296 ATMaSPHERIC FACTOR IN EVOLUTION. 



(6) It could only have been in very recent years thatr 

 man and many other animals, as at present constituted, could 

 have lived. 



(7) In the future, we would expect plant life gradually to 

 dwindle as carbon gets scarcer and scarcer. 



(8) We would expect the highest power of cerebration with 

 the most complete oxidation of the blood. In the future, then 

 the atmospheric condition will be even more favourable tO' 

 mental activity than at present ; owing to the increase of oxygen 

 in the air. 



There are still certain deductions of more general applica- 

 tion than those already given. 



(9) Plant life under water, being less directly under the 

 influence of the air changes, would change less than the aerial- 

 plants. 



(10) The lungs (or breathing organs) of animals, would, 

 as the oxygen became more plentiful, be expected to diminish 

 in proportion to the rest of the body. 



(11) Similarly we would expect the lungs of plants (namely,. 

 their leaves) to increase in size relatively to the size of the plant,, 

 as the carbon dioxide in the air got less and less. 



(12) The temperature of animals should rise as the oxygen 

 in the air increases, due to improved oxidation. We thus see 

 the reason of the change from cold-blooded to the warm-blooded 

 animals of the present day. 



In conclusion I will indicate some directions in which ex- 

 periments might be made. 



Plants : We often hear of the experiment of growing 

 plants under electric light, and of passing electric currents 

 through the soil in which they are growing. Experiments, 



too, on the effects of various manures, that is foodstuffs, given 

 to the roots of plants are being continually tried. But I have 

 not heard of what one would think would be the most natural 

 way of stimulating plant growth, and that is by increasing its 

 aerial food, the carbon dioxide in the air. 



In experimenting on the effects of increased quantities of 

 carbon dioxide in the air the quantity of carbon dioxide would 

 have to- be gradually increased, and that of oxygen correspond- 

 ingly decreased; and the more gradual the change, the greater 

 would be the ultimate effect on the plants. One would expect 

 primitive plants to respond most readily to this experiment. 

 After a certain point was reached in the relative proportions of 

 carbon dioxide and oxygen we would expect the most highly 

 developed, that is the most recently evolved plants to die. 



Animals : Experiments made on similar Unes with animals 

 we would expect to produce similar results. As the carbon 

 dioxide increased, and oxygen became less, soon the more 

 highly developed animals would die. Suppose this experiment 

 to be carried on indefinitely and with infinitely small changes 

 in the proportions of carbon dioxide to oxygen, we would 

 expect ultimately to get back to the most primitive form of 

 animal life. 



