Looking next to the extinction of animals, we find that the species found in the oldest deposits containing human remains are in part still extant. Others which are locally extinct we know existed in Europe until historical times, that is, within the last two thousand years. How long previously to this the others became extinct we have no certain means of knowing, though it seems probable that they disappeared gradually and successively. We have, however, farther to bear in mind the possibility of cataclysms or climatal changes which may have proved speedily fatal to many species over large areas. In any case we have this certain fact that, though the time elapsed has been sufficient for the extinction of many species, it does not seem to have sufficed to effect any noteworthy change on those that survived. Farther, we may consider that time is only one factor in this matter, and not the one which is the efficient cause of change, since we know no reason why one species of animal should not continue to be reproduced as long as another, but for the occurrence of physical changes of a prejudicial character. We have still remaining the changes which have taken place in the erosion of valleys since the caverns were occupied. Dupont informs us that the openings of some of the caverns once flooded by rivers are now in limestone cliffs two hundred feet above the water, while no appreciable lowering of the bottoms of the ravines is taking place now. This would in some contingencies put back the period of filling of the caves to an indefinite antiquity. But then the questions occur--Was there once more water in the rivers or more obstruction at their outlets, or was the erosive power greater at one time than now, or were the river valleys excavated in still more ancient time, and partly filled with mud when the water entered the caves, and may this mud have been since swept away? So, in like manner, the waters flowing in the channels near Brixham Cave and Kent's Hole were apparently about seventy feet higher in times of flood than at present, but the time involved is subject to the same doubts as in the case of the Belgian caves. Hughes has well remarked that elevations of the land, by causing rivers to form waterfalls and cascades, which they cut back, may greatly accelerate the rate of erosion. Farther, there is the best reason to believe that in the glacial period many old valleys were filled with clay, and that the modern cutting consisted merely in the removal of this clay. Belt has shown in a recent paper[139] good reason to believe that this is the case with the Falls of Niagara, and that the cutting actually effected through rock within the later Pleistocene and modern period has been that only of the new gorge from the whirlpool to Queenstown, the main part of the ravine being of older date and merely re-excavated. This would greatly reduce the ordinary estimate of time based on the cutting of the Niagara gorge.