HOW OLD IS THE EARTH? 163 



era, which then would comprise some one hundred thousand years. 

 Comparing the Tertiary era with the Quaternary, however, I can 

 not agree with Prof. Dana's estimate that the latter was a third 

 as long as the former, and am quite at a loss to discern evidences 

 justifying that view. The best means for learning their ratio I 

 think to be found in the changes of faunas and floras since the 

 beginning of the Tertiary era, using especially the marine mol- 

 luscan faunas as most valuable for this comparison. Scarcely 

 any species of marine mollusks have become extinct or undergone 

 important changes during the Glacial and recent periods; but 

 since the Eocene dawn of the Tertiary nearly all of these species 

 have come into existence. Judged upon this basis, the Tertiary 

 era seems probably fifty or a hundred times longer than the Ice 

 age and subsequent time ; in other words, it may well have lasted 

 two million or even four million years. Taking the mean of these 

 numbers, or three million years, for Cenozoic time, or the Quater- 

 nary and Tertiary ages together, we have precisely the value of 

 Prof. Dana's ratios which he himself assumes for conjectural 

 illustration, namely, forty-eight million years since the Cambrian 

 period began. But the diversified types of animal life in the 

 earliest Cambrian faunas surely imply a long antecedent time for 

 their development, on the assumption that the Creator worked be- 

 fore then as during the subsequent ages in the evolution of all 

 living creatures. According to these ratios, therefore, the time 

 needed for the deposition of the earth's stratified rocks and the 

 unfolding of its plant and animal life must be about a hundred 

 million years. 



Reviewing the several results independently reached through 

 the geologic estimates and ratios supplied by Wallace, Dana, and 

 Davis, we are much impressed and convinced of their approximate 

 truth by their somewhat good agreement among themselves, 

 which seems as close as the nature of the problem would lead us 

 to expect, and by their all coming within the limit of one hundred 

 million years which Sir William Thomson estimated on physical 

 grounds. This limit of probable geologic duration seems there- 

 fore fully worthy to take the place of the once almost unlimited 

 assumptions of geologists and writers on the evolution of life, that 

 the time at their disposal has been practically infinite. No other 

 more important conclusion in the natural sciences, directly and 

 indirectly modifying our conceptions in a thousand ways, has 

 been reached during this century. 



