LXXXVI PRESIDENTIAL ADDRESS. 



or 3 per cent. Surely no possible arrangement in the distribution 

 of land and water can reasonably account for the extinction of no 

 less than 97 per cent, of the known species of animal life as we 

 advance from the Carboniferous to the Permian. Surely some 

 great cosmic cause, such as variation in the amount of the sun's 

 heat received at different geological periods, must have contributed 

 to bring about this remarkable result. Figure 7 shows the 

 theoretical fluctuation of temperature at various geological periods. 

 The time intervals are based on (1) assumed thicknesses for the 

 geological formations similar to those given in Sollas' Age of the 

 Earth; (2) an assumption that equal thicknesses of strata accumu- 

 lated in equal time, obviously a very loose assumption. 



CURVE SHOWING APPROXIMATE RANGE OF TEMPERATURE IN TEMPERATE LATITUDES IN GEOLOGICAL TIME 



Siturian 



Carboniferous 



Miocene ReceQt 



Permo- Carboniferous Pie 



Otoaal C 



■/S'Com/'F 



Divisions on horizontal line refer to time based on reletive thicknesses of strata 

 formed at the various geological periods the latter after fro f WJ Sollijs 



Fig. 7. 



SUMMARY. 



From the evidence at present available, the following pro- 

 visional conclusions may be deduced: — 



That the reason for the great climatic and biological differences 

 between the North and the South Poles respectively is mainly 

 geographical, dependent that is on the present distribution of land 

 and water, and the modifications which they introduce into the 

 circulations of air and water in either Hemisphere. 



In reference to the control of Australian weather at present by 

 Antarctica, the existence of that large continent, with an average 

 elevation of about 6,000 feet, acts as a great refrigerator in the 

 Southern Hemisphere, and so causes extremes which would not 

 otherwise exist between South Polar and Equatorial temperatures. 



This factor tends to increase the rapidity of air circulation in 

 the Southern Hemisphere, the acceleration being greatest during 

 the winter months, when the isothermal, and consequently isobaric, 

 grades between Antarctica and the belts of ocean to the north of 

 it are steepest. 



