The Stoics and the Christians extended its application to the whole of living humanity.

But in late years the rule has received a vastly greater extension by the inclusion of the unborn generations of the future.

This principle of duty to posterity is a direct corollary of the idea of Progress.

In the recent war that idea, involving the moral obligation of making sacrifices for the sake of future ages, was constantly appealed to; just as in the Crusades, the most characteristic wars of our medieval ancestors, the idea of human destinies then in the ascendant lured thousands to hardship and death.
The present attempt to trace the genesis and growth of the idea in broad outline is a purely historical inquiry, and any discussion of the great issue which is involved lies outside its modest scope.

Occasional criticisms on particular forms which the creed of Progress assumed, or on arguments which were used to support it, are not intended as a judgment on its general validity.