DIAGRAM ILLUSTRATING VARIOUS ARRANGEMENTS OF DETERMINERS IN A SINGLE CHROMOSOME PAIR 1 and 2, Pure; 3, Hybrid.DIAGRAM ILLUSTRATING VARIOUS ARRANGEMENTS OF DETERMINERS IN A SINGLE CHROMOSOME PAIR1 and 2, Pure; 3, Hybrid.
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 penetratesthe 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.
CHROMOSOMES IN CELL DIVISION Figure 1, chromosome splitting in ordinary cell division, in which each determiner splits in half, contrasts with Figure 2, reduction division, in which the chromosomes of the pair are simply pulled apart.CHROMOSOMES IN CELL DIVISIONFigure 1, chromosome splitting in ordinary cell division, in which each determiner splits in half, contrasts with Figure 2, reduction division, in which the chromosomes of the pair are simply pulled apart.
Figure 1, chromosome splitting in ordinary cell division, in which each determiner splits in half, contrasts with Figure 2, reduction division, in which the chromosomes of the pair are simply pulled apart.
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 studiesof 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 wehave 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 thatcauses 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.
CARING FOR THE BABY’S EARS Important for adults and babies alikeCARING FOR THE BABY’S EARSImportant for adults and babies alike
THE BABY’S FOOTPRINT A means of positive identificationTHE BABY’S FOOTPRINTA means of positive identification
BABIES PHYSICALLY AND MENTALLY ACTIVEBABIES PHYSICALLY AND MENTALLY ACTIVE
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 thatshould 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 amale. 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 cameabout, 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 thatare 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 ofchildren 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.
EVERYONE is familiar with the beguilingly helpless picture the tiny baby presents. The disproportionately large head, with aimlessly rolling eyes and toothless mouth, the frail and delicate limbs, waving in the air or clutching spasmodically at anything within reach, the expressionless face, on which, for the first few days, only sensations of discomfort are registered, all mark a creature whose survival hinges absolutely on unremitting care; a far cry from the competent self-sufficiency of the average person of mature years. These surface marks of helplessness are by no means the most significant. Buried from view beneath the soft and velvety skin are characteristics of even greater meaning to those on whom falls the responsibility for the rearing of the new life.
At the time of birth the bony skeleton is very incomplete; there is a spot just above the forehead where the skull bones have not yet grown completely together, leaving a space where the brain is protected from injury only by the overlying skin. This spot can be detected easily in very young infants by its pulsations in time with the heartbeats. In most of the other bones the deposit of lime salts to which they owe their stiffness has gone only a little way, so that it would be impossible for the child to stand or walk even though it knew how.In this respect the contrast between the human infant and the new-born of such animals as horses or cattle is very striking, for the latter walk stumblingly from almost the moment of birth, and efficiently within a few hours thereafter.
Not only is the infant devoid of teeth, but in various other regards his digestive apparatus is undeveloped. Not only is he unable to chew solid food, but he could not digest most of it if it were served already chewed. During the early months of life the child is emphatically a one-diet being. His alimentary tract deals successfully with the mixture of proteins, fats, and sugars of which milk consists, provided the proportions are substantially those of mother’s milk and the quantity at a feeding is not too great. He can do this because the enzymes needed for digesting these particular substances are manufactured by his digestive glands from the very beginning, and because the muscles of his stomach and intestines can churn and propel the soft curd into which the milk is converted as soon as it enters the stomach. The fact that cow’s milk sets into a tough curd accounts for much of the difficulty some babies have in thriving upon cow’s milk.
There is no starch in milk, and neither the saliva nor the pancreatic juice of the infant contains the enzyme by which starch is digested. It is wholly useless to begin feeding starch-containing foods until this enzyme begins to be manufactured, which usually takes place when the child is about eight months old. Even then the introduction of starch into the diet should be gradual and cautious.
Modern science has discovered no better food for infants than mother’s milk, and no substitute more satisfactory in general than suitably modified cow’sor goat’s milk. Under modern conditions of life, particularly in cities where milk has to be transported over long distances, and where much time necessarily elapses before the milk can be placed in the hands of the consumer, there is serious danger of contamination of the milk with disease germs. In all enlightened communities this danger is fully recognized, and the entire milk supply, so far as possible, but that part of it destined for the feeding of children in particular, is safeguarded by all available means. A fairly reliable index to the degree of enlightenment of any community is the quality of the milk which its children receive. One recent discovery of considerable importance, and incidentally an interesting illustration of the way in which correct procedure may be hit upon in advance of the scientific knowledge which justifies it, is the finding that orange juice contains one of the dietary accessories, known as vitamines, essential to health, and present in raw milk, but destroyed when milk is heated. For years, physicians had recommended orange juice for babies. It is of vital importance when the milk must be pasteurized.
Both the heartbeat and the breathing in the young child are much more rapid than in the grown person. It is believed that this quickness of heart action and of breathing rate are related to the smaller size of the infant as compared with the adult and are of no very marked significance. At any rate it is true in general that the smaller the animal the more rapidly does its heart beat and the more quickly does it breathe. A very noticeable fact about young children is the susceptibility to all sorts of influences of the mechanism by which the breathing is controlled. Every passing interest reflectsitself in heightened breathing. Violent emotion often leads to such extreme overbreathing as to drain the child’s blood of considerable of its store of carbon dioxide, whereupon the rapid breathing gives place to prolonged holding of the breath. Many a young mother has been seriously alarmed by the length of time her offspring in a tantrum is able to refrain from drawing its breath. Contrary to the appearance of things, which would indicate that the child is holding its breath out of spite, and in the hope of getting even with its parent, the cessation of breathing is largely or wholly automatic, indicating the way in which nervous and chemical influences have interacted to suspend the respiration.
