Individual == egg + non-parental environment; but
for most mammals, including man—
Individual == egg + intra-maternal environment + non-parental environment.
This condition in mammals introduces a complicating factor which is likely to obscure the whole issue unless we bear it constantly in mind. In other words, we must discriminate sharply, in the discussion of inheritance in man, for instance, between two classes of influences which may exist in the infant at birth, that is, which are congenital; namely, those which were truly inherent—were in the germ-cells—at the very inception of the young individual, and (2) those which might later have been derived from either parent by the yet unborn offspring. The latter are not regarded as truly hereditary. Since certain diseases or their effects belong here we occasionally find a physician using the term inheritance for such prenatal influences, but the more careful ones now employ the term transmission to discriminate between such conditions and true inheritance. In its biological usage inheritance always refers to germinal constitution and never to any condition that may be thrust on a developing organism before birth. It is clear, then, that congenital conditions are not all necessarily cases of inheritance.
Three Fundamental Questions.—To get at the question of the inheritance of body modifications with the least confusion, let us examine it in the form of three fundamental questions, as follows:
1. Can external influences directly affect the germ-cells?
2. Can external influences, operating through the intermediation of the parental body, affect the germ-cells? If so, is the effect a specific and a permanent one which persists in succeeding generations independently of external influences similar to those which originally produced it? Only such a condition as this would rank as the inheritance of a somatic modification.
3. Can the appearance of new characters be explained on any other ground, or on any more inclusive basis, than through the transmission of somatic acquirements, or do organisms possess heritable characters which are inexplicable as inheritance of such modifications?
Obviously the only way the question can be settled is through careful experimentation in which all possible sources of error have been foreseen and guarded against. Much experimental work has been undertaken for the solution of this problem as the goal and we may therefore select typical ones of these experiments and apply the results toward answering our three questions.
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.