CHAPTER XX

THE PERPETUATION OF THE RACE

We have tried to take up one by one the chief things that happen in the body, but thus far have emphasized their importance entirely in connection with bodily well-being; that is, we have seen how the body maintains itself against the competition of other living things and in a world full of hazards. Before leaving the subject entirely, a short account must be given of the way in which the _race_ is maintained upon the earth as distinct from the maintenance of individuals. Early in the book we showed that every one of us starts life as a single cell which, by dividing and subdividing, along with continuous growth, finally develops into our large and complicated body. This cell is really the union of two cells; one furnished by each parent; in the body of the parent it formed part of what is known as the _germinal tissue_, this name being applied because the cell from which we start may be spoken of as the germ of life. As the cell from which the body is to come begins to develop, the cells formed from it quickly become different one from the other, so that very early it can be distinguished that certain cells are to form the future nervous system, others the future muscles, and so on. One of the earliest of these groups of cells to become distinguishable is that which is to become the germinal tissue in the newly formed individual. A great deal of importance has been attached of recent years to this fact that the germinal tissue is set aside almost from the beginning of development, and when we remember that the next generation will be derived from this germinal tissue and from no other tissue, we see that there is a very close relationship of the germinal tissue from generation to generation. The importance of this is in connection with heredity, namely, with the question of the resemblance of the child to the parent. We all know that sometimes children resemble their parents very closely, at other times there is almost no resemblance at all. The main fact of heredity is, as we have just seen, that the child comes from germinal tissue of the parent and not from the parent as a whole. We can think of an individual as a complex body carrying within itself and nourishing an independent group of cells which are to serve as the starting point for the next generation, but with which the individual himself has nothing to do except to provide nourishment. A little later we shall try to show just why we think of the germinal tissue as so little dependent on the rest of the body. What we want to do first is to point out how this notion must affect our views of heredity. If the germinal tissue is simply harbored and nourished by the body, but otherwise quite independent of it, it follows as a matter of course that things the parent does, provided that they do not disturb the nourishment of the germinal tissue, can have very little effect upon it. For example, anything that the parent may achieve during his lifetime cannot be passed on by heredity to his offspring. This is entirely contrary to the idea of heredity which is held by nearly everybody. Just a little over one hundred years ago a Frenchman by the name of Lamarck published writings in which he argued very strongly in favor of the inheritance of “acquired characters,” meaning by this that the achievements of parents could be and were transmitted to the offspring. It is evident that this would be a very great advantage; the progress of the race would be secure if the children could inherit directly the achievements of their parents. Of course, it might work the other way around and children might inherit unfavorable acquisitions of their parents as well as favorable. Since the time of Lamarck there has been not only hot discussion of his ideas but also a vast amount of experimentation to find out whether acquired characters actually can be inherited. One of the most famous experiments is that of a man who cut off the tails of mice at birth and did this generation after generation in the hope of being able to produce a race of tailless mice; after a great many generations he abandoned the attempt with the conclusion that the artificial removal of tails from parents would not cause the offspring to be born in the tailless condition. More recently similar experiments have been carried out on kinds of animals that have much shorter lives and in which the generations are correspondingly more frequent. In some cases thousands of generations have been studied without any indication that changes brought about during the lifetime of the parents can be transmitted to the offspring.

One of the first questions that is sure to be asked as soon as this inheritance of acquired characters is denied, is how changes can then be brought about; the answer to that question is found in the experience of animal and plant breeders. One fact of nature which should be emphasized, is that individuals are never exactly alike except perhaps in the case of so-called identical twins which have a special heredity spoken of in Chapter V. Even brothers and sisters always have pretty marked differences. These differences among individuals are some of them probably more or less accidental results of the way in which the germ develops into the complete body; others are the result of differences in the germ cells themselves. The distinction between these is that where the germ cells are different, the difference will be transmitted by heredity to future generations; if it is a modification that comes on in connection with development and is not due to a difference in the germ cell, it will not show itself in future generations. What the animal and plant breeders have to do is to watch for changes in their stock, and when the kinds that they are looking for appear, they breed them in the hope that the desired features will prove to be hereditary, so that a race can be established showing them. A very striking example of this kind of breeding is in the development of hornless cattle. Of late years it has been the practice to dehorn cattle regularly and this dehorning has never caused the offspring to be born without horns, but on the other hand it has happened occasionally that calves have been born which were naturally hornless and since this absence of horns was due to a difference in the germ tissue, the character was found to be hereditary and it has been possible to establish breeds of hornless cattle. Thus by selecting cases where a change occurred naturally in the germ tissue, a result has been obtained which could not possibly be gotten by any amount of work directly upon the parents.

