Popular Science Monthly/Volume 48/March 1896/The Study of Inheritance II
|THE STUDY OF INHERITANCE.|
IT may be well to remind those who are not familiar with statistical reasoning that a type may exhibit the influence of inheritance and yet be of no value as a basis for generalization on inheritance.
The bullet type shows the influence of aim, but if we use it to test the accuracy of aim or the excellence of the rifle we may be led astray if some other influence, such as the weight of the bullet, act on all or on a majority of the shots and escape detection. In this case the type may seem to prove that the rifle is inaccurate or improperly sighted when it is not, and we can not assume that because a type shows the influence of aim it is a test of aim.
So a characteristic or a group of characteristics of living things may conform to the mathematical law of deviation from a mean, and may thus form a type, and this type may show the influence of inheritance, without being a safe basis for generalization regarding inheritance.
This may be illustrated by an example. If we were to tabulate the prices of all the horses sold within a given period, we should undoubtedly find that they would conform to a type: that there is a mean or average price; that the horses which fetch more than this price are equal in number to those which fetch less, and that the prices group themselves about the mean according to the law of error. If the term be long enough to include several generations, we shall find that inheritance or "blood" has a marked influence on price, and that the children of high or moderate or low priced parents are much more likely than horses selected at random to bring the same price. This type will exhibit the influence of inheritance, but it will be of no value in studying inheritance unless we can in some way separate the influence of blood from the influence of supply and demand which has far more to do with the average price and with the type.
That the price of horses is, on the whole, determined like that of other commodities is obvious, and it is also obvious that the type may be changed by events which have no relation to inheritance, such as the application of electricity to street cars.
A change of this sort, such as took place when steam replaced stage coaches, is a "sport" or sudden and fundamental change of type, but this may also be changed by slight and gradual modification with the slow growth of a complicated civilization and an increased demand for horses.
As inheritance has an influence on the price of horses, what will be the result if we destroy the children of all horses which fetch less than +2 of Galton's scheme, and breed only from that fourth of the whole which sell for more than 75° of his centesimal scale?
We may at first get fancy prices for our expensive stock, but if selection cease with this first step, and we supply as many colts as before, while the demand remains unchanged, the price will "revert" to the type, and the mean will become the same as it was.
Does this prove that those qualities in horses for which money is paid have "retrograded to mediocrity" in these descendants of high-priced parents? It proves nothing of the sort, for the qualities which command a price are one thing and the price another. Even if the horses have much more of these qualities than the old stock, the price will still be fixed by the ratio between demand and supply, and while blood will tell in use it will not tell in price.
It is clear, then, that characteristics of living things which are influenced by inheritance may conform to a type which exhibits "specific stability," "regression to mediocrity," an occasional "sport," and all the other properties of the types which Galton has studied, without furnishing proof that "inherited" qualities behave in the same way. To prove this we must cancel, or neutralize, or make allowance for all the factors which have an influence upon the type, except "inheritance."
Galton's generalizations upon the laws of inheritance from the statistical study of finger prints rests upon the belief that the patterns are inherited. If they are not, they can teach nothing of inheritance. He proves by statistics that they are, to some degree, dependent either directly or indirectly upon inheritance, just as the price of horses is, but this is not enough. To warrant his deductions he must prove that inheritance is the controlling factor in determining the type; that, in the long run, all the other factors will balance; and this, it seems to me, he fails to prove. He has studied in one hundred and fifty fraternal couples or children of the same parents the frequency with which the same pattern occurs on the same digit of both, and he finds that when marked on a scale in which indicates no resemblance and 100° the greatest possible relationship, they show 10° of relationship. This number is great enough to prove the influence of inheritance, but it is too small to show that the patterns are themselves directly inherited, and it seems to indicate that they are indirectly influenced by some other inherited characteristic, such perhaps as the ratio between the growth in the embryo of the ball of the finger and that of the nail.
Inheritance is unfortunately a word which is not always used with scientific precision. Most of the qualities which give a horse its value, as compared with other horses, are due to breeding, but this word has many meanings. Orlando says: "His horses are bred better; for besides that they are fair with their feeding, they are taught their manage, and to that end riders dearly hired." The "breeding jennet, lusty, young, and proud," seems to be a wild mare, with no breeding in the first sense, and the horse which did not lack what a horse should have, "round-hoofed, short-jointed, fetlocks shag and long. Broad breast, full eye, small head, and nostrils wide. High crest, short ears, straight legs, and passing strong. Thin mane, thick tail, broad buttocks, tender hide," is a thoroughbred.
Recent speculations have enforced attention to the difference between these meanings of the word. In the last sense breeding is the influence of ancestry, and it may practically be treated as synonymous with the word ancestry.
In the first sense breeding, broadly used, is that influence of the ontogenetic environment for which that most objectionable term "acquired characters" has been thoughtlessly adopted.
