SUMMARY OF THE LIFE OF THE PLEISTOCENE 369 



cent of extinct species. These groups consist of highly complex organisms 

 which have undergone rapid evolution. The vertical geological range of many 

 of the species and groups is noteworthy; the data are not yet full enough, how- 

 ever, to permit of generalizations, except in a few instances. 



I. The Flora 



The flora of the Pleistocene differs but little from that of the same area 

 today. At different times during the interglacial intervals the cold temperate 

 plants have been driven southward and their remains have been found to the 

 south of their normal habitat of today. Many species of plants preserved in 

 the Don beds of Toronto (Sangamon interval) as well as in later postglacial 

 deposits, are also found in the Pleistocene of South Carolina, Maryland and 

 Virginia. 2 Likewise, during a period of warmer climate a subtropical and 

 temperate flora flourished at a higher northern latitude than at the present 

 time. As has already been stated, it is probable that each interglacial interval 

 experienced five periods of climatic variation — (1) Arctic, (2) cold temper- 

 ate, (3) temperate (perhaps subtropical), (4) cold temperate, (5) Arctic. 

 This climatic variation has been observed in only a few places, notably at 

 Toronto and Chicago. The flora of the former locality is the most extensive 

 of any known Pleistocene locality within the glaciated area. The plant remains 

 also attest the presence of swamps, lakes, rivers, and vast forested areas, com- 

 parable with the same territory as it was before the advent of the White Man. 



Among the plants, trees predominate, perhaps because the woody leaves 

 and stems were more readily preserved. Enough of the other branches of the 

 plant kingdom have been preserved to indicate that they also were abundantly 

 represented. 



II. The Fauna. 



I. THE MOLLUSCA 



Almost half of the life of the Pleistocene belongs to the group of mollusks 

 owing, doubtless, to the hard shelly skeleton which is easily preserved. As 

 in the case of the plants, the most complete molluscan fauna occurs at Toronto 

 and Chicago. A greater or less number of species have also been observed in 

 many other places. The Sangamon interval is the most prolific in the remains 

 of the naiades, evidencing pronounced lake conditions in at least one locality 

 (Toronto). This type of mollusk also flourished during the Aftonian and the 

 Yarmouth intervals. The species from these early deposits show that little 

 change has taken place in this group during the glacial episodes. 



The fresh water univalves are abundantly represented in all strata, many 

 species being found in all, or nearly all of the interglacial intervals; notably, 



2 Berry, Joum. Geol., XV, pp. 338-349, 1909. 



