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 he found them on those rare occasions when, as he says, “he was troubled with a looseness.” But not for a moment did he guess that his trouble was caused by those little beasts, and from his unimaginativeness and his carefulness not to jump to conclusions modern microbe hunters--if they only had time to study his writings--could learn a great deal. For, during the last fifty years, literally thousands of microbes have been described as the authors of hundreds of diseases, when, in the majority of cases those germs have only been chance residents in the body at the time it became diseased. Leeuwenhoek was cautious about calling anything the _cause_ of anything else. He had a sound instinct about the infinite complicatedness of everything--that told him the danger of trying to pick out one cause from the tangled maze of causes which control life.... The years went by. He tended his little dry-goods store, he saw to it the city hall of Delft was properly swept out, he grew more and more crusty and suspicious, he looked longer and longer hours through his hundreds of microscopes, he made a hundred amazing discoveries. In the tail of a little fish stuck head first into a glass tube he saw for the first time of all men the capillary blood vessels through which blood goes from the arteries to the veins--so he completed the Englishman Harvey’s discovery of the circulation of the blood. The most sacred and improper and romantic things in life were only material for the probing, tireless eyes of his lenses. Leeuwenhoek discovered the human sperm, and the cold-blooded science of his searching would have been shocking, if he had not been such a completely innocent man! The years went by and all Europe knew about him. Peter the Great of Russia came to pay his respects to him, and the Queen of England journeyed to Delft only to look at the wonders to be seen through the lenses of his microscopes. He exploded countless superstitions for the Royal Society, and aside from Isaac Newton and Robert Boyle he was the most famous of their members. But did these honors turn his head? They couldn’t turn his head because he had from the first a sufficiently high opinion of himself! His arrogance was limitless--but it was equaled by his humility when he thought of that misty unknown that he knew surrounded himself and all men. He admired the Dutch God but his real god was truth: