No. 569] ECTOPARASITE, 



•259 



mouth parts to do it with, and are perhaps even more 

 specialized in their physiological adaptations to their 

 host than the biting lice, most of the general remarks 

 made concerning the Mallophaga will apply to the suck- 

 ing lice also. 



In their peculiar special relations to their hosts as per- 

 manent ectoparasites on them, wingless, and reluctant to 

 migrate even with opportunity, and so fitted physiologi- 

 cally to their parasitic life that they can not live for more 

 than a few hours (or, at most, and exceptionally, days) 

 off the bodies of their hosts, the Anoplura and Mallophaga 

 are alike. And hence the conditions and problems of 

 their distribution and species-forming are practically the 

 same for the two groups. 



The thesis that I have maintained, on a basis of the 

 conditions presented by the bird-infesting Mallophaga, 

 I now wish to test by the conditions presented by the 

 mammal-infesting Mallophaga and Anoplura. This thesis 

 is, in fewest words, that the host distribution of these 

 wingless permanent ectoparasites is governed more by 

 the genetic relationships of the hosts than by their geo- 

 graphic range, or by any other ecologic conditions. The 

 fact, proved by abundant cases, that two host species of 

 wholly distinct geographic range and with no possible 

 opportunity for contact such as would permit of the 

 migration of wingless parasites from one to the other, 

 may have, nevertheless, one or more parasitic species 

 common to them both, is associated almost always with 

 the further fact that these common hosts are closely 

 related genetically. They are most often of the same 

 genus or of closely allied genera; they are almost cer- 

 tainly always of the same subfamily or family. The ex- 

 planation for the possibility and the reality of this inter- 

 esting host distribution I find in the hypothesis that the 

 common parasite species has persisted unchanged from 

 a common ancestor of the now divergent but allied host 

 kinds. 



Also, if it be true that genetic relationship is the deter- 



