probably much greater than that of the descendants of one generation, every descendant would in the course of time deviate unfavourably in some one character from the type of the species, and then either all the descendants would be eliminated or the type would become unstable. But neither of these things happens, and there are undoubtedly species which remain constant for long periods of time, therefore the assumption must be false and every variation of a determinant does not of necessity go on in the same direction without limit. I therefore suppose that although slight variations are ceaselessly taking place upwards or downwards in all determinants, even in constant species, the majority of these turn again in the other direction before they have attained to any important degree of increase, at least in the germ-plasm of all species which have had a definitely established equilibrium for thousands of generations. In such a germ-plasm, or to speak more precisely, in the id of such a germ-plasm, marked fluctuations in the nutritive stream will not be likely to occur as long as the external conditions are unchanged, but slight fluctuations, which will not be wanting even here, may often alternate and turn in an opposite direction, and thus the upward movement of a determinant may be transformed into a downward one. Every determinant is surrounded by several others, and we can imagine that the regular nutritive stream which we have assumed may be partially dammed up by a slight enlargement of the determinant, and that this will drive the surplus back again. But however we may picture these conditions, which are for all time outside of the sphere of observation, the assumption of a self-regulation of the germ-plasm, up to a certain degree, cannot be regarded as inconceivable or unphysiological. But there are limits to this self-regulation; as soon as the increase or decrease of a determinant attains a certain degree, as soon as it has got beyond the first slight deviation, it overcomes all obstacles, and goes on increasing in the direction in which it has started. This must happen even in the case of old and constant species, and frequently enough to admit of an apparent capacity for adaptation in all directions. Every part of a species can vary beyond the usual individual fluctuations, and as this is possible only by means of intra-germinal processes, we must assume that even in the case of germ-plasms which have long remained in a state of stable equilibrium there may occasionally be marked fluctuations in the nutritive stream, and thus more than usually pronounced variations of the determinants affected by it will occur. These yield the material for new adaptations if they are in the direction of fitness, or they are eliminated either by the chances of reducing division or by personal selection if new adaptations are not required.