Popular Science Monthly/Volume 37/July 1890/Human Heredity
PROFESSOR OF NATURAL HISTORY IN UNION COLLEGE.
IN common speech we use the term heredity as signifying simply that principle by which the qualities of parents are transmitted to their children. We give the term a meaning broad enough to cover facts which come within our ordinary notice. We see that the features of children—the shape of the brow and nose, the color of the hair and eyes—bear a resemblance to those of the parents; as they grow older we notice not only physical but also psychical resemblances—the temperament, tastes, and aptitudes are more or less like those of the parents. We find an explanation of these likenesses in the principle of heredity; and, as no evidence of any deeper operation of such a principle comes within our ordinary observation, we limit it to these particulars. It is true that occasionally we are reminded that the principle may extend to a second generation; we see the traits of the grandparent reappearing in the child, this being most noticeably true in respect to certain bodily disorders, as scrofulous diseases and certain forms of insanity. But we seldom think that the principle of heredity operates through more than the two or three generations of our immediate ancestors, or that any other qualities than those which are specifically peculiar to us—that mark our individuality of body or mind—come to us by it.
A little reflection, however, must convince us that this principle works more deeply. Those qualities that distinguish us as members of a nationality—whence come they? As Americans we pride ourselves that there is something distinctive about us, that places us in a different category from Englishmen and Frenchmen. Whence come these national characteristics? They were possessed by our fathers and our grandfathers, and the immediate inference, therefore, is that they come to us by inheritance. Of course, we have to consider that the fathers who were the founders of the nation did not inherit the American characteristics, since we must regard them as the original possessors of them. The fact seems to be that national characteristics originate in external causes, but once established they are perpetuated by inheritance. It may be urged, of course, that external causes operate upon succeeding generations as well as the antecedent one, as evidenced in our nationality by its rapid absorption of foreign stock. No doubt the direct influence of our institutions is a constant force in the development of the national characteristics, and goes a great way toward Americanizing citizens of foreign birth even in a single generation. But to native-born Americans this influence simply adds itself to that of inheritance, and does not diminish its strength or importance. Taking a general view of the question, the case becomes very obvious. It will scarcely be disputed that national characteristics manifest themselves definitely, not only in the temperament, ideas, etc., of the people, but also in their bodily features. We speak of the English type of facial features, the German type, etc., and every one appreciates that these terms express real distinctions. Moreover, we know that these types have existed a long time, slightly variable, no doubt, but never losing the main lines. It is scarcely necessary to say that this continuance of type rests on heredity. The case is precisely the same as that of the continuance of the family likeness, only the family is larger and the features less distinctive, though, in the long run, they are more faithfully conserved.
Lying deeper than those characteristics that mark us as members of a nationality are others that mark us as members of one of the great races of the human family. The term race has different significations according to its use, whether referring to distinctions chiefly of an anatomical character (though connoting others), as the Caucasian, Mongolian, etc., races, or to distinctions based more directly on differences of lineage, as the Celtic, Teutonic, etc., races. For the purpose in hand we use it in the former sense, dividing mankind into the usual five races, Caucasian, negro, Indian, etc. Now, it is obvious that our race characteristics come to us in the same way as our national and family characteristics: we get our white skin and orthognathous skulls by inheritance just as truly as our more specific bodily features, only these qualities come to us from ancestors more remote. We need not concern ourselves here how they obtained them, nor whether they were acquired suddenly in a single generation or gradually through many generations. The point to be insisted upon is that, race characteristics once established, they are transmitted by inheritance through all succeeding generations. Of course, the principle applies not only to merely anatomical features, but to mental traits as well. The peculiarities of mental constitution that make the Caucasian the most progressive race are handed down by inheritance just as truly as the color of the skin and the shape of the skull.
Still deeper than the race characteristics—more fundamental than they—are those that mark us as members of the human family itself. Our convoluted brains, our power of verbal speech, certain of our intellectual and our moral faculties, these are the qualities that belong to us in common with all men, and that distinguish us from the highest animals. Whence came these qualities? It can only be answered that they came from the fathers of the race, having been transmitted by the law of heredity through the successive generations to us. It matters not whether the race originated in a single pair or had a multiplex origin, nor are we here concerned how our progenitors came to possess these qualities at all; the fact at hand is that, once having lived, they transmitted to their descendants down to us their distinctive human qualities.
The facts to which our attention has now been given are summarized as follows:
All the qualities of our human nature come to us by inheritance.
Those qualities which are strictly individual—the "skin-deep" qualities—come to us from our immediate ancestors, our parents and grandparents.
Those qualities which are less specific, which we have in common with others who live under the same laws and institutions, and generally under similar physical conditions, come to us from ancestors more remote, though quite within historic time.
