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THE VOYAGE OF EVOLUTION 111 



that in large areas the amphibians could not return to the water 

 to lay their eggs. Hence many types perished. Only those 

 persisted which became true reptiles whose eggs are able to hatch 

 upon the land. The crisis had at last been weathered. 



Not till millions of years later did the next great step in 

 evolution occur. That step was the rise of the warm-blooded 

 mammals. We do not find their fossil record until the time known 

 as Upper Triassic, but they must have originated farther back, 

 apparently in the Permian. The date of the Permian Period is 

 estimated as anywhere from ten million to two hundred million 

 years ago. The break between the types of life before and after 

 this great crisis is the most profound in the history of evolution. 

 It is therefore highly important to find that this was also the 

 time of the greatest changes of climate. Vast glaciers descended 

 to sea level within thirty degrees of the equator. Perhaps at no 

 other time during the evolution of man's ancestors has there been 

 such a succession of cold, stormy glacial epochs alternating 

 sharply with mild, interglacial epochs. 



Let us consider the effect of such climatic stress upon other 

 forms of life as well as upon our ancestors. Previous to the 

 Permian Period the vegetation of all parts of the earth's surface, 

 including even the far north, was much alike. In general the 

 lands were covered with forests, averaging perhaps forty feet 

 in height but with some trees towering a hundred feet. Schu- 

 chert describes it as a forest of rapid growth, of soft and even 

 spongy woods, in which evergreen trees with comparatively small, 

 needle-like leaves were prominent. Associated with these were 

 thickets of rushes, also of very rapid growth, which in habit 

 resembled modern cane brakes and bamboo thickets. Here and 

 there stood majestic treelike ferns, while many smaller ferns 

 and similar plants thrived in the shady places or climbed among 

 the trees. Flowers of a certain sort were sparingly present, but 

 of insignificant size and unattractive color. Spores took the 

 place of seeds to such a degree that when the trees and ferns 