In his earlier writings Galton, borrowing, I suppose, from the Tempest, uses the word "nurture" to designate it, and this term is so apt and expressive that it should not pass out of use, for it may be given a definite technical meaning without violence to its ordinary use.
Using nurture instead of acquired characters for the influence of the environment of the individual, we may speak of the two elements of breeding as ancestry and nurture.
At the present day it is obvious that our studies of inheritance can have little value unless we distinguish between these two factors, for many naturalists hold that there is good ground for questioning whether the effects of nurture are ever inherited, and most naturalists admit the possibility that the value of these two factors may be very different.
If breeding is to be studied by the statistical method for the purpose of exhibiting the laws of inheritance, we must employ types in which we can separate the effects of ancestry from the effects of nurture; for if we make use of types which do not admit of this analysis, our results may tell us no more of inheritance than the scheme of prices tells us of the value of blood in horses.
If, as many teach, inheritance is equivalent to ancestry, and nurture is not inherited, no type in which these two factors are combined can tell us anything about inheritance.
It seems probable from Galton's data regarding the resemblance between the finger marks of fraternal couples that this is due to nurture in the broad sense of the word, and not to inheritance, and there is ample evidence that the value in breeding of a given parental characteristic does depend upon its origin, and that one due to nurture has a very different value from one which is itself inherited.
Of the 2,459 deaf pupils of the American Asylum, nearly six hundred have married and have become the parents of over eight hundred children, of whom 104, or more than twelve per cent, were born deaf—a ratio which is great enough to prove that inheritance has some influence. Analysis of the records shows clearly, however, that these deaf children are not uniformly distributed among the married pupils of the asylum, but that the result is influenced by the character of the parental deafness. From 283 of the 590 marriages no children are reported, while from three other families no report is made except that all the children hear, so that the 811 child en which are reported are from only 304 families, and in many of them only one parent was deaf. Of the 101 children of forty of these marriages none are reported as deaf, and all but eleven are reported as hearing, and the 710 children are from the remaining 264 marriages. In fifty-two of the marriages both father and mother were congenitally deaf, and these are the parents of forty-eight out of the 104 congenitally deaf children, but they are the parents of only 151 of the total number of 811 children, and nearly thirty-two per cent of all the children of these congenitally deaf parents are congenitally deaf.
In two of the groups in which the marriages may be classified the number of marriages and the number of children are about equal, but there is a most remarkable difference in the number of deaf children.
In fifty-five marriages, with 139 children, both parents are reported as adventitiously deaf, while in fifty-two marriages, with 151 children, both were congenitally deaf. In the latter group fifty-two children, or 31·78 per cent, are congenitally deaf, only eighty-eight are stated to hear, and no facts are given about the hearing of fifteen of them. In the first group only four of the 139 children, or 3·87 per cent, are reported as congenitally deaf, 129 are reported as hearing, and six are not reported.
I have divided all the marriages into four groups: In one all the children hear; in the second from five to six per cent are deaf; in the third from twelve to eighteen per cent are deaf; and in the fourth 31·78 per cent are deaf. In the first group, in which all the children hear, five of the marriages, with eighteen children, are between a hearing husband and a wife who is adventitiously deaf; one marriage with four children between a hearing man and a woman the source of whose deafness is unknown; six marriages, with thirteen children, where the wife hears and the husband is adventitiously deaf; twenty-three marriages, with fifty-one children, where husband is adventitiously deaf and wife deaf from unknown causes; two marriages, with six children, where both were deaf from unknown causes; one marriage, with four children, where husband is deaf from unknown causes and wife hears; and two marriages, with five children, where wife is congenitally deaf and husband deaf from unknown causes. None of the 101 children of these forty marriages are reported as deaf.
In the second group, where from five to six per cent of the children are deaf, eighty-seven are the children of thirty-seven marriages where the husband was congenitally and the wife adventitiously deaf; and 139 are the children of fifty-five marriages where both husband and wife were adventitiously deaf.
In the third class, where from twelve to eighteen per cent of the children are congenitally deaf, 124 are children of fifty-one marriages where husband was adventitiously and wife congenitally deaf; sixty-six were children of sixteen marriages of hearing husband and congenitally deaf wife; seventy-two were children of twenty-six marriages where wife hears and husband is congenitally deaf; and seventy-one of twenty-nine marriages of congenitally deaf husband with deaf wife of unknown origin. In all the families of this group one parent was congenitally deaf.
In the fourth class, where 31·78 per cent of the children are congenitally deaf, all the parents in the fifty-two marriages with one hundred and fifty-one children are congenitally deaf.
While too few to give quantitative results, these statistics prove that it is the congenital and not the adventitious deafness which is transmitted.
Of the fifty-two families in which both parents are congenitally deaf, twenty-three have congenitally deaf children.
