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saults. There were some beings that moved sluggishly and looked like wee bent sticks, nothing more, but that Dutchman squinted at them till his eyes were red-rimmed—and they moved, they were alive, no doubt of it! There was a menagerie in his mouth! There were creatures shaped like flexible rods that went to and fro with the stately carriage of bishops in procession, there were spirals that whirled through the water like violently animated corkscrews. . . .

Everybody he could get hold of—as well as himself—was an experimental animal for that curious man. Tired from his long peering at the little beasts in his own mouth, he went for a walk under the tall trees that dropped their yellow leaves on the brown mirrors of the canals; it was hard work, this play of his, he must rest! But he met an old man, a most interesting old man: "I was talking to this old man," wrote Leeuwenhoek to the Royal Society, "an old man who led a very sober life, who never used brandy nor tobacco and very seldom wine, and my eye chanced to fall on his teeth which were badly grown over and that made me ask him when he had last cleaned his mouth. I got for answer that he had never cleaned his teeth in his whole life. . . ."

Away went all thought of his aching eyes. What a zoo of wee animals must be in this old fellow's mouth. He dragged the dirty but virtuous victim of his curiosity into his study—of course there were millions of wee beasties in that mouth, but what he wanted particularly to tell the Royal Society was this: that this old man's mouth was host to a new kind of creature, that slid along among the others, bending its body in graceful bows like a snake—the water in the narrow tube seemed to be alive with those little fellows!

You may wonder that Leeuwenhoek nowhere in any of those hundreds of letters makes any mention of the harm these mysterious new little animals might do to men. He had come upon them in drinking water, spied upon them in the mouth; as the years went by he discovered them in the intestines of frogs and horses, and even in his own discharges; in swarms