THE WILLOWS AND POPLARS 85 



regarding it as reduced by evolution from a more complex type. 

 Whichever view is correct the willows undoubtedly appear early 

 in the geological record. 



The oldest Tertiary, or Eocene, deposits have furnished about 

 25 species of willows, the records including all of the continents of 

 the Northern Hemisphere. Willows had now reached Greenland, 

 where five different species have been discovered. Other 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 more equable during Eocene times since we find many 

 sub-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 temperate types. 



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 flourished 

 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 different 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, hke that furnished by the tiny lake basin at Floris- 

 sant in the Colorado Rockies we find an abundance of willows — ^five 

 having been described from Florissant. They are equally abund- 

 ant in the lake beds and elsewhere throughout Europe. A few 

 are known from eastern Asia and in America they occur in Vir- 

 ginia 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 descend- 

 ants are known from Asia Minor to Spain, but unfortunately for 

 our history the American PHocene deposits are for the most part 



