment supplied by the community are of greater importance than the innate differences. We shall find reason to conclude in later chapters that small innate differences do exist, and, if this is so, then the interpretation of these results is not at variance with the general, though necessarily vague, conclusions derived from the evidence previously given, i.e. that the influence of the environment as represented by the variations actually occurring in the elements of the surroundings hitherto mentioned is small.

8. There is another class of factors which may be summed up under the heading of disease. It was remarked that disease may produce notable results among species in a state of nature. 347Compared, however, with the state of things among men, and especially in the later stages of history, disease is rare among such species. Among men it comes to assume a peculiar importance.

Disease results both from the attacks of parasites and from other causes. The various classes of disease will be referred to in the next chapter. We are here concerned only with the results, and we may think of the disease as affecting particular organs in the body and as affecting the general functioning of the organs. Every organ in the body is liable to be attacked by disease and the modifications produced are in many cases familiar. Thus all physical and mental characters may be directly and to almost any degree modified by disease. Again it is known that the result of disease in children is to inhibit growth, and that the growth thus lost is not subsequently made up. Disease may thus be said to draw upon the capital and not upon the income of children.^[1577] We may here confine ourselves to some notice of the effect of disease on the general functioning of the bodily organs which though not so familiar is more important from our point of view.

The functioning of the bodily organs has been found in late years largely to depend upon certain glands—known as the endocrinous or ductless glands—in a manner and to a degree altogether unsuspected. The thyroid gland, for example, manufactures a secretion which is essential to the proper growth and normal metabolic functions of the whole body. If it is removed from a child, the whole body is stunted and mental deficiency results. Certain maladies, such as goitre, cretinism, and others are known to be connected with a diseased condition of the thyroid. Profound modifications of both physical and mental characters may thus follow when these glands cease to function normally, which is known to be the case in certain specific diseases and may arise in other ways which are not fully understood.

Temperament depends upon the general functioning of the bodily organs and upon the actual condition of the nervous system. Disease, whether it takes the form of a failure of the ductless glands to function as they should, or some other form, always affects the general functioning of the body and thus has a direct bearing upon temperament. Thus ‘we know now’, 348says Mr. McDougall, ‘that defect of the functions of this organ (thyroid) may reduce any one of us to a state of mental apathy bordering upon idiocy, and that its excessive activity produces the opposite effect and may throw the mind into an over-excitable condition verging upon maniacal excitement. Again we know that certain diseases tend to produce specific changes of temperament, that phthisis often gives it a bright and hopeful turn, diabetes a dissatisfied and cantankerous turn. It is clear that in some such cases of profound alteration of temperament by bodily disorder the effects are produced by means of the chemical products of metabolism, which, being thrown out of the diseased tissues into the blood and reaching the nervous system by way of the blood-stream, chemically modify its processes. It is probable that every organ in the body exerts in this way some influence upon our mental life, and that temperament is in large measure the balance or resultant of all these many contributory influences.’^[1578]

Thus in addition to the more obvious direct effects of disease upon mental and physical characters, there are the more subtle and profound effects upon temperament, and in our general summing-up we shall find reason to attribute no small importance to temperament in its influence upon progress. It thus becomes of importance to observe that in particular regions of the world certain chronic and non-lethal diseases are very common. Clearly the result of disease upon temperament is greatest when it takes the form, not of kill or cure—which is on the whole the case in temperate climates—but of chronic non-lethal maladies which are common in tropical climates, especially in Africa. The hook-worm disease is an African disease which has been introduced into America. The hook-worm (Ancylostomum duodenale) is an internal parasite which attaches itself to the lining of the intestine and causes bleeding and anaemia. Death may ensue, but the patient usually lives a long time. It is a most debilitating disease; it is said to affect some 50 per cent. of the inhabitants of some tropical and sub-tropical countries. Even as a mild infection it is the cause of much invalidity and heavy economic loss. The laziness and degeneracy of the ‘poor white trash’, who are said to belong to the purest-blooded English stock in the United States, are due not so much to the environment^[1579] and heredity 349as to the ravages of these parasites. In a public lecture, delivered before the University of Pennsylvania, in 1915, Allan Smith, citing the factors which have caused the Southern States to lag in American progress, mentioned that the hook-worm disease ‘stands with malaria as worse than wars and the devastations of battles and worse than all other pathogenic agencies in combination’. Through the influence of these diseases ‘the men and women of the South, bred from the best colonial stock, offspring of pioneers, with the blood of English gentry and of continental cavaliers in their veins, sank lower and lower in physical degeneration and squalor, were derided and denounced as lazy and shiftless and condemned in popular opinion as worthless and a disgrace’.^[1580]

