96 PROCEEDINGS OF THE ACADEMY OF [1892. 



oxygen, is incapable of supporting combustion and closely assails 

 the limits of mammalian life. 



If an animal possessing a given extent of lung surface and pas- 

 sages must, as now constituted, transmit through those passages 100 

 lbs. of air to obtain the necessary 23 lbs. of oxygen, then if some 

 appreciable quantity, say for example, one per cent, by Aveight of 

 CO2 be added to the atmosphere, the animal must pass through its 

 organs 101 lbs. of air to obtain the same rej)aratory result, and so 

 on, in similar proportion, and this result will be accelerated if such 

 addition of carbon dioxide be accompanied by the positive abstrac- 

 tion of oxygen as in the case of the combustion of fixed carbon. 

 But under the continuing effects of diminishing oxygen and increas- 

 ing carbon dioxide, there must come a time when the growing 

 atmos2Dheric modification can no longer be met by adaptation of 

 individuals, but only by a permanent increase of the rapidity of 

 respiration, or an established enlargement of the respiratory organs ; 

 and an animal materially and permanently changed in either 

 respect would amount to a new form, adapted to the new conditions 

 but unable to exist in the old, and only producible by the extinc- 

 tion of existing species no longer fit, and the evolution of favorable 

 variations into new species. 



Hence if it seems probable that existing types of vegetation could 

 only absorb materially increased quantities of carbon by developing 

 specific and generic changes, we may with still more confidence con- 

 clude that animals adapted to prevailing atmospheric conditions, 

 can only acquire the necessary modifications of their respiratory 

 and associated organs by means equally radical. As the principal 

 air-breathing forms, including most reptiles and all birds and mam- 

 mals, slowly succeeded the atmospheric changes of the carbon-fixing 

 period, so under the influence of converse modifications it seems 

 justly inferable that they being the most specialized and least 

 adaptable as individuals, must in turn give place to specific substi- 

 tutions. 



Of course the extent of modification in cellular surface or rapidity 

 of respiration to which existing forms can adapt themselves without 

 such specific destruction and substitution, must chiefly be learned 

 from actual experiments by competent hygienists. 



But whatever may be the value of such speculations upon 

 ultimate consequences, it is certain that with the large and con- 

 stantly accelerating increase in the annual rate of production and 



