Inquiry into the problems of heredity is beset with many difficulties, of which not the least is the temptation to argue about the possible, or the probable, rather than to keep in the lines of observation. Setting out from a laborious and beautiful series of investigations into the anatomy of the Hydromedusæ, Weismann came to think that the organic material from which the sexual cells of these animals arose was not the common protoplasm of their tissues, but a peculiar plasm, distinct in its nature and possibilities. In the course of several years, Weismann not only continued his own investigations in the many directions that his conception suggested, but made abundant use of that new knowledge of the nature and properties of cells which has been the feature of the microscopy of the last decade. His theory of the germplasm gradually grew, undergoing many alterations, so that even in its present form he regards it as tentative. Neglecting the numerous modifications and accessory hypotheses by which he has sought to adapt the theory to the phantasmagorial complexity of organic nature, the main outline of the theory is as follows: A living being takes its individual origin only where there is separated from the stock of the parent a little piece of the peculiar reproductive plasm, the so-called germplasm. In sexless reproduction one parent is enough; in sexual reproduction equal masses of germplasm from each parent combine to form the new individual. The germplasm resides in the nucleus of cells, and Weismann identifies it with the nuclear material which microscopists have named chromatin, on account of the avidity with which it absorbs certain dyes. Like ordinary protoplasm, of which the bulk of cell-bodies is composed, the germplasm is a living material, capable of growing in bulk without alteration of structure, when it has access to appropriate food. But it is a living material much more complex than protoplasm. In the first place, the mass of germplasm which is the starting-point of a new individual consists of several, sometimes of many, pieces termed _ids_, each of which contains all the possibilities--generic, specific, individual--of a new organism. Each _id_ is a veritable microcosm, possessed of a historic architecture that has been slowly elaborated during the multitudinous series of generations that stretch backwards in time from every living individual. This microcosm, again, consists of a number of minor vital units called _determinants_, which cohere according to an orderly plan. A _determinant_ exists for every part of the adult organism which is capable of being different in different individuals. And, lastly, each _determinant_ consists of a number of ultimate particles called _biophores_, which eventually pass into the protoplasm of the cells in which they come to lie and direct the vital activities of these cells. A most important part of the theory is what it supposes to occur during the