f 



i 



126 DOCTRINE OF EVOLUTION ! 



an adjustment of life to inorganic environmental in- 

 fluences. While it may seem unjustifiable to speak of j 

 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 i 



so important not only in purely scientific respects, i 



but also in connection with the analysis of human i 



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 j 

 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 living may dry up, and these organisms j 

 may be killed by the bilKon. 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 j 



and the thirsts of the desert. Unknown thousands I 



perish at sea from storm and shipwreck, while the | 

 recorded casualties from earthquakes and volcanic 



eruptions and tidal waves have numbered nearly one j 



hundred and fifty thousand in the past twenty-eight j 



years. The effects of inorganic influences upon all | 



forms of organic hf e 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 