The child is born with all its muscles in place, and all fully formed in that every muscle fiber the child will ever have has been produced previous to birth. In fact, as soon as the full equipment of muscle fibers has been laid down the body loses the power to form more, so that if, through injury, one is so unfortunate as to have some of his muscle fibers destroyed he will have to get along for the rest of his life with those that are left. The gaps in the muscle tissue that are produced by injuries are filled up by a kind of connective tissue known as scar tissue. The muscle fibers are all present, but smaller and weaker than they will be later.
The connections between muscles and nerves are also pretty well established at the time of birth, so that the body and limbs can be moved freely, even if not at all efficiently.
Not only are the motor nerves formed and in connection with the muscles at and even before birth, but the sensory nerves and most of the central nervous system are ready to begin functioning aswell. Some reflex actions, including a few that require quite elaborate muscular and nervous coordination, are performed very shortly after birth. One famous example, that has been much cited as tending to reinforce the belief in man’s descent from tree-dwelling ancestors, is the curling of the fingers about a slender rod that is pressed against the palms. In most very tiny babies the grasp thus secured is strong enough so that the child can be raised and held in the air, supported wholly by its own grip. Sneezing, which is really a very complex act, requiring accurate cooperation on the part of many muscles, is done successfully by very young infants. The reflex of sucking, which is of paramount importance, in that without it the child would almost inevitably starve, is present practically from birth. Another important early reflex is that of crying. It is a curious thought that this reflex, by which bodily discomfort is made known, and through which relief may be summoned, is revealed at the very instant of birth, in connection with the drawing of the first breath, while the contrary reflex of laughter, by which bodily well-being is expressed, puts in an appearance only after some weeks or even months.
It is difficult to determine just how far the sense organs have arrived in their development at the time when the infant begins its independent existence. That touch and those senses related to bodily discomfort, of which pain is most important, are operative from the first is shown by the occurrence of the reflexes described above, which are brought into action by those particular senses. There is good reason to believe that the sense of hunger comes into play within two or three days at thelatest. The sense of thirst does not have much chance to reveal itself early in life, for with a diet exclusively liquid, and feedings separated by only a few hours, there is no reason why the child should become thirsty. As the feedings become less frequent, and particularly as solid food begins to be added, there is real danger that the child may be insufficiently supplied with water.
Muscle sense and the equilibrium sense, if present at all, must be in a very imperfect condition at first. They seem to differ from the senses described thus far in that they depend on practice for their development. At any rate the bodily movements are largely aimless in the beginning, and it will be observed that the baby has the appearance of experimenting with its extremities, placing them repeatedly in particular positions and seeming to gain precision thereby. The eye movements, and especially those by which both eyes are focused on a single object, depend for their accuracy on the working of muscle sense. In the estimation of the parents a distinct mark of progress is registered the first time the baby follows a movement with its eyes. As soon as it does this accurately, and also brings both eyes to bear on any object, its muscle sense is known to be in efficient operation, so far, at least, as the eye muscles are concerned. Equilibrium sense first shows itself when real balancing motions of the body are made.
The senses of taste and smell may be operative to some degree in very young children, but it is doubtful whether they have either the breadth or the acuteness that will be shown later in life. Recognition of disagreeable tastes or smells seems to appear earlier than appreciation of agreeable. This is inline with the general fact that the self-protective reactions are developed very early.
Hearing and sight are probably in working order practically from birth. It is customary to test the sight of the new-born by passing a light directly in front of the eyes. If sight is present there will be some appreciable eye movement, suggesting that the eyes are attempting to follow the moving light. There is no reason to believe that there is any such thing as definite looking at objects thus early. So long as the eyes continue to roll aimlessly about, and before they begin to focus accurately, they are more likely concerned with distinctions between light and shadow, than with perceptions of form or size. In general we may say of the senses that those concerned immediately with bodily discomfort are about as fully developed at birth, or shortly thereafter, as they ever will be, while those that have to do with the general adjustments of the body to its environment reach full efficiency more gradually.
The higher parts of the brain, especially those concerned on the one hand with the mental life (the cerebrum), and on the other with the complicated reflex acts involved in locomotion (the cerebellum), are not ready to begin active functioning when the child is born. Indeed some parts of the cerebellum do not take on final form for from eight months to two years afterward. It is thought by some that the question of whether a child will learn to walk early or late depends, in part at least, on how soon his cerebellum reaches complete development.
Most parents are fully alive to the importance of abundant warm covering when their children are to be taken out into the cold, but there is much less appreciation of the harm that may be done by toomuch clothing in extremely hot weather. Of special importance is the avoidance of exposure to sharp drops in temperature. The adult adjusts himself more or less automatically to these, whereas the infant does so to only a limited extent.
Finally, the hold of the infant upon life, that quality that we know as ruggedness or vitality, is much slighter than it will be after a few years. Not only is the susceptibility to many kinds of infectious diseases very much greater, and the power of resisting them very much less, but the ill effects of poisons, whether taken in with the food or breathed in with the air, are more pronounced. Thus the vitiated air of slum dwellings, saturated with the effluvium from unwashed bodies and unclean clothing, while trying enough for the average grown person, is deadly for all but the toughest babies. Even in the ordinarily well-kept home, especially in the winter time when ventilation is apt to be neglected, the air within the house tends to become unsatisfactory from the standpoint of the infant’s welfare.
The wise parent, and wise he must be at this time, relaxes his care in just proportion as the child achieves ability to do things for himself. Since bodily development is more rapid than mental the close supervision of food, clothing, and physical occupation is necessary only during the early years, but the task of building up, through the slow processes of education, the sort of mind which will be able to do its proper share in dealing with the difficulties which confront the coming generations is one to which may well be devoted the best thought and effort not only of the parents, but of organized society as a whole.