TENDEL’S LAWS OF HEREDITY 381 
Mendel’s experiments.—What Mendel sought to discover was the 
law of inheritance in hybrid varieties, and he selected for experiment 
the edible pea (Pisum sativum). The trial plants, he says, must 
Possess constant differentiating characters, and must admit of easy 
artificial pollination; the hybrids of the plants must be readily fertile, 
and readily protectable from the influence of foreign pollen. These 
conditions were afforded by peas, and twenty-two varieties or sub- 
species of pea were selected, which remained constant during the eight 
years of the experiments. Whether they were called species, or sub- 
species, or varieties, is a matter of convenience; the names Pisum 
quadratum, P. saccharatum, P. umbellatum, etc., do in any case repre- 
sent groups of similar individuals which breed true inter se. It should 
be noted that these peas have the particular advantage, for experi- 
mental purposes, that they are habitually self-fertilised—in North 
Europe, at least. 
In studying the different forms of peas, Mendel found that there 
were seven differentiating characters which could be relied on: 
1. The form of the ripe seeds, whether roundish, with shallow 
wrinkles or none, or angular and deeply wrinkled: 
2. The colour of the reserve material in the cotyledons—pale 
yellow, bright yellow, orange, or green; 
3. The colour of the seed-coats, whether white, as in most peas 
with white flowers, or grey, grey-brown, leather brown, with or with- 
out violet spots, and so on; 
4. The form of the ripe pods, whether simply inflated, or con- 
stricted, or wrinkled; 
5. The colour of the unripe pods, whether light or dark green, or 
vividly yellow, this colour being correlated with that of stalk, leaf- 
veins, and blossoms; 
6. The position of the flowers, whether axial or terminal; and 
4. The length of the stem, whether tall or dwarfish. 
Mendel’s results; the Law of Dominance.—Having defined the 
differentiating characteristics of the varieties, Mendel proceeded to 
make crosses between these, investigating one character at a time. 
Thus, pollen from a pea of the round-seeded variety was transferred 
to the stigma of a pea of the angular-seeded variety, the stamens of the 
artificially pollinated flower being, of course, removed before they 
were ripe. The same was done all along the line. _ 
What was the result in the hybrid or cross-bred offspring? It was 
found that they showed one of each pair of contrasted characters, to 
