OCTOBER, 1902. 
THE MIGRATION OF BIRDS. 
183 
instinct. It will probably be found, that every gradation still ex- 
ists 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 restricted area in which they breed, and live 
there the whole year round, and those other cases in which the 
two areas are absolutely separated" (i6). 
This theory exerted much influence upon the students of the 
time and is still often quoted as an explanation. 
Weismann presented his theory in a quite lengthy discussion in 
1878 (17). His first step is to show why birds migrate at all, 
and he arrives at the conclusion that it is mainly due to an ab- 
sence of food in the northern and southern portions of the geo- 
graphical distribution of birds at opposite seasons of the year. 
He conceives the idea that the original home of the birds is in 
the tropics, and that from here they dispersed into the temperate 
and friged zones, met with unfavorable conditions in winter and 
were forced to seek new areas. A struggle for existence ensued, 
resulting in the survival of those that returned to the south. 
Through natural selection the phenomenon of migration then be- 
came an instinct. 
"Let us suppose, for example." he says, "that the waxwing 
had not become an inhabitant of Russia, but was living summer 
and winter alike in Germany, slowly multiplying and therefore 
gradually extending its range further north. 
''Now we will imagine a flock of these birds to have colonized 
further north. In the very first winter they would find their 
food becoming scarce, and would be compelled to wander about 
in search for it ; all, that is to say, which had taken a wrong di- 
rection. Only those which, whether by accident or by remember- 
ing the way they had come, took a southerly course, would have 
any prospect of outliving the winter. 
'Tn each succeeding winter, therefore, a selection would take 
place among the northern colonists, and only those would re- 
main alive which had migrated southward. As these alone would 
remain to perpetuate in the next year, this habit of a southerly 
flight would be transmitted to their descendants, and so a race 
would arise predisposed by habit not to wander hither and thither 
16. Nature, vol. 10, p. 4.59. 
17. Conterap. Review, vol. 34, p. .531. 
