We shall have occasion in the next chapter to enumerate some of the human unit characters whose heredity has been traced and which have been found to Mendelize, but we may mention here a few Mendelizing units in other organisms in order to give some idea of the kind of character which behaves as a unit and of the range of the forms which have been found to show Mendelian phenomena in their heredity. Among the higher animals one might mention the absence of horns in cattle and sheep; the "waltzing" habit of mice and the pacing gait of the horse; length of hair and smoothness of coat in the rabbit and guinea pig; presence of an extra toe in the cat, guinea pig, rabbit, fowl; length of tail in the cat; and in the common fowl such characters as the shape and size of the comb, presence of a crest or a "m.u.f.f," a high nostril, rumplessness, feathering of the legs, "frizzling" of the feathers, certain characters of the voice, and a tendency to brood.
Among plants may be mentioned such characters as dwarfness in garden peas, sweet peas, and some kinds of beans; smoothness or p.r.i.c.kliness of stem in the jimson weed and crowfoot; leaf characters in a great variety of plants; in the cotton plant a half dozen characters have been found to Mendelize; seed characters such as form and amount of starch, sugar, or gluten; flat or hooded standard in the sweet pea; annual or biennial habit in the henbane; susceptibility to a rust disease in wheat. We should not fail to mention that scores of color characters are known to Mendelize, such as hair or coat color and eye color in animals and the colors of flowers, stems, seeds, seed-coats, etc., in plants. The list of Mendelizing traits in different organisms now extends into the hundreds and is increasing almost weekly.
Before leaving the subject of Mendelism we should say that the phenomena, as described above in the Andalusian fowl and guinea pig, are among the simplest known. And while such simple formulas serve to describe the phenomena of heredity in a large number of instances, yet in a great many other cases the descriptive formulas are more complicated. We cannot in this place describe any of these complications. For a full discussion of these and of the whole subject of Mendelism the interested reader is referred to Professor Bateson"s work on "Mendel"s Principles of Heredity" (1909). It must suffice to say here that in color heredity, for example, such ratios as 9:3:4 or 12:3:1 in the second filial generation instead of the more frequent 1:2:1 or 3:1 are explainable upon essentially the same relations as these simpler and more typical ratios. And further, many less usual Mendelian phenomena, which we cannot undertake to describe here, are a.s.sociated with what the specialist technically terms "s.e.x limitation," "gametic coupling," and the like.
It is often said that the Mendelian formula has a very limited applicability to human heredity. This is probably true if we consider carefully the grammatical tense in which this statement is made. And yet it is almost certainly true that heredity in man is to be described by this law. This apparent paradox is easily explained. The only characters whose history in heredity follows this formula are the unit characters. A complex trait is not heritable, as a whole, but its components behave in heredity as the separate units. It is perfectly well known that we are deeply ignorant regarding this phase of human structure. Our ignorance here is not the necessary kind, however, it is merely due to the newness of the subject--we have not had time to find out. How can we say that a complex trait is or is not inherited according to some form of Mendel"s law when we do not know the nature of the units of which it is composed? We can make no statements about the Mendelian inheritance of such a trait until it is factored into its units. A considerable number of human characteristics are really known to be heritable according to this formula, enough so that several general rules of human heredity have been formulated. But it is also quite within the range of possibility that some traits really do not follow this law, although it cannot yet be said definitely that this is or is not the case. On the whole, then, we cannot, for the next few years, expect too much from the application of Mendel"s laws to human heredity, however much this is to be regretted.
Shall we then decline to say anything about the heredity of the great bulk of human characteristics? By no means: we have seen that in our bagatelle board we talk very definitely about the distribution of all the peas, though only about the probable history of one pea. Mendel"s law deals with individual inheritance. When we cannot apply this formula we have left still the possibility of talking about human heredity in the group as a whole. That is to say, we have left the opportunity of describing heredity by the statistical methods, with the crowd, not the individual, as the unit. Since we are forced into extensive use of this formula by our present and temporary ignorance of the applicability of Mendel"s rule we must get a clear notion of how the statistical method is applied in this matter.
