86 THE PASSENGER ipIGEON IN PENNSYLVANIA 



families of our great forests and left records upon the 

 Triassic rocks, before the existing mountain chains 

 were upheaved from the Tertiary plains. Changes of 

 climate eliminated many families of the trees, and 

 finally the deciduous forms were evolved. 



It is now believed by some scientists that a small 

 addition to the carbon dioxide in the air would so im- 

 prison earth-heat that temperate conditions would be 

 restored nearly to the poles of the earth, as was the 

 case when the tropical trees grew in far northern 

 regions of America ; and when the great plains existed, 

 before the great mountains were here, subject to rapid 

 erosions of modern times, imprisoning the carbonic 

 acid gas, as bicarbonates, in the waters that spread over 

 the depressions of the plain. During the rest period 

 monocarbonates formed in the water, releasing part of 

 the carbon dioxide, to be absorbed into the air. The 

 reverse would be a Glacial period that would follow 

 any time, when erosions increased upon earth for any 

 considerable period of time ; as, in that case, a per- 

 centage of atmospheric carbon dioxide would be taken 

 from the air, leaving less to check radiation of heat 

 from the earth. 



This is merely an academic illustration, in an effort 

 to explain the manner in which the changes upon earth 

 occur; to give the reason for disappearing races of 

 animals and birds ; because their food supplies are 

 affected, as a family of trees or plants declines and 

 new forms are born. There are such changes, pro- 

 gressing toward consummation, around us all the time ; 



