192 ON THE INTEEACTION OF NATURAL FORCES. 



appear to us, what is it in comparison with the time 

 during which the earth carried successive series of rank 

 plants and mighty animals, and no men ; during which in 

 our neighbourhood the amber- tree bloomed, and dropped 

 its costly gum on the earth and in the sea ; when in Sibe- 

 ria, Europe, and North America groves of tropical palms 

 flourished ; where gigantic lizards, and after them ele- 

 phants, whose mighty remains we still find buried in 

 the earth, found a home? Different geologists, pro- 

 ceeding from different premises, have sought to esti- 

 mate the duration of the above-named creative period, 

 and vary from a million to nine million years. The 

 time during which the earth generated organic beings 

 is again small when compared with the ages during 

 which the 'world was a ball of fused rocks. For the 

 duration of its cooling from 2,000° to 200° Centigrade 

 the experiments of Bishop upon basalt show that about 

 350 millions of years would be necessary. And with re- 

 gard to the time during which the first nebulous mass 

 condensed into our planetary system, our most daring 

 conjectures must cease. The history of man, therefore, 

 is but a short ripple in the ocean of time. For a much 

 longer series of years than that during which he has 

 already occupied this world, the existence of the present 

 state of inorganic nature favourable to the duration of 

 man seems to be secured, so that for ourselves and for 

 long generations after us we have nothing to fear. But 

 tlie same forces of air and water, and of the volcanic 

 interior, which produced former geological revolutions, 

 and buried one series of living forms after another, act 

 still upon the earth's crust. They more probably will 

 bring about the last day of the human race than those 

 distant cosmical alterations of which we have spoken, 

 forcing us perhaps to make way for new and more com- 

 plete living forms, as the lizards and the mammoth 



