For years we were in doubt as to the cause of diphtheria. Half a dozen different theories were advanced, bad sewerage, foul air, overcrowding; but it was not until shortly after the Columbus-like discovery, by Robert Koch, of the new continent of bacteriology, that the germ which caused it was arrested, tried, and found guilty, and our real knowledge of and control over the disease began. This was in 1883, when the bacteriologist Klebs discovered the organism, followed a few months later (in 1884) by Loeffler, who made valuable additions to our knowledge of it; so that it has ever since been known as the Klebs-Loeffler bacillus. This put us upon solid ground, and our progress was both sure and rapid: in ten years our knowledge of the causation, the method of spread, the mode of assault upon the body-fortress, and last, but not least, the cure, stood out clear cut as a die, a model and a prophecy of what may be hoped for in most other contagious diseases. Great as is the credit to which bacteriologists are entitled for this splendid piece of scientific progress, there was another co-laborer, a silent partner, with them in all this triumph, an unsung hero and martyr of science who deserves his meed of praise--the tiny guinea-pig. He well deserves his niche in the temple of fame; and as other races and ages have worshiped the elephant, the snake, and the sacred cow, so this age should erect its temples to the guinea-pig. From one of the most trifling and unimportant,--kept merely as a pet and curiosity by the small boys of all ages,--he has become, after the horse, the cow, the pig, and the sheep, easily our most useful and important domestic animal. It may be urged that he deserves no credit, since his sacrifice--though of inestimable value--was entirely involuntary on his own part; but this should only make us the more deeply bound to acknowledge our obligation to him. By a stern necessity of fate, which no one regrets more keenly than the laboratory workers themselves, the guinea-pig has had to be used as a stepping-stone for every inch of this progress. Upon it were conducted every one of the experiments whose results widened our knowledge, until we found that this bacillus and no other would cause diphtheria; that instead of getting, like many other disease-germs, into the blood, it chiefly limited itself to growing and multiplying upon a comparatively small patch of the body-surface, most commonly of the throat; that most of its serious and fatal results upon the body were produced, not by the entrance of the germs themselves into the blood, but by the absorption of the toxins or poisons produced by them on the moist surface of the throat, just as the yeast plant will produce alcohol in grape juice or sweet cider.