44 Origin of the British Flora. 



borne out by the evidence above mentioned. We have no 

 indication in our Tertiary or later deposits of a number of 

 alternating Glacial and Interglacial Epochs, such as are 

 required on the theory of Croll * adopted by Professor 

 James Geikie.-|- On the other hand, the evidence is 

 perfectly clear that this country saw two cold Epochs, and 

 certain indications make one suspect that there may have 

 been a third, less rigorous. The exact succession of events 

 is at present very difficult to follow ; for it is unsafe to 

 compare isolated records, which belong to different regions, 

 and may not belong to the same period. We need more 

 excavation and close examination of localities such as 

 Hoxne and the Selsey Peninsula, where several stages can 

 be studied in chronological order, with no possibility of 

 mistakes in the succession. 



The wind-borne 'loess' of Central Europe, with its 

 desert or sand-dune mollusca and mammals, belongs 

 apparently to the second cold period just alluded to. 

 Only slight indications of this dry climate have been 

 discovered in Britain, and, though it may have marked an 

 important stage in the building up of our flora, we know 

 little about its plants in the south, while nearer the glaciated 

 area those found are common Arctic forms. It is always 

 difficult to obtain botanical evidence of a bygone period 

 of drought, for desert-plants seldom find their way into 

 lacustrine deposits, and porous sub-aerial deposits like 

 drift-sand or loess are the worst possible for the preserva- 

 tion of plant-remains, though they may be full of calcareous 

 fossils. 



The South of England during the second period of 



glaciation seems to have suffered from dry cold winters, 



which froze the ground unprotected by snow, and allowed 



the summer rains to fall on soils rendered impervious by 



* Climate and Time. t Great Ice Age. 