Of the thirty-seven families in which the husbands are congenitally deaf and the wives adventitiously deaf, two have deaf children—four in one family and one in the other.
Of the fifty-one families in which the fathers were adventitiously deaf and the mothers congenitally deaf, seven produced deaf children, and nine of the congenitally deaf children came from two families.
There are fifty-five families in which both parents are adventitiously deaf, and from these have sprung four congenitally deaf children—one in each of four families.
Four of the sixteen families in which the husbands hear and the wives are congenitally deaf have deaf children.
In five families out of the twenty-six in which the husbands are congenitally deaf and the wives hear, there are children born deaf.
Six of the twenty-seven families in which the husbands were congenitally deaf and the state of the hearing of the wives is unknown produced congenitally deaf children.
Of the twenty-six families in which both parents are deaf and have congenitally deaf children, there are five families in which one of the parents has one deaf parent, seventeen families in which both parents have deaf relatives of the same generation, four in which one parent has deaf relatives of the same generation, and five in which neither parent has deaf relatives of the same generation.
Of the twenty-six families in which both parents are congenitally deaf and have hearing children only, there is none in which either parent has a deaf parent, so far as reported, twelve families in which both parents have deaf relatives of the same generation, eleven families in which one parent has deaf relatives of the same generation, and three families in which neither parent has deaf relatives of the same generation.
This illustration proves that the origin of an individual peculiarity has much to do with the question of its inheritance, and that we can not be sure that statistical data illustrate inheritance unless we can separate phenomena of ancestry from those of nurture.
Furthermore, in order to prove that children always revert to the mean or type of the race, and are on the average more mediocre than their parents, we must prove that this is the case when both parents have the same inherited peculiarity.
Galton shows that this is true of the stature of children both whose parents were tall or both short, but he has not shown that it is true when the peculiarity in the stature of both parents is the same inherited peculiarity. He points out that stature may be affected by diversity in the thickness of more than one hundred bodily parts, and it is plain that if the extra height of a tall father is due to a long femur for example, the chances are a hundred to one that the femur of the tall mother is normal and that her extra height is due to some other peculiarity—thick intervertebral bodies, for example.
There is statistical evidence from other sources to prove that if both parents have long femurs and have brothers and sisters with long femurs, the children, instead of reverting to mediocrity, may on the average be expected to have femurs very much above the mean, and that some of them may have them longer than either parent.
Many facts in our stock of information regarding domestic animals and cultivated plants show that hereditary peculiarities are often very persistent independently of selection, and the experience of all breeders shows that this tendency is greatly intensified when both parents have the same inherited peculiarity.
Not only is this the case, but it may he proved by many observations that the normal or type to which the average children of exceptional parents tend to revert may itself be rapidly modified.
In proof of this I refer to the following experiments in selection by Fritz Müller (Ein Zuchtungs-versuch an Mais. Kosmos, 1886, 2, i, p. 22):
Yellow corn is very variable in many respects. The number of rows of kernels on the cob is usually from eight to sixteen; cobs with ten or twelve rows being the most common, while one with eighteen or twenty rows is very seldom found. After searching through several hundred cobs Fr. Müller found one ear with eighteen rows, but none with more.
In 1867 he sowed, at different times, and in such a way as to prevent crossing, (1) seed from the cob with eighteen rows; (2) the seed from the finest sixteen-rowed ear; and (3) the seed from the finest fourteen-rowed ear. In 1868 he sowed (1) seed from a sixteen-rowed ear which had grown from seed from a sixteen-rowed ear; (2) seed from an eighteen-rowed ear from sixteen-rowed seed; and (3) seed from an eighteen-rowed ear from eighteen-rowed seed. In 1869 he sowed (1) seed from an eighteen-rowed ear with eighteen-rowed parents and grandparents; (2) seed from a twenty-rowed ear with eighteen-rowed parents and grandparents; and (3) seed from a twenty-two-rowed ear from seed from an eighteen-rowed ear produced from seed from a sixteen-rowed ear. The results are given in the accompanying table:
|Number of rows on cob from which seed were taken.||1867.||1868.||1869.|
|Total number of cobs produced.||658||385||205||1,789||262||460||2,486||740||373|
|8-rowed cobs||0·3||. . . .||0·5||0·1||. . . .||. . . .||. . . .||. . . .||. . . .|
|10-rowed cobs||14·4||3·0||1·0||1·4||0·8||0·2||0·1||. . . .||. . . .|
|20-rowed cobs||. . . .||0·1||0·3||0·3||1·2||4·4||3·9||2·8||4·8|
|22-rowed cobs||. . . .||. . . .||0·3||. . . .||0·8||0·2||0·5||0·8||1·0|
|26-rowed cobs||. . . .||. . . .||. . . .||. . . .||. . . .||. . . .||. . . .||. . . .||0·5|
It will be seen from this table that the number of ears with few rows decreases very rapidly in plants grown from seed taken from ears with many rows, and that the greater the number of rows on the ear from which the seed is taken the smaller is the number of ears produced with a small number of rows. It is also plain that, as the number of rows on the ear from which the seed was taken increases, the number of ears produced with a large number of rows increases, and that we have in each case a very considerable number of ears which equal their parents and a few which excel them, even when the parent seeds are far beyond the maximum for all ordinary corn. Fritz Müller says he has never, under ordinary conditions, except in three instances, found an ear with more than eighteen rows, and Darwin puts the maximum at twenty rows; yet we have among the children of seed from a twenty-two-rowed ear no less than 4·8 per cent, or eighteen ears out of 373 with twenty rows, and one ear out of 373 with twenty-six rows, and it will also be seen that the number of children which equal their parents increases in each case in each successive generation.
