to admit that the other embryonic forms might also produce disease. The embryo must, therefore, be regarded as harmless; the adult worm is the pathogenic agent.
Now, the first difficulty we have to get over is the fact, which is generally admitted, that in the great majority of instances the adult worm appears also to be harmless. Sir P. Manson states:
"There is nothing in the life-history of F. sanguinis hominis and its relation to the human host incompatible with the perfect health of the latter. The amount of injury done by the immature parasite in its travels towards its permanent abode is so trifling that no serious disease can possibly result from it. The mature animal itself lies extended in a vessel, and is perfectly adapted by its size and shape for the situation which it occupies; it creates no irritation, and the small amount of obstruction which it may give rise to is readily compensated for by a rich anastomosis. The embryos move along with the lymph, and, being no broader than the corpuscles, readily pass the glands and enter the general circulation. Hence they give rise to no trouble but circulate as easily as the blood corpuscles. In fact, the parasite seems in every respect well calculated to live in perfect harmony with its host and not at all likely to be the cause of serious injury or disease."
Primrose, in an interesting article, states "that the victims of filarial infections do not necessarily suffer inconvenience from the presence of the worm or its embryos. In fact, it would appear to be the exception for pathological lesions to manifest themselves in persons thus infected." My own experience, as the result of many hundred examinations, coincides with this, and it is unnecessary to labour this point—we may take it as undisputed.
Now if we have a disease, the alleged causal agent of which is in the great majority of instances harmless, if the presence of the F. bancrofti in the lymphatics and the embryos in the blood is not necessarily productive of disease—if, in addition to this, we find exactly similar diseases with the same clinical phenomena occurring in non-tropical countries where filariasis can be excluded with certainty, it is evident that, to carry conviction, the actual association of the parasite with the given disease must be of the clearest and most definite description, and must