What we have tried to make clear thus far is that heredity is absolutely a matter of the condition of the germinal tissue. What we have now to do is to see what the germinal tissue is like and how the process of heredity actually goes on. In Chapter V something was said about the nucleus; it will be remembered that the nucleus contains a substance known as _chromatin_, which, as seen under the microscope, looks like a tangled skein or network of fine threads, but is in reality a number of tiny structures known as _chromosomes_. We now know that these chromosomes are the actual controllers of heredity. Of course we do not know at all how they exert their control; what we do know is that when certain factors are present in the chromosomes of the germinal tissue of the parents, the offspring derived from that germinal tissue will have traits that would not be present if the chromosomes had been different.

Space does not permit us to tell at length how the facts that are now to be described were discovered. They date from the gardening experiments of an Austrian monk by the name of Mendel, who for many years grew ordinary garden peas and studied from season to season the varieties that appeared. The facts of heredity that he found to be true of peas, have since been shown to apply just as well to ourselves, and since this book deals with human beings we shall try to make the description apply directly to human heredity. The first thing to get clearly in mind is that every single thing about any one of us which can or does differ from the corresponding feature in another person, may be a _hereditary trait_, and, if it is, will have a factor controlling it somewhere among our chromosomes. Since the number of hereditary traits is legion, including not only the size and shape of all parts of our bodies inside and out, but our mental peculiarities and moral tendencies as well, there must be a huge number of controlling factors, or _determiners_, as they are often called. As a matter of fact, there are probably not quite so many determiners as traits, because a single determiner may govern more than one trait. But even so, the number of determiners is too large for comfort in trying to describe their working. The best way to go about it is by pretending that we are not so complicated as we really are. We shall do this by setting the number of different determiners human beings may possess at fifty-two, not because that is anywhere near their real number, but because it is the number of combined large and small letters in the alphabet, and we propose to use letters to stand for determiners, as an easy way to keep them separate in the description.

Every cell in our bodies has its nucleus with its equipment of chromosomes. An interesting fact already spoken of in Chapter V is that according to our best knowledge the chromosomes in any cell of any one of us are exactly like those in all the other cells of the same person. The chromosomes in the muscle cells are exactly like those in the nerve cells, and both correspond exactly with the chromosomes of the ordinary cells in the germinal tissue. In every one of these cells the chromosomes are arranged in pairs. It happens that in human beings the number of pairs is twenty-four, with one pair incomplete in some of us for a reason that will be explained presently; this is an awkwardly large number for our present purpose, since we have allotted only fifty-two determiners altogether, so we shall do some more pretending and set the number of pairs at nine. One more change will have to be made from the real state of affairs before we can go on with the description; this is to suppose that our various bodily features can show only one difference, instead of the many of which they are really capable. For example, we shall suppose that the hair can be either black or light, but never red; the nose can be Roman or Greek, but never Irish. By making this supposition, we can let the large letters stand for one set of hereditary determiners and the small for the same features but with exactly contrary traits. According to this arrangement, if we had two persons side by side one of whom had only large-letter determiners in his chromosomes and the other only small, they would have the same general human make-up, but in every possible detail one would be the exact opposite of the other.

It has been proven by complicated studies which we cannot take time to describe that the determiners are grouped in the different chromosomes in a definite plan. We saw a moment ago that the chromosomes are in pairs. This pairing is an essential part of the arrangement, for every hereditary bodily feature actually has two determiners governing it, which lie in corresponding positions in the two chromosomes of the pair. To illustrate how this works out, let us suppose that the chromosomes of the first pair contain determiners A, B, and C. Each of these three determiners will be present in both members of pair number one, and if this is true of any cell it will be true of all the cells, and in any other human being either they or the corresponding small-letter determiners, a, b, and c, will occupy pair one of the chromosomes. Furthermore, they will lie in a row within the chromosomes, always in the same order; thus if A and C are at the ends with B in the middle in one chromosome, every other chromosome that contains these three determiners will have them in the same order.

