314 EVOLUTION AND ANIMAL LIFE 



possession, and their descendants are 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. Local pecu- 

 liarities disappear with wide association, and are intensified 

 when individuals of similar peculiarities are kept together. 

 Should later migrations of the original 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 

 on 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. Distinctness is in direct proportion to isolation. 

 What is true in this regard of the fauna of any region as a whole, 

 is likewise 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. 



The laws governing the distribution of animals are reducible 

 to three very simple propositions. Every species of animal is 

 found in every part of the earth having conditions suitable for 

 its maintenance unless: 



(a) Its individuals have been unable to reach this region, 

 through barriers of some sort; or, 



(6) Having reached it, the species is unable to maintain 

 itself, through lack of capacity for adaptation, through severity 

 of competition with other forms, or through destructive con- 

 dition of environment ; or, 



(c) Having entered and maintained itself, it has become 

 so altered in the process of adaptation as to become a species 

 distinct from the original type. 



