In regard to the perpetuation of the advantages gained by means of this power of adaptation, it is clear in those cases in which the young are nourished during their embryonic life by the mother, that, in this way, the young may be rendered immune to a certain extent, and there are instances of this sort recorded, especially in the case of some bacterial diseases. Whether this power can also be transmitted through the egg, in those instances in which the egg itself is set free and development takes place outside the body, has not been shown. In any case, the effect appears not to be a permanent one and will wear off when the particular poison no longer acts. It is improbable, therefore, that any permanent contribution to the race could be gained in this way. Adaptations of this sort, while of the highest importance to the individual, can have produced little direct effect on the evolution of new forms, although it may have been often of paramount importance to the individuals to be able to adapt themselves, or rather to become able to resist the effect of injurious substances. The important fact in this connection is the wonderful latent power possessed by all animals. So many, and of such different kinds, are the substances to which they may become immune, that it is inconceivable that this property of the organism could ever have been acquired through experience, no matter how probable it may be made to appear that this might have occurred in certain cases of fatal bacterial diseases. And if not, in so many other cases, why invent a special explanation for the few cases? We may defer the general discussion of the rôle that external factors have played in the adaptation of organisms, until we have examined some of the theories which attribute changes to internal factors. The idea that something innate in the living substance itself has served as the basis for evolution has given rise to a number of different hypotheses. That of the botanist Nägeli is one of the most elaborately worked out theories of this sort that has been proposed, and may be examined by way of illustration. Nägeli’s Perfecting Principle Nägeli used the term _completing principle_ (“Vervollkommungsprincip”) to express a tendency toward perfection and specialization. Short-sighted writers, he says, have pretended to see in the use of this principle something mystical, but on the contrary it is intended that the term shall be employed in a purely physical sense. It represents the law of inertia in the organic realm. Once set in motion, the developmental process cannot stand still, but must advance in its own direction. Perfection, or completion, means nothing else than the advance to complicated structure, “but since persons are likely to attach more meaning to the word _perfection_ than is intended, it would perhaps be better to replace it with the less objectionable word _progression_.”