ORIGIN AND DIFFERENTIATION 541 



for any such conclusion. As pointed out several times before, it must 

 have followed that if the sun had dominated earth atmosphere through- 

 out measurable geologic time to the extent it now does, temperatures 

 would have been distributed by latitude ; hence the solution hinges on the 

 correctness of the biologic interpretation of a non-zonal arrangement 

 prior to the Pleistocene. If this has been shown from trustworthy evi- 

 dence admitting of no other conclusion — as indeed I believe it has — then, 

 as Abbot has stated in a recent letter to me, ^^one must follow with the 

 conclusion that the earth^s heat supply in those times did not come prin- 

 cipally (as now) from a source subtending a small solid angle." 



It now follows that several solutions may be suggested. These have 

 been formulated b}^ Abbot as follows : 



"1. The sun was shut off by clouds and the heat came from within the earth. 



"(a) Internal heat persisted. 



"{!)) Radio-active heat was available. 

 "2. The sun was effectively bigger. 



"(a) Sun then an extensive nebula. 



"(&) Sun's rays reflected by extensive nebula." 



Heat from a bigger sun. — The second of these suggested solutions, 

 namely, that which postulates an effectively larger sun, has had a number 

 of advocates. For instance, it is given quite favorable consideration by 

 Dr. Abbot in his well-known book on "The Sun,'^ but he writes me that 

 he does not now think so well of it as he formerly did. 



The present sun is about 866,000 miles in diameter. It is giving up 

 energy at a tremendous rate, which it is possible to measure with great 

 accuracy. The manner in which the sun^s heat is maintained is not cer- 

 tainly known, but the hypothesis that finds widest acceptance, that of 

 Helmholtz, assumes it to be due to the gravitational infall of matter 

 at its outer rim, by the transfer of motion into heat. It has been cal- 

 culated that an infall of this kind of 250 feet per annum would be suffi- 

 cient to maintain the present loss of energy. This, of course, implies 

 that the sun must have had a definite beginning, just as it shows it to be 

 tending toward an unmistakable end; in fact, Newcomb has calculated 

 that at the present rate of loss of energy the sun, in seven million years, 

 will be reduced to one-half its present size. Astronomers are not agreed 

 as to the probable age of the sun, though, so far as I can learn, few of 

 them incline to regard it as more than one hundred million years old, 

 and many have hesitated to concede more than fifty million years to it;^ 

 existence. This is totally at variance with the demands recently made as 

 to the age of the earth, for it is impossible to conceive of the earth as 



