might have had to be begun over again.  But near Chalons-on-the-Marne, in the year 451, in one of the most obstinate struggles of which history preserves the record, the career of the “Scourge of God” was arrested, and mainly by the prowess of Gauls and of Visigoths whom the genius of Rome had tamed.  That was the last day on which barbarism was able to contend with civilization on equal terms.  It was no doubt a critical day for all future history; and for its favourable issue we must largely thank the policy adopted by Caesar five centuries before.  By the end of the eighth century the great power of the Franks had become enlisted in behalf of law and order, and the Roman throne was occupied by a Frank,—­the ablest man who had appeared in the world since Caesar’s death; and one of the worthiest achievements of Charles the Great was the conquest and conversion of pagan Germany, which threw the frontier against barbarism eastward as far as the Oder, and made it so much the easier to defend Europe.  In the thirteenth century this frontier was permanently carried forward to the Vistula by the Teutonic Knights who, under commission from the emperor Frederick II., overcame the heathen Prussians and Lithuanians; and now it began to be shown how greatly the military strength of Europe had increased.  In this same century Batu, the grandson of Jinghis Khan, came down into Europe with a horde of more than a million Mongols, and tried to repeat the experiment of Attila.  Batu penetrated as far as Silesia, and won a great battle at Liegnitz in 1241, but in spite of his victory he had to desist from the task of conquering Europe.  Since the fifth century the physical power of the civilized world had grown immensely; and the impetus of this barbaric invasion was mainly spent upon Russia, the growth of which it succeeded in retarding for more than two centuries.  Finally since the sixteenth century we have seen the Russians, redeemed from their Mongolian oppressors, and rich in many of the elements of a vigorous national life,—­we have seen the Russians resume the aggressive in this conflict of ages, beginning to do for Central Asia in some sort what the Romans did for Europe.  The frontier against barbarism, which Caesar left at the Rhine, has been carried eastward to the Volga, and is now advancing even to the Oxus.  The question has sometimes been raised whether it would be possible for European civilization to be seriously threatened by any future invasion of barbarism or of some lower type of civilization.  By barbarism certainly not:  all the nomad strength of Mongolian Asia would throw itself in vain against the insuperable barrier constituted by Russia.  But I have heard it quite seriously suggested that if some future Attila or Jinghis were to wield as a unit the entire military strength of the four hundred millions of Chinese, possessed with some suddenly-conceived idea of conquering the world, even as Omar and Abderrahman wielded as a unit the newly-welded power of the Saracens