Thus far we have planned our diagrams as though only large letter determiners were concerned; but we saw a moment ago that there is a complete set also of small-letter determiners, which control a set of contrary hereditary traits, and we intimated that these will sometimes be found in the chromosomes in positions corresponding with those occupied by the equivalent large-letter determiners. It might happen, for example, that pair one of the chromosomes would contain large-letter determiners A, B, and C, while pair two would contain small-letter determiners a, b, and c. Evidently the person in whom this combination was present would differ from one all of whose chromosomes contained large-letter determiners, since part of his traits would be established by small-letter determiners.

We are now ready to go back to the germinal tissue and trace the process of heredity as it actually works out in the developing offspring. In the ordinary cells of the germinal tissue, as we have already seen, the chromosomes are exactly like those in the other cells of the body. Cell multiplication goes on actively in the germinal tissue of both parents; in the mother this leads to the production of germ cells which are called eggs, and in the father to the production of cells that are called sperm. During the process certain changes occur, so that neither eggs nor sperm are exactly like the original cells of the germinal tissue. If we look back at the description of cell division in Chapter V, we shall recall that the chromosomes split lengthwise and are pulled apart. Now that we have learned about determiners, we will realize that every determiner splits in half, because otherwise there would not be an equal distribution of determiners between the two cells. Much of the cell division in the germinal tissue is precisely like this, but at a certain stage in the production of both eggs and sperm there is one cell division in which the pairs of chromosomes are simply pulled apart, without there having been any previous lengthwise splitting. The result of this is to leave the resulting cells with only half as many chromosomes as the other cells of the body have. Some further changes take place in these cells before they become ripe eggs or ripe sperm, but there is no further disturbance of the chromosomes, so that eggs and sperm contain only one member of each pair, instead of both members, as do all other body cells.

The first step in development is the coming together of the egg cell with one sperm in the process that we call fertilization. The sperm penetrates the egg and its chromosomes line themselves up with the egg’s, restoring the pairing that is present regularly. Immediately afterward the cell divisions begin that make up development, and in all of these the usual lengthwise splitting of determiners takes place. Every cell in our body contains its pairs of chromosomes, one member of each pair tracing back directly to the egg cell while the other traces directly to the sperm. Thus half of our determiners came from the maternal germ tissue and half from the paternal.

We shall now begin to see how heredity works out. Suppose chromosome one in the egg has large-letter determiners A, B, and C, while the corresponding chromosome in the sperm has small-letter determiners a, b, and c. When these line up after fertilization, restoring pair one, we have A opposite a, B opposite b, and C opposite c. Since we have supposed the large and small letters to stand for contrary hereditary traits we introduce here a conflict and must ask at once how it is settled. One of the things the monk Mendel worked out in his studies of heredity in peas was this particular problem. He found that where there is conflicting heredity one of the determiners usually dominates over the other, and when this happens the trait in the offspring will be like that of the parent which contributed the dominant determiner. To illustrate: suppose A is dominant over a, then in the case in question the offspring will be like the mother in feature A. In some kinds of animals and plants conflicting characters blend in the offspring, producing an intermediate appearance. A good example of this latter case is in the common flower, the four-o’clock. In this plant white blossoms and red blossoms are due to determiners that occupy the same positions in the chromosomes; therefore, if white and red flowered plants are crossed, these conflicting determiners come together when the sperm and egg chromosomes pair. Since neither determiner dominates over the other, the color of the flowers in the offspring is neither white nor red, but pink.

