We shall find here, as we find nearly always, that the truth lies somewhere between the extremes of two warring schools. Our race has been existing for a long time in a certain environment, and its very existence implies superiority to that environment. The weaklings, for whom its hardships were too severe, were weeded out; hostile parasites invaded their blood-stream and conquered and devoured them. But those who survived were able to make in their blood-stream the substances known as anti-bodies, the "opsonins," to help the white blood corpuscles devour the germs. As the result of their victory, we carry those anti-bodies in our system, which gives us immunity to those particular diseases, or at any rate gives us the ability to have the diseases without dying. Every time we go into a street car, we take into our throat and lungs the germs of tuberculosis. Examination proves that we carry around with us in our mouths the germs of all the common throat and nose diseases, colds, bronchitis, tonsilitis. No matter what precautions we might take, no matter if we were to gargle our throats every few minutes, we could never get rid of such germs. And they wage continual war upon the body's defenses; they batter in vain upon the gates of our sound health. But take us to some new environment to which we are not accustomed; take us to Panama in the old days of yellow fever, or take us to Africa, and let the tsetse fly bite us, and infect us with "sleeping sickness." Here are germs to which our systems are not accustomed; and before them we are as helpless as the ancient knights-at-arms, who had conquered everything in sight, and ruled the continent of Europe for many hundreds of years, but were wiped off the earth by a chemist mixing gunpowder.