Thus the seed planted in 1867 from an eighteen-rowed ear produced 12·6 per cent of eighteen-rowed children. The eighteen-rowed ear planted in 1868 from an eighteen-rowed parent produced 18·2 per cent of eighteen-rowed children, and the eighteen-rowed seed planted in 1869 from eighteen-rowed parents and grandparents produced 18·6 per cent of eighteen-rowed children. The series is 12·6 per cent, 18·2 per cent, and 18·6 per cent.
The rapid change which took place in the "type" after only three years of selection is well shown by the following table, which gives the dominant number of rows at each sowing, and also the percentage of ears which had this number:
|1867,||12||rows||48||per cent.||1868,||14||rows||35·4||per cent.|
The minimum for the third generation is equal to the mean for the first; the mean for the third generation, sixteen rows, is very near the maximum for ordinary corn, and the maximum for the third generation is far beyond the maximum for the grandparents, and much beyond the maximum for the parents.
No one can dispute the well-known fact that this sort of pedigree selection for a single point quickly grows less and less effective, and soon reaches a maximum; but this is no proof of any "principle of organic stability," or anything else except the truth that long ages of natural selection have made the organism such a unit or coordinated whole that no great and continuous change in one feature is possible, unless it is accompanied by general or constitutional change.
We must not forget, in addition, that, in a state of Nature, selection is neither for one feature, nor is it pedigree selection, or breeding from the fittest.
It is the extermination of the unfit, and unfitness may come from the imperfect co-ordination of the whole, or from defect in any quality whatever.
It is undoubtedly true that many of our domesticated races can be proved to have arisen as "sports," and that no great change of type can be effected, by the methods of the breeder, without sports; but there seem to be both evidence and theoretical ground for holding that, in this particular, artificial selection furnishes no measure of natural selection.
It seems to me that, notwithstanding the great value of Galton's data, they fail to prove that the "principle of organic stability" owes its existence to anything except past selection; that regression to mediocrity occurs when ancestry is studied uncomplicated by nurture; that the "mid-parent" is anything else than the actual parent; that "sports" are fundamentally different from the ordinary differences between individuals; or that natural selection is restricted to the preservation of sports.
Our tendency to believe that a type is something more real and substantial than the transitory phenomena which exhibit it is deeply rooted in our minds.
As the very nature of this belief renders disproof of it impossible, we can feel little surprise at its appearance and reappearance time after time in the history of thought, although science is based upon the well-warranted opinion that, whether types are real or unreal, we know them only as generalizations or abstractions constructed by our minds out of our experience of the orderly sequence of phenomena.
In zoölogy and botany the conception of species is unquestionably valid and justifiable, and as its most obvious characteristic is its persistency, as contrasted with the fleeting procession of evanescent individuals, we can not wonder at the vitality of the belief that specific types of life are more real than the individual animals, although Darwin's work has done away with whatever evidence may at one time have seemed to support this belief.
To the further question, whether specific types are inherent in living matter or external and objective to it, Darwin answers that they are both; that they are inherent, insomuch as all their data, or "events," are properties of the physical basis of life; but that they are external, inasmuch as the agreement of the "events" with the "law of frequency of error" is the effect of the environment.
Biology is not a closed science, and Darwin's view of the matter is not proved; possibly it is not provable; but its great value is in the proof that there is no shadow of evidence for any other view.
When embryologists talk about the doctrine of evolution in embryology as antagonistic to the doctrine of epigenesis; when biologists seek for the origin of species in "laws of variation" which are not the outcome of selection; when they talk about a "principle of organic stability" which does not owe its origin to the same agency—it seems to me that they fail to grasp the significance of Darwin's work, and that they are wandering from the only path in which we can have any well-grounded hope for progress—the path which takes its departure from that conception of specific types which leads us to seek for the origin of the "events" which exhibit the type in the physical properties of living matter, and to seek in the order of Nature external to the organism for the origin of the "law of error" which forms a type out of these events.