THE GEOLOGICAL EECOKD 233 



tion, with a variety of nourishing products, in foliage, fruit, 

 and flower, never before available. 



Now here we have a tremendous series of special develop- 

 ments of life-forms simultaneous in all parts of the earth, 

 affecting both plants and animals, insects and vertebrates, 

 whether living on land, in the water, or in the air, all contem- 

 poraneous in a general sense, and all determining the transi- 

 tion from a lower to a very much higher grade of organisation. 

 Just as in the first such great step in advance from the '' age 

 of fishes " to the '' age of reptiles " we see reason to connect 

 it with the change from a more carbonised to a more oxygen- 

 ated atmosphere, produced by the locking up of so much carbon 

 in the great coal-fields of the world ; so, I think, the next groat 

 advance was due to a continuation of the same process by a 

 different agency. Geologists have often remarked on the pro- 

 gressive increase in the proportion of limestone in the later 

 than in the earlier formations. In our own country w^e see a 

 remarkable abundance of limestone during the Secondary era, 

 as shown in our Lias, Oolites, Portland stone, and Chalk rocks ; 

 and somewhat similar conditions seem to have prevailed in 

 Europe, and to a less extent in Xorth America. As limestone 

 is generally a carbonate of lime, it locks up a considerable 

 amount of carbon which might otherwise increase the quantity 

 of carbonic acid in the atmosphere ; and as lime, or its metallic 

 base, calcium, must have formed a considerable portion of the 

 original matter of the earth, solid or gaseous, the continued 

 formation of limestone through combination with the carbonic 

 acid of the atmosphere must have led to the constant diminu- 

 tion of that gas in the same way that the formation of coal 

 reduced it. 



It seems probable that when the earth's surface was in a 

 greatly heated condition, and no land vegetation existed, the 

 atmosphere contained a much larger proportion of carb(m- 

 dioxide than at present, and that a continuous reduction of 

 the amount has been going on, mainly through the extraction 