The method is the same as that employed by the statistician in measuring the relatedness of any two series of varying phenomena. If two quant.i.ties or characteristics are so related that fluctuations in the one are accompanied in a regular manner by fluctuations in the other, the two quant.i.ties or characters are said to be correlated. For instance, the temperature and the rate of growth of sprouting beans are related in such a way that increase in the former is accompanied in a regular way by increase in the latter; or the width and height of the head, or the total stature and the length of the femur similarly vary regularly together so that they are said to be correlated to a certain extent which can be measured. This correlation may result from the fact that one condition is a cause, either direct or indirect, of the other; or there may be no such causal relation between the two phenomena, both resulting more or less independently from a common antecedent condition or cause.
This phenomenon of correlation is not limited among organisms to the comparison of two or more different characters in a single series of individuals; it is applicable also to the comparison of two series of individuals with respect to the same characteristic. Thus we may compare the stature of a series of fathers with the same measurement in their sons. It is this form of correlation with which we are particularly to deal here. While it is not necessary to understand just how this subject is dealt with by the statistician we should know one or two of the elementary principles involved, in order to appreciate the statistical form of many statements about heredity.
The stature of men may be said to vary usually between limits of 62 and 76 inches, the average height being about 69 inches. In the complete absence of heredity in stature we should find that fathers of any given height, say 62 or 63 or 76 inches would have sons of no particular height but of all heights with an average of 69 inches, the same as in the whole group. Or if stature were completely heritable from one generation to the next the _total generations being the units compared_, then 62 or 63 or 76 inch fathers would have respectively sons all 62, 63, and 76 inches tall. When we examine the actual details of the resemblance we find, as a matter of fact, that neither of these possibilities is actually realized. What we do find is that fathers below or above the average height have sons whose average height is also below or above the general average but not so far below or above the general average as were the fathers. If we measured a large number of pairs of fathers and sons with respect to stature we should find each generation with a variability such as that ill.u.s.trated in Fig. 3 of the stature of mothers, the limits here, however, being about 62 and 76 inches. But if we measured all the sons of 62-inch fathers they would be found to vary say from 62 to only 69 inches, averaging about 66 inches. Similarly 63-inch fathers would have sons from 62 to 70 inches tall, averaging about 66.5 inches, or 76-inch fathers might have sons from 69 to 76 inches in height, averaging about 72 inches, and so on for fathers of all heights. In general, then, we may say that fathers with a characteristic of a certain plus or minus deviation from the average of the whole group have sons who on the whole deviate in the same direction but less widely than the fathers, although the fact of variability comes in so that some few of the sons deviate as widely as, or even more widely than, the fathers, others deviate less widely than the fathers from the average of the whole group. This is the general and very important statistical fact of _regression_.
The phenomenon of regression may be made somewhat clearer by the aid of a simple diagram--Fig. 10. Here are plotted first the heights, by inches, of a group of fathers, giving the series of dots joined by the diagonal _AB_. Next are plotted the average heights of the sons of each cla.s.s of fathers: 62-inch fathers give 66-inch sons, 63-inch fathers 66.5-inch sons, 64-inch fathers 67-inch sons, and so for all the cla.s.ses of fathers. These dots are then joined by the line _EF_.
This is the _regression line_. Had it been the case that there was no regression in stature the different cla.s.ses of fathers would have had sons averaging just the same as themselves and the line representing the heights of the sons would have coincided with the line _AB_. Or if regression had been complete the fathers of any cla.s.s would have had sons averaging about 69 inches--just the same as the average of the whole group--and the line representing their heights would have had the position of _CD_ in the diagram. As a matter of fact, however, neither of these possibilities is actually realized and the regression line _EF_ is approximated in an actual series of data. A similar relation has been found for many characters other than stature.
[Ill.u.s.tration: FIG. 10.--Diagram ill.u.s.trating the phenomenon of regression. Explanation in text.]
The fact of regression is of considerable importance for the theory of evolution as well as for the subject of Eugenics when describing the phenomena of heredity in this statistical manner in whole groups without paying attention to particular individuals. Regression is found in all characteristics observed in this way, psychic as well as purely physical. "The father [i. e., fathers] with a great excess of the character contributes [contribute] sons with an excess, but a less excess of it; the father [fathers] with a great defect of the character contributes [contribute] sons with a defect, but less defect of it."
