CHAPTER XIV. 

 PARASITISM. 



A simple calculation will show that the unrestricted 

 multiplication of any living organism would cause it to 

 cover the entire surface of the earth in the course of a 

 relatively short time. No organism has, however, met 

 with such numerical success. The earth is tenanted by 

 a highly diversified flora and fauna in which, though 

 certain forms greatly preponderate over others in num- 

 bers, all are restricted by certain conditions that may 

 not be overcome. 



The first of these has to do with the unequal condition 

 of the earth's surface where variations of temperature, 

 moisture, chemical composition of the soil, exposure to 

 violent winds and deluges of rain determine that great 

 numbers of living things of many different kinds shall 

 thrive in hospitable localities, diminishing numbers in- 

 habit less hospitable localities, and none at all be found 

 in the most inhospitable localities. 



The second have to do with the behavior of the living 

 things toward one another, which, though appearing 

 harmonious upon cursory observation, is found upon 

 critical examination to be an unending interference 

 which Darwin has aptly described as the " struggle for 

 existence." 



In this perpetual strife it is easily conceived that those 

 forms best adapted to the exigencies of the situation will 

 survive while their less fortunate fellows must fail; 

 hence the "struggle for existence" results in the " sur- 

 vival of the fittest." 



These subjects are treated at considerable length else- 

 where, but for an intelligent understanding of the subject 



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