Courses in germ sociology are therefore of prime necessity. How do germs act? On what do they live? Why do they move from place to place? What causes them to become extinct? With few exceptions, germs migrate for the same reason as man,--search for food, love of conquest, and love of adventure. When there is plenty of food they multiply rapidly. Full of life, overflowing with vitality, they move out for new worlds to conquer. Like human beings, they will do their best to get away from a country that provides a scanty food supply. Like men and women, they starve if they cannot eat. Like boys and girls, they avoid enemies; the weak give way to the strong, the slow to the swift, the devitalized to the vitalized. Human sociology imprisons, puts to death, deprives of opportunity to do evil, or reforms those who murder, steal, or slander. Germ sociology teaches us to do the same with injurious germs. We imprison them, we take away their food supply, we kill them outright, or we starve them slowly. They have a peculiar diet, being especially partial to decomposing vegetable and animal matter and to what human beings call dirt. By putting this diet out of their reach we make it impossible for them to propagate their kind. By placing poison within their reach or by forcing it upon them we can successfully eliminate them as enemies. As the president of Mexico restored order "by setting a thief to catch a thief," so modern science is setting germs to kill germs that harm crops and human stock. Of utmost consequence is it that the body's germ consumer--its pretorian guard--be always armed with vitality ready to vanquish every intruding hostile germ. If we are false to our guard, it will turn traitor and join invaders in attacking us. But here, as in dealing with evils that originate with human beings, an ounce of prevention is worth a ton of cure. The most effectual way to eliminate germ diseases is to remove the cause--the food supply of disease germs. The fact that many germs are plants, not animals, does not weaken the analogy, for weeds do not get a chance in well-tilled soil.