are recessive to the normal. Both conditions are heritable as the diagrams show, though the chart of the ateliotic type is incomplete and does not make clear its recessive character.

Deafness may be due to some sickness or accident but it is often the result of the inheritance of some peculiar ear formation. No one could consider deaf-mutism as anything but an undesirable affliction. It was known many years ago that the condition was more common among the children of affected families than in the population at large, and many family charts have now been made. These show that the condition may be passed on even though neither parent is affected, i.e., deaf-mutism is a recessive.

Hemophilia is one of the sex-linked traits, as before mentioned. The blood does not clot properly and hence even a slight cut may be dangerous or even fatal. In the family here charted the first six sons died as a result of this fact. It is to be noted that the daughters do not show the condition. They will transmit it to their sons but not to the daughters unless they chance to marry men who are also carriers.

5 PEARSON, K. o. c., Pl. X, Fig. 62.

• Ibid., Pl. X, 58.

TTTTTT

INHERITANCE OF HEMOPHILIA

7

These illustrations clearly indicate that a number of physical characters, which must be considered as defects in comparison with the normal, not merely exist in many individuals but are passed along generation after generation. The list is by no means complete. We have all seen albinos who are characterized by an absence of pigment cells so that the skin looks chalky white and the eyes red owing to the blood. This is a recessive condition. On the other hand, the affliction known as hypotrichosis, hairlessness accompanied by loss of teeth, seems to be a dominant. Many others might be mentioned.

Until recently we have assumed that these conditions were peculiar to the individual. Now with the development of biology and accumulation of material from many sources we are coming to believe that direct inheritance has been a larger factor than we have thought. Whenever individuals possessing some peculiar characteristic have settled in an isolated community, geographically or socially sharply separated from other settlements, we are likely to find that these characteristics have been perpetuated and are much more common than in other communities. Thus, in America, Martha's Vineyard was the home of a line of deaf-mutes who have wandered over the country. A colony of "bleeders," originally developing in Sullivan County, Pennsylvania, has formed new 7 PEARSON, K. o. c., Pl. XXXV, Fig. 392.

settlements in Minnesota, South Dakota and California. Long Island and Fairfield County, Connecticut, were the original seats of much of the Huntington's chorea. As our social surveys are multiplied we find evidence that such minor groupings are far more common than was realized.

Important as we may consider the above mentioned defects we will all admit that mental defects are much more serious from the standpoint of society, particularly if it appears that they are rooted in heredity.

Unfortunately in the present state of knowledge, our information is based almost wholly upon the actions of men, and we know almost nothing of the nature or significance of brain differences. It seems that these things which we consider as attributes of man, memory and the higher forms of thought, depend upon "The human cerebral cortex. . . a superficial layer of the brain with a thickness varying from one and a half to five millimeters and covering an average of 2,352 centimeters" (about one and a half square feet). This cortex is estimated to weigh about 658 grammes. It is composed chiefly of blood vessels, supporting tissues and nerve cells. The blood vessels and supporting tissues are merely mechanical accompaniments of an apparatus, the real functions of which are carried on by the nerve cells. These cells have been carefully studied, their arrangement and distribution made out, and it is estimated that in a single cortex their number is not far from 9,200,000,000. Notwithstanding this prodigious number, these cells and their processes represent only two per cent of the total weight of the cortex; in other words, the cortical nerve cells and their processes in the average man weigh about thirteen grammes. This amount represents a little less than a cubic inch of ma

terial, or to be more accurate, it will just fill a cube whose edge is 2.35 centimeters. In a man who weighs approximately one hundred and fifty pounds, this amount of substance would represent about one five-thousandth of his total weight, yet this very small proportion of his body serves him as the material basis for a whole life of intelligent activity and is the part of the nervous system chiefly concerned in yielding that almost impalpable product, human personality.'

To some extent the different parts of the cortex have control over different reactions and hence differences in ability may rest on different organization of the cortex and may be passed along from one generation to another. There are many unsolved questions here and about all we can be sure of is "no brain, no mind." The new-born child may have all the brain cells it will use, but the connecting links must be developed; or is it better to say, perhaps, that the child must learn to use the connecting links and that this process is what we mean when we speak of "the formation of habits"? This cortex is easily injured and hence the checking of development may come therefrom or via the road of heredity. All we need remember here is that the man is not merely an enlarged edition of the child. An adult with the proportions of a child would be a caricature. Our minds as well as our bodies are changed by the experiences of life. We are in part what we are today because of the things we have done.

A further difficulty arises from the belief, held even today, that the body is but the house inhabited by a spirit which once entered into it and will some day depart again. In older days it was even held that some foreign spirit

8 PARKER, G. H. Biology and Social Problems, pp. 34-35.

might enter in and take possession if the original inhabitant was found absent, or might even dispossess the original owner. Hence the actions of a person might be attributed to possession by "evil spirits," and that chapter of human history dealing with demoniacal possession is both interesting and harrowing.

It has long been seen that actions of men were in some way dependent upon circumstances, upon education and upon health, hence such words as "afflicted" or "gifted " are rather vague. There has come, nevertheless, recognition that the human race grades from those of the lowest mental caliber to those of the highest. Using present terms we may roughly classify men into the following groups:

The Subnormal-The Feeble-minded: Idiot, Imbecile, Moron.

Normal-minded - The Common People.

Supernormal

The Distinguished, the Genius. Before we attempt to consider these separate classes we must again emphasize the difference between endowment and attainment. Though we cannot put our hands upon the actual physical traits, we know that men differ by nature as well as by nurture. In every-day life it may be impossible to determine the actual contribution of these two factors. This backward child in the schoolroom may have a normal mind which has been poorly trained, or it may be weak through lack of food. It may be suffering from disease, or it may have been crippled by some accident. The distinguished man may be one of only ordinary capacity who has been unusually well trained, or has had the superior opportunities which wealth and social position bring, or who accidentally has done that which has brought renown; or he may be mentally far above the