THE DISTRIBUTION OF SPECIES. 



195 



thus native to the islands. But, isolated from the great 

 mass of their species and bred under new surroundings, 



these island birds come to differ from 

 Effects of migra- their nt and stin more frQm the 



tion on species. . . 



great mass of the land species of which 



their ancestors were members. Separated from these, 

 their individuality would manifest itself. They would 

 assume with new environment new friends, new foes, 

 new conditions. They would develop qualities peculiar 

 to themselves qualities intensified by isolation. " Mi- 

 gration," says Dr. Coues, "holds species true; localiza- 

 tion lets them slip." This would be more exactly the 

 truth should we say that localization holds peculiarities 

 true ; migration lets them slip. Local peculiarities dis- 

 appear with wide association, and are intensified when 

 individuals of similar peculiarities are kept together. 

 Should later migrations of the original land species come 

 to the islands, the individuals surviving would in time 

 form new species, or, more likely, mixing with the mass 

 of those already arrived, their special characters would 

 be lost in those of the majority. 



The Galapagos, first studied by Darwin, serve to 



us only as an illustration. The same problems come up 



in one guise or another in all questions 



Effects of f g e0 or ra phical distribution, whether of 



isolation. 



continent or island. The relation of 



the fauna of one region to that of another depends on 

 the ease with which barriers may be crossed. Distinct- 

 ness is in direct proportion to isolation. What is true in 

 this regard of the fauna of any region as a whole is like- 

 wise true of any of its individual species. The degree of 

 resemblance among individuals is in direct proportion 

 to the freedom of their movements, and variations within 

 what we call specific limits is again proportionate to the 

 barriers which prevent equal and perfect diffusion. 



