=External Influences May Directly Affect the Germ-Cells.--=There is evidence that under special conditions external influences may in certain organisms affect the germ-cells, but that this occurs commonly is extremely doubtful. For example, Professor MacDougal, by treating the germ-cells of the evening primrose with various solutions, such as sugar, zinc sulphate and calcium nitrate, has apparently succeeded in producing definite germinal mutations. He injected the solution into the ovary of the flower the forenoon of the day at the close of which pollination would occur. He reports that in this way changes were produced in the germ which found expression in new and permanent characters. Professor Tower has experimented for a number of years with various species of _Leptinotarsa_, the potato beetle. By varying the conditions of temperature, humidity and atmospheric pressure when females were laying their eggs, he reports having produced variations in the young which came from these eggs although the mothers themselves were not changed. According to Professor Tower slight increase or decrease in these environmental factors stimulated the activity of the color producing ferments, giving rise to melanic or darker individuals. Greater increase or decrease, inhibited them and produced albinos. He found also that at times the same stimulus might show different results in different eggs. The effect, therefore, is a general and not a specific one. Ordinarily the eggs of these beetles are laid in batches. When one of these batches was laid and left under normal conditions, the usual form of young hatched from it, but other batches from the same female under abnormal conditions resulted in the production of atypical forms. For example, a normal two-brooded form became five-brooded. The commonest modification was the production of various color types. These once established, according to Professor Tower, behave as independent, inheritable units. The experiments of Doctor Bardeen with X-rays and of others with X-rays, radium and other agents on the sperm and ova of amphibia show that these are very susceptible to injurious influence at or near the time of fertilization. =Such Effects Improbable in Warm-Blooded Animals.--=However possible it may be to bring about germinal changes in invertebrata or lower vertebrata by such external agents as temperature and the like it is obvious that the probability of such extrinsic influences affecting the germ-cells of warm-blooded animals is very remote indeed. In the latter the germ-cells are more or less distant from the exterior and are at practically a constant temperature. Such experiments, therefore, beyond showing the possibility of producing changes in germ-cells, do not have very direct bearing on the problem of how inheritable variations are produced in man. In his case about the only avenue of approach through which germ-cells might be influenced is the blood or lymph.