XVI HEREDITY IN MAN

1. Having touched upon the influence of the environment in producing modifications, we have now to approach the subject of the selection of characters, both mental and physical. It is proposed to consider in this chapter what it is that is inherited, and in the two following chapters to consider the results of selection.

What is meant by saying that any character is inherited is clear from former chapters. There are certain predispositions in the germinal constitution of every individual—predispositions to give rise to certain characters under certain conditions and to other characters under other conditions. There is a predisposition, for instance, in the species of primrose referred to in Chapter XIV to give rise to red flowers at a certain temperature and to white flowers at a different temperature, the colour of the flower being equally inherited in both cases. We may therefore take as our starting-point the fact that there are in the constitution of every individual many such predispositions.

From the results of breeding experiments upon Mendelian lines certain conclusions have been reached which carry us farther. These experiments have shown that certain unit-characters can be isolated which behave in a certain manner when crossed, and from the manner in which they behave it is deduced that they depend upon unit-factors in the germinal constitution which are invariable. The number of unit-characters which have been detected in any one species is not large; it is true that in one species of insect about two hundred unit-factors have been found, but even this number can only form a small proportion of the total number if it is the case that the germinal constitution consists entirely of unit-factors which on crossing give the Mendelian ratio. This is the working hypothesis which is put forward as the explanation of inheritance, and which it is the object of investigators now to test. We may consider for a moment what it is that this hypothesis implies.

357We have to think of the germinal constitution as containing a very large number of factors—predispositions in other words. The characters to which these factors give rise may be the characters which we see when we examine any individual, but more often they are not. There are, for instance, cases in which the colour of a flower or of the coat of a mammal are unit-characters but more often such visible characters are the product of two or more unit-factors. There are many cases now known in which the visible character is in this fashion due to the presence of several unit-factors. With regard to this hypothesis in general all that can be said is that at present no facts are known which are definitely in contradiction to it. It is the only hypothesis which holds the field.^[1593]

Up to the present very few unit-factors have been distinguished in man. It has been found that brachydactyly—a peculiar malformation of the hand—presenile cataract, tylosis—a thickening of the hands and of the soles of the feet—epidermolysis bullosa—a blistering of the skin—and night blindness behave as simple Mendelian characters. So too does eye colour, pigment in front of the iris being dominant to its absence—in other words, brown, green, and hazel being dominant to pure grey and blue. There is also some reason for thinking that musical ability is a recessive character. On the hypothesis outlined above it must be supposed that most of the visible characters of man are the product of several unit-factors which have not yet been identified. What appears to happen when matings take place between people of different colour, stature, and so on is that there is a blending of characters in the offspring. Such appearances are not incompatible with the hypothesis—the assumption being that many unit-characters are concerned.

This question as to the ultimate nature of inheritance has been introduced here because of its inherent interest. It is not as a matter of fact strictly relevant. What we require to know is rather what predispositions are present in the germinal constitution. It is this which is essential; the precise manner in which the predispositions are represented in the gametes, whether by 358unit-factors or not, is not essential to the present inquiry. We require to be able to give some answer to such questions as whether disease, intellect, and temperament are represented in the germinal constitution or not, and we can find an answer to these questions without concerning ourselves with the ultimate nature of inheritance.

2. Since studies of inheritance on Mendelian lines do not at present enable us to say what human characters are inherited, we are dependent for our knowledge of this subject chiefly upon biometry. The method pursued by biometricians is as follows. The particulars with respect to any character—say stature—are noted both for the parents and for the children. The average degree of resemblance between parents and children can thus be measured and expressed numerically. If the resemblance was complete, if, that is to say, in respect of any character children exactly resembled their parents, the fact would be expressed by saying that the correlation was equal to unity. When the resemblance is less, the fact is expressed by representing the degree of resemblance as a fraction. It must be emphasized that this method merely measures the average degree of resemblance; it does not without further inquiry tell us how far that resemblance is due to inheritance. So far as such degrees of resemblance are found, it is quite possible, supposing that we had no further knowledge of the subject at all, that they might arise as follows. It might be that all men had the same germinal constitution, and that the degree of resemblance was due to the fact that fathers and their children were brought up under more or less similar surroundings. Though this fact should be borne in mind, such an explanation of the resemblances found in the cases that will be quoted in what follows is as a matter of fact shut out, and we may accept the correlations as a measure of the degree of likeness due to inheritance.

