Evolution coincided with another important development.

History shows that all great periods of civilization have at their back sources of energy.

In the civilizations of the past such sources of energy have come from the enslavement of conquered peoples or from commerce, or more direct forms of robbery, which have enabled a favored class to appropriate for its purposes the results of the work of others.

While these sources have not been absent in the development of our civilization, the great source of energy has come from the rapid, and usually wasteful and reckless, utilization of the stored energy of the earth.
The almost incredible advance in medical and other forms of scientific knowledge and the utilization of this knowledge is largely due to the greater forces which we have become possessed of.
Disease plays such a large part in the life of man and is so closely related to all of his activities that the changes in this period must have exerted an influence on disease.

We have already seen that within the period we have obtained knowledge of the causes of disease and the conditions under which these causes became operative.