Any individual whose chromosome pairs contain conflicting determiners is called a “hybrid”; there may be every degree of hybridism, from the simplest, in which all the chromosome pairs are alike except one, up to the most complete, in which all the pairs are unlike. It is easy to tell a hybrid by its appearance in the cases in which there is blending inheritance, but not so easy when one trait dominates over the other, for then the hybrid will look like the parent that furnished the dominant determiner. The sure way to detect hybrids is by the study of their offspring. Suppose we have two parents, both of whom are hybrids in respect to chromosome pair one. This pair in both will contain determiners A, B, and C lined up opposite a, b, and c. Since we have supposed the large letters to be dominant over the small, both will have the same appearance, dependent on the presence in their chromosomes of A, B, and C. Since in the formation of eggs and sperm the chromosome pairs are pulled apart, half the eggs produced by the mother will contain determiners A, B, and C, and the other half a, b, and c, and half the sperm of the father will similarly contain one set and half the other. Since it is an absolute matter of chance which sperm encounters which egg in fertilization we can safely conclude that in the long run all the chances will be realized equally. Calling, for convenience, the large-letter eggs E and the small-letter e, and, similarly, the large-letter sperm S and the small-letter s, this means that E can be fertilized either by S or s, and e also by either S or s. The possible combinations are ES, Es, eS, and es. In terms of actual determiners these combinations are ABCABC, ABCabc, abcABC, and abcabc.

The combinations just given represent the possible offspring from a pair of hybrid parents. If we look them over, we see at once that only half of them are hybrid, namely, combinations ABCabc and abcABC; the other half are pure breed, the chromosome pairs being exactly alike; but these pure breeds are of two kinds, one having only large-letter determiners, the other only small-letter. If it is a case where the large letters dominate over the small, the large letter pure breeds and the hybrids will look alike, but the small-letter pure breeds will look different, since in them the traits governed by the small-letter determiners have a chance to show themselves. A very good illustration of this is seen in eye color in human beings. Brown eyes are dominant over blue; in other words, the determiner that causes eyes to be brown dominates over that responsible for blueness in cases where both come together in hybrids. A person who has brown eyes may be either a pure breed in that respect or may be a hybrid; there is no way to tell the difference from the appearance; but if the brown-eyed person has offspring, and any of them turn up with blue eyes, it is proof positive that the parent is a hybrid so far as eye color is concerned. Moreover, the blue-eyed child is not a hybrid in this respect; his brown-eyed brethren may or may not be; in the long run two-thirds of them will prove so; the other third will be pure breed, having in their chromosome pairs only brown-eye determiners.

Where the hybrids differ from the pure breeds, as in the case of the four-o’clocks, given earlier, it is easy, of course, to tell which are pure and which are hybrid. When pink four-o’clocks are interbred, the chromosomes will combine just as described above for hybrids, since the plants that have pink flowers are hybrid. One-half the offspring will have pink flowers, showing that they are hybrid; one-fourth will have white flowers, proving that in them the white-flower determiners have separated out, and the other fourth will have red flowers, because in them the red-flower determiners are the only kind present. This and similar experiments have been tried hundreds of times, and whenever the numbers of offspring have been great enough to allow the chances to equalize, the proportion of different kinds of offspring has always agreed almost exactly with expectation. Of course in human beings the families are not large enough for this always to work out accurately, but even so the agreement is often striking.

In practical animal breeding the blended inheritance just described is not very useful, for even though a blended character might appear which is just what the breeder has been looking for, it will not occur in more than half the offspring and can not ever be depended on to show itself in any particular individual. This explains largely why pure-bred stock is always more desirable than hybrid, and why breeders strive so eagerly to obtain desired traits in pure-bred animals. In plants, blended characters are much more valuable, for two reasons; first, because the offspring are so numerous that even though half of them come out pure, and so lack the desired blend, there are enough left that have it to make the crop worth while; and second, because propagation by cuttings is possible in very many kinds of plants, which means that the same plant is kept going in hundreds of places, and for tens or even hundreds of years. A trait that is desirable can be perpetuated indefinitely by this means, even though it may be a blending of several hereditary traits, which would separate out in a few generations by ordinary means of propagation.

