ORIGIN AND DIFFERENTIATION 557 



Others on broad, low plains; some on small oceanic islands, others along 

 seacoasts or on isolated mountains in the middle of a great continent. 



Hypothesis of the variation w the atmospheric content of carbon- 

 dioxide. — The general proposition that a change in the amount of carbon- 

 dioxide in the atmosphere might be sufficient to initiate glaciation was 

 suggested by Tyndall more than half a century ago, and in recent years 

 it has received able advocacy by xirrhenius and by Professor Chamberlin. 

 The present normal content of the atmosphere is about .03 per cent. 

 According to calculations by Arrhenius, an increase of carbon-dioxido 

 to .09 per cent would be sufficient to raise the temperature in the polar 

 regions by from 12° to 16° ¥., giving a temperate climate, while a 

 decrease by about one-half of the present amount would be sufficient to 

 bring about a period of glaciation similar to that of the Pleistocene. 



The slight changes required are so very modest in amount that it must 

 be confessed this constitutes a very attractive hypothesis, especially after 

 one has followed Professor Chamberlin's exposition; but when it is 

 critically examined a number of objections obtrude that seem to remove 

 it from the category of major factors in the initiation of glaciation or 

 deglaciation. What seems to me to be a most formidable objection is 

 the rapid and repeated fluctuations of the carbon-dioxide content of the 

 atmosphere that must be postulated to account for the retreat and re- 

 advance of glacial activity during the Pleistocene ice age, to say nothing 

 of those of other geologic times. "Surely," as Edgeworth David *^ has 

 said, "there is hardly scope here for a world-wide variation of carbon- 

 dioxide in the earth^s atmosphere repeating itself so frequently over sucJi 

 short intervals of time." That is to say, that why there should have 

 been this intermittent activity, separated by millions of years in the 

 earlier ages, and the sudden fluctuations in the Pleistocene, separated at 

 most by a few thousands of years, is not apparent; also, it should not 

 be forgotten that according to Schloesing,"^^ and supported by Dittmar, 

 the ocean exercises a powerful and potent control over the amount of 

 carbon-dioxide in the atmosphere. The manner of this action and 

 reaction is set forth by Gregory ^^ as follows : 



"If the amount of carbonic acid in the atmosphere is diminished, the bicar- 

 bonat^s in the sea are dissociated; the gas thus liberated is thus poured into 

 the air, until the former equilibrium between the tension of the carbonic acid 



^0 T. W. Edgeworth David : Conditions of climate at different geological epochs. Cong, 

 geol. internat, Compt. rend., Tenth Session, 1906 [1907], p. 477. 



" Schloesing : Sur le Constance de la Proportion d'acide carbonique dans I'air. 



Compt. Rend., vol. 90, 1880, p. 140. 



« J. W. Gregory : Climatic variations, their extent and causes. Cong. geol. internat., 

 <:ompt. rend.. Tenth Session, 1906 [1907], p. 419. 



