378 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1910. 



peculiar sense it would seem that this might be true of carbon di- 

 oxide, for it is one of the least of the constituents and is one of 

 the most active chemically, and has come thereby to be preeminently 

 the critical constituent of the atmosphere. Some small proportion 

 of this element is altogether necessary to plant life, and so to animal 

 life dependent on plant life, while a large proportion of carbon 

 dioxide would be fatal to air-breathing animals. If the three or 

 four hundredths of one per cent now present in the air were lost 

 all life would go with it; if it were increased to a few per cent the 

 higher life would be suppressed or radically changed. And yet the 

 theoretical sources of supply are abundant enough for imaginable 

 disaster of the one order, while the agencies of depletion have theo- 

 retical efficiency enough for imaginable disaster of the other order. 

 But neither Scylla nor Charybdis has swallowed up the living king- 

 dom. There seems little escape from the conclusion that ever since 

 the birth of air-breathing life, some 30,000,000 or 40,000,000 years 

 ago, let us say, the interplay of the opposing agencies of supply 

 and depletion has been so balanced that neither fatal-excess nor fatal 

 deficiency has been permitted to cut short the history of the higher 

 life. 



The dangers of excess or of deficiencj^ of the other constituents 

 of the air are indeed less narrow as named in percentages, but they 

 are scarcely less real in theoretical possibilities. 



The well-being of life is also hemmed in between a suitable pro- 

 portion of moisture, dependent on an adequate water surface, on the 

 one hand and a diluvial excess on the other. Universal deluges 

 and universal deserts would alike be disastrous to our race. A few 

 thousand feet more of water depth or a few thousand feet less would 

 alike exclude our race and seriously restrict the class of life to which 

 we belong. 



In even a more serious way the habitability of the earth is condi- 

 tioned on a range of mean temperature of some such measure as 100° 

 C, roundly speaking. The higher life is in fact confined to a nar- 

 rower range. This is scarcely 5 per cent of the range of natural 

 temperatures on the earth and a still smaller per cent of the range 

 in the heavens. A few miles above us and a few miles below us 

 fatal temperatures prevail. It is deeply significant that the thermal 

 states of the narrow zone of life on the face of the earth should have 

 been kept within so close a range as to permit millions of species to 

 follow one another in forming the great genealogical lines which lead 

 continuously up from the primitive types to the present ones without 

 breakage of continuity in all the ages, while the prevailing tempera- 

 tures a few miles below them and a few miles above them, as well as 

 in space generally, would have been fatal. Wliile this constant and 

 necessary supply of heat has come from the sun, the control of tem- 



