We have come to the climax of the eugenist's preliminary argument; if the main differences between human beings are not due to anything in the environment or training, either of this or previous generations, there can be but one explanation for them.

They must be due to the ancestry of the individual—that is, they must be matters of heredity in the ordinary sense, coupled with the fortuitous variations which accompany heredity throughout the organic world.

We need not limit ourselves, however, to the argument by exclusion, for it is not difficult to present direct evidence that the differences between men are actually inherited by children from parents. The problem, formally stated, is to measure the amount by which the likeness of individuals of like ancestry surpasses the likeness of individuals of different ancestry. After subtraction of the necessary amount for the greater likeness in training, that the individuals of like ancestry will have, whatever amount is left will necessarily, represent the actual inheritance of the child from its ancestors—parents, grandparents, and so on.

Obviously, the subtraction for environmental effects is the point at which a mistake is most probable. We may safely start, therefore, with a problem in which no subtraction whatever need be made for this cause. Eye color is a stock example, and a good one, for it is not conceivable that home environment or training would cause a change in the color of brothers' eyes.

The correlation[30] between brothers, or sisters, or brothers and sisters—briefly, the fraternal resemblance—for eye-color was found by Karl Pearson, using the method described in Chapter I, to be .52. We are in no danger of contradiction if we state with positiveness that this figure represents the influence of ancestry, or direct inheritance, in respect to this particular trait.

Suppose the resemblance between brothers be measured for stature—it is .51; for cephalic index, that is, the ratio of width of skull to length of skull—it is .49; for hair color—it is .59. In all of these points, it will be admitted that no home training, or any other influence except heredity, can conceivably play an important part. We could go on with a long list of such measurements, which biometrists have made; and if they were all summed up it would be found that the fraternal correlation in these traits as to the heritability of which there can be no dispute, is about .52. Here is a good measure, albeit a technical one, of the influence of heredity from the near ancestry. It is possible, too, to measure the direct correlation between a trait in parent and the same trait in offspring; the average of many cases where only heredity can be thought to have had any effect in producing the result, is .49. By the two methods of measurement, therefore, quite comparable results are obtained.

So much work has been done in this subject that we have no hesitation in affirming .5 to represent approximately the average intensity of heredity for physical characters in man. If any well-marked physical character be measured, in which training and environment can not be assumed to have had any part, it will be found, in a large enough number of subjects, that the resemblance, measured on a scale from 0 to 1, is just about one-half of unity. Of course, perfect identity with the parents is not to be expected, because the child must inherit from both parents, who in turn each inherited from two parents, and so on.

So far, it may be said, we have had plain sailing because we have carefully chosen traits in which we were not obliged to make any subtraction whatever for the influence of training. But it is evident that not all traits fall in that class.

This is the point at which the inheritance of mental traits has been most often questioned. Probably no one will care to dispute the inheritance of such physical traits as eye-color. But in considering the mind, a certain school of popular pseudo-psychological writers question the reality of mental inheritance, and allege that the proofs which the geneticist offers are worthless because they do not make account of the similarity in environment or training. Of course, it is admitted that some sort of a mental groundwork must be inherited, but extremists allege that this is little more than a clean slate on which the environment, particularly during the early years of childhood, writes its autograph.

We must grant that the analysis of the inheritance of mental traits is proceeding slowly. This is not the fault of the geneticist, but rather of the psychologist, who has not yet been able to furnish the geneticist with the description of definite traits of such a character as to make possible the exhaustive analysis of their individual inheritance. That department of psychology is only now being formed.

We might even admit that no inherited "unit character" in the mind has yet been isolated; but it would be a great mistake to assume from this admission that proof of the inheritance of mental qualities, in general, is lacking.

The psychologists and educators who think so appear either to be swayed by metaphysical views of the mind, or else to believe that resemblance between parent and offspring is the only evidence of inheritance that can be offered. The father dislikes cheese, the son dislikes cheese. "Aha, you think that that is the inheritance of a dislike for cheese," cries the critic, "but we will teach you better." An interesting example of this sort of teaching is furnished by Boris Sidis, whose feelings are outraged because geneticists have represented that some forms of insanity are hereditary. He declaims for several pages[31] in this fashion:

"The so-called scientific method of the eugenists is radically faulty, in spite of the rich display of colored plates, stained tables, glittering biological speculations, brilliant mathematical formulÆ and complicated statistical calculations. The eugenists pile Ossa on Pelion of facts by the simple method of enumeration which Bacon and the thinkers coming after him have long ago condemned as puerile and futile. From the savage's belief in sympathetic, imitative magic with its consequent superstitions, omens, and taboos down to the articles of faith and dogmas of the eugenists we find the same faulty, primitive thought, guided by the puerile, imbecile method of simple enumeration, and controlled by the wisdom of the logical post hoc, ergo propter hoc."

