THE POSITION OF THE EARTh's AXIS. 113 



vered within the Arctic Circle^ very much resembling the 

 floras of the same time from the temperate regions of 

 Europe ; and these also indicate a much greater warmth. 



Many of these plants are unable to resist severe cold, 

 besides requiring a warm summer; and it seems difficult to 

 accept the fact of these flowering and ripening their seeds 

 where the winters are so long and the summers are so short, 

 and, apart from the lower temperature, where the amount 

 of light is so much diminished. 



Having put forward the grounds upon which a more 

 satisfactory explanation is desired of the warm periods 

 preceding the glacial condition, we must now examine the 

 position in which geologists are placed by the physicists. 



When the vast extension of the glaciers was appreciated, 

 various theories were started to explain the phenomena. 

 Some thought variation of interstellar space was the cause, 

 others the obliquity of the ecliptic, others that different 

 geographical distribution of land and sea would explain it ; 

 and this has had great influence, but does not give a 

 sufficient explanation : and the increase in the eccentricity 

 of the earth^s orbit was made the explanation ; but this was 

 refuted, as the heat received by proximity to the sun in 

 perihelion is balanced by quicker motion. The explanation 

 which is now generally accepted is that of Mr. Croll*, that 

 the glacial periods were brought about indirectly from an 

 increase in the eccentricity of the earth's orbit, modified 

 by the obliquity of the ecliptic. In the longer and colder 

 winter more snow fell, which the summer could not melt 

 away; so that the snow-covered earth got little of the 

 warmth of the sun, as so much was required to melt the 

 snow. 



It will be seen that Mr. CrolFs theory requires constantly 



* Various articles in the ' Philosophical Magazine ;' and ' Climate and 

 Time,' 1875. 



SER. III. VOL. VI. I 



