MIGRATION. 795 



"We must also remember that distribution and migration are dis- 

 tinct phenomena, and that while the geographical distribution of 

 birds shows clear indications of the effect of past geological changes 

 in the distribution of land and water, migratory birds are kept, like 

 other birds, from invading other provinces than their own by com- 

 petitors and enemies rather than by geographical barriers. 



As so many birds move toward the poles of the earth to lay 

 their eggs, and toward the equator to spend the winter, the 

 view that their two homes have been drawn apart by changes of 

 climate seems probable at first sight, but the rule is not universal, 

 for many of the great breeding grounds of sea birds are in temperate 

 or tropical regions. The petrels, albatrosses, terns, gulls, and many 

 other birds pass most of their life scattered over the ocean, but this 

 affords no nesting place, while the wastes of water which keep car- 

 nivorous mammals and reptiles and other enemies of nestling birds 

 from the remote and desolate rocks and sand bars of the open ocean 

 are no obstacle to them. These spots are so secure that birds born in 

 them are much more likely to survive than those born on the shores 

 of inhabited lands, so that it has come about that all or nearly all 

 of the modern members of these groups are descended from ancestors 

 which shunned the dangerous nesting places, not because acquired 

 habits have become hereditary, nor because their feeding ground 

 and their nesting place have been drawn apart by geological change, 

 but because all which did not instinctively lay in safe places the few 

 eggs which are all their fitness for continuous and rapid flight per- 

 mits, have been exterminated. These birds now gather from all 

 parts of the ocean on the few widely scattered rocks and islands 

 where their young are safe, and the periodic assemblies of innumer- 

 able multitudes of wandering sea birds in the " rookeries " are 

 true migrations, for they are as regular as the almanac in the time 

 of arrival and departure, although their feeding ground is almost as 

 extensive as the ocean and the food supply has nothing to do with 

 their movements, and although they do not reach the " rookeries " 

 by a single path. 



In this case the needs of reproduction are the controlling influ- 

 ence, and the site of the " rookery " has been fixed by its safety; and 

 while it is difficult to say how far the birds are guided by knowledge 

 of the danger of other places, the well-known tameness of sea birds 

 in their breeding places, and their apparent ignorance of the exist- 

 ence of enemies seem to show that they are quite unconscious of the 

 advantages of the chosen spot, and that they resort to it automatically 

 or naturally not because they know its safety, but because they owe 

 their survival and existence to the fact that it is safe. 



Zoologists are far too ready to resort to the boundless fields for 



