History is the biography of the mind of man, and its educational value is in direct proportion to the completeness of our study of the individuals through whom this mind has been manifested. I have tried to take you back to the beginnings of science, and to trace its gradual development, which is conditioned by three laws. In the first place, like a living organism, truth grows, and its gradual evolution may be traced from the tiny germ to the mature product. Never springing, Minerva-like, to full stature at once, truth may suffer all the hazards incident to generation and gestation. Much of history is a record of the mishaps of truths which have struggled to the birth, only to die or else to wither in premature decay. Or the germ may be dormant for centuries, awaiting the fullness of time. Secondly, all scientific truth is conditioned by the state of knowledge at the time of its announcement. Thus, at the beginning of the seventeenth century, the science of optics and mechanical appliances had not made possible (so far as the human mind was concerned) the existence of blood capillaries and blood corpuscles. 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.