The correlations between parents and children and between children of the same parents have been determined in respect of many characters—both mental and physical—and found to be about 0·5. Thus, in respect of height the correlation between father and son was found to be 0·514, between father and daughter 0·510, between mother and son 0·494, between mother and daughter 0·507, between brother and brother 0·511, between sister and sister 0·537, between brother and sister 0·553. So too the average parental 359correlation in respect to eye colour is 0·495, and the average fraternal correlation is 0·475. With regard to ability the fraternal correlation is 0·46, the correlation between sisters 0·47, and that between brother and sister 0·44.

All that has been said with regard to the nature of fertilization and development lead to a similar conclusion. In the processes of maturation and fertilization we can detect a mechanism whereby the bearers of inherited qualities are transmitted from parent to child. In development we see how, when the appropriate stimulus plays upon the fertilized egg, an adult member of the species grows up. It is only when we suppose that within the fertilized egg there are certain predispositions in respect of every character which are derived from the parents that we can in any way understand how adult individuals come into being.^[1594] And it must be emphasized that, however much opinions may differ regarding the precise nature of the mechanism of inheritance, there is no difference regarding the fact of inheritance. All biologists are agreed on this subject up to a point; the matters in debate are not strictly relevant here.

3. There are certain points, however, which, in view of the form which the discussion in the following chapters will take, require further treatment. As regards physical characters, it is sufficient to think of all such characters whether great or small as inherited. But with regard to disease it may be as well to consider further to what degree it can be said to be inherited.

What we call disease falls under two headings: disease due to the attacks of parasites and disease due to structural defects or weaknesses. Parasitic infections are of post-conceptional acquirement; they may be acquired, it is true, before birth, but such an acquirement is strictly analogous to the ‘catching’ of a disease after birth. In the sense, therefore, that parasites are not transmitted from the parent to the ovum, as the germinal constitution is transmitted, disease is never inherited.

It has been shown, however, that in the case of certain diseases, such as tuberculosis, a definite susceptibility to contract the disease is inherited. In other words, given an equal exposure to infection, some men do not ‘catch’ the disease, some only 360experience a mild form, and others die rapidly of it. It is probable that there are innately given degrees of susceptibility to all diseases which are caused by the invasion of the body by small, chiefly unicellular, parasites, which produce their harmful effects by the secretion of toxins. The same does not apply to the diseases caused by the attacks of the larger multicellular parasites, such as tapeworms, which, at any rate to a large extent, produce their harmful effects directly by causing lesions of the tissues. As to how far we have to think of different kinds of susceptibility to different diseases, or how far diseases go in classes, so that susceptibility to one disease goes with susceptibility to another disease and vice versa, little is known, but it is probable that in some cases at least there is a linkage of susceptibilities, that, for instance, susceptibility to scarlet fever is to some extent linked with susceptibility to measles. We must suppose that there are structural peculiarities which underlie these different susceptibilities. Though various hypotheses have been put forward, we are still ignorant as to their nature. But whatever the exact nature of these structural differences, it is clear that they are inherited like any other physical character.

