204 ISLAND LIFE 



there is strong negative and some positive evidence of 

 alternating warmer and colder conditions, not glacial, 

 contained not only in English Eocene, but all Tertiary- 

 beds throughout the world.^ In the case of marine faunas 

 it is more difficult to judge, but the numerous changes in 

 the fossil remains from bed to bed only a few feet and 

 sometimes a few inches apart, may be sometimes due to 

 change of climate ; and when it is recognised that such 

 changes have probably occurred at all geological epochs 

 and their effects are systematically searched for, many 

 peculiarities in the distribution of organisms through 

 the different members of one deposit may be traced to 

 this cause. 



General View of Geological Climates as dependent on the 

 Physical Featitres of the Earth's Surface. — In the pre- 

 ceding chapters I have earnestly endeavoured to arrive at 

 an explanation of geological climates in the temperate and 

 Arctic zones, which should be in harmony with the great 

 body of geological facts now available for their eluci- 

 dation. If my conclusions as here set forth diverge consid- 

 erably from those of Dr. Croll, it is not from any want of 

 appreciation of his facts and arguments, since for many 

 years I have upheld and enforced his views to the best of 

 my ability. But a careful re-examination of the whole 

 question has now convinced me that an error has been 

 made in estimating the comparative effect of geographical 

 and astronomical causes on changes of climate, and that, 

 while the latter have undoubtedly played an important 

 part in bringing about the glacial epoch, it is to the former 

 that the mild climates of the Arctic regions are almost 

 entirely due. If I have now succeeded in approaching to 

 a true solution of this difficult problem, I owe it mainly to 

 the study of Dr. Croll's writings, since my theory is entirely 

 based on the facts and principles so clearly set forth in his 

 admirable papers on " Ocean Currents in relation to the 

 Distribution of Heat over the Globe.'' The main features 

 of this theory as distinct from that of Dr. Croll I will now 

 endeavour to summarise. 



Looking at the subject broadly, we see that the climatic 

 1 Geological Magazine, 1877, p. IS^. 



