XXVI 

 THE PROBLEM OF PARASITISM 



ONE of the perplexing shadows in the pleasant 

 picture of animate nature is the frequency 

 of parasitism. To some minds it appears as a blot 

 spoiling the whole script. But without denying 

 that there is some warrant for practical, aesthetic, 

 and ethical recoil, we think that much of this is due 

 to lack of perspective. Let us take a rapid survey 

 of the facts. Thousands of living creatures, both 

 plants and animals, live in or on others, bound 

 up with them in a brutally direct nutritive depend- 

 ence and incapable of living in any other way. 

 Uninvited non-paying boarders they are, who 

 make their hosts no return for the hospitality 

 enjoyed. When we think of the " minor horrors of 

 war," regarding which Dr. Shipley has written so 

 admirably, of yard-long tapeworms and plump 

 maw-worms in their inglorious life of ease, of mites 

 and ticks innumerable, of fish-lice and flukes, of 

 rusts and mildews and other parasitic fungi, and 

 so on down to the microscopically minute bacilli 

 and trypanosomes, we are appalled at the number 

 and diversity of parasites. It is some relief to find 

 that no backboned animals are parasitic unless it 

 be the hags (Myxine) which sometimes burrow 



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