154 Slou.jc City Academy of Science and Letters. 



ten thousand parts of the atmosphere. A very small 

 increase of this percentage of carbonic acid in the air 

 would be fatal to the highly organized life now existing. 

 Let us now examine the remains of animal life of the 

 past that we have found as fossils and see if they, too, 

 will tell us the same story of the ancient condition of the 

 earth. We very frequently read in the papers of men los- 

 ing their lives by going down into wells or cisterns which 

 have what is called "bad air" in them, of terrible mine 

 disasters where scores of miners perish because of choke 

 damp. These accidents are all caused by an excess of 

 carbonic acid gas, which, being heavier than air, settles 

 in the bottoms of wells or mines. This teaches us that 

 the animal life now existing cannot be maintained in 

 an atmosphere where the proportion of carbonic acid 

 is very much greater than in the air we breath. In trac- 

 ing the life of our earth backward we must expect to 

 find a decrease in the number of our present forms of 

 organic animal life, replaced by others lower down in the 

 scale of being. Up to the close of the Carboniferous Age, 

 we find no remains of birds or mammals. The animal 

 life, with the exception of a few insects, was wholly aqua- 

 tic or amphibian, and a large proportion was marine. 

 Dana says of this period: "During the progress of the 

 carboniferous period there was, then, a using up and 

 storing away of the carbon of the superfluous carbonic 

 acid, and, thereby, a more or less perfect purification of 

 the atmsopliere, and a diminution of its density. In 

 early times there was no aerial animal life on the earth, 

 and as late as the Carboniferous period there were only 

 reptiles, myriapods, spiders, insects and pulmonate mol- 

 lusks. The cold-blooded reptiles, of low order of vital 

 activity correspond with these conditions of the atmos- 

 phere. The after ages show an increasing elevation of 

 grade, and variety of type in the living species of the 

 land." If we go back still further in the life history of 

 the earth, as shown by paleontology, we shall find still 

 more proof of the development animal life has passed 

 through because of the changing environment. The first 

 certain fossils are found in the Cambrian rocks. These 

 rocks rest unconformably on the crystalline rocks of the 



