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THE POPULAR SCIENCE MONTHLY. 



rapidity of descent of the curve. The abscissas correspond to the equal 

 decrements in equal times ; the ordinates, to the fall in heat-retaining 

 power. Up to the heavy line, counting from the right, the ordinates 

 are proportional to the numbers in Professor Tyndall's table (page 354), 

 which were obtained by measurements. The others are estimated by 

 continuing the curve to the left. If we suppose the first abscissa on 

 the right represents the Tertiary, the next three will represent the 

 Mesozoic ; the next twelve the Palaeozoic, and an unknown number 

 beyond will represent the Archaean. 



7 s yft T 



The Uppee Cueve is foe the Ethee, and the Loweb One foe Olefiant Gas. 



All that is known about the changes of temperature in the geologi- 

 cal climate is derived from observing the changes in the plants and 

 animals. These changes were world-wide, and hence were due to a 

 world-wide cause. None sufficiently broad has been imagined other 

 than a change of temperature, or a change in the purity of the atmos- 

 phere. The peculiar life of the Palaeozoic lasted several times as long 

 as that of the Mezosoic ; and that, in turn, lasted several times as long 

 as that of the Tertiary. Or, calling the last 1, the ratio was 12 : 3 : 1. 

 The millions of years of the Palaeozoic brought a certain increase of 

 purity and decrease of temperature, and were followed by an almost 

 complete extermination of species, owing to their non-adaptation to 

 the new conditions. The Mesozoic, although only one fourth as long, 

 brought probably quite as great a change, and was followed by " one 

 of the most complete exterminations on record." Through the Eocene 

 and Miocene, although not more than one fifteenth as long as the pre- 

 ceding, the fall of temperature was actually greater than in them all 

 combined, and this was far exceeded in the rest of the Tertiary, for, in 

 the Miocene, cypresses and magnolias grew within 10J of the pole, 

 and, at the end of the Pliocene, that dread winter began to set in 

 which is known as the Glacial epoch. There was a correspondingly 

 rapid change in the plants and animals. Whether it is a matter of 

 accident, or whether they stand in relation of cause and effect, the 



