is, however, another method of approach which must be mentioned. In 1846 the Swiss Quetelet in his "Letters on the Theory of Probability" applied statistics to biological problems and offered evidence to show that variation took place in accordance with that law. This method was greatly developed in England by a cousin of Darwin, the late Francis Galton. Its chief advocate today is Karl Pearson to whom we are indebted for the name of the science, biometrics. Inasmuch as this is a method of studying phenomena and involves complicated higher mathematics it would be out of place to discuss it extensively here but we may consider some of the results claimed by its advocates. Inasmuch as it deals with averages based on records of large number of cases, it indicates what will happen on the average rather than the results of any particular case. Perhaps the best known of the conclusions of the biometricians is the "law of ancestral inheritance." To use Galton's words: "The two parents between them contribute, on the average, one-half of each inherited faculty, each of them contributing onequarter of it. The four grand-parents contribute between them one-quarter, or each of them one-sixteenth, and so on, the sum of the series 12-14-8-16. being equal to 1, as it should be." In view of the later discoveries that some ancestors seem to make no contributions in given cases, it has been suggested that this law shows us rather "the average amount of resemblance between an individual and particular ancestor." Galton also thought he found that on the average the children of given parents would be more mediocre than the parents themselves, that is, would approach the group average, and this is called "the law of regression." This idea may be shown by the following table:

..

As Galton puts it: "A fundamental distinction may exist between two couples whose personal faculties are naturally alike. If one of the couples consists of two gifted members of a poor stock, and the other of two ordinary members of a gifted stock, the difference between them will betray itself in their offspring. The children of the former will tend to regress, those of the latter will not." The value of some of these observations will be elsewhere considered. Here it is sufficient to note that some able students believe that valuable generalizations may be gotten by statistical methods. It appears that there are some discrepancies between their results and those of the biologists. Just how these are to be reconciled is a matter for the future to decide. For the present we must welcome any methods which give promise of revealing facts whether, in the existing state of knowledge, we can combine them or not.

Our journey has now brought us to the place where one of the greatest stumbling blocks known threatens to upset us, or divert us into bypaths, unless in some way we can remove it.

Do the things we see in men result from "nature or nurture," to use Galton's expression? Are they due to heredity or environment, to blood or to breeding, to use more common terms? We can avoid much useless controversy if we realize that it is foolish to debate the relative importance of two essentials for there is no standpoint from which they can be so viewed and considered. Both are present and without either the organism is unthinkable. Water and air are alike necessary to the organism. The student then has to discover, if he can, what water or air brings to the body. He must learn what of a man's equipment is due to his inheritance, what to his training. The two are constantly fusing in such fashion that sharp separation even is often extremely difficult. We must not forget moreover that the student in any given field is naturally prone to think, as time goes on, that the factors he is specially studying are more important than the others. Thus he tends to exaggerate unconsciously enough the rôle of his own interests. Furthermore he must often obtain a hearing for his views in spite of the opposition of others, and this is not always easy. It is clear that the popular notions must be modified in many ways. The older conception was that the characters of the body in some way got into the germ cells therein produced. Now we know that this is an inversion of the facts. From the physical side it makes no difference (usually at least) what the ancestors did, but it makes tremendous difference what they were. To what extent this race ancestry is modified generation after generation, either for better or worse, will be considered in the next chapter.

If the word heredity is to be used in our discussions, it must be given a definite meaning. Originally inheritance referred to the property received from the preceding generation. This we may consider its legal use. If this inheritance of property is so arranged that the bulk goes to the eldest son it may easily happen that great social differences are soon apparent even though the physical ancestry is the same. Because a child is born into a given social environment, among given people with whom he lives, and from whom he gets his language, customs and standards, we say that he inherits them. This is "social heredity." The medical man frequently uses the word "hereditary" when he means congenital, which is purely a chronological term referring to the date at which a given character appears, and in this sense has nothing to do with the origin of the characters. The medical man further offends by speaking of certain diseases as inherited when he means only that they are transmitted to the child before birth. The venereal diseases are caused by specific organisms which may enter the body at any time and it is not correct to speak of them as inherited. We must be very careful then if we would avoid ambiguity to give a definite meaning to the term. Conklin has suggested the following: "Heredity may be defined as the appearance in offspring of characters whose differential causes are found in the germ cells." Strictly speaking we do not inherit eyes, feet or arms for the miniatures of these do not exist in the germ cell; but there is something within the cell which causes the development of the parts of the body. In this sense heredity will be used in these pages unless otherwise indicated.

SUGGESTIONS FOR READING

BATESON, W. Mendel's Principles of Heredity. 1909.

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Problems in Genetics. 1913.

CASTLE, W. E.

Breeding.

Heredity in Relation to Evolution and Animal 1911.

CONKLIN, E. G.

Heredity and Environment. (2nd Edit.) 1916. DARBISHIRE, A. D. Breeding and the Mendelian Discovery. 1911.

DONCASTER, L. Heredity in the Light of Recent Research. 1911.

HERBERT, S. First Principles of Heredity. 1910.

Lock, R. H. Variation, Heredity and Evolution (2nd Ed.). 1909.

LOEB, J. Artificial Parthenogenesis and Fertilization. 1913. MINOT, C. S. Modern Problems of Biology. 1914.

REID, G. A. Principles of Heredity. 1905.

THOMSON, J. A. Heredity. 1908.

DEVRIES, H. Species and Varieties: Their Origin by Muta

tion. 1905.

WALKER, C. E. Hereditary Characters and Their Modes of

Transmission. 1910.

WALTER, H. E. Genetics. 1913.
WEISMANN, A. The Germ Plasm. 1893.