BIOLOGICAL FOUNDATIONS OF SOCIOLOGY 533
vidual unites with a germ-cell from another. The compound cell thus formed, the fertilized ovum, multiplies by dividing and redividing many times the one cell into two cells, till a new individual, a new cell-colony a man, for instance arises. During multiplication, differentiation in form and function occurs among the cells, so that ultimately the individual is compounded of many different kinds of cells muscle-cells, skin-cells, gland- cells, nerve-cells, and so forth. In the fertilized ovum is a dot, the nucleus. In the nucleus are ultra-microscopical dots and threads of a substance which, when multiplication occurs, displays remark- able movements and is divided, seemingly with great precision, among the daughter-cells. In this way it passes, apparently with little change, from the germ-cells of the parent to those of the offspring. This substance has been identified, with a high degree of probability, as the germ-plasm, the bearer of heredity. We need not pin our faith to any theories as to the composition of the germ-plasm; but some such substance there must be some substance which is the bearer of heredity. If this theory of the transmission of the germ-plasm from germ-cell to germ-cell be correct and all the evidence indicates that it is correct the child resembles his parent, not because his several parts are derived from similar parts of his parent his head from his parent's head, his hand from his parent's hand, and so on but because his germ-plasm is derived by direct descent from the parental germ-plasm, and therefore is very similar. The nature of the germ-plasm, therefore, determines the nature of the indi- vidual that arises from the germ-cells. Thus from one variety of germ-plasm proceeds a man, and from another a rabbit. When a species undergoes evolution, the germ-plasm undergoes gradual change. When we improve our domestic plants and animals, the alteration is always in essence a germinal change. It is the germ- plasm that is the main fact to be grasped in a study of heredity. All the characters, all the physical and mental parts, of a living being are either "inborn" or "acquired." An inborn character is one which comes to the individual "by nature," as part of his natural inheritance. An acquired character, on the other hand, is a modification of an inborn character caused, as a
