Here and there, in the history of human thought and action, we find periods to which the epithet ‘great’ may properly be applied—in Greece the fourth century before Christ; in England the Elizabethan age; in the domain of science the seventeenth century, the ‘century of genius’, to which we now come.

It would be very undiscerning to suppose that such a period of greatness could arrive as a mere accident, a specially brilliant galaxy of exceptional minds just happening to be born at one particular epoch. Mental ability is believed to be transmitted in accordance with the laws of heredity, in which case the laws of probability will see to it that no abrupt jump occurs from one generation to the next. Thus a period of greatness must be attributed to environment rather than to accident; if an age shows one particular form of greatness, external conditions must have encouraged that form. For instance, the sixteenth century was an age of great explorers because conditions then specially favoured exploration; the pioneering voyages of Columbus, Vasco da Gama, Cabot, Magellan and others had drawn attention to the wealth of new territory awaiting discovery, while men had learned to build ships which could defy the worst fury of the ocean.