140 J. Croli— Cause of Mild Polar Climates. 
of heat and cold. Whenever the area became warmer, the descen- 
dants of semitropical forms would gradually creep farther and far- 
ther north, whilst the descendants of cold-loving plants woul 
i neve 
whilst the cold-loving plants would return to the area from which 
their ancestors had been driven out. In each case there would 
be some lingering remnants of the retreating vegetation (thoug 
perhaps existing with diminished vigor) growing alongside of the 
earliest arrivals of the incoming vegetation. ‘ 
Such is a possible explanation of our finding these plant-remams 
commingled together. It must be borne in mind that it is not s0 
much the mean temperature of a whole year which affects the po* 
sibility of plants growing in any locality, as the fact. of what are 
the extremes of summer and winter temperature.” * 
This is ant the explanation given by the commingling 
of subtropical and arctic floras and faunas of deposits belonging 
to the Glacial epoch. The causation in the two cases was 10 
fact the same in principle, differing only in the conditions ut 
der 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 
a we but moderately cool, and the warm periods exceed 
ingly hot. 
8 Wallace, who refers to Mr. Gardner’s views approvingly, 
says:—‘In the case of marine faunas it is more difficult t0 
judge, but the numerous changes in the fossil remains from be 
to bed, only a few feet and sometimes a few inches apart, may 
be sometimes due to change of climate ; and when it is recog 
nized that such changes have probably occurred at all geo 
logical 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.”t 
To prevent having thus to admit the existence of alternate 
warmer and colder periods during Tertiary times, Mr. Searles 
V. Wood, Jun., proposed another theory, which, stated in his 
own words, is the following :— 
“'The remains upon which the determination of this flora have 
been based are drifted, and not those of a bed in sitw like the coal 
seams, and the whole of the Hampshire Eocene is connected wit 
* Geological Magazine, 1877, p. 25. + Island Life, p. 197. 
