=Persistence of Mendelian Factors Argues Against Such a Mode of Inheritance.--=On the face of things, the apparent inviolability of Mendelian factors which may remain unexpressed in the germ for one or many generations--indeed the whole matter of genotypical differences in the gametes of the same individual--shows the improbability of somatic interference with the germ-plasm. But notwithstanding this, because of the great importance of the issue, it is well to review in some considerable detail the various phases and possibilities of the question. =Experiments on Insects.--=Some of the attempts to secure evidence of the transmission of personally acquired parental modifications in insects are very interesting. Many insects in the larval stages, particularly just after pupation seem to be especially susceptible to external influences. They have been much used, therefore, for purposes of experiment. It has long been known that differences in size, in color and even in the shape of wings can be produced by various agents if applied at this period of development. From the standpoint of heredity, however, the important consideration is to determine if these experimentally induced changes have been reflected on to the germ-cells so that they reappear in the offspring of the modified individuals. It has been found that in some cases where male and female are of different color, the color of the female can be changed to that of the male by altering the conditions of temperature. In certain cases types can be changed by cold so that they resemble varieties of the same species found farther north, and by heat, varieties found farther south. But not all individuals of a given lot are affected, and often different individuals of the same kind show different effects. Moreover, in some cases the same aberrations were produced by heat as by cold. This indicates that it is not so much a question of specific effects as a general physiological change, apparently mainly a matter of direct influence of temperature on the chemical composition of the pigments. The Countess von Linden in fact has shown that the extracted pigments can be made to undergo the same changes of color in a test-tube by heat and cold as in the pupæ. But there is no evidence that the germ-cells of the living insect were affected in a specific way. In a small fraction of the offspring of such modified individuals abnormalities appeared, but these were not always of the same kind as those which had been produced in the parent. That is, there was no evidence of a trait or character having been acquired by the body and handed on to the germ-cell. Where an effect was produced on the germ-cell it was probably produced directly as in the first cases discussed.