be brought out in a manner to admit of no possible doubt Under such conditions mere inference is a somewhat uncertain prop to lean upon.
Now in order to get over the diflficulty of the parasite under ordinary conditions, which, it is evident, is in the minds of most observers who have written on the subject, we must assume that some change takes place in the parasite or in the individual, something must be added by means of which the parasite acquires pathogenic properties, and various theories have been brought forward, the principal of which I may mention:—
1. That some filarial diseases, such as abscess and lymphangitis are due to death of the parent worm.
2. That elephantiasis and chyluria are due to impaction of the parent worm in some part of the lymphatic system—in other words, that it is merely a mechanical obstruction. It has been further suggested that the impaction of the worm produces haemorrhage and consequent organisation of the clot, with narrowing of the lymph trunk.
3. This suggestion carries the great authority of Sir P. Manson, and has been put forward in a very convincing manner: that elephantiasis takes place as the result of obstruction of the lymphatics from the impaction in the afferent lymph vessels of a gland, of the immature ova from an aborting female. But in addition to this lymphangitis must occur. Let us briefly consider these theories seriatim:
1. That certain filarial diseases—abscess and lymphangitis —are due to the death of the parent worm. Now, of course, the presence of a foreign substance in the human body will not necessarily produce inflammatory action, it must become septic, and we must at once admit the possibility of an adult worm becoming affected with a streptococcus and thus produce an abscess and secondary lymphangitis, though the streptococcus might produce it without
