reoccupied the ground they had lost, and these annual oscillations 
becoming greater and greater as the glacial age gradually gave 
way to an interglacial or post-glacial age, the conditions of migra¬ 
tion which we observe today became established by long repeated 
practice. 
The Dixon-Braun A third aiicestral-habit theory was developed 
independently in 1900 by Braun, who observed 
migrating birds extensively for several years in Constantinople 
and in 1892 by the English ornithologist, Dixon. The Dixon- 
Braun theory postulates that the center of distribution of birds, 
that is, their original home as a class, was not in the north as the 
Deichler-Jager theory assumes, but in the tropics. The reason 
for this conclusion lies in the fact that many genera of our migra¬ 
tory birds are most largely represented by tropical species which 
do not migrate at all. For example, there are many more species 
of flycatchers remaining throughout the year in the tropics than 
migrating north, indicating that the original distribution-center 
from which flycatchers in general have spread must have been in 
the tropics where they are now most at home. As a result of over¬ 
population or famine in times past these tropical birds have been 
forced to travel to less crowded and more favorable localities for 
food. Relief could be found only toward the north since overpop¬ 
ulation is most likely to occur during the spring breeding season at 
a time when the northland is just released from the rigors of win¬ 
ter. Thus, according to the Dixon-Braun theory the first migra¬ 
tion was a spring migration while according to the Deichler-Jager 
theory the first migration was a fall migration. The Dixon-Braun 
theory further supposes that the original spring migrants, having 
been forced north by over-population are in turn compelled as 
winter comes on to retreat south into the overcrowded tropics or 
perish, only to repeat the experiment of finding relief in the north 
as soon as the advent of spring allows. In this way the old birds 
perform again what in their experience had proved to be a success¬ 
ful experiment, while the young birds go along with them and 
learn the habit. 
The Kobelt- A recent attempt has been made by Duncker 
Duncker Theory in Bresslau (1905) to Combine elements of the 
two theories last mentioned. Duncker accepts the classification 
of birds made by Kobelt (1902) into summer-excursionists (Som- 
merfrischler) and zvinter-zvanderers (Winter fluchter). The for¬ 
mer are birds whose home was originally in the south but who 
now make an annual excursion (Badereise!) north in order to 
breed returning home again as soon as this function is accom¬ 
plished, while the latter comprise those whose home was always in 
