THE GEOLOGIC FUNCTIONS OF LIFE. 639 



may be regarded as in some sense the basal material or the fundamental 

 food of the organic kingdom, and hence it plays a radical role in the 

 life-history of the earth. 



AYater, and the constituents of water, oxygen and hydrogen, play 

 a larger part quantitatively, but a less distinctive part. 



Nitrogen is also an essential element, and usually stands next to 

 carbon, oxygen, and hydrogen in quantity. 



These, it will be noted, are all atmospheric constituents, and the 

 material of life is, therefore, dominantly atmospheric. This is even 

 true of aquatic life, for it lives largely on the atmospheric constituents 

 dissolved in the water. The function of life, considered from the mate- 

 rial point of view, is not only fundamentally concerned with the atmos- 

 phere, and intimately dependent on its conditions, but its most impor- 

 tant material effects appear to lie in its modification of the constitution 

 of the atmosphere. 



The non-atmospheric factors. — The atmospheric constituents are 

 not, however, the only elements intimately connected with the hfe 

 function. Compounds of sulphur, phosphorus, potassium, sodium, 

 chlorine, iron, calcium, magnesium, silicon, and other elements are 

 more or less essential to the life of many organisms, or are employed 

 by them for their skeletons, coverings, etc. Incidentally, nearly all 

 the common elements become intimately related to living organisms 

 either in the relations of active elements in their physiological func- 

 tions, or of passive elements in their structure or in their auxiliary 

 parts. 



Three Classes of Effects. 



Out of life processes grow three rather distinct classes of results: 

 (1) changes in the amounts and proportions of the constituents of the 

 atmosphere and, to some slight extent, of the hydrosphere and litho- 

 sphere; (2) aid or hindrance to inorganic processes, such as disinte- 

 gration, erosion, and deposition; and (3) distinctive products, either 

 (a) of organic matter that would not have come into the existing com- 

 bination but for life, such as peat, lignite, amber, etc., or (5) of special 

 forms of inorganic matter that would not have arisen but for hfe, such 

 as coral deposits, shell-marl, diatom ooze, etc. 



(1) Changes in the composition of the atmosphere. 

 The succession of modifications which the atmosphere has under- 



