142 J. Croll—Cause of Mild Polar Climates. 
theory of recurring glacial epochs, answer this question em- 
phatically in the negative. This belief as to the non-existence 
of glacial conditions during the Tertiary period is, of course, 
based wholly on negative evidence; and this negative evidence 
though strong is by no means perfectly conclusive, and cer- 
tainly not worthy of the weight which has been placed upon it. 
n Chap. xvii of ‘Climate and Time,’ I have endeavored to 
show that although much has been written on the imperfection 
of geological records, yet the imperfection of those records in 
regard to the past glacial epochs has not received the attention 
at times exceptionally high, as, for example, 2,500,000 and 
850,000 yearsago; but whether a glacial epoch occurred at these 
in aphelion. At the periods 2,500,000 and 850,000 years ag% 
for example, those physical agents would no doubt produce 
* Phil. Mag., February, 1884; American Journal of Science, February, 1984. 
