294 POPULAR LECTi A'A.S' AND ADDRESSES. 



sphere and less in the southern then than now. 

 And whatever effect on the seasons or climates of 

 the two hemispheres is produced by the change in 

 a few thousand years from being nearest to the sun 

 in winter to being nearest to the sun in summer, 

 much greater effect must have been produced when 

 through the secular variation of the eccentricity 

 of the earth's orbit the difference of distance has 

 been three or four times as great as at present, 

 as it has been probably more than once in the last 

 million years. But what the precise character of 

 this effect may be, or whether even at its greatest 

 it can have been distinctly sensible among the 

 undoubtedly potent influences depending on the 

 distribution of land and sea, or whether it can 

 have been preponderant in giving rise to alter- 

 nations of glacial and milder epochs as argued 

 by Mr. Croll in the interesting and suggestive 

 illations of his Climate and Time, are very 

 difficult questions which have not hitherto been 

 validly dealt with and which deserve the most 

 careful consideration. 



As to changes of the earth's axis, I need not 



