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." s 



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 

 Im^ast 1 ^ 64 of distribution and migration of both animals 

 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 



