UNIFORMITY OF SURFACE TEMPERATURE 121 



solar systems. It there is life on other planets, circumstances there 

 also ought not only to have permitted some sort of comparable in- 

 organic reactions leading to proto-life of some sort, but on top of 

 that they also must have been of a nature constant enough to permit 

 subsequent preservation, and, of course, evolution, of that particular 

 form of life on its heavenly body. 



On our own earth, to return to our subject, our type of life has 

 been protein-based over at least 2 billion years. Now protein is a 

 subtile substance. It survives neither freezing nor heating over any 

 geologic length of time. It follows that over these 2 billion years the 

 mean yearly temperature of the surface of the earth cannot have 

 varied more than a score of degrees centigrade. Or, in other words, 

 over this period it has been remarkably stable. 



This pronouncement may well be in need of qualification for the 

 general reader, who, by familiarity with normal geologic literature, 

 has perhaps become convinced of the strong variations in temperature 

 in the geologic past, leading, for instance, to the Ice Ages. 



It is true, of course, that there were Ice Ages in the past. Pre- 

 sumably even now we do not live in a postglacial age but in an 

 interglacial period between the last Ice Age and the next one. A 

 future Ice Age may well be expected for geologic reasons, although, 

 incidentally, it might be possible that man prevents its coming, 

 willingly or even unwillingly, through an excess of industrialization. 



So, there have been Ice Ages, several of them occurred in the 

 recent geological past, over the last million years or so. Others oc- 

 curred further back, between 200 million and 250 million years ago, 

 during late Carboniferous and early Permian systems. Still other Ice 

 Ages are known to have occurred in late pre-Cambrian times, prob- 

 ably around 600 million years ago. There are, moreover, almost 

 certainly still older Ice Ages, still less known. As a counterpart to 

 these Ice Ages, there have been warmer periods too. These are far 

 more difficult to detect geologically, but one might well speak of 

 Heat Ages. As an example, let us cite the late Permian, when, 

 closely following upon the early Permian Ice Age, a Heat Age pro- 

 duced such strong evaporation in many parts of the earth that most 

 of our primary rock-salt deposits were formed at that time. 



So there have repeatedly been Ice Ages in the geological past, 

 whilst, although less well known, there also have been Heat Ages. 



