ATMOSPHERIC VARIATION AS A FACTOR IN ORGANIC 



EVOLUTION. 



By David Traill, M.A., M.B., Ch.M., B.S. 



Before entering on the subject proper of my l-'aper, I will 

 enunciate two propositions. 



The first is : TJie animal kingdom is dependent on the vege- 

 table ; and conversely, the vegetable is dependent on the animal. 



The truth of the first part of this proposition will at once be 

 acknowledged, but the second part, namely that plant life is ulti- 

 mately dependent on animal life, is not as self-evident. Indeed 

 at first one is apt to say " What a riotous time of it plants would 

 have if there were no animals to destroy and devour them." But 

 the good time would be only of short duration. Animals are con- 

 tinually increasing the carbon dioxide in the air, plants remove 

 some of this carbon from the air ; part of the carbon is buried 

 and permanently removed from the air. In this way plants, if 

 alone on the earth, would gradually diminish the quantity of 

 carbon in the air, and ultimately exhaust it, when all plant life 

 would come to an end. 



Thus we see that the vegetable kingdom is ultimately 

 dependent on the animal. 



The other proposition is : (i) The more oxygen and the less 

 carbon dioxide in the air the livelier and brisker the life of the 

 animal living in that air; (2) the greater the appetite, but (3) the 

 shorter the life; and conversely the less oxygen {within certain 

 and safe limits) and the more carbon dioxide in the air, then (i) 

 the more sluggish the life, (2) the smaller the appetite; but {3) the 

 longer the life. 



This proposition is self-evident when we consider (i) the effect 

 of pure oxygen on the burning of a fire and (2) the close analogy 

 between the life of an animal and the life of a fire. 



I shall now take up the subject of " Atmospheric Variation," 

 and may here state that it is almost solely with the oxygen and 

 carbon dioxide in the air that I shall deal, especially with their 

 relative proportions to one another, and more especially with the 

 variation of the quantity of carbon in the atmosphere. 



I have chosen three special points in time, and I shall specially 

 consider the relative proportions of oxygen and carbon dioxide to 

 one another at these times. The three points of time are (i) at 

 the beginning, (2) at the present time, (3) at the end. 



(i) At the beginning. The " beginning " for us means the 

 time of the first birth of organic life on our planet. What was the 

 relative proportion of oxygen to carbon dioxide at that time? At 

 the beginning, after this world of ours had solidified, and its crust 

 had cooled sufficiently for organic life to start, the probability is 

 that there was very little free oxygen in the air, most probably 

 all of the present free oxygen being tied up with carbon in the 

 form of carbon dioxide. Lord Kelvin was of this opinion. All the 

 elements had been at a great heat, and had gradually cooled. The 



