- 1. We should be able to prove the invariable association of the parasite with the disease produced.
- 2. We should be able to remove the parasite and grow it outside the human body.
- 3. We should be able to re-introduce it into the human body or that of some lower animal and there reproduce a similar disease.
- 4. We should be again able to demonstrate the presence of the parasite.
With reference to the first, I shall show presently the invariable association of F. nocturna has not being clearly proved; indeed, its absence in the case of elephantiasis is cited as one of the proofs of its association with the disease, and this difficulty requires explanation.
The second condition has undoubtedly been fulfilled by the proof that the parasite grows outside the human body in the mosquito, and that the human host is reinfected by the medium of the proboscis. But, of course, it is not possible to experiment on the human being in the case of a parasite which may produce a dangerous disease, nor to introduce it into one of the lower animals, as a human parasite does not necessarily flourish in any of them, so that the production of the disease, experimentally, and the recognition of the parasite afterwards—the third and fourth conditions—are still wanting. We find, then, that up to the present we fail in several of the conditions laid down, and we are forced to endeavour to come to our conclusion by means of deductions from definitely ascertained facts in the human body and the possible behaviour and effect of the parasite.
And at the outset, in the case of filariasis, we are met by several difficulties. In the first place, it is not maintained, so far as I have been able to ascertain, that the presence of the embryo of F. nocturna is the pathological factor in the production of disease : if that were so we should also have