Now, whatever the actual extent of this regression is in a group we need to know how uniformly it occurs for all the cla.s.ses of different deviations from the general average, that is, we need to know whether the extreme groups regress to the same relative extent as do those nearer the general average; and, further, we need to know how nearly the sons of fathers of any certain height are grouped about their own average. In other words, we should know, first, whether the regression of the sons of 62 and 76 or 67 and 71 inch fathers is proportionately the same in each case, and, second, to what extent the sons of 62-inch fathers vary, whether they vary as do the fathers of 62-inch sons, and so for each group. This kind of information we get by calculating what is called the _coefficient of heredity_. The calculation of this coefficient is a complicated process which it is unnecessary to describe here. It must suffice to say that a numerical coefficient can readily be determined, which will express the average closeness and regularity of the relationship between all the plus and minus deviations from the group average in fathers and the corresponding plus and minus deviations from the group average of their sons with respect to a given characteristic. This coefficient of heredity may vary between 0.0 and 1.0. When it is 0.0 there is, on the whole, no regularity in the relationship, i. e., no heredity; when it is 1.0 there is, on the whole, complete regularity, i. e., heredity is complete. Neither of these values is ever actually found in determining coefficients of heredity in the parental relation; these are usually between 0.3 and 0.5. It should be emphasized again that this comparison is between whole groups and not between individuals, and that it fails to allow for the distinction between fluctuations and true variations. And, further, it should be noted that the information derived from such a coefficient is defective in that it takes into account only the relationship between the son and one parent; the maternal relation is just as important but this has to be determined separately. There is no satisfactory method of determining the relation between children and both parents at the same time.
The coefficient of heredity is, therefore, an abstract numerical value which gives us a fairly precise estimate as to the probable closeness of the relation between deviations from the group average of any character in two groups of relatives. The coefficient of _correlation_ is, in general, a measure of the relation between two different characteristics or conditions in a single group of individuals. The method of its determination and its limiting values are the same as for the coefficient of heredity.
By experience the coefficients of heredity and correlation in general are found to have the following significance:
0.00- no relation.
0.00-0.10--no significant relation.
0.10-0.25--low; relation slight though appreciable.
0.25-0.50--moderate; relation considerable.
0.50-0.75--high; relation marked.
0.75-0.90--very high; relation very marked.
0.90-1.00--nearly complete.
1.00--complete relation.
One further point remains to be considered, which applies not so much to coefficients of heredity as to coefficients of correlation in general, i. e., to the relatedness of two different characters or series of events in a single group of cases or individuals. This is that coefficients of correlation may be either positive or negative.
That is, the real limits of the value of the coefficient are plus one and minus one. The example given above of stature of fathers and sons gives a positive coefficient. Whenever the deviation from the average of one group is accompanied in the second group by a deviation in the same direction, the coefficient is positive. A negative correlation means that deviation from the average in a given direction in the first group is accompanied in the second group by a deviation in the opposite direction. If we imagine that as one measurement increased above its average a second related measurement decreased below its average the correlation in such a case would be negative. For instance, if we measured the relation between the number of berry pickers employed and the quant.i.ty of berries remaining unpicked, in a number of different fields we would get a negative correlation coefficient. Some organisms are formed in such a way that increase in one dimension, such as length, is a.s.sociated with decrease in another, such as breadth; measurement of the relatedness of these dimensions would give a coefficient of correlation that might be very high, indicating a considerable relation in the deviations, but it would be negative. In an instance of negative correlation the relation is that of "the more the fewer." As we shall see presently, a negative correlation may be just as important and significant as a positive correlation.
The application of the principles of heredity to our subject of Eugenics is of such great importance that it is reserved for separate consideration in the next chapter. We may, therefore, devote the remainder of this chapter to the consideration of data of another kind, which are commonly treated by this same method of determining correlation coefficients between two sets of varying phenomena in order to determine whether there is any actual relation between them or not. This will serve to ill.u.s.trate the use of this method.
We shall turn then to the subject of differential or selective fertility in human beings and consider its relation to Eugenics. As a starting point we may take the self-evident statement that a group of organisms will tend to maintain constant characteristics through successive generations only when all parts of the group are equally fertile. If exceptional fertility is a.s.sociated with the presence or absence of any characteristic the number of individuals with or without that trait will either increase or diminish in successive generations, and the character of the distribution of the group as a whole will gradually become altered, the average moving in the direction of the more fertile group. Or if infertility is so a.s.sociated, then the average of the whole group moves away from that condition. Eugenically, then, we should ask whether in human society there is at present any such a.s.sociation of superfertility or infertility with desirable or undesirable traits. It is obviously the aim of Eugenics to bring about an a.s.sociation of a high degree of fertility with desirable traits and a low degree of fertility with undesirable characteristics.
