64 POPULAR LECTURES AND ADDRESSES. 



ture is put the better ; but the farther back the 

 time of the heating, the hotter it must have been. 

 The best for those who draw most largely on time 

 is that which puts it farthest back, and that is the 

 theory that the heating was enough to melt the 

 whole. But even if it was enough to melt the 

 whole, we must still admit some limit, such as fifty 

 million years, one hundred million years, or two or 

 three hundred million years ago. 1 Beyond that we 

 cannot go. The argument described ( 19) above 

 regarding the earth's rotation shows that the earth 

 has not gone on as at present for a thousand 

 million years. Dynamical theory of the sun's heat 

 renders it almost impossible that the earth's sur- 

 face has been illuminated by the sun many times 

 ten million years. And when finally we consider 

 underground temperature we find ourselves driven 

 to the conclusion in every way, that the existing 

 state of things on the earth, life on the earth, all 

 geological history showing continuity of life, must 

 be limited within some such period of past time as 

 one hundred million years. 



and Tail, Appendix 1). (r.) 



