species (forms of life having a certain similarity and related by descent) appear "all at once" by mutations.

In support of this theory, Professor De Vries takes the case of a certain evening primrose which has shown sudden and repeated leaps with a remarkable subsequent constancy. The Chelidonium majus laciniatum appeared suddenly in the year 1590 in the garden of an apothecary at Heidelberg, and has remained constant ever since. These experiments and observations have lead to another theory of descent. It is now held by a school of biologists represented by De Vries, that species have arisen by this discontinuous process, in which each new unit, "forming a fresh step in the process, sharply and completely separates the new form as an independent species from that from which it sprang." The new species originates from the parent species without any visible series of transitional forms. It can perhaps be made more clear by figure 3. The figure represents by A B the direct line of descent from which the parent B has sprung. Now with the usual fluctuating or continuous variation, the offspring of B would not be likely to have the same average (of any trait) as their own parents, but an average much nearer the average of the whole group to which the parents belonged. But in the case of the "sport," whose origin we are explaining, the offspring C of B will start a new and independent line of descent. That is, the offspring D of C, will not have an average nearer that of B than C was, but will have an average nearer that of their parents C. Thus the "sport" C, has established a new group type round which there will be fluctuating or continuous variation.

Galton has illustrated the process by analogy, but from another point of view. The polyhedron may be

19 A

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.

C

D

In 1900, when De Vries in Holland, Correns in Germany, and Tschermak in Austria independ- FIGURE 3. Diagram illusently, and almost simultaneously,

trating a Mutation.

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

D(R)

R

R

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

FIGURE 4. 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, 34 to 12 feet high, the offspring were all tall. The character of tallness which appeared in the hybrid generation (F1), to the exclusion of dwarfness, was called by Mendel the "dominant" character, the other was

called the "recessive" character. But this was not all. By self-fertilizing the tall cross-bred peas (this corresponds to inbreeding in animals), giants and dwarfs appeared among their progeny in the average proportions of 3 to 1.

2

2

Now when the dwarfs of this F, generation were selffertilized, it was observed that all of their offspring (F) were dwarfs. Moreover, successive generations bred from them were also all dwarfs. These are called recessives, since they are "pure" as regards dwarfness. But when the giants of the F2 generation were selffertilized, it was discovered that their offspring 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, produced by allowing the crossbred forms or hybrids (F1) to self-fertilize, consisted of one-quarter pure dominants, one-half impure dominants, and onequarter recessives.10

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.