There are several more things in heredity that must be taken up while we are on the subject, so we shall have to return to the chromosomes for a while. We have seen that there are several determiners to each chromosome; for convenience, we assigned three apiece to our chromosomes, except the ninth, which has to get along with two; but in reality the number to each chromosome is often much greater. This grouping of the determiners, several to a chromosome, carries an interesting consequence with it, in that all the hereditary traits controlled by one chromosome have to go together in reproduction. In the example we have already used A, B, and C are together; therefore any individual that shows character A must show B and C as well. The most striking instances of this are certain traits that are bound up with sex, but we cannot describe these further until we have looked into the heredity of sex, which we shall do in a minute. First a word must be said about occasional exceptions that turn up to the rule that we have just stated. In the study of thousands of specimens now and then one has been found in which there has evidently been a swapping about of determiners. We can illustrate the situation by supposing chromosome one is found to contain determiners A, b, and C, instead of A, B, and C; one small-letter determiner has traded places with a large. Of course, the effect of this is to permit different combinations of hereditary traits than ordinarily occur, and at the present time students of heredity are actively engaged in following this up to see how it happens, and what advantage can be taken of it. This crossing over of determiners from one chromosome to another takes place only among such as are actually in contact at times within the nucleus as seen under the microscope, which confines it to the members of corresponding pairs.

In man, and in many of the lower animals, sex is a hereditary character. That means that there is a determiner for it which is grouped with other determiners in one of the chromosomes. In man the determiner is for femaleness; there is no special determiner for producing the male sex; it is produced whenever the female determiner is missing from one chromosome of the pair, and this is brought about by having the whole chromosome that should make up this pair absent. At the beginning of this chapter the fact was mentioned incidentally that a good many of us have only 47 chromosomes, instead of the 48 that are characteristic of human beings. The distribution is really almost exactly half and half, for all males have 47 and all females 48. This means that the cells of the germinal tissue of females have 24 complete pairs, while the corresponding cells in males have only 23 complete pairs and one chromosome over. This extra chromosome is the one that contains the determiner for femaleness; each of the chromosomes of pair 24 in females contains this determiner also. These are often spoken of as sex chromosomes.

Now when in the course of the production of egg cells within the mother’s germinal tissue the pairs of chromosomes are pulled apart, each separate cell, and so each egg, will contain the full number of chromosomes, 24, including the sex chromosome. But when the same thing happens in the course of the production of sperm only every other one will have the full number; the remaining half having only 23, and all of this half lacking the chromosome that contains the determiner for femaleness. There are, then, always equal numbers of two kinds of sperm, one with 24 and the other with only 23 chromosomes. If the egg is fertilized by a sperm containing 24, including the sex chromosome, the pairing of chromosomes is complete in the egg, and the offspring will be a female; if, on the other hand, the fertilizing sperm is one that contains only 23 chromosomes the pairing in the egg will be incomplete; the single sex chromosome of the egg will not be paired with a corresponding one from the sperm and the egg will develop into a male. Since it is a pure chance whether fertilization will be accomplished by a sperm of 24 or one of 23 chromosomes, we should expect the sexes to appear in exactly equal numbers, taking the world as a whole. As a matter of fact, whenever extensive birth data have been accumulated they have shown a very slight excess of male births over female. We are not able to explain this at the present time. It is possible that the 23 chromosome sperms are a little more vigorous for some reason than those that have 24, and so are able to fertilize slightly more than their share of eggs.

We spoke a moment ago of hereditary traits whose determiners are bound up in the sex chromosomes. All such behave interestingly in heredity for the simple reason that they can never be transmitted from father to son, but only from father to daughter. This is because, as we have just seen, the sex chromosome in the sperm always causes the egg which that sperm fertilizes to develop into a female. The single-sex chromosome which males possess invariably comes from the mother. An interesting example is the common type of color blindness known as Daltonism. Normal color vision is hereditary and the determiners which establish it are in the sex chromosomes. Occasionally a person is found in whom these determiners are defective. If this person is a male, he will be color blind, but if a female not, unless both sex chromosomes are defective in this regard, since normal color vision is dominant over color blindness; so if one sex chromosome is normal the vision will be also. The woman, in this case, will be a hybrid with respect to color vision; one of her sex chromosomes containing a normal determiner, the other a defective.

