GEOLOGICAL EXTERMINATIONS. 



175 



has occurred in regard to the constitution of the atmosphere or 

 the waters of the earth. It is absolutely necessary for animal life 

 to breathe the atmosphere, whether by taking in the air 

 mixed with the water as fishes take it in, through their respiratory 

 gills, or air-breathing animals, by their lungs or trachea?. Even the 

 existence of plants requires that there should be a certain regulated 

 amount of oxygen in the air, and that there should be a 

 proportionate quantity of nitrogen gas (which latter is neutral in 

 its action) and an extremely minute quantity of carbonic acid gas.* 

 If you place a plant in an atmosphere — and these are results of 

 experiments at the Eoyal Botanic Gardens, Kew, and at other 

 places — too richly endowed with carbonic acid gas, that plant becomes 

 sickly and dies. Therefore there is no doubt that at the time when 

 the great abundance of coal-plants flourished on the surface of the 

 earth, there would, not be such an excess of carbonic acid gas in the 

 atmosphere as to prevent those plants from living. When we are 

 dealing with mere plants, we may say for the sake of argument 

 that plants possibly may have been able to survive with a greater 

 quantity of carbonic acid gas in proportion to the oxygen in the air. 

 But alongside these coal-plants, we know that in the Carboniferous 

 period there were abundant examples of amphibian reptiles, fishes, 

 mollusca, Crustacea, and insects, and that those creatures spent their 

 lives, some in the water, some among the plants, and many of the 

 insects in flying in the air, as, for example, the great dragon-flies, 

 some of which were twenty-four inches across from tip to tip 

 of their wings. Insects, which were all air-breathers, w^ere 

 abundant in the Coal-period. Then there were land-snails, which 

 Sir William Dawson found in hollow tree-trunks of the Sigillarise 

 in the coal-beds of Xovia Scotia ; there were also many " cock- 

 roaches." It is surprising that cockroaches having begun in the 

 Coal-period, should have continued living to the present day 



* Oxygen by volume, 20 - 96 or ^th. 

 Nitrogen by volume, 79*00 or |-ths. 

 Carbonic acid by volume, 0*04 or -^oo- 

 The amount of carbonic acid gas is extremely variable, 3 parts in 10,000 

 is the proportion in the open country, 5 parts in towns, and as much as 

 30 parts in 10,000 of air in overcrowded rooms. More than this acts 

 pouonously on animal life. 



