THE CAUSE OF THE GLACIAL PERIOD. 305 



heat of the sun and other fixed stars by supposing it to be 

 simply transformed motion. According to this theory, 

 the original form of force imparted to the universe was 

 that exerted in setting in motion innumerable dark bodies, 

 which from time to time have collided with each other. 

 The effects of such collisions would be to transform a large 

 amount of motion into heat and its accompanying forms 

 of molecular force. The violence of the compact of two 

 worlds would be so great as to break them up into the 

 original atoms of which they are composed, and the heat set 

 free would be sufficient to keep the masses in a gaseous con- 

 dition and cause them to swell out into enormous propor- 

 tions. From that time on, as the heat radiated into space, 

 there would be the gradual contraction which we sup- 

 pose is going on in all the central suns, accompanied, of 

 course, with a gradual decline of the heat-energy in the 

 system. 



Now, it is well known that the earth and the solar 

 system in their onward progress pass through trains of 

 meteorites. The tails of some of the comets are indeed 

 pretty clearly proved to be streams of ponderable matter, 

 through which, from time to time, the minor members of 

 the solar system plunge, and receive some accession to 

 their bulk and weight. The shooting-stars, which occa- 

 sionally attract our attention in the sky, mark the course 

 of such meteorites as they pass through the earth's atmos- 

 phere, and are heated to a glow by the friction with it. 

 It has been suggested, therefore, that the sun itself may at 

 times have its amount of heat sensibly affected by such 

 showers of meteorites or asteroids. Upon this theory the 

 warm period of the Tertiary epoch, for instance, may 

 have been due to the heat temporarily added to the sun 

 by impact with minor astronomical bodies. When, after- 

 wards, it gradually cooled down, receiving through a long 

 period no more accessions of heat from that source, the 

 way was prepared for the colder epoch of the Glacial 



