That was the celebrated preparation 606, and it rejoiced in the name: “Dioxy-diamino-arsenobenzol-dihydro-chloride.” Its deadly effect on trypanosomes was as great as its name was long. At a swoop one shot of it cleaned those fell trypanosomes of the mal de Caderas out of the blood of a mouse--a wee bit of it cleaned them out without leaving a single one to carry news or tell the story. And it was safe! So safe--though it was heavily charged with arsenic, that pet poison of murderers. It never made mice blind, it never turned their blood to water, they never danced--it was safe! “Those were the days!” muttered old Kadereit, long after. Already in those days he was growing stiff, but how he stumped about taking care of the “Father.” “_Those_ were the days, when we discovered the 606!” And they were the days--for what more hectic days (always excepting the days of Pasteur) in the whole history of microbe hunting? 606 was safe, 606 would cure the mal de Caderas, which was nice for mice and the hindquarters of horses, but what next? Next was that Paul Ehrlich made a lucky stab, that came from reading a theory with no truth in it. First Paul Ehrlich read--it had happened in 1906--of the discovery by the German zoölogist, Schaudinn, of a thin pale spiral-shaped microbe that looked like a corkscrew without a handle. (It was a fine discovery and Fritz Schaudinn was a fantastic fellow, who drank and saw weird visions. I wish I could tell you more of him.) Schaudinn spied out this pale microbe looking like a corkscrew without a handle. He named it the _Spirocheta pallida_. He proved that this was the cause of the disease of the loathsome name. Of course Paul Ehrlich (who knew everything) read about that, but it particularly stuck in Ehrlich’s memory that Schaudinn had said: “This pale spirochete belongs to the animal kingdom, it is not like the bacteria. Indeed, it is closely related to the trypanosomes.... Spirochetes may sometimes turn into trypanosomes....” Now, it was hardly more than a guess of that romantic Schaudinn that spirochetes had anything to do with trypanosomes, but it set Paul Ehrlich aflame. “If the pale spirochete is a cousin of the trypanosome of the mal de Caderas--then 606 ought to hit that spirochete.... What kills trypanosomes should kill their cousins!” Paul Ehrlich was not bothered by the fact that there was no proof these two microbes were cousins.... Not he. So he marched towards his day of days. He gave vast orders. He smoked more strong cigars each day. Presently regiments of fine male rabbits trooped into the Georg Speyer House in Frankfort-on-the-Main, and with these creatures came a small and most diligent Japanese microbe hunter, S. Hata. This S. Hata was accurate. He was capable. He could stand the strain of doing the same experiment a dozen times over and he could, so nimble was this S. Hata, do a dozen experiments at the same time. So he suited the uses of Ehrlich, who was a thorough man, do not forget it!