In no class of cases would the transmission of an acquired character superficially appear so probable as in those where power of resisting the attack of a pathogenic organism is acquired in the lifetime of the zygote. The possession of such a power is moreover a distinction comparable with those which differentiate varieties and species. It is due to the development in the blood of specific substances which pervade the whole fluid. This development is exactly one of those "appropriate responses to stimuli" which naturalists who incline to regard adaptation as a direct consequence of an environmental influence might most readily invoke as an illustration of their views. And yet all evidence is definitely unfavourable to the suggestion of an inheritance of the acquired power of resistance. Such change as can be perceived in the virulence of the attacks on successive generations may be most easily regarded as due to the extermination of the more susceptible strains, and perhaps in some measure to variation in the invading organisms themselves, an "acquired character" of quite different import. The specific "anti-body" may have been produced in response to the stimulus of disease, but the power to produce it without this special stimulus is not included in the germ-cells any more than a pigment. All that they bear is the _power to produce_ the anti-bodies when the stimulus is applied. If we could conceive of an organism like one of those to which disease may be due becoming actually incorporated with the system of its host, so as to form a constituent of its germ-cells and to take part in the symmetry of their divisions, we should have something analogous to the case of a species which acquires a new factor and emits a dominant variety. When we see the phenomenon in this light we realise the obscurity of the problem. The appearance of recessive varieties is comparatively easy to understand. All that is implied is the omission of a constituent. How precisely the omission is effected we cannot suggest, but it is not very difficult to suppose that by some mechanical fault of cell-division a power may be lost. Such variation by unpacking, or analysis of a previously existing complex, though unaccountable, is not inconceivable. But whence come the new dominants? Whether we imagine that they are created by some rearrangement or other change internal to the organism, or whether we try to conceive them as due to the assumption of something from without we are confronted by equally hopeless difficulty.