First, let us look at certain data gathered relative to the size of the family in both normal and pathological stocks (Table II). In order that a stock or family should just maintain its numbers undiminished through successive generations and under average conditions, at least four children should be born to each marriage that has any children at all.
TABLE II
_Fertility in Pathological and Normal Stocks._ (From Pearson)
NATURE OF MARRIAGE. NO. IN AUTHORITY. (Reproductive period.) FAMILY.
Deaf-mutes, England Schuster Probably complete 6.2 Deaf-mutes, America Schuster Probably complete 6.1 Tuberculous stock Pearson Probably complete 5.7 Albinotic stock Pearson Probably complete 5.9 Insane stock Heron Probably complete 6.0 Edinburgh degenerates Eugenics Lab Incomplete 6.1 London mentally defective Eugenics Lab Incomplete 7.0 Manchester mentally defective Eugenics Lab Incomplete 6.3 Criminals Goring Completed 6.6 English middle cla.s.s Pearson 15 years at least, begun before 35 6.4 Family records--normals Pearson Completed 5.3 English intellectual cla.s.s Pearson Completed 4.7 Working cla.s.s N.S.W. Powys Completed 5.3 Danish professional cla.s.s Westergaard 15 years at least 5.2 Danish working cla.s.s Westergaard 25 years at least 5.3 Edinburgh normal artisan Eugenics Lab Incomplete 5.9 London normal artisan Eugenics Lab Incomplete 5.1 American graduates Harvard Completed 2.0 English intellectuals Webb Said to be complete 1.5
All childless marriages are excluded except in the last two cases. Inclusion of such marriages usually reduces the average by 0.5 to 1.0 child.
The table given shows clearly what stocks are maintaining, what increasing, and what diminishing their numbers.
This subject has been investigated recently in a rather extensive way by David Heron, for the London population. Heron concentrated his attention upon the relation of fertility in man to social status. He used as indices to social status such marks as the relative number of professional men in a community, or the relative number of servants employed, or of lowest type of male laborers, or of p.a.w.nbrokers; also the amount of child employment pauperism, overcrowding in the home, tuberculosis, and pauper lunacy. Twenty-seven metropolitan boroughs of London were canva.s.sed on these bases, which are certainly significant, though not infallible, indices to the character of a community. His results are shown in the briefest possible form in Table III.
TABLE III
_Correlation of the Birth Rate with Social and Physical Characters of London Population._ (From Heron.)
CORRELATION COEFFICIENT.
With number of males engaged in professions -.78 With female domestics per 100 females -.80 With female domestics per 100 families -.76 With general laborers per 1,000 males +.52 With p.a.w.nbrokers and general dealers per 1,000 males +.62 With children employed, ages 10 to 14 +.66 With persons living more than two in a room +.70 With infants under one year dying per 1,000 births +.50 With deaths from pulmonary tuberculosis per 100,000 inhabitants +.59 With total number of paupers per 1,000 inhabitants +.20 With number of lunatic paupers per 1,000 inhabitants +.34
This table gives the results of the calculation of coefficients of correlation between the birth rates and the conditions enumerated. We may just recall that this coefficient is a measure of the regularity with which the changes in two varying conditions or phenomena are a.s.sociated: and further that a coefficient of 1.0 indicates perfectly regular a.s.sociation, 0.75 a very high degree of regularity. The first line of the table then, for example, means that when these twenty-seven districts were sorted out, first, with reference to the number of professional men dwelling in them, and then with reference to their respective birth rates, there was found a very high degree of regularity (coefficient of correlation = -.78) in the a.s.sociation of these two conditions--birth rate and number of professional men. Here is a very close relation, _but_, the sign of the coefficient is _negative_. The significance of this negative sign is that among the communities studied those where the number of professional men is the larger show always, at the same time, the lower birth rates. Coming to the second line of the table, it seems fair to a.s.sume that the number of servants employed in a district in proportion to the total number of residents or families there, gives a fairly though not wholly satisfactory indication of the social character of the community.
Measurement of the actual relation between the proportional number of servants employed in a community and the birth rate in that community, gave practically the same result as in the case of the number of professional men. The more servants employed in a district the lower its birth rate. Two methods of measuring this relation gave essentially the same result; comparison of the birth rate with the ratio of domestics, first to the number of families, second to the number of females, gave -.76 and -.80 respectively--very high coefficients and both negative.
