color (red) does not completely dominate the other. In this case the impure dominants show a color (pink) which is a blend of the colors of the parental generation.

This remarkable mode of inheritance has been demonstrated to hold for a great diversity of organisms: in mice, rats, rabbits, guinea pigs, cattle, poultry, canaries, snails, silk-moths; in beans, maize, wheat, barley, and stocks. In cattle, for example, hornlessness is the dominant and presence of horns the recessive character. In wheat, rough and red chaff are the dominant and smooth and white chaff the recessive characters.11

It is difficult to draw definite conclusions from the study of human inheritance on account of the great complexity of the human organism. Man is the result of the intermixture of so many different stocks that there are no "pure lines." Since experiment is out of the question, observation must be relied upon. But the rate of increase of the human species is slow (about 60 generations of men since the Christian era began), and the number of offspring are few. In spite of these difficulties studies have been made with the result that certain human traits appear to be inherited in accordance with Mendelian proportions.12 For example, 13

12 Boas, F.-The Mind of Primitive Man, 1911, p. 78.

13 Davenport, C. B.—Heredity in Relation of Eugenics, pp. 31, 66, 77, 175.

Mendel's theory, like Weismann's, has the idea of germinal continuity for one of its foundations. It conceives of the hereditary relation as one between the parental and filial germ cells and not between the bodies of parent

FIGURE 6. Mendelian Inheritance in Four-o'clocks.

and offspring. Mendelism explains the organism as built up of a number of definite and separably inheritable characters. Variation seems to consist in the presence or omission of elementary factors. Thus, the white sweet pea was brought about in the variation by which one of the color factors was dropped out. Variation is not always the progression from a lower degree of com

plexity to a higher, but the reverse may be true.14 Thus experimental breeding has shown that the white coat of the horse is not a simple character, but is due to several independently inheritable factors.15

Besides Mendel's Law, there are Galton's two statistical laws of inheritance: the Law of Regression and the Law of Ancestral Inheritance. The Law of Regression as first expounded was based upon measurements of the stature of over 900 English persons.16 Galton found that the stature of an individual is determined by the racial type to which the parents belong, modified, however, by a tendency to revert to a type intermediate between the special variations represented by the parents. For example, if the father is very tall, and the mother somewhat taller than the average, the children tend to develop a stature which is somewhat near the racial type, but at the same time dependent upon an intermediate value located between the stature of the mother and that of the father. This law appeared to hold for the inheritance of stature, eye-color, and artistic ability. Biologists have criticized Galton's laws on the ground that they lump together both inherited and non-inherited variations. The Law of Ancestral Inheritance showed that on the average each parent contributes 1/4 of every inherited faculty, each grandparent 16, and so on. More recent studies have shown that the intensity of heredity for each parent may be expressed by about 3.17 The principle may be made clear by quoting from Dr. Boas: "Provided the mother differs in her stature by an amount of 9 cm. from the racial norm,—for instance, if

14 Thomson & Geddes, op. cit., p. 137.

15 Davenport, op. cit., p. 24.

16 Galton, op. cit., chs. vi. and vii.

17 Pearson, K.-"On the Laws of Heredity in Man," Biometrika, vol. ii, p. 357 et seq.; and Boas, F.—“Heredity in Anthropometric Traits,” Amer. Anthropologist, N. S., vol. ix, p. 453 et seq.

she is 9 cm. taller than the average individual, then we may expect the child to be one-third of 9 cm., or 3 cm., above the average. It will thus be seen that if both parents differ in the same direction from the average, the effect of both will be cumulative; and if both differ from the average of their people by the same amount, the joint effect of the two parents may be expressed by the coefficient of about two-thirds. In case, for instance, both father and mother should be 9 cm. above the type average, we should expect the child to be about two-thirds of 9 cm., or 6 cm., above the average.

99 18

These variations, inherited in accordance with the laws that have been outlined, form what Professor Thomson has called the "raw materials" of evolution. Of the origin of these variations we know little. In the microcosm of the germ cells there goes on a process of extraordinarily intricate permutation and combination. Weismann supposes that there is a struggle within the germ cell between rival hereditary items. However this may be, there is much research still necessary before we can hope to speak in dogmatic fashion of the origin of variations.

SUPPLEMENTARY READINGS.

BOAS, F.-The Mind of Primitive Man.

CRAMPTON, H. E.-The Doctrine of Evolution.

DARWIN, C.-The Origin of Species.

DAVENPORT, C. B.-Heredity in Relation to Eugenics.

GALTON, F.-Natural Inheritance.

KELLICOTT, W. E.-The Social Direction of Human Evolution.

METCALF, M. M.-Organic Evolution.

PUNNETT, R. C.-Mendelism.

ROMANES, G. J.-Darwin and After Darwin, I The Darwinian

Theory.

18 Boas, op. cit., p. 82.

THOMSON, J. A. & GEDDES, P.-Evolution (Home University

Library).

THORNDIKE, E. L.-Individuality.

WEISMANN, A.-The Evolution Theory.

ERRATA

Page 17, lines 19 to 27, should read

mother and that of the father. This law of regression was found to hold for the inheritance of stature, eye-color, and artistic ability. In accordance with another Galton theory it was found that the influence upon the individual of inheritance was about 1/4 from each parent and 1/16 from each grandparent. More recent studies have shown that the intensity of heredity for each parent may be expressed by about 1/3. The principle may be made clearer by quoting from Professor Boas: "Provided the