GENETICS, the science which deals with the coming-into-being of organisms. It is one of the fundamental assumptions of modern biology, amounting almost to an axiom, that no organisms at the present time originate de notro,.but only from previously existing similar organisms, which either divide to form new individuals by processes called vegetative or asexual, or else produce new individuals from single cells, the so-called reproductive cells. Examples of reproductive cells are the spores of ferns and mosses and the eggs and sperm cells of the higher plants and higher animals. In some cases the egg-cell of a flowering plant or of an animal, like the spore of a fern, may by it self produce a new individual, but in the great majority of the higher plants and animals (in cluding man and his domestic animals) a new individual arises only from the union of two unlike reproductive cells, an egg-cell and a sperm-cell. These when united, an egg-cell with a sperm-cell, a process which constitutes the *fertilization) of the egg, form a zygote (a joining-together of two reproductive cells) and the zygote grows into a new individual. The fact that an individual so formed resembles its parents is called heredity. It is due, other things being equal, to the similarity in nature of the reproductive cells from which an indi vidual arises and the reproductive cells from which its parents arose. The connection is closer than most persons suppose. In reality the parent does not produce the child nor even the reproductive cell which functions in its origin. The parent is himself merely a by product of the fertilized egg (or zygote) out of which he arose. The direct product of the zygote is other reproductive cells, similar to those from which it arose. Thus the zygote begins its development by dividing into two cells, these divide into four, and so on until a great number of cells arise, among which differences in form and function can be recog nized. Some become bone cells, some muscle cells, some nerve cells, others blood cells, and thus the body is produced. But a very few cells derived from the zygote do not change into anything. They remain, like the zygote from which they arose, reproductive cells and when they leave the body it will be as repro ductive cells capable of forming new zygotes. Hence heredity (that is, the resemblance be tween parent and child) depends upon the close connection between the reproductive cells which formed the parent and those which formed the child, one being the immediate and direct product of the other. This principle of the *continuity of the germinal substance) (re productive cell material) is one of the founda tion principles of genetics. It shows why body changes produced in a parent'by environ mental influences are not inherited by the off spring. It is because offspring are not the product of the parent's body but only of the germinal substance which that body harbors. Changes produced in the body alone are called acquired characters to distinguish them from characters dependent upon the nature of the germinal substance itself, which form the basis of heredity. That *acquired characters are
not follows as a logical necessity, if the principle of germinal continuity is sound. To August Weismann belongs the credit for first making this clear. He may thus be re garded as one of the founders of genetics. His contribution consists of the two points (1) that the body is a product of the germinal sub stance plus environment, but is sot itself the producer of germinal substance but merely harbors it, inheritance being from germ cell to germ cell; (2) that changes in the body pro duced by the environment (*acquired char acters)) are not inherited, since the body is not a vehicle of inheritance. In qualification of these principles, it should be stated that the possibility is admitted, though not conclusively proved, that the environment may directly modify the structure of the inheritance, sub- stance and thus indirectly affect nheritance, but if so this is not accomplished through the mechanism.of bodily changes (*acquired char acters)) but only of germinal changes. Genetics assumes the validity of the evolutionary hypothesis, that changes in germinal substance do from time to time occur and that thus organisms come into being which differ from their If this did not occur, the organisms of to-day would be substantially identical with those of 10,000,000 years ago, which we know from the geological record is not the case. To Charles Darwin belongs the credit for establishing beyond question the fact of organic evolution, that is, the fact that or ganisms vary that such variations as best meet the requirements of the environment sur vive while others perish. Thus from age to age organisms change more or less extensively, though retaining unmistakable resemblances to their progenitors. In terms of Weismann's principles, the variations of organisms which result in evolution have their origin in changed structure of the germ-cells, but the natural selection which determines survival is made upon the bodies which harbor the germ-cells. The practical result is the same as if the genii cells themselves were subjected to competitive elimination rather than the bodies which rep resent them as by-products. This more com plete analysis of the process of evolution through natural selection was made by Weis mann after Darwin's death. Darwin rested with the demonstration of the fundamental fact that organisms vary genetically (that is, as regards the factor of heredity) and that such variations through natural selection result in evolution. He above all others is therefore the founder of genetics. Weismann carried the analysis of the method of evolution an important step farther. He also called attention to the chromosomes of the cell nucleus as the prob able *bearers of heredity,) these being the com ponents of the germ-cell in which structural changes probably occur which result in herit able changes of body form.