The Science of Human Nature, William Henry Pyle [best pdf ebook reader .txt] 📗

is dependent upon likeness in structure. The young robin acts as does the old robin, because the nervous mechanism is the same, and therefore a similar stimulus brings about a similar response.

Most of the scientific work in heredity has been done in the study of the transmission of physical characteristics. The main facts of heredity are evident to everybody, but not many people realize how far-reaching is the principle of resemblance between parent and offspring. From horses we raise horses. From cows we raise cows. The children of human beings are human. Not only is this true, but the offspring of horses are of the same stock as the parents. Not only are the colts of the same stock as the parents, but they resemble the parents in small details. This is also true of human beings. We expect a child to be not only of the same race as the parents, but to have family resemblances to the parents—the same color of hair, the same shape of head, the same kind of nose, the same color of eyes, and to have such resemblances as moles in the same places on the skin, etc. A very little investigation reveals likenesses between parent and offspring which we may not have expected before.

However, if we start out to hunt for facts of heredity, we shall perhaps be as much impressed by differences between parent and child as we shall by the resemblances. In the first place, every child has two parents, and it is often impossible to resemble both. One cannot, for example, be both short and tall; one cannot be both fair and dark; one cannot be both slender and heavy; one cannot have both brown eyes and blue. In some cases, the child resembles one parent and not the other. In other cases, the child looks somewhat like both parents but not exactly like either. If one parent is white and the other black, the child is neither as white as the one parent nor as black as the other.

The parents of a child are themselves different, but there are four grandparents, and each of them different from the others. There are eight great grandparents, and all of them different. If we go back only seven generations, covering a period of perhaps only a hundred and fifty years, we have one hundred and twenty-eight ancestors. If we go back ten generations, we have over a thousand ancestors in our line of descent. Each of these people was, in some measure, different from the others. Our inheritance comes from all of them and from each of them.

How do all of these diverse characteristics work out in the child? In the first place, it seems evident that we do not inherit our bodies as wholes, but in parts or units. We may think of the human race as a whole being made up of a great number of unit characters. No one person possesses all of them. Every person is lacking in some of them. His neighbor may be lacking in quite different ones. Now one parent transmits to the child a certain combination of unit characters; the other parent, a different combination. These characteristics may not all appear in the child, but all are transmitted through it to the next generation, and they are transmitted purely. By being transmitted purely, we mean that the characteristic does not seem to lose its identity and disappear in fusions or mixtures. The essential point in this doctrine of heredity is known as Mendelism; it is the principle of inheritance through the pure transmission of unit characters.

An illustration will probably make the Mendelian principle clear. Let us select our illustration from the plant world. It is found that if white and yellow corn are crossed, all the corn the first year, resulting from this crossing, will be yellow. Now, if this hybrid yellow corn is planted the second year, and freely cross-fertilized, it turns out that one fourth of it will be white and three fourths yellow. But this yellow consists of three parts: one part being pure yellow which will breed true, producing nothing but yellow; the other two parts transmit white and yellow in equal ratio. That is to say, these two parts are hybrids, the result of crossing white with yellow. It is not meant that one can actually distinguish these two kinds of yellow, the pure yellow and the hybrid yellow, but the results from planting it show that one third of the yellow is pure and that the other two thirds transmit white and yellow in equal ratio.

The main point to notice in all this is that when two individuals having diverse characteristics are crossed, the characteristics do not fuse and disappear ultimately, but that the two characteristics are transmitted in equal ratio, and each will appear in succeeding generations, and will appear pure, just as if it had not been crossed with something different. The first offspring resulting from the cross—known as hybrids—may show either one or the other of the diverse characteristics, or, when such a thing is possible, even a blending of the two characteristics. But whatever the actual appearance of the first generation of offspring resulting from crossing parents having diverse characteristics, their germ-cells transmit the diverse characteristics in equal proportion, as explained above.

When one of the diverse characteristics appears in the first generation of offspring and the other does not appear, or is not apparent, the one that appears is said to be dominant, while the one not appearing is said to be recessive. In our example of the yellow and white corn, yellow is dominant and white recessive. And it must be remembered that the white corn that appears in the second generation will breed true just as if it had never been crossed with the yellow corn. One third of the yellow of the second generation would also breed true if it could be separated from the other two thirds.

