114 THE FOUNDATIONS OF ZOOLOGY 



which do not leave the feeding area at the proper time. Now 

 if we suppose that the two areas were, for some remote ancestor 

 of the existing species, coincident, but by geological or climatic 

 changes gradually diverged from each other, we can easily under- 

 stand how the habit of incipient and partial migration at the 

 proper season would at last become hereditary, and so firmly 

 fixed as to become what we term an instinct. It will probably 

 be found that every gradation still exists in various parts of the 

 world, from a complete coincidence to a complete separation of 

 the breeding and subsistence areas, and when the natural history 

 of a sufficient number of species is thoroughly worked out, we 

 may find every link between species which never leave a re- 

 stricted area where they breed and live the whole year round, 

 to those other cases in which the two areas are absolutely 

 separated." 



Modern zoology owes its basis to the work of Wallace and Dar- 

 win on the distribution of birds, which, in their hands, has led to 

 a revolution in our conceptions of nature, and has given so much 

 weight to all their utterances on the subject that no one would 

 venture to differ from them inconsiderately, although, when we 

 try to interpret the language which Wallace here uses in the 

 light of his other works, it seems very doubtful whether he has 

 carefully weighed the words in which he here states that " the 

 habit of incipient and partial migration " may " at last become 

 hereditary." We must also bear in mind that migration and dis- 

 tribution are distinct 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, migra- 

 tory birds, like other birds, are kept from invading other provinces 

 than their own by competitors and enemies, rather than by geo- 

 graphical barriers. As so many birds move towards the poles of 

 the earth to lay their eggs, and towards the equator to spend 

 the winter, the view that their breeding area and their subsistence 

 area have gradually become widely separated by changes of cli- 

 mate seems probable at first sight, but this rule is not universal, 

 for many of the great breeding grounds of sea-birds are in tem- 

 perate or tropical waters. The petrels and albatrosses, terns, gulls, 

 and many other birds pass most of their lives scattered over the 



