Lastly, there remains the only theory on which we have positive evidence. It is the theory that the bacilli contained in the fæces of the flea are deposited on the skin and then find their way into the wound made by the piercing organ. They may be helped in this by the rubbing and scratching which follow on the bite of the flea. We know, of course, that plague bacilli are present in abundance in the fæces of fleas taken from plague-sick rats, and that such fæces are infective to guinea-pigs both by cutaneous and by subcutaneous inoculation. Experiments were made to discover whether the pricks made by fleas were of sufficient size to allow plague bacilli to enter the body, no other damage to the skin being done. Healthy fleas, confined in a test-tube, were allowed to feed on a small part of a guinea-pig’s abdomen, the hair of which had been cropped close without injuring the skin Immediately afterwards a few drops of the septicæmic blood of a rat which had died of plague, or of a virulent culture of plague bacillus, were lightly spread over the part. Many successful infections were obtained in this way. Similar experiments were made in which the plague culture was first spread on the skin, and, afterwards, healthy fleas were allowed to feed on the same spot. Successful infections were also obtained by this means. Two facts then seem to be demonstrated beyond doubt: first, that the puncture made by a flea will allow the bacillus to gain access to an animal’s body and to infect it; secondly, that there is a possibility of infection by the fæces of fleas. As to whether this is the usual process the highest authorities are not ready to express any opinion. The safest course appears to be to kill fleas but to avoid rubbing them in. Good work was done during the recent outbreak of plague in San Francisco when the energies of an army of men were directed to controlling and destroying the rat population. Enormous numbers of rats were killed, their breeding places were destroyed and everything was made as uncomfortable for them as possible. Men of science were at the same time engaged in collecting and examining the fleas from many thousands of rats. The great success of the work confirmed the soundness of the theory on which it was based. The spread of the most terrible of epidemic diseases was controlled and prevented by knowledge. At San Francisco the fleas of man, rats, mice, dogs, cats, ground-squirrels and gophers were studied. It was found there, as elsewhere, that while each species of flea has its particular host few are unwilling or unable to attack man and other animals when the host dies. There is good reason to believe that during the last outbreak of plague in Manchuria the fleas carried the bacillus from the marmots (_Arctomys_) to man.