Jenner could not have added to his "Inquiry" a study on immunity; Sir William Perkin and the chemists made Koch technique possible; Pasteur gave the conditions that produced Lister; Davy and others furnished the preliminaries necessary for anaesthesia.

Everywhere we find this filiation, one event following the other in orderly sequence--"Mind begets mind," as Harvey (De Generatione) says; "opinion is the source of opinion.

Democritus with his atoms, and Eudoxus with his chief good which he placed in pleasure, impregnated Epicurus; the four elements of Empedocles, Aristotle; the doctrines of the ancient Thebans, Pythagoras and Plato; geometry, Euclid."(2) (2) Works of William Harvey, translated by Robert Willis, London, 1847, p.

532.
And, thirdly, to scientific truth alone may the homo mensura principle be applied, since of all mental treasures of the race it alone compels general acquiescence.

That this general acquiescence, this aspect of certainty, is not reached per saltum, but is of slow, often of difficult growth,--marked by failures and frailties, but crowned at last with an acceptance accorded to no other product of mental activity,--is illustrated by every important discovery from Copernicus to Darwin.
The difficulty is to get men to the thinking level which compels the application of scientific truths.