We have therefore to note that (a) susceptibility to specific infections is inherited. If gross anomalies, such as polydactyly and syndactyly (malformations of the hand) are to be accounted as diseases, then we may note that (b) gross anomalies are inherited. (c) Minor anomalies are also inherited. Under this head come haemophilia, due to some peculiarity in the blood-vessels, which results in their not contracting as they should and to the absence of coagulating power in the blood, albuminuria, due to some defect in the filtering apparatus in the kidney, albinism, myopia, icthyosis, and others, all of which are clearly due to structural peculiarities. Further, we may note that (d) ‘other conditions due, it would seem, to disturbances of metabolism, underlying which may very possibly be finer anatomical variations, have for long been noted as tending to be inherited; such as obesity, diabetes, gout, and chronic rheumatism’.^[1595] Lastly (e) certain nervous diseases are inherited. We may distinguish two classes of nervous disease—the homeomorphic and the heteromorphic. ‘In the former the offspring show the same lesions and 361symptoms as the parents. These are cases more particularly of lack of development or of premature atrophy of certain groups of nerve cells.’^[1596] In the latter the parent may suffer from one kind of disease, while the offspring may exhibit one or more of a group of other diseases. This is attributed to a lack of development of the highest nerve-centres as a whole. There is a lack of ‘perfect stability and co-ordination of various parts so that according to the strains to which the individual members of family are subjected, now one, now the other series of centres may show itself unable to respond adequately, and one or the other form of mental disturbance and nervous disease may result. Here are to be included the condition of insanity, familial epilepsy and the neuroses.’^[1597] In other words, we sometimes find stocks in which there is a nervous weakness, which may manifest itself in very various ways, including hysteria, epilepsy, inability to control impulse, delusion, and so on. Alcoholism is sometimes spoken of as though it was to be regarded as an irresistible impulse to drink. Probably we should rather imagine that a condition of general nervous weakness may at times manifest itself in the form of a loss of control with regard to the use of alcohol. According to Mott ‘there can be no doubt that neurasthenics, epileptics, imbeciles, degenerates, eccentrics, and potential lunatics—all those indeed with an inherent narrow margin of highest control—possess a marked intolerance to the effects of alcohol’.^[1598] Certain facts are, however, very puzzling. Suicide, for example, would seem to be a manner in which general nervous weakness may manifest itself. Nevertheless, the tendency to suicide appears sometimes as a very definite and peculiar disease which manifests itself generation after generation at a certain age. Perhaps in these cases we should rather see an example of how the same outward circumstances—here the knowledge of the family history—tends to cause a general nervous weakness to manifest itself in the same way, rather than an example of a specific nervous weakness.^[1599]

4. What we know as temperament stands half-way between physical and mental characters. As we have seen, temperament depends upon influences exerted by the functioning of the bodily organs on the nervous system and upon the peculiarities of the 362nervous system itself. Though on this account temperament or nervous tone is peculiarly susceptible to environmental influences, a certain condition of nervous tone is innately given. We must suppose the actual structure of the nervous system to be largely inherited, and we must suppose that the manner in which the bodily organs function is largely due to their innate organization. We have thus to think of a certain temperament as always given in the germinal constitution. An extreme example is an innate defect of the thyroid gland which can produce any degree of mental apathy. Again, such characters as excitability, rapidity of response, and differences in respect of fatiguability and recuperation are largely inherited.

Habit is best considered under this heading. The essential fact about habit is that, if the nervous system is stimulated in a certain fashion, so that a stimulus passes along certain paths, the next stimulus of the same kind will produce an impulse which passes more easily along those paths. Thus in time the same result, whatever it may be, is achieved by the application of a weaker stimulus. This fact, which is of great importance, suggests an actual modification of structure—an actual fashioning of a path. Whether this is so or not, we do not know. What is noticeable here is that the ease with which habits are formed undoubtedly differs from man to man, and we have again to postulate predispositions in the germinal constitution which tend towards a definite degree of development of the power of the formation of habits.

5. The greatest difficulties arise when dealing with the purely mental characters. There is a general agreement among biologists that mental characters are inherited just as physical characters are inherited. As we have pointed out, there is evidence to this effect derived from studies on biometric lines. The difficulties arise when we attempt to give more precision to the statement that mental characters are inherited. One method of approaching this problem is to analyse mental behaviour, and to ascertain if possible what faculties there are which cannot be explained in terms of other faculties. Such faculties are the least that we must suppose to be given in the germinal constitution. It does not, of course, follow that irreducible characters are Mendelian characters. In all probability they are due to the presence of many Mendelian factors. This analysis is a matter of great 363difficulty, and there is at present no agreement among psychologists as to what ultimate faculties of the mind are given out of which the characters we observe are compounded. Modern analysis leads to the idea of mental process as the activity of a subject. But though we may not think of the mind merely as a bundle of faculties, we have to attribute certain faculties to the subject, and the question arises as to what these faculties are.

