INSECTS OF WESTERN NORTH AMERICA 



249 



thereby has effected an intrusion of a predominantly subtropical 

 group into the cool temperate belt. There is in these data no sugges- 

 tion as to when this extension of range developed, but the lack of 

 formation of distinctive northern species in the group suggests 

 that it occurred relatively recently. 



Adding together the known data concerning cool-adapted insects, 

 one gets the definite impression that at three points in geologic 



o 



o 



a. 

 E 



25 



20 



15 



10 



CRETACEOUS 



CENOZOIC 



PLEISTOCENE 



Fig. 14. Suggested temperature trends during the latter part of 

 Cretaceous and Cenozoic time, combining data for the Cretaceous from 

 Lowenstam and Epstein (1954), the Cenozoic from Durham (1950), and 

 the Pleistocene from Emiliani (1955). These authors stress that the values 

 represent trends and should not be construed as indicating absolute 

 climatic measurements. 



time — middle Cretaceous, Paleocene, and Pleistocene — conditions 

 were such that extensive dispersal of cool-temperate forms took 

 place. Various estimates of temperature trends during Mesozoic 

 and Cenozoic agree closely (Fig. 14), but they indicate that periods 

 of widespread cool conditions were of relatively short duration. 

 During the long intervening periods when warmer climates prevailed 

 over the lowlands, we must presume that the cool-temperate biota 

 existed in higher elevations in the montane regions. Two sets of 

 data from the western region of North America fit well into such a 

 hypothesis. 



1. The relatively flat portions of the present day northern tundra 



