ATMOSPHERIC FACTOR IN EVOLUTION. 295 



known. About 20 per cent, of the air is oxygen and only about 

 .04 per cent, carbon dioxide : that is there is about 500 times 

 more oxygen than carbon dioxide. How has this come about? 

 How has the carbon dioxide been so tremendously reduced in 

 quantity? It has all been done by the action of plants, through 

 the ages, on the carbon dioxide in the air. In the green parts 

 of plants, that is in the presence of chlorophyll and in the presence 

 of sunlight, the carbon dioxide of the air was split up into carbon 

 and oxygen : the carbon was absorbed by the plant, and helped 

 to form its leaves, twigs and branches. Some of these leaves, 

 twigs and branches ultimately fell or were blown into water, 

 where they became water logged and sank to the bottom. There 

 they were preserved from contact with the air and its decaying 

 influence, and ultimately helped to form a coal-bed. There 

 is only one point to discuss with reference to the oxygen and 

 carbon dioxide in the air at the present time. Is the carbon 

 dioxide in the air still decreasing now as it has done for untold 

 ages in the past? 



Of late years a new process has been started by man : a pro- 

 cess which acts in a contrary direction to that which Nature has 

 been pursuing from the beginning. Man has unearthed the 

 carbon buried by Nature, and 'has given it back to the air as 

 carbon dioxide. Never was there a time when man was so 

 extravagant in the use of Nature's stores. This is truly an 

 animal age, with its almost innumerable animals, giving out 

 carbon dioxide into the air. This is, therefore, an age, too, of 

 the manufacture of carbon dioxide, just as the Carboniferous 

 period must have been a vegetable age and an age' of the 

 manufacture of oxygen. Our question then is this: — Is man's 

 extravagance such as to check for a time the gradual diminu- 

 tion by Nature of the air's supply of carbon? To assist us in 

 answering this question let us again take Great Britain and 

 Ireland, and again let us assume that the air above them was 

 shut off from the rest of the air ; and let us suppose that all the 

 carbon in the enclosed space was converted into coal, the result- 

 ing amount would be about equal to two years' output at the 

 present time. In other words, assuming that for these two 

 years the plant and animal action on the carbon of the air 

 iDalanced one another, and assuming that the two years output 

 of coal were all burned in the enclosed area, the quantity of 

 carbon dioxide in the air would be doubled at the end of the 

 two years. Considering this fact, I am decidedly of opinion 

 that at the present time the quantity of carbon in the air is on 

 the increase, and will continue to increase for some hundreds or 

 it may be thousands of years. This is a point which will be 

 or at least ought to be definitely determined in a few years 

 by means of careful experiments made, say, yearly, simply for 

 the express purpose of determining this point. 



But possibly the variation may be too small to determine 

 chemically. Now sometimes, when chemistry fails to give evidence 

 of a point, Physiology steps in. Let us see what Physiology 

 has to say on this point. We remember the proposition that 

 the less oxygen and the more carbon dioxide in the air, in which 