There is less difficulty with regard to the affective and conative faculties, that is to say, those that are connected with feeling and striving, than with regard to the cognitive faculties. Psychologists are to some extent agreed as to what instincts can be recognized. McDougall, for instance, gives the following list, associating in each case with the impulse an emotion representing the conative or affective aspect.^[1600] (1) Instinct of flight and emotion of fear. (2) Instinct of repulsion and emotion of disgust. (3) Instinct of curiosity and emotion of wonder. (4) Instinct of pugnacity and the combative emotion. (5) Instinct of self-assertion and the emotion of elation. (6) Instinct of self-abasement and the emotion of subjection. (7) Parental instinct and the tender emotion. To the remaining instincts which he names a special emotion is less definitely attached. These instincts are those of reproduction, sexual jealousy, female coyness, gregariousness, acquisition, and construction. This list is not exhaustive; there are other instincts of less importance, such as the instinct which tends to make a boy at a certain age leave his home. The list, however, includes all the more important instincts.

There are a number of other emotions often described as primary which McDougall describes as complex. Thus, according to his view admiration is a combination of wonder and self-abasement. When fear is added, we have the emotion of awe, and when gratitude is added we have the emotion of reverence. Gratitude is itself a combination of tender emotion and self-abasement. In a similar fashion loathing, fascination, and envy can be explained. According to this view, therefore, we should not think of separate predispositions towards these complex emotions as existing in the germinal constitution; we should think of them as determined by the predispositions towards the simpler emotions.

364McDougall further describes three general or non-specific tendencies which have sometimes been classed as instincts.^[1601] Sympathy, or the sympathetic induction of the emotions, describes the fact that instinctive behaviour incites similar behaviour in the observer. Suggestion is defined as a process of communication resulting in the acceptation with conviction of the communicated proposition in the absence of logically adequate grounds for its acceptance. Imitation, which has often been used to include sympathy and suggestion, is in the limited sense adopted by McDougall, the tendency to copy the bodily actions of some one else. Finally there is the tendency to play.

Apart, therefore, from temperament and the faculties connected with cognition, we have to recognize predispositions towards certain instincts and certain general tendencies. Without question they differ in strength from man to man; without question such differences are in large part attributable to varying predispositions in the germinal constitution, and we must presume that such differences are in some way connected with differences in nervous structure or organization. The subject is one of great difficulty, and nothing can be affirmed regarding it with any certainty. The ultimate modes of feeling seem to be those of pleasure and displeasure, and of excitement and depression. It is scarcely possible to stop at this point, and it seems that we have to go on and attribute the more specific forms of feeling or emotions to primary faculties incapable of further analysis. So, too, conation or striving seems to be distinguishable into striving towards and into striving away from an object. Again as with feeling it appears that we have to go farther and presume certain more clearly-defined faculties as indicated above.

6. The difficulties are still greater with regard to cognition. Analysis seems to bring us to three ultimate faculties—those of judgement, of comparison, and of association.^[1602] Judgement consists in affirming or denying, and upon this faculty all higher reasoning is built. Memory should perhaps be considered as a special aspect of this faculty—as the power to think of an object over again, and to affirm or deny it to be the same object. This faculty, together with the faculty of comparing, enables that process of distinction and systematization to be performed upon which all the higher developments are based. To these 365faculties of analysis and synthesis are to be added the faculty of association, whereby objects come to be thought of in groups according to the sequence in which they came to be presented to the mind quite apart from their intrinsic affinities or distinctions. Thus, in addition to the faculties whereby the mind attains some grasp of the constitution of the world of objects, there is a faculty which in some degree mirrors the history of the world.

