AND ITS INHABITANTS 55 



We are living at a time when the earth has marked climatic 

 differences, varying between icy polar climates and hot moist 

 or dry tropical conditions. This has, however, not always 

 been the case, for not long ago geologically the temperature 

 of the entire earth was even colder than it is now, although 

 most of the climates of the past have been warm and fairly 

 equable the world over (see Fig. 3). The ancient plants and 

 animals are "self-registering thermometers" with regard to 

 the climates of the past, and they indicate that warm climates 

 persisted during long geological ages, and that even though 

 there were at all times zonal belts and fluctuations in the tem- 

 perature, the polar areas were usually inhabited, as is now 

 well known, by plants and animals of kinds that were adjusted 

 to winterless environments. These temperature fluctuations 

 were greatest during the closing and opening stages of the 

 periods and eras. 



The very long warm times were separated by short periods 

 of cool to cold climates. Geologists now know of seven 

 periods of decided temperature changes {earliest and latest 

 Proterozoic, Silurian, Permian, Triassic-Lias, Cretaceous- 

 Eocene, and Pleistocene) ^ and of these at least four (those in 

 italics) were glacial climates). Cooled climates occur when 

 the lands are largest and most emergent, during the closing 

 stages of periods and eras, and cold climates nearly always 

 exist during or Immediately following the times when the 

 earth is undergoing most marked mountain making (see 



Fig- 3). 



Origin of the earth! s zvaters. There was a time when the 

 earth was too small in mass to hold a hydrosphere, the 

 envelope of water that lies beneath the atmosphere and above 

 the rocky surface of the planet. As the earth grew in mass, 

 it became more and more possible for it to have standing 

 bodies of water and clouds of water-vapor floating with the 

 winds of the atmosphere. All of this water, the newer geology 



