194 LIFE OF THE PLEISTOCENE 



It will be seen that the greater number of species is characteristic of the 

 Mississippi Valley, 16 genera and 31 species being from this region, while but 

 3 genera and 4 species are from the eastern fauna, and only 3 of these are 

 strictly confined to the Atlantic faunal region. The inference to be drawn 

 from these data is obvious and is in complete accord with Dr. Walker's re- 

 marks on the distribution of the recent naiades of the same region. 228 Equally 

 interesting data could be provided from a study of the distribution of other 

 groups of animals. 



3. VARIATION IN CLIMATE AS EVIDENCED BY THE BIOTA 



Some years ago, Dr. T. C. Chamberlin 229 made the following statement, 

 "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 deposit- 

 ing tract just outside the limit of the invading ice, the full series for the ad- 

 vancing stage 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. " 



As remarked by Dr. Chamberlin, this theoretical succession is rarely per- 

 fectly represented. In several places, however, as at Chicago and some 

 other places, a partial record has been preserved, and a characteristic biota 

 is represented, abundantly supporting the statement of Dr. Chamberlin. At 

 Chicago a series of deposits are superimposed one upon another, which contain 

 the biota of several climates. The lowest, and therefore the oldest of the 

 retreating series, contains two spruces, Picea canadensis and Picea mariana, 

 a tamarack, Larix laricina, the balsam fir, Abies balsamea, the arbor vitae, 

 Thuja occidentalis, and the balsam poplar, Populus balsamifera, as well as a 

 molluscan fauna characteristic of a cold-temperate or even subarctic zone. 

 The deposit overlying this cold fauna is filled with an abundant and varied 

 fauna characteristic of a temperate climate as warm as, or even warmer, than 

 that of today. 



That there was a period during which the climate of the region immediately 

 adjacent to the lower Great Lakes was somewhat warmer than at the present 

 time is apparently evidenced by the presence of a peccary (Platygonus com- 

 pressus) in both Michigan and northern New York. The Megalonyx also lived 

 in Ohio. Deposits in northern New Jersey contain a flora the species of which 

 indicate a period of higher temperature. Of the 9 species represented in this 

 deposit, 4 do not now range north of southern Maryland, and 1 (Zizyhus) is 

 mainly tropical and is not now represented in the northern coastal plain (Vide 



*« Nautilus, XXVII, pp. 21, 30, et seq. 



229 Chamberlin and Salisbury, Geology, III, p. 487. 



