222 THE GEOLOGICAL HISTORY OP PLANTS. 



allowing of the growth of a temperate flora in Greenland. 

 As Dr. Brown has shown,* and as I have elsewhere 

 argued, the absence of light in the Arctic winter is no 

 disadvantage, since, during the winter, the growth of 

 deciduous trees is in any case suspended ; while the con- 

 stant continuance of light in the summer is, on the con- 

 trary, a very great stimulus and advantage. 



It is a remarkable phenomenon in the history of gen- 

 era of plants in the later Mesozoic and Tertiary, that the 

 older genera appear at once in a great number of specific 

 types, which become reduced as well as limited in range 

 down to the modern. This is, no doubt, connected with 

 the greater differentiation of local conditions in the mod- 

 ern ; but it indicates also a law of rapid multiplication of 

 species in the early life of genera. The distribution of the 

 species of Salisburia, Sequoia, Platanus, Sassafras, Lirio- 

 dendron, Magnolia, and many other genera, affords re- 

 markable proofs of this. 



Gray, Saporta, Heer, !N"ewberry, Lesquereux, and 

 Starkie Gardner have all ably discussed these points ; but 

 the continual increase of our knowledge of the several 

 floras, and the removal of error as to the dates of their 

 appearance, must greatly conduce to clearer and more 

 definite ideas. In particular, the prevailing opinion that 

 the Miocene was the period of the greatest extension of 

 warmth and of a temperate flora into the Arctic, must 

 be abandoned in favour of the later Cretaceous and 

 Eocene ; and, if I mistake not, this will be found to ac- 

 cord better with the evidence of general geology and of 

 animal fossils. 



In these various revolutions of the later Cretaceous 

 and Kainozoic periods, America, as Dr. Gray has well 

 pointed out, has had the advantage of a continuous stretch 

 of high land from north to south, affording a more sure 



* " Florula Discoana," 