Those qualities which are still more general, which we have in common with others of the same general physical features, came from ancestors much more remote, whose records are lost in prehistoric time.
Those qualities which are broadly anthropological, which we possess in common with all members of the human family, came to us from the original progenitors of the race.
We have thus far considered only the strictly human qualities of our nature. We have now to consider whether the operation of the law of heredity extends also to the animal qualities. Let us first notice those which man possesses in common with the highest animals. They are, a vertebral column, giving form and flexibility to the body; two pairs of limbs for prehension and locomotion, mammary glands supplying food for the young; a four-chambered heart and double blood-circulation; and, finally, a well developed nervous system, with sense-organs, placing the animal in conscious relation with the external world. Does the principle of heredity by which, as we have seen, all our anthropological qualities have come to us, give us also these zoölogical qualities? The point here to be enforced is, that if the answer to this question is not in the affirmative, then there is a break in that law, the operation of which we have seen to extend from the most specific to the most general anthropological qualities. In considering whether there is such a break, the special point of inquiry is whether the two classes of qualities, the anthropological and the zoölogical, are different from each other in kind. For, if they be the same in kind, the presumption is that the law operates in respect to both; in other words, that there is no break. The anthropological qualities are, as we have seen, partly physical—the shape of the skull, the structure of the brain, the color of the skin—and partly psychical, the intellectual and moral faculties. The zoölogical qualities are also partly physical and partly psychical: of the former class, the general structure of the body and the particular structure of organs, as the heart, lungs, glands, etc.; of the latter class, the faculties of intelligence. It appears, therefore, that the zoölogical qualities are the same in kind with the anthropological, and the inference, therefore, is that the law of heredity extends also to them. That is to say, as we proceed, step by step, from the most specific to the most general qualities peculiar to man, and then by the next step pass to those qualities which we possess in common with the highest animals, we find that the last term of the series is the same in kind with the others, and all the reasons that lead us to conclude that the law of heredity extends successively through the first terms of the series lead us to conclude that it extends also to the last.
The fact that not all the anthropological qualities have their zoölogical prototypes does not at all affect the force of the inference. We may allow, for example, that the moral faculties are strictly anthropological; but this does not detract from the evidence that the intellectual faculties came from the zoölogical prototypes, any more than the fact that the Italian people have dark complexions detracts from the evidence that they descended from Caucasian progenitors. In other words, the possession of specific qualities by a class—qualities not received by inheritance—affects in no way the evidence that the general qualities of the class were received by inheritance. We have recognized this principle at each step in the present discussion. For example, we saw that the national characteristics of a people arise from other causes than inheritance, but this did not lead us to conclude that the race characteristics of the same people were not inherited. In fact, every person affords in his own facial features an illustration of this principle. The expression of our countenances, whether intelligent or dull, cheerful or grave, etc., has been determined by the circumstances of our lives—education, etc.; but the anatomical features—color of eyes, shape of nose, etc.—are inherited from our parents.
Our conclusion, therefore, is that the evidences that man has inherited his anthropological qualities apply equally as well to his having inherited his zoölogical qualities.
Below those qualities which man has in common with the higher animals are others which he possesses in common with the lower animals also. These are chiefly anatomical and physiological in character, such as the possession of bodies whose structural units consist of cells, organs which perform the functions of alimentation, reproduction, etc. The reasoning already employed leads us to conclude that these lower animals were the ancestors of the higher, and transmitted to them-the qualities which the two classes possess in common. For example, both the higher and lower animals possess an alimentary canal—a tube running through the body for the reception and digestion of food. We conclude that this alimentary canal was not obtained by the higher animals through external causes, but by inheritance from the lower animals.
We have, finally, to consider those qualities which man, and both the higher and lower grades of animals, possess in common with the very lowest animals. These lowest animals, consisting in respect to their physical characters simply of minute jelly particles, destitute of organization, agree with the higher animals only in their physiological properties. These are essentially only two, nutrition and reproduction. These, indeed, are the two absolutely fundamental and essential properties of any living organism. Without the one, the life of the individual ceases; without the other, the life of the species. From the biological point of view, the carrying out of these two functions of nutrition and reproduction is the sole end of existence of any living being. Animals differ from one another—they occupy a lower or higher place in the scale of life—according to the advantages of organization enabling them to carry out these functions. The special powers possessed by animals which at first sight seemed to be ends in themselves, are seen by a moment's reflection to be only subservient to these two great ends. In birds, for example, both those powers depending upon structural perfection, as flight, vision, song, and plumage, and those depending upon a highly developed nervous system, as the instincts of migration and nest-building, serve, in the end, wholly to better enable the animals to maintain their own life and that of their species—to carry out the functions of nutrition and reproduction. Thus, rapid flight and keen vision enable them to procure food; melodious notes and brilliant plumage are sexual attractions; while migration and nest-building are directly connected with nutrition and reproduction respectively. From these considerations it is seen that, to the biologist, the simplest animals—the animate jelly-particles—are beings of far higher rank than their physical simplicity would indicate, since they carry on the same life-processes that other animals do, only lacking parts and organs subserving the operation of these processes. It is, therefore, only to assume that like proceeds from like to suppose that from these simplest animals the higher forms received by the law of heredity the two powers of their being which all possess in common—nutrition and reproduction. The differences subsisting between these lowest animals and the higher are not differences relating to the life-characters, but merely to the physical forms in which life manifests itself.