Now if resemblance between parent and offspring were, as Dr. Sidis supposes, the only evidence of inheritance of mental traits which the eugenist can produce, his case would indeed be weak. And it is perfectly true that "evidence" of this kind has sometimes been advanced as sufficient by geneticists who should have known better. But this is not the real evidence which genetics offers. The evidence is of numerous kinds, and several lines might be destroyed without impairing the validity of the remainder. It is impossible to review the whole body of evidence here, but some of the various kinds may be indicated, and samples given, even though this involves the necessity of repeating some things we have said in earlier chapters. The reader will then be able to form his own opinion as to whether the geneticists' proofs or the mere assurances of those who have not studied the subject are the more weighty.

1. The analogy from breeding experiments. Tame rats, for instance, are very docile; their offspring can be handled without a bit of trouble. The wild rat, on the other hand, is not at all docile.

W. E. Castle, of Harvard University, writes:[32] "We have repeatedly mated tame female rats with wild males, the mothers being removed to isolated cages before the birth of the young. These young which had never seen or been near their father were very wild in disposition in every case. The observations of Yerkes on such rats raised by us indicates that their wildness was not quite as extreme as that of the pure wild rat but closely approached it."

Who can suggest any plausible explanation of their conduct, save that they inherited a certain temperament from their sire? Yet the inheritance of temperament is one of the things which certain psychologists most "view with alarm." If it is proved in other animals, can it be considered wholly impossible in man?

2. The segregation of mental traits. When an insane, or epileptic, or feeble-minded person mates with a normal individual, in whose family no taint is found, the offspring (generally speaking) will be mentally sound, even though one parent is not. On the other hand, if two people from tainted stocks marry, although neither one may be personally defective, part of their offspring will be affected.

This production of sound children from an unsound parent, in the first case, and unsound children from two apparently sound parents in the second case, is exactly the opposite of what one would expect if the child gets his unsoundness merely by imitation or "contagion." The difference can not reasonably be explained by any difference in environment or external stimuli. Heredity offers a satisfactory explanation, for some forms of feeble-mindedness and epilepsy, and some of the diseases known as insanity, behave as recessives and segregate in just the way mentioned. There are abundant analogies in the inheritance of other traits in man, lower animals and plants, that behave in exactly the same manner.

If mental defects are inherited, then it is worth while investigating whether mental excellencies may not also be.

3. The persistence of like qualities regardless of difference in environment. Any parent with open eyes must see this in his own children—must see that they retained the inherited traits even when they left home and lived under entirely different surroundings. But the histories of twins furnish the most graphic evidence. Galton, who collected detailed histories of thirty-five pairs of twins who were closely alike at birth, and examined their history in after years, writes:[33] "In some cases the resemblance of body and mind had continued unaltered up to old age, notwithstanding very different conditions of life;" in other cases where some dissimilarity developed, it could be traced to the influence of an illness. Making due allowance for the influence of illness, yet "instances do exist of an apparently thorough similarity of nature, in which such differences of external circumstances as may be consistent with the ordinary conditions of the same social rank and country do not create dissimilarity. Positive evidence, such as this, can not be outweighed by any amount of negative evidence."

Frederick Adams Woods has brought forward[34] a piece of more exact evidence under this head. It is known from many quantitative studies that in physical heredity, the influence of the paternal grandparents and the influence of the maternal grandparents is equal; on the average one pair will contribute no more to the grandchildren than the other. If mental qualities are due rather to early surroundings than to actual inheritance, this equality of grandparental influence is incredible in the royal families where Dr. Woods got his material; for the grandchild has been brought up at the court of the paternal grandfather, where he ought to have gotten all his "acquirements," and has perhaps never even seen his maternal grandparents, who therefore could not be expected to impress their mental peculiarities on him by "contagion." When Dr. Woods actually measured the extent of resemblance to the two sets of grandparents, for mental and moral qualities, he found it to be the same in each case; as is inevitable if they are inherited, but as is incomprehensible if heredity is not largely responsible for one's mental make-up.

