CAUSE OF MILD POLAR CLIMATES. 161 



palaeontology. Mr. J. Starkie Gardner, a geologist 

 who has had great experience in the fossil flora of the 

 Tertiary deposits, says that such alternating warmer 

 and colder conditions are supported by strong negative 

 and some positive evidence, found not only in English 

 Eocene, but in all Tertiary beds throughout the world. 

 In the Lower Bagshot of Hampshire have been found, 

 he states, feather- and fan-palms, Dryanclva, beech, 

 maple, Azalea, laurel, elm, acacia, aroids, cactus, ferns, 

 conifers, Stenocarpus, and plants of the pea tribe, 

 together with many others. The question which 

 presents itself to one's mind, he remarks, is, how is 

 it possible that the tropical forms, such as the palms, 

 aroids, cactus, &c, could have grown alongside of the 

 apparently temperate forms, such as the oak, elm, 

 beech, and others ? Mr. Gardner's explanation is as 

 follows : — 



"Astronomers, having calculated the path of the revolution 

 of the earth in ages past, tell us that in recurring periods 

 each hemisphere, northern and southern, has been successively 

 subject to repeated cyclical changes in temperature. There 

 have been for the area which is now England many alter- 

 nations of long periods of heat and cold. Whenever the 

 area became warmer, the descendants of semi-tropical forms 

 would gradually creep further and further north, whilst the 

 descendants of cold-loving plants would retreat from the 

 advancing temperature, vice versa. Whenever the area 

 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. 



M 



