THE GEOLOGIC FUNCTIONS OF LIFE. 641 



bustion and all kinds of ordinary decay, and this constitutes a part of 

 the organic residue. Animals never completely oxidize all the organic 

 matter they take into their systems; their bodies never entirely con- 

 sume themselves. A like statement may be made respecting those 

 plants that feed on organic matter. That which animals and plants 

 leave imoxidized is indeed more or less preyed upon by other animals 

 and plants, and relatively little escapes fuial reoxidation, but there is 

 a remnant, and this constitutes another part of the organic residue. The 

 more conspicuous forms of the organic residue are found in the mucks, 

 peats, lignites, coals, organic oils, and gases, but in addition there is 

 not a little disseminated organic matter in nearly all the sedimentary 

 rocks; in the aggregate, this probably amounts to more than the dis- 

 tinct organic deposits. 



The meaning of the organic residue. — All the unoxidized, or incom- 

 pletely oxidized, carbon in the organic residue implies that oxygen has 

 previously been separated from this residual carbon by plants and given 

 to the atmosphere, and hence has been a source of atmospheric enrich- 

 ment in oxygen. The amount thus contributed is equal to that which 

 is required to restore the residual carbon to its original state of oxida- 

 tion. So, in a similar way, the unoxidized hydrogen in the organic 

 hydrocarbons and like compounds implies that oxygen has been sep- 

 arated from the hydrogen of water and given to the atmosphere, and 

 hence this also is a source of atmospheric enrichment in oxygen. It 

 seems safe, therefore, to conclude that the action of life, taken as a 

 v\diole, has increased the free oxygen of the atmosphere. 



While not here under consideration, it is not to be forgotten that 

 inorganic processes involving the same atmospheric constituents have 

 been in operation concurrently with the organic processes, and that 

 they have also affected the amounts and proportions of the atmospheric 

 constituents. Rocks have been oxidized in greater or less measure at 

 the expense of the atmospheric oxygen, and hence when the total atmos- 

 pheric problem is considered, there arises the question whether the 

 amotmt of oxygen in the atmosphere has been increased or diminished 

 during geological history, when the balance is struck between the inor- 

 ganic and the organic actions. The probabilities seem to us to strongly 

 favor the view that organic action has preponderated, and that the 

 oxygen has been increased beyond its primitive amount, but that it 

 has fluctuated during kno^m geological history. The reasons for this 

 ^iew will appear in the historical chapters. 



