76 LOUIS AGASSIZ 



England in 1834 and 1835 he was re- 

 ceived as an ichthyologist ; but when he 

 went again, in 1840 and in 1846, it was 

 to find everywhere traces of extinct gla- 

 ciers, to explain in a word phenomena 

 that had puzzled everybody, and to con- 

 vince some of the English geologists out 

 of hand. The theory of an ice age grew : 

 the sheet of ice asserted to have passed 

 over some parts of Europe assumed more 

 and more startling dimensions, while 

 ridicule and opposition diminished in 

 inverse ratio. To-day no one doubts 

 the main facts : the explanation is yet 

 to be given. 



So far as we know, Agassiz formed 

 no opinion about the possible causes of 

 a glacial period. It is one of those 

 large questions unencumbered by data 

 which invite irresponsible theorising. 

 Periodic changes in the elongation of 

 the earth's orbit or in the inclination of 

 its axis might cause a cosmic summer 

 and winter (in which case the ice age 



