_The functional independence of the elements or units of the body. Variability and inheritance._--Variability generally results “from changed conditions acting during successive generations.” The influence is exerted on the sexual system, and if extreme, impotence tends to be produced. Bud-variation proves that “variability is not necessarily connected with the sexual system.” The inherited effects of use and disuse of parts imply that the changes in the cells of a distant part of the body affect the reproductive cells, so that the being produced from one of these cells inherits the changes. “Nothing in the whole circuit of physiology is more wonderful.” “Inheritance is the rule and non-inheritance the anomaly.” Inheritance follows laws, such as the tendency for a character to appear at corresponding ages in parent and offspring. Reversion “proves to us that the transmission of a character and its development ... are distinct powers.” Crossing strongly induces reversion. “Every character which occasionally reappears is present in a latent form in each generation.” * * * * * [Sidenote: PANGENESIS.] The hypothesis of pangenesis attempts to explain and connect together all the facts and conclusions which have been summarised in the preceding pages. This hypothesis assumes that each one of the countless cells of which the body of a higher animal is composed throws off a minute gemmule which, with those derived from other cells, exists in the body, and when supplied with nutriment multiplies by division. Each gemmule is capable of ultimate development into a cell similar to the one from which it, either directly or indirectly, arose. Each cell of the body dispatches its representative, as it were, to each single germ-cell, and this explains how it is that the latter possess the power of reproducing the likeness of the parent body. But the germ-cells also receive dormant gemmules which may remain undeveloped until some generation in the remote future. The development of the gemmules into cells depends on their union with the developing cells which precede them in the order of growth. Gemmules are thrown off during each stage of growth and during maturity. This hypothesis of pangenesis is so called because the whole body is supposed to produce the elements from which new individuals arise, the germ-cells being only the union of these elements into clusters. The fact that hybrids may be produced by grafting, that the pollen can act on the tissues of the female plant, and the male germ-cells on the future offspring of the female, implies that the reproductive material can exist and the reproductive processes take place in the tissues, and that they are not confined to the germ-cells.