3 i8 THE POPULAR SCIENCE MONTHLY. 



off-shore islands, like the Bermudas, the remainder drowned in 



the sea. 



Of the immigrants to the Galapagos the majority doubtless 



die and leave no sign. A few will remain, multiply, and take pos- 

 session, 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 indi- 

 viduality would assert itself. They would assume with new envi- 

 ronment new friends, new foes, new conditions. They would de- 

 velop qualities peculiar to themselves— qualities intensified by 

 isolation. " Migration/' says Dr. Coues, " holds species true ; 

 localization lets them slip/' This would be more exactly the 

 truth should we say that localization holds peculiarities true; 

 migration lets them slip. Local peculiarities disappear by 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 surviv- 

 ing would in time form distinct 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 Mr. 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 relations of the fauna of different regions are intimate in 

 direct relation to the ease by which barriers may be crossed. Dis- 

 tinctness 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 move- 

 ment, and variation within what we call specific limits is again 

 proportionate to the barriers which prevent equal and perfect dif- 

 fusion. 



The various divisions or realms into which the surface of the 

 earth may be divided on the basis of the differences in animal life 

 each has its boundary in the obstacles offered to the spread of the 

 average animal. Each species broadens its range as far as it can. 

 It struggles knowingly or not to overcome the barriers of ocean 

 or river, of mountain or plain, of woodland or desert, of moist- 

 ure or drought, of cold or heat, of lack of food or abundance of 

 enemies, whatever these barriers may be. Were it not for these 

 barriers, every species would become what only man now is, prac- 

 tically cosmopolitan. Man is pre-eminently the barrier-crossing 

 animal. The degree of hindrance offered by any barrier to the 



