218 SECRETS OF ANIMAL LIFE 



condenses from the atmosphere, percolates through 

 the soil, with the help of the associated carbon 

 dioxide dissolves mineral matter, flows to the sea, 

 and rises again in cloud. Thus it regulates the 

 temperature of land and sea, mobilizes and dis- 

 perses large quantities of elements, gives the sea 

 an approximate constancy of composition, canalizes 

 the earth, produces great terrestrial diversity, and 

 so on. So from Professor Henderson's panegyric 

 on water as a factor in evolution we pass naturally 

 to Professor Chamberlin's emphasis on the relative 

 uniformity which the meteorological cycle and 

 other processes have ensured. " Perhaps there is no 

 fact in the earth's career more remarkable than the 

 fidelity with which the very narrow ranges of 

 temperature, and the not less narrow ranges of 

 atmospheric constituents essential to the evolution 

 of life, have been maintained, while oscillations 

 within these permissible ranges have freely pre- 

 vailed. These limits and these oscillations were 

 perhaps as imperative for life's origin as for its 

 prolonged maintenance." The steadiness of the 

 physico-chemical environment, which has been 

 friendly to the ascent of life, is not inconsistent 

 with there being a gradual change in soil and in 

 climate which may have served as a spur to the 

 evolution of organisms. Organic evolution has had 

 its pulse, responding to ameliorating or depress- 

 ing external conditions, just as our individual pulse 

 responds to a sunny or a gloomy day. " Does not 

 a man," John Burroughs said, " imply a cooler 



