I began to suspect that he was trying over some phrases for a coming lecture; but it was early yet and I could not decently make an excuse for leaving him. I took a fresh cigar and let him go on without interruption. “It always seems strange to me how little the man in the street knows of the things around him. The microscopic world has no existence as far as his mind is concerned. A grain of dust is too small for him to notice; it must blow into his eye before he appreciates that it has perceptible size at all. And yet, all about him and within him there lives this wonderful race of beings, passing to and fro in his veins as we do in the streets and avenues of a great city; coming to birth, going about their concerns, falling ill and dying, just as men do in London at this hour. Think of the battles, the victories, and the defeats which take place minute by minute in the tiniest drop of our blood; and the issue of the war may be the life or death of one of us. They talk of the struggle for existence; but the real struggle for existence is going on within us and not in the outer world. Phagocyte against bacterium--that is where the fitness of an organism comes to its ultimate test. A slight hitch in the reinforcements, a minute’s delay in bringing numbers to bear, and the keystone is out of the edifice; nothing is left but a ruin. “It always reminds me of those frontier skirmishes--a mere handful of troops engaged on either side--upon the issue of which the fate of an empire may depend. Get a new set of enemies, some novel type of bacteria with fresh tactics which the phagocytes cannot cope with--and down comes a human being. It strikes wonder into me, that, you know. A human body is so colossal in comparison with these bacteria that they can have no idea even of our existence; and yet they can destroy the whole machinery upon which our life depends. It’s almost as if a few shots fired in Africa could crumble the whole Earth into an impalpable dust. “And it is not only within us that these struggles are going on. When you came in, I was just studying some specimens of organisms which are equally vital to us. Come over here to the microscope, Flint, and have a look at them yourself.”