4o6 NOTES OF A NATURALIST. 



month is lower by less than three and a half degrees of Fahrenheit 

 than it is at Coimbra in Portugal, one and a half degree nearer 

 the equator, in the region which receives the full warming effect 

 of the Gulf-stream. 



I should have expressed myself ill in the preceding pages if I 

 should be supposed to deny that, in his writings on this subject, 

 Mr. Croll has made an important contribution to the physics of 

 geology. He has, in my humble opinion, been the first to 

 recognize the full importance of one of the agencies which, 

 under possible conditions, may have profoundly affected the 

 climate of the globe during past epochs, although I do not 

 believe that, in the present state of our knowledge, we can 

 safely draw those positive inferences at which he has arrived. 

 Even those who are unable to accept any portion of his theory 

 as to the causes of past changes of climate must feel indebted 

 to his writings for numerous valuable suggestions, and for the 

 removal of many popular opinions which his acute criticism has 

 shown to be untenable. 



