No evidence whatsoever in favor of the liberation of living units from the body cells, their accumulation in the germ cells and fusion there of like with like gemmules is to be found in the modern work on the germ cells. More over, a critical examination, especially in Weismann's hands, of the evidence on which was rested the entire theory of inheritance of acquired characters, has shown that this evi dence was in part mythical and in part due to uncritical deductions.
Beginning in 1883 August Weismann in a series of essays that were in part speculative, yet backed' by a constant appeal to observation and experiment, attacked the prevailing idea that characteristics acquired by the individual are transmitted to the germ cells and may re appear in the offspring. The germ cells had been shown in many cases to separate them selves at an early stage in the development of the embryo from the rest of the cells, remain ing in an unspecialized condition while the re maining cells from which the body of the individual is to be formed were differentiating. The germ cells become later the essential parts of the ovary and testis respectively. In origin, therefore, they are independent of the rest of the body and have never been a constituent part of it. They are protected and nourished by the body, but not otherwise influenced by it. The germ tract is the imperishable stream which throws off in each generation the body cells whose destiny is to maintain the germ cells. All new modifications arise primarily in the germ cells, and first show themselves as characters in the individuals that develop from these germ cells. Evolution is germinal in origin and not somatic as had been earlier taught. This idea of the origin of new char acters is held almost universally to-day by biologists. Heredity is due, therefore, to the conservation in the germ plasm of those ele ments that have appeared in it from time to time — the old as well as the new. The germ plasm is the capital of the race of which the interest only is spent in each generation in producing new individuals.
Darwin held not only that somatically characters are transmitted, but also that new characters arise in other ways, in the main through the action of the outer world on the germ cells themselves. To this source he attributed many of the differences shown by individuals of any race or species. Darwin knew that some of these differences were due directly to the environment in which the indi vidual was reared and were not inherited, but others he thought were in the germ stream and hence were transmitted (inherited). Darwin thought that the latter variations were different from the larger differences that sometimes ap pear and which were known to the breeder as "sports' (now as "mutations"). The former he believed to furnish the principal material on which natural selection acts to bring about evolution, while the larger and sudden changes or "sports' rarely if ever furnished the ma terials for evolution. To-day it appears that no such distinction exists and that all varia tions, whether large or small, that are inherited, arise in the same way, viz., as sudden changes (mutations) in the germ-plasm. Galton, whose studies in heredity, particularly those of human heredity, are based on much the same kind of evidence in regard to variation as that to which Darwin appealed, appreciated more fully the significance of discontinuity in the origin of new characters, but in his treatment of the inheritance of characters he made no sharp dis tinction between the methods of transmission of •blended)) inheritance and "mosaic)' inheritance and handled both by the same mathematical method. Nevertheless, Galton's introduction of
exact mathematical and quantitative methods has had an important bearing on all recent work. Its later development, especially by Pearson and the school of Biometry, has furnished the Mendelian school with formulae indispensable for testing the validity of much of their data.
Galton reached the conclusion that the two parents of a given individual contribute to it on the average half of its inheritance; the four grandparents collectively one-fourth of its in heritance, the great-grandparents one-eighth, etc. The assumptions on which this theory rests have been shown (see below) to be wrong, for the procedure by which the hered itary elements are sorted out in successive gen erations is incompatible with such an assump tion in individual cases. Individual analyses rather than study of mass phenomena gave the successful method through which the real hereditary mechanism was discovered by Men del in 1865_ Mendel's two laws of heredity have been shown to hold not only for many plants, but for man and other animals, for birds, amphibia, fish, molluscs, crustacea, insects, etc. It has been found that Mendel's laws apply not only to characteristics of cultivated plants and animals, not only to superficial things such as color, but to the characters of wild animals, to differences that distinguish species, and to the most funda mental attributes of living things. Mendel's law of segregation states that the elements that the two parents contribute to the offspring constitute and that the members of each pair separate in the formation of the germ cells in the offspring so that each germ cell contains one member only of each pair. For example, Mendel crossed a race of edible peas having green seeds to one having yellow seeds. All of the offspring seeds were yellow. Yellow dominates green. If plants from these hybrid peas are self-fertilized (or crossed to each other) they produce both yellow and green the ratio of three yellows to one green. iese green peas are pure and never produce anything but green peas. The yellow peas, however, were found to be of two kinds, some were pure yellow always producing all yellow descendants, others were hybrids, producing both yellow and green peas in the ratio of three to one. Taken altogether these second generation peas appeared in the proportion of one pureyellow, two hybrid yellows, one pure green. M'endel pointed out that if the original green grandparent contributed an element for green and the yellow grandparent an element for yellow, these contrasted elements form a pair in the hybrid—a pair whose members separate (segregate) from each other when the germ cells (gametes) are produced. In con sequence, half of the ovules will contain the element for yellow and half the element for green; likewise half of the pollen grains will contain the element for yellow and half the element for green. Chance meetings of the ovules and pollen, thus: Ovules Green Yellow T Pollen Green Yellow will give 1 green green; 2 green yellow; 1 yellow yellow.