280 DISCUSSIONS IN COSMOLOGY. 



elapsed since life began on the globe, for we do not 

 know the total amount of rock removed by denudation ; 

 but we have data perfectly sufficient to show that it 

 began far more than twice 20 million years ago. 



But if the present order of things has been existing 

 for more than 20 million years, then the sun must have 

 been illuminating our globe for that period, and, if so, 

 then there must have been some other source than that 

 of gravitation from which the sun derived its energy, 

 for gravitation, as we have seen, could only have sup- 

 plied the present rate of radiation for about one-half 

 that period. 



It is perfectly true, as has been stated, that the 

 length of time that the sun could, by its radiation, have 

 kept the earth in a state fit for animal and vegetable 

 life, must have been limited by the store of energy in 

 the form of heat which it possessed. But it does not 

 follow as a necessary consequence, as is generally sup- 

 posed, that this store of energy must have been limited 

 to the amount obtained from gravity in the condensa- 

 tion of the sun's mass. The utmost that any physicist 

 is warranted in affirming is simply that it is impossible 

 for him to conceive of any other source. His inability } 

 however, to conceive of another source cannot be 

 accepted as a proof that there is no other source. But 

 the physical argument that the age of our earth must 

 be limited by the amount of heat which could have 

 been received from gravity is in reality based upon 

 this assumption — that, because no other source can be 

 conceived of, there is no other source. 



It is perfectly obvious, then, that this mere negative 

 evidence against the possibility of the age of our 

 habitable globe being more than 20 or 30 million years 

 is of no weight whatever when pitted against the 

 positive evidence here advanced, that its age must be 

 far greater. 