But the sign changes and becomes positive when we come to other comparisons. When we count the relative number of p.a.w.nbrokers and general dealers, of "general laborers" (that is, men without a trade and without regularity of occupation and employment), of employed children between the ages of ten and fourteen, of persons living more than two in a room, when we consider the infant death rate, the death rate from pulmonary tuberculosis, and the relative number of paupers,--then we find the signs of the coefficients are all positive, and on the average the coefficients are more than 0.50--a moderate to high degree of regularity of the relation. The districts characterized by the larger numbers of such individuals or by higher death rates of these kinds, are at the same time the districts where the birth rates are the higher.
In a word, then, Heron found that the greater the number of professional men, or of servants employed in a community, the lower the birth rate--a very high degree of negative correlation. On the other hand, the more p.a.w.nbrokers, child laborers, pauper lunatics, the more overcrowding and tuberculosis, the higher the birth rate--a high degree of positive correlation. Little doubt here as to which elements of the city are making the greater contributions to the next generation. There may be some doubt, however, so let us consider two possible qualifications of these results. First, is not the death rate also higher among these least desirable cla.s.ses? Yes, it is. Is it not enough higher to compensate for the difference in the birth rates, so that after all the least desirable cla.s.ses are not more than replacing themselves? No, it is not. After calculating the effect of the differential death rate among these different social groups it still remains true that the _net_ fertility of the undesirables is greater than the _net_ fertility of the desirables: the worst cla.s.ses are in reality more than replacing themselves numerically in such communities; the most valuable cla.s.ses are not even replacing themselves. Second, is not this the same condition that has always existed in these districts? Why any cause for supposing that this is going to bring new results to this society? Has not such a condition always been present and always been compensated for somehow?
Fortunately, Heron is able to compare with these data of 1901 similar data for 1851, and is able to show that every one of these relations has changed in sign since that date--in fifty years. The significance of this change in sign is probably clear. It means here that in London sixty years ago there was a high degree of regularity in the relation such that the more professional men and well-to-do families the community contained, the higher the birth rate; that ten years ago this had all become changed so that the more of these desirable families found in a district the lower is the birth rate. It means that sixty years ago the relation was such that the more undesirables numbered in a district, the lower its birth rate; ten years ago the more undesirables, the higher the birth rate, and the coefficients of 1901 are unusually high, indicating great closeness and regularity in this relation. Heron is further able to show that as regards number of servants employed, professional men, general laborers, and p.a.w.nbrokers in a district, the intensity of the relationship has _doubled_, besides changing in sign, in the period observed. It is not necessary to review the history of this change nor to discuss the causes involved, but it is necessary to take into account for the immediate future the fact of the change.
Sidney Webb has recently published an account of the birth-rate investigations undertaken by the Fabian Society with a view to determine the causes leading to the rapidly falling birth rate in England. During the decade previous to 1901 the number of children in London actually diminished by about 5,000, while the total population increased by about 300,000. As far as they bear upon this phase of the subject his results fully confirm these we have been considering. The falling off is chiefly in the upper and middle cla.s.ses, in the cla.s.ses of thrift and independence, and it has occurred chiefly during the last fifty years. Webb cannot find that this is due to any physical deterioration in these cla.s.ses; it is due to a conscious and deliberate limitation of the size of the family for what are thought prudential and economic reasons.
An actual reduction in the number of children may not be an unmixed evil. A falling birth rate may be a good sign. This is partly a question for the political economist. "Suicide" may be a socially fortunate end for some strains. But when, in either a rising or a falling birth rate, we find a differential or selective relation, then the subject is eugenic. If the higher birth rate is among the socially valuable elements of each different cla.s.s the Eugenist can only approve; to bring about such a relation is one of his aims. What we really find, however, is the undesirable elements increasing with the greatest rapidity, the better elements not even holding their own.
