GREAT STEPS IN ORGANIC EVOLUTION 889 



and had thus been beyond all possibility of re-colonisation, except 

 by creatures that could fly or swim or drift. 



We naturally speak of these latest glacial periods as "the Great 

 Ice Age", for Europe is still in process of recovering from their 

 inexorable sifting. We know that the changes they brought about 

 were colossal. About two million square miles of Europe and four 

 million square miles of North America were glaciated; indeed, about 

 a fifth of the total land surface of the globe was then ice-covered, 

 though not all at once. Asia and Africa seem to have been but little 

 affected ; but we must not forget to include Greenland and Antarctica, 

 where so much ice still remains. But one of the many striking 

 conclusions of modern geology is that before the Great Ice Age 

 there were many Ice Ages, some of them greater than the Great. 

 There are traces of ice-sheets for most of the geological periods, 

 there appear to have been big-scale glaciations during the Huronian, 

 the early Cambrian or late Pre-Cambrian, and the Permo-Carboni- 

 ferous times — the last-mentioned being the greatest of all. It used 

 to be supposed that the earth was becoming gradually colder down 

 the ages, but the Pre-Cambrian is now regarded as probably the 

 coldest chapter in the earth's history. It used to be a nightmare 

 that our earth would become more and more like an ice-house till 

 all life came to an end; but now the geologists tell us that the 

 inhabitants of a planet that weathered the Permo-Carboniferous 

 Ice Ages need not be too timid about any others, grave though 

 these cannot but be. 



All these questions are discussed in a masterly way in Prof. 

 Coleman's Ice Ages: Recent and Ancient (1926), and one of the many 

 interesting facts that he emphasises is the irregularity in the 

 occurrence of these catastrophic interruptions of equability. "They 

 seem to be spaced in a haphazard way, and there is no certain 

 evidence of a rhythmic swing, after so many millions of years, from 

 mild to cold and back again." Not a few thoughtful men have 

 advanced theories by means of which the coming and going of Ice 

 Ages have for a time seemed "fully explained". But the problem 

 seems to become more, not less, difficult. For the theory that dom- 

 inant glaciation is a hint of the gradual refrigeration of the earth 

 is not easily squared with the Pre-Cambrian Ice Ages. The theory 

 that glaciation is the natural consequence of the elevation of parts 

 of the earthcrust above the level of perpetual snow appears an 

 enormous postulate when we think of the vast areas involved at 

 the same time in the two greatest Ice Ages. The occurrence of wide- 

 spread glaciation has been connected by Wegener and others with 

 the supposed drifting of continents; by others with the supposed 

 wandering of the poles ; and by others with supposed great changes 

 in the flow of ocean currents. Shrewd investigators have appealed 

 to changes in the condition, if not the composition, of the atmo- 



