OTHER THEORIES OF SPECIES-FORMING. 241 



fest in a comparison of offspring and parents, but under 

 normal conditions held in check or lost (unless directly 

 advantageous) by crossing among less closely related indi- 

 viduals. For example, 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 

 community. 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 con- 

 ditions of life on this 'island' are practically identical with 

 the conditions on other similar 'islands' other eagles in- 

 habited by other individuals of the same parasite species, so 

 there is no influence working to produce a wide divergence 

 of the members of these various isolated groups of indi- 

 viduals 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, 

 inconsiderable, but taken altogether a condition in the life 

 of the whole order exerting an influence which has the 

 readily recognisable result of creating a great number of 

 small variations within species limits. 



"The results, manifest to any student of the group, of 

 these two opposing influences are to render difficult the divi- 

 sion of the order into distinct genera on account of the gen- 

 eral similarity of structure, and to make difficult the defini- 

 tion of species on account of the many slight variations 

 among the individuals from different bird individuals."' 



The study of geographic distribution and its influences on 



species-forming has not been limited, of course, to living 



Geographic organisms alone. In fact, the geologic study 



isolation studied of distribution and migration of both animals 



among past 



animals, and plants has given us some of our most 



important facts touching the problem of the influence of 

 isolation on species-transformation. In an interesting paper 



