NEW MALLOPHAGA. 53 



crossing among less closely related individuals. For ex- 

 ample the individuals of a parasite species on a bird of 

 long life and non -gregarious and monogamous habits, 

 like an eagle, live very much the life of an isolated com- 

 munity. There must be many years of in-and-in breed- 

 ing. It is like island life. The result is certain: the 

 members of this isolated group will soon differ from the 

 specific type in noticeable particulars. On the other 

 hand, the conditions of life on this "island" are practi- 

 cally identical with the conditions on other similar 

 "islands" — other eagles — inhabited by other individuals 

 of the same parasite species, so there is no iufluence 

 working to produce a wide divergence of the members of 

 these various isolated groups of individuals of the same 

 species. Now this isolation of groups of individuals is in 

 some degree an incident of the life of all Mallophaga ; in 

 some instances it is considerable: in others, inconsider- 

 able, but taken altogether a condition in the life of the 

 whole order exerting an influence which has the readily 

 recognizable result of creating a great number -of small 

 variations within species limits. 



We have noted now two influences resulting from the 

 peculiar habits of the Mallophaga which are somewhat 

 opposed to each other. One influence, due to the uni- 

 form (as far as relation to parasite goes) conditions of the 

 habitat, the body of the host, tending to preserve essen- 

 tially unchanged the type-forms of the parasites; the 

 other influence, due to the isolation of groups of individ- 

 uals and the consequent close breeding, tending to foster 

 and fix small variations. The results, manifest to any 

 student of the group, are to render difficult the division 

 of the order into distinct genera on account of the general 

 similarity of structure, and to make difficult the definition 

 of species on account of the many slight variations among 



