386 THE WORLD OF LIFE 



which are essential to their contiiiTied existence. It is also a 

 constituent of almost every mineral and rock, and is estimated 

 to form about 47 per cent of the whole mass of the globe. The 

 next most abundant elements are silicon, aluminium, and iron, 

 which form 25, 8, and 7 per cent respectively of the earth- 

 mass. Then follow calcium, magnesium, sodium, and potas- 

 sium, contributing from about 4 to 2 per cent of the whole; 

 while no other element forms so much as one per cent, and the 

 majority probably not more than one-fiftieth or one-hundredth 

 of one per cent. 



The gases, hydrogen and nitrogen, are, however, exceedingly 

 important as forming with oxygen the atmosphere and the 

 oceans of the globe, which by their purely physical action on 

 climate, and in causing perpetual changes on the earth's sur- 

 face, have rendered the development of the organic w^orld pos- 

 sible. These ten elements appear to be all that were necessary 

 to constitute the earth as a planet, and to bring about its varied 

 surface of mountain and valley, rivers and seas, volcanoes and 

 glaciers ; but in order to develop life, and thus clothe the earth 

 with ever-growing richness of vegetation and ever-changing 

 forms of animals to be sustained by that vegetation, four other 

 elements were required — carbon, sulphur, phosphorus, and 

 chlorine — but these being either gaseous or of very small spe- 

 cific gravity, and thus existing (perhaps exclusively) near the 

 earth's surface, comparatively little of them was needed. 



Elements in Protoplasm in Order of their 

 Abundance {approximately). 



List of the More Important Elements 



Elements in the Earth in Order of their 

 Quantity {approximately). 



per cent. 



1. Oxygen 47 



2. Silicon 25 



3. Aluminium 8 



4. Iron 7 



5. Calcium 4 



6. Magnesium 3 



7. Sodium 2.5 



8. Potassium 2.5 



9. Hydrogen (?) 0.1 



10. Nitrogen (?) 0.1 



All others (?) 0.8 



100 



