We might expect on _a priori_ grounds that not only the chance fluctuations of nutrition within the germ-plasm would cause its elements to vary in this or that direction, but that there would also be influences of a more general kind, especially those of nutrition and climate, which would in the first place affect the body as a whole, but with it also the germ-plasm, and which would therefore bring about variations, either in all or only in certain determinants. In this case all the individuals would vary in the same way, because all would be similarly affected by the same causes of change. This is actually the case; it is indubitable that external influences, such as those emanating from the environment or media in which species live, are able to cause direct variation of the germ-plasm, that is, permanent, because hereditary variations. We have already referred to this process and called it 'induced germinal selection.' That such influences of environment may bring about changes in _individual_ organisms is obvious enough; that, for instance, good nutrition makes the body strong and vigorous, that too abundant food makes it fat and causes degeneration, that insufficient food lessens its stamina and vigour, are well-known facts. We have to inquire, on the one hand, to what extent such influences are able to cause changes in the individual body in the course of a lifetime, and, on the other hand, more particularly, how far such changes or modifications of the soma can call forth corresponding variations in the determinant system of the germ-cells, and whether and under what circumstances they may be transmitted; for where this is not the case there can be no permanent hereditary variation of the whole species, and the variation will only persist as long as the conditions which gave rise to it endure, and will disappear again with these. The influence of nutrition as a cause of variation has often been over-estimated. The old statement which has gone the round of the textbooks since the time of John Hunter, that the stomach of carnivores may be transformed by vegetable diet into a herbivore stomach, is absolutely unproved. Brandes at least, who not only subjected all the statements in the literature on this point to a critical investigation, but also instituted experiments of his own, regards the statement as altogether unfounded. All the 'cases' cited, in which the stomach of a gull or of an owl fed on grain became transformed into an organ with stronger muscles and covered with horny plates, depend, according to Brandes, upon inexact observation. There can therefore be no question of any inheritance of this fictitious stomach-transformation, and the idea that such a fundamental histological adaptation as the alleged transformation of the stomach of the grain-eating bird should arise as a direct effect of the food is wholly without foundation.