4. Persistence of unlike qualities regardless of sameness in the environment. This is the converse of the preceding proposition, but even more convincing. In the last paragraph but one, we mentioned Galton's study (cited at some length in our Chapter I) of "identical" twins, who are so much alike at birth for the very good reason that they have identical heredity. This heredity was found to be not modified, either in the body or the mind, by ordinary differences of training and environment. Some of Galton's histories[35] of ordinary, non-identical twins were also given in Chapter I; two more follow:

One parent says: "They have been treated exactly alike; both were brought up by hand; they have been under the same nurse and governess from their birth, and they are very fond of each other. Their increasing dissimilarity must be ascribed to a natural difference of mind and character, as there has been nothing in their treatment to account for it."

Another writes: "This case is, I should think, somewhat remarkable for dissimilarity in physique as well as for strong contrast in character. They have been unlike in mind and body throughout their lives. Both were reared in a country house and both were at the same schools until the age of 16."

In the face of such examples, can anyone maintain that differences in mental make-up are wholly due to different influences during childhood, and not at all to differences in germinal make-up? It is not necessary to depend, under this head, on mere descriptions, for accurate measurements are available to demonstrate the point. If the environment creates the mental nature, then ordinary brothers, not more than four or five years apart in age, ought to be about as closely similar to each other as identical twins are to each other; for the family influences in each case are practically the same. Professor Thorndike, by careful mental tests, showed[36] that this is not true. The ordinary brothers come from different egg-cells, and, as is known from studies on lower animals, they do not get exactly the same inheritance from their parents; they show, therefore, considerable differences in their psychic natures. Real identical twins, being two halves of the same egg-cell, have the same heredity, and their natures are therefore much more nearly identical.

Again, if the mind is molded during the "plastic years of childhood," children ought to become more alike, the longer they are together. Twins who were unlike at birth ought to resemble each other more closely at 14 than they did at 9, since they have been for five additional years subjected to this supposedly potent but very mystical "molding force." Here again Professor Thorndike's exact measurements explode the fallacy. They are actually, measurably, less alike at the older age; their inborn natures are developing along predestined lines, with little regard to the identity of their surroundings. Heredity accounts easily for these facts, but they cannot be squared with the idea that mental differences are the products solely of early training.

5. Differential rates of increase in qualities subject to much training. If the mind is formed by training, then brothers ought to be more alike in qualities which have been subject to little or no training. Professor Thorndike's measurements on this point show the reverse to be true. The likeness of various traits is determined by heredity, and brothers may be more unlike in traits which have been subjected to a large and equal amount of training. Twins were found to be less alike in their ability at addition and multiplication, in which the schools had been training them for some years, than they were in ability to mark off the A's on a printed sheet, or to write the opposites to a list of words—feats which they had probably never before tried to do.

This same proposition may be put on a broader basis.[37] "In so far as the differences in achievement found amongst a group of men are due to the differences in the quantity and quality of training which they had had in the function in question, the provision of equal amounts of the same sort of training for all individuals in the group should act to reduce the differences." "If the addition of equal amounts of practice does not reduce the differences found amongst men, those differences can not well be explained to any large extent by supposing them to have been due to corresponding differences in amount of previous practice. If, that is, inequalities in achievement are not reduced by equalizing practice, they can not well have been caused by inequalities in previous practice. If differences in opportunity cause the differences men display, making opportunity more nearly equal for all, by adding equal amounts to it in each case should make the differences less.

"The facts found are rather startling. Equalizing practice seems to increase differences. The superior man seems to have got his present superiority by his own nature rather than by superior advantages of the past, since, during a period of equal advantage for all, he increases his lead." This point has been tested by such simple devices as mental multiplication, addition, marking A's on a printed sheet of capitals and the like; all the contestants made some gain in efficiency, but those who were superior at the start were proportionately farther ahead than ever at the end. This is what the geneticist would expect, but fits very ill with some popular pseudo-science which denies that any child is mentally limited by nature.