One further aspect of the result of the smaller family remains to be considered. Are the various members of a single family approximately similar in their characteristics or are the earlier born more or less likely to be particularly gifted or particularly liable to disease or abnormal condition? Or is there no rule at all in this matter? There is much evidence that the incidence of pathological defect falls heaviest upon the earlier members of a family. Consider, for example, the presence of tuberculosis. We should ask, in families of two or more, are the tubercular members, if any, as likely to be the second born or third or tenth as to be the first born? The data are tabulated in Fig. 11, _A_. The distribution of family sizes being what it is in the number of families investigated and tabulated, we should expect that there would be about 65 tubercular first born, 60 tubercular second born, and so forth, on the basis of its average frequency in the whole community, provided the chances are equal that any member of the family should be affected with tuberculosis. What we actually find, however, is that 112 first born are affected, about 80 second born, and after that no relation between order of birth and susceptibility to tuberculosis. That is, susceptibility to tuberculosis is double the normal among first born children. The same thing is true for gross mental defect. Fig. 11, _B_, shows that the ratio of observed to expected insane first born children is about 4 to 3. Such a relation has long been known to criminologists and frequently commented upon. Fig. 11, _C_, gives a definite expression to the facts here. Whereas, in the number of families observed about 56 criminal first born were to be expected, the number actually found is about 120; for the second born the corresponding numbers are about 54 and 78, and after that no marked relation is found between order of birth and criminality. For albinism (Fig. 11, _D_) the expected and observed numbers among first born are about 185 and 265, second born 165 and 190, and thereafter no definite relation. It remains to be seen whether a similar relation holds for the unusually able and valuable members of a family; something has been said on both sides here, but there are available at present no data sufficiently exact to be worthy of consideration.
[Ill.u.s.tration: FIG. 11.--Diagrams showing the relation between order of birth and incidence of pathological defect.
(From Pearson).]
We have here a result that has very important bearings upon the value to the race of the large family and of the danger of the small family.
The small family of one, two, or three children contributes on the average much more than its share of pathological and defective persons. No matter just now what the causes are, they seem to be more or less beyond remedy. The result for the future, however, must be reckoned with. This relation has important bearings upon the custom of primogeniture as well as upon the eugenic values of the large family.
In conclusion let us give a few sentences only slightly modified from Pearson"s "Grammar of Science." The subject of differential fertility is not only vitally important for the theory of evolution, but it is crucial for the stability of civilized societies. If the type of maximum fertility is not identical with the type fittest to survive in a given environment, then only intensive selection can keep the community stable. If natural selection be suspended there results a progressive change; the most fertile, whoever they are, tend to multiply at an increasing rate. In our modern societies natural selection has been to some extent suspended; what test have we then of the ident.i.ty of the most fertile and the most fit? It wants but very few generations to carry the type from the fit to the unfit. The aristocracy of the intellectual and artizan cla.s.ses are not equally fertile with the mediocre and least valuable portions of those cla.s.ses and of society as a whole. Hence if the professional and intellectual cla.s.ses are to be maintained in due proportions they must be recruited from below. This is much more serious than would appear at first sight. The upper middle cla.s.s is the backbone of a nation, supplying its thinkers, leaders, and organizers. This cla.s.s is not a mushroom growth, but the result of a long process of selecting the abler and fitter members of society. The middle cla.s.ses produce relatively to the working cla.s.ses a vastly greater proportion of ability; _it is not want of education, it is the want of stock which is at the basis of this difference_. A healthy society would have its maximum of fertility in this cla.s.s and recruit the artizan cla.s.s from the middle cla.s.s rather than _vice versa_. But what do we actually find? A growing decrease in the birth rate of the middle and upper cla.s.ses; a strong movement for restraint of fertility, and limitation of the family, touching only the intellectual cla.s.ses and the aristocracy of the hand workers! Restraint and limitation may be most social and at the same time most eugenic if they begin in the first place to check the fertility of the unfit; but if they start at the wrong end of society they are worse than useless, they are nationally disastrous in their effects. The dearth of ability at a time of crisis is the worst ill that can happen to a people. Sitting quietly at home, a nation may degenerate and collapse, simply because it has given full play to selective reproduction and not bred from its best. From the standpoint of the patriot, no less than from that of the evolutionist and Eugenist, differential fertility is momentous; we must unreservedly condemn all movements for restraint of fertility which do not discriminate between the fertility of the physically and mentally fit and that of the unfit. Our social instincts have reduced to a minimum the natural elimination of the socially dangerous elements; they must now lead us consciously to provide against the worst effects of differential fertility--a survival of the most fertile, when the most fertile are not the socially fittest.
The subject before us ill.u.s.trates the direct bearing of science upon moral conduct and upon statecraft. The scientific study of man is not merely a pa.s.sive intellectual viewing of nature. It teaches us the art of living, of building up stable and dominant nations, and it is of no greater importance for the scientist in his laboratory, than for the statesman in council and the philanthropist in society.
III
HUMAN HEREDITY AND THE EUGENIC PROGRAM