compared with an organism. They "have this cardinal fact in common, that if either is disturbed without transgressing the range of its stability,

it will tend to re-establish itself," that is, if tipped to the right or left it will fall back upon the original side, but if the range is passed, it will topple over into a new position of stability. This illustrates a mutation. There is now a new position of stability or average condition about which there will be fluctuation.

In the present state of our knowledge of variations we are unable to say dogmatically whether species B have arisen by the slow accumulated adjustments of fluctuating variation or by the more rapid process of mutation. In support of the first theory there are numerous cases where species are connected by intermediate grades. There is much experimental evidence to support the second theory.

In 1900, when De Vries in Holland, Correns in Germany, and

C

D

Tschermak in Austria independ- FIGURE 3. Diagram illustrating a Mutation. ently, and almost simultaneously,

reached results from the experimental study of heredity which have modified our views of the origin of species, the whole subject of heredity took on added interest.

9 Galton, op. cit., p. 28.

This increased experimentation and interest led to the discovery of a buried paper written in 1865 by Gregor Mendel, an Austro-Silesian abbot. It proved to be a disclosure of great importance. Mendel had experimented in his garden upon the common edible pea. The law of

FIGURE 4.

giants 25% giants 50 % giants (impure) 25% dwarfs dwarfs (pure) (pure)

(pure) (pure)

Diagram of Mendelian Inheritance in the Pea, where D stands for the Dominant Character, D(R) for the Impure Dominant, and R for the Recessive Character.

heredity which he discovered was ridiculed at the time of the writing of his paper and the discovery was to all intents and purposes lost to science until about 1900.

The remarkable results of Mendel's experiments upon the common pea were as follows. He found that when he crossed a giant variety of 6 to 7 feet with a dwarf variety, 3 to 12 feet high, the offspring were all tall. The character of tallness which appeared in the hybrid generation (F,), to the exclusion of dwarfness, was called by Mendel the "dominant" character, the other was

called the "recessive" character. But this was not all, when the tall cross-bred peas were left to self-fertilize, which corresponds to close inbreeding in animals, among their progeny there were giants and dwarfs in the average proportions of 3 to 1.

2

When the dwarfs of this F2 generation were allowed to self-fertilize, their offspring (F) were all dwarfs, and further generations bred from them were also all dwarfs. They are called pure recessives, being "pure" as regards dwarfness.

2

But when the giants of the F2 generation were left to self-fertilize, their offspring (F) were of two kinds: one-third of them (pure dominants) produced giants only; two-thirds of them (impure dominants) produced giants in the proportion of 3 to 1. Thus the F2 generation, resulting from the self-fertilization of the cross-bred forms or hybrids (F1), consisted of 25 per cent. pure dominants, 50 per cent. impure dominants, and 25 per cent. pure recessives.10

2

The law will be made clear by examining Figures 4, 5 and 6 in which the inheritance of the waltzing trait is shown for mice, and the inheritance of colors is shown. for red and white four-o'clocks.

Figure 5 shows how the waltzing character is recessive and absence of this character is dominant. In the first generation a normal mouse (represented in black) is crossed with a waltzing mouse (represented in white). The result is all normal mice in the first filial (hybrid) generation. When two mice of this generation are crossed, they yield waltzing mice in the proportion of one waltzing to three normal mice. When the waltzing mice of this generation are mated they yield waltzing

10 Thomson & Geddes, op. cit., p. 129.

mice alone. This is because they are pure recessives. But some of the normal mice produce only normal mice, these are pure dominants, while others of the normal mice produce normal and waltzing mice in the proportion of three normal to one waltzing mice, these are impure

dominants.

FIGURE 5. Mendelian Inheritance in Mice.

This law does not mean that if there were only four offspring of a mouse in the first filial generation F1, one would be normal and would breed only normal, two would be normal but would breed both normal and waltzing mice, and one would be waltzing and would breed only waltzing mice. It might of course happen this way. What it means is that on the average, if one were to study a great number of matings of normal and waltzing mice, the offspring would possess the waltzing trait in the proportion indicated. It does not enable one to make a dogmatic prediction about a small group of brother and sister mice.

Figure 6 shows the inheritance of color in which one

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

Mendelian inheritance as applied to human characters has not yet been worked out, if indeed inheritance of human characters can be established at all in accordance with this law. Dr. C. B. Davenport has found that when both parents have pure blue eyes all of the children have pure blue eyes; when one parent has pigmented iris while the other has blue, either all the children will show no blue eyes or else half of them will be blue-eyed; when both parents have brown iris either all the children will have brown iris or else about a quarter will be blueeyed.12 Professor Boas found that among Indian halfbloods an extensive series of measurements of width of face showed a decided tendency in the children to resemble either the Indian race or the White race. The feature of Mendelian inheritance which brings about the occurrence of mixed characters in the first hybrid generation was not found, but instead of this a decided tendency of reversion to either type and a comparative rarity of intermediate forms.13

11 Thomson & Geddes, op. cit., p. 132.

12 Davenport, C. B.-Heredity in Relation to Eugenics, p. 31,

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