In the second paragraph, primitive medicine is first described and then modern medicine is described by contrast. One of the masses is built of cut stone, the other of poured concrete. One took 50,000 men twenty years to build, the other will take 5,000 men six years, in a task not only three times greater but vastly more complex and dangerous. Both structures relied on the labor of those who would otherwise have been unemployed. Pyramids were houses for the dead.
Dams are centers of energy for the living.
It is better, I think, to live in the age of the Great Dams than in the age of the Great Pyramids. They are in complete opposition to one another. The former involves the belief that disease is caused by supernatural forces. He recognizes that knowledge is the sole means of preventing it. The measures he relies upon both to prevent and cure disease are those which have resulted from scientific investigation and which have been proved to be effective by experience.