acquired traits in any true sense of the term. They are, and for ever remain, foreign elements, just as a rifle bullet that finds its way into the tissues is, and for ever remains, a foreign element. The secondary infection of a germ or of an embryo through the parent, therefore, no more implies the transmission of an acquired trait than the passage of a rifle bullet through the body of the parent into the embryo implies it. If, however, the effect produced by the passage of the bullet on the parent's body were afterwards reproduced in the unwounded developing body of the child, we should then have a true example of the transmission of an acquired trait; so also, if the effects produced on the tissues of the parent by the microbes and toxins of a zymotic disease were reproduced in the non-infected developing body of the child, we should have another example of the transmission of an acquired trait. But this certainly never happens: no non-tubercular child of a parent who has phthisical cavities in his lungs ever reproduces those cavities in the process of development; never are the pock-marks in the pitted skin of one who has suffered from small-pox reproduced during development by the non-infected child.
Following a line of argument somewhat different from that commonly used, and writing at a time when the microbic origin of many diseases was unknown, Mr. Herbert Spencer said—
