This fact was shortly and precisely stated by Galton in 1875, and by Weismann in his “Studies in the Theory of Descent,” published in English in 1882. In 1883, in his essay upon Life and Death,[35] Weismann looked upon the germ plasm as the substance of the germinal or reproductive cells, and on p. 148 he defines “germ” as follows: “I should propose to include under this term every cell, cytode, or group of cells which, while not possessing the structure of the mature individual of the species, possess the power of developing into it under certain circumstances.” So far Weismann was a fact man, and gave to the facts observed their true and full significance. Since that time, however, he has speculated upon the nature of the germ plasm. To him it consists of ultimate living particles, to which he assigns various and specific purposes, and groups them at will, group within group, like nests of Chinese boxes. The biophores (his conceived units) have the capacity of growth and reproduction--a generous concession indeed. They are groups of chemical molecules, far beyond the highest powers of the microscope, and cannot therefore be investigated by our senses; they are conceptions, not perceptions. These biophores are arranged in groups, called determinants, one for every part of the adult which is capable of variation; groups of determinants are termed “ids,” and groups of ids are termed “idants,” the last being visible in the ovum as a brightly-stained rod of unclear matter. By the multiplication, differentiation, and disintegration of these various groups, the adult body is formed, and Weismann is prepared to explain every step.