THE PLEISTOCENE OR GLACIAL PERIOD. 487 



then living in low latitudes, with forms now tropical. The sifting influ- 

 ences of the to-and-fro movement of the sharply differentiated climatic 

 zones, seem to have sorted out the mixed assemblage, or to have forced 

 them into special adaptations, or both, so that to-day most species are 

 confined to definite climatic zones. This was not universal, however. 

 Certain forms seem to have met the stress of the times by becoming 

 adapted to various climatic conditions. This versatility of adaptation 

 finds its highest expression in man, but in this case it is secured by 

 extraneous means not available to the lower creatures. Seasonal 

 oscillations are met by birds and certain other animals by seasonal 

 migration. The cases of versatile adaptation are, however, quite 

 inferior in number to those adapted to limited climatic zones only. 

 Superposition of cold and warm faunas and floras in the record. — 

 The to-and-fro movement of the faunas and floras introduced into the 

 record exceptional superpositions of faunas upon one another. The 

 succession was orderly but unusual. Where a complete record could 

 be made, as in a depositing tract just outside the limit of the invading 

 ice, the full series for the advancing stages of an ice invasion should 

 embrace a succession of faunas and floras ranging from the temperate, 

 through cold-temperate and sub-arctic, to the extreme arctic types, 

 while a full record of the retreating stages of the ice should embrace 

 the same series reversed. Such an orderly superposition should ideally 

 be repeated as often as there were ice invasions of the requisite mag- 

 nitude. In every interglacial period, therefore, there should be embraced 

 ideally a series of forms ranging from the arctic to the most tem- 

 perate compatible with the interglacial conditions, and thence back- 

 ward to the arctic. It is important to observe this range in interpreting 

 the fossils of interglacial deposits, for the presence of arctic and sub- 

 arctic faunas and floras in the lowermost and uppermost portions of 

 an interglacial series does not necessarily preclude the occurrence of 

 temperate forms in its middle part. Care in observing the exact hori- 

 zons from which fossils come is obviously required to avoid mingling 

 distinct groups. It is obvious so delicate and so changeable a record 

 would only be perfectly preserved under exceptionally favorable con- 

 ditions. No series having such ideal completeness has yet been described, 

 but series embracing sufficient representatives of cold and warm 

 climates are known to justify this ideal conception, and to make it 

 the working basis of observation, record, and interpretation. 