It is probable that in addition to these ultimate faculties, there may be many other faculties which cannot be reduced to aspects of the former, and have therefore to be considered also as primary. However that may be, what is important for our present purpose is that the faculty commonly known as ability is undoubtedly innate. It may be very variously composed of specialized forms of these three primary faculties, and perhaps of other faculties combined in very many ways. Every degree of ability is known and there are many forms in which it is specialized, such as in musical, artistic, and other directions. It has been shown that a high development of general ability is correlated with a high development of any special ability. In other words, we have to think of a genius with some marked talent as possessing on an average a high general level of ability. There is ample evidence of the inheritance of ability, both derived from the examination of large numbers of cases and from the analysis of family histories. This evidence also shows that musical and other forms of special ability are inherited. We have therefore to think of predispositions in the germinal constitution, which give rise to abilities of all kinds, although a more correct analysis may refer them to some combination of the ultimate faculties enumerated above, much as certain complex emotions are to be referred to the combination of certain primary emotions, and not thought of as given as such in the germinal constitution.

There are other characters which do not appear to be covered by any extension of the term ability. Such, for instance, are will and self-control. In its essence will seems to be the rising up of some dominating impulse which controls or harmonizes the feelings. The weaker impulse somehow overcomes the stronger. The will is variously explained, and in modern psychological writings often in such a manner as to exclude the idea of a special faculty. It is not necessary here to go into these explanations of 366the origin of will. It is clear that, if it has to be referred to the working of complex faculties which are themselves explicable in the terms of more ultimate faculties, it is in a sense innately given. Undoubtedly will is in a sense inherited. However much will may be a product of the environment in the second sense, there is clearly an innate tendency towards the development of a will of a certain form and strength. Whether therefore it is an ultimate faculty or not, it is due in some measure either to a definite innate predisposition or to such a combination of other innate predispositions as give rise to the manifestation of a will of a certain nature.

Finally, we may look at the whole problem of the inheritance of mental characters from another point of view. It is known that certain areas in the brain associated with particular functions are differently developed in different individuals, and that there are differences in the speed at which the nervous impulse travels which are doubtless due to anatomical peculiarities and so on—that there are, in fact, differences in the physical basis which underlies the manifestation of mental characters. It is thus easy to understand in a general way how mental characters which are based upon the nervous organization are inherited. A man may inherit a brain in which certain regions are of relatively large size, or a nervous organization by which impulses are swiftly conducted, and thus we can understand how quickness of response, power of concentration, readiness of association, type of mind (whether emotional or intellectual) are inherited.

7. This brief inquiry into what is given in the germinal constitution leads to the conception of large numbers of predispositions which under the stimulus of the environment develop into the characters that we observe. This is true at least of physical characters. In physical characters we observe the result of the play of the environment upon certain predispositions. With regard to mental characters the position is somewhat different. Into mental characters, as presented to us—into ability, for instance—there enters the influence of the environment in the second sense. When we are judging the ability of a man from a practical point of view, we are judging a character into the make-up of which has entered not only certain predispositions and a certain environment—using environment in the sense of the complement to inheritance—but also the results of the influence 367of the environment in the second sense, such as habit and what may be regarded as tools, namely, modes of thought and so on. In order to get at the characters we must disentangle or allow for these tools. This difficulty does not arise in the case of physical characters. When measuring the physical strength of two men we are not likely to allow one man to throw an object with a throwing-stick and another to use his own unaided strength. But we are apt when comparing ability to forget that one man may have learnt to distinguish between the categories when another may not have done so. To this difficulty we shall refer again later.

These predispositions have their basis in the germinal constitution and are therefore inherited. It is probable that we should regard the germinal constitution as consisting of unit-factors. However that may be, it is clear that either a differential birth-rate (reproductive selection) or a differential death-rate (lethal selection), will change the average germinal constitution of the race among which they are operative. Bearing this in mind, we may inquire when and where in human history we can detect reproductive and lethal selection at work. And we must remember that in the first place not merely obvious factors such as war have been and are at work, but also that subtle changes in social life and social organization may have profound effects, and that in the second place not merely obvious characters may be favoured or eliminated, but the least striking and most minute (though not necessarily the least important) characters both mental and physical may be similarly affected.

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