There are many other tropical and sub-tropical diseases of a somewhat similar nature. Among them may be mentioned the ‘yaws’—marked by various eruptions probably due to a spirochaete—the guinea-worm, which lives buried in connective tissue; bilharzia, a trematode worm which lives in the bladder and infects nearly 50 per cent. of the inhabitants of Egypt; and elephantiasis, due to a worm related to the guinea-worm. The manner and degree in which these various diseases affect the nervous tone varies very much. In general the result is debility and a lowering of the nervous tone. So too malaria is a cause of debility, and the main reason for the decadence of Ancient Greece has been sought in this disease.^[1581]

9. The results of the inquiry so far, though illuminating, are indefinite. This is largely because we are ignorant regarding innate differences. The consideration of the evidence as to identical twins offers a way out of this difficulty. It is known that there are two kinds of twins. There are the so-called identical twins, between whom there is a very close resemblance, and other twins between whom there is no greater resemblance than between any two children of the same parents. It was formerly thought that there was a sharp distinction between the two kinds of twins—identical twins arising from a single ovum, which completely separated into two halves during early development, each half giving rise to one child, ordinary twins arising from the simultaneous fertilization of two ova. If this was so, then identical twins would always have approximately the same 350germinal constitution, whereas other twins would have germinal constitutions as different as on the average are those of the offspring of the same parents. That identical twins do thus at times arise is practically certain, but it has recently been shown that the supposed sharp distinction between identical and other twins does not exist.^[1582] This may be explained by various assumptions. It may be supposed that some ova are binucleate or that some ova are penetrated by two spermatozoa; again the earlier or later separation of the blastomeres may have a bearing upon the position. Under any of these circumstances twins intermediate between identical twins and ordinary twins would arise—twins, that is to say, not having approximately similar germinal constitutions, but germinal constitutions more nearly alike than those of ordinary twins. However this may be, it is sufficient for our purpose to note that so-called identical twins are always more alike in their germinal constitutions than are ordinary twins and may quite frequently have practically similar germinal constitutions. It, therefore, follows that differences between identical twins must be very largely of the nature of modifications. It is thus of great interest to ask in what these differences consist.

Galton collected data regarding thirty-five cases of identical twins. He summarizes the information about them up to the time at which they left the family circle as follows. ‘In a few of these not a single point of difference could be specified. In the remainder, the colour of the hair and eyes were almost always identical; the height, weight, and strength were nearly always so. Nevertheless I have a few cases of a notable difference in height, weight, and strength, although the resemblance was otherwise very near. The manner and personal address of the thirty-five pairs of twins are usually described as very similar, but are accompanied by slight differences of expression, familiar to near relatives though unperceived by strangers. The intonation of the voice when speaking is commonly the same, but it frequently happens that the twins sing in different keys.’^[1583] He goes on to say that ‘both twins are apt to sicken at the same time in no less than nine out of the thirty-five cases. Either the illnesses, 351to which I refer, were non-contagious, or, if contagious, the twins caught them simultaneously; they did not catch them the one from the other. This implies so intimate a constitutional resemblance, that it is proper to give some quotations in evidence,’^[1584] and he proceeds to give detailed evidence. Later he remarks on the ‘similarity in the association of ideas. No less than 11 out of the 35 cases testify to this. They make the same remarks on the same occasions, begin singing the same song at the same moment and so on.’^[1585] With regard to tastes and dispositions Galton says that ‘in 16 cases—that is in nearly one-half of them—these were described as closely similar; in the remaining 19 they were much alike, but subject to certain named differences. These differences belonged almost wholly to such groups of qualities as these: the one was more vigorous, fearless, energetic, the other was gentle, clinging, and timid; or the one was more ardent, the other more calm and placid; or again the one was the more independent, original, and self-contained; the other the more generous, hearty, and vivacious. In short the difference was that of intensity or energy in one or other of its protean forms; it did not extend more deeply into the structures of the characters. The more vivacious might be subdued by ill health, until he assumed the character of the other; or the latter might be raised by excellent health to that of the former. The difference was in the key-note, not in the melody.’^[1586] Galton sums up the evidence as follows: ‘It follows from what has been said concerning the similar dispositions of the twins, the similarity in the association of their ideas, of their special ailments, and of their illnesses generally, that the resemblances are not superficial, but extremely intimate. I have only two cases of a strong bodily resemblance being accompanied by mental diversity, and one case only of the converse kind.’^[1587]

It has to be remembered that up to this period the twins had been reared under very similar conditions indeed; Galton in fact says that they had been reared ‘exactly alike’.^[1588] He goes on to ask what changes were produced when they left the family and went out into the world. He sums up the result of his inquiries into this point as follows: ‘Here are 35 cases of twins who were “closely alike” in body and mind when they were 352young, and who have been reared exactly alike up to their early manhood and womanhood. Since then the condition of their lives has changed; what change of nurture has produced the most variation?... They (the 35 cases) showed me that in some cases the resemblance of body and mind had continued unaltered up to old age, notwithstanding very different conditions of life; they showed me that in other cases the parents ascribed such dissimilarity as there was, wholly or almost wholly to some form of illness.’^[1589] ‘We may, therefore, broadly conclude that the only circumstance, within the range of those by which persons of similar conditions of life are affected, that is capable of producing a marked effect on the character of adults, is illness or some accident which causes physical infirmity.’^[1590] Galton then turns to consider the details regarding twenty cases of unlike twins and he finds that in spite of similar surroundings no growing resemblance can be traced.^[1591]

