"When the human soul has lost its strength through laziness, incredulity, and the gangrenous vices that follow an excess of civilization, it can be retempered only in blood. Certainly there is no easy explanation of why war produces different effects in different circumstances. But it can be seen clearly enough that mankind may be considered as a tree which an invisible hand is continually pruning and which often profits from the operation. In truth the tree may perish if the trunk is cut of the tree is overpruned; but who knows the limits of the human tree? What we do know is that excessive carnage is often allied with excessive population, as was seen especially in the ancient Greek republics and in Spain under the Arab domination."