20 EDWARD W. BERRY 



Arctic lands also shared this invasion, since willows have been 

 found in beds of this age in Alaska, at the mouth of the Mackenzie 

 River, in Iceland and Spitzbergen. The climate of the earth 

 seems to have been warmer during Eocene times since we find 

 many tropical plants in the Mississippi valley and as far north 

 as southern England, and the Arctic lands at this time were 

 clothed with dense forests of a temperate type. 



The Oligocene, which succeeds the Eocene in the Tertiary 

 sequence, was a time of prevailingly marine deposition in North 

 America so that few fossil plants are known and there is only 

 one willow among them, although doubtless willows still flour- 

 ished since they are common in succeeding deposits. In Europe 

 about half a dozen species are known from the Oligocene rocks. 



The next period — the Miocene — was a time of great variety 

 and luxuriance of tree growth. Between forty and fifty differ- 

 ent willows are known and the actual number in existence must 

 have been much greater for when we get a glimpse into the 

 past in an otherwise unknown area, like that furnished by the 

 tiny lake basin at Florissant in the Colorado Rockies we find an 

 abundance of willows — five have been described from Florissant. 

 They are equally abundant in the lake beds and elsewhere 

 throughout Europe. A few are known from eastern Asia and in 

 America they occur in Virginia on the East coast and in Oregon 

 and California on the Pacific coast. 



The Miocene was succeeded by the Pliocene period, a time 

 during which the forests of the Miocene continued practically 

 unchanged. Many willows whose characters foreshadow their 

 existing descendants are known from Asia Minor to Spain, but 

 unfortunately for our history the American Pliocene deposits 

 are for the most part marine marls so that the American Plio- 

 cene plant record is a blank, although we know that the familiar 

 types must have been present since willows are abundant in 

 the next or Pleistocene period. 



The Pleistocene, or period of continental glaciers, was an 

 epic time for all plants and animals, for it was a time during 

 which ice sheets many feet in thickness gradually accumulated 

 in northern America and Europe and in the more elevated moun- 



