The theories as to the transmission or non-transmission of acquired characters may be better understood by reference to schemes No. 1, 2 and 3. Scheme No. 1 represents the theory of =Pangenesis=, which teaches that reproductive cells are not formed from pre-existing reproductive cells, but by the body cells themselves. Darwin taught that all the cells of the body, such as skeletal-cells, muscle-cells, nerve-cells, and so on, are continually giving off infinitely small cell germs or gemmules, which have the power of growing and forming cells exactly like themselves. These gemmules have a great affinity for one another, and, circulating in the blood in countless numbers, they finally come together in the reproductive glands and form the reproductive cells. On this theory the fact of the transmission of acquired characters can readily be appreciated, and it can easily be understood how the parent molds the child. Suppose, for instance, that the parent, by exercise, has become a skillful athlete. In him certain muscles have become greatly developed and strengthened. During all the time of the exercise of these muscles, the modifying muscle cells have been continually giving off to the blood modified gemmules, which collect in the reproductive cells and make it possible for the offspring to develop into an athlete because the modified gemmules develop into modified muscles like those of the athlete. Scheme 1 shows the absence of any arrow like those shown in schemes 2 and 3, directly connecting germ-cell with germ-cell; this means that in this theory there is no continuity of the germ-cells. But arrows are seen extending from the various body-cells (skeletal, glandular, etc.) to the germ-cell; this means that the germ-cells are formed by influences or gemmules emanating from the various body cells. Scheme 2 teaches that a germ-cell (when fertilized, of course) can produce many cells, some of which differentiate, finally, into skeletal cells, some into glandular, some into muscle and nerve cells, and some into new germ-cells; so that an animal or plant, I, is formed. In like manner a germ-cell of animal, I, can give rise to the germ and body cells of animal, II, and so on indefinitely. This scheme shows that there is a direct =continuity of the germ-cells=; and it also shows that the germ-cells are entirely _insulated_, as it were, from the body-cells (skeletal, glandular, etc.), inasmuch as no influences (arrows) extend from the body-cells to the germ-cells. This means that the transmission of acquired characters, bodily, mental, moral, etc., is impossible. It means, in other words, that none of the advantages gained by a parent in the course of his life can be handed on to his offspring by heredity. There are many biologists and pathologists who teach this theory as the correct one.