VARIABILITY.--Changed conditions may lead to irregularity in the number of gemmules derived from various parts of the body; deficiency in number might cause variation in any part by leaving some of the cells free to unite with allied gemmules.
The direct action of surroundings, or the effect of use or disuse on a part, may cause corresponding modifications of the gemmules, and through these of the part in the succeeding generation. “A more perplexing problem can hardly be proposed,” and yet it receives an explanation on this hypothesis. Such causes must, as a rule, act during many generations before the modification reappears in the offspring. This may be due to the unaltered gemmules derived from earlier generations, and their gradual replacement by the increasing number of altered gemmules.
Variation in plants is much more frequent in sexually produced than it is in asexually produced individuals. This may be due to the absence in the latter of that great cause of variability, changes in the reproductive organs under altered conditions. Furthermore, the former alone pass through the earlier phases of development, when structure is most plastic and yields most readily to the causes inducing variability.
The stability of hybrids and of many varieties when propagated by buds, as compared with their reversion to the parent form when propagated by seed, remains inexplicable.
Hence variability is explained as due (1) to the irregularity in number of gemmules, to their transpositions, and redevelopment when dormant; and (2) to their actual modification and the gradual replacement by them of unaltered gemmules.
INHERITANCE.--The non-transmission by heredity of mutilations, even when repeated for many generations, as in docking the tails of certain domesticated breeds, may be explained by the persistence of gemmules from still earlier generations. The cases of inheritance when mutilations are followed by disease, as in Brown-Séquard’s experiments on guinea-pigs, may be due to the gemmules being attracted to the diseased part and there destroyed.
The disappearance of a rudimentary part, together with its occasional reappearance by reversion, is to be understood by the existence of ancestral gemmules, for which the corresponding cells have, except in the cases of reversion, lost their affinity. When the disappearance is final and complete, the gemmules have probably perished altogether.