GEOLOGICAL EXTERMINATIONS. 



169 



the ages. If we may judge from the flora, it reached its present 

 quality in the Pliocene. The change since that period appears 

 to have been an increase of its amount. 



It seems, therefore, from the rock-record, that the free oxygen 

 in the atmosphere, the purity of the water of the seas, and the 

 quality of the soil went on, side by side, ever increasing from 

 the dawn of life, till at last the loss and gain became equal near 

 this end of the world's history. In short, there was from period 

 to period a great improvement, world-wide and permanent, in 

 the life-sustaining powers of these biological factors, and always 

 towards present conditions. 



Those at the first were prohibitive of any form of life ; we 

 know what they are now. Intermediate in time the conditions 

 made only intermediate progress, and the rocks show organisms 

 intermediate between the earliest and latest forms. 



Was this coincidence merely a matter of chance, or was it an 

 instance of cause and effect ? It appears to me to be the latter, 

 because an intimate relation exists between species and their 

 environment. Leaving out of account temperature, which seems 

 during by far the larger part of geological time to have been 

 pretty much the same everywhere, what other environments 

 existed besides those which we are now considering ? Is it not 

 reasonable to suppose that changes in them would seriously 

 affect both plants and animals ? That floras and faunas dis- 

 appeared admits of no doubt, and, so far as I am aware, no 

 other causes possessing the world-wide extent and permanency 

 required are known. 



Nor are we confined to theoretical reasoning. Not a few 

 facts are established which have an important bearing on the 

 question. Birds and small mammals, placed in an atmosphere 

 having two or three times the normal amount of oxygen, do not 

 long survive. Fishes brought out of water are said to die from 

 the great amount of oxygen they are compelled to inhale. 

 Plants too highly manured lose the power of reproduction. 

 The seed does not form, or if it forms, does not mature. Even 

 corals are sensitive to the purity of the water in which they 

 live, and if that is materially changed they die. If this is true 

 of present species, is it unreasonable to believe that organisms 

 made for the atmosphere, water and soil of their native period, 

 would die out when these had greatly changed ? In this pro- 

 gressive improvement in the quality of these biological factors 

 is found, it seems to me, at least one cause — perhaps the cause 

 — of the disappearance of species and of their never reappearing. 

 Organisms to-dav have the power of adapting themselves to 



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