10. In attempting to sum up our conclusions on this subject, we may first ask what influence is to be attributed to the environment and then ask what bearing such influence has upon the main problem under review in these later chapters. It has always to be recollected that changes in the environment may have the most extreme results. Some examples were given of experiments upon developing animals, and it was shown that in the case of fish, for example, monsters of various kinds can be produced. So too doubtless extreme modifications could be produced in the case of man and are occasionally produced by untoward surroundings and by such customs as those of the distortion of the head and of the feet. What we want to know, however, is not what modifications can be produced, nor what exceptional modifications sometimes arise, but what changes are induced by the variations in the environment which usually occur.

The answer is that, putting aside disease among the factors and leaving out for the moment temperament among the characters, such variations as occur are of little importance. This applies to both physical and mental predispositions and includes the effect of not only such factors as climate and so on but also of such factors as are summed up in a good or bad home environment. 353It is probable that physical predispositions are on the whole more susceptible to such changes as occur than are mental predispositions. There is no reason, for instance, to think that the normal variation of any factor which influences mental predispositions produces as much effect as does use upon muscular development. But it is also further clear that, though we must, keeping aside for the moment questions connected with disease and with temperament, think of such modifications as occur as not of much importance, nevertheless these modifications are of greater importance than among species in a state of nature closely related to man. Consider for a moment muscular development. Side by side in the same street may live a man whose daily work is wholly sedentary and whose muscles are in consequence undeveloped, and a man who works in a mine or engineering shop and develops his muscles accordingly. Nowhere among vertebrate animals in a state of nature are such differences found.

Though but little is definitely known on the subject, it seems that temperament is more susceptible to changes in the environment than are other mental or physical characters. Changes of climate, for example, appear to produce more marked changes in temperament than in any other character. Temperament is certainly very susceptible to the influence of disease, which may also profoundly affect all characters both mental and physical. It is probably in its effects upon temperament that disease has its chief importance. We may even have to recognize in the effects produced by certain tropical diseases a serious hindrance to progress in tropical countries. It is difficult, however, to disentangle from the effects of disease in this sense the effects which it produces in the sense awaiting consideration in later chapters. Clearly the wide prevalence of disease, where such is the case, must form an important factor in the surroundings which the mind has for the subject-matter of its activities—but on this and on allied subjects there will be more to say later.

We have already in what has just been said referred to one kind of modification in its effect upon progress. Upon this subject in general it may in the first place again be emphasized that, so far as is at present known, there are no grounds for believing that modifications of the kind considered in this chapter give rise to mutations. From this it follows that the results 354of modifications are not cumulative. Unless in every successive generation the modification is induced, it will not reappear. Secondly these modifications on the whole tend, as in the case of the effect of disease upon temperament, to be connected with a condition of things that acts as a drag upon progress rather than as a spur to progress. The reason for this is clear in the light of what has been said in the last chapter. It was there shown that the germinal constitution among species in a state of nature has come to be through the action of selection of such a kind that under certain stimuli it gives rise to a certain form. This form is that which is best adapted to meet the normal features which characterize the niche in the organic world which the species occupies. Large modifications only become apparent when the species or certain members of it are no longer subject to their normal environment. To meet the new surroundings a somewhat different germinal constitution would give a better response. In other words the most satisfactory conditions are those under which considerable modifications do not arise.

Lastly we may again refer to a point that has already been alluded to. Great stress has been laid by certain authors, for instance by Professor Ridgeway, upon the influence of the environment upon man. Ridgeway in one passage speaks as follows: ‘My argument was, and is, that as the ice sheet receded, man passed upwards from the south or south-east into Europe, and settled in the three southern peninsulas, gradually spreading northwards over the Alps and extending eventually up to the Baltic. As they gradually spread upwards, under the influence of the environment (and in the environment I, of course, include food), they grew less dark, those of them who settled permanently along the axis of the Alps tending to have shorter skulls, while those who had passed upwards earliest became the most blonde and tallest people in the world.’^[1592] It is clear from other passages that Ridgeway is including two very different things under the phrase—the influence of the environment. He is thinking of the production of modifications such as have been studied here, and also of the elimination of certain types under the influence of the environment. He does not make it as clear, as is perhaps desirable, to which of these two factors he attributes most importance. But what is more relevant to our argument is that to include 355the results of selection under the heading of the influence of the environment is somewhat misleading. Selection is a wholly different matter and, unless it is carefully distinguished from other factors, confusion of thought results. The effects which follow from elimination owing to climate and food should not be summed up under the heading of the influence of the environment; in this book they are considered in Chapters XVII and XVIII.

356