It is not here claimed that Mendelism is a universal principle, that all characteristics are transmitted in this way. However, the results of the numerous experiments in heredity lead one to expect this to be the case. Most of the experiments have been with lower animals and with plants, but recent experiments and statistical studies show that Mendelism is an important factor in human heredity, in such characteristics as color of hair and eyes and skin, partial color blindness, defects of eye, ear, and other important organs.

The studies that have been made of human heredity have been, for the most part, studies of the transmission of physical characteristics. Very little has been done that bears directly upon the transmission of mental characteristics. But our knowledge of the dependence of mind upon body should prepare us to infer mental heredity from physical heredity. Such studies as throw light on the question bear us out in making such an inference.

The studies that have been more directly concerned with mental heredity are those dealing with the resemblances of twins, studies of heredity in royalty, studies of the inheritance of genius, and studies of the transmission of mental defects and defects of sense organs. The results of all these studies indicate the inheritance of mental characteristics in the same way that physical characteristics are transmitted. Not only are human mental characteristics transmitted from parent to offspring, but they seem to be transmitted in Mendelian fashion.

Feeble-mindedness, for example, seems to be a Mendelian character and recessive. From the studies that have been made, it seems that two congenitally feeble-minded parents will have only feeble-minded children. Feeble-mindedness acts in heredity as does the white corn in the example given above. If one parent only is feeble-minded, the other being normal, all of the children will be normal, just as all of the corn, in the first generation after the crossing, was yellow. But these children whose parents are the one normal and the other feeble-minded, while themselves normal, transmit feeble-mindedness in equal ratio with normality. It works out as follows: If a feeble-minded person marry a person of sound mind and sound stock, the children will all be of sound, normal mind. If these children take as husbands and wives men and women who had for parents one normal and one feeble-minded person, their children will be one fourth feeble-minded and three fourths of them normal.

To summarize the various conditions: If a feeble-minded person marry a feeble-minded person, all the children will be feeble-minded. If a feeble-minded person marry a sound, normal person (pure stock), all the children will be normal. If the children, in the last case, marry others like themselves as to origin, one fourth of their offspring will be feeble-minded. If such hybrid children marry feeble-minded persons, one half of the offspring will be feeble-minded. It is rash to prophesy, but future studies of heredity may show that Mendelism, or some modification of the principle, always holds true of mind as well as of body.

Little can be said about the transmission of particular definite mental traits, such as the various aspects of memory, association, attention, temperament, etc. Before we can speak with any certainty here, we must make very careful experimental studies of these mental traits in parents and offspring. No such work has been done. All we have at the present time is the result of general observation.

Improvement of the Race. Eugenics is the science of improvement of the human race by breeding. While we can train children and thereby make them much better than they would be without such training, this training does not improve the stock. The improvement of the stock can be accomplished only through breeding from the best and preventing the poor stock from leaving offspring. This is a well-known principle in the breeding of domestic animals.

It is doubtless just as true in the case of human beings. The hygienic and scientific rearing of children is good for the children and makes their lives better, but probably does not affect their offspring. We should not forget that all the social and educational influences die with the generation that receives them. They must be impressed by training on the next generation or that generation will receive no influence from them. The characters which we acquire in our lifetime seem not to be transmitted to our children, except through what is known as social heredity, which is merely the taking on of characteristics through imitation. Our children must go through all the labor of learning to read, write, spell, add, multiply, subtract, and divide, which we went through. Moral traits, manners and customs, and other habits and ideals of social importance must be acquired by each successive generation.

Heredity versus Environment. The question is often asked whether heredity or the influence of environment has the most to do with the final outcome of one’s life. It is a rather useless question to ask, for what a human being or anything else in the world does depends upon what it is itself and what the things and forces are that act upon it. Heredity sets a limitation for us, fixes the possibilities. The circumstances of life determine what we will do with our inherited abilities and characteristics. Hereditary influences incline us to be tall or short, fat or lean, light or dark. The characteristics of our memory, association, imagination, our learning capacity, etc., are determined by heredity. Of course, how far these various aspects develop is to some extent dependent upon the favorable or unfavorable influences of the environment. What is possible for us to do is settled by heredity; what we may actually do, what we may have the opportunity to do, is largely a matter of the circumstances of life.

In certain parts of New England, the number of men who become famous in art, science, or literature is very great