140 THE TREES AND SHRUBS [LECT. 



the gradual sinking of temperature which has taken 

 place in the crust of the globe, down to the glacial 

 period, and its subsequent elevation during that in 

 which we live, suggest causes for the disappearance 

 of certain species, and for the substitution of others, 

 which may be deemed sufficient to afford an ade- 

 quate solution of the problem. 



Nevertheless, if these effects are exclusively due 

 to climate, certain other conditions, at least, be- 

 sides that of temperature, must be concerned in 

 producing them. 



Take, for instance, the case of the Wettingtonia. 



This tree, or one nearly allied to it, existed gene- 

 rally throughout Europe during the Miocene period. 



The Lignites of Bovey Tracey, in Devonshire, are 

 supposed to be principally made up of its remains, 

 and an allied species is widely distributed over 

 Europe and America, from Italy to Greenland, 

 during the Miocene period, when the climate is 

 supposed to have been almost sub-tropical. 



At present, its only native habitat is California, 

 yet it does not seem to have been limited to this 

 one region by the gradually decreasing temperature, 

 since in that case it would not have survived the 

 glacial period, nor would it prove capable of re- 

 sisting the present winters of Great Britain. 



Mr. Batetnan, who has introduced this tree largely 

 into his plantations in Cheshire, considers it hardier 

 than the Deodara ; yet from some mysterious cause 

 unconnected with temperature, all his specimens 

 were blighted in the spring of 1864, whilst the 