6. Direct measurement of the amount of resemblance of mental traits in brothers and sisters. It is manifestly impossible to assume that early training, or parental behavior, or anything of the sort, can have influenced very markedly the child's eye color, or the length of his forearm, or the ratio of the breadth of his head to its length. A measure of the amount of resemblance between two brothers in such traits may very confidently be said to represent the influence of heredity; one can feel no doubt that the child inherits his eye-color and other physical traits of that kind from his parents. It will be recalled that the resemblance, measured on a scale from 0 to 1, has been found to be about 0.5.

Karl Pearson measured the resemblance between brothers and sisters in mental traits—for example, temper, conscientiousness, introspection, vivacity—and found it on the average to have the same intensity—that is, about 0.5. Starch gets similar results in studying school grades.

Professor Pearson writes:[38]

"It has been suggested that this resemblance in the psychological characters is compounded of two factors, inheritance on the one hand and training and environment on the other. If so, one must admit that inheritance and environment make up the resemblance in the physical characters. Now these two sorts of resemblance being of the same intensity, either the environmental influence is the same in both cases or it is not. If it is the same, we are forced to the conclusion that it is insensible, for it can not influence eye-color. If it is not the same, then it would be a most marvelous thing that with varying degrees of inheritance, some mysterious force always modifies the extent of home influence, until the resemblance of brothers and sisters is brought sensibly up to the same intensity! Occam's razor[39] will enable us at once to cut off such a theory. We are forced, I think, literally forced, to the general conclusion that the physical and psychical characters in man are inherited within broad lines in the same manner, and with approximate intensity. The average parental influence is in itself largely a result of the heritage of the stock and not an extraneous and additional factor causing the resemblance between children from the same home."

A paragraph from Edgar Schuster[40] may appropriately be added. "After considering the published evidence a word must be said of facts which most people may collect for themselves. They are difficult to record, but are perhaps more convincing than any quantity of statistics. If one knows well several members of a family, one is bound to see in them likenesses with regard to mental traits, both large and small, which may sometimes be accounted for by example on the one hand or unconscious imitation on the other, but are often quite inexplicable on any other theory than heredity. It is difficult to understand how the inheritance of mental capacity can be denied by those whose eyes are open and whose minds are open too."

Strictly speaking, it is of course true that man inherits nothing more than the capacity of making mental acquirements. But this general capacity is made up of many separate capacities, all of these capacities are variable, and the variations are inherited. Such seems to us to be the unmistakable verdict of the evidence.

Our conclusions as to the inheritance of all sorts of mental capacity are not based on the mere presence of the same trait in parent and child. As the psychological analysis of individual traits proceeds, it will be possible to proceed further with the study of the inheritance of these traits. Some work has been done on spelling, which is particularly interesting because most people, without reflection, would take it for granted that a child's spelling ability depends almost wholly on his training. Professor Thorndike's exposition[41] of the investigation is as follows:

"E. L. Earle ('03) measured the spelling abilities of some 800 children in the St. Xavier school in New York by careful tests. As the children in this school commonly enter at a very early age, and as the staff and methods of teaching remain very constant, we have in the case of the 180 pairs of brothers and sisters included in the 600 children closely similar school training. Mr. Earle measured the ability of any individual by his deviation from the average for his grade and sex, and found the coefficient of correlation between children of the same family to be .50. That is, any individual is on the average 50% as much above or below the average for his age and sex as his brother or sister.

"Similarities of home training might account for this, but any one experienced in teaching will hesitate to attribute much efficacy to such similarities. Bad spellers remain bad spellers though their teachers change. Moreover, Dr. J. M. Rice in his exhaustive study of spelling ability ('97) found little or no relationship between good spelling and any one of the popular methods, and little or none between poor spelling and foreign parentage. Cornman's more careful study of spelling ('07) supports the view that ability to spell is little influenced by such differences in school or home training as commonly exist."

This is a very clear-cut case of a definite intellectual ability, differences in which might be supposed to be due almost wholly to the child's training, but which seem, on investigation, to be largely due to heredity.

The problem may be examined in still greater detail. Does a man merely inherit manual skill, let us say, or does he inherit the precise kind of manual skill needed to make a surgeon but not the kind that would be useful to a watchmaker? Is a man born merely with a generalized "artistic" ability, or is it one adapted solely for, let us say, music; or further, is it adapted solely for violin playing, not for the piano?

