DISHARMONIES AND OTHER SHADOWS 581 



cular adhesive suckers and sometimes attaching hooks; it 

 can thrive with a minimum of oxygen; it has a mysterious 

 ' anti-body ' which prevents it being digested by its host ; it 

 is exceedingly prolific; and it is self-fertilising. The tape- 

 worm may be ugly, but it is very well-adapted ; it may be 

 repulsive, but in the technical biological sense, relative to 

 the given conditions, it is ' fit '. 



Such are a few of the most important facts in regard to 

 parasitism. Let us now inquire why the prevalent inter-rela- 

 tionship seems to many a dark shadow. Parasitism is re- 

 pulsive for three reasons: (1) because we dislike to see fine 

 organisation devastated, (2) because many parasites produce 

 an unpleasant aesthetic impression, and (3) because the life 

 of ease and sluggish dependence grates on our ethical sense 

 or on our idea of an organism. 



(1) Many people resent the fact that a contemptible mi- 

 crobe may kill a genius before he comes of age, and that 

 paltry flies put a drag on the wheel of the chariot of civili- 

 sation. It is abhorrent that fine organisation should be 

 spoiled by intrusive parasites, but it is necessary to look all 

 round the facts, (a) In a multitude of cases the parasites 

 do not greatly trouble their hosts, a modus vivendi has been 

 established. If the host be of a weakly constitution or en- 

 feebled by lack of food, the parasites hitherto trivial may 

 get the upper hand and bring about the death of the host. 

 But this sifting will make for racial health, and cannot be 

 called abhorrent. (6) Mortality from parasites is in most 

 cases a consequence of organisms entering a new area and 

 becoming liable to attack by creatures to which they can 

 offer no natural resistance, or a consequence of the introduc- 

 tion of the parasite into a new area where it finds new 

 hosts abnormally susceptible to it. Cattle introduced into a 



