328 THE WEALDEN VALLEY. 



This may seem a low rate, but we must bear in mind that 

 along any line of coast there are comparatively few points 

 which are suffering at one time, and that even on these when 

 a fall of cliff has taken place, the fragments serve as a pro- 

 tection to the coast until they have been gradually removed 

 by the waves. The Wealden Valley is twenty-two miles in 

 breadth, and on these data it has been calculated that the 

 denudation of the Weald must have required more than 

 150,000,000 of years. 



There can be no doubt about the interest of these calcula- 

 tions, and they have also the great merit of giving definition 

 to our ideas. We must not, however, attribute to them a 

 value which has been distinctly disclaimed even by their 

 authors. " Dans tous les cas," says M. Morlot, " il doit etre 

 bien entendu que Tauteur n' expose le present calcul que 

 comme une premiere imparfaite et hasardeuse tentative, sans 

 valeur absolue en elle-meme, tant qu'elle n'aura pas ete 

 verifiee au moyen d'autres essais du meme genre." More- 

 over, we must remember that these estimates are brought 

 forward not as a proof, but as a measure, of antiquity. Our 

 belief in the antiquity of man rests not on any isolated calcu- 

 lations, but on the changes which have taken place since his 

 appearance ; changes in the geography, in the fauna, and in 

 the climate of Europe. Valleys have been deepened, widened, 

 and partially filled up again; caves through which sub- 

 terranean rivers once ran are now left dry ; even the con- 

 figuration of land has been materially altered, and Africa 

 finally separated from Europe. 



Our climate has greatly changed for the better, and with 

 it the fauna has materially altered. In some cases, ' for in- 

 stance, in that of the hippopotamus and of the African ele- 

 phant, we may probably look to the diminution of food and 

 the presence of man as the main causes of their disappearance ; 

 the extinction of the mammoth, the Elephas antiquus, and the 



