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 
their parents, and still more from the 
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 
of geographical distribution, whether of 
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 faunaof 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. 
Effects of migra- 
tion on species. 
Effects of 
isolation. 
