It is plain that the grosser parasites of the human body, tapeworms and the like, could not be explained or understood without the help of feeding-experiments on animals. By this method, and by this alone, their life-history was discovered. They were known to Aristotle and to Hippocrates; but nothing was understood about them. They were never studied, for this among other reasons, that men believed in spontaneous generation; and the presence of lower forms of life inside human bodies was attributed to the fault of the patient, or the work of the devil. Then, at last, Redi (1712), and Swammerdam (1752) in his _Bibel der Natur_, struck at the doctrine of spontaneous generation, saying that it did not apply to insects; and in 1781 Pallas boldly declared that the internal parasites of man came out of eggs, like insects, and not "of themselves." It would be a good theme for an essay--_The paralysing effect, on medicine and surgery, of the doctrine of spontaneous generation_. Rudolphi (1808) and Bremser (1819) opposed Pallas; and von Siebold (1835) and Eschricht (1837) supported him. Then came the great students of this part of biology--Cobbold, Busk, Davaine, van Beneden, Leuckart, Küchenmeister. In 1842, Steenstrup had discovered, in certain insects, the alternation of generations; in 1852, Küchenmeister proved that the generations of internal parasites are similarly alternate. By feeding carnivorous animals with "measly" meat, he produced tapeworms in them; and by feeding herbivorous animals with the ova of tapeworms, he made their muscles "measly." The feeding of animals was the only possible way to understand the bewildering transformations and transmigrations of the thirty or more entozoa to which flesh is heir. This chapter of pathology makes up in tragedy what it lacks in romance; for these animal parasites have killed whole hosts of people. Take, for instance, the _trichina spiralis_, a minute worm discovered in 1835 encysted in countless numbers in the muscles of the human body; it was studied by Virchow, Leuckart, and others, by feeding-experiments on animals, and was proved to come from infected half-cooked ham and pork, and to make its way from the alimentary canal all over the body. The name of trichiniasis or trichina-fever was given to the acute illness that came of the sudden dissemination of these myriad parasites into the tissues. Trichiniasis had killed hundreds of people by a most painful death; outbreaks of it, in Germany and elsewhere, had swept through villages like cholera or plague: then Leuckart and Virchow traced it to its source, and it was stopped there--_Above all things, we must shut off the sources of the infection_--the butchers' shops were kept under sanitary inspection, people were warned against half-cooked ham and pork, and there was an end of it.