280 Dr. J. Oroll on the Cause of 



became gradually colder, the heat-loving plants would, from one 

 generation to another, retreat further and further south, 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 (though perhaps 

 existing with diminished vigour) growing alongside of the earliest 

 arrivals of the incoming vegetation. 



" 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 pos- 

 sibility 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 by the commingling 

 of subtropical and arctic floras and faunas of 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 approv- 

 ingly, says : — " In the case of marine faunas it is more diffi- 

 cult 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 recognized that such changes have probably occurred 

 at all geological epochs, and their effects are systematically 

 searched for, many peculiarities in the distribution of or- 

 ganisms through the different members of one deposit may 

 be traced to this cause "f« 



To prevent having thus to admit the existence of alternate 

 warmer and colder periods during Tertiary times, Mr. Searles 

 V. Wood, Juia., proposed another theory, which, stated in his 

 own words, is the following : — 



" The remains upon which the determinations of this flora have 

 been based are drifted, and not those of a bed in situ like the coal- 

 seams, and the whole of the Hampshire Eocene is connected with 

 the delta of a great river which persisted throughout the accumula- 

 tion of the various beds, which aggregate to upwards of 2000 feet 

 in thickness. This river evidently flowed from the west, through 

 a district of which the low ground had a tropical climate; but like 

 some tropical rivers of the present day, such as the Brahmaputra, 

 the Megna, the Ganges, &c, it was probably fed by tributaries 



* Geological Magazine, 1877, p. 25. 

 f Island Life, p. 197. 



