Recurring Changes in the Universe. 159 



are bound to believe that some process of recurrence must 

 exist, whereby useful change and activity are continued in the 

 universe, and the purposeless end of a changeless chaos pre- 

 vented — and that we should seek for the explanation of this, 

 not so much with the view to prove the fact thereby, but 

 rather as a satisfaction or confirmation of a fact we already 

 had logical grounds for believing to exist. 



That recurring changes exist in the universe seems to have 

 been the conviction of Sir W. Grove. He remarks relative to 

 this subject (' Gorr. of Physical Forces,' page 67): — "En- 

 larged observation may prove that phenomena seeming to 

 tend in one direction will turn out to be recurrent, though 

 never absolutely identical in their recurrence ; that there is 

 throughout the universe gradual change, but no finality; .... 

 that no star or planet could at any time be said to be created 

 or destroyed, or to be in a state of absolute stability, but that 

 some may be increasing, others dwindling away, and so 

 throughout the universe, in the past as in the future." 



Humboldt also says, as regards this point (Preface to 

 'Cosmos - '): — "I would therefore venture to hope that an 

 attempt to delineate nature in all its vivid animation and ex- 

 alted grandeur, and to trace the stable amid the vacillating 

 ever-recurring alternation of physical metamorphoses, will not 

 be wholly disregarded at a future age." 



It has been pointed out by Dr. James Croll, in a paper 

 published in the Quarterly Journal of Science for July 1877, 

 that Helmholtz's gravitation theory of the origin of the sun's 

 heat is not' alone sufficient to account for a past duration of 

 the sun's heat in harmony with geological evidence as to the 

 ao-e of the earth. It is of course evident that all the o-eolo- 

 gical history of the earth must be comprised within the limit 

 of the age of the sun. In this paper elaborate geological evi- 

 dence is given tending to prove conclusively that as a limit, 

 the age of the earth must be very much (at least three times) 

 greater than the time the store of heat could have existed in 

 the sun, if the origin of this heat were solely gravitation. For 

 it is a known fact (based upon direct experimental evidence 

 of the loss of solar heat by radiation) that the store of heat, if 

 it had resulted from gravitation alone, would only have sufficed 

 for about 20,000,000 years. Dr. Croll remarks regarding 

 this point (page 317): — " We have not sufficient data to de- 

 termine how many years have 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 



Now in proving that the antiquity of our habitable globe must 