Galton, in his pioneer studies, sought for data on this question. In regard to English judges, he wrote: "Do the judges often have sons who succeed in the same career, where success would have been impossible if they had not been gifted with the special qualities of their fathers? Out of the 286 judges, more than one in every nine of them have been either father, son or brother to another judge, and the other high legal relationships have been even more numerous. There can not, then, remain a doubt but that the peculiar type of ability that is necessary to a judge is often transmitted by descent."

Unfortunately, we can not feel quite as free from doubt on the point as Galton did. The judicial mind, if that be the main qualification for a judge, might be inherited, or it might be the result of training. Such a case, standing alone, is inconclusive.

Galton similarly showed that the sons of statesmen tended to be statesmen, and that the same was true in families of great commanders, literary men, poets and divines. In his list of eminent painters, all the relatives mentioned are painters save four, two of whom were gifted in sculpture, one in music and one in embroidery. As to musicians, Mendelssohn and Meyerbeer are the only ones in his list whose eminent kinsmen achieved their success in other careers than music.

Havelock Ellis, who likewise studied British men of genius, throws additional light on the subject. "Painters and sculptors," he found, "constitute a group which appears to be of very distinct interest from the point of view of occupational heredity. In social origin, it may be noted, the group differs strikingly in constitution from the general body of men of genius in which the upper class is almost or quite predominant. Of 63 painters and sculptors of definitely known origin, only two can be placed in the aristocratic division. Of the remainder 7 are the sons of artists, 22 the sons of craftsmen, leaving only 32 for all other occupations, which are mainly of lower middle class character, and in many cases trades that are very closely allied to crafts. Even, however, when we omit the trades as well as the cases in which the fathers were artists, we find a very notable predominance of craftsmen in the parentage of painters, to such an extent indeed that while craftsmen only constitute 9.2% among the fathers of our eminent persons generally, they constitute nearly 35% among the fathers of the painters and sculptors. It is difficult to avoid the conclusion that there is a real connection between the father's aptitude for craftsmanship and the son's aptitude for art.

"To suppose that environment adequately accounts for this relationship is an inadmissible theory. The association between the craft of builder, carpenter, tanner, jeweller, watchmaker, woodcarver, ropemaker, etc., and the painter's art is small at best, and in most cases is non-existent."

Arreat, investigating the heredity of 200 eminent European painters, reached results similar to those of Ellis, according to the latter's citation.

Arithmetical ability seems similarly to be subdivided, according to Miss Cobb.[42] She made measurements of the efficiency with which children and their parents could do problems in addition, subtraction, multiplication and division, and could copy a column of figures. "The measurements made," she writes, "show that if, for instance, a child is much quicker than the average in subtraction, but not in addition, multiplication or division, it is to be expected that one at least of his parents shows a like trait; or if he falls below the average in subtraction and multiplication, and exceeds it in addition and division, again the same will hold true of at least one of his parents." These various kinds of arithmetic appear to be due to different functions of the brain, and are therefore probably inherited independently, if they are inherited at all.

To assume that the resemblance between parent and offspring in arithmetical ability is due to association, training and imitation is not plausible. If this were the case, a class of children ought to come to resemble their teacher, but they do not. Moreover, the child sometimes resembles more closely the parent with whom he has been less associated in daily life.

From such data as these, we conclude that mental inheritance is considerably specialized. This conclusion is in accord with Burris' finding (cited by Thorndike) that the ability to do well in some one high school study is nearly or quite as much due to ancestry as is the ability to do well in the course as a whole.

To sum up, we have reason to believe not only that one's mental character is due largely to heredity, but that the details of it may be equally due to heredity, in the sense that for any particular trait or complex in the child there is likely to be found a similar trait or complex in the ancestry. Such a conclusion should not be pushed to the point of assuming inheritance of all sorts of dispositions that might be due to early training; on the other hand, a survey of the whole field would probably justify us in concluding that any given trait is more likely than not to be inherited. The effect of training in the formation of the child's mental character is certainly much less than is popularly supposed; and even for the traits that are most due to training, it must never be forgotten that there are inherited mental bases.

If the reader has accepted the facts presented in this chapter, and our inferences from the facts, he will admit that mental differences between men are at bottom due to heredity, just as physical differences are; that they are apparently inherited in the same manner and in approximately the same degree.