162 DISCUSSIONS IN CLIMATOLOGY. 



" Such is a possible explanation of our finding these plant- 

 remains commingled together. It must be borne in mind 

 that it is not so much the mean temperature of a whole 

 year which affects the possibility of plants growing in any 

 locality, as the fact of what are the extremes of summer and 

 winter temperature." * 



This is precisely the explanation given of the 

 commingling of sub-tropical and Arctic floras and 

 faunas in deposits belonging to the Glacial Epoch. 

 The causation in the two cases was, in fact, the same 

 in principle, differing only in the conditions under 

 which it operated. In the case of the Glacial Epoch 

 the cold periods were intensely severe and the warm 

 periods but moderately hot ; whereas in regard to the 

 Tertiary cold periods they were but moderately cool, 

 and the warm periods exceedingly hot. 



Mr. Wallace, who refers to Mr. Gardner's views 

 approvingly, says : — " 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." f 



To avoid having thus to admit the existence of 

 alternate warmer and colder periods during Tertiary 

 times, Mr. Searles V. Wood, Jun., proposed another 

 theory, which is thus stated in his own words : — 



" The remains upon which the determinations of this flora 

 have been based are drifted, and not those of a bed in situ 



* "Geological Magazine," 1877, p. 25. 

 f " Island Life," p. 197. 



