more slieltered locations, while grassland invaded 

 the drier areas (Moss 1^44). The numerous lakes 

 in the Great Basin shrank in size or entirely dried 

 up, and desert I)iota sjiread both far to the Xorth and 

 high up onto the mountains. Northern species were 

 eliminated from the tops of many southern moun- 

 tains. 



With the conung of cooler, moister climate in the 

 subsequent Sub- Atlantic period, the prairie peninsula 

 receded, leaving populations of biota, relict today, 

 behind. Beech once again spread westward, followed 

 by hemlock : hemlock re-established its dominance in 

 the Northeast. In the northern states there is some 

 evidence that spruce again spread southward. Many 

 of the lakes of the Great Basin refilled with water. 



Within historic time, smaller fluctuations in cli- 

 mate are known to have occurred. These have been 

 determined from growth rings of the giant sequoia 

 trees, lake levels, records of past civilizations such as 

 that of the Maya of Yucatan, as well as inferences 

 from historical documents. In western United States 

 these fluctuations have been dated as follows (Brooks 

 1949) : 



Wet. 500-250 b.c. Dry. a.d. 1100-1300 



Drv. 250-100 b.c. Wet. .\.d. 1300-1400 



Wet, 100 B.c.-A.D. 200 Drx. A.n. 1450-1550 



Drv. .^.D. 300-800 Wet. a.d. 1550- 

 Wet. A.D. 900-1100 



during the xerothermic 



Information given by 



1950, Halliday 1937). The 



sble extent to which the prairie mas- 



during this period (Schmidt 1938). 



Mountain glaciation, especially in the .\!ps and 

 Iceland, was extensive between 1600 and 1850, but 

 glaciers all over the world have been shrinking since 

 then at a very rapid rate. During the last hundred 

 years, mean annual temperatures have increased 0.5° 

 to 2.2°C, and the sea level has risen about 6 cm 

 (2.5 in.) (Flint 1947, Baum and Havens 1956). 

 This amelioration of climate has permitted the north- 

 ward dispersal of birds and other animals in recent 

 years into Ontario (Urquhart 1957), Iceland (Gud- 

 mundsson 1951), northern Europe (Kalela 1949, 

 Haftorn 1958), and in the sea (Taylor et al. 1957). 

 Other species will doubtless follow in the future : 

 northern communities are not presently saturated 

 with the variety of species they could support. This 

 is true of aquatic communities as well as terrestrial 

 ones. For instance, the fresh-water fish fauna of 

 North .America is most highly developed in the Mis- 

 sissippi River system. The impoverished variety of 

 the fish fauna northward and northeastward is in 

 large part due to the failure of fish species to bypass 

 land barriers and to disperse into otherwise suitable 

 waters in these regions since the retreat of the glacier. 

 A northward movement of fauna may be expected to 

 continue until the carrying capacity of the ecosystems 

 is reached, or until there is another reversal in the 

 climate. 



SUMMARY 



At the beginning of the Tertiary Era, 60-70 

 million years ago, the North American continent was 

 widely covered with epicontinental seas, marshes, and 

 lakes. Scattered mountain ranges occurred in the 

 Rocky Mountain region, but these had been greatly 

 eroded by Oligocene time. Rainfall was heavy and 

 temperatures mild. Tropical conditions extended 

 across the continent to 49° North latitude in the 

 West, and 37° North latitude in the East; temper- 

 ate climates obtained nearly to the North Pole. 



Three principal floras occurred during early Ter- 

 tiary time. The Neotropical-tertiary flora was co- 

 extensive with the tropical climate. The Arcto-ter- 

 tiary flora consisted of a temperate unit, largely 

 deciduous forest, and a boreal unit, preponderantly 

 coniferous species ; this flora extended to within 

 eight degrees latitude of the North Pole and across 

 the Bering land bridge into Eurasia. The Madro-ter- 

 tiary flora first appeared during the Eocene in scat- 

 tered dry sites on the lee sides of high ridges in 

 northern Mexico and southwestern United States, 

 but did not become well developed until the Miocene. 



Beginning in the Miocene and increasing in in- 

 tensity through the Pliocene into early Pleistocene, 



Paleo-ecology 291 



