6 HOMOIOTHERMISM 



^An atmosphere containing much carbon dioxide and 

 water-vapor should give the earth a warm and equable 

 climate. Such climates apparently did prevail at certain 

 times during the earth's early history, and also at cer- 

 tain times later. 



Temperature of the Modern Earth. — Organisms now 

 live at a time when the earth has marked climatic dif- 

 ferences, varying between the frigid polar climates and 

 the tropical humid or dry conditions. The earth has been 

 colder than it now is, and at times the climates of the 

 past have been warm and fairly constant the world over. 

 However, there probably have been, usually if not at all 

 times, zonal belts and fluctuations in temperature. This 

 is indicated by the types of ancient animals or plants, 

 from which may be deduced and plotted the temperature 

 curves of the past. Thus it is indicated that warm cli- 

 mates persisted during long geological ages and that the 

 polar regions were then usually inhabited by plants and 

 animals which were adjusted to winterless environments. 

 Apparently temperature fluctuations were often greatest 

 during the opening and closing of the periods and eras. 



Very long, warm times were separated by shorter 

 periods of cool to cold climates. Geologists have deter- 

 mined that there have been about seven periods of de- 

 cided diferences in temperature as indicated by the fossil 

 remains in sedimentary rocks and by the characters of 

 the rocks themselves: 



1. Early Proterozoic 5. Triassic to Lias 



2. Late Proterozoic 6. Cretaceous to Eocene 



3. Silurian 7. Pleistocene 



4. Permian 



At the beginning and ending of the Proterozoic, 

 Permian, and Pleistocene there were cold periods accom- 



