SOME MODERN VIEWS 19 



by geological and climatic changes gradually divero'ed 

 from each other, we can easily understand how the habit 

 of incipient and partial migration at the proper seasons 

 would at last become hereditary, and so fixed as to be 

 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 separa- 

 tion of the breeding and subsistence areas ; and when 

 the natural history of a sufficient number of species in all 

 parts of the world is thoroughly worked out, we may find 

 every link between species which never leave a restricted 

 area in which they breed and live the whole year round, 

 to those other cases in which the two areas are absolutely 

 separated. The actual causes that determine the exact 

 time, year by year, at which certain species migrate will 

 of course be difficult to determine." 



The advances made in our knowledge on this subject 

 since Dr Wallace published this illuminating explanation 

 in 1874, all go to strengthen his views, so much so that 

 a good deal which was then regarded as hypothetical is 

 now generally accepted, while the rest may be considered 

 as distinctly probable. 



Dr Weismann ^ has also expressed some interesting 

 and helpful views as to how the present distribution 

 of migrants in Europe came about. He is of opinion 

 that many of the birds which now inhabit the central 

 and northern regions of Europe were during the glacial 

 period wanting, because the climate was too severe. 

 They must, therefore, he considers, have come sub- 

 sequently from the south, and with the gradual rising 

 of the temperature there must have been a corresponding 

 steady but very gradual influx of birds into the north. 



I Co7ttemporary Revinu^ Feb. 1 879. 



