126 DOCTRINE OF EVOLUTION 



an adjustment of life to inorganic environmental in- 

 fluences. While it may seem unjustifiable to speak of 

 heat and cold and sunlight as enemies, the direct 

 effects produced by these forces are to be reckoned 

 with no less certainty than the attacks of living foes. 



The three divisions of the struggle for existence are 

 so important not only in purely scientific respects, 

 but also in connection with the analysis of human 

 biology, that we may look a little further into their 

 details, taking them up in the reverse order. Re- 

 garding the environmental influences, the way that un- 

 favorable surroundings decimate the numbers of the 

 plants of any one generation has already been noted, 

 and it is typical of the vital situation everywhere. 

 English sparrows are killed by prolonged cold and snow 

 as surely as by the hawk. The pond in which bacteria 

 and protozoa are hving may dry up, and these organisms 

 may be killed by the billion. Even the human species 

 cannot be regarded as exempt from the necessity of 

 carrying on this kind of natural strife, for scores and 

 hundreds die every year from freezing and sunstroke 

 and the thirsts of the desert. Unknown thousands 

 perish at sea from storm and shipwreck, while the 

 recorded casualties from earthquakes and volcanic 

 eruptions and tidal waves have numbered nearly one 

 hundred and fifty thousand in the past twenty-eight 

 years. The effects of inorganic influences upon all 

 forms of organic life must not be underestimated in view 

 of such facts as these. 



In the second place, the vital struggle includes the bat- 

 tles of every species with other kinds of living things whose 

 interests are in opposition. The relations of protozoa 



