304 MAN AND THE GLACIAL PERIOD. 



during the Tertiary period, and continued up to the he- 

 ginning of the Glacial epoch, was produced hy the influ- 

 ence of the warmer stretches of space through which the 

 whole solar system was moving at that time ; while the 

 Glacial period resulted from the influence upon the earth 

 of the colder spaces through which the system subsequently 

 moved. 



While it is impossible absolutely to disprove this 

 hypothesis, it labors under the difficulty of having little 

 positive evidence in its favor, and thus contravenes a 

 fundamental law of scientific reasoning, that we must have 

 a real cause upon which to rest our theories. In endeav- 

 ouring to explain the unknown, we should have something 

 known to start with. But in this case we are not sure 

 that there are any such variations in the temperature of 

 the space through which the solar system moves. This 

 theory, therefore, cannot come in for serious considera- 

 tion until all others have been absolutely disproved. As 

 we shall also more fully see, in the subsequent discussion, 

 the distribution of the ice during the Glacial period was 

 not such as to indicate a gradual extension of it from the 

 north pole, but rather the accumulation upon centres many 

 degrees to the south. 



Closely allied with the preceding theory is the sup- 

 position broached by some astronomers that the sun is a 

 variable star, dependent to some extent for its heat upon 

 the impact of meteorites, or to the varying rapidity with 

 which the contraction of its volume is proceeding. 



It is well known that when ■ two solid bodies clash to- 

 gether, heat is produced proportionate to the momentum 

 of the two bodies. In other words, the motion which is 

 arrested is transformed into heat. Mr. Croll, in his last 

 publication * upon the subject, ingeniously attempted to 

 account for the gaseous condition of the nebulge and the 



* Stellar Evolution and its Relation to Geological Time. 



