18 ENTOMOLOGY FOR MEDICAL OFFICERS 



By compounding these different phenomena we get some 

 broad idea of the normal methods of self-preservation 

 that are open to any species of microparasite whose natural 

 habitat is the living blood of a vertebrate animal, provided 

 that other busy blood-sucking parasites are at work on the 

 outside. 



The r61e of the intermediate " carrier " host makes such an 

 impression on the imagination that we are apt to forget that 

 even when the adaptation between microparasite, inter- 

 mediate (carrier) host, and final or destined host is perfected, 

 there must be many failures and hitches. A given micro- 

 parasite must often get into the stomach of the wrong blood- 

 sucking parasite and be digested. A given blood-sucker 

 must often inject the spores of a microparasite into the 

 wrong final host, where they perish. Perhaps the failures 

 may be more numerous than the successes, but this is 

 allowed for by the enormous fertility of the parasite that has 

 to be passively transferred. A cynic has said of man that in 

 every corporate action he finds the right way only after 

 taking every conceivable wrong turning and walking into 

 every pitfall. Probably it is much the same with these 

 adaptive parasites, that in addition to all their other difiSculties 

 have to hit a particular bridge from host to host What 

 appears to the easy-chair scientist as an elegant stream, may 

 be a seething cauldron of eddies and cross-currents. In the 

 language of the poets, chaos lies very close beneath the 

 orderly surface that we love to contemplate as we " mutter 

 the comfortable word ' evolution.' " 



v.— Classification of Arthropod Parasites and 

 Carriers that Affect Man. 



From the facts sampled in the foregoing section, it is 

 evident that it is by no means easy to define either a parasite 

 or a carrier; so that the following attempt to classify the 

 Arthropoda that are detrimental to the human organism is 

 to be regarded merely as a suggestive artifice. 



The species that, whatever else they may do, exert some 

 direct effect upon the tissues of man, may be classified broadly 

 as (a) Accidental and Casual Parasites; (d) Discretionary 