This works out in heredity as follows: If a color-blind man is married to a woman who has no color blindness in her heredity, none of his children will be color-blind because he cannot transmit the sex chromosomes which carry the determiners for color blindness to his sons, but only to his daughters; all the latter will be hybrid with respect to the character, since all of them come from fertilized eggs which received sex chromosomes from the sperm. If these daughters, in turn, marry men who are free from color blindness, some of their sons may be color-blind, but none of their daughters can be. The only way in which women can be color blind through inheritance is by descent from color-blind fathers and from mothers who are either themselves color-blind or are hybrid with respect to the trait. The result of this difference in the heredity of the two sexes is to make color blindness many times as frequent among men as among women. In round numbers four men out of every hundred have this type of color blindness, while only six or seven women in ten thousand show it.

We have left for discussion only one topic dealing with heredity, but this is the most baffling of all, since it deals with the problem of how the various kinds of determiners came into existence. It is evident that if given one parent with all large-letter determiners and the other with all small-letter, we might, in the course of many generations, get a great variety of combinations and so a great many different-appearing individuals. But unless we have various kinds of determiners to start with, there is no way in which this can be done. We do not pretend to know very much about how the innumerable determiners that are in existence came about, but we have one clue that is thought to point the way. In some animals, and many plants, descendants put in their appearance from time to time that are so different from their ancestors as not to be accountable according to ordinary laws of heredity. These have long been known, and the name of “sport” has been applied to them by breeders. Since the facts about determiners have been learned, it has been clear that these “sports” cannot have all their determiners like those present in their parents, and it has come to be believed that occasionally spontaneous changes take place within individual determiners. Since the determiners are undoubtedly complex chemical structures, we know of no reason why this might not happen. Probably it is much more common an occurrence in some kinds of plants and animals than in others. The name of “mutant” has been applied to the plant or animal in which this has happened, and the process is called “mutation.” It is important since it is the most likely way in which the innumerable kinds of determiners that are now in existence came into being.

We suppose that since life first put in its appearance on the earth there have been uncounted mutations, a vastly greater number than are now represented by determiners. Many of the mutants could not compete with their brothers and sisters of ordinary descent and so promptly died, but occasionally it might happen that a mutant would be as well fitted for life as its relatives, in which case it would establish itself, and in course of time become ancestor to a whole line like itself. If this happened often enough, and time were allowed for it to work out, all the kinds of plants and animals that are now in existence might have come by descent from a very few ancestors. The geological history of the earth shows that there has been plenty of time, even though valuable mutations did not occur oftener than once in a thousand years.

Our description of racial perpetuation should be finished by an account of the development of the fertilized egg. Snugly ensconced within the body of the mother, in an organ devoted solely to the purpose, the egg passes rapidly through the early stages, living at first on fats and proteins stored within itself. After these are exhausted it draws supplies from the body fluids of the mother. In the course of a few weeks it has developed its own conveyer system, with its own beating heart and its own stock of blood. There is never any actual mingling of the blood of the developing child with that of the mother; capillaries of the maternal circulation come into intimate contact with capillaries of the circulation of the child. Here interchange of all sorts of material goes on; food and oxygen pass from the mother’s blood to the child’s and waste materials from the child’s blood to the mother’s. During all this time the mother is eating, breathing, and excreting wastes for two. She cannot bring any nervous influences to bear on the developing child, since there is no connection between her nervous system and the child’s; she can, however, influence it chemically through the blood. Poisons that get into the blood of the mother can pass from it into the blood of the child. These may be the poisons of auto-intoxication, or drugs that the mother has taken. In either case they may do the child harm. We do not know very much about this, but it may be that a considerable percentage of children that are born with abnormalities that are not hereditary come by them through chemical influences received from the mother’s blood.

When the development of the child has gone far enough so that it can do its own breathing, feeding, and discharging of waste materials, it is expelled from the body of the mother in the process that we know as birth. This does not imply by any means that parental care and responsibility are at an end. Food, protection, and warmth must be provided. Education must be attended to, for the nervous system of the new-born infant is absolutely undeveloped. It has, through heredity, certain possibilities of achievement; their realization hinges upon the bringing to bear of worth-while influences. Upon the attainment of maturity the child will be expected to assume his place in society, and society has a right to the best that he is able to offer. In preparation for this it is the duty, both of the parents and of society itself, to provide throughout the formative years as nearly as possible the environment best suited to the development of those traits which make for usefulness. Environment cannot overcome the limitations of heredity, but environment can bring out the best that is in us.