GEOLOGICAL EXTERMINATIONS. 



167 



nent, and always working towards present conditions, and, 

 therefore, rendering a return of former species impossible. 



But the question arises, was there such improvement ? 



The early atmosphere must have contained an enormous 

 amount of carbonic acid, or, as it is now called, carbon-dioxide, 

 for though at one time carbon and oxygen were kept disasso- 

 ciated by the intense heat, yet as the temperature fell, a point 

 was reached when their usual affinities brought them together 

 till one was exhausted. Probably some free oxygen was left, 

 because after that — I know not how soon — there were found 

 protozoa, and these, like all other animals in the water and on 

 the land, require such oxygen. 



A very large part of the carbon dioxide had united with 

 lime and other bases, forming insoluble deposits, before the 

 Eozoic. The atmosphere was improved by the operation, 

 but became no richer in oxygen ; for this vegetation was neces- 

 sary. When there began to be even the lowest plants, the 

 oxygen of the dioxide commenced to be freed from the carbon, 

 and returned to the atmosphere, while the other element, the 

 carbon, gave suitable material to plants and their dependents, 

 the animals. All those then living, and all that have come 

 after them, whether now living or buried in the earth as 

 graphite, coal, lignite, oil, gas, or in other forms, existed at the 

 opening of the Eozoic period as carbon-dioxide. Hence its 

 atmosphere was poorer in free oxygen by the amount necessary 

 to turn all that carbonaceous matter back to dioxide, and this, 

 the chemists tell us, is eight pounds of oxygen to only three of 

 carbon ; or, to put it in another way, one pound of carbon will 

 turn into dioxide two and a half pounds of oxygen, or, more 

 exactly, two and two-third pounds. 



It is impossible to determine the amount of carbon which 

 has passed through plants and animals, but from what has 

 already been found in the comparatively small part of the 

 earth's crust which has been examined, and from what we may 

 reasonably suppose has been carried into the sea, it was sufficient 

 to hold a very large part, probably by far the largest part of 

 the present free oxygen, as carbon-dioxide. 



In every hundred pounds of the present atmosphere there 

 are about twenty-three pounds of free oxygen. At the begin- 

 ning of the Eozoic period, when all subsequent organisms 

 existed only as carbon-dioxide, the amount of free oxygen in 

 one hundred pounds of the atmosphere was very small, perhaps 

 not more than one or two pounds. The atmospheric improve- 

 ment since has been enormous. The carbon-dioxide is now 



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