As we know of no simpler organisms than these, and can not conceive that life could manifest itself in any simpler forms, we must regard them as the primordial animals—the progenitors of the animal kingdom.
The conclusion which we reach, then, is that not only all man's distinctively human qualities came to him by inheritance, but also all his purely animal qualities. The former came from human ancestors, the latter from animal ancestors. And as with the former, so with the latter; the more specific came from ancestors less remote, the more general from ancestors more remote. The most general, the absolutely fundamental and essential, came from the primordial living beings.
The animals of the first life-period were succeeded by others which, as we have seen, possessed not only the physiological characters of the primordial organisms, but also certain anatomical characters not received by inheritance, enabling them to carry out the physiological processes more perfectly. If for the sake of simplicity we consider the animals of the first and second life-periods to be those which we have already designated as the lowest and the lower animals respectively, then the latter received by inheritance from the former their functions of nutrition and reproduction, and acquired the special organs of alimentation and reproduction by which these functions were the better carried on. The question whence these new characters came need form no part of our present inquiry. For our purpose it will be sufficient to say that they resulted from external causes, it being understood that it is not intended to preclude the idea of the agencies in question being natural causes. The fact here to be set forth is that these animals of the second life-period transmitted, by the law of heredity, these characters that first appeared in them, along with those which they had received by inheritance, to their descendants. It is not to be supposed, of course, that the characters were preserved unmodified as to details, but only that their general nature, both as to structure and use, were retained. The animals of the third life-period—which we may consider those we have called the higher animals—therefore possessed at the outset all the characters of the first, together with those that were peculiar to the second. They, in their turn, under the influence of external causes, came to possess new characters—a vertebral column, four-chambered heart, etc.—while those which they had received by inheritance from their forerunners of the second period attained in them a higher development; in their turn, too, they transmitted their advanced organization to the succeeding order of beings—that is, to the human race. This same process continued through the successive generations of the human family. The distinctively human qualities acquired at the outset, together with the accumulation of inherited animal qualities, were handed down to the races that succeeded. They, in turn, bestowed all that had been bequeathed to them, together with their newly acquired race characteristics, to their descendants. Finally, the national characteristics, which in our time we may suppose to include all the traits that characterize civilized man, were differentiated.
Civilized man, therefore, inherits the accumulation of benefits that have come from the operation of the law of heredity through the long ages since life began upon the earth. In a deeper sense than we commonly think, we are the heirs of all the ages.
Man does not come into his full inheritance at the beginning of his existence. It is a fact of exceeding significance that, at the beginning of embryonic life, our bodies consist of nothing more than a single cell, precisely similar to the minute organisms with which life began upon the earth. It is as if man acknowledged the debt which he owes to these primordial living beings. But it is not only to the primal form of life that he makes this confession of affinity; for, as is well known, the successive stages of embryonic development represent the succession of type forms of animal life as they appeared upon the earth. Thus, man comes into his inheritance by degrees. At the beginning of his existence he possesses the characters of the primal forms of life; a little later, those of the second life-period—such as belong to the lower animals; still later, those of the third life-period—such as belong to the higher grade of animals. At a considerable time before birth he has already come into possession of all the animal qualities, and at birth the human physical characters are present. Then follows a more perfect development of the physical characters, and at the same time the acquirement of the higher human characteristics—the power of speech and the mental and moral faculties. Thus, in the unfolding embryo and in the growing child we have recorded in dim but unmistakable characters the history of the life of the earth.
A suggestion, looking to the future, here presents itself. The same agencies out of which has come the progress of the past are in operation now. It is, therefore, only in the course of nature that there should be a further progress. And as respects man, according to a law that has governed in the past, namely, that the most recently acquired characters of a type are most subject to progressive change, we may expect that advancement will be chiefly in respect to his higher powers—his intellectual, moral, and spiritual nature.