ON THE INTERACTION OF NATURAL FORCES. 171 



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

 for long generations after us we have nothing to fear. But the 

 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 complete 

 living forms, as the lizards and the mammoth have given place 

 to us and our fellow-creatures which now exist. 



Thus the thread which was spun in darkness by those who 

 sought a perpetual motion has conducted us to a universal law 

 of nature, which radiates light into the distant nights of the 

 beginning and of the end of the history of the universe. To 

 our own race it permits a long but not an endless existence ; it 

 threatens it with a day of judgment, the dawn of which is still 

 happily obscured. As each of us singly must endure the 

 thought of his death, the race must endure the same. But 

 above the forms of life gone by, the human race has higher 

 moral problems before it, the bearer of which it is, and in the 

 completion of which it fulfils its destiny. 



