Unfavourable conditions merely constitute the battlefield, the microbe is the bullet that wounds. " It is not the disease tuberculosis that comes into the world with certain individuals, or with successive children of the same family, but the aptitude to contract it should external conditions favour."—Prudden. The consumptive diathesis, if by that is understood comparatively feeble powers of resisting the tubercle bacilli, may be either of the inborn kind or of the acquired kind, or it may be a compound of both. That only the inborn kind, or only that part which is inborn of a diathesis compounded of both kinds, is transmissible, is decisively proved by the fact already dwelt on at length, that races that have long and severely been afflicted by tuberculosis are more resistant to it than races which have not been so afflicted.
More than any other disease syphilis bas furnished arguments to the supporters of the Lamarkian theory. The microbe producing it is apparently so minute, and of such little virulence, that alone of all pathogenic organisms it is able to take up its abode in or on the spermatozoon without destroying it, and so to accompany the latter in its long and devious journey to the ovum; for thus only is explainable the well-known fact that a healthy mother may bear a syphilitic child to an infected father. On the other hand, parents, both of whom are infected, may have for offspring a healthy child, as in the case mentioned by Mr. Jonathan Hutchinson (Syphilis, p. 412), when of twins born to parents both of whom were suffering from the disease in a virulent stage, one twin was diseased and the other healthy. Here, as regards the healthy child, not only cannot the spermatozoon have received and conveyed infection, but the ovum as well as the developing embryo must have escaped it, a combination of events which must necessarily be rare. The limiting membranes of the ovum
